Liquefied Strength from CPT under High Overburden Stress: A Non-linear Critical State Approach

Risk of flow liquefaction is a concern for any structure composed of loose, saturated sands and is a particular concern for large tailings facilities composed of sand-like tailings. Current methods to estimate the large strain (liquefied) undrained strength rely heavily on case histories and CPT data. However, most case histories of flow liquefaction failures occur at relatively low vertical effective confining stresses (< 300 kPa), and there is a growing tendency to construct large structures (e.g., tailings dams) composed of loose saturated sands that have effective stresses significantly higher than most case histories. An extension of the critical state approach suggested by Jefferies and Been (2016) has been applied that incorporates the non-linear nature of critical state lines to account for increasing confining stresses. Data from published case histories and laboratory testing have been used to evaluate the updated approach and results are compared to the simplified approach recommended by Robertson (2022). Practical suggestions are provided on how to account for beneficial effects of increasing confining stress on the liquefied undrained strength ratio. The updated approach shows that the slope of the critical state line (λ10) has a major role in estimating liquefied undrained strength and that λ10 can be sensitive to changing confining stress. Comments are provided regarding uncertainty in estimating key parameters and advice given on parameter selection within a risk-informed approach.