Significance of J1 and J2

Dear members,

I need some information about the implication or significance of J1 and J2 in granular materials not mathematical represantation which I have.Your feedback will be highly appreciated.Thanks in advance.

P = Mean stress or Hydrostatic stress = (σ₁₁ + σ₂₂+ σ₃₃)/3

J₁ = -3 * p

 J₂ = 1/6 *{(σ₁₁ - σ₂₂)² + (σ₂₂ - σ₃₃)² + (σ₃₃ – σ₁₁)²}