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Basements: Shape shifting solutions

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Complicated basement geometry and challenging neighbourly relationships has called for a novel support solution for a new development in Islington, north London.

Change is coming to Islington, in north London, but the only hint that most people on the main thoroughfare will have about this is the marketing suite for the new Islington Square development.

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Two levels of props have been used for the bottom up construction

But if you wander down a side street, you might see a large lorry squeezing down a seemingly too-small road and disappear behind a hoarding onto a major construction site.

Islington Square is being developed by Sager and centres around the former North London Royal Mail sorting and delivery centre with a scheme that is a mix of renovation and new build. It is the new section – and the way it wraps round the old – that has created a basement construction challenge for contractor Henry Construction Projects.

Sager’s re-imagination of the former Royal Mail site will deliver “homes with character” that range from studio apartments to four bedroom complexes, with some penthouses featuring rooftop swimming pools and a £3.5M price tag. The development will also include 16,000m2 of retail space with shops, restaurants, a six screen Odeon and a gym.

The development is split into three parts with the façade of the Edwardian sorting office being retained in front of a new structure on block A, while block B involves a single storey retained façade, topped with a modern zinc-clad structure Block C, which Henry is currently working on, is a complete new build over a deep basement.

“The basement is 6,000m2 in plan and 14m deep with a 1,200mm thick base slab,” says Henry Construction Projects project manager Noel Reeves. “There is a half mezzanine floor which will be followed by a ground floor slab.”

Piling work for the retaining wall was undertaken by Central Piling in late 2016 and is formed by a contiguous piled wall with 205, 750mm diameter piles at 950mm centres, constructed to 20m depth into the London Clay.

“We undertook piling for other parts of the Islington Square project but the work for block C was the most challenging,” says Central Piling managing director Steve Hadley. “The 14m deep basement was approaching the limit of what is possible with a continuous flight augur (CFA) rig.

“To reduce deviation of the piles, a guide wall was used and some redundancy was built into the pile design, effectively meaning that one in four piles could be ignored in terms of moment and shear capacity.

This meant that if some of the piles were found to be encroaching too far into the excavation they may be trimmed back, but the verticality we achieved meant that was not necessary.”

“The 14m deep basement was approaching the limit of what is possible with a continuous flight augur rig”

Steve Hadley, managing director, Central Piling

Reeves describes the basement as one of the most challenging he has worked on in terms of both the technical and logistical challenges. “It is a busy area with limited access – all deliveries come down a 7m residential street with parked cars on either side,” he says.

The space restrictions are not only limited to site access – the basement is landlocked on all sides except one and is U-shaped in plan. Henry’s neighbours include a theatre, a restaurant and the marketing suite for the Islington Square development. The shape and access challenges, combined with the depth of excavation, meant that bespoke propping was needed so Henry called in ground support specialist Groundforce.

Here, both levels of props are fixed to steel interface posts that span both slabs, creating challenges which resulted in the prop levels being installed in two stages.

Construction of the capping beam itself was challenging, according to Reeves, and called for the underpinning of some of the neighbouring buildings before the beam could be completed and basement propping could begin.

Groundforce first got involved in the project 12 months before the first props were delivered to site.

“Methodology discussions delayed the start of work but were essential to the smooth running of the basement excavation,” says Groundforce general manager Mark Whitmore. “Shape was a real issue.

“The use of slab spanning interface posts was challenging and it is the first time we have used them, but the knowledge gained here could benefit future projects as it has worked well.

“The other challenge was the concentration of props on the corners of the inner part of the U-shape.

There are three mounted very close together on one corner and two on the other.”

Reeves adds that the close location of the steel stubs for the props meant detailed analysis of the reinforcement in the capping beam was vital and called for a high level of accuracy when during the placement and casting process.

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The excavation is u-shaped in plan, which has presented some challenges

“The reinforcement sections had to be lifted in individually and grouted into place before the capping beam was cast,” he says.

In total there are 32 props supporting the excavation – 15 in the upper level and 17 in the lower. Most are 250t capacity, but there are several 500t ones. Some are on 1,200mm diameter supertube, which Whitmore says is the largest proprietary solution on the market.

The prop angles in one area of the bottom level called for fixed connections and the steel corbels were specifically made for the job because of the angles needed to secure the 500t props.

Other props are supported on single pile gallows brackets that have a curved radius backplate which are grouted into place and secured with a minimum of two anchors.

Props were not the only solution for the basement support – there is also a curved section of wall that called for a cast insitu concrete ring beam to be constructed, Reeves says this element was designed by IDH.

At peak, there were six excavators in the basement pit and the final stage of excavation was undertaken with a telescopic clamshell at ground level to allow the base slab construction to start.

Excavation of the basement was completed in August.

First prop removal was expected in mid-September but the final prop is not expected to be removed until January – almost a year after the first was installed.

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