Vol 21 No 2 (10-2011)
MTR Contract 705
MTR Contract 705
To meet the demand for better public transport service in Western District, construction of MTR West Island line has been 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, including the MTR Contract 705.
Contract 705 was awarded in December 2009 to Gammon Construction Ltd. The scope of construction includes construction of Kennedy Town Station, overrun tunnel and ventilation shaft near the end of the overrun tunnel. Several facilities within the district have to be demolished or relocated for the construction: Ex-Kennedy Town Swimming Pool was demolished in December 2010 while a new swimming pool was re-provided at Sai Cheung Street North and was opened in May 2011; Kennedy Town Ex-Police Quarters was demolished to facilitate the excavation and construction of a ventilation shaft; Forbes Street Playground has been occupied as a temporary construction site. Moreover, conservation of Forbes Street tree wall and the Old and Valuable Trees within the construction scope have been conducted. The station is expected to be completed in 2014 and opened for traffic.
Kennedy Town Station
The Kennedy Town Station box will be 20m deep underground and 240m in length with upper and bottom levels. There will be 3 entrances which will be located at Rock Hill Street, Forbes Street and Smithfield. Bottom-up construction method is adopted for the station structure using bored piles and pipe piles. To control the groundwater during station excavation, temporary grout curtains and pump-out wells have been installed. Due to site constraints, the station box is being excavated in sections from west to east. Early excavation on the western end enabled an early start in constructing a section of the overrun tunnel in the mixed ground at the western end of the station. The underground station will span across Smithfield, one of the main routes to Kennedy Town, has to be remained open to traffic at all times. Temporary traffic management schemes have been implemented at different stages as construction progresses to allow the diversion of utilities and staged construction under the road. Utility diversions included temporary diversion of a 1.9m by 2.3m box culvert. Whilst for the ventilation shaft, mechanical excavation is adopted with temporary rock support.
To enable the construction of the eastern part of the station, the Ex-Kennedy Town Swimming Pool was demolished in advance and was relocated at Sai Cheung Street. The demolition was carried out in stages to allow decommissioning period of the plant room and diversion of the adjacent box culvert. Hydraulic breakers were used instead of pneumatic breakers to reduce noise impact from demolition. To further minimize environmental impact to the community, acoustic decking was installed over the station excavation works. A noise enclosure was also erected above the tunnel portal at the western end of the station.
Besides the station box, a new park and a Green Minibus boarding and alighting area will be built at the site. Site formation, piling works and acoustic deck installation of the station are in progress. At the same time, Forbes Street Playground was demolished for excavation and piles works. Excavation work of the overrun tunnel and construction of station box has commenced at a football court adjacent to the playground. In order to improve the accessibility of the station, 2 lifts and a covered pedestrian link including lifts and escalator will be constructed at Rock Hill Street and Sands Street respectively.
Alignment and construction of the Overrun Tunnel
The overrun tunnel is about 650m long with diameter ranging from 6m to 12m. The tunnel will run to the west of the station, under the basketball courts at Forbes Street and under terrain located south to Ka Wai Man Road. Drill and blast method is adopted to excavate a majority of the tunnel starting from ventilation shaft and adit. A section of the tunnel within the mixed-ground portal of the station is constructed by mechanical excavation and is commenced from the station box using temporary support from canopy pipes, face dowels and lattice arches. To control groundwater inflow, grouting was carried out ahead of the excavation face. For the purpose of excavating the shaft at the end of the overrun tunnel, Kennedy Town Ex-Police Quarter Block A & Block C were demolished. A noise enclosure was built atop the shaft to reduce nuisance and dust.
The drill and blast method adopted in building the overrun tunnel was also used in other contracts of the West Island Line, including the construction of cavern structures of Sai Ying Pun Station and Hong Kong University Station, a section of eastbound tunnel between Sheung Wan Station and Sai Ying Pun Station, as well as underground magazine at Victoria Road. Extensive investigation of the geographical features, history of development and impact on nearby buildings was carried out in advance. Assessment reports were approved by relevant government departments. To ensure all blasting works are safe, all works are governed by the stringent control of relevant government departments and qualified professionals. Furthermore, a noise enclosure was built over the shaft to further reduce nuisance and dust during construction. Notifications of blasting works have been issued to residents, schools, hospitals and nearby parties or organizations beforehand.
There are many Old and Valuable trees in Western District; therefore, MTR Corporation has planned tree protection since the preliminary design stage of West Island Line project. One of the measures adopted to protect the trees is to employ a certified arborist to supervise all tree conservation and tree transplanting works in this project.
As the Forbes Street tree wall is located near the construction site, which could make the hundred-year-old tree wall vulnerable to changes and impacts during construction. The valuable Forbes Street tree wall is comprised by tall Chinese Banyans and is the largest tree wall in Hong Kong. MTR Corporation adjusted the location of the Kennedy Town Station regarding the proximity of Forbes Street tree wall and Old and Valuable Trees in Western District. By shifting eastwards from the original location below Forbes Street Playground and the tree wall to a location below Ex-Kennedy Town Swimming Pool, potential damages to the trees will be reduced.
Other protective measures include building the overrun tunnel 11m below the tree wall to ensure all tree roots stay unaffected during construction. Before any work commenced, temporary support structures and hoardings were set up along the tree wall for protection. All arboriculture works in the project has been supervised by a certified arborist, following guidelines developed by Professor Jim, C.Y. from Hong Kong University. During construction, the arborist also monitors the conditions of trees.
For trees lay within construction sites of West Island Line, they have been transplanted to suitable sites before any construction work started. However, some of the trees are in poor conditions that replantation was considered priority in handling them. MTR Corporation will replant compensatory trees in recreational areas and public areas after completion of the West Island Line project. After consultation with various government departments and arborist, MTR Corporation considers transplanting or replanting in the following locations:
‧ Roadside planter along Connaught Road West;
‧ Roadside planter and receptor site at Shing Sai Road;
‧ King George V Memorial Park;
‧ Pok Fu Lam Road Playground;
‧ Greening Master Plan site at Fung Mat Road planned by Civil Engineering and Development Department; and
‧ Drainage Services Department Pumping Station at Sheung Wan.
In spite of tree transplanting, re-provisioned recreational areas will also be provided in the district, including:
‧ Centre Street Market Sitting-out Area; ‧ Hill Road Rest Garden; and ‧ Forbes Street and Sand Street Area.
Temporary barging point at Sai See Street
Excavated rocks generated from the excavations of Kennedy Town Station and the overrun tunnel have been shipped away in order to diminish effects to the land traffic. Fully enclosed conveyor belt system connecting the construction sites and the barging point at the ex-abattoir in Sai See Street is used. Another temporary barging point is situated at Western District Public Cargo Working Area. It is expected that approximately 550,000 m3 of excavated materials generated from the West Island Line project are processed at two barging points.
Different design and construction methods were adopted according to ground conditions, nearby buildings and environment. Re-provisions of facilities and protection of trees within the district have been carried out to minimize possible nuisance to the community. It is estimated that West Island Line will alleviate the traffic congestion problem and improve accessibility of the district.
Courtesy acknowledgement: MTR Corporation for providing the above information and photos