The prediction and back analysis of excavation behaviour in Oxford Clay.

Abstract

The work described in this thesis involved the prediction and back-analysis of ground movements and pore water pressures around a temporary excavation, lOm deep, 105m long and 35m wide (50m long and lOm wide at its base) in the heavily overconsolidated Oxford Clay. An experimental programme was carried out which was designed to investigate the nonlinearity and the anisotropy in the soil's response. A series of computer controlled triaxial stress path tests were carried out on specimens of Oxford Clay trimmed from block samples. The initial anisotropic stress state resulted in a highly stress path dependent stiffness, and the stressstrain behaviour was closely linked to the time at which the specimen had been held at constant stress prior to a change in loading. In the triaxial apparatus, shear wave propagation tests using square wave input functions into bender element apparatus were significantly affected by near-field effects and by additional wave components. Tests carried out using sine wave inputs provided a far more consistent output allowing correlation analyses and easier visual identification of the travelling shear wave. The shear modulus of the soil at very small strains could then be determined. A stress path dependent, non-linear, cross-anisotropic elastic model was developed and implemented into the finite element program CRISP. A genuine prediction of the Elstow excavation was carried out while the instrumentation data from the site investigation were temporarily withheld. It was found that the horizontal displacements were modelled satisfactorily but that the vertical displacements were in error by as much as 2 or 3 times. This discrepancy was attributed to volumetric changes suggested by the instrumentation data. A parametric analysis was carried out in which the effects of the initial stress state, the degree of anisotropy, and the degree of non-linearity were investigated. This showed that although it was possible to improve the accuracy of the prediction locally, it was not possible to improve on the overall pattern of behaviour predicted by the first non-linear cross-anisotropic analysis