For years Kennedy Town residents have clamoured for an extension of the MTR Island Line to their district, but viability issues have prevented it from going ahead. Until now, that is. In June 2005 the Government gave the MTR Corporation the green light to proceed with the planning of West Island Line (WIL), setting the stage for the development of the 3 km extension to the old and historic district of Western.

After two years of public consultation, a final scheme was submitted for the Government's approval and an October 2007 gazette under the Railway Ordinance paved the way for the project's implementation.

Alignment
What is being built, though, is quite different from what was originally envisaged back in the 1980s. According to WIL design manager Stephen Hamill, the original alignment ran along Des Vouex Road West before swinging round towards Kennedy Town, close to the praya. Two factors, however, led to the alignment being shifted deeper underground and moved inland: the wish to minimise disruption to the community caused by cut-and-cover excavation under the original scheme; and changes to the district's development pattern, with more buildings rising further up the hill.

The approved scheme is designed to serve both uphill and downhill areas, with the three underground stations at Sai Ying Pun, Hong Kong University and Kennedy Town featuring entrances sited and connected to extensive networks of pedestrian subways for convenient access from different catchments.

"People don't want to walk 400-500 m underground," Mr Hamill observed. Locating the stations and their associated subways and entrances therefore involved not only technical considerations such as the trains' turning radius and ground conditions - Sai Ying Pun Station was moved three to four times to accommodate the geology - but also the need to minimise walking distance as much as possible.

For Sai Ying Pun Station, for example, 12 to 13 different locations were originally considered for station entrances. Following a series of public consultations, workshops and District Council meetings, MTR was able to narrow the choices down to six. The approved station entrances will form part of a pedestrian network that will facilitate access to destinations in the Mid-Levels area as well as the area along Queen's Road West through the unpaid area of the stations. The scheme also includes pedestrian walkways, footbridges, escalators and footpaths to improve mobility and connectivity within the district.

Private land was taken up on only one entrance, on Sai Woo Lane, where it was required for the construction of a launch shaft. All the adits leading to entrances are to be excavated by bored tunnelling techniques. Drill-and-blast would be used primarily where the adits are constructed below the rock head, which at Sai Ying Pun and Hong Kong University Stations is mainly granite. Where there is soft ground, adit excavation, such as the one leading to the Ki Ling Lane entrance of Sai Ying Pun Station, ground freezing will be employed to stabilise the soil during excavation.

To facilitate mechanised tunnelling and avoid utilities, piles and other structures, WIL has been placed deep underground, with many areas being as deep as -30 mPD. Hong Kong University Station is so deep that using escalators to transport passengers between a ground level entrance at 50 mPD to the platform level at -30 mPD would be inefficient in terms of both speed and the footprint required. MTR's solution is to use high-capacity lifts capable of operating at 4 m/sec to transport up to 28 passengers each time. At Haking Wong Building at the University of Hong Kong, two banks of four lifts each will serve two levels. To improve passenger flow, the lifts will have entrances and exits on opposing sides.

Contract packaging
The project was divided into 13 civil works contracts, 12 E&M systemwide contracts, two building services contracts and one supply-only contract. An advance contract was awarded for the excavation of two shafts on Kennedy Town Praya and Hill Road, which will be used for removing spoil, to minimise pollution. A fully enclosed conveyor belt will be used to convey the spoil from the Kennedy Tower Praya shaft to a barging point for removal by sea, to minimise impacts on the environment and road traffic.

The shaft occupies a former lorry park which is to be transformed into a reprovisioned swimming pool the first phase of which is due to open in early 2011.

A contract for the excavation of tunnels between Sheung Wan and Sai Ying Pun Stations involves the use of a slurry tunnel boring machine (TBM) to carry out soft ground tunnelling for 70% of the project; the remaining 30% will be excavated in hard rock. MTR has accepted the contractor's proposal for an alternative construction method involving the use of a tunnel dismantling machine to remove the existing reinforced concrete lining of the Island Line overrun tunnel. Developed in France specifically for this contract; the machine will simultaneously remove the lining and fill the void with concrete to facilitate subsequent TBM tunnelling.

Contract 704, involving excavation of the tunnels between Sai Ying Pun and Hong Kong University Stations, will be the largest contract on the project. The 14 m high, 22-23 m span station caverns will be fully drained structures constructed by drill-and-blast in predominantly granite with some geological faults expected to be encountered. Extensive grouting will be applied before a permanent lining is put up.

The last section of tunnel, between Hong Kong University and Kennedy Town Stations, will also be excavated by drill-and-blast in granite and volcanic rock. Contract 705, for the construction of Kennedy Town Station, will include the excavation of an overrun tunnel by cut-and-cover. This contract will be carried out in two phases: construction of the western half will start first with the eastern half to commence later so that the existing public swimming pool can remain in use until the new one opens within two years.

A temporary underground magazine located away from the built-up areas is being built under Contract 708, to store explosives for the drill-and-blast excavations.

Two contracts are related to works entrusted by the government, for the reprovisioning of a public swimming pool and the David Trench Rehabilitation Centre respectively. The new swimming pool, which will be located close to Kennedy Town Praya, will be a futuristic structure covered in ETFE, the same material used on the Water Cube in Beijing for the Beijing Olympics. It will have enhanced facilities, including a covered multi-purpose pool and jacuzzis.

To make way for the construction of new entrances for Sai Ying Pun Station, the existing David Trench Rehabilitation Centre on Bonham Road will be relocated to the ex-Upper Level Police Station on High Street.

A Grade III historic building, the police station was built in 1935 and features a mix of Art Deco and classical architectural styles. It was vacated by the Hong Kong Police in 2005; the relocation of the rehabilitation centre now presents an opportunity for the adaptive re-use of a heritage structure. The reprovisioning works will also involve the construction of a new annex to accommodate a new clubhouse and physiotherapy facilities for the centre.

According to Mr Hamill, the way the contracts were packaged was determined by a wish to give more contractors the opportunity to win work and the logistics of spoil removal.

"Our strategy was to spread the work around so more contractors can be involved," Mr Hamill said. "Contract 704 is probably larger than we would have liked but if it was split into two contracts one would have come out in the middle of the district, so we decided to have a larger contract to improve the mucking out."

Target cost
The poorest ground occurs at Sai Ying Pun Station, at the interface between Contracts 703 and 704, where a small diameter pilot tunnel will be excavated for site investigation. As it is a heavily built-up area, the 703 contractor has been asked to undertake advance works for the adjoining contract, to minimise interface problems. Access shafts for the tunnels and station to be used by the 704 contractor will be used by the 703 contractor until the former comes on board.

Both 703 and 704 are target cost contracts. This means that, between the critical milestone dates set by the Corporation, the contractors are not bound by a fixed schedule but instead have a free hand to change their working methods to achieve the target cost.

Under the target cost arrangement, the contract award process for Contract 703 was conducted in two stages. During the first stage, two contractors were shortlisted for further negotiations. MTR then spent four months working with both developing their construction methods. At the end of this process, for which both were paid, a contract was awarded to the contractor who worked best with the Corporation.

The winning contractor came up with the idea of using an additional shaft and a lining dismantling machine to save time, and together the two sides developed a design that could be executed using the contractor's methods.

The additional shaft, which is right beside the original shaft in Sai Woo Lane Playground, allows the contractor to construct a TBM launch adit prior to building demolition on the originally proposed TMB launch shaft site while remaining within the construction site's footprint. It is an alternative that offers considerable programme security against the original designer's proposal.

The target cost approach was not applied to all the contracts on WIL, however, because not all of them justified its adoption.

"Target cost contracts are of benefit only where there's sufficient complexity and the contractors can save time and cost by changing the sequence of works or construction methods," Mr Hamill explained. "The other contracts don't have as much scope for innovation."

Tree protection
While no stranger to target cost contracting, one aspect of the project is new to the Corporation: protection of a tree wall.

"When we first designed Kennedy Town Station it was much closer to the tree wall on Forbes Street, but when we went and looked at it we realised we couldn't get it so close to the beautiful wall," Mr Hamill said.

After engaging both local and overseas experts to advise on the best way to preserve the tree wall, extensive measures were drawn up to ensure the construction and operation of WIL would not affect the health of the trees.

The running tunnel was set a -5 mPD, well below the tree wall, and an exclusion zone was introduced within which no construction activity or storage of plant or materials is permitted. Only recharge wells are allowed within the zone, to ensure the trees are hydrated, and extensive temporary support comprising a mixture of pipe piles and bored piles was erected to protect the wall from movement during the construction period. Struts will be preloaded to avoid imparting too much load on the part of the site close to the wall, which will be under piezometer monitoring 24 hours a day.

As Kennedy Town is prone to flooding, dewatering pumps will be set up for temporary ground water control during construction. Provisions have also been made to control ground water after construction is completed.

"We have designed the station with a drainage blanket to maintain the groundwater regime, to prevent a damming effect on the surrounding areas, which may lead to flooding," Mr Hamill explained. "We also studied the impact of the drainage design on the tree wall, which needs to maintain its water regime."

Elsewhere on this project, 82 mature trees will be transplanted. A specialist contractor was hired for the transplantation and 20-25 trees have already been moved to date.

Construction work started in July 2009. Eight major contracts have been awarded so far. Contract 704, the biggest civil works contract on the project, will be awarded in March 2010.

Cross section of Sai Ying Pun Station

Cross section of Hong Kong University Station

Tight space: the site where the access shaft at Hill Road is being constructed

Construction at Shing Sai Road for the reprovisioned swimming pool

Artist's impression of the new swimming pool

The old Upper Level Police Station as it looks now (left) and an artist's impression of the reprovisioned David Trench Rehabilitation Centre (right)

The Forbes Street tree wall

WIL contracts awarded to date