A clay soil of thickness H is allowed to drain on the top boundary through a thin sand layer. A vertical stress of σ is applied to the clay. The excess pore water pressure distribution is linear in the soil layer, with a value of ut at the top boundary and ub (ub > ut) at the bottom boundary. The excess pore water pressure at the top boundary is not zero because the sand layer was partially blocked. Derive an equation for the excess pore water pressure distribution with soil thickness and time.