Undrained strength from CPTu in brittle soils: A numerical perspective

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Static liquefaction of soils that have a brittle undrained response (hydraulic fills, mine tailings or sensitive clays) may lead to sudden failures of large consequence. Given the importance of undrained failure, obtaining precise estimates of peak and residual yield strength is important. The CPTu plays a major role in the geotechnical characterization of these geomaterials and so do CPTu-based estimates of undrained strength. Most of the methods available for CPTu-based estimation of undrained strength are empirical, based on correlation with other laboratory or field tests. When such correlations are established difficulties appear due to variable disturbance affecting the reference laboratory samples and parasitic effects, such as unaccounted for partial drainage during penetration or unknown side friction, affecting the cone results. Such difficulties are not present when using numerical simulation. The paper builds upon a series of CPTu simulations using a model able to represent brittle undrained failure. Confounding factors such as partial drainage and cone side friction are systematically varied to examine their effect on the results. The results are then employed to examine the performance of several empirical methods frequently employed to obtain peak and residual strength from CPTu.