Greenfield ground response to EPBM tunnelling in London Clay

Abstract

Earth-pressure-balance machines (EPBMs) are frequently used in preference to more conventional non-pressurised tunnel-boring machines (TBMs) to minimise ground movements in the urban environment. The greenfield ground response to EPBM tunnelling in London Clay from a field monitoring research study is presented and discussed. Vertical and horizontal displacements and pore pressure changes were measured with an extensive array of surface and subsurface instruments. Surface settlement troughs observed above the tunnels were small (w_max < 10 mm) and can be modelled using the commonly adopted inverse Gaussian curve, but a complex kinematic mechanism took place within the subsurface ground. In the near vicinity of the pressurised closed-face tunnel-boring machine an ‘expanding' displacement field was observed, in contrast to the ‘contracting' field often observed and associated with open-face tunnelling. This expanding response is dependent on TBM variables, such as face pressure, and should be contemplated when predicting ground movements and assessing subsurface structures such as piles and existing tunnels.