Multiple choice

A 10 m thick clay layer is underlain by a sand layer of 20 m depth (see figure below). The water table is 5 m below the surface of the clay layer. The soil above the water table is capillary saturated. The value of gsat is 19 kN/m3. The unit weight of water is gw. If now the water table rises to the surface, the effective stress at a point P on the interface will

  1. increase by 5$\gamma w$

  2. remain unchanged

  3. decrease by 5 gw

  4. decrease by 10 gw
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

 $\text{Effective stress = total stress - pore water pressure}\\ \text{We know that rise of water due to capillary action acts as s surcharge for the calculation of effective pressure. So, when the water table rises upto ground level surcharged will be zero and reduction of the effective stress will be equal to $5 \gamma_w\$ so reduced by \$5 \gamma _w.$}$

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