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Island race Creation of the airport platform has been one of the major civil engineering projects of the decade.

At the peak of its site preparation contract, Chek Lap Kok was simultaneously one of the most intensive dredging opeations underaken in the world, and the fourth busiest open cast mine.

Around threequarters of the 1,248ha airport platform is reclaimed from the sea. The remaining area is the result of mining the original islands of Chek Lap Kok and its neighbour Lam Chau, which fell within the proposed platform footprint.

In total the contract involved the movement of 350M.m3 of materials, the equivalent to 10t/s over the 31 month period in which the reclamation was essentially completed. At peak production the platform was being enlarged at 15ha a week.

The total fill requirement of just under 200M.m3 represents only about 60% of the total volume of material moved. This is because 69M.m3 of soft compressible marine mud was first removed from the platform footprint, and an additional 66M.m3 of mud had to be removed to gain access to sand borrow pits.

The platform is made up with 107M.m3 from mining Chek Lap Kok and Lam Chau and 10M.m3 of imported rock excavated from nearby contracts. The remaining 76M.m3 is marine sand imported from borrow areas within Hong Kong territorial waters.

Airport Platform Contractors joint venture won the site preparation contrct on a HK9bn (£700M) fixed price lump sum bid.

Construction work started in December 1992 with mining and marine based activities operating simultaneously. Working westward from Chek Lap Kok, the platform reached Lam Chau in December 1993, and was essentially complete by summer 1995.

Before filling could take place the largest collection of dredgers ever assembled at one place started removal of soft marine muds.

In the main, marine mud was removed by trailer suction hopper dredgers to within 3m of the final dredge level, and finished by cutter suction dredgers. Grab dredgers operated adjacent to the coastline of Chek Lap Kok and Lam Chau islands and the island of Lantau.

Meanwhile, blasting operations continued around the clock with the two mining contractors, Yamazaki from Japan and Australia's Roche Brothers, moving 120,000t a day. Blasting was carried out twice a day using 50t of explosives for each blast. During the work, 12 Ingersoll Rand blasthole rigs drilled 2,700km of blast holes, and 16 excavators loaded the fleet of 67 dumptrucks. They hauled fill to the tip face and together the trucks clocked up 12.3 million kilometres.

Mined rock from Chek Lap Kok was end-tipped off the rapidly enlarging island, while rockfill was bottom dumped from barges.

The stability of the end-tipped rockfill face - which was up to 30m high - at first caused concern. Stability was improved where necessary by placing a 2m to 5m thick bottom-dumped sand blanket on the dredge surface. The site preparation contract also included 11.5km of mostly sloping armoured seawall, using 1t,3t and 5t rock won from Chek Lap Kok using specially designed blasts.

Finally the whole platform was capped with a 2m layer of compacted sand or completely decomposed granite.

Great care was needed for filter relationships between the bulk fill and capping layer. To avoid the need for a graded filter medium where, for instance, the sand cap overlies coarser rockfill, a geotextile spearator was placed at the base of the capping layer.

The integrity of this geotextile was and will remain an important factor for the stability of the platform. Tight control of follow on activities ensured that any separator damage was repaired.

Paul Wheeler is editor of NCE's sister magazine Ground Engineering. The full story of the platform's construction appeared in a GE supplement in December 1996. Copies, £5, tel: (0171) 505 6606.

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