How do slip elements work in CRISP?
The slip element in Crisp is discussed in section 3.10 of the Technical Reference Guide of Sage-Crisp, and the properties are in section 4.7.3 of the manual.

Goodman's 8-noded slip element is used in Crisp. This is detailed in: Goodman, Taylor and Brekke "A model for the Mechanics of Jointed Rock" in Journal of Soil Mechanics and Foundations Division, ASCE, Vol 94, SM3 May 1968, pp. 637-659.

A more detailed paper on this model is by Ghaboussi, et. al "Finite Element for Rock Joints and Interfaces" same journal as above in SM10 October 1973, pp 833-848.

In Crisp the properties are: C, phi,
Kn (normal stiffness) = E(1-v)(1+v)(1-2v)
Ks (shear stiffness) =G
Ksres (residual shear stiffness) = about 100 to 1000 times lower than Ks
t (element thickness) = between L/10 and L/100 where L is element length.

The program starts with the elastic properties. When the shear stress exceeds the limiting value (c+sigma(n).Phi), it sets the shear stiffness to the residual value (Ksres). Also if the normal stress becomes negative (tensile), the program assumes that the soil and structure are moving apart, then the normal and shear stiffnesses are reduced by a factor of 10e5.