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Piling: West End starring role

ilona rose house

Bachy Soletanche’s current run in the West End involves multiple piling techniques and some challenging logistics. GE visited site to find out more.

In the heart of London’s popular tourist destination of the West End, a new development is getting ready to take centre stage as it comes out of the ground next year. Reaching that milestone on the Ilona Rose House site has called for some major piling work that has been undertaken in close proximity to theatres, a church and the famous Foyles bookshop.

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Doing the razzle dazzle: The piling work is being undertaken in heart of the West End

Soho Estates is developing the site on Charing Cross Road, which was once part of Foyles, as a new building which will house shops, restaurants and office space for creative industries. Cantillon is the main contractor for the below ground works with Bachy Soletanche undertaking the £4.4M ground engineering elements.

The new development wraps around a Grade II listed building at 14 Greek Street, which will be protected and restored, and a façade next to the building is also being retained to maintain the street scene on Greek Street. The development will create a new “oasis” on Manette Street, which connects Charing Cross Road and Greek Street, and new public spaces within courtyards in the new development.

Cantillon started on the demolition in spring last year and will take the building to the first floor level but the contract for the remaining eight storeys has not yet been awarded. Bachy moved onto site in February 2018 and is on track to complete the foundations this autumn.

The ground engineering solution for the new development has called for a number of different techniques, including secant piled walls, bearing piles including those that will double as geothermal piles and tower crane base support and minipiles in low headroom areas.

“Ground conditions at the site are formed by London Clay to 33m but there is a 1m to 1.5m thick sand layer at that level, which overlays another 10m of soft London Clay before the Thanet Sand is encountered at around 44m below ground level,” says Bachy Soletanche project manager Rob Cunningham.

According to Cunningham, it is not the ground conditions that have been a challenge on the site but the close proximity to other buildings and the Northern Line running tunnels.

There is another, rather unusual, logistical challenge that the site team could yet face – a funeral procession through the site. The chapel of House of St Barnabas on Manette Street is so tight against the hoarding that in the event of a funeral, the hearse would have to pull into the site gates and the coffin be carried back out of the site’s pedestrian entrance in order to make it through the chapel’s gates.

site view

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The site is one the largest to undergo piling in central London this year

Fortunately, so far, Bachy has been able to focus solely on the ground engineering challenges.

There are two parts to the secant piled wall which will support the 18m deep, four storey basement – the main wall defines the perimeter of the site but there is also a second inner wall close to the Charing Cross Road side of the site.

“The double wall on the eastern side is due to the close proximity of the Northern Line tube tunnels, which run just outside the site boundary and are 33m below ground level,” says Cunningham.

To span over the Northern Line, the secant piles in this part of the site are 17m long, whereas the 880mm diameter piles are being bored to 23.5m over the rest of the of the site.

In total there are 405 secant piles being constructed and the total linear length of the secant piled wall is 235m.

Work on the bearing piles started just after the secant wall work got underway. In total there are 121 bearing piles and vary in diameter depending on location within the development.

The pile design was undertaken by Bachy but the locations of the piles was fixed by Tier Consult.

“The total loading of the structure has not changed but there have been a few changes to the distribution of the loads, which has called for pile sizes to be changed,” explains Cunningham.

In total, there will be 25 600mm diameter, 76 750mm diameter and 20 900mm diameter piles that are being constructed to an average depth of 52.5m with polymer support.

Around 80% of the bearing piles will be geothermal ones to cool and heat the building and the pipework on the piles will be linked to four 185m boreholes drilled by ground source heating specialist G-Core. The pipework loops were spliced onto the pile reinforcement cages by G-Core staff working closely with Bachy’s on site team.

Installing the loops was a time-consuming process, especially with the 23m long reinforcement cages being delivered to site by F Brazil in three pieces before being assembled on site.

“To speed up the installation process, we drilled a “rat hole” on site that allowed the reinforcement cage and loop assembly work to be undertaken while each pile was constructed,” says Cunningham.

The assembly was further complicated by the different cage designs depending on the location of the pile – there were three type of cage design for the secant wall and 17 for the bearing piles depending on the connections for the floor slabs.

“The top of the bearing piles features an 18m dummy section to account for where the basement will be excavated out,” says Cunningham.

Getting plant and concrete onto site has been a challenge due to access that minimises the length of deliveries to 15m, which is partly what drove the three-part cage design for the bearing piles.

For the piling rig, it meant that special dispensation was needed from Westminster City Council and Camden Council to track the rig into site from Charing Cross Road. Getting consent was one issue, but crowd control to bring the rig across pavements in the heart of the West End was another.

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The minipile work was focused in the area behind the retained façade on Greek Street where the structural steel supporting the brick structure restricted headroom

The one way system around the site and heavy traffic has huge potential to delay Hanson’s concrete deliveries. The recent hot weather has further complicated the mix design and resulted in a change in additives to ensure both flow and workability.

Bachy is using three different concrete mixes – a P300 for the female secant piles and a C32/40 for the male to form a hard-firm wall and a C28/35 mix for the bearing piles.

Nonetheless, Bachy’s on site concrete testing facility has helped to ensure the quality is being maintained despite the climatic and delivery challenges.

The minipile work was focused in the area behind the retained façade on Greek Street where the structural steel supporting the brick structure restricted headroom.

The minipiles were undertaken by Bachy’s minipiling division Specialist Geotechnics in June. In total 34 450mm diameter minipiles were installed to depths of up to 25m with 2m centres.

Access for the rig to the minipiling site was across the main site but difference in ground level between the two parts of the site called for the minipiling rig to be craned into position. This was not an easy task as the structural steel supports meant the gap to lift the rig over the secant pile wall was just 3m.

While at 40m by 70m, the site is probably one of the largest sites in London to undergo piling work this year, the use of a Bauer BG28/36 and BG39 to deliver the work, along with managing the minipiling work has been complex logistically. The aim to undertake have two days been female and male secant pile construction has come under pressure to be maintained due to the faster curing rate resulting from the hot summer.

Although the end of piling is in sight, the logistics are not getting any simpler and Bachy will soon be sharing the site with WJ Groundwater which is undertaking installation of the dewatering wells for the excavation phase.

Bachy expects to complete its work by the end of September and Cantillon is due to finish its element of the work next year to clear the way for the above ground works.

 

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