Contaminated ground and gas venting added complications to work on Derby’s Multi-sports Arena.
Project: Derby Multi-sports Arena
Client: Derby City Council
Contract value: £28m
Region: East Midlands
Main contractor: Bowmer & Kirkland
Geoenvironmental consultant: Idom Merebrook
Piling subcontractor: BBGE
Steelwork subcontractor: Billington Structures
Derby is steeped in history as a manufacturing and engineering powerhouse and the automotive, railway and aeronautical engineering industries have all helped put their mark on this patch of the East Midlands.
Now, a section of former railway sidings and waste tip to the east of the city centre is helping build its profile.
In the mid-1990s, huge amounts of money were invested in regenerating that industrial wasteland, reinventing it as business park that would attract the next wave of investment to the city.
Anchored by Derby County Football Club, the Pride Park business centre has become a byword for successful regeneration.
Now Derby City Council is looking to reinforce that success by developing an iconic multi-use sports centre alongside the existing Pride Park stadium.
Being built by locally based contractor Bowmer & Kirkland, the facility is one of the UK’s first Olympic legacy projects and will include a 250 m indoor cycling velodrome which can be converted into a 5,000-capacity concert arena.
The £28m scheme is supported by a slice of £3m of funding granted to Derby City Council by Sport England through its Iconic Facilities Olympic and Paralympic Legacy Fund.
As a local contractor, it was a project Bowmer & Kirkland was keen to win.
“There’s no doubt that it is going to be a major addition to the city and we wanted to be part of its delivery,” says Bowmer & Kirkland regional director Neil Brook.
Industrial dumping site
Even before the B&K team had tendered for the design-and-build scheme, it put in a huge amount of upfront work on developing a strategy for the full investigation of potential contamination of the site.
“We knew that work carried out during the original regeneration had included the installation of a bentonite cut-off barrier wall to help manage water flow and that landfill gases were being vented”
Neil Brook, Bowmer & Kirkland
For many years the site had been used for industrial dumping, and although some remediation work had been carried out during the 1990s, the team understood there were still concerns over potential contaminants in the ground.
“We knew that work carried out during the original regeneration had included the installation of a bentonite cut-off barrier wall to help manage water flow and that landfill gases were being vented,” Mr Brook says.
“We put in a lot of effort to help mitigate the risk and I think the client was impressed by that.”
The B&K team had been looking at the project for the best part of three months before it was awarded the contract in September 2012, during which time it worked on those measures to reduce risk as far as possible.
Derbyshire-based consultant Idom Merebrook had worked on massive regeneration projects on industrial waste sites – including the Olympic Park – in the past and was happy to lend a hand on working around the legacy of industrial pollutants to provide Derby with a healthier legacy.
“There was the potential for the site to carry significant constraints, as well as the issue of working within the zone of influence of an area of active groundwater containment and landfill gas generation,” says Idom Merebrook director Simon Edwards.
This potential for contamination and the management of any landfill gases across the site has featured heavily in the project’s final design.
Piling through contaminated ground
A forest of almost 1,300, 235 sq mm precast concrete piles have been driven some 8 to 12 m through the landfill layer and into the Mercia Mudstone bedrock beneath.
These foundations, installed by Balfour Beatty Ground Engineering, were chosen to negate any possibility of contaminated arisings being brought to the surface as they were installed.
They also enable the ground beam and pile caps to support the main structure without influencing the existing landfill layer.
“Using traditional [drainage] systems, we would be looking at some very deep excavations by the time we had got across the site”
Neil Brook, Bowmer & Kirkland
The drainage design has been rejigged to reduce the amount of disturbance caused to the existing layer of landfill during construction.
Here the team is installing a series of low-impact shallow filter and pumped drains, rather than the traditional gravity system initially intended.
“Using traditional systems, we would be looking at some very deep excavations by the time we had got across the site,” Mr Brook says.
“We wanted to limit our risk, so we made the decision to keep everything shallow at around 2 m deep and use a pumped system. It has also proven very cost-effective.”
But the nature of the site means it is not just water mitigation measures that need to be considered.
By building on a landfill site, the team has had to incorporate measures for dealing with the possibility of gases leaching through the 200 mm-thick reinforced concrete slab and into the building itself.
Clean air blanket
The Idom Merebrook team has designed a low-energy, clean air blanket system, which injects clean air through the slab and into the 6F2 stone sub-base layers beneath.
“There is a membrane beneath the slab too with sensors in the slab to monitor methane and carbon dioxide gas build-up levels. It is a very good system”
Simon Edwards, Idom Merebrook
By generating this positive pressure through the stone, any potential contaminants are diluted and able to vent freely.
“There is a membrane beneath the slab too, with sensors in the slab to monitor methane and carbon dioxide gas build-up levels,” Mr Edwards says. “It is a very good system.”
With less than a year to go before the completed building is handed over to the client, the site team is focused on making the structure weathertight and fully clad (see box below) so that the citizens of Derby have another iconic building in which they can take pride.
Steel frame catches the eye
Derby’s latest addition to its sporting heritage is being partly bankrolled by Sport England’s Iconic Facilities Fund.
Architect FaulknerBrowns has certainly made sure the building will deliver on the iconic front.
Its futuristic diamond shape features ‘eyelid’ windows at each end, with a steel skeleton supporting the inner frame for the four-floor structure.
The reception area at level one will include access to the changing areas and sports deck itself, while at levels two and three there will be gym and fitness facilities as well as multifunction rooms and dance studios. Level four is for plant.
Installed by Yorkshire-based specialist contractor Billington Structures, the roof trusses which span across the main velodrome and sports hall are 84 m long and 5 m deep.
These are pre-cambered and form the framework onto which the standing seam roof – rolled on site with the longest section measuring 114 m – is fastened.
The walls are also curved, both in plan and in elevation, making the job of setting out the support nodes for the aluminium shingle cladding painstakingly time-consuming.
“Every single point has to be co-ordinated and measured with the total station,” says B&K assistant quantity surveyor James Lawrence.
“It takes a long time, but it has to be spot on if we are to get the correct finish.”
To meet the required design, the straight lines of steel frame have been installed with a secondary frame providing the curve of the structure fixed to this main structural frame.
The secondary frame stands off the main one by as much as 600 mm.
Corrugated steel sheets are fixed to this secondary frame to provide the curve, before a vapour barrier is fitted beneath insulation panels and a plywood top sheet.
Eventually, the aluminium shingles are fitted to provide a total wall cladding thickness of 288 mm. The roof thickness stands at 488 mm.
With sheet curtain walling and faceted glazing panels across the ‘eyelid’ windows, the eventual BREEAM Very Good rating is on target to be hit, largely through the quality of the building’s insulation and the combined heat and power plant on site.
Laying the wooden track
Despite its reputation as a railway and engineering city, there will be no harping back to the glory days of rail when Derby’s latest section of track is laid.
The 250 m curved wooden track of the indoor velodrome will be comprised of small slithers of Siberian Pine measuring just 40 mm wide by 40 mm thick.
Each of these sections of pine will be fixed at 600 mm centres to a structurally graded timber frame that supports the track.
It will be a painstakingly difficult job, with each section needing to be perfectly chamfered so that it fits neatly alongside its neighbour.
Any slight mis-measurement risks the surface splintering, while if the timber of both the track surface and support frame is not perfectly acclimatised to the building, there is a risk there could be some buckling.
“We are bringing the timber in early so that it is given time to acclimatise,” Mr Brook explains. “It should take around a month to do that. Then once the track is in place we can go back in to finish the sports courts in the centre of the track.”