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Diaphragm Wall Barrette and Associated Works CENTRAL – WAN CHAI BYPASS TUNNEL (CAUSEWAY BAY TYPHOON SHELTER SECTION) Hong Kong

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Diaphragm Wall Barrette and Associated Works CENTRAL – WAN CHAI BYPASS TUNNEL (CAUSEWAY BAY TYPHOON SHELTER SECTION) Hong Kong Diaphragm wall Barrette and associated works CENTRAL – WAN CHAI BYPASS TUNNEL (CAUSEWAY BAY TYPHOON SHELTER SECTION) Hong Kong General view he Central – Wan Chai Bypass FINANCING: Government of the Special Administrative Region of Hong Kong and Island Eastern Corridor Link is a 4.5 km long dual tree-lane CLIENT: Hong Kong Highway Department trunk road with a 3.7 km long tunnel which forms part of an East-West stra- MAIN CONTRACTOR: China State Construction tegic route along the north shore of Engineering (Hong Kong) Ltd Hong Kong Island. ENGINEER: AECOM ASIA COMPANY LIMITED The whole Central - Wan Chai By- pass project comprised 10 contract packages. The foundation works on QUANTITIES OF WORKS: Contract 15, which was awarded to - Diaphragm wall length: 1,244 m BSGL in April 2010, comprised the - Concrete volume: 82,443 m3 portion located between Causeway - Reinforcement weight: 23,127 tonnes Bay and North Point across the - Pre-excavation ground treatment area: 30,954 m2 Hong Kong Yacht Club. - Slurry wall: 5,900 m2 - Rock fissure/Contact grouting: 3,198 2m The works - Pumping tests: 3 Various technical options were ini- tially contemplated by the local au- thorities for the construction of the 621 underground highway. After detailed tion, a grout curtain was installed plant and equipment. The diesel analysis and due consideration to on both faces of the excavation. was pumped from land to the “wor- various technical and environmental Furthermore, in specific areas, king island” through a specially constraints, the Highway department ground treatment using slurry wall installed pipe line. selected a cut and cover method with technique was required to remove Altogether, 10 barges and 10 tug installation of a 1.50 m thick perma- the temporary seawalls installed boats were mobilized to deliver dai- nent diaphragm wallsocketed into the to form the working platforms. ly materials, plant and consumable Moderately Decomposed Granite on to site. both sides of the future highway. Marine logistics The depth of the wall varied from The foundation works were car- A very tight programme 14 m to 74 m. The excavation had ried out in 3 separate visits to In addition to marine vessels, heavy to be carried out through the com- site. For each of these visits, the specialist plants were also mobilized to pletely decomposed granite with works were executed from reclai- site. Up to 2 Hydrofraises, 9 grabs, 8 many boulders and corestones med artificial islands with no service cranes (up to a capacity of 250 being encountered above bedrock land access. As a consequence, tonnes) were working concurrently on level. all manpower, plants, raw mate- the 200 m x 80 m platform. rials and consumables had to be An off-site steel yard was also set-up A very specific construction brought by sea: in order to produce 160 tonnes of bent sequence. • the concrete was supplied by and fixed steel per day and allow a Due to environmental constraints, conventional truck mixers trans- production of 5 to 6 panels per week dredging and reclaiming perma- ported to site on specially desig- (3,200 m3 of concrete a week). The 3rd nently lands off-shore Hong Kong ned barges able to accommodate visit to site was particularly intense Island raises strong opposition 8 to 12 trucks mixers, with 28 000 m2 of diaphragm wall and from the community and does not • at peak, 15,000 litres of die- barrettes completed in 4.5 months. constitute anymore an acceptable sel were required daily for BSGL construction method. Consequently, the construction of the underground CONSTRUCTION OF CUT & COVER TUNNEL - highways had to adopt a very speci- METHOD AND SEQUENCE fic construction sequence relying on the installation of temporary reclai- med working platform. The sequence of works was as follow: • within the Yacht Club Area ins- tallation of “reclaimed islands” to be used as temporary working platforms • From these platforms, installa- tion of the permanent diaphragm 1- Diaphragm wall and barrette construction on reclaimed island walls and barrettes to form “cut and cover boxes” • excavation of the boxes with installation of temporary struts and construction of the permanent rein- forced 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 2- Excavation of the boxes The diaphragm wall and barrettes being excavated through the uncon- trolled 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 excava- 3- Reinstate of original state 621.
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