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.
Response of a double-decked metro tunnel to shield driving of twin closely under-crossing tunnels
Abstract Twin tunnels on Shenzhen metro Shekou Line were driven by EPB shields through weathered granite gneiss closely under-crossing a double-decked tunnel embedded in strongly weathered granite gneiss on Luobao Line. The performance of the existing tunnel as affected by under-crossing tunnels was continuously monitored by means of an automatic high-performance total station measuring system. Displacements of the existing tunnel caused by under-crossing tunnels in different under-crossing stages were measured. The measured data were timely processed and fed-back to related operators to realize an information-oriented project. Based on the measured data discussions and analyses were carried out in terms of the impact of the under-crossing shield tunnelling on the existing tunnel, effect of rainfall infiltration on the performance of the existing tunnel, allowable deformations of the existing tunnel, possibility of reducing the number of measuring points in a monitoring cross section and the influence on the existing tunnel caused by twin tunnels. Thanks to the staggered single tunnel under-crossing and other favorable factors, the measured deformations of the existing tunnel were well below the predefined allowable values. Under the guidance of monitoring, the under-crossing project was smoothly completed without interruption of metro traffic. This is a valuable example for evaluating the shield tunnelling induced effects on adjacent existing tunnels and establishing criteria to guide shield tunnelling under similar conditions.
Mechanised Tunnelling in Urban Areas
This article provides introductory material covering important issues regarding mechanized tunneling for urban areas in particular. The authors list a number of hazards with excavating in these conditions, such as: surface and subsurface utilities, man-made ground, dense surface infrastructure, mixed geologies, poor ground conditions, shallow overburden, ground settlements, and the presence of groundwater among them. Additionally, the authors provide four guidelines for these projects as an outline. These are first to minimize disturbances to the surface, provide for all sewage systems and super structures, avoid the collapse of the tunnel face, and carefully mind limitations of the specifications of the surface settlement. The authors also discuss the need for increased international focus on risk management and overall excavation in urban settings, provide an outline for a book-length treatment of the subject that they have authored, and close with a description of their Risk Management Plan (RMP) as a key component of their methodology.