SEDL / STP / STP740-EB / STP28763S

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

Mayne, PW
Geotechnical engineer and senior geotechnical engineer, Law Engineering Testing Company, Washington, D.C.

Swanson, PG
Geotechnical engineer and senior geotechnical engineer, Law Engineering Testing Company, Washington, D.C.

Pages: 21    Published: Jan 1981

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Abstract

The results of isotropic and anisotropic consolidated-undrained shear tests (CIU, CK⁰, 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 (K⁰) 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, K⁰, 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 (Cc: 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.