Systems Geotechnique tackles tricky geology on Welsh hospital
08:30 11 Feb 2009
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Systems Geotechnique faced nightmarish geological conditions when tackling the foundations of a new mental health unit in Bridgend. Paul Thompson explains.
It is not the most inspiring site in the world. Shoehorned behind the Princess of Wales Hospital in Bridgend and a row of recently built semi-detached houses this car park is soon to become the latest facility to help youngsters with mental health issues in South Wales.
In a two-phase project totalling more than £21m,main contractor BAM Construct will build a new Child and Adolescent Mental Health Unit at the hospital alongside a new staff car park and access road.
But the area is littered with geological faults, soft spots and voids, and buildings are plagued by the effects of settlement and subsidence. In a bid to combat the effects of these faults BAM Construct has brought in piling specialist Systems Geotechnique to provide a solid base for the project.
At the moment, it is contracted to install a series of almost 300 end-bearing piles under a £960,000 contract across the site of what will become the three-level, two-storey staff car park. It's the first phase of what Systems Geotechnique's contracts manager Nick Cordon hopes will progress into the second phase of the project, the hospital extension itself.
"At the moment, we are only contracted for the first phase of the work, the piling for the second phase - the mental health unit - is still under discussion but we are hopeful we will be able to take that phase too," he says.
The Systems Geotechnique team is using 275mm outer-diameter temporary steel casings the length of the pile to install its design. Despite the voided, fractured nature of the ground BAM Construct decided early on that there was little point in carrying out full-grouting works around the site, due to the ground being so badly voided that it would be impossible for an economic grouting solution to be installed.
Instead, the project team looked at a piled solution and the engineers at Systems Geotechnique developed a reinforced concrete pile that would deliver the required 600KN working load, through some 20m of dodgy ground and into the sound underlying limestone bedrock.
Poor ground condition
"The ground is very poor. There is lots of silt and soft clay down to a depth of around 16m then there is 4m or so of heavily fractured limestone before we hit the bedrock proper at around 20m," explains Cordon.
Indeed, so poor is the ground that the piles have been designed to ensure that the all the working load is born through the pile end which is socketed some 3m into the bedrock. Thanks to a plastic sleeve inserted within the steel pile casing none of the loading is transferred through friction against the surrounding earth.
"It is actually designed to feature a negative skin friction. If there is even the slightest load borne through friction it tends to draw the ground down around the pile," Cordon says.
The piles themselves are set in groups of three or four on a regular grid pattern around the site and are bored through a temporary steel casing with a down-the-hole hammer rig, to the full 23m. A 5m-long 40mm steel GEWI bar from specialist manufacturer DYWIDAG Systems is then lowered into the centre of the pile shaft and into the 3m rock socket.
A spacer welded on the GEWI bar holds the main reinforcement in position, and instead of a standard rebar pile cage the Systems Geotechnique team is using a 178mm x 8mm grade 550 circular hollow section tube to transfer the load and to counter any bending that may result from any future ground movement. This tube projects 75mm into the pile cap.
Lowered around the steel tube is a 20mm thick, 225mm outer diameter plastic debonding sleeve which provides the negative friction resistance for the pile while a further short section of three B20 steel rebars, linked using B10 bars at 300mm centres, is suspended into the top section of the pile and will act as connecting bars for the pile cap reinforcement.
Clawing back lost time
Grouted using a standard 40N grout mix through a tremmie pipe the project team has been steadily clawing back time lost thanks to the unexpectedly poor ground conditions. The team found that the pile casings, driven by a fleet of Klemm 709 and 708 piling rigs alongside a Fondedile FF5, were difficult to extract due to the cohesiveness of the silty, clayey ground conditions.
The longer the casings stay in the ground while the pile is cast the more the ground pressure builds around them, making them difficult to pull out, so difficult in fact that the team sheared the head from the rig. In the end, by speeding up the whole installation process the team were able to pull the casings early enough - generally after a couple of hours - to avoid any problems during their extraction.
"We found that the quicker we pulled the casings the easier it was. It enabled us to catch up on some of the time we had lost," says Cordon.
Working a 10-hour shift the 20-strong Systems Geotechnique site team have been on site since the middle of November and are releasing the piles in stages for Abergavenny-based groundworking contractor Griffiths to follow on behind, breaking them down and casting the pile caps. It is a work method that will soon see them complete the 280 individual piles required under the first phase of the project - and move the delivery of a dedicated mental health unit for Bridgend's children and adolescents closer.
Wham BAM
The two-phase project to develop a new Child and Adolescent Mental Health Service unit at the Princess of Wales Hospital in Bridgend, south west Wales, will see main contractor BAM Construct build a three-level staff car park, construct a new, non-emergency vehicle access route from the existing Coity bypass that runs along the site as well as the CAMHS unit itself.
With the main extension to the hospital not due to start on site until later this year the project team is focused on delivering the multi-storey car park first so that it frees up much needed ground space to build the extension. It will also ensure that there is reduced off-site parking in the surrounding residential areas.
Funded entirely by the Welsh Assembly Government under two separate phases the scheme will ensure client Abertawe Bro-Morgannwgg University NHS Trust has some of the highest quality mental healthcare units available when it opens in 2010.
Project factfile
- Scheme: Princess of Wales Hospital, Bridgend CAMHS, car park and access road.
- Client: Abertawe Bro-Morgannwgg University NHS Trust
- Main contractor: BAM Construction
- Value: CAMHS unit - £12.5m
- Car park and access road - £8.9m
- Piling subcontractor: Systems Geotechnique
- Piling contract value: £960,000
- Groundworking subcontractor: Griffiths