Question

For the following soil profile, when the water table elevation is lowered to the level shown:

(a). What is the change in the effective stress at Point A?

(b). What will be the consequence of the effective stress change?

Assume that the total unit weight γ of the sand layer above the water table will decrease 5% from its saturated unit weight after lowering the water table.

Answer 1

Solution

(a). change in the effective stress at Point A

For initial ground water table:

For lowered ground water table:

(b). Consequence of the effective stress change.

When the water table lowered, the effective stress changes. In particular, the effective stress increases because the pore water pressure decreases as the water table level drops. This increasing of effective stress could lead to ground settlement since the volume of soil decreases as the ground water level drops.