Development of pore water pressure in the vicinity of SCLSCLSCL tunnels in London Clay - Liverpool Street Station

When tunnelling in low permeability soils such as London Clay, the surface above the tunnel experiences an immediate (undrained) settlement caused by the excavation, followed by a slow secondary settlement, or consolidation settlement, that could extend for a long period of time.

According to Wongsaroj et al. (2013) [1], Mair (2008) [2] and Gourvenec et al. (2005) [3] the long-term settlement could be caused by the new tunnel acting as a drain, creating permanently reduced pore pressures around the infrastructure and resulting in an additional consolidation of the clay. It is assumed that the drainage effect happens if the lining is not at least 50 to 100 times less permeable than the surrounding ground and, in the studied case, the current sprayed concrete lining and the clay have quite similar permeability. Although an attempt of verifying reduced pore pressures around an old segmental lining tunnel at Kensington was done by Gourvenec et al. (2005) [3], this effect could not be clearly confirmed.