CEX6230 GEOTECHNICS Dear Student:

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1 CEX6230 GEOTECHNICS Dear Student: Geotechnics (CEX6230) discusses design and construction aspects of Geotechnical Engineering. Even though this course requires a pass in CEX4230, I find that many learners still lack the necessary pre-knowledge on some basic concepts. Many find it difficult to relate knowledge to practice. As a result the number eligible to sit the final examination and those who pass the final examination is comparatively low; hence the only way to remedy this situation is by learning the concepts you ve skipped at lower levels and to pay attention to what you learn. Day Schools are meant to discuss subject related issues, hence you would benefit only if you come prepared to do so. At the first day school we will discuss how we should set about learning. We need to understand how learning outcomes are linked to learning and assessment. In this regard, we will see how Unit aims and objectives, and session objectives can be effectively used during self-learning. Learning is expected to be continuous and last minute cramming does not quite help you both at learning and at assessment. Therefore, it is important that you get your doubts clarified without any delay, in order to proceed at a uniform pace. Past performance statistics: Description Number enrolled during registration 76 (100%) 75 (100%) 100% 100% Number eligible to sit the final exam 34 (44.7%) 21 (28%) 20.4% 9% The statistics show an increase in learner performance, which I believe is due to strengthening of Continuous Assessment. Many learners fail to gain eligibility since they still are unable to relate knowledge, and rather continue to memorise structured information. Laboratory and Design activities are the two most important activities. These activities are assessed based on reports and the viva-voce examination. Viva-voce examination also tests your ability to relate concepts that you have learnt in basic Strength of Materials, basic Hydraulics and Soil Mechanics. You should also be able to explain your observations and conclusions. Plagiarism refers to presenting others work without acknowledging their contribution. Components of continuous assessment need your input and not the input of others. Learners can avoid being penalized by not making their assignments, design and laboratory reports available to other learners. We consider that meeting deadlines and scheduled appointments are entirely the responsibility of the learner. Learners be advised that due dates are strictly adhere to, without exceptions. Assignments will be accepted only at the Department of Civil Engineering, Nawala. Please ensure that you submit them before the stipulated deadlines. Assignments are collected in the assignment box, located in Block I. If you wish to mail in your assignment, it should be sent through registered post to reach us before the date of submission. You may be required to account for possible postal delays. The laboratory activity groups (i.e. Groups 1 and 2) are scheduled earlier than before. You are required to forward to me via your own your name, registration number and preferred laboratory group before 30 th June Those who do not have s could go to the nearest regional/study centre computer facility and setup you own account. The design assignment will be ed to you on a later date. You may contact me if you wish to schedule an appointment [ patnaweera@yahoo.com; Mobile: ]. With best wishes, Dr. H.G.P.A. Ratnaweera Academic Coordinator, CEX6230 1

2 TUTOR MARKED ASSIGNMENT 1 1. A Consolidated-Undrained triaxial test series was performed on undisturbed samples of an overconsolidated clay. The results are as follows: Cell pressure (kpa) Deviatoric Stress at failure (kpa) Pore water pressure at failure (kpa) a) Assuming that no back pressure is applied complete the following table. (6 points) Cell pressure End of Consolidation (kpa) At failure (kpa) (kpa) p p q p p q b) Plot the stress path parameters on a graph sheet. Connect these points appropriately to give the Total Stress Path (TSP) and the Effective Stress Path (ESP) for the above four tests. (6 points) c) The Mohr-Coulomb failure envelope on p q space takes the form q = p sin φ + c cos φ. Determine the shear strength parameters with respect to effective stress. (4 points) d) Determine the shear strength parameters with respect to total stress. (4 points) 2. a) An Unconsolidated Undrained triaxial loading test is to be performed on an undisturbed sample obtained from location A (refer figure). i) Estimate the average cell pressure you wish to apply during the test. (4 marks) ii) Explain how you would check whether the soil is fully saturated. (4 marks) b) A Consolidated Undrained test is performed on a normally consolidated soil specimen. The sample is tested at a cell pressure of 150kPa. The deviatoric stress, σ 1 -σ 2, at failure was 405kPa. The pore water pressure at failure was 70kPa. i) Plot the Total Stress Path and the Effective Stress Path for the said test. (6 marks) ii) Determine φ total and φ. (6 marks) 3. Figure below shows the Mohr s circles obtained for UC and UU tests performed on fully saturated undisturbed soil samples obtained from the same location. Both these tests give the same Undrained Cohesion, C u. 2

3 a) Explain why the Mohr s circles of Total Stress I and II gives the same Mohr s circle of Effective Stress. (4 points) b) Explain how dissipation of pore-water pressure is prevented in UC and UU tests. (4 points) c) An Unconsolidated Undrained triaxial loading test was performed on a Normally Consolidated Clay soil at the cell pressure of 200kPa. The Deviatoric Stress at failure was found to be 119kPa. The pore-water pressure at failure was found to be 110kPa. i) Determine the Undrained Shear Strength. (2 points) ii) Estimate the friction angle φ u, with respect to total stress, i.e. assuming C u = 0. (4 points) iii) Determine the friction angle φ, with respect to effective stress. (4 points) iv) If a UC test was performed on a similar sample, determine the failure stress. (2 points) 4. Final Examination 2009/2010 Q1 (20 points). 5. Using sketches describe the following: (2.5x8 = 20 points) a) Application of a constant cell pressure to a soil specimen during a conventional triaxial loading test. b) Application of a deviatoric stress to a soil specimen during a conventional triaxial loading test. c) Application of a back pressure to a soil specimen during a conventional triaxial loading test. d) Measuring `B-value of a soil specimen during a conventional triaxial loading test. e) Measuring A f of a soil specimen during a conventional triaxial loading test. f) Maintaining drained condition during a CD triaxial loading test. g) Ensuring undrained condition during a UU triaxial loading test. h) Ensuring undrained condition during a UC test. TUTOR MARKED ASSIGNMENT 2 1. Final Examination 2009/2010 Q2 (20 points). 2. Final Examination 2009/2010 Q3 (20 points). 3. Final Examination 2009/2010 Q5 (20 points). 4. Final Examination 2009/2010 Q7 (20 points). 5. A gravity retaining wall retains a fill sloping at 20 0 (refer figure). A trial slip surface has been drawn, with θ = The tension crack depth z 0 is estimated at 1.75m. Soil properties are as follows: γ bulk = 17 kn/m 3 ; c = c w = 10kPa; φ = 24 0 ; δ = 16 0 a) Sketch the active soil wedge showing all forces acting on it, including water forces in the tension cracks. Clearly show all relevant dimensions. (8 points) b) Calculate the magnitudes of all forces; show their directions and lines of action. (8 points) c) Sketch to scale the vector polygon and determine the force on the back of the wall. (4 points) 3

4 TUTOR MARKED ASSIGNMENT 3 1. Final Examination 2009/2010 Q4 (20 points). 2. Final Examination 2009/2010 Q6 (20 points). 3. Final Examination 2009/2010 Q8 (20 points). 4. a) State the limitations in using Taylors Charts for the analysis of the stability of slopes. (6 points) b) An excavation is to be made to a depth of 8m at an angle of 45 0 in a layered soil. (refer Figure) where the soil is excavated to the bottom of the top layer. The trial failure surface used in the analysis is shown. The properties of the two soil layers are given below: Layer 1: γ = 18.6 kn/m 3 ; φ = 15 0 ; c = 14.5 kpa. Layer 2: γ = 18.6 kn/m 3 ; φ = 10 0 ; c = 25 kpa. Water table is well below the bottom of the excavation. Using Bishops Simplified method of slices, compute the Factor of Safety corresponding to the given trial failure surface. You may ignore effects due to tension cracks, (14 points) 5. A smooth sheet pile wall is to retain 4.2m of sandy soil. Figure below shows the ground profile of the construction site. a) State the allowance for minimum surcharge and the allowance for unplanned excavation. (2 points) b) Calculate the design shear strength parameters to be used in your design. c) Give a sketch showing all design parameters and design dimensions. d) Figure also shows the deflected pattern of the sheet-pile wall. Identify the active and passive zones along the sheet-pile wall. e) Assuming that the same static water levels are maintained on both sides of the sheetpile wall, draw the resultant stress diagram, indicating principal stress values. (6 points) f) If dewatering is done during construction of the basement, explain how you would determine the force due to seepage pressures. 4

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7 THE OPEN UNIVERSITY OF SRI LANKA BACHELOR OF TECHNOLOGY (CIVIL) LEVEL 06 Final Examination 2008/2009 CEX6230 GEOTECHNICS Time allowed: Three Hours Date: Sunday, 04 th April, 2010 Time: Answer five questions. All questions carry equal marks. 1. Table Q1 lists test data for a Consolidated Undrained Triaxial (CU) Test, performed on a clay soil specimen. The test was performed at a cell pressure, σ 3 of 150kPa. a) Compute p, p and q for the given range of strain values (4 points) b) Plot q vs. p and q vs. p on the same graph sheet. (4 points) c) State whether the soil is normally-consolidated or over-consolidated. (1 point) d) Determine the shear strength parameters with respect to total stress. (2 points) e) Determine the shear strength parameters with respect to effective stress. (2 points) Figure Q1 shows the principal stresses of a soil element subjected to a vertical stress increase. u 0 is the static pore water pressure while u is the excess pore water pressure due to deviatoric loading. f) Plot on the same graph sheet, points (p,q) and (p,q) for `before loading. Plot the total stress path g) Plot on the same graph sheet, points (p,q) and (p,q) for `end of loading. Plot the effective stress path. You may assume that expected undrained behaviour is similar to what you ve observed in the CU loading test. h) State whether the soil element is `safe during short term and during long term. (1 point) Table Q1 ε 1 σ 1 σ 3 kpa u kpa p kpa p kpa q kpa Figure Q1 7

8 2. Figure Q2 shows a plot of void ratio versus effective consolidation stress, for a soil specimen representative of a compressible soil stratum. The in-situ (field) compression curve is OABC. a) State the in-situ effective overburden stress. (2 points) b) Explain how you would determine e 0. c) Explain how you would establish point B on the in-situ compression curve. d) Compute in-situ Compression Index and Recompression Index. e) During construction, the 9m thick clay stratum described above is subjected to a vertical stress increment of 75kPa. Compute the consolidation settlement. f) The geotechnical engineer recommends pre-loading to reduce later settlements and that the fill should be placed in 15kPa increments. She recommends that 90% of primary consolidation needs to be completed prior to placing the next layer of fill. Compute the time taken (in years) for 90% of settlement, for each layer to occur. The average c v for the load range considered is 2.8 x 10-3 cm 2 /s. T 90 = You may assume double drainage. g) The geotechnical engineer also recommends that pore-pressure monitoring wells to be placed at suitable locations. This is to verify whether observations support the theoretical analysis. Explain how you would make this comparison. Figure Q2 3. Explain/discuss the following: (You are encouraged to use sketches when necessary. Limit your description to a maximum of five sentences.) a) Figure Q3(a) shows laboratory compaction curves obtained for various soil types. Discuss your observations. Figure Q3(a) b) Sketch the phreatic surface for the earth dam with base permeable at downstream end. Figure Q3(b) 8

9 a) Compute the distance of the resultantt load from Column A. (4 points) b) Using the result obtained in 5(a), determine the equired footing length L and the locations of columns to ensure that the resultant force falls within the middle third of the footing. It is recommended that the footing should extend 0.6m beyond the centreline of each column. (4 points) c) Compute the minimum footing width B required to maintain a safety factor of 2.5 against possible bearing capacity failure. (6 points) d) State whether the selected footing dimensions satisfy design requirements. Use SPT-N = 25 (6 points) 9 c) Relative Density, Shear Strength and Angle of Internal Friction are related words. Explain how these parameters are related to each other. d) Figure Q3(d) shows the state of stress in a two-dimensional soil element. Discuss the influence of pore water on normal and shear stresses. Figure Q3(d) 4. a) Figure Q4(a) shows variations in wall pressure with wall movement. State the expressions for pressures p 0, p a and p p for a cohesionless backfill. b) Compute the respective coefficients of lateral earth pressures for a sandy soil with φ = c) Explain how wall pressures change based on l/h. (4 points) Figure Q4(a) d) A 5m high retaining wall holds a masss of dry cohesionless soil with its horizontal surface level with the top of the wall. The retaining walll has a smooth vertical back. The soil carries a uniformly distributed surcharge load of 10kN/m. It weighs 20kN/m 3 and has a φ = Determine the active thrust (i.e. force) on the back of the wall, per meter length of wall. (10 points) 5. Figure Q4 shows a rectangular foundation supporting two columns A and B, with centrelines separated by a distance of 2m. The dead and live loads acting on the two columns are as follows: Column Dead load (kn) Live load (kn) A B

10 Figure Q5 6. Explain/discuss the following: (You are encouraged to use sketches when necessary. Limit your description to a maximum of five sentences.) a) Figure Q6(a) shows a strip footing founded on a sandy soil. Discuss the effect of water table i) on Ultimate Bearing Capacity of the soil and ii) on allowable bearing capacity of the soil to satisfy settlement requirements. Figure Q6(a) b) The stress-strain behaviour of a stiff clay or a sand yields a peak strength and a residual strength, which gives us φ peak and φ residual values. Discuss how you would use these values when analysing a slope for stability. c) Discuss the use of Standard Penetration Test N in designing a shallow foundation. d) Figure Q6(d) shows an anchored sheet pile wall with free earth support sytem. Show the forces and reactions contributing to equilibrium of the wall. Sketch the deflected shape. 10

11 Figure Q6(d) 7. Figure Q7 shows two sheet-pile walls to be driven in a homogeneous river bed. The trench level is located 2.2m below the river bed. It is found that water in the trench needs to be pumped out at a rate of 5 litres per minute per meter length of trench. a) Compute the total head difference that contributes to seepage. (2 points) b) Sketch the flow net in accordance with stipulated rules. Use the sheet provided. (8 points) c) Show potential values corresponding to equipotential lines. (2 points) d) Show no-flow boundaries. (1 point) e) Determine the point at which maximum hydraulic gradient occurs. You may indicate this point on a sketch. (2 points) f) Estimate the maximum hydraulic gradient. Compute the factor of safety against piping. g) Estimate the Coefficient of Permeability of river sand. (2 points) 11

12 Figure Q7 8. A ready-made garment warehouse store of size 10m x 10m is to be constructed on a reclaimed land (Refer Figure Q8). The structural engineer has sought your opinion on the foundation type to be used. A strip footing of 1m width is required to carry a super-structure stress of 60kPa, while a raft foundation is required to carry a pressure to 25kPa. Both footing types are to be located at a depth of 0.5m from ground surface. a) For the strip footing sketch the pressure bulb corresponding to 0.1 σ v, where σ v is the applied stress at founding level. b) For the raft footing sketch the pressure bulb corresponding to 0.05 σ v, where σ v is the applied stress at founding level. c) Compute the consolidation settlement for both foundation types. You may assume that C c /(1+e 0 ) is equal to 0.1. d) Discuss your conclusions. e) Discuss the influence of water table on allowable bearing capacity of both footing types. 12

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