A layer of sand extends from ground level to a depth of 11m and overlies a layer of clay, of very low permeability, 8m thick. The water table is 7m below the surface of the sand. The saturated unit weight of the sand is 19kN/m3 and that of the clay 20kN/m3; the unit weight of the sand above the water table is 16kN/m3. Over a short period of time the water table rises of 3m and is expected to remain permanently at this new level. Determine the effective vertical stress at depths of 8m and 12m below ground level (a) immediately after the rise of water table and (b) several years after the rise of the water table.