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EJM – 094

I Semester M.E. (Civil) Degree Examination,March 2013 (Semester Scheme) 2K8Gt 104/2K8 EQ 104:ADVANCED SOIL MECHANICS Geotechnical Engineering (Common to GT/EQ) Time : 3 Hours

Max. Marks : 100

Instructions : 1) Answer any five full questions. 2) Missing data, if any, may be assumed suitably. 1. a) What are clay forming minerals ? Briefly explain the structure of clay forming minerals. (10) b) Explain the meaning of “Texture” and “Structure” of a soil.

(5)

c) Distinguish between “Black cotton soil” and “Laterite” from engineering point of view. (5) 2. a) Write an explanatory note on “Base exchange capacity” of soil, bringing out its importance. (10) b) What is “diffused double layer” ? Explain “Gury-Chapman diffused double layer theory”.

(10)

3. a) Bringout the usefulness of compaction test in the laboratory in soil engineering practice. (6) b) Discuss the effect of compaction on soil properties.

(6)

c) The soil from a borrow pit is at a bulk unit weight of 17.1 kN/m3 and a water content of 12.6 %. It is desired to construct an embankment with a water content of 18 % at 19.6 kN/m3 unit weight. Determine the quantity of soil to be excavated from the borrow pit and the amount of water to be added for (8) every 100 m3 of compacted soil in the embankment. 4. a) Discuss briefly the different factors influencing the shear strength of soils. b) What is “stress path” ? Explain.

(5) (5)

c) Two identical samples A and B of a normally consolidated clay were tested. Sample A was subjected to an unconfined compression test and it failed when the deviator stress was 200 kN/m2. Sample B was tested under UU condition with a cell pressure of 300 kPa. Prior to shear, sample B had no pore pressure but during shear pore water pressure built up to 200 kPa. What was the pore water pressure at failure in sample A ? What is the effective angle of internal friction for this clay ? (10) P.T.O.

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5. a) Discuss why ultimate strength of a loose sand sample and a dense sand sample consolidated to the same effective stress are the same, whereas, the peak strengths are different. (5) b) Why is the sand thought to be a “friction type” material ? Can saturated sand exhibit a φ =0 ? if so, under what circumstances ? (5) c) The effective shear strength parameters of a soil used in the construction of an earthen bund are c = 6 kPa and φ = 20 ° . Its unit weight is 18.2 kN/m3. The pore pressure parameters A = 0.45 and B = 0.85. If the height of the fill has been raised from 2 to 5 m, find the shear strength of the soil at the base of the bund. Assume the lateral pressure at any point as one-third of the vertical pressure. (10) 6. a) Write brief critical notes on “settlement of foundations”.

(5)

b) How does the construction period affect the time-rate of settlement of a structure ? What is the “effective loading period” ? (5) c) A footing 15 m square is located 1.5 m below the surface of a uniform soil deposit of unit weight 20 kN/m3. The void ratio of the soil is 0.8 and its compression index is 0.07. If the total thickness of the deposit, which is underlined by rock stratum is 3.5 m, compute the consolidation settlement of the footing when it carries a load of 225 kN. Use trapezoidal stress distribution (2H : 1V) and consider four layers. (10) 7. a) Explain and compare Boussinesq and Westergaard theories for stress distribution.

(10)

b) Write step by step procedure for analysing the stability of the U/S slope of an earth dam by Swedish method of slices. Bring out the effect of sudden draw down on the stability of the slope. (10) 8. Write short notes on the following : i) Pre-consolidation pressure ii) Creep in soils iii) Newmark’s influence chart iv) Secondary consolidation. ———————

(4×5=20)