Question 1Multiple choiceGeotechnical Engineering
A level site is underlain by a deep sand deposit. The groundwater table is 3 m below the ground surface. Above the water table the sand has a moist unit weight of 18 kN/m³; below the water table its saturated unit weight is 20 kN/m³. Take the unit weight of water as 9.81 kN/m³ and neglect capillary rise. What is the effective vertical stress at a depth of 7 m below the ground surface?
- A.
40.8 kPa
- B.
65.3 kPa
- C.
94.8 kPa
- D.
134 kPa
Worked solution
Effective stress is the total vertical stress minus the pore water pressure: . The soil skeleton carries only the effective stress.
Total vertical stress at 7 m is the weight of everything above, layer by layer, using the moist unit weight above the water table and the saturated unit weight below it:
Pore water pressure is hydrostatic below the water table, so it depends only on the depth below the water table (7 − 3 = 4 m), not on the depth below the ground surface:
Effective stress:
The same result comes from using the buoyant unit weight for the submerged sand: .
Effective vertical stress ≈ 94.8 kPa (option C)
The most tempting wrong answer is D (134 kPa), which is the total stress; it ignores the pore water pressure that supports part of the overburden. B (65.3 kPa) subtracts a full 7 m of water pressure as if the water table were at the ground surface. A (40.8 kPa) counts only the submerged layer and forgets the 3 m of moist sand above the water table.