The thickness of a clay layer is 8 m, and its volumetric weight is 18 kN/m3. It is covered by a layer of very permeable sand, having a thickness of 4 m, a saturated volumetric weight of 20 kN/m3, and a dry volumetric weight of 16 kN/m3. The phreatic surface coincides with the soil surface. However, in the thick sand layer directly below the clay layer the groundwater head is at a level of 4 m above the soil surface. Calculate the effective stress in the center of the clay layer.