Ground movements during diaphragm wall installation in clays

Abstract

Diaphragm walls are being used increasingly in connection with building basements and road improvement schemes, particularly in urban areas. Many of these walls retain overconsolidated clay, and the magnitude of the lateral stresses which will act on the wall under service conditions is uncertain. One of the reasons for this is that, although the initial in situ lateral effective earth pressures in an overconsolidated clay deposit will be high, they will be affected to some extent by the process of installation of the wall. Stress relief which occurs during installation should be taken into account, since it will influence the starting point for analysis of the post-construction behaviour. Ground movements which occur during installation are important in their own right, and might for a diaphragmtype retaining wall be more significant than those which occur during and after excavation in front of the wall. The investigation of this problem using a centrifuge modelling technique is the principal aim of the current research. An extensive literature review has been carried out to collate field data concerning the stress history and in situ lateral stresses of overconsolidated clay deposits. These were used to confirm that the proposed centrifuge modelling technique would achieve realistic stress states and changes in stress. A series of centrifuge tests has been carried out at the London Geotechnical Centrifuge Centre (operated jointly by Queen Mary & Westfield College and City University), on samples of overconsolidated speswhite kaolin, simulating the effects of excavation under a slurry trench and concreting the diaphragm wall. The background to the tests, and the geometry and design of the model are discussed. The influence of the groundwater level and panel width on ground movements and changes in pore water pressures during diaphragm wall installation have been investigated, and the results are presented. The centrifuge test results are compared with field data from various sites. The development of a simplified analytical method is presented, which may be used to estimate the installation effects of diaphragm walls in clay. The results of this analysis are compared with the centrifuge test results and field data. Finally, some areas of continuing uncertainty are highlighted and some suggestions for further research are made.