How can I model a gabion wall (or retaining wall) in Slide?

For gravity walls such as gabions or rockeries, we suggest using material layers with material properties equivalent to the material making up the walls.  For examples of MSE walls, see Examples #87-94 in the Slide Verification Manual Part 3.  

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Slide Slope Stability Verification #87:  Three-Tiered Wall

For modeling the actual retaining wall material, the user basically has two choices.  Infinite strength, in which case this forces all failure surfaces below the wall.  Concrete, in which case a failure surface can go through or under the wall.  The only issue then is the shear strength of concrete.

The modeling of the concrete really depends on the state of stress applied to the concrete.  The shear strength of the concrete/shotcrete depends very much on the unconfined compressive strength and age of the concrete and there are a variety of methods/codes that structural engineers use to relate them.  For example, if the allowable shear stress for unreinforced concrete is around 0.17 x sqrt(f'c), for 20MPa (3000psi) concrete, the shear strength is 0.75MPa (100psi).  Under low stress, this shear strength could be used as the cohesion with zero friction since at low normal stress, the resulting increase in strength due to friction has little effect on the overall strength.