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2 In many situations, soil itself is used as a construction material Highway embankments Railway embankments Earth dams Highway / Airfield pavements Backfilled trenches Landfills When soil is used as foundation material, it is desirable that the in-place material possess certain properties The purpose of compaction is to improve the soil such that it has physical properties appropriate for a particular project THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

3 A simple ground improvement technique, where the soil is densified through external compactive effort (mechanical energy). Compactive effort Soil grains get rearranged more closely Volume of air voids get reduced unit weight of soil is increased + water =

4 Compaction of soils is achieved by reducing the volume of voids. It is assumed that the compaction process does not decrease the volume of the solids or soil grains. Load g soil (2) > g soil (1) Air Air Soil Matrix Water Solids Compressed soil Water Solids g soil (1) = W T1 V T1 g soil (2) = W T1 V T2

5 Maximum shear strength occurs approximately at minimum voids ratio (achieved by compaction and consolidation). Larger voids if left may get filled with water, which reduces shear strength. Compaction increases the soil strength. Large air voids may lead to compaction under working loads. To prevent that during life of a structure, it s better to reduce those air voids before construction of structure. Compaction reduces settlements under working loads. Compaction reduces compressibility of the soil. Compaction makes water flow through soil more difficult Compaction reduces permeability of the soil. Compaction can prevent liquefaction during earthquakes

6 Consolidation is another phenomena through which densification of soil takes place. Difference between compaction and consolidation Compaction Consolidation 1. It is almost an instantaneous phenomenon It is a time-dependent phenomenon 2. Densification is due to reduction in volume of air voids at a given water content Volume reduction is due to expulsion of pore water from voids THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

7 Degree of compaction of a soil is measured in terms of dry unit weight (γ d ), i.e. the amount of soil solids that can be packed in a unit volume of soil Origin The fundamentals of compaction of fine-grained soils are relatively new. R.R. Proctor in the early 1930 s was building dams for the old Bureau of Waterworks and Supply in Los Angeles, and he developed the principles of compaction in a series of articles in Engineering News-Record. In his honor, the standard laboratory compaction test which he developed is commonly called the Standard Proctor Test. Objective of laboratory compaction The objective of a laboratory compaction test is to determine the proper amount of mixing water to use when compacting the soil in the field and the resulting degree of denseness which can be expected from compaction at this optimum water. 7

8 In the test, a specified amount of compactive effort is applied to a constant volume of soil mass. In laboratory compactive effort is transferred to soil by dropping a hammer (impact compaction) In field, compactive effort is transferred to soil by means of passing roller and/or dropping weight on a given volume of soil. For lab, the mass of the hammer, height of drop, number of drops, number of layers of soil, and the volume of the mould are specified and are as mentioned in further slides. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

9 Standard Proctor test equipment Das, 1998

10 Soil Compaction in the Lab: 1- Light Compaction Test 2- Heavy Compaction Test Procedure stepwise Light Compaction Test Heavy Compaction Test Equipments

11 Objective is to obtain the compaction curve and define the optimum water content and maximum dry unit weight for a specific compactive effort. (Standard Proctor) Light compaction IS 2720:PartVII 3 layers 25 blows per layer 2.6 kg hammer 310 mm drop hammer (Modified Proctor) Heavy compaction IS 2720:PartVIII 5 layers 25 blows per layer 4.9 kg hammer 450 mm drop 1000 ml compaction mould Required for testing soils susceptible to crushing 11

12 Equipment Handle collar (mould extension) Sleeve guide Cylindrical soil mould Hammer for compacting soil Base plate

13 Equipment Handle collar (mould extension) Sleeve guide Cylindrical soil mould Hammer for compacting soil Required for testing soils susceptible to crushing Base plate Mould volume (cc) Hammer mass (kg) Light Heavy Hammer drop (mm)

14 A soil at a selected water content is placed in layers into a mould of given dimensions, with each layer compacted by 25 blows of a (2.5 or 4.9 kg) hammer dropped from a height of 12 in or 18 in(310 mm or 450 mm). The resulting dry unit weight is determined. The procedure is repeated for a sufficient number of water contents to establish a relationship between the dry unit weight and the water content of the soil. This data, when, plotted, represents a curvilinear relationship known as the compaction curve or moistureunit weight curve. The values of water content and standard maximum dry unit weight are determined from the compaction curve as shown later THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

15 The object of compaction is to reduce the void ratio, or to increase the dry unit weight. g dry G s g w 1 e

16 The object of compaction is to reduce the void ratio, or to increase the dry unit weight. g dry G s g w 1 e In a compaction test bulk unit weight and moisture content are measured. The dry unit weight may be determined as follows W Wt of Solids Wt of Water Ws Ww g bulk V TotalVolume V

17 The object of compaction is to reduce the void ratio, or to increase the dry unit weight. g dry G s g w 1 e In a compaction test bulk unit weight and moisture content are measured. The dry unit weight may be determined as follows W Wt of Solids Wt of Water Ws Ww g bulk V TotalVolume V g bulk 1 W W V w s W s

18 The object of compaction is to reduce the void ratio, or to increase the dry unit weight. g dry Gs 1e g w Gsg w wg 1 S s In a compaction test bulk unit weight and moisture content are measured. The dry unit weight may be determined as follows g bulk W V Wtof Solids Wtof Water TotalVolume W s W V w W 1 w s W s g bulk (1 w)g dry V W dry 1 bulk w

19 From the graph we determine the optimum moisture content, w opt or m opt that gives the maximum dry unit weight, (g dry ) max. Copyright

20 Dry unit weight ( d ) Soil grains densely packed - good strength and stiffness - low permeability d, max OMC (optimum water content) 20 Water content

21 To understand the shape of the curve it is helpful to develop relations between g dry and the percentage of air voids, n a. n a Va (%) 100 V

22 To understand the shape of the curve it is helpful to develop relations between g dry and the percentage of air voids, n a. n a Va (%) 100 V na V w V V s

23 To understand the shape of the curve it is helpful to develop relations between g dry and the percentage of air voids, n a. g dry n a Va (%) 100 V na VwV s V na ( W W )(1 ) W W s w bulk s w g 100 1w V(1 w) ( V V )(1 w) s w

24 To understand the shape of the curve it is helpful to develop relations between g dry and the percentage of air voids, n a. g n a Va (%) 100 V na V dry w V V s na ( W W )(1 ) W W s w bulk s w g 100 1w V(1 w) ( V V )(1 w) s w V W s s, Gsg w V w W g w w ww g w s

25 To understand the shape of the curve it is helpful to develop relations between g dry and the percentage of air voids, n a. (%) 100 V V n a a V V V n s w a ) )(1 ( ) 100 )(1 ( ) (1 1 w V V n W W w V W W w w s a w s w s bulk dry g g w s w w w w s s s ww W V G W V g g g, 1 ) 100 (1 G w G n s w s a dry g g

26 If the soil is saturated (n a = 0 or S =100%) and g dry Gs 1e g w Gsg w wg 1 S s

27 Dry unit weight If the soil is saturated (n a = 0 or S = 100%) and g dry Gs 1e g w Gsg w wg 1 S s Impossible Zero-air-voids line (ZAV line) S = 100% S = 50% S = 75% S = 90% M oisture content

28 Proctor established that compaction is a function of four variables: (1)Dry unit weight ( d ) or dry unit weight g d. (2)Water content w (3)Compactive effort (energy E) (4)Soil type (gradation, presence of clay minerals, etc.) For standard Proctor test E = E Weight of hammer 2.5kg(9.81m / s kJ / m 3 Height of drop of hammer 2 Volume of mould )(0.30m)( Number of blows per layer Number of layers layers)(25blows / layer) 3 m For modified Proctor test E = kj/m 3

29 Water content of soil The type of soil being compacted The amount of compactive energy used Method of compaction Except type of soil, other factors can be controlled in field

30 Water acts as a lubricant Too much takes up space does not allow bonding Too little same compactive effort, lower compaction OMC - The moisture content of the soil at which maximum unit weight can be achieved for a given amount of compactive effort OMC of fine grain soils is higher than coarse grain soils

31 Lambe s theory using diffused-double layer Not applicable to coarse grained soils Attractive force van der Walls force between two soil particles Repulsive force due to the diffuse double layer of adsorbed water tending to come close At low water content, Net force is attractive, flocculated structure results Compactive effort isn t enough to move particles about Low dry unit weight is result As water content increases Diffuse double layer expands allowing particles to slide over each other (Dispersion) Closer packing results At OMC, double layer expansion is complete maximum dry unit weight Beyond OMC No expansion of double layer with addition of water Water replaces soil grains Dry unit weight decreases THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

32 Dry unit weight inc re a s ing c o m p a c tive e ne rg y zero-air-voids line Moisture content Increasing compactive effort results in: Lower optimum water content Higher maximum dry unit weight 34

33 Dry unit weight (γ d ) Compaction curves for different efforts Line of optimum Water content 35

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35 Typical Values g dry ) max (kn/m 3 ) w opt (%) Well graded sand SW 22 7 Clayey sand SC Low plasticity clay CL Non plastic silt ML High plasticity clay CH G s is constant, therefore increasing maximum dry unit weight is associated with decreasing optimum moisture contents Do not use typical values for design as soil is highly variable

36 Higher dry densities are associated with lower OMCs. Well graded coarse grained soils compact to high dry unit weights, especially if they contain some fines. Good packing of soil solids. Poorly graded or uniform sands lead to the lowest dry unit weights. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

37 Sand has relatively high unit weight when dry, The unit weight is maximum, at saturation point which corresponds to OMC. The compaction curve is S shaped. Bulking of sand (ref. Figure) due to capillary tension in pore water. Maximum unit weight results when soil is either dry or completely saturated. Sands are usually compacted either in a dry state or in a saturated state by flooding with water (very small difference). THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

38 Compaction of gravel is very difficult particularly when the percentage of gravel is more than that of fines. Though gravel have high dry unit weight in general. Clay is a difficult material to compact because of its fine grained particles which pose high inter-particle friction. Maximum dry unit weight tend to decrease with increase in plasticity. Silt behaves slightly better than clay and slightly easier to compact than clays. It gets compacted to higher unit weight than clay at lower OMC. The compaction curve is normally parabolic shape THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

39 Mode of compaction influence the shape and position of compaction curve. Ideally, laboratory test must reproduce a given field compaction procedure. Field compaction kneading or rolling type compaction. Laboratory tests dynamic-impact type compaction. Divergence in the OMC and maximum dry unit weight, but not enough to warrant different lab test simulating different field procedures. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

40 Figure. Comparison of field and laboratory compaction (after Lambe and Whitman, 1969) 4. Laboratory static compaction 5. Field compaction by rubber tired load, 6 passes 6. Field compaction by Sheeps-foot roller, 6 passes THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

41 Effect on Soil Structure Dry of optimum Wet of optimum Soil tends to have a flocculated structure on the dry side of optimum, and dispersed (oriented) structure on wet side of optimum Structure of soil at A and E more flocculated than at C and D, respectively (same dry density) Structure of soil at E more oriented than at A (for same w) THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

42 Effect on Permeability Marked decrease in permeability that accompanies an increase in moulding water content on the dry side of the optimum water content. Due to improved orientation of particles which results in size of voids. A minimum permeability occurs at water contents slightly above OMC (Lambe, 1958). After this a slight increase in permeability occurs. (This is due to the effect of a decrease in the γ d which is more pronounced than the effect of improved orientation). Increasing the compactive effort decreases the permeability of the soil. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

43 Effect on Compressibility At low stresses the sample compacted on the wet side is more compressible than the one compacted on the dry side (Interparticle bonds in a flocculated structure (dry side of OMC) do not allow the particles to be displaced). However, at high applied stresses the sample compacted on the dry side is more compressible than the sample compacted on the wet side. (Once the bonds are broken, flocculated structure, with it s larger volume, can undergo large volume change). THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

44 Effect on Swelling Soil on dry of OMC has higher water deficiency and a more random particle arrangement. Hence, soil at point A swells more than at point C Compaction can reduce swelling tendency of soil Effect on Shrinkage Particle arrangement at points C and D is more ordered. Soils compacted on the wet of optimum shrink more upon drying than those compacted Effect on Construction of pore water pressure dry of optimum. Compactive effort on loose fills will induce pore water pressures Soils compacted on the wet of optimum has higher pore water pressure compared to that compacted dry of optimum. 46

45 Effect on Stress-Strain Relationship Soils compacted dry of optimum have strong inter-particle bonds, so resist deformation from deviator stress. The stress-strain curve rises steeply to a peak and then falls off as interparticle bonds are broken. Soil compacted wet of optimum have a flatter stress-strain curve without any peak (inter particle bond already broken) THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

46 Minimum permeability occurs at or slightly above OMC. Low permeability wet of OMC. Project Compaction water content Reason Core of an earth dam Wet of optimum To reduce permeability and prevent cracking in core Homogeneous earth dam Dry of optimum To have a stronger soil and to prevent build-up of high pore water pressure Subgrade of pavement Wet of optimum To limit volume changes Less pore water pressure build-up for soil compacted dry of optimum Less swelling THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

47 Selection of compaction and procedure of compaction depends on the characteristics of the soil to be compacted. Field compaction depends on (1) thickness of lift (layer) (2) type of roller (3) number of passes of the roller (4) intensity of pressure on the soil Copyright

48 Smooth Wheeled Roller Compacts effectively to mm; therefore, place the soil in shallow layers (lifts) 50

49 Vibrating Plates for compacting very small areas effective for granular soils 51

50 Sheepsfoot Roller Provides kneading action Very effective on clays 52

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53 Impact Roller Provides deeper (2-3m) compaction. e.g., air field 55

54 - pounding the ground by a heavy weight Suitable for granular soils, land fills. Pounder (Tamper) Crater created by the impact (to be backfilled)

55 Pounder (Tamper) Mass = 5-30 tonne Drop = m Copyright

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57 Suitable for granular soils Practiced in several forms: vibro compaction stone columns vibro-replacement Vibroflot (vibrating unit) Length = 2 3 m Diameter = m Mass = 2 tonnes (lowered into the ground and vibrated)

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61 For densifying granular soils Fireworks? Aftermath of blasting 64

62 Dry unit weight Relative Compaction = γ d(field) /γ d(max) Accept Reject Moisture content (a) > 95% of (modified) maximum dry unit weight

63 Dry unit weight Dry unit weight During construction of soil structures (dams, roads) there is usually a requirement to achieve a specified dry unit weight. Accept Reject Accept Reject Moisture content (a) > 95% of (modified) maximum dry unit weight Moisture content (b) >95% of (modified) maximum dry unit weight and w within 2% of w opt

64 (1) End product specification (2) Method specification a systematic exercise where you check at regular intervals whether the compaction was done to specifications. e.g., 1 test per 1000 m 3 of compacted soil Minimum dry density Range of water content Field measurements (of d ) obtained using Sand replacement Proctor needle 68

65 Proctor needle 69

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67 d Compaction specifications w d,field =? w field =? Compare! Proctor needle Sand replacement compacted ground 71

68 (a) Specify the compaction criteria for the field. (b) Recommend field compaction equipment that would achieve the desired compaction. (c) Specify an appropriate quality control test. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

69 Strategy Because the soil is a gravelly sand, it is best to specify compaction dry of optimum. * Important note: If soil is highly plastic, then it is best to specify compaction wet of optimum as volume changes are reduced (reduced swelling) on wet side of OMC. Solution Step 1: Determine maximum dry unit weight and optimum water content. The maximum dry unit weight and optimum water content are 19.6 kn/m 3 and 5.8%, respectively. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

70 Step 2: Specify dry unit weight and water content. Specify 95% standard Proctor test to be compacted dry of optimum. γ d = 18.6 kn/m 3 ; w = 4.4% THAPAR UNIVERSITY, PATIALA Tuesday, September 6,

71 Step 3: Determine field method of compaction. The soil contains a larger proportion of sand than gravel. From Table 5.1, a vibrating roller is excellent for sand and good for gravel. Specify a vibrating roller. Step 4: Specify quality control equipment. Either the sand cone or the proctor needle is suitable. THAPAR UNIVERSITY, PATIALA Tuesday, September 6,