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Piling on the pressure

Contractors had substantially to redesign the foundations and construction methods for Nam Cheong station so they could undercut the competition

Nam Cheong station is probably the most challenging of all of West Rail's 15 civils packages. 'The station is located on either side of the airport rail line and under the West Kowloon Expressway, ' explains Kowloon Canton Railway Corporation's West Rail general manager, construction Jaya Jesudason. 'The main transport artery to the airport runs right through the site and we have to make sure neither is disrupted by the work.'

To make matters worse, a water main, electricity cables and cooling vents for the mass transit railway have also had to be relocated. In the water main's case this has meant jacking new pipe underneath the live Airport Express Line (AEL).

Contractor Balfour Beatty Zen Pacific joint venture has the HK$2.24bn contract to build the station. Work involves building platforms for a new station on the Tung Chung line, which shares track with the Airport Express at this point. This station will also be the first in Hong Kong to have a shared KCRC/MTR concourse where passengers can change between the two systems conveniently.

Lack of height beneath the Expressway viaduct means that concourses linking West Rail and the new Tung Chung line platforms have to be built underneath the live AEL tracks. Excavation work also has to dodge foundations for piers supporting the busy Expressway viaduct.

Winning the job was never going to be easy, especially in Hong Kong's highly price competitive contracting market. To do so the joint venture recruited consultant Robert Benaim Associates and foundations specialist Bachy to interrogate the client's design and come up with cost saving alternatives to undercut the opposition.

The result was a major rethink of the foundation design originally devised for KCRC by consultant Hyder. 'We looked very hard at the whole foundation aspect of the job and came up with a solution which did away with the diaphragm walled perimeter of the station in favour of excavation within sheet piles based on top down construction, ' says BB Zen Pacific project director Rod Ashman.

Work is now taking place within a 27m deep sheet piled wall installed around the station footprint. Sheet piles were driven through sandy reclaimed ground and into the decomposed granite material beneath. The site was then dewatered.

After casting the station floor slab, underground excavation could begin. Support for the sheet pile wall will come from diagonal struts taking horizontal loads from the wall to the ground floor and basement slabs.

The first struts to go in are 16m long and will be inserted as excavations reach 3.9m below ground floor level. Second tier struts are shorter at 10.5m and connect the sheet piling with the basement slab 9.7m below ground. Eventually the sheet piled wall will be beefed up with insitu concrete, allowing the struts to be removed.

Benaim director Robert Cook claims the solution also gives KCRC the advantage of a relatively maintenance free structure. 'Diaphragm walls are not always leak proof. In Hong Kong there have been some large maintenance bills on similar stations where diaphragm walls were preferred.'

Benaim and foundations specialist Bachy also looked at ways of saving money on piling within the station site. Nam Cheong is on reclaimed land so original foundation designs were based around bored piles taken down to bed rock, often as far as 90m underground.

'We decided that bored piles were not the optimum solution, ' says Ashman. Instead the joint venture decided to use shallower friction enhanced barrettes for what is thought to be the first time in Hong Kong.

Friction barrettes are formed in the normal way but the reinforcement cages incorporate pipes through which grout can be pumped into the surrounding ground. The grout creates extra skin friction, eliminating the need for deeper foundations.

This allowed BB Zen Pacific to replace the deepest bored piles, leaving only those which keyed into relatively high level bed rock.

'Friction barrettes go down to between 40m and 50m while end bearing bored piles would have been 90m, ' says Cook. Resulting savings on pile lengths were considerable. Jesudason says the contractor has probably cut the total length of piles inserted into the ground at Nam Cheong by between 15% and 20%.

Redesigning the foundations brought extra challenges for the joint venture. 'In putting forward an alternative we had to redesign the whole job, ' says Ashman. This meant getting the new design approved by the Hong Kong buildings department. None of this was allowed to affect the two year civils programme, as delays attract liquidated damages of HK$4.3M a day up to a maximum of 10% of the contract sum.

It took five months from the time BB Zen Pacific won the contract in October 1999 to get the station redesigned and approved. Piling started in earnest last March, and is now nearing the end of a tight 12 month programme, with more than 75% of the barrettes and bored piles now completed.

As construction moves towards the end of its first year, contractors have begun the riskiest part of the work - excavation beneath the live Airport Express tracks and between the West Kowloon Expressway piers.

This has involved instrumenting viaduct piers and track to ensure that even the slightest movement is picked up.

For this work, BB Zen Pacific had the advantage of being able to draw on Benaim's experience of working on Airport Express.

Its engineers knew the AEL base slab had been cast with connections for the future West Rail station and used this knowledge in designing the construction sequence for Nam Cheong.

Benaim suggested the contractor use the connections to temporarily key the AEL slab into the station's ground floor slab, using it as a support during excavations underneath. After excavation the airport line slab will be disconnected with loads transferred to internal walls. It is hoped this will reduce vibration damage caused by Airport Express trains.

At the moment Nam Cheong is approaching peak construction.

Around 1,000 people are working on the site and this is expected to rise to some 1,500 over the next few months.

Work has progressed rapidly, putting the four year combined civils and electrical and mechanical works contract on schedule for completion in 2003. This is important as it was put on the West Rail critical path. 'Work is progressing well, ' says KCRC's Jesudason. 'In terms of engineering challenges Nam Cheong was one of our key concerns.'

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