A layer of clay 4m thick lies between two layers of sand each 4m thick, the top of the upper layer of sand being ground level. The water table is 2m below ground level but the lower layer of sand is under artesian pressure, the piezometric surface being 4m above ground level. The saturated unit weight of the clay is 20kN/m3 and that of the sand 19 kN/m3; above the water table the unit weight of the sand is 16.5 kN/m3. Calculate the effective vertical stresses at the top and bottom of the clay layer.