A sand, with ( c = 0 ) and ( ϕ = 28⁰ ), is on the limit of failure. The principal stress ( σ₁ = 55 ) kPa. Compute the principal stress ( σ₃ ).
a) ( c = 0 )
b) ( ϕ = 28⁰ )
c) ( σ₁ = 55 ) kPa
d) ( σ₃ )