A layer of sand extends from ground level to a depth of 9 m and overlies a layer of clay, of very low permeability, 6 m thick. The water table is 6 m below the surface of the sand. The saturated unit weight of the sand is 19 kN/m3 and that of the clay 20 kN/ m3; the unit weight of the sand above the water table is 17 kN/m3. Over a short period of time, the water table lowers by 3 mands is expected to remain permanently at this new level. Determine the effective vertical stress at depths of 8 and 12 m below ground level 

(a)immediately after the lowering of the water table 

b)Several years after the lowering of the water table.

Explain your answer for each part.