Chapter 11 Compressibility of Soil

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1 Page 11 1 Chapter 11 Compressibility of Soil 1. The compression of soil layers as a result of foundation or other loadings is caused by (a) deformation of soil particles. (b) relocation of soil particles. (c) expulsion of water and air from the void spaces. (d) all of the above. 2. The elastic settlement of the ground is caused by the deformation of soil (a) without a change in its water content. (b) with a decrease in its water content. (c) with an increase in its water content. (d) in its dry condition. 3. When a saturated soil layer is subjected to a stress increase, the porewater pressure (a) decreases. (b) increases. (c) remains constant. (d) becomes zero. 4. The drainage caused by an increase in the porewater pressure in saturated sandy soils (a) takes place slowly. (b) takes place very slowly. (c) takes place quickly. (d) depends on the amount of water present in the soil. 5. Which of the following soils can have elastic and consolidation settlements occurring simultaneously? (a) Clayey and silty soils (b) Sandy soils (c) Gravelly soils (d) Both (b) and (c) 6. Which of the following settlements of ground surface as a result of foundation or other loadings is an immediate settlement? (a) Elastic settlement (b) Consolidation settlement (c) Both (a) and (b) (d) None of the above 7. The settlement which occurs more or less in a short time after the foundation is subjected to the structural load is called (a) primary consolidation settlement. (b) elastic settlement. (c) secondary consolidation settlement. (d) both (b) and (c).

2 Page A uniformly loaded, perfectly flexible footing resting on a saturated clay will have a (a) sagging elastic settlement profile. (b) uniform elastic settlement profile. (c) triangular elastic settlement profile. (d) trapezoidal elastic settlement profile. 9. The contact pressure at the base of a uniformly loaded, perfectly rigid footing resting on a saturated clay will be (a) smaller at the centre of the footing. (b) uniform throughout the width of the footing. (c) greater at the centre of the footing. (d) unpredictable. 10. The Poisson s ratio of saturated clays is (a) zero. (b) equal to 0.5. (c) less than 0.5. (d) greater than The elastic settlement of a footing is (a) directly proportional to the net load per unit area at the base level of the footing. (b) directly proportional to the width of the footing. (c) inversely proportional to the average Young s modulus of elasticity of the foundation soil. (d) all of the above. 12. The ratio of elastic settlement of a rigid footing to that of a flexible footing at its centre is approximately (b) (c) (d) The Young s modulus of elasticity of the hard clay can be equal to (a) 10 MN/m 2 (b) 30 MN/m 2 (c) 60 MN/m 2 (d) none of the above. 14. The Poisson s ratio of soils has a range of (a) 0 to 0.5. (b) 0.2 to 0.5. (c) 0 to 1. (d) None of the above.

3 Page Select the correct statement. (a) Consolidation settlement in clay is generally equal to the elastic settlement. (b) Consolidation settlement in clay is generally smaller than the elastic settlement. (c) Consolidation settlement in clay is generally several times greater than the elastic settlement. (d) Consolidation settlement in clay is generally negligible. 16. If a load is placed on the frictionless watertight piston of a model that consists of a cylinder with a spring at its centre and filled with water, and the drainage of water is not allowed by opening the valve attached to the cylinder, the entire load will be taken by (a) the spring. (b) the water. (c) both the spring and the water in a fixed proportion depending on the their individual compressibility values. (d) None of the above. 17. Theoretically, the consolidation of clayey soil ends at time (a) t = 0 (b) t = 6 months (c) t = 1 year (d) t = 18. At the end of consolidation of clayey soil, which of the following is correct? (a) Total stress increase = effective stress increase (b) Excess porewater pressure = 0 (c) Both (a) and (b) (d) Total stress increase = excess porewater pressure 19. In the one-dimensional laboratory consolidation test, the soil specimen is kept in (a) air-dried condition. (b) oven-dried condition. (c) partially saturated condition. (d) fully saturated condition. 20. During secondary consolidation of clayey soils, (a) expulsion of porewater pressure does not take place. (b) some deformation takes place because of the plastic readjustment of soil fabric. (c) both (a) and (b). (d) none of the above. 21. For normally consolidated clays, the overconsolidation ratio (OCR) is (b) 1. (c) smaller than 1. (d) greater than 1.

4 Page On a semi-logarithmic plot, the virgin compression curve is (a) a straight line. (b) approximately a straight line. (c) a parabolic curve. (d) a circular curve. 23. The slope of the void ratio versus logarithm of effective pressure is called (a) compression index. (b) coefficient of compressibility. (c) coefficient of consolidation. (d) coefficient of volume compressibility. 24. If the liquid limit of undisturbed clay is 40%, its compression index will be approximately 21. (b) (c) 0.3. (d) none of the above. 25. Select the correct statement. (a) Swell index = compression index. (b) Swell index > compression index. (c) Swell index < compression index. (d) Swell index << compression index. 26. If the plasticity index of a clay is 20%, its compression index will be approximately 05. (b) (c) (d) At the end of primary consolidation of soil, some settlement is observed because of the plastic adjustment of soil fabrics, which is usually termed (a) creep. (b) elastic settlement. (c) consolidation settlement. (d) immediate settlement. 28. Secondary consolidation settlement (usually termed creep) is more important than primary consolidation settlement in (a) sandy soils. (b) silty soils. (c) clayey soils. (d) organic and highly compressible inorganic soils. 29. Which of the following is not an assumption of Terzaghi s one-dimensional consolidation theory? (a) The clay-water system is homogeneous. (b) Saturation is incomplete. (c) Compressibility of water is negligible. (d) Darcy s law is valid.

5 Page The time factor is defined as (a) (b) (c) (d) none of the above where symbols have their usual meaning. 31. If the average degree of consolidation is 30%, the time factor will be (b) (c) (d) The coefficient of consolidation of soil generally (a) does not vary with a change in liquid limit of soil. (b) increases as the limit liquid of soil increases. (c) decreases as the liquid limit of soil increases. (d) does not have a regular trend of variation with changes in the liquid limit of soil. 33. If a 3-m-thick layer (double drainage) of saturated clay under a surcharge loading underwent 90% primary consolidation in 75 days, the coefficient of consolidation will be (b) cm/s. (c) cm 2 /s. (d) cm 2 /s. 34. In order to accelerate the consolidation settlement of the clay layer during precompression, sand drains are constructed by drilling holes through the clay layer and filling them with (a) highly permeable sands. (b) silts. (c) clays. (d) all of the above. 35. Which of the following requires drilling? (a) Prefabricated vertical drains (b) Sand drains (c) Both (a) and (b) (d) None of the above

6 Page 11 6 Answers, Hints and Discussion 1. (d) Discussion: Consolidation settlement is caused by the expulsion of water from the void spaces of soil; compaction of the soil layers by rollers or other compacting equipment causes soil compression/deformation/elastic settlement by the expulsion of air from the void spaces of the soil. 2. (a) 3. (b) 4. (c) 5. (d) 6. (a) 7. (b) 8. (a) Discussion: (b) is correct for a uniformly loaded, rigid footing resting on the saturated clay. 9. (a) Discussion: (b) is correct for a uniformly loaded, perfectly flexible footing resting on the saturated clay. 10. (b) 11. (d) Hint: See Eq. (11.1). 12. (b) Hint: See Eq. (11.9). 13. (a) Hint: See Table (b) Hint: See Table (c) 16. (b) Discussion: (b) is correct because water is incompressible; (a) is correct when drainage of water gets stopped when the valve is kept opened; and (c) is correct when the drainage of water keeps taking place when the valve is opened. 17. (d) 18. (c) 19. (d) 20. (c) 21. (b) 22. (b) 23. (a) 24. (b) Discussion: Eq. (11.33):

7 Page (d) 26. (d) Discussion: Eq. (11.37): (a) Hint: See Sec (d) 29. (b) 30. (a) 31. (b) Discussion: Eq. (11.60): (c) 33. (c) Hint: See Example (a) 35. (b)

Compressibility of Soil. Chapter 11

Compressibility of Soil. Chapter 11 Compressibility of Soil Chapter 11 TOPICS INTRODUCTION ELASTIC SETTLEMENT Stress distribution in soil masses CONSOLIDATION SETTLEMENT Fundamentals of consolidation Calculation of 1-D Consolidation Settlement