To alleviate traffic congestion in the western district of Hong Kong Island, a Mass Transit Railway (MTR) West Island Line (WIL) has been constructed, involving two single-track underground tunnels of 1.62 km total length.
Approximately 800 m of the tunnels were constructed using a 6.39 m diameter slurry mix shield tunnel boring machine (TBM) in alternating soil and rock conditions, passing beneath a number of high-rise buildings that had been constructed on old reclaimed land. Deep shafts were also constructed close to these existing buildings to receive and launch the TBMs. In addition to the new works, an existing over-run rail tunnel of 5.6 m to 5.8 m in diameter and 132 m in length was dismantled using a bespoke tunnel dismantling machine.
WSP* provided specialist geotechnical engineering services to the contractor for the unprecedented geotechnical challenges involved in the complex excavation interfaces associated with TBM tunneling, tunnel dismantling and shaft excavation works. Adopting the latest digital engineering technologies, WSP* used 3D finite element modelling to analyse these complex geotechnical problems during both tender and design phases, and to back-analyse the performance of the works during construction.
The original design from the client stipulated that advanced protective grouting was required from ground surface beneath the buildings or around the tunnels at several locations along the tunnel route. However, installing the design numbers of vertical/inclined grouting tubes from street level in a densely populated area with a multitude of high-rise buildings would be near impossible.
WSP* investigated the necessity of the advance grouting, finding that limitations of the original assessment methods had resulted in overly conservative protective measures. WSP*’s use of 3D finite element modelling rigorously analyzed the interaction between the ground and the building foundations to overcome these limitations, revealing that only one-third of the locations would require the advance protective grouting measures before commencing the TBM drives. Additionally, WSP*’s analyses revealed that, by adopting strict control of TBM slurry confinement pressures during tunnel excavation, it was possible to delete the remaining areas of advanced grouting, saving a vast quantity of time and grout materials, and the carbon dioxide emissions associated with fuel for plant operations.
The significant environmental and social benefit secured by deletion of grouting works also reduced traffic disruption, which would otherwise have caused huge amounts of carbon emissions and would have greatly inconvenienced the public for several months. The construction program was also reduced by up to three months.
Construction of the WIL commenced in 2009 and train service on the line was in full operation by March 2015, providing a safe, reliable and environmentally friendly mode of transport to the western district.
WSP* was a finalist at the 2019 Year in Infrastructure Awards in the Geotechnical Engineering category, for the work on this project.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.