The influence of bored piles on existing tunnels - A case study

In the urban environment, pile foundations are frequently constructed in locations very close to existing tunnels. Tunnels carrying transport networks, moving walkways, escalators and services can often tolerate only minimal movements. Construction and loading of piles causes ground movements and stress changes that can adversely affect the tunnels. Tunnel owners, such as London Underground (LUL) therefore place restrictions on the construction of deep foundations close to their tunnels. The restrictions may consist of one or more of the following: maximum allowable tunnel deformations; stress changes in the lining; or clear distances between the tunnel and the piles (Chudleigh et al, 1999). The problem of pile-tunnel interaction was recognised as early as the 1940s, when pile foundations for the Royal Festival Hall on the Southbank in London were rejected partly because of the possibility of vibrations causing damage to the adjacent tunnels (Measor & New, 1951). Over 30 years ago, during the construction of the Victoria line on the LUL network, the problem was of major concern and Morgan & Bartlett (1969) stated that These [multi-storey buildings] require bored piles up to 6ft [1.8m] in diameter carried down to the level of the underground railways. London Transport is very concerned about this because of the possibility of distortion and damage to its tunnels. However, the actual impact of piles on tunnels is still not well understood and there are very few published case records (Benton & Phillips, 1991 and Chapman et al, 2001). This paper presents a case study in which the influence of the construction and loading of bored pile foundations on existing LUL tunnels was measured. This work is part of an Engineering and Physical Sciences Research Council (EPSRC) funded research project in collaboration with LUL and Geotechnical Consulting Group (GCG) in which the interaction problem is investigated by means of field measurements and finite element analyses. The finite element analyses are not presented here.