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Advisory Council on the Environment

Quarterly Report on Progress of SSDS Stage I Works
(as at early January 1999)

(ACE Paper 5/99)
for information

Background

The Strategic Sewage Disposal Scheme (SSDS) Stage I comprises a series of deep sewage tunnels to collect and transfer sewage from the urban Kowloon and north east Hong Kong Island to a new centralized sewage treatment works at Stonecutters Island for treatment. The treated effluent is to be discharged via an interim outfall to the more open waters west of Victoria Harbour. The works for Stage I commenced in April 1994 and the new sewage treatment works was commissioned in May 1997. However, due to delays in the completion of the deep sewage tunnel system, only 25% of the projected sewage flow (i.e. flow from the nearby North West Kowloon area) is being delivered to the sewage treatment works for treatment at the moment.

2.The interim outfall, which is a 1.7km, 5m diameter tunnel in rock at 90m depth, was completed satisfactorily. The six sewage collector tunnels at similar depths were originally scheduled for completion in mid 1997. Unfortunately, the original tunnel contractor unilaterally suspended works in all tunnels in mid 1996. Despite being given every opportunity, the contractor did not recommence works and sought to re-negotiate the two contracts involved. This finally led to the forfeiture of the contracts in December 1996. The tunnel completion works were subsequently relet in three separate contracts with two tunnels per contract. The contract for completing the western tunnels (i.e. the two tunnels running from Kwai Chung and Tsing Yi to Stonecutters Island), i.e. Contract No. DC/96/20, was awarded in July 1997. The other two contracts for completing the eastern tunnels (i.e. the four tunnels running from Tseung Kwan O and Chai Wan to Stonecutters Island), Contract Nos. DC/96/17 and DC/96/18, were awarded in January 1998.

Progress of Tunnel Completion Works

Western Tunnels

3.The contractor of Contract No. DC/96/20 is Skanska International Civil Engineering AB (hereafter Skanska). Excavation for the two western tunnels restarted 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 Skanska 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 resumed.

4.At present, excavation for the tunnel from Kwai Chung to Tsing Yi using drill and blast method is about 95% completed and is progressing well. With the present progress rate, excavation of this tunnel would be completed by end January 1999.

5.Excavation for the tunnel from Tsing Yi to Stonecutters Island using a tunnel boring machine (TBM) is now about 25% complete. The work also experiences some difficulties and delays when it hit zones of weak rock in February 1998 and again in the recent two months. Progress through these zones has been slow because of the need for extensive ground treatment and installation of temporary steel supports. We are working hard to deal with the problems

Eastern Tunnels

6.The contractor of Contract No. DC/96/17 (Tunnels from Tseung Kwan O and Chai Wan to Kwun Tong) is Paul Y. - SELI Joint Venture (hereafter PYS). The contractor of Contract No. DC/96/18 (Tunnels from Kwun Tong to To Kwa Wan and To Kwa Wan to Stonecutters Island) is Gammon - Kvaerner Joint Venture (hereafter GK). Since award of contracts in January 1998, both contractors have been actively mobilizing resources and constructional plant to restart excavation works of the four tunnels. Like Skanska in Contract No. DC/96/20, both contractors 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.Excavation in the tunnel from Tseung Kwan O to Kwun Tong started in August 1998, but full swing production did not commence until mid November 1998. The initial progress rate is satisfactory and at present about 12% of the tunnel has been excavated. In the tunnel from Chai Wan to Kwun Tong, PYS have completed the blasting of the 160m long TBM launching chamber at Chai Wan, and TBM excavation is expected to commence in February 1999.

8.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 18% and 8% of the respective tunnel lengths have been excavated. Whilst GK is going through the initial learning curve of excavation, the TBM in the tunnel from Kwun Tong to To Kwa Wan has met several bands of weathered rock. The tunnelling progress has been affected to some extent due to the need for ground stabilization works. We expect that the progress will pick up after the initial learning phase is passed.

 

Summary

9.Various problems have arisen since commencement of the three completion contracts causing a delay of about eight months to the overall programme. It is worthwhile to note that tunnelling work are carried out in the ground which, being a natural material, is variable. Dealing with possible unforeseen variability is part and parcel of the tunnelling operation itself. The completion contractors have already demonstrated that they can deal with these difficult ground conditions in a professional and safe manner. No doubt the progress of the tunnel excavation will be largely determined by the actual geological conditions, but all are doing their best to try to achieve the target of completing the SSDS Stage I project in the year 2000.



Drainage Services Department
January 1999


 

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