A layer of saturated clay has a thickness of 4 m, and a volumetric weight of 18 kN/m3. Above this layer a sand layer is located, having a dry volumetric weight of 16 kN/m3 and a saturated volumetric weight of 20 kN/m3. The groundwater level is at a depth of 1 m below soil surface, which is the top of the sand layer. There is no capillary rise in the sand, and the pore pressures are hydrostatic. Calculate the average effective stress in the clay, in kPa.