Measured short-term subsurface ground displacements from EPBM tunnelling in London Clay

Subsurface ground displacements from the construction of the twin-bore Crossrail tunnels in London Clay by earth pressure balance machines (EPBMs) are presented and discussed, complementing a companion paper by the authors that focused on the surface response. Both papers report vertical and horizontal displacements, in this case measured using comprehensive arrays of instruments installed within boreholes in Hyde Park, London. The Crossrail tunnels are deeper than those cited in most UK case histories concerning tunnelling in stiff clay. Clear insights were gained into subsurface displacement mechanisms: an 'inward' displacement field was observed around the Crossrail tunnel construction, in contrast to the 'outward' displacement field that developed around the shallower Channel Tunnel Rail Link tunnels constructed east of London using similar EPBMs in London Clay. This has important implications when estimating subsurface displacements using currently available empirical methods. Appraisal of the EPBM operational variables suggests that the relative magnitude of face and tail grout pressures to overburden stress is the key factor contributing to the opposing senses of the observed displacement fields. Earlier tunnelling-induced strain softening of the London Clay is evident from greater subsurface incremental volume losses and settlement trough width parameters relating to subsequent tunnel construction.