The team originally looked at using piles to 3.5m depths at 7.5m centres with pile caps. However, there were concerns that the vast number needed for the site would take too much time and potentially pose a health and safety risk because of the holes that would litter the site.
Instead two types of piles have been designed for Pinderfields that will be built as one steel and one concrete block reaching up to six storeys. Each block will be separated by a four-storey atrium. The concrete structure will house an adult in-patient unit and the steel block will include a critical care and accident and emergency department.
Holding-down bolts or starter bars are being installed into these piles, which will support the steel or concrete columns of the building respectively. This means no pile caps or ground beams are necessary.
Stent Foundations came up with the plan to install the system and is working on the £1.35M contract for the joint venture, which comprises Balfour Beatty Construction Northern and Haden Young. Building Design Partnership is the project architect and lead designer, while White Young Green is structural engineer and came up with the pile load requirements.
"Basically you've got everything coming down the one pile," says Stent senior contracts engineer Filipe Pamment.
"Because of that we're taking on board part of the ground works contract that we don't usually get involved in. We look at it [the piled foundation] as one unit instead of two separate parts [a pile and a pile cap]."
Balfour Beatty project design director for the joint venture Robert Hughes says this is important because it avoids the possibility of differential settlement associated with the alternative option of using pads supported by four different piles. "The way it's been designed, a single pile takes the place of a piled foundation," says Hughes.
Although Stent has installed holding-down bolts with continuous flight auger piling previously, Pamment says this is the first time the company has done the same with large diameter piles more suitable for penetrating strong rock.
Because each pile is doing morework, both types have been designed as large diameter monopiles – ranging from 1.05m to 1.2m. Half of the 542 total piles will use the holding-down bolt system for the steel hospital block and half will involve using the starter bars for the concrete block.
As well as designing the large diameter piles to cope with high loads – up to 4500kN – the size was also determined by the holding-down bolts and starter bars.
For the piles that will support the steel frame, a 600mm2 base plate holds the steel bolts and for the concrete block the pile houses typically about 3.05m long starter bars. These comprise six or eight 25mm to 40mm diameter bars in a 175mm to 350mm diameter formation. A 1.65m portion is embedded into the pile with the remaining 1.3m extending into the 450mm2 concrete columns above. The starter bars will provide a connection to the steel structure above.
Each of the parts installed into the foundations must not interfere or clash with the reinforcing bars. A 75mm tolerance meant the piles had to be designed with an even larger diameter than was required to take the loads.
The piles found into rock – typically mudstone, but in some places sandstone – at depths of between 5m and 10m. There is up to 500mm of made ground on the 84,000m2 brownfield site, overlying about 500mm of weak rock and then stronger rock. Each pile sockets 1m into the stronger rock.
Piling work started in August and is expected to be complete by the end of this month. Two Casagrande B250 rigs drill down with the auger to loosen the soil before switching to a turning tool to spin 4m to 5m long casings down to the rock. A barrel tool is used to remove tougher rock and a cleaning bucket removes the spoil.Two Kobelco cranes (a 60t and 75t) drop the reinforcing bars into each borehole. Each cage has eight or 10 bars in a cylinder with an outside diameter of between 900mm or 1050mm. Once these are in place, the concrete is cast and steel casings removed.
Groundwater has meant that some piles need to be installed with a tremie system, which takes longer than for the others. About one in five piles are affected by thegroundwater and site workers have to ensure the pipes stay in the concrete as it is poured to keep the pile sealed from the water.
The presence of groundwater has also called for the use of a 150mm-275mm thick bentonite mat to go underneath the concrete slab in places. Pamment says each pile has a specific design – each reinforcement cage is numbered to correspond with a particular pile – which has made the work more challenging.
"There's a lot of variation in the piles, with 122 different combinations of pile diameters and lengths," he says. "Although it is a fairly big project for us in terms of the money, it's become a much bigger job because of the complexity of it. Doing all the sequencing and scheduling for each pile has taken a long time to organise."
Site workers install the starter bars and holding-down bolts by hand once the concrete is poured. Each system's positioning has to be accurate to within 10mm so two checking engineers are working with each rig team. The engineers have testing equipment that checks the position, height and level of both systems in every pile.
Before work started on the foundations contract Pamment says the team decided to build a dummy pile so site workers could see how each pile needed to be built.
"It was slower at the start because it takes a long time to get the site workers to understand the process," he says. However, the rate of production was expected to be five piles per rig each day and now, even with the additional time needed with the tremie, the team has increased production to an average six piles per rig.
Pinderfields is expected to open in 2010, with the 20,000m2 Pontefract site due to be ready by 2009. Consort Healthcare – a healthcare company and joint venture set up between Balfour Beatty and Royal Bank of Scotland – is responsible for the build and 35-year operation concession for the hospitals.