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A game of two halves

West Rail Construction

Construction of West Rail has been tackled in two distinct halves. While the 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 Hong Kong's 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.

'Structures needed to be wide enough for large objects to pass and to allow plant access for dredging, ' says Yuen. Modelling carried out by consultant Binnie Black & Veatch resulted in the channel being widened by an average of 5m-12m to compensate for construction within it.

The invert was also lined with concrete to improve flow and make maintenance easier. Meanwhile, the nullah was partially dammed in its lower reach to hold back tides up to 2.5m high.

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

There are year round flows in the nullah and 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 executed 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 off guard in January 2000. A flash flood overtopped the blockwork coffer dam that had been put up around a piling rig down in the channel 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.'

Although construction had been halted and plant removed from the nullah, flooding in April 2000 had wider ramifications when it spilled over surrounding land, Yuen adds. 'Nearby villages were flooded to depths of up to 600mm and people were up in arms with West Rail because they believed we had caused it.

While we weren't completely innocent, we found that downstream there was another contractor who had built a coffer dam across the nullah and hadn't removed it.' A huge public charm offensive was needed to win round the dampened communities, though.

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 pounding level to safeguard against settlement or collapse, ' says Yuen. 'It takes a couple of months to install a single pile.'

At Long Ping station there are 44 piles, a third of which are long, equating to 32 rig months of drilling; and Yuen Long station had 219 piles, half of which are long. A challenging job turned into something of a nightmare when piles hit cavity after cavity - the longest was over 120m.

'To cope with the situation 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.'

And KCRC also 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 HK$100M (US$13M) 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. 'Prebores were carried out at every pile location to check that there was sufficient sound rock without cavities below the founding level.

Interface cores were performed at the base of one third of the piles to give assurance that a sound interface was being achieved between the base of the piles and bedrock. Then sonic testing was carried out on every completed pile to check for voids and defects in the pile concrete - with long, 3m diameter piles, different parts of the pile are subject to different ground water flows, and concrete can be washed into surrounding soils.

This has always been the case, but of course, the more you test the more faults you find. Voids were backfilled by pressure grouting - it's not difficult. But it all cost time, and all became subject to contractual claims.'

Slimmed down and doubled up

At 13.4km long, West Rail's viaduct is a mega-structure 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 Maeda-Chun 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 post-tensioned 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.

Siu Hong squeeze

'Of all the parts that make up the northern section, Siu Hong Station has been the most difficult, ' Yuen says. 'It is overlooked by an estate housing 18,000 people - in fact the whole of the surrounding area is dominated by high rise blocks. It is built in a nullah [drainage channel].

And the station is not served by any public roads, which means we've had to build two new passenger transport interchanges in the nullah as well. Finally, the station's in an existing light rail triangle - we expect 60% of passengers at Siu Hong will come from light rail.'

To make it easy to lift materials for the station into the light rail triangle a largely steel design was chosen - prefabricated elements could be broken down into lightweight sections and bolted or welded together insitu.

However, any construction within 2.5m of the light rail line was restricted to night time possessions between 1.30am and 5am. Hammering bolts home as elements were lifted into place put the construction team in direct conflict with sleeping residents.

With support from KCRC's senior management, Yuen was able to secure eight exemptions from night time noise limits in order to get the station's major structural elements into place.

Crane operators and banksmen had a tough challenge: Lifting over the light rail line was allowed only so long as its overhead power cable and track were protected from harm. A railmounted shield was installed to cover the line, while lifts were limited to 2t and had to be carried out within maximum heights. 'Operating parameters were extremely tight, ' Yuen sums up.

The upshot is that, of all nine stations on West Rail, Siu Hong took the longest to build and was the last to be completed. During electrification, signalling and station fit out phases, Yuen has had to overlap work that would normally be carried out discretely to achieve the opening schedule.

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 a combination of 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.The shorter Kwai Tsing tunnels, undertaken by Dragages-Zen Pacific joint venture, consist of 1.7km of drill and blast in good rock with a high overburden. 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, but this was nothing out of the ordinary, says Fung. 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.

Dragages-Zen 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.'

Grouting was needed for the construction of three cross passages in soil, to ensure ground stability during excavation.

'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.

Stations offer their own challenges Relatively, construction of the southern section's three stations at Nam Cheong, Mei Foo and Tsuen Wan West was more complex than the tunnelling, says CN Fung, project manager for the works.

'Tsuen Wan West is built in made land - in fact part of the station footprint had to be reclaimed before construction could start. The site contains a number of sea walls that we had to excavate through. We also had to remove or relocate ferry piers, roads, footpaths and utilities.'

For good measure, three major drainage culverts with sections ranging from 10m 2to26m 2had to be diverted around the site, while extensive consolidation grouting was required to minimise risk of settlement in the piers of a highway viaduct crossing the Tsuen Wan West station site.

The station itself is built on two levels, half below and half above ground. Large diameter bored pile and diaphragm wall foundations are founded on bedrock and are big enough to allow high rise towers to be built on the station superstructure in future.

Tsuen Wan West station has been built through new and existing fill with piles and diaphragm wall foundations founded in rock at depths of up to 30m. The platform slab is roughly 20m below ground level, and the piled foundations play an important role in resisting uplift forces. The station was built bottom up, with the diaphragm walls supported by heavy bracing.

There were concerns during construction for the stability of the adjacent Tsuen Wan elevated bypass, which was originally built as a marine structure on driven steel piles.

Contingency plans were laid to jack the structure up if settlement was detected, reports Fung.

'Nam Cheong is effectively two stations rolled into one, ' says West Rail director Ian Thoms. Here, West Rail crosses the MTR Tung Chung line. 'The only obvious thing to do was build an interchange, so in the course of building a station for KCRC, MTR got one too - what a freebie!' he exclaims.

Nam Cheong Station is built under the MTR Airport Express Line, which is at grade, with a common concourse for both lines beneath. It is a complex configuration in its own right, but the Airport Express train also whizzes through Nam Cheong on a ballasted trackslab. For maximum complexity the site is in recently reclaimed ground.

'The reclamation was only 10 years old, so relatively little consolidation had taken place.

Ground settlement was a major concern, ' says Fung.

Construction was guaranteed to speed up the consolidation process: 'We were installing bored and percussion piles; there would be a lot of heavy plant moving around. There was also a major issue with dewatering, ' says Fung. This caused concern, as the expressway viaduct is founded on friction piles.

Dewatering was controlled and a system of 24 hour, real time settlement monitoring installed - claimed to be another first for Hong Kong. As soon as construction was completed the ground was recharged with water to halt movement - a total of 75mm settlement was measured.

Mei Foo Station is 'like a vertical sandwich', Fung jokes. It sits directly above MTR tunnels and underneath an expressway viaduct. It is hemmed in to either side by the viaduct's piers, by the foundations of high rise apartment blocks and by drainage culverts. 'Its alignment is tight; it had to be built to millimetre tolerances, ' Fung sums up. The constraints of the site have given Mei Foo curved platforms, on the minimum possible radius.

Construction was open cut, and extensive measures were taken to prevent settlement of the structures jostling up against it.

'We piled extensively over the footprint of the station to support the station structure itself, and constructed the station within a series of very small cofferdams, so that the working area was isolated as far as possible from the surrounding ground, ' Fung reports.

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