A strip footing 1.5m wide is locate at a depth of 0.75m in a sand of unit weight 18kN/m ³ , the water table being well below foundation level. The shear strength parameters are c’ = 0 and Φ’ = 80. The footing carries a load of 500. kN/m. Determine the factor of safety with respect to shear failure (a) if the load is vertical (b) if the load at 10° to the vertical.