I'm not confident either. Chameau and Hassiotis have a methodology for pinning slide planes:
Hassiotis, Sophia, Chameau, J.L. [1988] Stabilization of Slopes Using Piles, Joint Highway
Research Project, Purdue University, Report No. FHWA/IN/JHRP-84/8
Hassiotis, Sophia, Chameau, J.L...