Question

A layer of clay of thickness 12.5 m is underlain by sand. The ${\gamma }_{sat}$ of the clay is 18.5 $kN/m^3$. When the depth of an open trench excavated in the clay reached 8 m, the bottom cracked and the water started entering the trench from below. The height to which water would have risen from the top of sand in a bore hole if it was drilled into sand prior to the excavation is ($Take{\gamma }_w=10kN/m^3$).

• A. 8.3 m
• B. 10.2 m
• C. 3.83 m
• D. 6.3 m

Answer 1

The correct option is A 8.3 m


For quick sand condition,

Buoyant weight = seepage force

4.5${\gamma }_{sub}=(x-4.5){\gamma }_w$

4.5(18.5 - 10) = (x - 4.5) $\times$ 10

x = 8.325 m.

Hence the correct option is (a).

Note :

Quick sand condition: In case of upward seepage flow, if the upward seepage force become equal to the buoyant weight of soil, the effective stress in the soil becomes zero.