= A long trench with vertical sides is to be excavated in soft saturated clay deposits (ou 0) to lay a sewage pipeline. If the maximum depth of the trench is 2 m, what should be the approximate undrained cohesion of the clay (Cu) to maintain a minimum safety factor of 3? Assume that the clay has a unit weight of 20 kN/m and that the groundwater table is sufficiently below the excavation depth.