Taking East Rail over the Cross Harbour tunnel to the new Tsim Sha Tsui East station is the most technically challenging of the East Rail extension projects.
On the face of it, extending East Rail 1km south from its existing Hung Hom terminus in Kowloon may seem like a lot of trouble for very little gain. Work involves taking the railway underground and across the top of the busy underground Cross Harbour tunnel before squeezing the line under some of the city's busiest roads in cut and cover tunnel.
The new terminus at Tsim Sha Tsui will also be underground, shoe horned into a cut and cover box between busy Salisbury Road, a multistorey car park and steep sided Signal Hill park.
But for KCRC, the new link is vital to its long term plans. For a start, the new terminus will be an interchange between the existing railway system and the Mass Transit Railway (MTR). As such, finishing the project before the East Rail's Ma On Shan extension opens in 2004 is imperative.
Without the option of switching to the MTR at Tsim Sha Tsui, the extra passengers attracted to East Rail by the new line could swamp the existing MTR interchange at Kowloon Tong, says East Rail Extensions director KK Lee.
In addition, the new terminus will form part of KCRC's planned Kowloon southern loop, which will connect its East and West Rail systems for the first time.
Building the Tsim Sha Tsui extension is so complex that KCRC has split the project into three, with Gammon taking a major role in building all of them, two in joint venture with Nishimatsu.
The joint venture has a HK-640M (US-82M) contract to build the tunnels between Hung Hom and the new terminus and the HK-1.56bn contract for the East Tsim Sha Tsui station.
Gammon has a separate HK-354M contract to build around 1km of pedestrian subway linking the new station with Mass Transit Railway's Tsim Sha Tui station.
This last contract is complicated by the need for a 65m stretch to run partly buried across the top of the on-shore section of the four lane Cross Harbour road tunnel between Kowloon and Hong Kong Island.
Getting the railway over the tunnel presented a major challenge to KCRC and its consultant Ove Arup. 'The brief was to produce a design which would allow either structure to be upgraded without disturbing the other, ' says KCRC Tsim Sha Tsui project manager David Corby.
As a result, the new tunnel will be built as what Corby describes as an 'underground floating viaduct'. Each end of the 65m stretch of tunnel running across the Cross Harbour Tunnel will rest on piles, to keep the two structures independent of each other.
But that only solves part of the problem. Once the new tunnel is complete, dewatering around the crossing point will stop, so the fill that originally weighed down the Cross Harbour Tunnel will need to be replaced.
Putting fill into the void between the TST extension tunnel base slab and the Cross Harbour Tunnel carried the risk that it would transmit vibration from trains into the 30 year old tunnel.
So instead, engineers decided to fill the gap with a water filled ballast tank. The fluctuating water level acts as a shock absorber for train induced vibration, keeping pressure on the Cross Harbour Tunnel roof constant.
South of the Cross Harbour Tunnel entrance, work on the new station and subways is in full swing. Eventually, a traffic management system will be needed to enable the contractor to build temporary platforms to carry vehicles over excavation work.
Drilling and blasting pedestrian subway tunnel through Signal Hill to meet a cut and cover tunnel link with the MTR is under way, as are cut and cover tunnels between the Signal Hill Tunnel and the MTR along narrow Mody Road, where temporary platforms for traffic enable excavation of the subways underneath.
For Corby, one of the biggest challenges facing KCRC and its Tsim Sha Tsui contractors is dealing with the vast array of underground cables and sewers.
'Hong Kong is totally high rise and therefore the proportion of services under the roads is proportionately huge, ' says Corby.
The lifting of height restrictions on buildings in Tsim Sha Tsui following the closure of the old Kai Tak airport has added to the problem, as utilities want to use the opportunity created by the KCRC work to upgrade cabling and sewerage so that they have the capacity to serve taller buildings in future.