Initiation and failure mechanism of base instability of excavations in clay triggered by hydraulic uplift

Abstract

Excavations in clay overlying an aquifer may cause catastrophic basal failure due to hydraulic uplift. Although case histories with hydraulic uplift failures are reported worldwide from time to time, the initiation and failure mechanism of the base instability are not well studied and understood. To address these two issues, dimensional analysis is firstly conducted to propose dimensionless groups (DGs) possibly relevant to this subject. Effects of these DGs on the initiation and failure mechanism of base instability are then investigated, by carrying out a series of finite element analyses, in which constitutive models and model parameters have been previously validated against centrifuge test results. It is revealed that the initiation and failure mechanism of base instability due to hydraulic uplift is mainly governed by a ratio of excavation width over the thickness of soft clay inside excavation (B/D). As excavation becomes narrower (i.e., B/D decreases), the hydraulic pressure (Pi) required to initiate uplift movement of clay inside excavation increases significantly (maximum percentage increase = 50%), due to increased effect of downward shear stress acting along soil–wall interface on basal resistance. Based on the parametric study, a calculation chart is developed for estimating Pi of excavations with varied B/D and undrained shear strength of clay. At basal failure caused by hydraulic uplift, the dominant failure mode changes from simple shear in relatively narrow excavations (i.e., B/D < 4) to combined modes of triaxial compression, triaxial extension, and simple shear in relatively wide excavations (i.e., B/D > 4).