CE6502 FOUNDATION ENGINEERING UNIT I SITE INVESTIGATION AND SELECTION OF FOUNDATION 1.1 TYPES OF BORING 3

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1 Sl.No Contents Page No. CE6502 FOUNDATION ENGINEERING UNIT I SITE INVESTIGATION AND SELECTION OF FOUNDATION 1.1 TYPES OF BORING TYPES OF SAMPLES IN-SITU TESTS GENERAL PENETROMETER TESTS STATIC CONE PENETRATION TEST 12 UNIT II SHALLOW FOUNDATION 2.1 INTRODUCTION 2.2 DIFFERENT TYPES OF FOOTINGS METHODS OF DETERMINING BEARING CAPACITY UNIT III FOOTINGS AND RAFTS 3.1 COMBINED FOOTING 54 UNIT IV PILE FOUNDATION 4.1 DESIGN METHODOLOGY FOR PILES CLASSIFICATION OF PILES POINTS TO BE CONSIDERED FOR CHOOSING PILES PILES IN SAND SETTLEMENT OF PILE GROUPS 78 UNIT V RETAINING WALLS 5.1 RETAINING WALL DIFFERENT TYPES OF RETAINING STRUCTURES COUNTERFORT RETAINING WALL 83 T SHARMILA Page 1

2 CE2305 FOUNDATION ENGINEERING L T P C OBJECTIVE At the end of this course student acquires the capacity to assess the soil condition at a given location in order to sugest suitable foundation and also gains the knowledge to design various foundations. UNIT I SITE INVESTIGATION AND SELECTION OF FOUNDATION 9 Scope and objectives Methods of exploration-auguring and boring Water boring and rotatory drilling Depth of boring Spacing of bore hole - Sampling Representative and undisturbed sampling sampling techniques Split spoon sampler, Thin tube sampler, Stationary piston sampler Bore log report Penetration tests (SPT and SCPT) Data interpretation (Strength parameters and Liquefaction potential) Selection of foundation based on soil condition. UNIT II SHALLOW FOUNDATION 9 Introduction Location and depth of foundation codal provisions bearing capacity of shallow foundation on homogeneous deposits Terzaghi s formula and BIS formula factors affecting bearing capacity problems - Bearing Capacity from insitu tests (SPT, SCPT and plate load) Allowable bearing pressure, Settlement Components of settlement Determination of settlement of foundations on granular and clay deposits Allowable settlements Codal provision Methods of minimising settlement, differential settlement. UNIT III FOOTINGS AND RAFTS 9 Types of foundation Contact pressure distribution below footings and raft - Isolated T SHARMILA Page 2

3 and combined footings Types and proportioning - Mat foundation Types, applications uses and proportioning-- floating foundation. UNIT IV PILES 9 Types of piles and their function Factors influencing the selection of pile Carrying capacity of single pile in granular and cohesive soil - Static formula - dynamic formulae (Engineering news and Hiley s) Capacity from insitu tests (SPT and SCPT) Negative skin friction uplift capacity Group capacity by different methods (Feld s rule, Converse Labarra formula and block failure criterion) Settlement of pile groups Interpretation of pile load test Forces on pile caps under reamed piles Capacity under compression and uplift. UNIT V RETAINING WALLS 9 Plastic equilibrium in soils active and passive states Rankine s theory cohesionless and cohesive soil - Coloumb s wedge theory condition for critical failure plane - Earth pressure on retaining walls of simple configurations Graphical methods (Rebhann and Culmann) - pressure on the wall due to line load Stability of retaining walls. TOTAL: 45 PERIODS TEXT BOOKS 1. Murthy, V.N.S, Soil Mechanics and Foundation Engineering, UBS Publishers Distribution Ltd, New Delhi, Gopal Ranjan and Rao, A.S.R. Basic and Applied Soil Mechanics, Wiley Eastern Ltd., New Delhi (India), REFERENCES 1. Das, B.M. Principles of Foundation Engineering (Fifth edition), Thomson Books / T SHARMILA Page 3

4 COLE, Bowles J.E, Foundation analysis and design, McGraw-Hill, Punmia, B.C., Soil Mechanics and Foundations, Laxmi publications pvt. Ltd., New Delhi, Venkatramaiah,C. Geotechnical Engineering, New Age International Publishers, New Delhi, 1995 UNIT I SITE INVESTIGATION AND SELECTION OF FOUNDATION Types of boring 1.Displacement borings It is combined method of sampling & boring operation. Closed bottom sampler, slit cup, or piston type is forced in to the ground up to the desired depth. Then the sampler is detached from soil below it, by rotating the piston, & finally the piston is released or withdrawn. The sampler is then again forced further down & sample is taken. After withdrawal of sampler & removal of sample from sampler, the sampler is kept in closed condition & again used for another depth. Features : Simple and economic method if excessive caving does not occur. Therefore not suitable for loose sand. Major changes of soil character can be detected by means of penetration resistance. These are 25mm to 75mm holes. It requires fairly continuous sampling in stiff and dense soil, either to protect the sampler from damage or to avoid objectionably heavy construction pit. 2.Wash boring: It is a popular method due to the use of limited equipments. The advantage of this is the use of inexpensive and easily portable handling and drilling equipments. Here first an open hole is formed on the ground so that the soil sampling or rock T SHARMILA Page 4

5 drilling operation can be done below the hole. The hole is advanced by chopping and twisting action of the light bit. Cutting is done by forced water and water jet under pressure through the rods operated inside the hole. In India the Dheki operation is used, i.e., a pipe of 5cm diameter is held vertically and filled with water using horizontal lever arrangement and by the process of suction and application of pressure, soil slurry comes out of the tube and pipe goes down. This can be done upto a depth of 8m 10m (excluding the depth of hole already formed beforehand) Just by noting the change of colour of soil coming out with the change of soil character can be identified by any experienced person. It gives completely disturbed sample and is not suitable for very soft soil, fine to medium grained cohesionless soil and in cemented soil. V T SHARMILA Page 5

6 1.1 Planning For Subsurface Exploration The planning of the site exploration program involves location and depth of borings, test pits or other methods to be used, and methods of sampling and tests to be carried out. The purpose of the exploration program is to determine, within practical limits, the stratification and engineering properties of the soils underlying the site. The principal properties of interest will be the strength, deformation, and hydraulic characteristics. The program should be planned so that the maximum amount of information can be obtained at minimum cost. In the earlier stages of an investigation, the information available is often inadequate to allow a firm and detailed plan to be made. The investigation is therefore performed in the following phases: 1. Fact finding and geological survey Reconnaissance 1. Preliminary exploration 2. Detailed exploration 3. Special exploration 1. Fact finding and geological survey Assemble all information on dimensions, column spacing, type and use of structure, basement requirements, and any special architectural considerations of the proposed building. Foundation regulations in the local building code should be consulted for any special requirements. For bridges the soil engineer should have access to type and span lengths as well as pier loadings. This information will indicate any settlement limitations, and can be used to estimate foundation loads. 2. Reconnaissance This may be in the form of a field trip to the site which can reveal information on the type and behavior of adjacent sites and structures such as cracks, noticeable sags, and possibly sticking doors and windows. The type of local existing structure may influence, to a considerable extent, the exploration program and the best foundation type for the proposed adjacent structure. Since nearby existing structures must be maintained, excavations or vibrations will have to be carefully controlled. Erosion in existing cuts (or ditches) may also be observed. For highways, run off patterns, as well as soil stratification to the depth of the T SHARMILA Page 6

7 erosion cut, may be observed. Rock outcrops may give an indication of the presence or the depth of bedrock. 3. Auger boring This method is fast and economical, using simple, light, flexible and inexpensive instruments for large to small holes. It is very suitable for soft to stiff cohesive soils and also can be used to determine ground water table. Soil removed by this is disturbed but it is better than wash boring, percussion or rotary drilling. It is not suitable for very hard or cemented soils, very soft soils, as then the flow into the hole can occur and also for fully saturated cohesionless soil. In general soil samples are categorized as shown in fig Types of samples Disturbed samples: The structure of the soil is disturbed to the considerable degree by the action of the boring tools or the excavation equipments. The disturbances can be classified in following basic types: Change in the stress condition, Change in the water content an Disturbed samples: The structure of the soil is disturbed to the considerable degree by the action of the boring tools or the excavation equipments. The disturbances can be classified in following basic types: Change in the stress condition, Change in the water content and the void ratio, T SHARMILA Page 7

8 Disturbance of the soil structure, Chemical changes, Mixing and segregation of soil constituents The causes of the disturbances are listed below: Method of advancing the borehole, Mechanism used to advance the sampler, Dimension and type of sampler, Procedure followed in sampling and boring. Undisturbed samples: It retains as closely as practicable the true insitu structure and water content of the soil. For undisturbed sample the stress changes can not be avoided. The following requirements are looked for: No change due to disturbance of the soil structure, No change in void ratio and water content, No change in constituents and chemical properties. 4 Requirement of good sampling process Inside clearance ratio The soil is under great stress as it enters the sampler and has a tendency to laterally expand. The inside clearance should be large enough to allow a part of lateral expansion to take place, but it should not be so large that it permits excessive deformations and causes disturbances of the sample. For good sampling process, the inside clearance ratio should be within 0.5 to 3 %. For sands silts and clays, the ratio should be 0.5 % and for stiff and hard clays (below water table), it T SHARMILA Page 8

9 should be 1.5 %. For stiff expansive type of clays, it should be 3.0 %. area ratio Recovery ratio Where, L is the length of the sample within the tube, H is the depth of penetration of the sampling tube. It represents the disturbance of the soil sample. For good sampling the recovery ratio should be 96 to 98 %. Wall friction can be reduced by suitableinside clearance, smooth finish and oiling. The non-returned wall should have large orifice to allow air and water to escape. 1.3.In-situ tests General The in situ tests in the field have the advantage of testing the soils in their natural, undisturbed condition. Laboratory tests, on the other hand, make use of small size samples obtained from boreholes through samplers and therefore the reliability of these depends on the quality of the so called undisturbed' samples. Further, obtaining undisturbed samples from noncohesive, granular soils is not easy, if not impossible. Therefore, it is common practice to rely more on laboratory tests where cohesive soils are concerned. Further, in such soils, the field tests being short duration tests, fail to yield meaningful consolidation settlement data in any case. Where the subsoil strata are essentially non-cohesive in character, the bias is most definitely towards field tests. The data from field tests is used in empirical, but time-tested correlations to predict settlement of foundations. The field tests commonly used in subsurface investigation are: Penetrometer test Pressuremeter test Vane shear testplate load test T SHARMILA Page 9

10 Geophysical methods 1.4. Penetrometer Tests : Standard penetration test (SPT) Static cone penetration test (CPT) Dynamic cone penetration test (DCPT) Standard penetration test The standard penetration test is carried out in a borehole, while the DCPT and SCPT are carried out without a borehole. All the three tests measure the resistance of the soil strata to penetration by a penetrometer. Useful empirical correlations between penetration resistance and soil properties are available for use in foundation design. This is the most extensively used penetrometer test and employs a split-spoon sampler, which consists of a driving shoe, a split-barrel of circular cross-section which is longitudinally split into two parts and a coupling. IS: gives the standard for carrying out the test. Procedure The borehole is advanced to the required depth and the bottom cleaned. The split-spoon sampler, attached to standard drill rods of required length is lowered into the borehole and rested at the bottom. The split-spoon sampler is driven into the soil for a distance of 450mm by blows of a drop hammer (monkey) of 65 kg falling vertically and freely from a height of 750 mm. The number of blows required to penetrate every 150 mm is recorded while driving the sampler. The number of blows required for the last 300 mm of penetration is added together and recorded as the N value at that particular depth of the borehole. The number of blows required to effect the first 150mm of penetration, called the seating drive, is disregarded. The split-spoon sampler is then withdrawn and is detached from the drill rods. The split-barrel is disconnected from the cutting shoe and the coupling. The soil sample collected T SHARMILA Page 10

11 inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory. The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a case, the N value can be recorded for the first 300 mm penetration. The boring log shows refusal and the test is halted if 50 blows are required for any 150mm penetration 100 blows are required for 300m penetration 10 successive blows produce no advance. Precautions The drill rods should be of standard specification and should not be in bent condition. The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear. The drop hammer must be of the right weight and the fall should be free, frictionless and vertical. The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a case, the N value can be recorded for the first 300 mm penetration. The boring log shows refusal and the test is halted if 50 blows are required for any 150mm penetration T SHARMILA Page 11

12 100 blows are required for 300m penetration 10 successive blows produce no advance. Precautions The drill rods should be of standard specification and should not be in bent condition. The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear. The drop hammer must be of the right weight and the fall should be free, frictionless and vertical. The height of fall must be exactly 750 mm. Any change from this will seriously affect the N value. The bottom of the borehole must be properly cleaned before the test is carried out. If this is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed soil. When a casing is used in borehole, it should be ensured that the casing is driven just short of the level at which the SPT is to be carried out. Otherwise, the test gets carried out in a soil plug enclosed at the bottom of the casing. When the test is carried out in a sandy soil below the water table, it must be ensured that the water level in the borehole is always maintained slightly above the ground water level. If the water level in the borehole is lower than the ground water level, quick' condition may develop in the soil and very low N values may be recorded. In spite of all these imperfections, SPT is still extensively used because the test is simple and relatively economical. it is the only test that provides representative soil samples both for visual inspection in the field and for natural moisture content and classification tests in the laboratory. SPT values obtained in the field for sand have to be corrected before they are used in empirical correlations and design charts. IS: recommends that the field value of N be corrected for two effects, namely, (a) effect of overburden pressure, and (b) effect of dilatancy. (a) Correction for overburden pressure T SHARMILA Page 12

13 Several investigators have found that the penetration resistance or the N value in a granular soil is influenced by the overburden pressure. Of two granular soils possessing the same relative density but having different confining pressures, the one with a higher confining pressure gives a higher N value. Since the confining pressure (which is directly proportional to the overburden pressure) increases with depth, the N values at shallow depths are underestimated and the N values at larger depths are overestimated. To allow for this, N values recorded from field tests at different effective overburden pressures are corrected to a standard effective overburden pressure. 1.5.Static cone penetration test At field SCPT is widely used of recording variation in the in-situ penetration resistance of soil in cases where in-situ density is disturbed by boring method & SPT is unreliable below water table. The test is very useful for soft clays, soft silts, medium sands & fine sands. Procedure By this test basically by pushing the standard cone at the rate of 10 to 20 mm/sec in to the soil and noting the friction, the strength is determined. After installing the equipment as per IS-4968, part III the sounding rod is pushed in to the soil and the driving is operated at the steady rate of 10 mm/sec approximately so as to advance the cone only by external loading to the depth which a cone assembly available. For finding combine cone friction resistance, the shearing strength of the soil q s, and tip resistance q c is noted in gauge & added to get the total strength LimitationsThis test is unsuitable for gravelly soil & soil for having SPT N value greater than 50. Also in dense sand anchorage becomes to cumbersome & expensive & for such cases Dynamic SPT can be used. This test is also unsuitable for field operation since erroneous value obtained due to presence of brick bats, loose stones etc. Geophysical exploration General Overview Geophysical exploration may be used with advantage to locate boundaries between different elements of the subsoil as these procedures are based on the fact that the gravitational, magnetic, electrical, radioactive or elastic properties of the different elements of the subsoil may be different. Differences in the gravitational, magnetic and radioactive properties of T SHARMILA Page 13

14 deposits near the surface of the earth are seldom large enough to permit the use of these properties in exploration work for civil engineering projects. However, the resistivity method based on the electrical properties and the seismic refraction method based on the elastic properties of the deposits have been used widely in large civil engineering projects. Different methods of geophysical explorations 1 Electrical resistivity methodelectrical resistivity method is based on the difference in the electrical conductivity or the electrical resistivity of different soils. Resistivity is defined as resistance in ohms between the opposite phases of a unit cube of a material. is resistivity in ohm-cm, R is resistance in ohms, A is the cross sectional area (cm 2 ), L is length of the conductor (cm). The resistivity values of the different soils are listed in table 1.4 Procedure Material Resistivity ( - cm) Massive rock > 400 Shale and clay 1.0 Seawater 0.3 Wet to moist clayey soils Table 1.4 : Resistivity of different materials The set up for the test is given in figure In this method, the electrodes are driven approximately 20cms in to the ground and a dc or a very low frequency ac T SHARMILA Page 14

15 current of known magnitude is passed between the outer (current) electrodes, thereby producing within the soil an electrical field and the boundary conditions. The electrical potential at point C is V c and at point D is V d which is measured by means of the inner (potential) electrodes respectively. where, is resistivity, I is current, (1.1.1) (1.1.2 ),, and are the distances between the various electrodes as shown in fig Potential difference between C and D = = - = ( ) ( ) If where, Resistance then resistivity is given as, ( ) Thus, the apparent resistivity of the soil to a depth approximately equal to the spacing of the electrode can be computed. The resistivity unit is often so designed that the apparent resistivity can be read directly on the potentiometer. T SHARMILA Page 15

16 In resistivity mapping or transverse profiling the electrodes are moved from place to place without changing their spacing, and the apparent resistivity and any anomalies within a depth equal to the spacing of the electrodes can thereby be determined for a number of points. approximately equal to the spacing of the electrode can be computed. The resistivity unit is often so designed that the apparent resistivity can be read directly on the potentiometer. In resistivity mapping or transverse profiling the electrodes are moved from place to place without changing their spacing, and the apparent resistivity and any anomalies within a depth equal to the spacing of the electrodes can thereby be determined for a number of points. Seismic refraction method General This method is based on the fact that seismic waves have different velocities in different types of soils (or rock) and besides the wave refract when they cross boundaries between different types of soils. In this method, an artificial impulse are produced either by detonation of explosive or mechanical blow with a heavy hammer at ground surface or at the shallow depth within a hole. These shocks generate three types of waves. Longitudinal or compressive wave or primary (p) wave, Transverse or shear waves or secondary (s) wave, Surface waves. It is primarily the velocity of longitudinal or the compression waves which is utilized in this method. The equation for the velocity of the p-waves and s- waves is given as, (1.2.1) (1.2.2) Where, E is the dynamic modulus of the soil, is the Poisson's ratio, T SHARMILA Page 16

17 is density and, G is the dynamic shear modulus. v These waves are classified as direct, reflected and refracted waves. The direct wave travel in approximately straight line from the source of impulse. The reflected and refracted wave undergoes a change in direction when they encounter a boundary separating media of different seismic velocities (Refer fig. 1.19). This method is more suited to the shallow explorations for civil engineering purpose. The time required for the impulse to travel from the shot point to various points on the ground surface is determined by means of geophones which transform the vibrations into electrical currents and transmit them to a recording unit or oscillograph, equipped with a timing mechanism. Assumptionshyj METHODS OF ANALYSIS LIMIT EQUILIBRIUM The so-called limit equilibrium method has traditionally being used to obtain approximate solutions for the stability problems in soil mechanics. The method entails a assumed failure surface of various simple shapes plane, circular, log spiral. With this assumption, each of the stability problems is reduced to one of finding the most dangerous position of the failure or slip surface of the shape chosen which may not be particularly well founded, but quite often gives acceptable results. In this method it is also necessary to make certain assumptions regarding the stress distribution along the failure surface such that the overall equation of equilibrium, in terms of stress resultants, may be written for a given problem. Therefore, this simplified method is used to solve various problems by simple statics. Although the limit equilibrium technique utilizes the basic concept of upper-bound rules. T SHARMILA Page 17

18 Of Limit Analysis, that is, a failure surface is assumed and a least answer is sought, it does not meet the precise requirements of upper bound rules, so it is not a upper bound. The method basically gives no consideration to soil kinematics, and equilibrium conditions are satisfied in a limited sense. It is clear then that a solution obtained using limit equilibrium method is not necessarily upper or lower bound. However, any upper-bound limit analysis solution will be obviously limit equilibrium solution. INTRODUCTION Partly for the simplicity in practice and partly because of the historical development of deformable of solids, the problems of soil mechanics are often divided into two distinct groups the stability problems and elasticity problems. The stability problems deal with the conditions of ultimate failure of mass of soil. Problems of earth pressure, bearing capacity, and stability of slopes most often are considered in this category. The most important feature of such problems is the determination of the loads which will cause the failure of the soil mass. Solutions of these problems are done using the theory of perfect elasticity. The elasticity problems on the other hand deal with the stress or deformation of the soil where no failure of soil mass is involved. Stresses at points in a soil mass under the footing, or behind a retaining wall, deformation around tunnels or excavations, and all settlement problems belong to this category. Solutions to these problems are obtained by using the theory of linear elasticity. Intermediate between the elasticity and stability problems are the problems mentioned above are the problems known as progressive failure. Progressive failure problems deal with the elastic- plastic transition from the initial linear elastic state to the ultimate failure state of the soil by plastic flow. The following section describes some of the methods of analysis which are unique with respect to each other. DIFFERENT METHODS OF ANALYSIS T SHARMILA Page 18

19 There are basically four methods of analysis: Limit Equilibrium. Limit Analysis. Method of Characteristics. Finite Element / Discrete Element Method. THEOREMS There are two theorems which are used for the various analyses. Some follow one theorem while some methods of analysis follow the other. They are the upper bound and the lower bound theorems. In the Upper bound theorem, loads are determined by equating the external work to the internal work in an assumed deformation mode that satisfies: Boundary deformation pattern. Strain and velocity compatibility conditions. These are kinematically admissible solutions. This analysis gives the maximum value for a particular parameter. In the Lower bound theorem, loads are determined from the stress distribution that satisfies: Stress equilibrium conditions. Stress boundary conditions. Nowhere it violates the yield condition. These are statically admissible solutions. This analysis gives the minimum value for a particular parameter. T SHARMILA Page 19

20 However by assuming different failure surfaces the difference between the values obtained the upper and lower bound theorems can be minimized. Rankine earth pressure (3) where is the unit weight of the soil (4) along a horizontal plane. at a depth x, integrating equation (3) and (4), Boundary conditions: if there is no surcharge, C=0, D=0 at x=0.. Hence (active conditions) or (passive conditions) This implies that in passive case, and in active case.where is the inclination of the major principle stress with the x direction. Determination of earth pressure coefficients T SHARMILA Page 20

21 (for active case, ) = (5) (6) from eqn(5) and (6), coefficient of active earth pressure similarly, in the passive case, (7) (8) from eqn(7) and (8), coefficient of passive earth pressure Inclination of failure plane The failure planes at particular plane will make an angle of of major principal stress. with the direction Fig.3.7 Inclination of failure planes Inclined Ground T SHARMILA Page 21

22 Considering the forces in the u and v directions, ( 9 ) (10 ) dividing eqn 9 by 10 and simplifying, T SHARMILA Page 22

23 thus, UNIT II SHALLOW FOUNDATION 9 2.1Introduction A foundation is a integral part of the structure which transfer the load of the superstructure to the soil. A foundation is that member which provides support for the structure and it's loads. It includes the soil and rock of earth's crust and any special part of structure that serves to transmit the load into the rock or soil. The different types of the foundations are given in fig Different types of footings fig. 2.1 Different types of footings T SHARMILA Page 23

24 2.3Methods of determining bearing capacity The various methods of computing the bearing capacity can be listed as follows: Presumptive Analysis Analytical Methods Plate Bearing Test Penetration Test Modern Testing Methods Centrifuge TestPrandtl's Analysis Prandtl (1920) has shown that if the continuous smooth footing rests on the surface of a weightless soil possessing cohesion and friction, the loaded soil fails as shown in figure by plastic flow along the composite surface. The analysis is based on the assumption that a strip footing placed on the ground surface sinks vertically downwards into the soil at failure like a punch. Fig 4.8 Prandtl's Analysis Prandtl analysed the problem of the penetration of a punch into a weightless material. The punch was assumed rigid with a frictionless base. Three failure zones were considered. Zone I is an active failure zone Zone II is a radial shear zone Zone III is a passive failure zone identical for Zone1 consist of a triangular zone and its boundaries rise at an angle with the horizontal two zones on either side represent passive Rankine zones. The boundaries of the passive Rankine zone rise at angle of with the horizontal. Zones 2 located between 1 and 3 are the radial shear zones. The bearing capacity is given by (Prandtl 1921) as where c is the cohesion and expression is the bearing capacity factor given by the T SHARMILA Page 24

25 Reissner (1924) extended Prandtl's analysis for uniform load q per unit area acting on the ground surface. He assumed that the shear pattern is unaltered and gave the bearing capacity expression as follows. if, the logspiral becomes a circle and N c is equal to,also N q becomes 1. Hence the bearing capacity of such footings becomes =5.14c+q if q=0, we get =2.57q u where q u is the unconfined compressive strength. Terzaghi's Bearing Capacity Theory Assumptions in Terzaghi's Bearing Capacity Theory Depth of foundation is less than or equal to its width. Base of the footing is rough. Soil above bottom of foundation has no shear strength; is only a surcharge load against the overturning load Surcharge upto the base of footing is considered. Load applied is vertical and non-eccentric. The soil is homogenous and isotropic. L/B ratio is infinite. T SHARMILA Page 25

26 Fig. 4.9 Terzaghi's Bearing Capacity Theory Consider a footing of width B and depth loaded with Q and resting on a soil of unit weight. The failure of the zones is divided into three zones as shown below. The zone1 represents an active Rankine zone, and the zones 3 are passive zones.the boundaries of the active Rankine zone rise at an angle of, and those of the passive zones at with the horizontal. The zones 2 are known as zones of radial shear, because the lines that constitute one set in the shear pattern in these zones radiate from the outer edge of the base of the footing. Since the base of the footings is rough, the soil located between it and the two surfaces of sliding remains in a state of equilibrium and acts as if it formed part of the footing. The surfaces ad and bd rise at to the horizontal. At the instant of failure, the pressure on each of the surfaces ad and bd is equal to the resultant of the passive earth pressure P P and the cohesion force C a. since slip occurs along these faces, the resultant earth pressure acts at angle to the normal on each face and as a consequence in a vertical direction. If the weight of the soil adb is disregarded, the equilibrium of the footing requires that (1) The passive pressure required to produce a slip on def can be divided into two parts, and. The force represents the resistance due to weight of the mass T SHARMILA Page 26

27 adef. The point of application of is located at the lower third point of ad. The force acts at the midpoint of contact surface ad. The value of the bearing capacity may be calculated as : (2 ) by introducing into eqn(2) the following values: Footing subjected to Concentric loading Problem 1 Shallow footing subjected to vertical load along with moment. Design a column footing to carry a vertical load of 40 t (DL+LL) and moment of 1000 Kg-m. 2.4.Design of the Column. i Fig Concentric & Non Concentric Footing T SHARMILA Page 27

28 400 mm Trial 1 Let assume b = 300 mm & D (L) = See chart 33 of SP-16. Assume Diameter of bar 20 mm. It shows for this trial No Reinforcement required, but practically we have to provide reinforcement. Trial 2 b = 250 mm, D = 300 mm. Fig Column Section T SHARMILA Page 28

29 2.5.Design of footing Size of the footing Fig 4.28 Details of the coulmn Let D=500mm For concentric footing; T SHARMILA Page 29

30 V=40 t =40*104 N, e=m/v=1000*104/40*104 =25 mm For no tension case: & B. Determination of L & B for different values of L L in m B in m L=6e=150mm Let provide footing size is 2.2 m*1.0 m. Check: = =16.94 t/m 2 = =19.92 t/m 2 iii Thickness of footing a. Wide beam shear Factored intensity of soil pressure, T SHARMILA Page 30

31 For critical section of wide beam shear: x=(2.2/2)-(0.3/2)-d=0.95-d Assuming P t =0.2%, and from table 16 of SP d =0 By trial and error method, d=0.45 m T SHARMILA Page 31

32 Fig 4.29 Section for wide beam shear and upward earth pressure diagram Punching shear (two way shear) Fig 4.30 Section for two way at a distance of d/2 from face of the column round Critical area= (1.1+4d) d m 2 IS: , =250/300=0.83 Ks=(0.5+ )=1.33>1.0 Therefore Ks=1.0 =40.0*1.5=60 t/m 2 (1.1+4d)*96.8= (0.3+d) (0.25+d) by trial and error, d=0.255 m =450 mm, D= /2=500 mm T SHARMILA Page 32

33 Flexural reinforcement Fig 4.31 Section for bending moment =18.35*1.5=27.53 t/m 2 =19.42*1.5=29.13 t/m 2 BM= {27.53*0.5*0.952} + {( )*0.95*2/3*0.95}= t.m Table I of SP-16, =0.193% For wide beam shear P t =0.2% =0.2*1000*450/100 Provide 16mm diameter torq mm c/c in both directions. According to clause of IS: 456 =2.2/1=2.2 T SHARMILA Page 33

34 in central band width=2/( +1)* total in short direction=2/(2.2+1)*1980= mm 2 Hence 16 mm in longer direction satisfied all criteria & 16 for central band. v Check for development length Clause Now length of bars provided, ( )/2= 950 mm< Provide extra development length of =87.5 mm say 90 mm on side of the footing. vi Transfer of load at base of column Clause 34.4 Permissible bearing pressure, qb=0.45*15=6.75 =675 t/m 2 =1*2.2=2.2 m 2 =0.3*0.25=0.075 m 2 =675*2.0=1350 t/m 2 Footing subjected to eccentric loading Problem 2 T SHARMILA Page 34

35 Design a non-concentric footing with vertical load =40t and moment = 2tm. Allowable bearing capacity=20t/m 2. = 15 N/mm 2. =415N/mm 2. Determination of size of column: P = 40t. => = 40 * 1.5 = 60t. M = 2tm. => = 2 *1.5 = 3tm. Trial I Let us assume footing size b= 250mm, D=350mm. (see chart for 0.15) Ref. Chart 33, SP-16 => or, p =0.9% = Provide 4 nos. 16 bars as longitudinal reinforcement and 8 c/c as transverse reinforcement. Determination of the size of the footing Depth of the footing assumed as D= 500mm. For non-concentric footing, Area required = Adopt a rectangular footing of size 2m * 1.1m and depth 0.5m. Eccentricity of footing = M/P= 50mm. T SHARMILA Page 35

36 Fig Elevation and Plan of a non-concentric footing Determination of design soil pressure R= soil reaction =P =40t. =40 / (2 * 1.1) = 18.2 t/m 2 < 20 t/m 2 Therefore, = 18.2*1.5 =27.3 t/m 2.=.273 N/mm 2. Determination of depth of footing: a. Wide beam shear: Consider a section at a distance d' from the column face in the longer direction. Assuming =0.2% for =15N/mm 2, =0.32N/mm 2. T SHARMILA Page 36

37 .B.d. =.B.( d) 0.32 * d = * (0.875 d) Therefore, d = m b. Punching shear: Fig Section for wide beam shear Critical area for punching shear: = 2* ( 350+d+250+d)*d = 4d(300 + d). Clause : (IS 456:2000) = 0.25/0.35 =0.71 = =1.21 >1.0 Therefore, take, =1.0. = 0.25* (15) 0.5 =0.968 N/mm 2 ' =. =0.968 N/mm * 4d* (0.3 +d) = *(0.35+d)8(0.25+d) d = 0.246m. T SHARMILA Page 37

38 Therefore, from the punching and wide beam shear criteria we get, d required is Fig Section for wide beam shear 403 mm. D required is ( /2)=453mm <500mm (D provided). OK. Flexural reinforcement: Design soil pressure (q) = 27.3 t/m 2 Bending moment at the face of the column in the longer direction =27.3 * / 2 =10.45 tm/m width. d provided = 450mm. For singly reinforced section, table 1, SP-16, p t =0.147 N/mm 2 Area of steel required = Spacing using 16 bars = 201*1000 / = 303 mm c/c. Provide 16 F bars as longitudinal 300mm c/c in longer direction. T SHARMILA Page 38

39 Cl (IS-456:2000) B = 2.0 / 1.1 =1.82 Area of steel in the longer direction = * 2 =1323 mm 2 Area of steel in the central band =2 / ( )* 1323 =938 mm 2 Spacing = mm. Provide 16 bars as longitudinal 200mm c/c in shorter direction in the central band. For remaining portion provide c/c. The central band width = width of the foundation =1100mm. Check for development length: Cl (IS 456 :2000) Now, length of bars provided =( )/2 = 825 mm.<. Extra length to be provided = ( ) = 212.5mm. Provide development length equal to 225mm at the ends. Transfer of load at the column footing junction : Cl (IS 456:2000) Assuming 2:1 load dispersion, Required L = { *500*2} =2350mm >2000mm. T SHARMILA Page 39

40 Required B = { *500*2} =2250mm >1100mm. = 2 * 1.1 =2.2 m 2. = 0.25 * 0.35 = m 2 Ö ( / ) = 5.01 > 2.0. Take as = q b * Ö (A 1 / A 2 ) = 675 * 2 = 1350 t/m 2. = 40*1.5/(0.25* 0.35) * { *0.05 / 0.35 } = 1273 t/m 2. < 1350 t/m 2. Therefore, the junction is safe. Actually there is no need to extend column bars inside the footing, but as a standard practice the column bars are extended upto a certain distance inside the footing. Design of strap footing: Example: The column positions are is as shown in fig As column one is very close to the boundary line, we have to provide a strip footing for both footings. Fig Strap footing Design of the column Column A: T SHARMILA Page 40

41 =750 KN Let = 0.8%, so, A x = 0.008A and A c = 0.992A, Where, A is the gross area of concrete. As per clause 39.3 of IS , 750 x 103 = (0.4 x 15 x 0.992A) + (0.67 x 415 x 0.008A) A = mm 2 Provide column size (300 x 300) mm 750 x 103 = 0.4 x 15 x (1- (pt/100)) x x 415x ( /100) x = 0.86%, = (0.86/100) x (300)2 = 774 mm 2 Provide 4 no's tor 16 as longitudinal reinforcement with tor 250 c/c lateral ties. Column B: =1500 KN Provide column size (400 x 400) mm 1500 x 103 = 0.4 x 15 x (1- ( /100)) x x 415x (pt/100) x = 1.24%, = (1.24/100) x (300)2 = 1985 mm 2 Provide 8 no.s tor 16 as longitudinal reinforcement with tor 250 c/c lateral ties. Footing design Let us assume eccentricity e = 0.9m. T SHARMILA Page 41

42 Fig Strap footing soil reaction Taking moment about line, x 5 x (5-e) = 0 Footing size: For footing A: Fig Footing sizes = 2( ) =2.4m. Assume overall thickness of footing, D = 600mm. For footing B: Assume square footing of size, = T SHARMILA Page 42

43 = 2.13m Provide (2.2 x 2.2)m footing. Analysis of footing Fig Analysis of footing Thickness of footing i) Wide beam shear: For footing A: T SHARMILA Page 43

44 Let us assume = 0.2%, so from table 16 of IS456, Assume in direction of, width of strap beam (b) is 500 mm. Fig Wide beam shear for footing A Shear = b d = q u (0.4 - d) For footing B: Let us assume (%) = 0.2%, so from table 16 of IS456, Assume in direction of, width of strap beam (b) is 500 mm. T SHARMILA Page 44

45 Fig Wide beam shear for footing B Shear = b d = q u (0.4 - d) > 600 mm depth earlier assumed. Increasing the width of the beam to 700 mm Fig Wide beam shear for footing B Let us assume (%) = 0.3%, so from table 16 of IS456, T SHARMILA Page 45

46 Shear = b d = q u ( d) < 600 mm depth earlier assumed. Safe ii) Two way shear: For column A: From clause of IS Critical perimeter x d x = x (critical area dotted area in fig. 4.42) So, shear equation becomes, Critical perimeter x d x = x (critical area dotted area in fig. 4.42) 2 ( d) d (96.8) = ( d) d = mm < 600 mm. T SHARMILA Page 46

47 Fig Wide beam shear for footing A For column B: From clause of IS Critical perimeter = 2 (0.4+d+0.4+d) = 4 (0.4+d) So, shear equation becomes, Critical perimeter x d x = x (critical area dotted area in fig. 4.43) 2 (0.4+d) d (96.8) = (0.4 + d) d = mm < 600 mm. Among all the required d values (for wide beam shear and two way shear criteria), Max. = 521 mm. = (20/2) + 40 = 571 mm So, provide D = 600 mm = 550 mm Reinforcement for flexure for footings (i) Design along the length direction: Comparing the moments at the column faces in both the footings (A & B), = tm (for Footing B) T SHARMILA Page 47

48 From table 1 of SP-16, = % (ii) Design along the width direction: (= t/m) < (= t/m) So, for design along width direction footing B ( ) is considered. Fig Bending along the width of footing B So, = % i. e. same as reinforcement along longer direction. But. From wide beam criteria = 0.3 %, (required) = (0.3/100) x (103) x (550) = 1650 mm 2. Provide c/c along both directions at bottom face of the footing A and B. Design of strap beam (i) Reinforcement for flexture: = tm (Refer fig. 4.45) From table 49 of SP-16, d'/d = 50/550 = 0.1, T SHARMILA Page 48

49 = 0.83 % and P c = 0.12 % (required on tension face) = (0.83/100) x 700 x 550 = mm 2, (required on compression face) = (0.12/100) x 700 x 550 = 462 mm 2, Provide (6+5=) 11 no.s Tor 20 at top of the strap beam and 4 no.s Tor 20 at bottom of the strap beam. (ii) Check for shear: V max = t < max = 2.5 N/mm 2 (for M15) (provided) = From table 61 of SP-16, = 0.57 N/mm 2 But, provide shear reinforcement for shear = ( acting ) = N/mm 2 = Vus = KN/cm From table 16 of SP-16, using 4L stirrups, (V us /d) = (11.144/2) = KN/cm From table 62 of SP-16, provide 4L-stirrups c/c near the column (upto distance of d=550mm from column face) and 4L-stirrups c/c for other portions. Check for development length From clause of IS , Development length = = For column A: Length of the bar provided = = 110mm < By providing 2 no.s 90 o bend the extra length to be provided = ( (8 x 20)) = 707 mm. T SHARMILA Page 49

50 In B direction length of the bar provided = Providing two 90 o bend, the extra length to be provided = ( (8 x 20)) = 517 mm. (ii) Check for shear: V max = t < max = 2.5 N/mm 2 (for M15) (provided) = From table 61 of SP-16, = 0.57 N/mm 2 But, provide shear reinforcement for shear = ( acting ) = N/mm 2 = Vus = KN/cm From table 16 of SP-16, using 4L stirrups, (V us /d) = (11.144/2) = KN/cm From table 62 of SP-16, provide 4L-stirrups c/c near the column (upto distance of d=550mm from column face) and 4L-stirrups c/c for other portions. Check for development length From clause of IS , Development length = = For column A: Length of the bar provided = = 110mm < By providing 2 no.s 90 o bend the extra length to be provided = ( (8 x 20)) = 707 mm. In B direction length of the bar provided = T SHARMILA Page 50

51 Providing two 90 o bend, the extra length to be provided = ( (8 x 20)) = 517 mm. For column B: Fig Development length for footing A Length of the bar provided = Providing one 90 o bend, the extra length to be provided = ( (8 x 20)) = 277 mm. Fig Development length for footing B (Along the length and width) Transfer of load at base of the column: For footing A: T SHARMILA Page 51

52 From clause 34.4 of IS , permissible bearing stress ( )= = ( )(1300)= mm 2 = (300 x 300) = mm 2 = 2 x 0.45 x x1500 = 1161 t//m 2 = (load on column/area of column) = (1.5 x 50)/(0.3)2 = t//m 2 < Safe. Fig Area of footing A considered for check of transfer of load at column base For Footing B: From clause 34.4 of IS , permissible bearing stress ( )= = (2200) 2 = mm 2 = (400 x 400) = mm 2 T SHARMILA Page 52

53 Fig Area of footing B considered for check of transfer of load at column base = 2 x 0.45 x x1500 = 1161 t/m 2 = (load on column/area of column) = (1.5 x 100)/(0.4) 2 =937.5 < Safe UNIT III FOOTINGS AND RAFTS 9 T SHARMILA Page 53

54 T SHARMILA Page 54

55 3.1.Combined Footing Design of =800kN =1000kN =20 t/m 2,M15, =415kN/m 2 Fig Loading on combined footing Column size: 400x400mm. See Fig 4.54 for details of footing. Column design Let pt=0.8% =.008A; =0.992A Clause.39.3 of IS A= mm 2 = mm 2, = mm 2 T SHARMILA Page 55

56 Provide footing of 400x400size for both columns. Using 8-16 as main reinforcement and as lateral tie Design of Footing Fig Forces acting on the footing Resultant of Column Load R =1800 kn acting 3.08m from the boundary. Area of the footing : Taking length L=6m, Depth of footing =0.9m,, Width of footing, =1.549m. Therefore, provide footing of dimension 6m x 1.6m Soil Pressure q = =18.75 t/m 2 < 20 t/m 2 OK. = t/m 2 Soil pressure intensity acting along the length =B x =1.6x =45t/m. R B =119.88kN, R C =150.12kN. T SHARMILA Page 56

57 Thickness of Footing i. Wide beam shear: Maximum shear force is on footing C,SF=115.02KN for percentage reinforcement =0.2% 0.32 x d x 1.6=45 [ d] d=1.1m for percentage reinforcement =0.6% 0.6 x d x 1.6=45 [ d] d=0.847m.d=900mm.ok. T SHARMILA Page 57

58 ii.two way Shear Footing i. Wide beam shear: Thickness of Maximum shear force is on footing C,SF=115.02KN for percentage reinforcement =0.2% 0.32 x d x 1.6=45 [ d] d=1.1m for percentage reinforcement =0.6% 0.6 x d x 1.6=45 [ d] T SHARMILA Page 58

59 d=0.847m.d=900mm.ok. ii.two way Shear Column B d=0.415m. Column A 2d[(0.4+d)+(0.42+d/2)] x 96.8= [(0.4+d)(0.42+d/2)] d=0.3906m =0.85mm =900mm, Flexural reinforcement Along Length Direction Table 1of SP16 =0.354% provided=0.6% =850mm.OK. =1.15N/mm 2 required=5100 mm 2 /mm Provide at top and bottom of the footing Along width direction T SHARMILA Page 59

60 T SHARMILA Page 60

61 Raft Footing Design the raft footing for the given loads on the columns and spacing between the columns as shown below. T SHARMILA Page 61

62 Fig 4.57 column locations and intensity of loads acting on the raft a) Column sizes Take size of the columns are as: 300*450 mm for load of less than 115 ton 450*450 mm for a load of greater than 115 ton Thickness of raft Two way shear The shear should be checked for every column, but in this case because of symmetry property checking for 115 t, 150 t, and 55 t is enough. For 150 t column T SHARMILA Page 62

63 Fig 4.58 section for two way shear for 150 t column IS: , =450/450=1.0 =(0.5+ )=1.0=1.0 Therefore =1.0 4(0.45+d)*d*96.8=150* (0.45+d)2 Therefore d=0.562 m For 115 t column Fig 4.59 section for two way shear for 115 t column 2(0.45+d d/2) d*96.8=115* (0.45+d)( d) T SHARMILA Page 63

64 Therefore d=0.519 m For 55 t column Fig 4.60 section for two way shear for 55 t column 2( d d) d*96.8=55* ( d+0.075)( d+0.15) Therefore d=0.32 m The guiding thickness is 0.562m and code says that the minimum thickness should not be less than 1.0m. let provide a overall depth of 1.1m=D = /2=1015mm. To calculate k & -Stiffness factors There are two criterions for checking the rigidity of the footing: Plate size used is 300*300 mm. For clays: =0.5, Take k=0.7 and B=30 cm Es=15.75 kg/cm 2 =1.575 N/mm 2 T SHARMILA Page 64

65 b=23.2*103 mm, a=12.8*103 mm 4, d=1015 mm =0.085<0.5 Therefore it is acting as a flexible footing. = * / = =9.75m If column spacing is less than 1.75/, then the footing is said to be rigid. Therefore the given footing is rigid. One criterion showing the footing is flexible and another showing that the given footing is rigid. Both are contradicting each other, so design the footing for both criterions. =5.607 T SHARMILA Page 65

66 T SHARMILA Page 66

67 Reinforcement in width direction From SP-16 graphs =0.102%, but minimum is 0.12%. =(0.12*1000*1015)/100=1218 mm 2 Provide 20 mm diameter c/c along shorter direction in bottom. Reinforcement in length direction T SHARMILA Page 67

68 Provide 20 mm diameter c/c in longer direction. Clause Provide 20 mm diameter 200 c/c in central band and 20 mm diameter c/c at other parts along shorter direction at bottom. Shear (wide beam shear criterion) In width direction 0.2 N/mm 2 < =0.123%, =0.27 N/mm 2 > (from table 61 of SP 16 by extrapolation) Therefore no shear reinforcement is required. =0.235 N/mm 2 < (0.27 N/mm 2 ) Therefore no shear reinforcement is required. Along the width direction T SHARMILA Page 68

69 Fig Shear Force and Bending Moment Diagrams of strips 1 and 4 In width direction: Strip1/4:- =141.2tm = =0.337N/mm 2 Strip2/3 T SHARMILA Page 69

70 Fig Shear Force and Bending Moment Diagrams of strips 2 and 3 Strip 2/3 =282.36tm = =0.364N/mm 2 Minimum =0.12%has to be provided. Provide in centre band and at other parts along the shorter direction. T SHARMILA Page 70

71 1. Shear check Along width direction:- For strip1/4: =76.35t = =0.185N/mm 2 <, OK. For strip 2/3: = t = =0.208N/mm 2 <, OK. Hence no shear reinforcement is required. Development Length = =1128.3mm At the ends, length of bar provided=150mm. Extra length to be provided= x20=818.3mm. Provide a Development length of 850mm 3. Transfer of load at the base of the column:- For end column; =2650X2725= x106mm 2 =300x450=135000mm But not greater than 2.0 T SHARMILA Page 71

72 = =13.5N/mm 2 = =4.07N/mm 2 <.OK. For 150t columns = =7.41N/mm 2 <.OK. For 115t columns 2, = =8.52N/mm 2 <.OK. T SHARMILA Page 72

73 CE6502-FOUNDATION ENGINEERING YEAR:III/SEM:V CIVIL ENGINEERING T SHARMILA Page 73

74 UNIT IV PILES DESIGN METHODOLOGY FOR PILES The detailed design methodology of piles is described in the following sections. REQUIREMENT FOR FOUNDATIONS DEEP Generally for structures with load >10, we go for deep foundations. Deep foundations are used in the following cases: Huge vertical load with respect to soil capacity. Very weak soil or problematic soil. Huge lateral loads eg. Tower, chimneys. Scour depth criteria. For fills having very large depth. Uplift situations (expansive zones) Urban areas for future large and huge construction near the existing building. 4.2CLASSIFICATION OF PILES 1. Based on material Timber piles Steel piles Concrete piles Composite piles (steel + concrete) 2. Based on method of installation Driven piles ----(i) precast (ii) castin-situ. Bored piles. 3. Based on the degree of disturbance Large displacement piles (occurs for driven piles) Small displacement piles (occurs for bored piles) 4.3.POINTS TO BE CONSIDERED FOR CHOOSING PILES Loose cohesion less soil develops much greater shaft bearing capacities if driven large displacement piles are used. Displacement effect enhanced by tapered shafts. Potential increased of shaft capacities is undesirable if negative friction is to be feared. (Negative friction is also called drag down force) High displacement piles are undesirable in stiff cohesive soils, otherwise excessive heaving takes place. Encountered with high artesian pressures on cased piles should be excluded. (Mainly for bridges and underwater construction) Driven piles are undesirable due to noise, damage caused by vibration, ground heaving. Heavy structures with large reactions require high capacity piles and small diameter cast-in-situ piles are inadequate. 4.4PILE CLASSIFICATION Friction piles. End bearing piles. Compaction piles.( Used for ground movement, not for load bearing ) Tension piles/anchored piles.(to resist upliftment) Butter piles (Inclined) --- +ve and ve. T SHARMILA Page 74

75 CE6502-FOUNDATION ENGINEERING YEAR:III/SEM:V CIVIL ENGINEERING Fig. 5.1 Direction of load is same as the direction of batter. (Rotation of pile) Raymond piles. (Driven cast-in-situ piles, first tapered shell is driven and then cast) Franki Piles (Driven cast-in-situ piles, first casing is driven upto 2m depth, then cast a block within that casing and then drive the block. When it reaches the particular depth, take out the casing and cast the piles.) Underreamed piles (bored cast-in-situ piles, bulbs used, hence not possible to install in loose sand and very soft clays.) PILES IN CLAY Zone of influence T SHARMILA Page 75

76 Fig.5.2 Driven piles in clay The heaving effect can be felt upto (10 15) D from the centerline of the pile. Due to driving load, pressure is generated and as a result heaving occurs. Afterwards with time, the heaved part gets consolidated and strength gradually increases as the material regains shear strength within 3 6 months time after the installation of the pile. This regain of strength is called thixotrophy. On the first day some part of the pile will be driven and on the second day some part of the pile may move up due to the gain of shear strength. This is known as the wakening of the pile. By the driving force, the extra pore pressure generated is (5 7) times the of the soil. Bearing capacity of the pile is 9. Hence due to this property, maximum single length of the pile theoretically can be upto 25m but 10-12m is cast at a time. Then by splicing technique the required hired length of the pile is obtained. Special types of collars are used so that the splices become weak points. Concrete below the grade M20 is never used. Pile Diameter Maximum length (m) à T SHARMILA Page 76

77 Fig.5.3 Generation of T SHARMILA Page 77

78 4.4.PILES IN SAND Fig.5.4a Driven piles in loose sand T SHARMILA Page 78

79 Fig.5.4b Improvement in f due to pile driving SETTLEMENT OF PILE GROUPS Assume 2V:1H dispersion for settlement of pile groups. Fig.5.5 Settlement of pile groups CODAL PROVISION SAFE LOAD ON PILES/PILE GROUPS ( Ref. IS: 2911 Part IV 1979 ) Single pile: 1. Safe load = Least of the following loads obtained from routine tests on piles : 2/3 of the final load at which total settlement is 12mm. 50% of the final load at which settlement is 10% of the pile dia.( for uniform dia. piles) and 7.5% of bulb dia. (for Underreamed piles) 2/3 of the final load at which net settlement is 6mm. Consider pile as column and find the total compressive load depending on the grade of concrete and dimensions. Eg. Consider a 300mm dia pile made of M20 concrete.. T SHARMILA Page 79

80 Therefore, ultimate load =. Fig 5.40 Multiple Under Reamed Pile Under reamed piles are bored cast-in-situ concrete piles having one or more number of bulbs formed by enlarging the pile stem. These piles are best suited in soils where considerable ground movements occur due to seasonal variations, filled up grounds or in soft soil strata. Provision of under reamed bulbs has the advantage of increasing the bearing and uplift capacities. It also provides better anchorage at greater depths. These piles are efficiently used in machine foundations, over bridges, electrical transmission tower foundation sand water tanks. Indian Standard IS 2911 (Part III) covers the design and construction of under reamed piles having one or more bulbs. According to the code the diameter of under reamed bulbs may vary from 2 to 3 times the stem diameter depending upon the feasibility of construction and design requirements. The code suggests a spacing of 1.25 to 1.5 times the bulb diameter for the bulbs. An angle of 45 0 with horizontal is recommended for all under reamed bulbs. This code also gives Mathematical expressions for calculating the bearing and uplift capacities. From the review of the studies pertaining to under reamed piles, it can be seen that ultimate bearing capacity of piles increases considerably on provision of underreamed bulbs (Neumann and P&g, 1955, Subash Chandra and Kheppar, 1964, Patnakar, 1970 etc.). Pile load capacity was found to vary with the number of bulbs and with the spacing ratio S / or S/d adopted (where S = distance between the piles, = diameter of under reamed bulbs and d = diameter of piles). Table summarizes the various recommendations made for the selection of S / and S/d for the optimum pile load capacity. It can be seen that some of these recommendations differ from those given in IS 2911 (Part III), Table: 5.6 of recommendations for S / and S/d for the optimum pile load capacity Recommendations of S/ & S/d values for under reamed piles s.no. Reference No. of Bulbs Spacing T SHARMILA Page 80

81 1. Patnakar (1970) 2 Agarwal and Jain (1971) 3 Sonapal and Thakkar (1977) 4 IS 2911(Part III 1980) 5 Ray and Raymond (1983) Pile capacity for one bulb increases25 percent, for two bulbs 600 percent, and for three bulbs700 percent over simple pile. More than two bulbs are not advisable For optimum capacity two bulbs S / = 6 or S/d = 15, far three bulbs, S / = 5 or S/d = For optimum capacity S / = 1.25 to For optimum capacity S / = 2.5 S / = 1.25 to Maximum value of S / = 1.24 to 1.5 The choice of an under-reamed pile in unstable or water-bearing ground is generally to be avoided. There is a danger of collapse of the under-ream, either when personnel are down the hole, or during concreting. Important Notes: On the basis of limited experimental studies conducted on model under reamed piles in cohesion less soil the following conclusions are drawn. 1. By providing under reamed bulbs the ultimate load capacities of piles increases significantly. 2. The ultimate load bearing capacities of the under reamed piles with angle of under reamed bulbs of 45 0 and zero are almost same. 3. Three or more under reamed bulbs are advantageous only when the spacing ratio (S / ) is two or less, and when (S / ) is greater than two, multi-under reamed piles do not have specific advantages. 4. The ultimate load bearing capacities of piles are maximum when the spacing between two under reamed bulb is 2.5 times the T SHARMILA Page 81

82 diameter of the under reamed bulb. It appears that the spacing between two under reamed bulbs suggested in (1.25 to 1.5 times) IS 2911(1980) is not the optimum, 5. The expression suggested in IS 2911(1980) can be used for predicting the ultimate load carrying capacity of under reamed piles with spacing ratio (S / ) less than UNIT V RETAINING WALLS RETAINING WALL Retaining walls are structures used to retain earth or water or other materials such as coal, ore, etc; where conditions do not permit the mass to assume its natural slope. The retaining material is usually termed as backfill. The main function of retaining walls is to stabilize hillsides and control erosion. When roadway construction is necessary over rugged terrain with steep slopes, retaining walls can help to reduce the grades of roads and the land alongside the road. Some road projects lack available land beside the travel way, requiring construction right along the toe of a slope. In these cases extensive grading may not be possible and retaining walls become necessary to allow for safe construction and acceptable slope conditions for adjacent land uses. Where soils are unstable, slopes are quite steep, or heavy runoff is present, retaining walls help to stem erosion. Excessive runoff can undermine roadways and structures, and controlling sediment runoff is a major environmental and water quality consideration in road and bridge projects. In these situations, building retaining walls, rather than grading excessively, reduces vegetation removal and reduces erosion caused by runoff. In turn, the vegetation serves to stabilize the soil and filter out sediments and pollutants before they enter the water source, thus improving water quality. In this section you will learn the following Gravity walls Semi Gravity Retaining Wall Special type of retaining walls Flexible walls Different Types of Retaining Structures On the basis of attaining stability, the retaining structures are classified into following: 1. Gravity walls : T SHARMILA Page 82

83 Gravity walls are stabilized by their mass. They are constructed of dense, heavy materials such as concrete and stone masonry and are usually reinforced. Some gravity walls do use mortar, relying solely on their weight to stay in place, as in the case of dry stone walls. They are economical for only small heights.. Semi Gravity Retaining Wall These walls generally are trapezoidal in section. This type of wall is constructed in concrete and derives its stability from its weight. A small amount of reinforcement is provided for reducing the mass of the concrete.this can be classified into two: Cantilever retaining wall Counter fort retaining wall Cantilever retaining wall Fig 6.3.Semi Gravity Retaining Wall This is a reinforced concrete wall which utilises cantilever action to retain the backfill. This type is suitable for retaining backfill to moderate heights(4m-7m). In cross section most cantilevered walls look like L s or inverted T s. To ensure T SHARMILA Page 83

84 stability, they are built on solid foundations with the base tied to the vertical portion of the wall with reinforcement rods. The base is then backfilled to counteract forward pressure on the vertical portion of the wall. The cantilevered base is reinforced and is designed to prevent uplifting at the heel of the base, making the wall strong and stable. Local building codes, frost penetration levels and soil qualities determine the foundation and structural requirements of taller cantilevered walls. Reinforced concrete cantilevered walls sometimes have a batter. They can be faced with stone, brick, or simulated veneers. Their front faces can also be surfaced with a variety of textures. Reinforced Concrete Cantilevered Walls are built using forms. When the use of forms is not desired, Reinforced Concrete Block Cantilevered Walls are another option. Where foundation soils are poor, Earth Tieback Retaining Walls are another choice. These walls are counterbalanced not only by a large base but also by a series of horizontal bars or strips extending out perpendicularly from the vertical surface into the slope. The bars or strips, sometimes called deadmen are made of wood, metal, or synthetic materials such as geotextiles. Once an earth tieback retaining wall is backfilled, the weight and friction of the fill against the horizontal members anchors the structure. 5.3.Counterfort retaining wall When the height of the cantilever retaining wall is more than about 7m, it is economical to provide vertical bracing system known as counter forts. In this case, both base slab and face of wall span horizontally between the counter forts. Fig. 6.5 Counter fort retaining wall T SHARMILA Page 84

85 3. Flexible walls: there are two classes of flexible walls. A. Sheet pile walls and B. Diaphragm wall A. Sheet Pile Walls Sheet piles are generally made of steel or timber. The use of timber piles is generally limited to temporary sdtructures in which the depth of driving does not exceed 3m. for permanent structures and for depth of driving greater than 3m, steel piles are most suitable. Moreover, steel iles are relatively water tight and can be extracted if required and reused. However, the cost of sheet steel piles is generally more than that of timber piles. Reinforced cement concrete piles are generally used when these are to be jetted into fine sand or driven in very soft soils, such as peat. For tougher soils, the concrete piles generally break off. Based on its structural form and loading system, sheet pile walls can be classified into 2 types:(i)cantilever Sheet Piles and(ii)anchored Sheet Piles 1. Cantilever sheet pile walls: Fig. 6.6.Cantilever sheet pile wall Cantilever sheet piles are further divide into two types: Free cantilever sheet pile It is a sheet pile subjected to a concentrated horizontal load at its top. There is no back fill above the dredge level. The free cantilever sheet pile derives its stability entirely from the lateral passive resistance of the soil below the dredge level into which it is driven. Cantilever Sheet Pile Wall with Backfill T SHARMILA Page 85

86 A cantilever sheet pile retains backfill at a higher level on one side. The stability is entirely from the lateral passive resistance of the soil into which the sheet pile is driven, like that of a free cantilever sheet pile. 2. Anchored sheet pile walls Anchored shet pile walls are held above the driven depth by anchors provided ata suitable level. The anchors provided for the stability of the sheet ile, in addition tomthe lateral passive resistance of the soil into which the shet piles are driven. The anchored sheet piles are also of two types. Fig. 6.7.Anchored sheet pile wall Free earth support piles. An anchored pile is said to have free earth support when the depth of embedment is small and the pile rotates at its bottom tip. Thus there is a point of contraflexure in the pile. Fixed earth support piles. An anchored sheet pile has fixed earth support when the depth of embedment is large. The bottom tip of the pile is fixed against rotations. There is a change in the curvature of the pile, and hence, an inflection point occurs. Diaphragm Walls Diaphragm walls are commonly used in congested areas for retention systems and permanent foundation walls. They can be installed in close proximity to existing structures, with minimal loss of support to existing foundations. In addition, construction dewatering is not required, so there is no associated subsidence. Diaphragm walls have also been used as deep groundwater barriers through and under dams. Diaphragm walls are constructed by the slurry trench technique which was developed in Europe, and has been used in the United States since the l940's. The T SHARMILA Page 86

87 technique involves excavating a narrow trench that is kept full of an engineered fluid or slurry. The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Slurry trench excavations can be performed in all types of soil, even below the ground water table. Cast in place; diaphragm walls are usually excavated under bentonite slurry. The construction sequence usually begins with the excavation of discontinuous primary panels. Stop-end pipes are placed vertically in each end of the primary panels, to form joints for adjacent secondary panels. Panels are usually 8 to 20 feet long, with widths varying from 2 to 5 feet. Once the excavation of a panel is complete, a steel reinforcement cage is placed in the center of the panel. Concrete is then poured in one continuous operation, through one or several tremie pipes that extend to the bottom of the trench. The tremie pipes are extracted as the concrete raises in the trench, however the discharge of the tremie pipe always remains embedded in the fresh concrete. The slurry, which is displaced by the concrete, is saved and reused for subsequent panel excavations. When the concrete sets, the end pipes are withdrawn. Similarly, secondary panels are constructed between the primary panels, and the process continues to create a continuous wall. The finished walls may cantilever or require anchors or props for lateral support. Fig Construction Stages of a Diaphragm Wall using Slurry Trench Technique. 4. Special type of retaining walls Gabion walls T SHARMILA Page 87

88 Gabion walls are constructed by stacking and tying wire cages filled with trap rock or native stone on top of one another. They can have a continuous batter (gently sloping) or be stepped back (terraced) with each successively higher course. This is a good application where the retaining wall needs to allow high amounts of water to pass through it, as in the case of riverbank stabilization. It is important to use a filter fabric with the gabion to keep adjacent soil from flowing into or through the cages along with the water. As relatively flexible structures, they are useful in situations where movement might be anticipated. Vegetation can be reestablished around the gabions and can soften the visible edges allowing them to blend into the surrounding landscape. For local roads, they are a preferred low-cost retaining structure. Fig. 6.9 (i) Gabion Wall T SHARMILA Page 88

89 Design Requirement for Gravity walls Gravity Retaining walls are designed to resist earth pressure by their weight. They are constructed of the mass, concrete, brick or stone masonry. Since these materials can not resist appreciable tension, the design aims at preventing tension in the wall. The wall must be safe against sliding and overturning. Also the maximum pressure exerted on the foundation soil should exceed the safe bearing capacity of the soil. So before the actual design, the soil parameters that influence the earth pressure and the bearing capacity of the soil must be evaluated. These include the unit weight of the soil, the angle of the shearing resistance, the cohesion intercept and the angle of wall friction. Knowing these parameters, the lateral earth pressure and bearing capacity of the soil determined. T SHARMILA Page 89

90 Fig-6.12a Fig-6.12b Fig. 6.12a shows a typical trapezoidal section of a gravity retaining wall. The forces acting on the wall per unit length are: Active Earth pressure. The weight of the wall ( ) The Resultant soil reaction R on the base. (or Resultant of weight & ).Strike the base at point D. There is equal and opposite reaction R' at the base between the wall and the foundation. Passive earth pressure acting on the lower portion of the face of the wall, which usually small and usually neglected for design purposes. The full T SHARMILA Page 90

91 mobilization of passive earth pressure not occurs at the time of failure so we not consider it. If we consider it then it shows resistance against instability. So if we ignore it then we will be in safer side. First decide which theory we want to apply for calculating the active earth pressure. Normally we calculate earth pressure using Rankine's theory or Coulomb's Earth pressure theory. For using Rankine's theory, a vertical line AB is drawn through the heel point ( Fig 6.12-b ). It is assumed that the Rankine active condition exist along the vertical line AB. While checking the stability, the weight of the soil ( ) above the heel in the zone ABC should also be taken in to consideration, in addition to the Earth pressure ( ) and weight of the wall ( ). But Coulomb's theory gives directly the lateral pressure ( the wall, the forces to be considered only ) on the back face of (Coulomb) and the Weight of the wall ( ). In this case, the weight of soil ( ) is need not be considered. Once the forces acting on the wall have been determined, the Stability is checked using the procedure discussed in the proceeding section. For convenience, the section of the retaining wall is divided in to rectangles & triangles for the computation of the Weight and the determination of the line of action of the Weight. For a safe design, the following requirement must be satisfied. No Sliding Horizontal forces tend to slide the wall away from the fill. This tendency is resisted by friction at the base. T SHARMILA Page 91

92 = Coefficient of friction between the base of the wall and soil (= tan ). = Sum of the all vertical forces i.e. vertical component of inclined active force. A minimum factor of safety of 1.5 against sliding is recommended. No Overturning The wall must be safe against overturning about toe. No Bearing Capacity Failure and No Tension First calculate the line of action of the Resultant force ( e ) from centre of the base. (No Tension will develop at the heel) T SHARMILA Page 92

93 The pressure at the toe of the wall must not exceed the allowable bearing capacity of the soil. The pressure at the base is assumed to be linear. The max. Pressure at the Toe & min at the Heel is given by: should be less than the Safe bearing capacity( ) of the soil & should not be Tensile in any case. Tension is not desirable. The tensile strength of the soil is very small and tensile crack would develop. The effective base area is reduced. T SHARMILA Page 93

94 2 Marks UNIT What are the information obtained in general exploration? Preliminary selection of foundation type depth of water, Depth, extent and composition of soil strata Engineering properties required disturbed or partly disturbed samples approximate values of strength and compressibility 2. Define significant depth? Exploration depth, in general it should be carried out to a depth upto which increase in the pressure due to structural loading is likely to cause shear failure, such depth is known as significant depth. For footing, depth of exploration =1.5B 3. What are the types of soil samples? Disturbed soil sample Undisturbed soil sample 4. What is the difference between disturbed and undisturbed soil sample? Disturbed soil sample Natural structure of soils get partly or fully modified and destroyed Undisturbed soil sample Natural structure and properties remain preserved 5. What are the disadvantages of wash boring? It is a slow process in stiff soil It cannot be used effectively in hard soil, rocks,etc. 6. What are design features that affect the sample disturbance? Area ratio, inside clearance, outside clearance, inside wall friction, method of applying force 7. What are the corrections to be applied to the standard penetration number? Overburden pressure Correction dilatancy correction 8. What are various methods of site exploration? Open exacavation, borings, geophysical methods, sub-surface soundings 9. What are the methods of boring? Auger borings, shell boring, wash boring, rotary boring, percussion boring. 10. Define area ratio? Area ratio is defined as the ratio of maximum cross sectional area of the cutting edge to the area of the soil sample 11. Define liquefaction of sand? The mass failure occurs suddenly, and the whole mass appears flow laterally as if it were a liquid such failure is referred to as liquefaction. T SHARMILA Page 94

95 12. How will you reduce the area ratio of a sampler? By increasing the size of the soil sample. 13. What is meant by a non- representative sample? Name the laboratory tests that could be conducted on this sample. Soil sample consists of a mixture of soil from different soil strata is called non- representative sample and the size of the soil grains and mineral constituents have changed. 14. Write the uses of Bore log Report. (i) Used to record the change of layer s depth. (ii) Used to record the water level. (iii)used to record the water quality in deeper levels. 15. Define detailed exploration. Detailed exploration follows as a supplement to general exploration when large engineering works, heavy loads, and complex and costly foundations are involved. A detailed exploration is meant to furnish information about soil properties such as shear strength, compressibility, density index, and permeability. 16.What are the limitations of hand augers in soil exploration? 1. Hand augers are not suitable for sands and gravels above the water table. 2. The sample is distributed and suitable for identification purposes only. 17. What are the guidelines in terms of inside clearance and outside clearance for obtaining undisturbed sample? An undisturbed sample is that in which the natural structure and properties remain preserved. The inside clearance should lie between 1 to 3 percent and the outside clearance. The walls of the sampler should be smooth and should be kept properly oiled. 18. List the various methods of soil exploration techniques. 1. Pits and trenches 2. Boring a) augur boring b) wash or water boring c) rotary boring d) percussion boring 3. Geophysical methods a) seismic refraction b) electrical resistivity 4. Standard penetration test 5. Static cone penetration test 19. Write short notes on Augur boring. An augur is a type of tool which is used for understanding the characteristics of the subsurface soil. Generally there are two types of augurs, a) Manually operated augur b) Mechanically operated augur 20. Define standard penetration number. The number of blows required to penetrate 300 mm of the split spoon sampler beyond a seating drive of 150mm is known as penetration number (N). T SHARMILA Page 95

96 21. List the various corrections to be carried out in SPT test. The two corrections are a) Dilatancy correction ( Silty sand) b) Over burden pressure correction ( Granular soil) 22. What are the uses of soil exploration? a) To select type and depth of foundation for a given structure b) To determine the bearing capacity of the soil of the selected foundation c) To investigate the safety of the existing structure d) To establish ground water level 23. What is soil exploration? The process of collection subsoil sample by an appropriate method to a needed depth and check those samples for knowing the properties is called soil exploration. 24. List the different types of samplers. a) Standard split spoon sampler b) Shelby and thin walled tube sampler c) Denison sampler d) Piston sampler e) Scraper bucket sampler 25. List the various parameters affecting the sampling disturbance. a) Area ratio b) Inside clearance c) Outside clearance d) Inside wall friction e) Position of non return wall f) Recovery ratio g) Methods of applying force 26. Write the advantages of SCPT over SPT. a) There is no need of hammering action, just pushing into the ground. b) No need of bore holes, it is carried out on the ground c) Engineering properties of the soil like permeability, Shear strength, Compressibility can be evaluated. 27. Write short notes on spacing of bore holes. The spacing of bore holes depends upon the variation of subsurface soil in the horizontal direction. The factors influencing the spacing of bore holes are, a) Type of soil b) Fluctuation of water table c) Load coming from structure d) Importance of the structure. e) Economical feasibility. T SHARMILA Page 96

97 UNIT What are components of total foundation settlement? Elastic settlement, consolidation settlement, secondary consolidation settlement 2. What are the types of shear failure? General shear failure, local shear failure, punching shear failure 3. What are assumptions in Terzaghi s bearing capacity theory? - the base of the footing is rough - the load on footing is vertical and uniformly distributed - the footing is continuous 4. List out the methods of computing elastic settlements? based on the theory of elasticity, Pressure meter method, Janhu Bjerram method, Schmentmann s method 5. What are the limitation of Terzaghi s analysis? - As the soil compresses, pi changes slight down ward movement of footing may not develop fully the plastic zones - Error due to assumption that the resultant passive pressure consists of three components is small 6. Define ultimate bearing capacity? Gross pressure at the base of the foundation at which the soil fails in shear is called ultimate bearing capacity. 7. Define net ultimate bearing capacity? Net pressure increase in pressure at the base of the foundation that causes failure in shear, is called as net ultimate bearing capacity 8. Define allowable bearing capacity? It is the net loading intensity at which neither the soil fails in shear nor there is excessive settlement detrimental to the structure 9. Write the expression for correction due to dilatancy submergence? Ne = 15 + ( No-15 ) 10.What are the requirements for a stable foundation? -must be safe from failure -must be properly located -must not settle or deflect sufficiently to damage the structure or impair its usefulness. 11. What are the factors which depends depth? Type of soil, size of structure, magnitude of loads, environmental conditions, etc T SHARMILA Page 97

98 12.Define net pressure intensity? It is the excess pressure, of the gross pressure after the construction of the structure and the original overburden pressure. 13. What are the zones used in the Terzaghi s bearing capacity analysis for dividing the failure envelope of the soil.? Elastic equilibrium zone, Radial Stress zone, plastic zone 14. Write the ultimate bearing capacity equation for the general shear failure of soil in Terzaghi s analysis for a strip footing. qu = c Nc + γdnq γb Nγ 15. Define Shallow foundation. If the depth of the foundation is less than its breadth, such foundation is known as shallow foundation. 17. Write down the equation for estimating the elastic settlement based on the theory of elasticity.? 18. When will the total settlement be completed in the case of cohesion-less soil? Once the construction is over, the total settlement is assumed to be completed. 19. Define differential settlement If any two points of the foundation base experiences different settlements then such settlement is known as differential settlement. 20. What type of shear failure of soil is more likely to happen in the case of very dense soil? usually punching shear failure and local shear failure may also be possible. 21. Write the ultimate bearing capacity equation for the general shear failure of soil in Terzaghi s analysis for a square footing. qu = 1.3 cnc + γd Nq γb Nγ 22. When will the Consolidation settlement get completed? In the case of cohesion-less soil, the consolidation settlement gets completed once the construction is over. But In the case of cohesive soil, the consolidation settlement takes place for several years. 24. Define Deep foundation If the depth of the foundation is equal to or greater than the breadth of the foundation such foundation is called as deep foundation. 25. For which type of foundation, Terzaghi s bearing capacity equation is applicable. Why? Shallow foundation only. Because the effect of the depth is not considered. T SHARMILA Page 98

99 UNIT III 1. Under what circumstances, a strap footing is adopted? When the distance between the two columns is so great, so that trapezoidal footing is very narrow and so it is uneconomical. It transfers the heavy load of one column to other column. 2. What is a mat foundation? It is a combined footing that covers the entire area beneath a structure and supports all the walls and columns. 3. Where mat foundation is used? It is used when the area of isolated footing is more than fifty percentage of whole area or the soil bearing capacity is very poor. 4. Define spread footing? It is a type of shallow foundation used to transmit the load of isolated column, or that of wall to sub soil. The base of footing is enlarged and spread to provide individual support for load. 5. What are types of foundation? shallow foundation, deep foundation 6. What are the footings comes under shallow foundation? spread footing or pad footings, strap footings, combined footings, raft or mat foundation 7. What are the footings comes under deep foundation? pile, caisons(well foundation) 8. Define floating foundation? It is defined as a foundation in which the weight of the building is approximately equal to the full weight of the soil including water excavated from the site of the building. 9. What is mean by proportioning of footing? Footings are proportional such that the applied load including the self weight of the footing including soil.the action are not exceeding the safe bearing capacity of the soil. 10. What are the assumptions made in combined footing? - the footing is rigid and rests on a homogenous soil to give rise to linear stress distribution on the bottom of the footing. - the resultant of the soil pressure coincides with the resultant of the loads, then it is assumed to be uniformly distributed. T SHARMILA Page 99

100 UNIT -IV 1. List out the type of pile based on material used? timber pile, concrete pile, steel pile, composite pile 2. How is the selection of pile carried out? The selection of the type, length and capacity is usually made from estimation based on the soil condition and magnitude of the load. 3. What is mean by group settlement ratio? The settlement of pile group is found to be many times that of a single pile. The ratio of the settlement of the pile group to that of a single pile is known as the group settlement ratio. 4. What are the factors consider while selecting the type of pile? -the loads -time available for completion of the job -availability of equipment -the ground water conditions -the characteristics of the soil strata involved 5. What are the type of hammer? drop hammer, diesel hammer, double acting hammer, single acting hammer, vibratory hammer 6. What is pile driver? Piles are commonly driven by means of a hammer supported by a crane or by a special device known as a pile driver. 7. What are methods to determine the load carrying capacity of a pile? - dynamic formulae - static formula - pile load test - penetration tests 8. What are the two types of dynamic formulae? - Engg. news formula - Hiley s formula 9. What is meant by single-under reamed pile? The pile has only one bulb is known as single under reamed pile. 10. Write down the static formulae? The static formulae are based on assumption that the ultimate bearing capacity Qup of a pile is the sum of the ultimate skin friction Rf and total ultimate point or and bearing resistance Rp. Qup=Rf+Rp Qup=Asrf+Ap.rp T SHARMILA Page 100

101 UNIT -V 1. Define conjugate stresses? The stress acting on the conjugate planes is called conjugate stresses 2. How do you check the stability of retaining walls? The wall should be stable against sliding The wall should be stable against overturning The base of the wall should be stable against bearing capacity failure 3. Define angle of repose? Maximum natural slope at which the soil particles may rest due to their internal friction, if left unsupported for sufficient length of time 4. Define theory of plasticity? The theory on which the condition of the stress in a state of a plastic equilibrium is called as theory of plasticity. 5. What are assumption in coulomb wedge theory? - the backfill is dry, cohesionless, isotropic, homogenous, - the slip surface is plane which passes through the head of the wall 6. How to prevent land sliding? Sheet piles, retaining wall may be used to prevent the land sliding 7. Write down any two assumptions of Rankine s theory? - semi infinite soil - cohesion-less backfill - homogenous soil - the top surface is a plane which may be inclined or horizontal. 8. Distinguish Coloumb s wedge theory from Rankine s theory? Rankine considered a soil particle at plastic equilibrium but Coulomb considered the whole soil mass. 16 Marks Unit 1 1. Explain any two methods of site exploration in detail? The various methods of site exploration may be grouped as follows: 1. Open excavations. Trial pits are the cheapest method of exploration in shallow deposits, since these can be used in all types of soils. Soils are inspected in the natural conditions and samples, disturbed and undisturbed can be conveniently taken. The cost of open excavation increases rapidly with depth. They are generally considered suiable for shallow depths(say upto 3 m). 2. Borings. The following are the various boring methods commonly used:(i) Auger boring.(ii) Auger and shell boring.(iii) Wash boring.(iv) Percussion boring.(v) Rotary boring T SHARMILA Page 101

102 3. Sub-surface soundings. The sounding methods consist of measuring the resistance of the soil with depth by means of penetrometer under static or dynamic loading. The penetrorneter may consist of a sampling spoon, a cone or other shaped tool. The resistance to penetration is empirically correlated with some of the engineering properties of soil, such as density index, consistency, bearing capacity etc. 4. Geo-physical methods. Geo-physical methods are used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. Geo-physical investigations involve the detecttion of significant differences in the physical properties of geological formations. 2. Explain wash boring method of soil exploration? The boring methods are used for exploration at greater depths where direct methods fail. These provide both disturbed as well as undisturbed samples depending upon the method of boring. In selecting the boring method for a particular job, consideration should be made for the following: The materials to be encountered and the relative efficiency of the various boring methods in such materials. The available facility and accuracy with which changes in the soil and ground water conditions can be determined. Possible disturbance of the material to be sampled. The different types of boring methods are: 1. Displacement boring. 2. Wash boring. 3. Auger boring. 4. Rotary drilling. 5. Percussion drilling. 6. Continuous sampling. 1. Displacement borings It is combined method of sampling & boring operation. Closed bottom sampler, slit cup, or piston type is forced in to the ground up to the desired depth. Then the sampler is detached from soil below it, by rotating the piston, & finally the piston is released or withdrawn. The sampler is then again forced further down & sample is taken. After withdrawal of sampler & removal of sample from sampler, the sampler is kept in closed condition & again used for another depth. Features : Simple and economic method if excessive caving does not occur. Therefore not suitable for loose sand. Major changes of soil character can be detected by means of penetration resistance. These are 25mm to 75mm holes. It requires fairly continuous sampling in stiff and dense soil, either to protect the sampler from damage or to avoid objectionably heavy construction pit. 2. Wash boring: It is a popular method due to the use of limited equipments. T SHARMILA Page 102

103 The advantage of this is the use of inexpensive and easily portable handling and drilling equipments. Here first an open hole is formed on the ground so that the soil sampling or rock drilling operation can be done below the hole. 3. Auger boring This method is fast and economical, using simple, light, flexible and inexpensive instruments for large to small holes. It is very suitable for soft to stiff cohesive soils and also can be used to determine ground water table. Soil removed by this is disturbed but it is better than wash boring, percussion or rotary drilling. It is not suitable for very hard or cemented soils, very soft soils, as then the flow into the hole can occur and also for fully saturated cohesionless soil. 4. Rotary drilling Rotary drilling method of boring is useful in case of highly resistant strata. It is related to finding out the rock strata and also to access the quality of rocks from cracks, fissures and joints. It can conveniently be used in sands and silts also. Here, the bore holes are advanced in depth by rotary percussion method which is similar to wash boring technique. A heavy string of the drill rod is used for choking action. The broken rock or soil fragments are removed by circulating water or drilling mud pumped through the drill rods and bit up through the bore hole from which it is collected in a settling tank for recirculation. If the depth is small and the soil stable, water alone can be used. However, drilling fluids are useful as they serve to stabilize the bore hole. Drilling T SHARMILA Page 103

104 CE6502-FOUNDATION 2305 FOUNDATION ENGINEERING YEAR: YEAR:III/SEM:V V CIVIL CIVIL ENGINEERING Fig.1.3 Rotary Drilling System 3. Explain about standard penetration test? In-situ tests General The in situ tests in the field have the advantage of testing the soils in their natural, undisturbed condition. Laboratory tests, on the other hand, make use of small size samples obtained from boreholes through samplers and therefore the reliability of these depends on the quality of the so called undisturbed' samples. Further, obtaining undisturbed samples from non-cohesive, granular soils is not easy, if not impossible. Therefore, it is common practice to rely more on laboratory tests where cohesive soils are concerned. Further, in such soils, the field tests being short duration tests, fail to yield meaningful consolidation settlement data in any case. Where the subsoil strataa are essentially non-cohesive in character, the bias is most definitely towards field tests. The data from field tests is used in empirical, but time-tested correlations to predict settlement of foundations. The field tests commonly used in subsurface investigation are: Penetrometer test Pressuremeter test Vane shear test Plate load test Geophysical methods Penetrometer Tests : Standard penetration test (SPT) Static cone penetration test (CPT) Dynamic cone penetration test (DCPT) Standard penetration test The standard penetration test is carried out in a borehole, while the DCPT and SCPT are carried out without a borehole. All the three tests measure the resistance of the soil strata to penetration by a penetrometer. Useful empirical correlations between penetration resistance and soil properties are available for use in foundation design. T SHARMILA Page 104

105 This is the most extensively used penetrometer test and employs a split-spoon sampler, which consists of a driving shoe, a split-barrel of circular cross-section which is longitudinally split into two parts and a coupling. IS: gives the standard for carrying out the test. Procedure The borehole is advanced to the required depth and the bottom cleaned. The split-spoon sampler, attached to standard drill rods of required length is lowered into the borehole and rested at the bottom. The split-spoon sampler is driven into the soil for a distance of 450mm by blows of a drop hammer (monkey) of 65 kg falling vertically and freely from a height of 750 mm. The number of blows required to penetrate every 150 mm is recorded while driving the sampler. The number of blows required for the last 300 mm of penetration is added together and recorded as the N value at that particular depth of the borehole. The number of blows required to effect the first 150mm of penetration, called the seating drive, is disregarded. The split-spoon sampler is then withdrawn and is detached from the drill rods. The split-barrel is disconnected from the cutting shoe and the coupling. The soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory. The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a case, the N value can be recorded for the first 300 mm penetration. The boring log shows refusal and the test is halted if 50 blows are required for any 150mm penetration 100 blows are required for 300m penetration 10 successive blows produce no advance. Precautions The drill rods should be of standard specification and should not be in bent condition. The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear. The drop hammer must be of the right weight and the fall should be free, frictionless and vertical. The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a T SHARMILA Page 105

106 distance of 450 mm. In such a case, the N value can be recorded for the first 300 mm penetration. The boring log shows refusal and the test is halted if 50 blows are required for any 150mm penetration 100 blows are required for 300m penetration 10 successive blows produce no advance. The height of fall must be exactly 750 mm. Any change from this will seriously affect the N value. The bottom of the borehole must be properly cleaned before the test is carried out. If this is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed soil. When a casing is used in borehole, it should be ensured that the casing is driven just short of the level at which the SPT is to be carried out. Otherwise, the test gets carried out in a soil plug enclosed at the bottom of the casing. When the test is carried out in a sandy soil below the water table, it must be ensured that the water level in the borehole is always maintained slightly above the ground water level. If the water level in the borehole is lower than the ground water level, quick' condition may develop in the soil and very low N values may be recorded. In spite of all these imperfections, SPT is still extensively used because the test is simple and relatively economical. it is the only test that provides representative soil samples both for visual inspection in the field and for natural moisture content and classification tests in the laboratory. SPT values obtained in the field for sand have to be corrected before they are used in empirical correlations and design charts. IS: recommends that the field value of N be corrected for two effects, namely, (a) effect of overburden pressure, and (b) effect of dilatancy. (a) Correction for overburden pressure Several investigators have found that the penetration resistance or the N value in a granular soil is influenced by the overburden pressure. Of two granular soils possessing the same relative density but having different confining pressures, the one with a higher confining pressure gives a higher N value. Since the confining pressure (which is directly proportional to the overburden pressure) increases with depth, the N values at shallow depths are underestimated and the N values at larger depths are overestimated. To allow for this, N values recorded from field tests at different effective overburden pressures are corrected to a standard effective overburden pressure. T SHARMILA Page 106

107 4. Explain any two important types of samplers Types of Samplers The samplers are classified as thick wall or thin wall samplers depending upon the area ratio. Thick wall samplers are those having the area ratio greater than 10 percent. Depending upon the mode of operation, samplers may be classified in the following three common types : (i) open drive sampler (including split spoon samplers), (ii) stationary piston sampler and (iii) rotary sampler. The open drive sample r is a tube open at its lower end. The sampler head is provided with vent +41s (valve) to permit water and air to escape during driving. The check valve helps to retain sample when the sampler is lifted up. The tube may be seamless or it may be split in two parts; in the latter case it is known as split spoon sampler. The stationary piston sampler consists of a Sample cylinder and the piston system. During lowering of the sampler through the hole, the lower end of the sampler is kept closed with the piston. When the desired sampling elevation is reached, ihe piston rod is clamped, thereby keeping the piston stationary, and the sampler tube is advanced down into the soil. The sampler is then Lifted up, with piston rod clamped in position. The sampler is more suitable for sampling soft soils saturated sands. Rotatory samplers are the core barrel type having an outer tube provided with cutting teeth and a removable thin wall liner inside. It is used for firm to hard cohesive soils T SHARMILA Page 107

108 and cemented soils. 5. Explain with neat sketch auger boring method of soil exploration. The following are the various boring methods commonly used: (i) Auger boring. (ii) Auger and shell boring. (iii) Wash boring. (iv) Percussion boring. (v) Rotary boring. (i) Auger boring Augers are used in cohesive and other soft soils aboye water table. They may either be operated manually or mechanically. Hand augers are used upto a depth upto 6 m. Mechanically operated augers are used for greater depths and they can also be used in gravelly soils. Augers are of two types: (a) spiral auger and (b) post-hole auger. FIG AUGER. Samples recovered from the soil brought up by the augers are badly disturbed and are useful for identification purposes only. Auger boring is fairly satisfactory br explorations at shallow depths and for exploratory borrow pits. (ii) Auger and shell boring Cylindrical augers and shells with cutting edge or teeth at Iower end can be used for making deep borings. Hand operated rigs are used for depths upto 25 m and mechanised rigs up to 50 m. Augers are suitable for soft to stiff clays, shells for very stiff and hard clays, and shells or sand pumps for sandy soils. Small boulders, thin soft strata or rock or cemented gravel can be broken by chisel bits attached to drill rods. The hole usually requires a casing. Fig shows a typical sand pump. T SHARMILA Page 108

109 CE6502-FOUNDATION 2305 FOUNDATION ENGINEERING YEAR: YEAR:III/SEM:V V CIVIL CIVIL ENGINEERING FIG SAND PUMP (iii) Wash boring Wash boring is a fast and simple method for advancing holes in all types of soils. Boulders and rock cannot be penetrated by this method. The method consists of first driving a casing through which a hollow drilled rod with a sharp chisel or chopping bit at the lower end is inserted. Water is forced under pressure through the dril rod which is alternativety raised and dropped, and also rotated. The resulting chopping and jetting action of the bit and water disintegrates the soil. The cuttings are forced upto the ground surface in the form of soil-water slurry through the annular space between the drill rod and the casing. The change in soil stratification could be guessed from the rate of progress and colour of wash water. The samples recovered from the wash water are almost valueless for interpreting the correct geo-technical properties of soil. (iv) Percussion boring In this method, soil and rock formations are broken by repeated blows of heavy chiesel or bit suspended by a cable or drill rod. Water is added to the hole during boring, if not already present and the slurry of pulverised material is bailed out at intervals. The method is suitable for advancing a hole in all types of solis, boulders and rock. The formations, however, get disturbed by the impact. (v) Rotary boring Rotary boring or rotay drilling is a very fast method of advancing hole in both rocks and soils. A. drill bit, fixed to the lower end of the drill rods, is rotated by a suitable chuck, and is always kept in firm contact with the bottom of the hole. A drilling mud, usually a water solution of bentonite, with or without other admixtures, is continuously forced down to the hollow dril rods. The mud returning upwards brings the cuttings to the surface. The method is also known as mud rotary drilling and the hole usually requires no casing. T SHARMILA Page 109

110 CE6502-FOUNDATION 2305 FOUNDATION ENGINEERING YEAR: YEAR:III/SEM:V V CIVIL CIVIL ENGINEERING Rotary core barrels, provided with commercial diamond-studded bits or a steel bit with shots, are also used for rotary drilhng and simultaneously obtaining the rock cores or samples. The method is them also known as core boring or core drilling. Water 15 circulated down drill rods during boring. FIG WASH BORING. 6. Describe the salient features of a good sub-soil investigation report? A borehole log should give details of the foreman driller s log, the observations of the supervising engineer and the results of any site tests. A typical borehole log is shown in Fig Trial pits, trenches and boreholes should be given reference numbers, located on plan, their ground level noted and the date of excavation recorded. It is advisable to record the following additional information: (1) Type of rig, diameter and depth of bore or width of bucket. (2) Diameter and depth of any casing used and why it was necessary. (3) Depth of each change of strata and a full description of the strata. (Was the soil virgin ground or fill?) (4) Depths at which samples taken, type of sample and sample reference number. (5) In situ test depth and reference number. (6) The levels at which groundwater was first noted; the rate of rise of the water; its level at start and end of each day. (When more information on permeability, porewater pressure, and the like is T SHARMILA Page 110

111 required, then it is vitally important that the use of piezometers should be considered.) (7) Depth and description of obstructions (i.e. boulders), services (drains) or cavities encountered. (8) Rate of boring or excavation (useful to contractors and piling sub-contractors as such information gives some guidance in ease of excavation or pile driving). (9) Name of supervising engineer. (10) Date and weather conditions during investigation. Fig. Example of a typical borehole log (BS 5930). 7.Explain various types of Samles. Also discuss various factors affecting quality of samples? Soil samples can be of two types: T SHARMILA Page 111

112 (i) Disturbed samples. (ii) Undisturbed samples. A disturbed sample is that in which the natural structure of soil gets partly or fully modified and destroyed although with suitable precautions the natural water content may be preserved. Such a soil sample should, however, be representative of the natural soil by maintainlng the original proportion of the various particles intact. An undisturbed sample is that in which the natural structure and properties remain preserved. The sample disturbance depends upon the design of the samplers and the method of sampling. To take undisturbed samples from bore holes properly designed sampling tools are required. The sampling tube when forced into the ground should cause as little remoulding and disturbance as possible. The design features of the sampler, that govern the degree of disturbance are (i) cutting edge (ii) inside wall friction and (iii) non-return valve. Fig. shows a typical cutting edge of a sampler, with the lower end of the sampler, with the lower end of the sampler tube. The following terms are defined with respect to the diameters marked in Fig. Fig. LOWER END OF A SAMPLER. The area ratio should be as low as possibie. It should not be greater than 25 percent; for soft sensitive soil, it should preferably not exceed to porcent. The inside clearance should lie between 1 to 3 percent and the outside clearance should not be much greater than the inside clearance. The walls of the sampler should be smooth and should be kept properly oiled so that wall friction is minimum. Lower value of inside clearance allows the elastic expansion of soil and reduces the frictional drag. T SHARMILA Page 112

113 The non-retum valve, invariably provided in samplers, should permit easy and quick escape of water and air when driving the sampler. UNIT -II 1. A Square footing of 1.2 m x1.2m rest at depth of 1m in saturated clay layer 4m deep. The clay is normally consolidated having an unconfined compressive strength of 40 kn/m 2. The soil has a liquid limit 30%, sat = 17.8 kn/m 3, W=28% and G = Determine the load which the footing can carry safely with a a factor of safety of 3 against shear. Also, determine the settlement if the footing is loaded with this safe load. Use Terzahi s analysis for bearing capacity. Ans: Since = 0, N c = 5.7, N q = 1and N = 0 Also, = = 17.8 C= q u /2 =40/2= 20 kn/m 2 q s = q nf + sat D F (or) 1 q s = [ 1.3 cn c + (Nq-1) B. N ] + sat D F (or) 1 q s = [ 1.3 x 20 x (1-1) + 0] = = 67.2 kn/m 2 Q s = q s x B 2 =67.2 x 1.2 x 1.2 =96.77 kn. Thickness of clay layer = 4 m. Depth of centre of clay layer, below footing Level =4/2 1 =1 m. Assuming load dispersion at 45 width of load spread = (1) = 3.2 m. Vertical stress increment due to foundation load = = = 9.45 kn / m x 3.2 Now, the consolidation settlement of footing is given by, C c 0 + S c = C H log e 0 0 Assume C = 1 T SHARMILA Page 113

114 e 0 = w sat G = 0.28 x 2.68 = 0.75 C c = 0.009(w L - 10) = 0.009(30-10) = 0.18 Initial over burden pressure at the centre of clay layer = 0 = sat Z = 17.8 x 2 = 35.6 kn / m c = x 4 log = m = 42 mm. 2. The results of two plate load tests for a settlement of 25.4mm are given. Plate diameter Load 0.3 m 31 kn 0.6 m 65 kn A square column foundation is to be designed to carry a load of 800 kn with an allowable settlement of 25.4 mm. Determine the size of the foundation using Housel s method. Ans: Given data: Q 1 = 31 kn Q 2 = 65 kn Q = 800 kn, d 1 = 0.3 m d 2 = 0.6 m 25.4 mm. To find: Size of the foundation using Housel s method. Solution: Q 1 = A 1 m + P 1 n Q 2 = A 2 m + P 2 n Q = A m + P n..(1)..(2)..(3) T SHARMILA Page 114

115 d 2 1 x A 1 = = = x 10-3 m d 2 2 x A 2 = = = x 10-3 m P1 = x d 1 = x 0.3 = m P2 = x d 2 = x 0.6 = m 31 = x 10-3 m n..(a) 65 = x 10-3 m n...(b) Solving equation A and B, M = 21.22, n = 31.3, Q = A m + P n 800 x B 2 x (31.31 x 48) B = 3.86 m say 4m x 4m Size of the foundation 4m x 4m. 3. A square footing for a column is 2.5 m x 2.5 m and carries a load of 2000 kn. Find the factor of safety against bearing capacity failure, if the soil has the following properties. C = 50 kn/m 2, =15,γ = 17.6 kn/m 3 N c 1 = 12.5, Nq 1 = 4.5, Nr 1 =2.5. The foundation is taken to a depth of 1.5 m. Data Size of column = 2.5 m x 2.5 m Load = 2000 kn C= 50 kn/ m 2 =15 γ = 17.6 kn/m 3 N c = 12.5, N q = 4.5, N γ = 2.5 D = 1.5 m Solution q nf =2/3 cn c S c + γ D (N q -1) S q BN γ S γ for square footing T SHARMILA Page 115

116 S c = 1.3 S γ =0.8 S q =1.0 q nf = 2/3x 50x12.5x x1.5(4.5-1)x1+0.5x17.6x2.5x2.5x0.8 = q nf = Actual load intensity q a =Load = 2000 Area 2.5x2.5 We know that q a = q s q s =qnf + γ D F x1.5= F F = 2.31 q a = 320 kn/m 2 4. Determine the probable settltement of a strip footing 1m wide transmitting a pressure intensity of 100 kn/m 2 at a depth of 1.5m below sand. If the plate load test shows a settlement of 5mm against100 kn/m 2. The sizeof the plate is 30cm x 30cm. Data B F =1m B P =0.3m q F = 100kN/m 2 q p =100kN/m 2 d= 1.5m p =5mm or 5x10-3 m Sol p = F B p (B F + 0.3) 2 B f ( B p +0.3) 5x10-3 = F 0.3(1+0.3) 2 1( ) 5x10-3 = 0.42 F F=0.012m Settlement of footing =12mm T SHARMILA Page 116

117 UNIT -V 1. A vertical excavation was made in a clay deposit having weight of 20 kn/m 3. It caved in after the depth of digging reached 4m. Taking the angle of internal friction to be zero, calculate the value of cohesion. If the same clay is used as a backfill against a retaining wall, up to a height of 8 m, calculate (i) (ii) Total active earth pressure, Total passive earth pressure. Assume that the wall yields for enough to allow Rankine deformation conditions to establish. Ans: The critical height H c of an unsupported vertical cut in cohesive soil is given by Equation: 4c H c = tan 45 + As = 0, tan = tan = 1 2 H c γ 4 x 20 c = = = 20 kn / m 2..(1) 4 4 (i) Total active earth pressure is given by Equation 1 P a = γ H 2 cot 2-2cH cot 2 1 = x 20 x (8) 2 x 1 (2 x 20 x 8) x 1 2 = = 320 kn/m. (ii) Total passive earth pressure is given by Equation 1 P p = γ H 2 tan 2 + 2cH tan 2 1 = x 20 x (8) 2 x 1 + (2 x 20 x 8) x 1 2 = = 960 kn/m. T SHARMILA Page 117

118 The soil beneath structures responsible for carrying the loads is the FOUNDATION. The general misconception is that the structural element which transmits the load to the soil (such as a footing) is the foundation. The figure below clarifies this point. TYPES OF FOUNDATIONS Foundations can be can be categorized into basically two types: Shallow and Deep. Shallow Foundations: These types of foundations are so called because they are placed at a shallow depth (relative to their dimensions) beneath the soil surface. Their depth may range from the top soil surface to about 3 times their breadth (about 6 meters). They include footings (spread and combined), and soil retaining structures (retaining walls, sheet piles, excavations and reinforced earth). There are several others of course. Deep Foundations: The most common of these types of foundations are piles. They are called deep because the are embedded very deep (relative to their dimensions) into the soil. Their depths may run over several 10s of meters. They are usually used when the top soil layer have low bearing capacity. T SHARMILA Page 118

119 DESIGN CONSIDERATIONS To perform satisfactorily, foundations must carry the loads (and moments) and have two main characteristics: 1. Be safe against overall shear failure (Bearing Capacity Failure). 2. Not undergo excessive displacement (Settlement). These conditions will insure that the foundation i.e. the soil is safe and can carry the loads without major problems. Therefore, when designing foundations, these two characteristic must be satisfied. In addition to satisfying the conditions for the foundation, the structural members (concrete, steel and/or wood) must be able to transfer the load to the soil without failing. In the case of concrete, two basic conditions must be satisfied: 1. No shear failure: This is satisfied by providing an adequate thickness of concrete. 2. No tension failure: This is satisfied by providing adequate steel reinforcement. This course covers the analysis and design (geotechnical and concrete design) of the basic and most commonly used types of foundations including both shallow and deep foundations. (c) (d) Fig. 1 Spread Footings: (a) Square, (b) Rectangular, (c) Wall (Strip) and (d) Circular T SHARMILA Page 119

120 BEARING CAPACITY OF SOIL Definitions Bearing capacity is the power of foundation soil to hold the forces from the superstructure without undergoing shear failure or excessive settlement. Foundation soil is that portion of ground which is subjected to additional stresses when foundation and superstructure are constructed on the ground. The following are a few important terminologies related to bearing capacity of soil. Ultimate Bearing Capacity (q f ) : It is the maximum pressure that a foundation soil can withstand without undergoing shear failure. Net ultimate Bearing Capacity (q n ) : It is the maximum extra pressure (in addition to initial overburden pressure) that a foundation soil can withstand without undergoing shear failure. q n = q f - q o Fig. 7.1 : Main components of a structure including soil T SHARMILA Page 120

121 CE6502-FOUNDATION Here, q o ENGINEERING represents the overburden YEAR:III/SEM:V pressure at foundation CIVIL level ENGINEERING and is equal to D for level ground without surcharge where is the unit weight of soil and D is the depth to foundation bottom from Ground Level Safe Bearing Capacity (q s ) : It is the safe extra load the foundation soil is subjected to in addition to initial overburden pressure. Here. F represents the factor of safety Allowable Bearing Pressure (q a ) : It is the maximum pressure the foundation soil is subjected to considering both shear failure and settlement. qn q s = + qo F Foundation is that part of the structure which is in direct contact with soil. Foundation transfers the forces and moments from the super structure to the soil below such that the stresses in soil are within permissible limits and it provides stability against sliding and overturning to the super structure. It is a transition between the super structure and foundation soil. The job of a geotechnical engineer is to ensure that both foundation and soil below are safe against failure T SHARMILA Page 121

122 and do not experience excessive settlement. Footing and foundation are synonymous. 3. Explain the various Modes of shear failure. Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the foundation soil. 1. General shear failure (Ref Fig. 7.1a) 2. Local shear failure (Ref Fig. 7.1b) 3. Punching shear failure (Ref Fig. 7.1c) Shear failure in foundation soil P curve in different foundation soils Fig : Footing on ground that experiences a) General shear failure, b) Local shear failure and c) Punching shear failure General Shear Failure This type of failure is seen in dense and stiff soil. The following are some characteristics of general shear failure. 1. Continuous, well defined and distinct failure surface develops between the edge of footing and ground surface. 2. Dense or stiff soil that undergoes low compressibility experiences this failure. 3. Continuous bulging of shear mass adjacent to footing is visible. T SHARMILA Page 122

123 4. Failure is accompanied by tilting of footing. 5. Failure is sudden and catastrophic with pronounced peak in P curve. 6. The length of disturbance beyond the edge of footing is large. 7. State of plastic equilibrium is reached initially at the footing edge and spreads gradually downwards and outwards. 8. General shear failure is accompanied by low strain (<5%) in a soil with considerable (>36 o ) and large N (N > 30) having high relative density (I D > 70%). Local Shear Failure This type of failure is seen in relatively loose and soft soil. The following are some characteristics of general shear failure. 1. A significant compression of soil below the footing and partial development of plastic equilibrium is observed. 2. Failure is not sudden and there is no tilting of footing. 3. Failure surface does not reach the ground surface and slight bulging of soil around the footing is observed. 4. Failure surface is not well defined. 5. Failure is characterized by considerable settlement. 6. Well defined peak is absent in P curve. 7. Local shear failure is accompanied by large strain (> 10 to 20%) in a soil with considerably low (<28 o ) and low N (N < 5) having low relative density (I D > 20%). Punching Shear Failure This type of failure is seen in loose and soft soil and at deeper elevations. The following are some characteristics of general shear failure. 1. This type of failure occurs in a soil of very high compressibility. 2. Failure pattern is not observed. T SHARMILA Page 123

124 4.Distinction between General Shear & Local or Punching Shear Failures The basic distinctions between general shear failure and punching shear failure are presented in Table 7.1. Table 7.1 : Distinction between General Shear & Local Shear Failures General Shear Failure Local/Punching Shear Failure Occurs in dense/stiff soil >36 o, N>30, I D >70%, C u >100 kpa Occurs in loose/soft soil <28 o, N<5, I D <20%, C u <50 kpa Results in small strain (<5%) Results in large strain (>20%) Failure pattern well defined & clear Failure pattern not well defined Well defined peak in P- curve No peak in P- curve Bulging formed in the neighbourhood of No Bulging observed in the footing at the surface neighbourhood of footing Extent of horizontal spread of Extent of horizontal spread of disturbance at the surface large disturbance at the surface very small Observed in shallow foundations Observed in deep foundations Failure is sudden & catastrophic Failure is gradual Less settlement, but tilting failure Considerable settlement of footing observed observed 5. Explain Terzaghi s bearing Capacity Theory Terzaghi (1943) was the first to propose a comprehensive theory for evaluating the safe bearing capacity of shallow foundation with rough base. T SHARMILA Page 124

125 Assumptions 1. Soil is homogeneous and Isotropic. 2. The shear strength of soil is represented by Mohr Coulombs Criteria. 3. The footing is of strip footing type with rough base. It is essentially a two dimensional plane strain problem. 4. Elastic zone has straight boundaries inclined at an angle equal to to the horizontal. 5. Failure zone is not extended above, beyond the base of the footing. Shear resistance of soil above the base of footing is neglected. 6. Method of superposition is valid. 7. Passive pressure force has three components (P PC produced by cohesion, P Pq produced by surcharge and P P produced by weight of shear zone). 8. Effect of water table is neglected. 9. Footing carries concentric and vertical loads. 10. Footing and ground are horizontal. 11. Limit equilibrium is reached simultaneously at all points. Complete shear failure is mobilized at all points at the same time. 12. The properties of foundation soil do not change during the shear failure Limitations 1. The theory is applicable to shallow foundations 2. As the soil compresses, increases which is not considered. Hence fully plastic zone may not develop at the assumed. 3. All points need not experience limit equilibrium condition at different loads. 4. Method of superstition is not acceptable in plastic conditions as the ground is near failure zone. T SHARMILA Page 125

126 Fig. 7.3 : Terzaghi s concept of Footing with five distinct failure zones in foundation soil T SHARMILA Page 126

127 Concept A strip footing of width B gradually compresses the foundation soil underneath due to the vertical load from superstructure. Let q f be the final load at which the foundation soil experiences failure due to the mobilization of plastic equilibrium. The foundation soil fails along the composite failure surface and the region is divided in to five zones, Zone 1 which is elastic, two numbers of Zone 2 which are the zones of radial shear and two zones of Zone 3 which are the zones of linear shear. Considering horizontal force equilibrium and incorporating empirical relation, the equation for ultimate bearing capacity is obtained as follows. Ultimate bearing capacity, q f = cn c + γdn q γBN If the ground is subjected to additional surcharge load q, then q f = cn c + ( γd + q) N q γBN γ γ Net ultimate bearing capacity, qn = cn c + γd( N q 1) γBN γ q n = cn + γdn γbn γ γd c q Safe bearing capacity, q = [ cn + γd( N 1) + 0.5γBN γ ] + γd Here, F = Factor of safety (usually 3) c = cohesion = unit weight of soil D = Depth of foundation q = Surcharge at the ground level B = Width of foundation N c, N q, N = Bearing Capacity factors s c q 1 F Table 7.2 : Bearing capacity factors for different N c N q N g N ' c N ' q N ' g T SHARMILA Page 127

128 6. Explain theeffect of shape of Foundation The shape of footing influences the bearing capacity. Terzaghi and other contributors have suggested the correction to the bearing capacity equation for shapes other than strip footing based on their experimental findings. The following are the corrections for circular, square and rectangular footings Circular footing q = 1.3cN + γdn + 0. γbn f c q Square footing q f = 1.3cN c + γdn q γBN Rectangular footing q B B = ( ) cn c + γdn q + (1 0.2 )0. γbn L L f 5 γ γ Summary of Shape factors Table 7.2 gives the summary of shape factors suggested for strip, square, circular and rectangular footings. B and L represent the width and length respectively of rectangular footing such that B < L. γ Table 7.3 : Shape factors for different shapes of footing Shape s c s q s Strip Square Round Rectangle B B ( ) 1 (1 0.2 ) L L T SHARMILA Page 128

129 7.5 Local shear failure The equation for bearing capacity explained above is applicable for soil experiencing general shear failure. If a soil is relatively loose and soft, it fails in local shear failure. Such a failure is accounted in bearing capacity equation by reducing the magnitudes of strength parameters c and as follows. 2 tanφ 1 = tanφ 3 c 1 = Table 7.3 summarizes the bearing capacity factors to be used under different situations. If is less than 36o and more than 28o, it is not sure whether the failure is of general or local shear type. In such situations, linear interpolation can be made and the region is called mixed zone. 2 c 3 Table 7.4 : Bearing capacity factors in zones of local, mixed and general shear conditions. Local Shear Failure Mixed Zone General Shear Failure < 28 o 28 o < < 36 o > 36 o N 1 c, N 1 1 q, N N m c, N m m q, N N c, N q, N 7.Factors influencing Bearing Capacity Bearing capacity of soil depends on many factors. The following are some important ones. 1. Type of soil 2. Unit weight of soil 3. Surcharge load 4. Depth of foundation 5. Mode of failure 6. Size of footing 7. Shape of footing 8. Depth of water table T SHARMILA 9. Eccentricity in footing load Page

130 Major disadvantages of field tests are Labourious Time consuming Heavy equipment to be carried to field Short duration behavior Plate Load Test Foundation Soil Sand Bags Platform for loading Dial Gauge Testing Plate Foundation Level Fig. 7.8 : typical set up for Plate Load test assembly 1. It is a field test for the determination of bearing capacity and settlement characteristics of ground in field at the foundation level. 2. The test involves preparing a test pit up to the desired foundation level. 3. A rigid steel plate, round or square in shape, 300 mm to 750 mm in size, 25 mm thick acts as model footing. 4. Dial gauges, at least 2, of required accuracy (0.002 mm) are placed on plate on plate at corners to measure the vertical deflection. 5. Loading is provided either as gravity loading or as reaction loading. For smaller loads gravity loading is acceptable where sand bags apply the load. T SHARMILA Page 130

131 6. In reaction loading, a reaction truss or beam is anchored to the ground. A hydraulic jack applies the reaction load. 7. At every applied load, the plate settles gradually. The dial gauge readings are recorded after the settlement reduces to least count of gauge (0.002 mm) & average settlement of 2 or more gauges is recorded. 8. Load Vs settlement graph is plotted as shown. Load (P) is plotted on the horizontal scale and settlement () is plotted on the vertical scale. 9. Red curve indicates the general shear failure & the blue one indicates the local or punching shear failure. 10. The maximum load at which the shear failure occurs gives the ultimate bearing capacity of soil. Reference can be made to IS The advantages of Plate Load Test are 1. It provides the allowable bearing pressure at the location considering both shear failure and settlement. 2. Being a field test, there is no requirement of extracting soil samples. 3. The loading techniques and other arrangements for field testing are identical to the actual conditions in the field. 4. It is a fast method of estimating ABP and P behaviour of ground. The disadvantages of Plate Load Test are 1. The test results reflect the behaviour of soil below the plate (for a distance of ~2Bp), not that of actual footing which is generally very large. 2. It is essentially a short duration test. Hence, it does not reflect the long term consolidation settlement of clayey soil. 3. Size effect is pronounced in granular soil. Correction for size effect is essential in such soils. 4. It is a cumbersome procedure to carry equipment, apply huge load and carry out testing for several days in the tough field environment. T SHARMILA Page 131

132 7.12 Problems & Solutions 1. A square footing is to be constructed on a deep deposit of sand at a depth of 0.9 m to carry a design load of 300 kn with a factor of safety of 2.5. The ground water table may rise to the ground level during rainy season. Design the plan dimension of footing given sat = 20.8 kn/m 3, N c = 25, N q = 34 and N =32. (Feb 2002) Data C = 0 F = 2.5 D = 0.9 m R W1 = R W2 = 0.5 = 20.8 kn/m 3 N c = 25 N q = 34 N = 32 P P 1 q s = = = 1.3 cn c + γd( N q 1) RW γBN γ RW 2 + γd = B B 3 B = 1.21 m 2. What will be the net ultimate bearing capacity of sand having = 36 o and d = 19 kn/m 3 for (i) 1.5 m strip foundation and (ii) 1.5 m X 1.5 m square footing. The footings are placed at a depth of 1.5 m below ground level. Assume F = 2.5. Use Terzaghi s equations. (Aug 2003) N c N q N By linear interpolation N c = 65.38, N q = 49.38, N = 54 at = 36 o Data B = 1.5 m D = 1.5 m T SHARMILA Page 132

133 Strip Footing qn = cn c + γd( N q 1) γBN q n = kpa Square Footing q = 1.3cN + γd( N 1) + 0. γbn n c q 4 q n = kpa γ γ 3. A square footing 2.5 m X 2.5 m is built on a homogeneous bed of sand of density 19 kn/m 3 having an angle of shearing resistance of 36 o. The depth of foundation is 1.5 m below the ground surface. Calculate the safe load that can be applied on the footing with a factor of safety of 3. Take bearing capacity factors as N c = 27, N q = 30, N = 35. (Feb 2004) Data C = 0 F = 3 B = 2.5 m D = 1.5 m = 19 kn/m 3 N c = 27 N q = 30 N = 35 q s = P A P = = 2 B 1 [.3cN + γd( N 1) R + 0.4γBN γ R ] + γd 1 c q W1 W 2 Safe load, P = q s *B*B = kn 4. A strip footing 2 m wide carries a load intensity of 400 kpa at a depth of 1.2 m in sand. The saturated unit weight of sand is 19.5 kn/m 3 and unit weight above water table is 16.8 kn/m 3. If c = 0 and = 35 o, determine the factor of safety with respect to shear failure for the following locations of water table. a. Water table is 4 m below Ground Level F T SHARMILA Page 133

134 b. Water table is 1.2 m below Ground Level c. Water table is 2.5 m below Ground Level d. Water table is at Ground Level. Using Terzaghi s equation, take N q = 41.4 and N = (Feb 2005) Data C = 0 = 35 o B = 2 m D = 1.2 m b = 19.5 kn/m 3 (bottom) t = 16.8 kn/m 3 (top) N c = 0 N q = 41.4 N = 42.4 Safe load intensity = 400 kpa q s = 1 [ cn + γd( N 1) R + 0.5γBN γ R ] + γd 400 = c q W1 W 2 a. Water table is 4 m below Ground Level R W1 = R W2 = 1 = 16.8 kn/m 3 F = 4.02 b. Water table is 1.2 m below Ground Level R W1 = 1, R W2 = = X F [ 16.8X 1.2X 40.4X1+ 0.5X19.5X 2X 42.4X 0.5] F = c. Water table is 2.5 m below Ground Level RW2 = 0.5(1+1.3/2) = X X 0.7 γ eff = = kn/m 3 2 T SHARMILA Page 134 F

135 1 400 = X F [ 16.8X 1.2X 40.4X1+ 0.5X17.745X 2X 42.4X 0.825] F = d. Water table is at Ground Level R W1 = R W2 = 0.5 = 19.5 kn/m = X F F = [ 19.5X 1.2X 40.4X X19.5X 2X 42.4X 0.5] A square footing located at a depth of 1.3 m below ground has to carry a safe load of 800 kn. Find the size of footing if the desired factor of safety is 3. Use Terzaghi s analysis for general shear failure. Take c = 8 kpa, N c = 37.2, N q = 22.5, N = (Aug 2005) d = 18 kn/m 3 c = 8 kpa F = 3 D = 1.3 m N c = 37.2 N q = 22.5 N = 19.7 P = 800 kn R W1 = R W2 = 1 q s = P A 47.28B P = = 2 B B B = m 3 (Assumed) 1 [.3cN + γd( N 1) R + 0.4γBN γ R ] + γd 1 c q W1 W = 0 6. A square footing 2.8 m X 2.8 m is built on a homogeneous bed of sand of density 18 kn/m 3 and = 36 o. If the depth of foundation is 1.8 m, determine F T SHARMILA Page 135

136 the safe load that can be applied on the footing. Take F = 2.5, Nc = 27, Nq = 36, N = 35. (Feb 2007) Data d = 18 kn/m 3 c = 0 (sand) F = 2.5 B = 2.8 m D = 1.8 m N c = 27 N q = 36 N = 35 P =? R W1 = R W2 = 1 q s = P A P = = 2 B P = q s *B*B = 6023 kn 1 [.3cN + γd( N 1) R + 0.4γBN γ R ] + γd 1 c q W1 W 2 7. A strip footing 1 m wide and a square footing 1 m side are placed at a depth of 1 m below the ground surface. The foundation soil has cohesion of 10 kpa, angle of friction of 26 o and unit weight of 18 kn/m 3. Taking bearing capacity factor from the following table, calculate the safe bearing capacity using Terzaghi s theory. Use factor of safety of 3. (July 2008) N c N q N 15 o o o F As = 28 o, the ground experiences local shear failure C = (2/3)X10 = 6.67 kpa tan = (2/3) X tan T SHARMILA Page 136

137 = o By linear interpolation, N c =15.79, N q =5.97, N =4.01 B = 1 m D = 1 m = 18 kn/m 3 Strip footing q s = 1 [ cn + γd N 1) + 0.5γBN γ ] + γd Square footing q s c ( q =94.96 kpa F 1 [.3cN + γd( N 1) + 0.4γBN γ ] + γd = 1 c q = kpa F 8. A square footing placed at a depth of 1 m is required to carry a load of 1000 kn. Find the required size of footing given the following data. C = 10 kpa, = 38 o, = 19 kn/m 3, N c = 61.35, N q = 48.93, N = and F = 3. Assume water table is at the base of footing. (July 2007) Data C = 10 kpa = 38 o B =? D = 1 m = 19 kn/m 3 N c = N q = N = F = 3 R W1 = 1 R W2 = 0.5 q s B 3 = P A P = = 2 B B 2 1 [.3cN + γd( N 1) R + 0.4γBN γ R ] + γd 1 c q W1 W = 0 B = 0.72 m F T SHARMILA Page 137

138 1.Determine the elastic settlement of a footing 3 m X 3 m resting on sandy soil given Es = kpa and = 0.3. Footing carries a load of 2000 kn. Take I = 0.82 (Feb 2002) q = 2000/3 2 = kpa B = 3 m I = 0.82 E = k Pa = 0.3 S I = m = 11 mm S I 2 1 µ qbi ρ E = 2.Estimate the immediate settlement of a concrete footing 1 m X 1.5 m in size, if it is founded at a depth of 1 m in silty soil whose compression modulus is 9000 kpa. Footing is expected to transmit unit pressure of 200 kpa. Assume I = 1.06, = 0.3 Data E = 9000 kpa = 0.3 q = 200 kpa B = 1 m I = 1.06 S I = m S I 2 1 µ qbi E = ρ 3.A series of plate load tests was conducted on three plates 300 mm, 450 mm and 600 mm square plates. The loads and corresponding settlements in the linear portions of P curves are as follows at a site. Find the immediate settlement of a footing 2 m X 2 m subjected to a load of 1000 kn. Table 8.8 : Details of Load settlement for different plate sizes Plate size (mm) Load (kn) Settlement (mm) 300 X X X T SHARMILA Page 138

139 Table 8.9 : Variation of qb with settlement for different plate sizes B (m) P (kn) q (kpa) S (m) qb (kn/m) S I 2 1 µ = qbi E 2 S 1 I µ = I qb E 2 1 µ I E ρ ρ ρ = ( kpa) qb (kn/m) Settlement (m) Fig. 8.3 : Variation of qb with settlement for different plate sizes Data B = 2 m q = 1000/(2*2) = 250 kpa T SHARMILA Page 139

140 S I 1 E I µ 2 = ρ qb S I = m Problem 4 The following are the results of plate load test on granular soil. Find the allowable bearing pressure if B = 2 m, Bp = 0.3 m, permissible settlement in field = 12 mm. Table 8.10 : Values of Load Settlement from Plate Load Test P (kn) (mm) Load (kn) Settlement (mm) P f = 50 kn; q f = kpa -9 Fig. 8.4 : Load Settlement curve for Plate Load Test data s s p f Bp ( B f + 0.3) = B f ( Bp + 0.3) 0.3( ) = 2( ) + s = 1mm p 2 2 T SHARMILA Page 140

141 Based on settlement Permissible plate settlement ~ 1 mm ABP = 32 kn/(0.3x0.3) = kpa Problem 5 The following results were obtained from a plate load test conducted on dry sandy stratum using square plate of 0.3 m width. Determine the settlement of square footing 1.5 m wide when the intensity of loading is 120 kpa. Table 8.11 : Values of Load Settlement from Plate Load Test Pressure (kpa) Settlement (mm) Data Sandy stratum B F = 1.5 m B P = 0.3 m S P = 3.2 mm Plate Load Test Result Settlement (mm) Soil Pressure (kpa) Fig. 8.5 : Load Settlement curve for Plate Load Test data T SHARMILA Page 141

142 8.5 Consolidation Settlement 1. It occurs due to the process of consolidation. 2. Clay and Organic soil are most prone to consolidation settlement. 3. Consolidation is the process of reduction in volume due to expulsion of water under an increased load. 4. It is a time related process occurring in saturated soil by draining water from void. 5. It is often confused with Compaction. 6. Consolidation theory is required to predict both rate and magnitude of settlement. 7. Since water flows out in any direction, the process is three dimensional. 8. But, soil is confined laterally. Hence, vertical one dimensional consolidation theory is acceptable. 9. Spring analogy explains consolidation settlement. 10. Permeability of soil influences consolidation. S S P F BP ( BF + 0.3) = BF ( BP + 0.3) ( ) = S F 1.5( ) + S = 8.89 mm F Table 8.12 : Compaction Vs Consolidation COMPACTION CONSOLIDATION 1. Man made 1. Natural 2. Volume reduction due to 2. Volume reduction due to expulsion of air expulsion of water 3. Sudden (Short duration) 3. Gradual 4. Dry density increases water 4. Dry density increases water content does not change content decreases 5. Applicable for unsaturated soils 5. Applicable for saturated soils 2 2 T SHARMILA Page 142

143 Time factor is obtained from the formulae shown below. It depends on the degree of consolidation. U TV = π 4100 Commonly time factor at 50 % and 90 % degrees of consolidation are used and are as mentioned below. (T V ) 50 = (T V ) 90 = T V = log (100 U %) 10 Problem 6 The total time taken for 50 % consolidation of clay layer is 4 years. What will be the time taken for 90 % consolidation? (Aug 2001) (T V ) 50 = (T V ) 90 = C V 0.848d t t ( TV ) = t d 2 ( TV ) = t 0.197d = 4 = years d 2 Problem 7 A layer of clay 8 m thick underlies a proposed new building. The existing overburden pressure at the center of layer is 290 kpa and the load due to new building increases the pressure by 100 kpa. C c = 0.45, = 50 %, G = Estimate the consolidation settlement. (Aug 2002) Data C c = 0.45 e o = H = 8 m o = 290 kpa = 100 kpa T SHARMILA Page 143

144 S c e o Cc = 1+ e o = ωg =1.355 σ o + σ H log 10 σ o 45 o (Dispersion angle) H H/2 B H/2 Fig. 8.9 : Concept of Load dispersion S c Cc = 1+ e o H log 10 σ o + σ σ o = m Problem 8 A normally consolidated clay layer is 18 m thick. Natural water content is 45 %, saturated unit weight is 18 kn/m 3, grain specific gravity is 2.7 and liquid limit is 63 %. The vertical stress increment at the center of clay layer due to foundation load is 9 kpa. Ground water table is at the surface. Determine the settlement. (Aug 2003) Data C c = e o = H = 18 m o = 162 kpa = 9 kpa T SHARMILA Page 144

145 CC e o σ = γ o 0.009( ω 10%) = = L = ωg =1.215 sat σ = 9 kpa Z = 18* 18 2 = 162kPa S c Cc = 1+ e o H log 10 σ o + σ σ o = m Problem 9 A square footing 1.2 m X 1.2 m rests on a saturated clay layer 4 m deep. L = 30 %, sat = 17.8 kn/m 3, = 28 % and G = Determine the settlement if the footing carries a load of 300 kn. σ = γ o sat Z = 17.8* 300 σ = ( ) C e o C H = 4 m L = = 35.6kPa kPa = 0.009( ω 10%) = 0.18 = ωg = 0.28* 2.68= S c Cc = 1+ e o H log 10 σ o + σ σ o = m Problem 10 A test on undisturbed sample of clay showed 90 % consolidation in 10 minutes. The thickness of sample was 25 mm with drainage at both top and bottom. Find the time required for 90 % consolidation of footing resting on 5 T SHARMILA Page 145

146 m thick compressible layer sandwiched between two sand layers. (Aug 2007) Data D lab = 25/2 = 12.5 mm D field = 5000/2 = 2500 mm t lab = 10/(60*24*365) years t t lab field d = d lab field 2 t t lab field t field dlab = d field = years 2 T SHARMILA Page 146

147 11.Determine the creep settlement in a sensitive clay of thickness 6 m given C = 0.01 when the laboratory sample 20 mm thick with double drainage experienced complete consolidation in 10 minutes. The life span of structure is 100 years. Data t sec = 100 yrs H = 6 m C = 0.01 t t t lab field field t d = d = 36X10 prim = t lab field 6000 = / 2 field mnts = 6.85yrs tsec t prim SS = C α H log10 = m t prim Fig : Details of oedometer test Problem 12 A 2 m X 2 m footing carrying a load of 1600 kn rests on a normally consolidated saturated clay layer 10 m thick below which hard rock exists. The life span of the structure is 150 years. Time taken for the completion of primary consolidation of 20 mm thick laboratory specimen with double drainage facility is 20 minutes. Find the total settlement, if the soil properties are as follows. Soil modulus 20 MPa, Poisson s ratio 0.45, Influence factor 0.9, Liquid Limit 50 %, Natural water content 25 %, Specific Gravity of T SHARMILA Page 147

148 grains 2.7, saturated density 20 kn/m 3 compression and coefficient of secondary Total Settlement, S = S I + S C + S S Data for Immediate Settlement E = kpa = 0.45 q = 1600/2 2 = 400 kpa B = 2 m I = 0.9 S I 2 1 µ qbi ρ E = S I = m = mm Data for Consolidation Settlement L = 50 % = 25 % G = 2.7 C c = 0.36 e o = H = 10 m o = 100 kpa = kpa C e o C = 0.009( ω 10%) = 0.36 L ωg 0.25* 2.7 = = = S 1 σ = γ H o sat 100kPa 2 = σ = S c P H 2 + B 2 Cc = 1+ e o 2 = 11.11kPa σ o + σ H log 10 σ o T SHARMILA Page 148

149 Data for Secondary Settlement t sec = 150 yrs H = d field = 10 m C = d lab = 10 mm t lab = 20 mnts t t lab field t prim d = d = t lab field field 2 = 38.05yrs S S = C α tsec t H log 10 t prim prim = m = 4. 7 mm Total Settlement, S = S I + S C + S S = = mm 8.7 Factors Influencing Settlement Many factors influence the settlement of foundation soil when a structure is built on it. The following are a few important factors to be considered in the evaluation of settlement. 1. Elastic properties of soil 2. Shape of footing 3. Rigidity of footing 4. Contact pressure 5. Width of footing 6. Compressibility characteristics of soil 7. Initial conditions of soil (Density, void ratio etc.) 8. Degree of saturation 9. Over Consolidation Ratio 10. Time available for settlement 11. Thickness of soil layer T SHARMILA Page 149

150 qq = QQ AA γγdd ff [5.16] Where QQ = dead weight of the structure and the live load AA = area of the raft In all cases, q should be less than or equal to qq all (net ). Example 1 Determine the net ultimate bearing capacity of a mat foundation measuring 45 ft 30 ft on saturated clay with cc uu = 1950 lb/ft 2, φφ = 0, and DD ff = 6.5 ft. Solution From equation (10) qq net (u) = 5.14cc uu BB DD ff LL BB = (5.14)(1950) = 12,307 lb/ft 2 Example What will the net allowable bearing capacity of a mat foundation with dimensions be of 45 ft 30 ft constructed over a sand deposit? Here, DD ff = 6 ft, allowable settlement = 1 in., and corrected average penetration number NN cccccc = 10. Solution From equation (13) qq all (net ) = 0.25NN cccccc DD ff SS BB ee 0.33NN cccccc SS ee qq all (net ) = 0.25(10) (6) (1) 2.67kip/ft 2 30 DIFFERENTIAL SETTLEMENT OF MATS T SHARMILA Page 150

151 1 Introduction A foundation is a integral part of the structure which transfer the load of the superstructure to the soil. A foundation is that member which provides support for the structure and it's loads. It includes the soil and rock of earth's crust and any special part of structure that serves to transmit the load into the rock or soil. The different types of the foundations are given in fig. 4.1 Different types of footings Fig. 4.1 Different types of footings If the soil conditions immediately below the structure are sufficiently strong and capable of supporting the required load, then shallow spread footings can be used to transmit the load. On the other hand, if the soil conditions are weak, then piles or piers are used to carry the loads into deeper, more suitable soil. Design Considerations: Must not settle excessively. Must be placed at depth sufficient to prevent damage from surface environmental effects (frost, swelling and shrinkage, erosion and scour). Must not cause failure of supporting soil (Bearing Capacity criteria). T SHARMILA Page 151

152 Advantages of using shallow foundation Cost (affordable) Construction Procedure (simple) Materials (mostly concrete) Labor (does not need expertise) Disadvantages of using shallow foundation Settlement Irregular ground surface (slope, retaining wall) Foundation subjected to pullout, torsion, moment. Shallow foundations are foundations where the depth of the footing ( ) is generally less than the width (B) of the footing. Deep foundations are foundations where the depth of the footing ( width (B) of the footing. ) is greater than the Footings : 1. Spread Footing: It is circular, square or rectangular slab of uniform thickness. Sometimes, it is stepped or haunched to spread the load over a larger area. When spread footing is provided to support an individual column, it is called Isolated footing as shown in fig.4.2. Fig. 4.2 Isolated (spread) footing T SHARMILA Page 152

153 2. Strap Footing: It consists of two isolated footings connected with a structural strap or a lever, as shown in fig The strap connects the footing such that they behave as one unit. The strap simply acts as a connecting beam. A strap footing is more economical than a combined footing when the allowable soil pressure is relatively high and distance between the columns is large. Fig. 4.3 Strap footing 3. Combined Footing: It supports two columns as shown in fig It is used when the two columns are so close to each other that their individual footings would overlap. A combined footing is also provided when the property line is so close to one column that a spread footing would be eccentrically loaded when kept entirely within the property line. By combining it with that of an interior column, the load is evenly distributed. A combine footing may be rectangular or trapezoidal in plan. Trapezoidal footing is provided when the load on one of the columns is larger than the other column. Fig. 4.4 Combined footing T SHARMILA Page 153

154 4. Strip/continuous footings A strip footing is another type of spread footing which is provided for a load bearing wall. A strip footing can also be provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other. In such a cases, it is more economical to provide a strip footing than to provide a number of spread footings in one line. A strip footing is also known as continuous footing. Refer fig. 4.5 Fig. 4.5 Strip footing 4. Mat or Raft footings: It is a large slab supporting a number of columns and walls under entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is large variation in the loads on individual columns. In this there are two types: Fig. 4.6 Typical Raft Foundation T SHARMILA Page 154