Question $6(20$ marks$)$

A concrete retaining wall is shown in Figure $3.$ The effective shear strength parameters of the

sandy soil are $=38$ and $c=0.$ The ground water table is $30$ m below ground surface. Unit

weight of the sandy soil is $18kN{m}^{-3}.$ The unit weight of the concrete is $24kN{m}^{-3}.$ The angle of

internal friction between the wall and soil is $=25.$

a$)$ Determine the factor of safety against tipping; and

b$)$ Determine the factor of safety against sliding.