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Bridging the gaps

Construction of West Rail's 13.4km viaduct is storming ahead despite numerous obstacles.

Travel around the western New Territories by car and you cannot help but notice piers and deck sections spreading across the skyline as West Rail's 13.4km viaduct starts to take shape. The £185.8M viaduct is the longest bridge of its kind in Hong Kong, taking West Rail all the way from the Kam Tin.

'It is on viaduct from the depot to Tuen Mun because the North West New Territories are susceptible to flooding, ' says Kowloon Canton Railway Corporation West Rail's general manager, construction Jaya Jesudason. 'From an operational point of view it is best to have the track on viaduct - an embankment would have created problems by retaining flood water.'

Originally KCRC had planned to let the viaduct as two separate contracts. The two contracts were finally awarded to MaedaChun Wo joint venture which came up with a cost saving alternative design.

Consultant Robert Benaim & Associates developed the alternative design to the one originally developed for KCRC by Arup as part of a value engineering exercise for Maeda-Chun Wo. Benaim designed out cost while allowing the viaduct to conform to the Hong Kong Environmental Protection Department's insistence that noise created by trains be below 64dbA within 25m of the structure.

Maeda-Chun Wo asked Benaim to look for a way of cutting down the amount of concrete used, so it could keep its tender price down. Benaim decided it could do this by narrowing the horizontal width of the box girder, and making the web thinner, after consulting Matthew Harrison of Cranfield University School of Mechanical Engineering and Southampton University's Institute of Sound & Vibration Research.

'To reduce the mass, we had to get the webs under the rubber bearings beneath the floating track slab, ' says Robert Benaim & Associates managing director Jeremy Cooper. Doing this is expected to reduce sound generated by trains running over the box girder which behaves like a hollow drum. It also enabled the contractor to reduce total amount of concrete needed for the structure from 360,000m 3to250,000m 3.Benaim also removed bearings at the top of each pier making the most common spans into monolithic longitudonal portals.

These rest on pairs of 800mm thick columns forming spans of between 30m and 35m.

'This reduces the bending moment at the underside of the pilecap caused by longitudonal loads, compared with the moment for a conventional pile with bearings, which acts as a simple cantelever, ' says Cooper.

Doing this has also halved the number of piles supporting the structure. 'In the conforming design, a typical pilecap has four 1.5m diameter piles. In the alternative design, a typical pile cap has only two 1.8m diameter piles, ' says Cooper.

In addition, the alternative design relies more heavily on the interaction between the piles and the ground to resist horizontal forces from the trains. Loads carried by the foundations were also reduced as a result of the decision to reduce the thickness of the box girder.

A decision to split viaduct construction between eight work fronts helped contractor Maeda-Chun Wo push ahead rapidly with deck erection last year following contract award in July 1999. So far it has erected 175 of the 600 precast, post tensioned, glued segmental spans.

The viaduct crosses highways, concrete lined drainage channels or nullahs and a river.

Piers also have to be inserted into several nullahs and when complete must not be allowed to obstruct storm water flows unduly.

'With the four pier solution there would have been more head loss in the nullah or river, ' says KCRC West Rail's viaduct construction manager Samuel Lo. To resolve the problem, dolphins are proposed to reduce the head loss.

So it can keep within the tight 27 month construction programme, Maeda-Chun Wo is working from eight points along the viaduct route. 'Rather than going for highly sophisticated launching equipment, our deck erection sub contractor opted for seven sets of less sophisticated underslung girders for ease of construction and relocation, ' says Maeda-Chun Wo project director Hugh Boyd. Deck erection specialist VSL is also using three erection methods.

Most of the viaduct is being constructed using 85m long, 100t self launching steel girders which cantilever off support brackets on the piers to support deck segments which are lifted onto them by crane.

At Kam Sheung Road where the viaduct incorporates a five track turnout, the contractor has been using a longitudinal steel support beam, slung between piers, using a crane to lift in segments. This can be shifted sideways to support erection of parallel decks.

Support girders are dispensed with for the longer, balanced cantilever spans across major obstacles like the Kam Tin river and Yuen Long bypass where a maximum of 80m spans are needed. Instead, precast sections are craned into position, glued and stressed in a symmetrical sequence.

When progress has been at its quickest, the contractor has been putting up one of the 30m spans every three days. 'This allows one day for lifting, one for gluing and temporary prestressing and one day for permanent prestress, load transfer and launch, ' says Boyd.

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