Numerical modelling of a deep circular excavation at Abbey Mills in London

Abstract

For several forthcoming tunnelling projects circular shafts will need to be constructed in London. Better understanding of their behaviour could result in more reliable movement predictions and efficient designs. A previous large-scale monitoring scheme of Thames Water's Abbey Mills shaft provided measurements of diaphragm wall deflections and bending moments. In this paper, the results of a parametric study in Flac2D are compared with the field data to understand the shaft's performance during construction. Very small wall movements are calculated and are confirmed by the inclinometer data. Previous researchers have proposed that the joints between the diaphragm wall panels reduce the circumferential wall stiffness. Out of several simulations with reduced hoop stiffness, the results obtained from the isotropic wall, that is, with strong joints, give closest agreement with the field measurements. In the initial design, the Chalk was allowed to yield below excavation level, but the field data indicate elastic behaviour. The elastic Chalk caused the wall to bend more sharply at the excavation level and hence bending moments exceeded the design predictions. Further simulations show that the wished-in-place assumption with a K0 of 1·0 in the Chalk was appropriate, and that, when the wall installation is modelled, the applied bentonite and wet concrete pressures during the construction process govern the performance of the shaft wall. These findings provide valuable information for the construction of future shafts in a similar geological setting.