Question

Assertion (A) : A clay layer of thickness. H, with single drainage suffers an ultimate settlement of S under increase in pressure of $\Delta p$. When a thin sand layer is present at its mid depth, the layer, under otherwise identical conditions, will have no change in ultimate settlements.

Reason (R) : The path of travel of water for the clay layer is H under single drainage. When a thin sand layer is present at mid depth of clay layer, the path of travel in lower half of the layer gets reduced to H/2 and that in upper half of the layer gets reduced to H/4. This considerably reduces the time required to realize ultimate settlement.

• A. both A and R are true and R is the correct explanation of A
• B. both A and R are true but R is not a correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Answer 1

The correct option is B both A and R are true but R is not a correct explanation of A

The ultimate settlement is given by

$S=m_v\Delta PH$

It does not depend upon type of drainage (single or double drainage) conditions. Therefore A is correct. The presence of thin sand layer reduces drainage path and settlement becomes fast.