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ON TIP TOE

PILING - Soft toe piles were the answer to complex ground conditions at a major new hospital development in Scotland. Alexandra Wynne finds out how the piles work.

Consultant engineers for the new £100M (2150M) Victoria Hospital in Glasgow knew from existing records that a number of geological faults made for unusual ground conditions.

Evidence of un-worked coal layers from new investigations meant they had to design a piling method that would bear the full load of the structure relying only on skin friction.

The new three storey hospital and basement car park in south Scotland is being built on a 4.15ha site across the road from the existing Victoria Hospital. Geological faults have over time forced ground to shift vertically, causing its strata to become muddled, according to consultant Faber Maunsell geotechnical director, Peter Boyd.

Up to 2m of made ground sits over 4m to 18m of a succession of glacial marine deposits, glacial till, limestone with silt stones, and coal layers, before stronger ground appears.

Although the consultant knew about the faults from previous investigations, it needed to know more about how the ground had been affected. 'A reasonable amount of site investigation had been done before we got involved, which painted a picture of what was below, but because of the faults we needed more information, ' says Boyd.

The company knew that stronger ground in the form of boulder clay and sandstones appears at 6m to 20m depths across the site, but new boreholes revealed evidence of un-worked coal within the glacial deposits above, he says. The thickness of coal layers ranged between 18mm and 750mm.

'On many sites you tend to have a good idea of where the coal lies at this point, ' says the company's structural regional director, Stuart Mason. 'But because of these faults, the coal is all over the place and you can't tell its exact location at any one point.'

Boyd and his team considered CFA piling for a cost effective method of installing foundations for the hospital's steel structure. But it was ruled out because of concerns that it would be unable to cope with possible obstructions in the ground.

Traditional bored piling emerged as the best option. 'We knew it was going to be quite dif cult to predict what sort of ground we would nd, ' says Boyd. 'Bored piling is more expensive but it was the only method suitable for this ground.' Main contractor Balfour Beatty Construction has sub-contracted its foundation division Stent to do the 22.65M piling work for the hospital.

Two rigs are being used to carry out the piling work, a Casagrande B160 and a Bauer BG36. Operators have to drill through soft ground to a strong layer of bedrock to create a suitable rock socket, says Boyd. A flight auger tool is used for drilling through softer ground before the switch is made to a rock boring tool that copes with harder rock.

Once depth is reached - most piles are about 12m to 14m long - a steel casing is rotated down by the rig to clay depth and site workers insert the reinforcement cage by hand. Stent pours concrete in and removes the steel casing once the concrete begins to cure, but before it hardens completely.

Mason says the team was concerned that despite efforts to create a secure rock socket, it might not know if a coal seam lies just below the toe of a pile situated in the rock.

Once pressure is applied from the load of the structure, it is possible the piles could crack through a few millimetres of rock and collapse into the softer material below.

'We took the decision to come up with a design purely focusing on skin friction, ' says Mason.

'That was our design philosophy but we had to find a way to prove it would work.' The strategy of enabling the building to be supported by skin friction piles alone is an unusual one, according to Boyd. 'We use a standard bored pile, but instead of going in at a regular depth for the socket of say, six inches [152mm], we created a much deeper one intothe rock using a coring drill bit that grinds 2m to 3m into the bedrock, ' he says. 'Effectively the end bearing capacity is ignored and any load the end can take is considered a reserve.' Stent carried out a pile load test at the end of December before installing any working piles to see whether the skin friction piles would be able to cope with the load pressure from the hospital and its car park.

To mimic the skin friction capacity required by working piles, a void was created at the base of the test pile. Rig operators drilled a 750mm diameter hole to about 13m deep and rotated temporary steel casing into the ground.

A dead weight helped to force a circular block of polystyrene to the bottom of the hole. Site workers inserted a reinforcement cage with a narrow pipe running directly to the polystyrene before pouring concrete in to form the pile.

After the concrete partially sets, the casing is removed and a solvent liquid poured through the narrow pipe. The polystyrene supports the concrete while it cures but is dissolved by the liquid to effectively create an air void, says Boyd.

Once the concrete cured completely, site workers set up a testing rig and used a hydraulic jack to apply increasing loads up to a factor of safety of 2.25. The maximum working load for any of the piles for the structure is 4200kN.

Designers predicted a ettlement of 10mm at working load. The test results showed the pile settling at only 6mm and at 9400kN settlement only 70mm.

Following the success of the soft toe test, the team decided on a second test to see how the piles would cope under temporary lateral pressure exerted during construction of the building.

Mason says reinforced concrete ground beams normally protect the piles and pile caps from lateral pressure during the construction phase.

However, a 350mm concrete slab forming the basement car park level provided an alternative to beams.

This slab would help counter the effects of buoyancy caused by relatively high groundwater.

The slab is expected to take up to a year to complete, but the schedule requires work to begin on the structure as and when sections of the concrete are ready.

Work to install the 257, 600mm and 135, 700mm working piles began at the start of this year and each pile takes about an hour to install. Piling work is due to be completed by the end of March.

Smaller diameter piles support areas with a lighter load, including the single storey energy centre and atrium sections of the main building. The rest of the main hospital building will be supported by the larger piles.

'Skin friction piling is expensive, ' says Mason. 'But the alternative of creating an investigation borehole for every pile you install would cost a lot more and it's just not practical.' The new hospital will open in 2009 and is expected to treat around 400,000 patients each year.

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