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A game of two halves


While West Rail's northern section is mainly on viaduct, the south is entirely in tunnel.

Water, piling and noise are the three issues that dominated my part of the project, ' says Gregory Yuen, project manager for the northern section.

At the far end of the line, Siu Hong and Tuen Mun Stations were both built in the Tuen Mun Nullah, or water course, as this was the only land available to KCRC. Nullahs flood during the rainy season from April to late September and are often also subject to tidal ingress and egress.

'Flash floods are quite capable of washing equipment away, ' Yuen states. This severely limited contractors' working time, and also largely dictated the design of the stations' footings - smaller numbers of large 3m diameter bored piles were preferred over larger numbers of smaller diameter piles.

Tuen Mun Nullah is 60m wide and small agricultural buildings are frequently borne downstream on flood waters. Hong Kong's Drainage Services Department was acutely worried that construction of piers in the watercourse would reduce its hydrological section, raising flood levels. It also wanted assurance that the piers would be spaced far enough apart to prevent large debris becoming wedged and damming the flow.

Modelling carried out by consultant Binnie Black & Veatch resulted in the channel being widened by an average of 5m to 12m to compensate for construction within it.

Piling contractors were forced to race ahead with work during brief six month windows.

But floods can happen even during the dry season, so all work had to be carried out behind temporary cofferdams, leaving half of the channel open. By the time temporary works had been completed contractors had a bare four and a half months to tackle their real objective - getting the foundations in.

'When operations started in July 1999 we were working to five day weather forecasts, but we were caught offguard in January 2000. A flash flood overtopped the blockwork cofferdam around a piling rig and knocked it out of action, ' recalls Yuen. 'When you've only got four months to achieve results, losing one month is a big issue. The incident threw work close to a year out, and the following year we got nearly every piling contractor in Hong Kong involved to pick up lost time.'

Piling challenges on West Rail have become the stuff of local legend. Contractors drilling into marble bedrock repeatedly hit cavities. 'Normally in Hong Kong granites you have to go into bedrock with at least 5m of competent rock below toe level.

But in cavities you have to go down until you have 17m of competent rock below founding level to safeguard against settlement or collapse, ' says Yuen. 'It takes a couple of months to install a single pile.'

At Long Ping station a challenging job turned into something of a nightmare when piles hit cavity after cavity - the longest was over 120m.

'We made design changes, ' recounts Yuen. 'We used fewer piles with bigger capping beams.

In some cases we used sacrificial casing to support the bottom of the shaft during concreting. We brought in more piling rigs, and more reverse cycle drills.'

KCRC achieved a breakthrough for Hong Kong construction by making the case for friction piles. 'The Buildings Department is conservative: It has allowed only end bearing piles since the 1960s, following some high profile collapses of buildings built on friction piles.

To prove that friction piles would be acceptable we had to do extensive bearing tests, but for us, building kilometres of viaduct and several stations, it was worth spending about £7.2M on research and development.'

West Rail, along with almost every other major construction project in Hong Kong, was rocked in 2000 by a short piling scandal: A piling subcontractor was found to have been faking certificates on high rise flats for the Housing Department.

'As a result, the whole supervision regime was tightened up, ' recounts Yuen. 'Pre-bores were carried out at every pile location to check that there was sufficient sound rock below the founding level. Interface cores were performed at the base of one third of the piles. Then sonic testing was carried out on every completed pile to check for voids and defects in the pile concrete. Voids were backfilled by pressure grouting - it's not difficult. But it all cost time, and all became subject to contractual claims.'

Underground obstacle course

'Tunnelling and excavation in Hong Kong throw up some absorbing challenges. For a start, there are wide variations in ground conditions - anything from hard, competent rock, through folded, faulted and decomposed rock, to sands, silts and made ground: Hong Kong's shoreline has been extensively reshaped by reclamation. For added interest, the densely populated terrain means there are foundations, other tunnels, cables and pipes to negotiate.

All were encountered on the three construction packages making up West Rail's 9.1km long southern section from Nam Cheong Station, excavated using bored, blasted, and cut and cover tunnel, says CN Fung, project manager for the works.

Dragages, which with Nishimatsu was responsible for the 5.5km long Tai Lam tunnel, used drill and blast to drive through more competent rock and complete what is now Hong Kong's longest transportation tunnel. Drill and blast is Hong Kong's standard excavation technique in good ground, says Fung. Steel arches, rock bolting and extensive grouting were needed in some transitional zones to support loose rock and stem large inward flows of water.

The drill and blast tunnel has a horseshoe section with up and down rail lines separated by a dividing wall.

For the 1.78km Tsing Tsuen tunnel connecting through to Tsuen Wan West, however, an earth pressure balance tunnel boring machine (TBM) was used in Hong Kong for the first time. DragagesZen Pacific suggested using a TBM as an alternative to cut and cover.

'The consultant's design was open cut, but by bringing in a TBM the contractor avoided the hassle of traffic and utilities diversions and the public uproar they would have created, ' says Fung.

'The twin bore TBM drives took us through soft ground with boulders, extensive stretches of reclamation, and completely decomposed ground - we were in earth pressure balance mode quite a bit of the time, although excavation itself was straightforward by TBM standards.

'What made it interesting was using a machine like this for the first time here and proving the technology, ' says Fung. Its rate of progress, averaging 10m a day, was considered such a success that the TBM is now being used by Dragages on the East Rail Lok Ma Chau spur line.

Slimmed down and doubled up

At 13.4km long, West Rail's viaduct is a megastructure composed of endlessly repeated 35m spans. Such a major component of the scheme offered significant opportunities for cost and time saving, delivered by consultant Robert Benaim.

'We started off with a competent but conventional design by Arup and Maunsell, consisting of reinforced concrete columns at 35m centres and a continuous concrete deck supported on bearings, ' says Gregory Yuen, project manager for the northern section. 'Benaim, working for contractor MaedaChun Wo, put forward an alternative design making the columns slimmer but doubling them up, while the deck was divided into discrete 35m spans. As a result we have a series of portal structures placed end-to-end, each performing as an independent structure. Because all thermal movement and deflection under live loading is taken up within the individual portals the design eliminated the need for bearings and expansion joints altogether.'

Support for the viaduct is provided by some 600 pairs of columns between 7m and 28m tall.

The deck is composed of more than 8,600, 2m deep by 2.5m long precast concrete sections. These were fabricated in mainland China and shipped in to Hong Kong by barge. Segments were run out along steel launching gantries and glued and posttensioned to form each span.

Deck sections were tensioned down to the column heads with Macalloy bars.

In just a handful of locations balanced cantilever construction was used to span distances greater than 35m, for example where the viaduct crosses rivers or major roads - 'anywhere it was difficult to lift deck segments from the ground', explains Yuen.

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