A case history of a deep basement in London Clay

Abstract

The case history of the deep excavation for the National Gallery extension in London is presented in this paper. Comparisons with data from other sites show that it is typical for that of similar projects in Central London. Class 1 predictions of the retaining wall behaviour (prior to construction) using the Model London Clay constitutive relationship considerably over-estimated wall and ground movements. Retaining wall and ground movements are also considerably over-predicted by analyses using a simple linear elastic/perfectly plastic soil model, despite optimistic parameters being assumed for the soils. Predictions made using the constitutive model BRICK are closer to the measured deflected shape, but are also higher than measured values. Parametric studies of the effect of various parameters suggest that a “best estimate” of the wall movements are still well in excess of those measured. It is concluded that these differences are due to three-dimensional effects and deficiencies in the model. The “beam-spring” computer software for retaining walls FREW gives similar results to the analyses using the simple model. Analyses of the same problem carried out by a different operator using another finite element code, but with the same constitutive model, yielded somewhat different results and highlight the need for careful interpretation of finite element analyses.