Cylindrical cavity expansion analysis under partially drained conditions for normalisation of excess water pressure in CPTU

TL;DR

This paper introduces a hydro-mechanical coupling model for cylindrical cavity expansion under partially drained conditions, improving interpretation of CPTU tests by normalizing excess water pressure effects in soils with intermediate permeability.

Contribution

It develops a novel analytical solution for cavity expansion considering partially drained conditions, linking soil behavior with excess water pressure normalization in CPTU testing.

Findings

The model accurately predicts excess water pressure effects in CPTU tests.

A new normalized penetration rate enhances soil parameter interpretation.

Backbone curves derived from the model match extensive experimental and numerical data.

Abstract

Cone tip resistance and excess water pressure (EWP) measured by piezocone penetration tests (CPTU) may be significantly affected by the partially drained effect in soils with intermediate permeability. To capture this effect, the paper proposes a straightforward, hydro-mechanical coupling solution for cylindrical cavity expansion under partially drained conditions. The mechanical behaviour of soils is modelled using the elastoplastic Tresca model and water flow within porous soils is assumed to obey Darcys law. Two partial differential equations (PDEs) are established in the elastic and plastic zones, respectively, transforming the cavity expansion analysis into a typical Stefan problem with dynamic boundary conditions (i.e. a moving boundary at the elastoplastic interface). An approximate solution for the PDEs is derived by leveraging the variable transformation method. Based on the new solution, a novel normalised penetration rate is defined considering the rigidity index of soils, with which a unique backbone curve for CPTU is found. Finally, the backbone curve is compared with a database comprising 109 in-situ experimental tests, 101 centrifuge modelling tests, and numerical simulation results. The proposed solution may provide a useful theoretical tool for interpreting the consolidation coefficient of fine-grained soils from the penetration stage of multi-rate CPTU, which can enhance the interpretation reliability for CPTU dissipation tests.