An in-tunnel jacking above tunnel protection methodology for excavating a tunnel under a tunnel in service

Abstract A section of the tunnel of Dongzhimen station on Beijing metro Airport Express closely under-passing the turn-back track tunnel in service through Dongzhimen station on Line 13 was constructed by a mining method, termed the ‘tunnel-column method’. Due to restrictions on the surface and subsurface settlement of structures in the area, an in-tunnel support approach was adopted using jacks controlled by programmable logic controllers (PLCs) in combination with the mining method. Jacks were arranged, grouped and slowly loaded in the new tunnel to support the overlying tunnel in service. The under-passing tunnel was excavated under the protections of the lifting jacks, and the shell composed of shotcrete and lattice girders. Performance of the tunnel in service due to the under-crossing tunnel was continuously monitored by means of a real-time and high-precision hydrostatic leveling system together with grating scales installed near the jacks. The measured data were timely processed, and fed back to operators to readjust the procedure if required. As a result of the in-tunnel jacking, the movement of the tunnel in service was below the allowable value, and the under-crossing project was successfully completed without interrupting the metro traffic.