TNAG-1062-FCO40-1312-Replacement-airport-for-Hong-Kong-1981 — Page 47

24

CIVIL ENGINEERING

The civil engineering study of a replacement airport for Hong Kong began in July 1973 with the publication of a working paper identifying 13 potential new airport sites of varying capabilities. On advice of the Steering Group convened for the long-term studies, a shortlist of six sites was defined. The Consultants later recommended one site for further evaluation. In August 1973, the Director of Civil Aviation, on advice of the Steering Group, authorized further studies and arranged for a limited engineering investigation of the site. These investigations were documented in a 1974 report as part of the Hong Kong Air Transport System studies. The current investi- gative reports present additional civil engineering work, based largely on data and analyses of the site area recently conducted for the PWD.

Land reclamation for the proposed airport site would be created by excavating Chek Lap Kok Island to approxi- mately 9 m above Principal Datum (Principal Datum is about 1.2 m below mean sea level). The excavated material (soil, weathered rock, and granite) would be used to construct a land reclamation extending to the west and south of the island. The critical engineering aspect of the site area is that the seabed is overlain with fine-grained marine sediments having poor characteristics

for a structural foundation. The low bearing capacity of this material could result in undesirable settlement of the land reclamation if these sediments are permitted to remain under the reclamation.

Engineering data for the site area is not as complete as the Consultants would have preferred, but judicious engi- neering analysis and PWD advice made it possible to reach preliminary conclusions on a satisfactory method of constructing the reclamation embankment. A tradeoff had to be reached between the least costly method of constructing the embankment (leaving the marine sedi- ments in place and accepting the long-term settlement) and the more costly and more rapid method (removing all of the sediments by dredging and constructing the embankment on the underlying strata). The result was a zoned reclamation programme where varying thicknesses of weak strata are left in place. Figure 16 illustrates the conclusions reached by the Consultants for a preliminary zoning plan based on assumptions of time available for consolidation of the fill structure before final construction of airport elements must be completed on the surface.

In simple terms, the initial phase of the airport would have the shortest available consolidation time. The programme would require dredging and disposal of the marine sediments to a thickness of about 3 m and constructing most of the embankment on this reduced layer. All sediments would be dredged under a protective seawall, which would then be constructed on the under- lying stratum of alluvium. Later phases of development