In Situ Stresses

P a g e | 9‐1 Chapter 9 In Situ Stresses 1. The total stress carried by water in voids, called porewater pressure, at a point within a saturated soil mass acts (a) in vertical direction. (b) in horizontal direction. (c) with unequal intensity in all directions. (d) with equal intensity in all directions. 2. The porewater pressure in voids of soil mass is called (a) neutral stress. (b) effective stress. (c) vertical stress. (d) total stress. 3. The sum of the vertical components of the forces developed at the points of contact of the solid particles per unit cross-sectional area of the soil mass is called (a) vertical stress. (b) total stress. (c) effective stress. (d) neutral stress. 4. The effective stress principle for the saturated soils provides an expression involving the total stress σ, the effective stress σ′ and the porewater pressure (aka neutral stress) u as (a) (b) (c) (d) none of the above. 5. The principle of effective stress was first developed by Karl Terzaghi in (a) 1925. (b) 1936. (c) 1943. (d) 1948. 6. Which of the following cannot be determined experimentally? (a) Total stress (b) Porewater pressure (c) Effective stress (d) All of the above 7. Which of the following depends greatly on effective stress? (a) Soil compressibility (b) Soil strength (c) Both (a) and (b) (d) Index properties of soil P a g e | 9‐2 8. The fluctuation of water table when it remains above the ground level causes (a) no changes in the total stress and porewater pressure at any point below the ground level. (b) no changes in the effective stress at any point below the ground level. (c) equal increase or decrease in total stress and porewater pressure at any point below the ground level. (d) both (b) and (c) 9. If the groundwater table coincides with the ground level and the saturated unit weight of soil is 19 kN/m3, the effective stress at a depth of 3 m below the ground level will be approximately (a) 19 kN/m2 (b) 28 kN/m2 (c) 57 kN/m2 (d) none of the above. 10. In Q. 6, the porewater pressure at a depth of 3 m below the ground level will be approximately (a) 10 kN/m2 (b) 19 kN/m2 (c) 29 kN/m2 (d) 57 kN/m2 11. If the groundwater table fluctuates, but it remains below the ground level, the effective stress at any point below the groundwater table (a) increases with a rise of the groundwater table. (b) decreases with a rise of the groundwater table. (c) tends to become zero with a rise of the groundwater table. (d) remains constant with a rise or fall of the groundwater table. 12. If water is seeping through a soil layer in the vertically upward direction, the effective stress at any point within the soil (a) will be lower than its static case without seepage. (b) will be higher than its static case without seepage. (c) may decrease to zero for a specific hydraulic gradient. (d) both (a) and (c). 13. For the upward water seepage through a soil mass, boiling or quick condition occurs when the hydraulic gradient equals (a) (b) (c) (d) where all the symbols have their usual meaning. P a g e | 9‐3 14. For most soils, the critical hydraulic gradient that causes quick condition is approximately (a) 0. (b) 0.5. (c) 1.0. (d) none of the above. 15. The seepage force per unit volume of soil is equal to (a) (b) (c) (d) where all the symbols have their usual meaning. 16. The factor of safety against heave on the downstream side of a hydraulic structure can be obtained using the concept of (a) structural design. (b) consistency limits. (c) seepage force. (d) porewater pressure. 17. If D is the depth of embedment of a sheet pile into submerged soil layer, the soil heaving generally occurs with a distance of (a) D/3 from the sheet pile. (b) D/2 from the sheet pile. (c) 2D/3 from the sheet pile. (d) D from the sheet pile. 18. The factor of safety FS against heaving (or piping) can be expressed as (a) FS = icr / iexit (b) FS = iexit / icr (c) FS = 2icr / iexit (d) FS = 2iexit / icr where icr = critical hydraulic gradient, and iexit = maximum exit gradient. 19. For the safety of the hydraulic structures, the recommended minimum factor of safety against soil heave is about (a) 1 to 2. (b) 2 to 3. (c) 3 to 4. (d) 4 to 5. 20. Use of a filter in the downstream side of the sheet-pile structure (a) increases the factor of safety against heave. (b) decreases the factor of safety against heave. (c) does not affect the factor of safety against heave. (d) both (a) and (b) depending on the thickness of filter. P a g e | 9‐4 21. The total stress at any point within a partially saturated soil mass consists of (a) effective stress. (b) pore air pressure. (c) porewater pressure. (d) all of the above. 22. The effective stress in partially saturated soils is given by: σ ′ = σ − u a + χ (ua − u w ) , the value of χ for saturated soil is (a) 0. (b) 0.5. (c) 1. (d) 2. 23. If d = diameter of the capillary tube, and hc = height of capillary rise, then 1 (a) hc ∝ d 1 (b) hc ∝ 2 d (c) hc ∝ d (d) hc ∝ d 2 24. If D10 = effective size of soil particles, and hc = height of capillary rise, then (a) hc ∝ D10 (b) hc ∝ D102 (c) hc ∝ 1 D10 (d) hc ∝ 1 D102 25. Select the correct range of capillary rise in fine sands. (a) 0.1 – 0.2 m (b) 0.3 – 1.2 m (c) 0.75 – 7.5 m (d) 7.5 – 23 m P a g e | 9‐5 Answers, Hints and Discussion 1. (d) 2. (a) 3. (c) 4. (b) 5. (a) 6. (c) Discussion: Effective stress is not a physically meaningful parameter, so it cannot be determined experimentally; however, it can be calculated using the effective stress principle. 7. (c) 8. (d) Hint: See Eq. (9.5), and its explanation. 9. (b) Discussion: Eq. (9.5): stress. 10. (c) Discussion: 3 9.8 3 19 29.4 11. (b) 12. (d) 13. (a) Hint: See Eq. (9.9). 14. (c) 15. (a) Hint: See Eq. (9.13) or Eq. (9.18). 16. (c) 17. (b) 18. (a) Hint: See Eq. (9.23). 19. (d) 20. (a) 21. (d) 22. (c) Discussion: (a) is correct for dry soil. 23. (a) Hint: See relationship (9.31). 24. (c) Hint: See Eq. (9.32). 25. (b) Hint: See Table 9.2. 9.8 29 27.6 / . 28 / . (c) is correct for the total