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Piling and foundations: Proving the design

Construction of new student accommodation in Shoreditch is now underway following some complex foundation analysis to demonstrate the safety for nearby infrastructure

tlc cph 7517

tlc cph 7517

The development will fill the whole site and is bounded by roads, other construction work and a rail line

The name Long Street might imply plenty of room but for one redevelopment on the road, that is definitely not the case with the development taking up almost every millimetre of space. The 55m by 35m site will be home to 221 student apartments spread over eight floors that are expected to be ready for the new academic year in 2019.

HG Construction is now working on taking the building out of the ground but delivering the foundations has not been a simple task. Central Piling has had to work round a sewer running under the site and gain permissions to work close to the rail viaduct next to it, as well as cope with the limited space.

HG recently bought a 25% share of Central and this is the first scheme to be delivered under the new partnership. Both Central managing director Steve Hadley and HG project director Andrew Carrington believe that the partnership helped them to manage the challenges of this scheme more effectively.

“As well as being surround by roads and other developments, the western boundary is formed by a brick built rail viaduct and a Thames Water sewer passes north to south across the site,” says Hadley. “The location of the site meant that impact assessments with both Transport for London and Thames Water were needed and called for significant design and planning before the project could move onto site.”

Given the site’s small dimensions and that the development will fill the entire plot, using three different pile sizes and two techniques along with the sequencing created by the permissions made the scheme challenging in terms of planning.

“The finished development will be 15mm within the boundary,” says Carrington.

The tight programme meant that piling work had to start while Central was still working with TfL to secure the Basic Asset Protection Agreement (Bapa) that would allow it to work on the western half of the site.

The Thames Water sewer was located at 10m depth with an access chamber 1.4m wide stepped out from the line of the tunnel and within the site but records were not clear about how the chamber extended to depth and connected with the tunnel.

The sewer is a brick built Victorian structure and HG undertook surveys before starting work on site and will undertake further surveys once the work is completed to ensure there has been no movement.

The 750mm diameter rotary piles are being used to take high loads and bridge them over the sewer.

“The piles are sleeved using slip coated tubing over the top 12m with the annulus backfilled with pea shingle to prevent them from putting any load onto the sewer,” said Hadley. “The initial design was for 40m deep piles with drilling fluid to support the bore during construction. However, there is limited space on site and is an expensive solution, so we worked with Cundall to reduce the factor of safety and undertook pile testing to prove the design.”

Central has recently started to undertake ground investigation work and drilled a 40m deep borehole to look closely at the ground conditions to ensure the alternative design was viable. The company also undertook another to 10m to understand the groundwater regime in the Terrace Gravels to guide the design of the contiguous piled wall.

According to Carrington, there had been four previous ground investigations at the site but they were old and didn’t have the detailed information needed for the pile design on the current development.

tlc cph 7488

tlc cph 7488

Central Piling worked with Cundall and HG Construction to come up with an alternative to the planned 40m deep piles to avoid the need to use bentonite support fluid

“The site is underlain by Terrace Gravels to 8m and then there is London Clay below,” explains Hadley. “The Lambeth Group comes in at about 26m.

“The drilling fluid would have been needed for deep piles as there was concern over whether the bore would stay open as there was water in the Lambeth Group.”

The cost and logistical issues with use of drilling fluid led Central and Cundall to develop a pile design that could end at 26m.

Carrington estimates the extra cost of the longer piles and fluid support needed to construct them was in the region of £100,000.

Central undertook extensive finite element analysis using Plaxis 3D to prove the design and the effect on the sewer before moving to the pile testing.

In total 13 750mm diameter piles have been installed around the sewer. The original design called for six but to 40m depth.

Central has installed 241 piles at the site – 13 rotary 750mm diameter piles for the sewer and a further four for the tower crane, 111 600mm diameter continuous flight auger (CFA) to 14m with full length cages for the contiguous piled wall and the remainder were 450mm diameter CFA load bearing piles to 18m to 23m.

tlc cph 7503

tlc cph 7503

Special permissions were needed to work close to the rail line and over the Thames Water sewer

“The basement will house a cycle store, plant room and lift pit and the contiguous piled wall for the excavation is designed to be a cantilevered, free standing, unpropped structure to aid the construction process,” says Hadley.

Piling started on site in mid-February with the load bearing piles nearest the road as Central had not yet secured the Bapa to allow work nearer the rail line.

“The Bapa called for finite element analysis of the basement excavation to prove that it would not impact on the viaduct and it was also carried out on the ground beam design to prove it would not affect TfL structures or other services,” says Hadley. “We also had to model every single pile and is part of a trend we’re seeing towards greater demand for in-depth analysis.”

With the Bapa in place by mid-March, Central was clear to start piling on the second half of the site and completed the work in mid-April. HG has now taken over the site to bring the building out of the ground.

 

 

 

 

 

 

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