The Critical-State Pore Pressure Parameter from Consolidated-Undrained Shear Tests

The results of isotropic and anisotropic consolidated-undrained shear tests (CIU, CK 0 , U) are used to determine the critical-state pore pressure parameter (Λ o ). The relative advantages of using the critical-state parameter (Λ o ) over Skempton's pore pressure parameter (A) and Henkel's parameter (a) are discussed. The effects of over-consolidation ratio (OCR) and initial stress state (K 0 ) on both Henkel's and Skempton's pore pressure parameters can significantly alter effective stress predictions of undrained strength. The critical-state parameter is independent of OCR, K 0 , and level of shear to failure, thus requiring only two basic soil constants in order to predict undrained strength: (1) the effective stress friction angle (Φ'), and (2) the critical-state pore pressure parameter (Λ o ). An extended critical-state model is developed using the equivalent pressure concept for overconsolidated states. The method then provides a simple analytical representation of undrained stress-strain behavior and pore pressure response for clays with different values of OCR. One additional soil constant (C c : the virgin compression index) is required in order to model stress-strain behavior. The validity of the critical-state theory is substantiated by data from over ninety different clay and silt soils reported in the geotechnical literature. Furthermore, the critical-state concepts are shown to encompass both total stress and effective stress methods under one unified theory.