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The project involves the reclamation of approximately 650 hectares of land north of the existing airport island and the construction of a new passenger building, with more than 280,000m2 of floor area. It will serve the 3,800m-long new runway together with supporting taxiway systems. The project was divided into different packages, including the provision of a new 2,600m-long Automated People Mover (APM) and an equally long high-speed Baggage Handling System (BHS), connecting the expanded Terminal 2 with the Third Runway Concourse. 

The alignment of the tunnels cross under the operational MTRC Airport Express Line railway (AEL) and so the box jacking methodology has been chosen to construct this section of the works. Two concrete boxes will be fully constructed in a shaft on the south side of the AEL embankment and subsequently jacked through the embankment to a receiving shaft to the north of the line. The AEL railway will remain in operation throughout the works. 

Mark Cabrita, Project Manager VSL-INTRAFOR, says:” Operating in a very critical area there are many challenging aspects of the project. The tunnels are to be built in reclaimed land less than 10 metres below the existing AEL and with the sea located only 30 metres away. VSL-INTRAFOR also has to operate in a congested site with very limited space and there are very stringent limits on ground settlement during the construction period to guarantee passenger safety on the AEL.

An extensive campaign of ground treatment was completed underneath the AEL in March 2020 to improve the soil conditions and reduce permeability to enable the excavation of the tunnels. The team has designed and built ‘grout blocks’ along the alignment of the tunnels to be built. These blocks were built using the ‘tubes-a-manchettes’ grouting method, with three types of grout: bentonite cement grout, microfine cement grout and chemical grout. 

During the course of project, the client – Airport Authority Hong Kong (AAHK) – requested VSL-INTRAFOR to carry out compensation grouting works underneath the existing AEL railway lines. This was needed to offset initial ground settlement that had already occurred during previous works. VSL-INTRAFOR injected grout inside the ground through horizontally drilled holes in order to create heave and ensure compliance with the stringent settlement limits. 

Jean-Christophe Gillard, Managing Director VSL-INTRAFOR, says: “The main contractor - China State Construction Engineering Hong Kong Ltd (CSCE) - was looking for complementary expertise in ground engineering and the box-jacking method. Capitalising on a strong experience on box jacking VSL-INTRAFOR is the only company in Hong Kong that is capable of offering the full range of services associated with the construction of such tunnels.”

Following completion of the ground treatment works the canopy was installed above the crown of the tunnels to control ground settlements during the excavation process. The canopy comprises a series of horizontal pipe piles, each approximately 30m in length. The horizontal pipe piles were installed with Micro TBM’s and the works were carried out in the daily 3-hour window that the AEL is not in operation.  

The jacking slab with its shear keys was constructed upon completion of the bulk excavation to final excavation level in the jacking shaft. The jacking slab provides a working platform for the construction of the tunnel boxes and also serves as the main structural element to resist the large thrust forces resulting from the box jacking operation, ensuring no load transfer to the surrounding excavation and lateral support structure. This arrangement generates significant tension in the slab, which is effectively compensated by the post tensioning. Ground anchors are provided to counteract the large forces that are transferred into the jacking slab. The installation of the ground anchors is now in progress and the stressing works are expected to be completed at the end of September 2021. 

The contract began on 15th June 2018 and the expected completion date for the box jacking and tunnel excavation is June 2022.

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Contact : Mark CABRITA
Senior Project Manager at VSL Intrafor Hong Kong
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