The relationship between the relative density, Dr, and the angle of friction, ', of a sand can

The relationship between the relative density, Dr, and the angle of friction, ϕ', of a sand can be given as ϕ' = 28 + 0.18Dr (Dr in %). A drained triaxial test was conducted on the same sand with a chamber-confining pressure of 150 kN/m2. The sand sample was prepared at a relative density of 68%. Calculate the major principal stress at failire.