For information
Legislative Council
Panel on Environmental Affairs
Panel on Planning, Lands & Works
The Progress of Tunnelling Works
under the Stage I of the Strategic Sewage Disposal Scheme
Background
1. There are seven deep tunnels with a total length of 25 km under the Stage I of the Strategic Sewage Disposal Scheme (SSDS). Among them, the interim outfall tunnel, which is 1.7 km in length and 5 m in diameter, has already been completed satisfactorily at a depth of about 100 m below the harbour. The other six sewage collection tunnels were originally scheduled for completion in mid 1997 under two contracts. These six tunnels comprise two western tunnels from Kwai Chung and Tsing Yi to Stonecutters Island and four eastern tunnels from Chai Wan and Tseung Kwan O to Kwun Tong, from Kwun Tong to To Kwa Wan and from To Kwa Wan to Stonecutters Island. The locations of these tunnels are shown at the attached drawing. In mid 1996, the original tunnel contractor responsible for both contracts unilaterally suspended works in all six tunnels, claiming impossibility in complying with the contract specification. This finally led to the forfeiture of the two contracts in December 1996.
2. Works for completing the tunnels were subsequently re-let under three separate contracts each for two tunnels. The contract for completing the western tunnels was awarded in July 1997 for completion in August 1999. The other two contracts for completing the eastern tunnels were awarded in January 1998 for completion in February and April 2000 respectively. All three contractors have actively resumed the tunnelling works upon award of the respective contracts.
Western Tunnels
3. The two western tunnels (i.e. Tunnel G from Kwai Chung to Tsing Yi and Tunnel F from Tsing Yi to Stonecutters Island) restarted excavation in late 1997. However, tunnelling works have suffered some delays mainly as a result of the breakdown of a vertical mucking system which was installed by the previous contractor at the Tsing Yi vertical shaft for removing spoil excavated from the two tunnels. The mucking system finally proved to be unsuitable for the works and the contractor had to stop tunnelling works for both tunnels in March 1998 to effect replacement of the mucking system. A replacement mucking system was completed in July 1998 and excavation for the tunnels has resumed since then.
4. Despite the delays caused by the replacement of the mucking system, excavation for the tunnel from Kwai Chung to Tsing Yi was completed in January 1999.
5. Excavation for the tunnel from Tsing Yi to Stonecutters Island using a tunnel boring machine is now about 25% completed. Apart from the delay caused by the replacement of the mucking system, the progress of tunnelling works has also been affected by the geological problems encountered. Since commencement of work in this tunnel, there were two incidents where weak rocks were encountered and some earth materials entered into the tunnel. These incidents had held up normal tunnelling operation. At present, the tunnel is still in a region of weak rocks. Because of the need to carry out extensive ground stabilisation work; installation of temporary steel supports and grouting ahead of excavation face so as to prevent excessive ground water inflow, the progress has been slower than expected. We are working hard to deal with the problem.
Eastern Tunnels
6. Since award of contracts in January 1998, the contractors have been actively mobilising resources and constructional plant to restart excavation works of the four eastern tunnels (i.e. Tunnels A-B and C from Chai Wan and Tseung Kwan O to Kwun Tong, Tunnel D from Kwun Tong to To Kwa Wan and Tunnel E from To Kwa Wan to Stonecutters Island). Like the western tunnels, the contractors of the eastern tunnels have suffered some delay in the early stages of their contracts due to the need to replace the vertical mucking systems left on site by the previous contractor.
7. For the tunnel from Chai Wan to Kwun Tong, the contractor has completed the blasting of the 160 metres long tunnel boring machine (TBM) launching chamber at Chai Wan, and TBM excavation is expected to commence in early 1999.
8. Excavation in the tunnel from Tseung Kwan O to Kwun Tong started in August 1998 and reached full swing production in mid November 1998. The initial progress rate is satisfactory and at present about 17% of the tunnel has been excavated.
9. Excavation in the tunnels from Kwun Tong to To Kwa Wan and from To Kwa Wan to Stonecutters Island commenced in August and September 1998 respectively. At present, about 21% and 10% of the respective tunnel lengths have been excavated. Whilst the contractor is optimising the matching of the TBM excavation work with the installation of the precast concrete lining, the TBM in the tunnel from Kwun Tong to To Kwa Wan has also met several bands of weathered rock. Tunnelling progress has been affected slightly due to the need for ground stabilisation works. We expect that the progress will pick up after the initial phase is passed.
Looking Ahead
10. Various obstacles have been encountered by the contractors since commencement of the three completion contracts. As a result, it is likely to take four to eight months longer to complete all the tunnels than provided for in the contracts. It is however worthwhile to note that the tunnelling work is carried out in rock which, being a natural material, could vary significantly in its degree of properties along such a substantial length. Dealing with possible unforeseen variability is part and parcel of the tunnelling operation itself. The current contractors have already demonstrated that they can deal with these difficult ground conditions in a professional and safe manner. From the results of previous site investigation work, we note that there are still possible bands of weak rock to be traversed. Progress of the remaining tunnel boring work will be determined by the actual geological conditions to be encountered. Our present estimate is that all tunnels under SSDS Stage I will be completed in year 2000.
January 1999
Works Bureau
