I am using exponent Drucker-Prager model at the moment, is there anyone can tell me where the follow exponent D-P eqation comes from originally?

(σ e)(σ e)=λ(σt)(σt)-3(λ-1)σmσt

Where λ= is hydrostatic stress sensitivity parameter and equal to σc/σt, σe is effective stress, σm is hydrostatic stress, σc, σt are stresses under compression and tension, respectively.