Assessment of Parameter Uncertainty Associated with Dip Slope Stability Analyses as a Means to Improve Site Investigations

Uncertainty is inherent in geotechnical design. In regard to estimating the stability state of dip slopes, most of the uncertainty lies in the geologic model assumed and the geotechnical parameters used in the evaluation. Biplanar (or active–passive) sliding in dip slopes occurs along a slope-parallel sliding surface with toe breakout occurring at the base of the failure. Internal shearing is required to facilitate kinematic release. All three of these release surfaces work together for the slope to fail, but with different degrees of importance depending on the dip-slope inclination. Increased efficiency and value with respect to the site investigation resources can be gained by working toward minimizing the uncertainty of those parameters that have the greatest bearing on the outcome of the slope stability analysis. This can be done quickly and inexpensively by performing scoping calculations facilitated by the use of Spearman rank correlation coefficients. This paper demonstrates that for shallow-dipping dip slopes, stability is primarily dictated by the shear strength of the slope-parallel sliding surface, and therefore, efforts should be focused on constraining the shear strength of this surface. For steep dip slopes, the shear strength related to the toe breakout and internal shear release surfaces becomes dominant, and therefore, the rock-mass shear strength and that for any adversely dipping persistent discontinuities should be the focus of the geotechnical investigation.