Example $8\mathrm{.}1$: The stresses at failure on the failure plane in a cohesionless soil mass were:

Shear stress $=4k\frac{N}{m^2}$; normal stress $=10k\frac{N}{m^2}.$ Determine the resultant stress on the

failure plane, the angle of internal friction of the soil and the angle of inclination of the failure

plane to the major principal plane.