A subsurface “drift and pile” protection scheme for the construction of a shallow metro tunnel

In the construction of a shallow running-tunnel for Beijing metro line No. 13, due to restrictions by the surface environment of the area, a subsurface protection scheme was applied: two small cross-section drifts with shotcrete linings were constructed at above each side of the subway running tunnel, and reinforced concrete piles were installed into the ground from within the drifts. The piles were rigidly connected with the floor slabs of the drift linings, so that the drifts could effectively help extend the height of the protected zone of ground. With no disturbance to the normal living of the residents in the area, the major objective of the scheme was to separate the negative effects of tunneling on two nearby high-rise residential buildings, one at each side of the tunnel, respectively. Relating to the scheme, this paper presents the relevant site conditions, design concepts and analyses, constructions and monitoring highlights.