Enhancement of the Brick constitutive model to incorporate viscous soil behaviour

Abstract

The Brick model is an advanced elasto-plastic constitutive model for soils and has become a recognised tool for modelling ground behaviour, following its implementation into the finite element program Safe. The model can reproduce many essential elements of soil behaviour, which is viewed from a strain-based perspective. An analogue for the model is that of a man walking around a room, pulling a number of bricks with strings of different lengths behind him. The room represents strain space, the man’s movement is the applied strain and each brick movement is the response of a proportion of the soil. The present Brick model, although advanced, does not account for viscous behaviour, including creep and strain rate dependent stiffness, which can be very significant for clay soils. The principal aim of the work was to incorporate such behaviour into the Brick model. Two main approaches were identified manipulating either the velocity of the bricks or the string lengths. Both approaches were implemented into the Brick model allowing a series of tests to be conducted into their predictive capabilities. Isotach strain rate behaviour was investigated with both models by simulating both constant and step rate of strain tests. Simulations of past experimental work into the combined effects of creep and recent stress history on clay soil stiffness were also conducted. It was demonstrated, in accordance with the experiments, that creep can erase the effects on the initial stiffness of recent stress history involving relatively short stress paths approaching the current state, though this was not true for longer approach paths. The experimental results were more correctly simulated using the model with manipulated, strain rate dependent string lengths, the SRD Brick model. The SRD Brick model was then implemented into Safe to allow the analysis of two case histories, thereby quantifying the influence that viscous effects can have. The first involved the analysis of surface displacements above the westbound tunnel forming part of the Jubilee Line extension beneath St James’s Park, London. The second analysis was concerned with the prediction of heave displacements of a deep basement in Horseferry Road, London. The SRD Brick model was able to significantly improve on the predictions given by the original Brick model in both cases