Recent foundations work has cleared the way for the historic St Joseph’s Gate building to gain a new lease of life as the latest London des-res.
Tree-lined driveways, mature parkland and an elevated position overlooking London to the south are set to offer the new residents of Berkeley Homes’ latest development a “life of luxury” according to the company’s sales pitch. However, these benefits, designed to attract discerning residents, presented challenges to recent piling work at the St Joseph’s Gate site in Mill Hill for foundations contractor Rock & Alluvium.
St Joseph’s Gate is a former seminary college which is grade II listed and has been empty since 2007 but is now being developed into apartments by Berkeley, with wings added for new-build apartments. Before the college conversion could get underway, piled foundations were needed for the new build elements of the scheme - and this is where Rock & Alluvium’s challenge started.
The challenge came in the form of the site’s topography – because of the sloping nature of the site, there were multiple piling platform levels, which made moving from position to position difficult
Just getting the rig and materials onto the site to complete the two-month long piling contract was an issue due to the tree-lined driveway.
“Some of the trees had tree preservation orders on them but most didn’t, however Berkeley wanted to maintain all the trees to keep the look of the entrance,” explains Rock & Alluvium head of operations Nick Dewey. “It was a tight fit for access but we just managed to get through.”
The company’s choice of rig for the scheme compounded the issue, but the decision proved to be a good one in terms of efficiency. The work was originally scheduled to be carried out using two piling rigs and completed in two stages, but Rock & Alluvium chose to debut its new Soilmec SF135 rig and it has proved to be so efficient that the second rig was not needed.
“We had a few commissioning issues but no more than with any new piling rig and they were quickly sorted,” says Dewey.
“The rig is bigger and has a more powerful rotary table, which has proved to be useful on this project, and the work has gone quickly and smoothly as a result.”
Contiguous piled walls
The scheme involved constructing two contiguous piled walls and a series of load bearing piles into London Clay.
The main challenge for the piling work was installing the 750mm diameter piles for the contiguous piled wall within 1.5m of the old college building, which was built in 1866. According to Dewey, the 14.5m deep piles will allow Berkeley to develop undercroft parking below the new extensions.
The new part of the development is split into four blocks. “We installed two contiguous piled walls for blocks D and G using 50, 750mm diameter piles,” says Dewey.
“We also installed another 75, 450mm diameter contiguous piles to 11.5m depth perpendicular to the existing building for Blocks D and G. For Block F we installed another 95, 450mm diameter contiguous piles to 15m.
“We also installed bearing piles for the blocks. These were a mix of 450mm diameter piles and, to cope with the shear loads, 750mm diameter piles. In total there were 102 of the smaller diameter piles and 39 of the larger sized piled, which were installed to depths of up to 22m,” he adds.
“The ground conditions were good, with gravels over the London Clay. There were a few perched water tables but we installed the piles without using casings.”
Steelwork for the piles was supplied by Lemon Groundwork Solutions. “In total we used 130t of steel - some was prefabricated, but the reinforcement cages for the 750mm diameter piles were tied on site,” explains Dewey.
“The concrete we used was a CT28 35 DC2 mix supplied by Cemex. In total we used 13,000m3 for the scheme.”
Originally Rock & Alluvium had planned to use a piling rig for each size of pile but chose to phase the work to allow the single Soilmec to complete the project without switching tools frequently.
“The challenge came in the form of the site’s topography,” explains Dewey. “Because of the sloping nature of the site, there were multiple piling platform levels, which made moving from position to position more difficult than usual.”
Piling work is now complete and work is underway to convert the college into luxury apartments.