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Bombs to Bronze: United House delivers delicate development

Mammoth demolition, archaeology, explosive ordnance logistics and vibration super-sensitivity all collided on a major inner-London residential project.

Project: The Exchange
Client: Notting Hill Housing
Contract value: £30m
Region: London
Main contractor: United House
Architect: PCKO Architects
Concrete frame subcontractor: Henry Construction Projects
Start date: May 2012
Completion date: November 2014

Building on tight and historic London sites is always challenging – but even more so when you are next to a large electricity substation.

United House has encountered precisely this on a project to build five residential blocks in Southwark.

The Exchange in Bermondsey Spa, for client Notting Hill Housing, will consist of 205 units from studio flats to townhouses and include ground-floor retail space.

It has been designed by architects PCKO to meet the requirements of Code for Sustainable Homes Level 4 and also to fit with historical buildings surrounding it.

Mammoth demolition

But before construction work could even begin, there was a huge five-month demolition phase to contend with.

The site was historically used as a tannery and sits in an area of London that was heavily bombed during the Second World War.

“We had to carry out the unexploded ordnance surveys. Only then could we disturb the ground”

David Martin, United House

When the project team took hold of the site, it contained post-war low-rise flats, depots and Southwark Council buildings. Everything had to be demolished to the underside of the ground-floor slab to allow archaeologists access.

A full report was written and pottery discovered dating back to the Bronze Age. But this was just the start.

Unexploded ordnance logistics

“Once we had clearance from the archaeologists for each section, we had to carry out the unexploded ordnance surveys,” says United House project manager David Martin. “Only then could we disturb the ground.”

Scheduling of work was affected by various sectional clearances, the order of break clauses in the development contract and the sensitivity of demolition.

“It took longer than anticipated,” Mr Martin says. “The site got quite complicated in terms of where we could work.”

Indeed, the frame for Block E was two floors up while existing buildings remained part-dismantled elsewhere on the site.

One particularly challenging area of demolition was where an existing building was supporting an external wall of the primary electricity substation serving much of south London.

Just to heighten the tension of this job, it took place during the 2012 Olympics. Site workers cut pockets in the wall of the substation and cast stiffeners in to allow steel columns to be put in to retain the wall.

Anti-vibration techniques

Much of the structure that was previously holding up the substation was then taken down by hand, using delicate techniques including diamond-chip drilling.

“We had to monitor vibrations and report to UK Power Networks daily,” Mr Martin says. “The fire system in the substation is levers and pulleys, which can be triggered by vibrations.”

“We had to stitch in their façade to their frame before removing our building”

David Martin, United House

A similarly sensitive task was taking down a building that was holding up a new development next door.

“We had to stitch in their façade to their frame before removing our building,” Mr Martin says. This was done using chemical fixing of metal rods and steel plates.

Even as demolition slowly took shape, there was a huge amount of work to be done under the ground, with a complex web of services criss-crossing the site from the substation and various previous buildings.

Electricity proximity

“There was a scanning exercise and also trench digging as we mapped out what was there, what could come out and what could stay,” Mr Martin explains.

“It was a significant volume of services. Some carried up to 10,000 volts, which tends to focus your attention”

David Martin, United House

“I’ve never worked in such close proximity to a substation like this – it was a significant volume of services. Some carried up to 10,000 volts, which tends to focus your attention,” he says.

All obstructions were removed and the site cleared to a depth of 3 m before foundations could be put in.

Hyper sensitivity

CFA piling was chosen because of the proximity of neighbouring properties to the site. “This way the risk of damage is limited to adjacent foundations, underground utilities and ground displacement,” Mr Martin says.

“If you stamped your foot alongside the measuring equipment it jumped up to 7 or 8 mm/sec”

David Martin, United House

Groundworks close to the substation were particularly sensitive. Less than 7 mm of vibration per second was allowed within 6 m of the substation. This was not easy to achieve.

“If you stamped your foot alongside the measuring equipment it jumped up to 7 or 8 [mm/sec],” Mr Martin says.

“We had to select appropriate methods and test them before we could do it for real. We ended up doing a lot more works by hand than we would have done.

“We dug out the last foot for foundations for the supporting steelwork by hand.”

Once foundations were in, services had to be co-ordinated. This job was on the critical path at an early stage due, in a roundabout way, to the site’s proximity to the Thames.

“We needed to consider flood risk, so permeable paving was used to contain water on site, as we could not have it running off site into the sewers,” says PCKO project architect Philip Harvey.

Although permeable paving is not currently adopted by the Highways Agency, the roads at The Exchange are being built to allow adoption if this becomes possible at a later date.

Services therefore had to be diverted away from the roads, pushing them up against the buildings, meaning they had to be in place early to allow scaffolding to go up for the frame.

“It was a case of getting everything ducted, getting an infrastructure in place early on and getting as many of the services in place as we could to let the scaffolding get up off the top,” Mr Martin explains.

“There was a lot of pressure at the front end on something that you would quite often do later on in the job. It was almost treated as a civils approach,” he says.

Frame complexities

Finally the frames could start: reinforced concrete was used as the most cost-effective way of meeting requirements. Traditional build was considered for the smaller blocks, but United House decided to take a uniform approach with concrete throughout.

Substantial insulation was required to meet the requirements of Code for Sustainable Homes Level 4.

Sealed units were cast-in for all soil and rainwater pipe penetrations, and 180 mm PIR insulation used in the roofs. The balcony slabs also had to be cranked to allow level thresholds to balconies without affecting storey heights post-planning.

All this made for a complicated concrete pour, which took 12 to 20 weeks per block and was carried out by Henry Construction Projects.

“It’s a case of keeping resources high on site and ensuring trade continuity. That’s the key to speed – keeping a flow of work for people”

David Martin, United House

Block D had to be built in two halves, as a secondary substation within its footprint could not be removed until a replacement had been created and connected in Block B.

As the frames went up, the project team focused on getting them watertight to allow the internal works to begin and start the drive towards completion.

One innovation was to install windows ahead of brickwork cladding. Elsewhere the new secondary substation was built in a concrete casing to allow it to be operational before Block B was watertight.

“There was a lot of discussion about detail,” Mr Harvey says. Liaison with the council was important to ensure relevant façades will match the nearby Bermondsey Library and the Municipal Building.

Biodiverse roofs, electric vehicle charging bays and solar panels will add to the sustainability credentials of the scheme.

Block E is due for completion in January, with the show units open to the public as Construction News visited last month. Works are continuing as anticipated on the other four blocks, with final handover expected in autumn 2014.

Soon this former tannery will be full of residents in new homes, who will mostly be blissfully unaware of the incredible complexities of this project on their short walks to London Bridge.

The need for speed

As well as sensitivity, speed has been of the essence on this scheme.

“Every tender interview you go to, one of the biggest drivers for the client is how quickly you can build the project,” Mr Martin says. “We are constantly looking at quicker ways of doing things.”

From the beginning, works were sequenced to get to the finish as efficiently as possible, from the heavy ducting of services to the early installation of windows.

Working hours are limited on the site, as it is in such a dense residential area.

“We are restricted to between 8am and 6pm Monday to Friday and 8am to 1pm at the weekend,” Mr Martin says.

“It’s a case of using that to the full, keeping resources high on site and ensuring trade continuity. That’s the key to speed – keeping a flow of work for people.”


Solid foundations

Demolition and foundations work took up a huge chunk of project architect Philip Harvey’s planning for the scheme.

“The first six months of design meetings were focused on getting the buildings down, on the parting walls and on the services,” he says.

Demolishing a wall being used to retain the substation was particularly sensitive.

“There was an initial proposal to use brick buttresses to hold up the substation wall,” he explains. “But they were going to be too big and encroach onto our site too much.

“Then the engineer suggested a steel goalpost arrangement, but we were not going to be able to weather the horizontal beam properly, so it would have looked awful after a couple of years.”

More vertical columns were chosen rather than the horizontal beams, and they were designed to allow planting of a green wall to improve views for residents.

“This worked better for the spec of the building,” Mr Harvey says.

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