Investigation and Design for Harbor Area Treatment Scheme Stage 2A - Sewage Conveyance System, Hong Kong Special Administrative Region

Harbor Area Treatment Scheme is one of the most important sewerage projects undertaken by the Government of the Hong Kong Special Administrative Region to clean up the Victoria Harbour. This paper will describe the sewage conveyance system of Stage 2A of the Scheme. The Stage 2A system consists of 21 km of tunnels at a maximum depth of 160m below sea level. The sewage tunnels will collect sewage from preliminary treatment works located in various districts on the northern and western sides of Hong Kong Island, and convey the collected sewage to the Stonecutters Island Sewage Treatment Work for treatment and disposal. Sewage flows via the deep tunnels under an inverted siphon system. The Stage 2A sewage conveyance system will go through some of the most densely populated and busiest trafficked areas on Hong Kong Island, as well as through difficult geological features.This paper will present the considerations in the hydraulic design of the conveyance system, methods of ground investigation along the proposed tunnel alignments together with the selection of construction method to deal with the anticipated geological conditions. One of the major challenges of the project is to control ground water drawdown during tunnel construction. The paper will describe the design and monitoring approach to address the issue of ground water control. Another major consideration is the maintenance aspect. This paper will discuss some of the design considerations to ensure that the efficiency of the sewage tunnels can be monitored continuously during operation.