If I understand the definition of cohesionless soil, it is like dry sand with no resistence to shear sliding. Is that what you are trying to simulate? This sounds like a recipe for non-convergence in a static structural run. You will need enough cohesion for numerical stability, in a static run. Would running this as a transient (with system damping) be an option or perhaps an explicit solver like LS-Dyna?
Perhaps there are additional tips in the references listed below (from Section 4.11.1.4 of the Material Reference Guide):
Simo, J. C., Kennedy, J. G., & Govindjee, S. (1988). Non-smooth multisurface plasticity and viscoplasticity. Loading/unloading conditions and numerical algorithms. International Journal for Numerical Methods in Engineering. 26(10), 2161-2185.
Simo, J. C. & Hughes, T. J. R. (1998). Computational Inelasticity. New York: Springer.
Roscoe, K. & Burland, J. B. (1968). On the generalised stress-strain behaviour of wet clay. Engineering Plasticity. 169(1), 535-609.
Wood, D. M. (1990). Soil Behaviour and Critical State Soil Mechanics. Cambridge: Cambridge University Press.
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