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}