Development of the safety control framework for shield tunneling in close proximity to the operational subway tunnels: case studies in mainland China
IntroductionChina’s largest cities like Beijing and Shanghai have seen a sharp increase in subway network development as a result of the rapid urbanization in the last decade. The cities are still expanding their subway networks now, and many shield tunnels are being or will be constructed in close proximity to the existing operational subway tunnels. The execution plans for the new nearby shield tunnel construction calls for the development of a safety control framework—a set of control standards and best practices to help organizations manage the risks involved.Case description Typical case studies and relevant key technical parameters are presented with a view to presenting the resulting safety control framework. The framework, created through collaboration among the relevant parties, addresses and manages the risks in a systematic way based on actual conditions of each tunnel crossing construction. The framework consists of six parts: (1) inspecting the operational subway tunnels; (2) deciding allowed movements of the existing tunnels and tracks; (3) simulating effects of the shield tunneling on the existing tunnels; (4) doing preparation work; (5) monitoring design and information management; and (6) measures and activation mechanism of the countermeasures. The six components are explained and demonstrated in detail.Discussion and EvaluationIn the end, discussions made involve construction and post-construction settlement of the operational tunnel, application of the remedial grouting to rectify excessive settlements of the operational tunnel, and use of the innovative tool of the optical fiber measurement for tunnel movement monitoring.ConclusionsIt is concluded that the construction movement of the tunnel can be controlled within 15 mm when the shield machine is <7 m in excavation diameter. The post-construction settlement of the tunnel buried in the very soft ground is much greater than its construction settlement, and last several years until reaching a final stable state. Two cases are outlined to demonstrate the feasibility of using the remedial grouting to reduce the long-term settlement of the operational tunnels. The more and more segmental tunnels being constructed, there is an increasing need of the optical fiber measurement for tunnel movement monitoring in the near future.
Remediation of Damaged Shield Tunnel Using Grouting Technique: Serviceability Improvements and Prevention of Potential Risks
AbstractShield tunnel damage induced by excessive and differential tunnel displacement has become a major threat to the operational safety of urban subways in China. Grouting offers an effective wa...
Behavior of the ground in rectangularly crossed area due to tunnel excavation under the existing tunnel
In this paper, the study was focused on the behavior of the near-surface ground in the tunnel-crossing zone and the behavior of the existing tunnel located above the newly excavated tunnel. Several largescale model tests were carried out in the 1/12 scale for the sample tunnel in practice where the upper tunnel has 8.4m cover over the crown and 7.2m diameter and the lower tunnel has 10.8m diameter .Test model was constructed in a large-scale test pit that was 4.0m wide, 3.8m high and 4.1m long. The ground density was controlled by sand raining for middle density. The existing upper tunnel lining had 60cm diameter and the 4mm thickness of steel. The existing lining was buried during the ground formation. The lower tunnel was excavated in 90cm diameter and supported by steel liner with 4.5mm thickness at each construction phase of 30cm advancing length. Measurements were carried out for the lining stresses, the upper tunnel displacements, the ground stresses, and the settlement of the crossed zone at each construction phase of the lower tunnel. All tests were verified by numerical back-analysis. Test results show that the longitudinal arching effect contributed considerably to the redistribution of the earth pressure and the settlement of the ground in crossed zone by the lower tunnel excavation. Model tests and numerical analyses show that upper tunnel give the disturbance to the stress flow produced by the longitudinal arching effect due to the excavation of lower tunnel. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. "Reprinted with permission from Elsevier".