Restrictions of CPTu Based Interpretation Methods and Impact Thereof on Limit Equilibrium Modelling

Abstract

There is a large reliance on piezometric cone penetration testing (CPTu) in the tailings industry to estimate strength parameters for assessing the stability of tailings storage facilities (TSFs). It is common practice to assess the post-liquefied stability of a TSF using a residual undrained shear strength ratio (USSR). In such an assessment the residual USSR is typically applied to all saturated tailings. The current methods available to analyse CPTu data, are largely based on correlations and assumptions, and therefore contain limitations. Due to the limitations of the methods, this could lead to either over- or un-conservative estimates of stability. In this study, three methods for determining liquefaction potential and residual undrained shear strength ratio (USSR) are compared, namely: Robertson (2022), Been and Jefferies (2016), and Olsen and Stark (2002). These methods differ in the way in which they apply cone resistance, sleeve friction, and dynamic pore pressure response to estimate residual USSR. Their limitations are highlighted and discussed. A hybrid method is proposed for applying the results to post-liquefaction stability analyses. The hybrid interpretation approach uses a combination of the methods to account for different ranges of effective overburden stress and to identify weaker and stronger layers in the tailings profile based on state parameter, residual USSR, and pore pressure response. This hybrid method was applied to a stability assessment of a decommissioned platinum tailings storage facility in South Africa. The results indicated that the hybrid interpretation resulted in a more realistic phreatic surface location and a more accurate failure plane than conventional interpretations.

Full Paper