At cut 9m deep is to be made in clay with a unit weight of $18\text{…

Question

At cut 9m deep is to be made in clay with a unit weight of $18\text{ kN/m}^3$ and cohesion of $27\text{ kN/m}^2$. A hard stratum exists at a depth of 18m below the ground surface. Determine from Taylor's charts if a $30^0$ slope is safe. If a factor of safety of 1.50 is desired, what is a safe angle of slope?