Study on Key Techniques for Shield Tunnel Passing under Metro Tunnel Closely in Launching Phase

A case of shield tunnel crossing under an operating metro is studied in this paper. The existing tunnel was in operation and the new shield tunnel was close to the existing one. The existing tunnel was closed to launching shaft, so the passing was in launching phase. The earth pressure balance had not yet been reached in the launching phase, and the boring face was unstable. Studies revealed that ground loss and water loss - which can cause the structure a great displacement or even failure - were the biggest threats to the existing tunnel. Therefore, a special outer sleeve which was used for TBM (tunnel boring machine) launching was fixed in the tunnel portal. The TBM can be sealed in launching phase by using the sleeve. At the same time, such countermeasures as ground improvement, adjustment of boring parameters and information feedback construction were employed. The monitored data showed that the countermeasures can effectively reduce the loss of ground and water and ensure safety of the existing tunnel.