Contaminated ground forced a redesign for a project on which Cardiff and Vale College students are gaining first-hand experience of construction.
Project: Cardiff and Vale College – Canal Parade campus
Client: Rightacres Property
Contract value: £28m
Contract type: JCT Design and build
Main contractor: Willmott Dixon
M&E subcontractor: Aecom
Fire & acoustics: Aecom
Of all the UK’s major cities, there can be few that have been so dramatically transformed over the past 15 years as the Welsh capital.
When the Cardiff Bay Development Corporation completed its work on the Cardiff Bay Barrage, it opened up a swathe of former industrial land that had been made attractive for developers to move into, transforming the city’s seafront.
At the same time, construction of the Millennium Stadium in the city centre kick-started its much-needed regeneration.
Now plans are afoot to develop the final section of land linking the two areas. Butetown, or ‘Tiger Bay’ as it is known locally, was once the home of singer Dame Shirley Bassey and the bustling centre of Cardiff’s dockside.
Now as part of the city’s ongoing redevelopment, one project will see main contractor Willmott Dixon build a new college campus for Cardiff and Vale College (CAVC) through its developer client Rightacres Property.
The site itself is, like much of the surrounding area, a relic of the heavy industry that used to service the nearby docks and allied trades.
A former chain testing and railway sleeper dipping facility, it had been reclaimed and largely decontaminated over the years.
Despite those efforts, initial site investigation reports indicated that areas of creosote contamination remained, forcing a change of piling method for the site team.
“There is 2 m of made ground over the top of the clay with perched water sitting on top of that. We needed to manage that water just in case of any contamination”
Darren Hancock, Willmott Dixon
The design was changed to a driven precast concrete pile solution, aimed at negating the potential of perpetuating contamination through the disturbance of hydrocarbons within the ground during the installation of continuous flight auger piles.
Specialist piling subcontractor BBGE agreed to drive the 450 mm by 450 mm precast piles in 5 m to 6 m section lengths down to 14.5 m.
A mechanical shoe connection between the sections ensures their integrity, while a steel nose on the leading length helps protect the pile and keep it on line.
In all there are 588 piles across the site. It is covered by a gas membrane, which is fixed around the piles and 500 mm-thick pile caps onto the 450 mm-thick slab.
“There is 2 m of made ground over the top of the clay with perched water sitting on top of that,” Willmott Dixon construction manager Darren Hancock explains.
“Pre-stressing means we can use fewer columns, giving greater spans for the laboratories”
Darren Hancock, Willmott Dixon
“We needed to manage that water just in case of any contamination.
“From the site investigation reports we realised we couldn’t use CFA piles, which left us looking at the precast solution.”
Wrap-around demands careful programme
The college campus features three reinforced concrete-framed buildings, which are linked together by steel bridges.
The central ‘pod’ building has been built under phase one of the construction programme. Phases two and three wrap around this central building to form the remainder of the facility.
“We had to programme it that way just so that we could get access to the building,” Mr Hancock says.
“[Its height] differs across the site as its rises and falls. It is two storeys at its lowest and six storeys at its highest.”
With a sustainability target of BREEAM Excellent, the building will feature a mixture of both mechanical and natural ventilation and will harness the thermal mass of the main reinforced concrete frame.
Shear walls and blade walls in the building are 450 mm thick and reach from 1 m to 3 m in height, with all the columns featuring a fair-faced finish.
Pre-stressing offers advantages
The pre-stressed concrete floor slabs will enable the flat soffit to be left exposed and offers proven time-saving advantages over other systems, according to Mr Hancock.
“Pre-stressing means we can use fewer columns, giving greater spans for the laboratories.
“They are also more slender, which means we get more headroom and – just as importantly – means we can strip them quickly, giving us more time security on the programme.”
“Approximately 80 per cent of the roof is clad using a Euroclad SF500 system and incorporates one of the largest solar roof arrays in Wales”
Darren Hancock, Willmott Dixon
Prestressing specialist subcontractor CCL has installed 50 km of cables during the slab-casting process, while frame subcontractor Thames Valley Construction has poured 4,000 cu m of concrete and tied more than 170 tonnes of steel reinforcement across the project.
The team used C40 OPC concrete to cast the slabs and C50 OPC concrete in the columns.
“We didn’t use blended mixes,” Mr Hancock points out. “We wanted OPC because it is better for early strength gain. When it was really cold we could up the cement content to maintain that.”
Huge solar array
Site workers cast the slabs using a Peri formwork system and in most cases removed it after 48 hours or when cube tests would show a 10 N strength.
Following this, the CCL team tensioned to the first design load before putting the final design load tension through after the concrete had gained 70 per cent of its final strength.
A 7 deg pitched roof across the building is supported on a steel frame, which has proven awkward to construct with the differing heights of the building’s concrete frames.
No two sections are identical, and a 1,850 sq m photovoltaic array is also being installed.
“There are lots of different steps and angles which has proven challenging,” Mr Hancock says.
“Approximately 80 per cent of the roof is clad using a Euroclad SF500 system and incorporates one of the largest solar roof arrays in Wales.”
With internal work now starting to race ahead, the team looks well positioned to hand over the project ready for next year’s influx of students.
Students get site experience
Corporate social responsibility is a subject taken very seriously by the construction team at the Cardiff and Vale College site.
It is doing all it can to make sure the local community benefits from its presence both during and after the construction process.
As well as the college itself, the team is taking various steps to ensure it leaves a lasting legacy for the city, the industry and the people of Butetown.
Principal among these is the provision of 200 hours of unpaid work specifically so Cardiff and Vale College students can come and gain actual, valuable experience in working in the construction sector.
It can be difficult to timetable the blocks of experience into the build schedule to suit both the college and the site team, but for Mr Hancock the effort is worthwhile.
“These students are the next generation of construction workers,” he says. “It is very important we do our best to enable them to gain real experience in a site environment.”
Those students can expect to get two blocks of experience on the site, but there are also five apprentices learning valuable lessons on the project in disciplines including carpentry and joinery, as well as plastering and drylining.
A faster façade
Making the structure as weatherproof as possible as soon as possible is the key driver for the CAVC project team.
With around 70 per cent of the scheme featuring a glass façade, including the fine dining restaurant with views across the city scape toward the Millennium Stadium, the team has developed a waterproof gasket that helps ensure a weathertight seal between window and structural frame.
It runs from behind the glazing unit and is fixed to the concrete face of the structure.
This simple measure ensures finishing and wet trades can get into areas of the building earlier than might otherwise be the case.
Mr Hancock is well versed on the delivery of schemes in the education sector, having worked on a host of projects since he began working for Willmott Dixon almost 10 years ago.
“I suppose it has become a bit of a speciality,” he says. “I’ve done lots of schools in my career and I enjoy the education sector. I know the constraints, the specifications, the standards.”
He admits that a particular focus of projects under his management is that of programme delivery.
He has identified the delivery of the façade as key to keeping the programme on track to welcome students to the building at the start of the academic year in autumn 2015.
“I will not over sail the project delivery date,” he says. “On any of my schemes that is one of my prime drivers.
“On the Cardiff and Vale College scheme the façade is important. Get it installed and get the building watertight as soon as possible and work from there.”