Plasticity Solutions of an Undrained Cavity Contraction for the Prediction of Soil Behaviour around Tunnels

Cavity contraction method has been used for decades for the design of tunneling and the prediction of ground settlement, by modelling the cavity unloading process from the in-situ stress state. Analytical solutions of undrained cavity contraction in a unified state parameter model for clay and sand (CASM) are used in this paper to predict the soil behaviour around tunnels. CASM is a critical state soil model with two additional material constants, which has the ability to capture the overall behaviour of clay and sand under both drained and undrained loading conditions. Large-strain and effective stress analyses of cavity contraction provide the distributions of stress/strain within elastic and plastic regions around tunnels. The effects of ground condition and soil model parameters are investigated from the results of stress paths and cavity contraction curves. Comparisons are also provided between the predicted and measured behaviour of tunneling, using data of centrifuge tunnel test in clay. To account for the effect of free ground surface, Loganathan & Poulos's method using undrained gap parameter is incorporated to estimate tunneling induced ground surface settlement.