● Midfield concourse advance work of APM Tunnel, Hong Kong International Airport
WT Chan & Associates Limited appointed by Fujita-Sun Fook Kong JV to carry out alternative design for the dewatering system for the construction of the APM tunnel construction for the Hong Kong International airport, midfield concourse.
The conforming design comprises of open excavation with pumping system for the construction of the AMP tunnel. The extent of the excavated area is approximately 190m (L) by 35m (W) and maximum 14.8m deep. During the excavation, it was found that the use of pumping system cannot effectively lower the water table.
WT Chan & Associates Limited was then appointed by the contractor to investigate the dewatering system to check if additional pumps are required to lower the water table. As the airport is a reclaimed land with various types of Filling materials known as Type A, B and C Fill, all with different permeability, also the excavated area is about 300m from the seashore, therefore the in-situ soil permeability can varies significantly in different areas. Extensive dewatering tests were carried out to determine the in-situ permeability, however, since only a few hundreds millimeters could be lowered by the pumps after a few hours of dewatering, also the water level within the observation wells were affected by the tidal levels, which made the interpretation of the results very difficult. This required a complicate interpretation of the readings, we finally determine that the number of pumps required to lower the water level to the excavation level are very excessive (>300 numbers), also we found that the water level at the site is about 2 hours delay from the sea water tidal levels, i.e., the sea water takes about 2 hours to flow into the site. We finally successfully design the alternative sheet-piling cutoff system for the excavation.