Risk management of SSDS stage I tunnel B - Tunnelling under the Eastern Harbour Crossing Tunnel in Hong Kong

Abstract

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 fifteen rectangular reinforced concrete box segments, each of 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. To identify and to manage the risk of potential movement when Tunnel B was approaching under the EHC, site specific ground investigation comprising of a 800 m long horizontal exploratory drill hole and geophysical survey had been carried out to identify the geological condition and groundwater regime ahead of the tunnel. The geophysical survey was specifically carried out to investigate any accretion of silt above the EHC box segments in order to properly model the correct boundary condition in a seepage analysis. Sensitivity analysis on the effect of different boundary conditions to settlement prediction using a full range of soil parameters has been considered. It is found from this study that the presence of a blanket of marine deposit (above the tunnel box segment) is an important factor that could increase the settlement prediction by many orders of magnitude as compared to the case without marine deposit. The marine deposit is a very low permeability material, which could act as a cap and retard or prevent the downward flow of the seawater from recharging into the system. The best estimate of ground settlement likely anticipated due to tunnelling was arrived, based on engineering judgment and experience consistent with other project studies. When Tunnel B was 600 m away from the EHC, additional probing and shortening of cuts were exercised; settlement and joint opening were continuously monitored on a bi-daily basis; and inflow into Tunnel B was measured daily. All of these measures were carried out in accordance with the risk management plan. Three different levels of warning systems (Alert, Action and Alarm) are described in this paper.