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Piling into Shepherd's Bush tube station

Spotlight - Secant piles are aiding the construction of new lift shafts to help West London's Shepherd's Bush tube station cope with an influx of passengers.

Shepherd's Bush in West London has never boasted the upmarket and prestigious reputation of its neighbouring hotspots Holland Park and Chiswick.

But regeneration plans for a high prole shopping centre based at White City look set to transform the district. With the construction work well underway, London Underground plans to rejuvenate the area's underground stations including the Central Line station, which lies at the southern entrance of the development.

A ajor development is underway to help the station cope with the 45,000 commuters who are expected to use it each day. The project involves a number of improvements such as the remodelling of the ticket hall and the construction of new lifts. New passageways and staircases will also increase the station's capacity.

Ground engineering specialist Bachy Soletanche was brought in by principal contractor Morgan Est Rail to construct two shafts for the new lifts.

The site is sandwiched between Shepherd's Bush Central Line underground station, the bus station and the busy city centre roads. Bachy Soletanche site supervisor Tom Stewart explains:

'Careful co-ordination was essential to the smooth running of the project, as both sides of the site were tight and surrounded by traffic and pedestrians most of the working day.

'Due to the site's location, the secant wall piling method was utilised, as it's an ideal solution to busy, urban locations. Not only does it provide minimal vibration and low noise levels but it also avoids the risk of construction-induced settlements to neighbouring structures.' The £1M plus piling contract was broken into two projects, the east shaft and the west shaft. Both shafts use the secant wall piling method, incorporating 900mm large diameter auger (LDA) piles, drilled to a depth of 19m for the east shaft, and 24m for the west shaft. As the west shaft is larger, 68 piles were constructed compared to the 56 installed at the east shaft.

Stewart says the LDA process is more time-consuming than other methods such as continuous ight auger (CFA) piling but it was chosen for its accuracy. He explained the piles needed 'a drilling tolerance of better than 1 in 200 verticality, a 300mm cut on the female piles, and required a concrete strength of 30n/mm 2 in the female piles.' The accuracy of the LDA with its shorter penetration could cope with the task more easily.

'Also, some of the piles required further strengthening before tunnelling work could begin between the two shafts, meaning 13 of the female piles from each shaft had full length steel beams inserted into the piles before concreting, ' Stewart says.

The female piles are installed before the male piles in the secant wall piling process. This means accuracy is essential when the male piles are nally bored into the two adjoining female piles. As a result, the position of the beams gave very little room for error as the male piles were cut full depth and with less than 50mm between the casings and the beams.

Choosing a heavy duty rig saved the project from delays after it hit existing piles on the east shaft only after work had already begun.

The piles had to be cored out to the full 19m depth before the contractor could install the new piles. Stewart says: 'Discovering the existing piles, and having to stop works to core them out was a lengthy process, but as the heavy duty rig was already on site the coring process allowed the shaft to be installed in the original position without any design changes.' Having completed the piling in March Bachy Soletanche has now been commissioned to undertake ground consolidation work to allow the tunnelling work to take place.

Trials for grouting work are being assessed and once complete a consolidation method will be decided upon.

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