Vol 21 No 1 (08-2011)
MTR West Island Line- Construction of the tunnels between Sheung Wan and Sai Ying Pun


To solve the traffic problem in Western District, the construction of MTR West Island Line commenced in July 2009 and is expected to be completed in 2014. The 3.3km long new railway will extend from Sheung Wan to Kennedy Town with three stations: Sai Ying Pun Station, Hong Kong University Station and Kennedy Town Station. All civil contracts have been awarded.

MTR Contract 703

Contract 703 involves the construction of the tunnels between Sheung Wan and Sai Ying Pun and was awarded to Dragages- Maeda-Bachy Soletanche Group Joint Venture. The scope of construction includes construction of two tunnels between Sheung Wan and Sai Ying Pun Stations, demolition of the existing overrun tunnel at Sheung Wan Station, and construction of shafts at Sai Woo Lane and King George V Memorial Park. The shaft at Sai Woo Lane will become part of the passenger entrances at Des Voeux Road West and Queen’s Road West after the construction is completed.

Information of the tunnels

Alignment: Eastbound Tunnel – Sai Ying Pun to Sheung Wan Crossover Box
Westbound Tunnel - Sai Ying Pun to the existing overrun tunnel near Queen’s Street
Length: Eastbound Tunnel - 765m
Westbound Tunnel - 470m
Diameter: ranges from 5.45 to 6.05m

Characteristics of the contract

Contract 703 is a target cost contract, which means the target cost had been agreed by both MTRC and contractor prior to the works. It allows greater flexibility on the contract administration by creating ideas on works proposal, in order to achieve programme benefits, enhance constructability, safety, environmental and quality control and allow cost savings on both design and construction. It also allows fairer risk allocation by gainshare (increased profit for the Contractor, saving for the Client) and painshare (loss of profit for the Contractor, cost for the Client) between MTR and the contractor.

Construction of the tunnels

The eastbound tunnel is 765m long, of which 542m located in soft and mixed ground will be excavated using a Tunnel Boring Machine (TBM) while the rest will be excavated using drill and blast method; the westbound tunnel is 470m long, of which 246m will be excavated using TBM and the rest will be excavated using drill and blast method.

The TBM is mainly being used in soft/mixed ground. It can bore through the rock and soil, remove the excavated materials and secure the tunnel wall at the same time. TBM excavation is carried out underground, limiting the disturbance to the surrounding ground and the traffic.

The westbound tunnel of the project ties in to the existing overrun tunnel which requires the dismantling of the 120m concrete lining using the Tunnel Dismantling Machine (TDM), and the backfilling of the existing overrun tunnel before the TBM drives in.

Challenges

As the construction sites are located in the densely populated area in Western District, impacts of the construction to the residents must be kept to a minimum. To suppress noise and vibration of blasting works, water was used as the blast ballast in construction of the shaft at King George V Memorial Park. Also, electronic detonators were used in blasting as they allow more control on the delay period assigned to each detonators, thus improve the blast quality by giving greater control over the sequencing of the initiation and minimize the vibration by avoiding simultaneous triggering detonations due to system error. The whole process occurred inside an 18m tall noise enclosure atop the shaft to further contain any noise being generated.

Contract 703 also involves construction works that connects to the existing Island Line. The WIL team has been engaged in extensive liaison and detail planning with the operating railway in order to minimize interface problems and ensure that the existing railway service will not be affected.

Environment-friendly

Major infra-structure construction including building of railways will inevitably cause some impact to the community in its vicinity. Different mitigation measures have been applied to minimize the impact of the construction works to the community and the environment.

To effectively control noise, most of the excavation works are carried out under acoustic deck or noise enclosure. For example, a noise enclosure has been built at King George V Memorial Park and an acoustic deck has been installed at Sai Woo Lane work site to minimize noise nuisance. For works that cannot be conducted under noise enclosure, such as the demolition and piling of buildings, other measures including noise barriers, acoustic fabrics and quiet plants have been used.

For dust control, construction sites are paved as much as possible to minimize dust emission source, with regular water spraying. All vehicles must be wheel-washed before leaving the sites; dump trucks are equipped with covers; dusty activities are also well-covered. Using water for the blast ballast in shaft excavation to absorb pressure helps reducing noise and dust.

Progress and Conclusion

The progress of blasting works at Sai Ying Pun is going well. Preparation for demolition of the existing overrun tunnel at Sheung Wan has commenced, while the TBM will be launched in October 2011. After the completion of the West Island Line project in 2014, the road network of the district will be less congested, road safety and quality of life of the residents will also be improved.

Courtesy acknowledgement:
MTR Corporation Limited
for providing the above information and photo