Risk management of SSDS stage I Tunnel B - tunnelling under the eastern harbour crossing tunnel in Hong Kong

摘要

Tunnel B of the stage I strategic sewage disposal scheme (SSDS) has an excavated diameter of 3.2 m and it was excavated with a hard rock tunnel boring machine 75 m below the eastern harbour crossing tunnel (EHC) in the Victoria Harbour of Hong Kong. The EHC is an immersed tube structure built in 1990 and the structure was made up of 15 rectangular reinforced concrete box segments, each about 125 m long, connected together by flexible joints that are under compression at working condition. The major risk that had been identified was that if any ground movement in the EHC tunnel boxes had occurred, as a result of groundwater drawdown due to large inflow into Tunnel B, the flexible joints connecting the two adjacent boxes of the structure could have lost its effectiveness in water-sealing with the consequence of seawater entering the boxes under full hydrostatic pressure. The EHC is a major trunk road in Hong Kong and should water seeps into the structure, traffic would have been devastated resulting in huge financial loss to the city.