The growth of slip surfaces in the progressive failure of over-consolidated clay

TLDR

In heavily over‑consolidated clays, shear stress peaks and then falls to a lower residual value as strain increases, causing slopes made from such clay to fail progressively many years after construction. The study aims to identify the time‑dependent mechanisms controlling the propagation speed of a concentrated shear band. Using fracture‑mechanics concepts such as the J‑integral and elastic analysis, the authors model shear‑band propagation, incorporating pore‑water diffusion, viscoelastic deformation, and diagenetic bond weathering to determine the end‑zone size and propagation speed. Sliding occurs on a concentrated slip surface where the mean resolved shear stress is markedly lower than the peak shear strength, and the analysis reveals a strong size effect with important implications for soil‑mechanics modeling.

Abstract

In heavily over-consolidated clays there is a marked peak in the observed relation between shear stress and shear strain. As the strain increases, the stress falls from a peak to a much smaller residual stress. Slopes made from such a clay often fail progressively many years after construction. Sliding occurs on a concentrated slip surface, and it is found that the mean resolved shear stress on that surface is markedly less than the peak shear strength. Concepts from fracture mechanics, and in particular the J -integral, are used to derive conditions for the propagation of a concentrated shear band of this kind. The results indicate the presence of a strong size effect, which has important implications for the use of models in soil mechanics. An elastic analysis makes it possible to determine the size of the end zone in which the shear stress on the shear band falls to its residual value. An attempt is made to assess the possible sources of the time-dependence governing propagation speed of the shear band. They include pore-water diffusion to the dilating tip of the band (which governs the rate at which local strength reductions can occur), viscoelastic deformation of the clay (which allows a gradual build-up of strain concentration at the tip of the band), and the weathering break-down of diagenetic bonds.