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

Abstract

The results of isotropic and anisotropic consolidated-undrained shear tests (CIU, CK0, U) are used to determine the critical-state pore pressure parameter (Λo). The relative advantages of using the critical-state parameter (Λo) 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 (Λo). 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.

Effect of Overconsolidation and Anisotropy of Kaolin Clay Using True Triaxial Testing

“…This relationship showed that the critical state was reached by the clay at a constant shear stress ratio, M(=qapi), which may be different from the peak shear stress ratio or the shear stress ratio at the peak shear stress level (Egan, 1977). Mayne and Swanson (1981) summarized the results from 90 different clay and silt soils reported throughout the geotechnical literature and confirmed the finding of Ladd and Foott (1974) that the normalized strengths (Sday'o) are a function of OCR. (6) Here 6,,0 is overburden pressure.…”

“…Using Eqs. 9and (10), the failure surface in q-p' plane (stress space) can be defined as: (11) During an undrained test the IX remains constant throughout the test, and for NC clay, the pre-consolidation pressure before shearing p6 is equal to IX. Therefore, the shear strength:…”

“…Equations (12) and (13) can be rearranged to show the normalization of undrained strength in Eq. (6) suggested by Mayne and Swanson (1981):…”

“…(14) Figure 12 shows the failure surface defined by Eq. (11) in q-p' plane for Ao= 0.5, 0.7, 0.9, and 1.0 with a constant value of Cf= 0.5. The strength of OC clay relative to the NC clay increases with increasing A o and reaches its maximum at the theoretical limit of Ao =1 when Cs = 0.…”

Functional soil-landscape modelling to estimate slope stability in a steep Andean mountain forest region

“…Unfortunately, we could not perform any measurements on our own. However, Vanacker et al (2003), who also investigated slope failure in Ecuador, assumed φ′ = 22°(tan φ′ = 0.4), but references stated by Mayne and Swanson (1981) would propose a higher φ′ value (35°, tan φ′ = 0.7), based on studies for siltic soils, which also dominate the research area. In conclusion we assume φ′ to range between 22 and 35°.…”