ABSTRACT
The aim of this paper is to describe the prediction and subsequent monitoring of ground movements during the construction of a 23m deep excavation at Queensberry House in London.
Queensberry House is located in the centre of London on Savile Row and is surrounded by old Georgian brickwork buildings. The geology at the site comprised 5.0m of gravels underlain by stiff, London Clay with the water table 5m beneath ground level. The development entailed the construction of a 7 storey building above a 13 half-storey, 23m deep basement. A contiguous piled wall solution with 750mm diameter piles at 900mm centres was proposed to form the basement.
An initial study was undertaken using Plaxis to evaluate the loads imposed on the retaining wall by the ground during construction of the basement and to determine the potential ground movements induced. Details of the mesh, method of construction adopted and the selection of parameters for the ground and the sub-structure are all discussed. Results of the analysis are presented in the form of predicted horizontal and vertical surface displacements. The resulting ground displacement profiles with distance from the excavation were used to assess the extent and severity of possible damage incurred to the surrounding buildings.
The predicted displacements from Plaxis are compared with those predicted by the Contractor, John Laing Ltd who undertook a subsequent finite element analysis using the CRISP package. The stiffness characteristics of London Clay used in the CRISP models were determined from the results of the main site investigation, in particular, from the self-boring pressuremeter and small strain triaxial tests.
In conjunction with the Plaxis and CRISP modelling, a monitoring program was adopted to measure the behaviour of the ground and the surrounding buildings during construction to ensure that they were within acceptable limits. The results of the surface displacements measured during construction are compared with those predicted by the Plaxis and CRISP packages prior to construction.
