A soil profile consists of 10 m of sand over 10 m of clay over rock as shown in the attached figure. The water table is at the sand/clay interface. For sand, Gs = 2.65, e = 0.58, and water content above the water table is 30%. For clay, Gs = 2.70, n = 0.35. What is the vertical effective stress at 15 m below the ground surface? (Gs = specific gravity, e = void ratio, n = porosity=e/(1 e))