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Holding the rains

Difficult ground conditions and the need to work around the rainy season have been the main challenges for contractors working on the two stations at the northern end of West Rail.

Contractors working on the two stations at the northern end of the line have had to contend with the potential for heavy disruption caused by monsoon rains.

Tuen Mun and Siu Hong stations, at the far end of the line, are being built adjacent to the viaduct which runs along most of the northern section's length.

Each station will be supported on around 200 concrete columns, which have begun to sprout from the floor of a concrete lined nullah or flood drainage channel.

During heavy rains these nullahs can fill to the brim, fed by runoff from monsoon storms. As the construction method involves casting around 200 columns and piers within a cofferdam built in the nullah, work is restricted to the dry winter months between October and the end of April. This is to ensure water is free to flow down the channel during monsoon storms.

When contractor HKACE joint venture, comprising HK Construction and Amec International, originally won the £119.5M contract in July 1999 the Hong Kong Drainage Services Department (HKDSD) agreed that work in the nullah could take place until the end of April 2000. The contractor then had to dismantle the cofferdam and remove plant and equipment to ensure there was no obstruction to storm water run during the summer rainy season.

'We have given the HKDSD an agreement that the work we undertake in the nullah will not increase flood risk, ' explains KCRC West Rail general manager, construction Jaya Jesudason.

That agreement was put to the test just before the end of the dry season when the rains came early. Storms swept across Hong Kong on 14 April 2000 producing 350mm of rain in under six hours - a one in 35 year event. Run off filled the nullah to the brim, overtopping the cofferdam and flooding the site.

Worse still, there was widespread localised flooding around Tuen Mun, leading some local inhabitants to blame the Kowloon Canton Railway Corporation work for making the flooding worse. A subsequent HKDSD investigation cleared West Rail of any blame.

Nevertheless, the HKDSD reviewed the working agreement for this winter, bringing forward the deadline for vacating the nullah by a month.

Although KCRC had put in place a mechanism to monitor the weather forecasts closely, and warns contractors of impending storms, the April flood also prompted KCRC and HKACE to review site evacuation procedures.

Constraints imposed by the HKDSD and a slow start up by the contractor have hampered progress at Tuen Mun, and so far only around 19% of the work is complete with only 35% of piles in place. Progress at Siu Hong, which is also being built by HKACE, has been slightly quicker with around 67% of piling complete and 20% of contract work finished. On both contracts HKACE aims to complete work in the nullah by this spring.

The threat of monsoon disruption has also influenced construction of Kam Sheung Road station in the low lying Kam Tin river valley, just north of the Lam Tin tunnel.

When Necso Entrecanales Cubiertas won the £43.8M contract at the end of October 1999 the race was on to get all the piling finished by the start of the rainy season. With piles in place and the suspended station concourse slab cast, it would be easier to work on the station superstructure.

The piling programme was tight. By the time the contractor got on site it was early December, which meant there were only 100 days to place 100 piles, anything from 10m to 30m deep, before the rains came. In the end the programme was achieved, allowing Necso to cast a suspended floor slab and get on with working on the station superstructure without having to work on saturated, low lying ground.

Necso project manager Rafael Rubio says that the contractor did its own pile checking on top of checks by KCRC to ensure work did not have to be done twice. Despite the rush to complete, only one pile had to be abandoned and replaced.

The strategy has enabled Kam Sheung Road to take the lead in the race to get the first station built. The structure was topped out in December enabling the first fit out contract to begin in January.

Some of the most unpredictable station work has been at the neighbouring Yuen Long and Long Ping stations, as West Rail swings towards the New Territories west coast from the Kam Tin valley. Here piles have to be bored into marble formations riddled with cavities. Hong Kong Building Department regulations insist that piles run through 20m of continuous marble.

This has turned out to be a nightmare for HKACE, which has the combined £156.6M contract for both stations. The joint venture's piling rigs have encountered cavities at less than 20m intervals and the contractor must be heaving sighs of relief that KCRC is taking on unforeseen ground risk. At Long Ping station a world record breaking 127m deep pile was bored before a continuous 20m thick layer of marble was encountered.

These problems have affected progress at both stations and while all viaduct piles for Yuen Long are complete, only 21% of station and viaduct piles at Long Ping are finished, even though the station substructure is due for completion by March.

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