Circle Line Stage 3 (CCL3) forms part of Singapore's enhanced Rapid Transit system and probably represents the most challenging section of underground railway lines designed so far in Singapore. The overall route alignment for CCL3 does not follow the major radial road easements but instead passes through highly built up areas comprising low rise housing and apartment block estates and crosses numerous other infrastructure. As a result of this, the tunnel alignment design had to deal with the inherent engineering difficulties associated with theses constraints which also included large numbers of existing piled structures, the extent of which is novel in Singapore. Alignment design was aimed at achieving the correct balance between four principal factors. The factors were firstly, the desire to provide the required ridership, secondly optimum railway geometry for operational purposes, thirdly, the constraints on tunnelling imposed by the existing ground conditions, impacts on existing infrastructure and the desire to avoid expensive construction methods and high risk underpinning wherever feasible. The fourth factor was that the alignment design sought to provide least impact on the public in terms of nuisance, safety and private land acquisition. The balance of these factors was paramount to the successful design of this project. The challenges of this project required the Client (Land Transport Authority) and their Designer (MWH and sub-consultants) to work closely and proactively in pursuing railway alignment options that called for major modifications to the envisaged alignment from the previous feasibility designs. The final design saw the elimination of one of the six stations and the original requirement for underpinning of 20 major structures reduced to one. Adopted tunnel construction methods were principally using face pressurised Tunnel Boring Machines, with limited cut and cover structures for track cross-overs and mined tunnels for cross-passages or temporary access. Ground conditions expected at tunnel horizon comprise Bukit Timah Granite Formation rock, soil and extensive mixed face conditions, Old Alluvium and Kallang Formation soils. The alignment design process involved the assessment of alignment options by using ground characterisation through investigation, study of existing structures and foundations, identification of obstacles to tunnelling and methods of advance, analysis of tunnelling effects on structures, design of mitigation works and railway considerations. Each option was tested in terms of impact on the general public, buildability and cost. State-of-the-art geotechnical analysis methods were used to investigate the behaviour of piled structures during tunnelling and the design of piled underpinning to low and high rise structures. Significant advantage was gained by adopting a Value Engineering approach and designing the alignment to pass beneath government buildings that were scheduled for refurbishment, allowing partial demolition and the extraction of the existing piles, which would have otherwise been in the path of the Tunnel Boring Machines, from the surface. A trial of these pile removal works was made prior to adopting the preferred pile extraction method. This paper discusses the engineering challenges faced, presents a methodology for finding the optimum design alignments and describes important aspects of the adopted alignment. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.
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Sep 1, 2011
Zhan Fei Gu + 3
Open Access