Question

A 10m thick clay layer is underlain by a sand layer of 20m depth (see figure). The water table is 5 m below the surface of clay layer. The soil above the water table is capillary saturated. The value of ${\gamma }_{sat}$ is 19 kN/$m^3$. The unit weight of water is ${\gamma }_w$. If now the water table raises to the surface. The effective stress at a point P on the interface will be

• A. Increase by 5${\gamma }_w$
• B. Remain unchanged
• C. Decreases by 5${\gamma }_w$
• D. Decreases by 10${\gamma }_w$

Answer 1

The correct option is C Decreases by 5${\gamma }_w$

In both the cases, the soil is saturated so total stress is constant when water table rise to surface (by rising 5 m, pore water pressure will be increased by 5${\gamma }_w$, hence effective stress will decrease by 5${\gamma }_w$).