Central – Wan Chai Bypass tunnel, Hong Kong

Due to environmental constraints, dredging and reclaiming perma­nently lands off-shore Hong Kong Island raises strong opposition from the community and does not constitute anymore an acceptable construction method. Consequently, a very speci­fic construction sequence relying on the installation of temporary reclai­med working platform has been chosen.

The sequence of works was as follow:

• within the Yacht Club Area ins­tallation of “reclaimed islands” to be used as temporary working platforms
• From these platforms, installa­tion of the permanent diaphragm walls and barrettes to form “cut and cover boxes”
• excavation of the boxes with installation of temporary struts and construction of the permanent rein­forced structure
• backfilling of the boxes up to sea bed level, demolition and removal of the diaphragm wall above seabed level and removal of the temporary platform to reinstate the site to its original state.

The diaphragm wall and barrettes being excavated through the uncon­trolled fill which was put in place without any specific compaction, ground treatment was required to prevent leakage of bentonite slurry and possibly trench instability and water pollution. For this reason, prior to start the diaphragm wall excava­tion, a grout curtain was installed on both faces of the excavation. Furthermore, in specific areas, ground treatment using slurry wall technique was required to remove the temporary seawalls installed to form the working platforms.

Marine logistics

The foundation works were car­ried out in 3 separate visits to site. For each of these visits, the works were executed from reclai­med artificial islands with no land access. As a consequence, all manpower, plants, raw mate­rials and consumables had to be brought by sea:

• the concrete was supplied by conventional truck mixers trans­ported to site on specially desig­ned barges able to accommodate 8 to 12 trucks mixers,
• at peak, 15,000 litres of die­sel were required daily for BSGL plant and equipment. The diesel was pumped from land to the “wor­king island” through a specially installed pipeline.

Altogether, 10 barges and 10 tug boats were mobilized to deliver dai­ly materials, plant and consumable to site.

A very tight programme

In addition to marine vessels, heavy specialist plants were also mobilized to site. Up to 2 Hydrofraises, 9 grabs, 8 service cranes (up to a capacity of 250 tonnes) were working concurrently on the 200 m x 80 m platform.

An off-site steel yard was also set-up in order to produce 160 tonnes of bent and fixed steel per day and allow a production of 5 to 6 panels per week (3,200 m3 of concrete a week). The 3rd visit to site was particularly intense with 28 000 m2 of diaphragm wall and barrettes completed in 4.5 months.