Tunnelling and compensation grouting at Bond Street, London

As part of the London Underground Bond Street station upgrade works, a Costain, Laing O'Rourke and London Underground One team is constructing over 450 m of tunnels using the sprayed concrete lining method. The risk of damage to overlying listed buildings from tunnel-induced settlement was mitigated through, among other measures, the use of compensation grouting. Installation of the compensation grouting array was observed to coincide with settlements of over 10 mm. The source of the settlement is attributed to the wet flush open bore method employed to install the tubes à manchettes. Subsequent heave and building settlements are correlated with changes in pore pressures beneath the array following grouting and tunnelling excavation phases, respectively. It is demonstrated that grouting causes large pore pressure increases which, while dissipating, coincide with ‘relaxation’ settlements. Analysis of surface and subsurface settlements indicates that the staged construction of a large-diameter tunnel reduces the ultimate volume losses. Deformations of the tunnel lining following spraying exceeded 25 mm, although this could not be directly attributed to compensation grouting.