Surface and subsurface structural response on the City of London cable tunnels project

Abstract

This paper presents surface and subsurface ground and structural response to the excavation of an urban cable tunnel within London clay and the Lambeth Group strata. Project specific tunnelling volume losses were estimated and found to be dependent on face advance and geology. The presence of adjacent buildings reduced predicted ‘greenfield’ settlements. The tunnel passed below a continuous section comprising basements, box rail tunnels and other structures. These structures generally responded at the level of their foundations. The presence of a pile group through which the tunnel passed only reduced surface settlement by ~50%. Passing below two existing segmentally-lined LUL tunnels these responded immediately and as predicted, although with somewhat increased trough widths indicating a stiffening effect. A multi-span bridge directly above the LUL tunnels on deep pier foundations, part of the Holborn Viaduct, settled more slowly and twice as much as anticipated. A possible general effect of tunnelling on heavily loaded foundations including end-bearing piles is discussed