It may be a small site at only 40m2, but Memorial Court at Clare College – part of the University of Cambridge – is getting bigger facilities with a new study centre and accommodation wing in an £8.5M facelift.
The site is flanked on one side by the University Library and to make the best use of the space available, a secant wall is being built to support an underground 150-seater auditorium. In addition, site workers are installing two different types of piles with a single rig to support the new buildings.
May Gurney is completing this work under a £287,000 piling subcontract for main contractor Haymills Construction and it started work on site in early August. Cameron Taylor is consultant for the project.
The basement lecture theatre will be 6m high (the equivalent of two typical storey heights) to accommodate the auditorium-style seating. Rig operators are installing a continuous flight auger (CFA) secant piled wall around the 16m by 18m perimeter. This comprises 39 alternating hard and soft piles with each of the hard ones intersecting the adjacent soft piles. All are 600mm in diameter and at between 700mm and 900mm centres. The piles act as a retaining structure for when the excavation takes place and as the eventual walls of the basement. But they also have to cope with groundwater on site.
"We're relying on this wall to keep the basement excavation as waterproof as possible," says Haymills site manager Nigel Phipps. This is made all the more challenging because of the depth of the retaining wall – the hard piles go down between 13.1m and 14.1m with the soft piles at 8m depths.
"It's very important that the piles are lined up with one another as much as possible," says May Gurney southern area manager Mike Cowan. "If there are any big gaps it could cause too much water to get through. We chose to use a guide wall all the way around the building – it helps make the piles go in a little bit straighter."
A shallow trench excavated at pile locations provides a casting area for the guide wall. Site workers then poured concrete between the trench walls and inserted a scallop-shaped shutter – either side of the trench – to create a guideline for the secant wall. This helps ensure that each wall pile goes in as straight as possible by providing a well for the auger to line up with.
Cowan says this is important when installing secant walls at deeper levels because if a pile is 1mm out of place near the ground level, the alignment will become increasingly exaggerated the lower down the pile goes. This means creating ever larger gaps between piles the further they go down.
A C35 concrete mix is being used to form the hard piles and a weaker concrete using bentonite in the mix creates the more pliable soft piles in between.
The sequence of piling is important for the secant wall with soft piles installed before the intersecting hard piles are formed. The latter must be built in a reasonable period following the soft piles to ensure they are still soft enough to drill through. Cowan says if the concrete is given too much time to cure, it would make the job of forming the hard piles too difficult.
The limited space on site and difficult single-lane access meant the subcontractor opted to use one rig to complete the remaining piling work.
May Gurney chose to use its LLamada P100TT rig to complete all the piling work – the secant wall, CFA piling for the accommodation block and rotary bored piles to support the auditorium building.Cowan says this is unusual because in these ground conditions normally only a rotary rig would be required for the latter piles. About 1m of made ground overlies 3m of sand and gravel before underlying Gault clay appears.
The rig spins an about 7m long steel casing into the ground before the auger drills down to a maximum depth of 20m. The auger removes the spoil, but because the steel casing is in place, the normal CFA process of pumping concrete through the auger's hollow stem to form the pile is unnecessary.
Phipps says: "It's like taking a sledgehammer to a nut – it [the CFA rig] is overkill for the rotary bored piles, but using the same rig for all the piling helps to keep the operation more simple."
The cut-off level for these piles is down to between 6m and 7m in depth because they will be supporting the basement construction. These are designed to cope with compression loads of 500kN and tension of 850kN. This is because the cut-off level is low in the ground and floatation forces must be taken into consideration.
A reinforcing cage of up to 17, T16 bars, with a maximum length of 15m, is suspended from the casing into the borehole, so the top of the reinforcement is below the cut-off depth, before concrete is poured in to form the 600mm diameter piles. The casing can then be removed and the area above the cut-off level is backfilled. This avoids any health and safety problems that might arise if the holes were left open around the site.
An additional 79 CFA piles are needed for the 12m by 38m accommodation block that will eventually be three storeys tall. These are designed as end-bearing piles down to a maximum depth of 16.9m and will be able to cope with loads of 480kN. Five CFA piles will also be installed to support an area where a 30t crane will sit while the building is constructed.
The CFA piles were considered a sensible option over a driven method because the ground conditions are ideal and the site is a conservation area with work going on just a few metres away from a listed Grade II building. This technique keeps down noise and vibration disturbance.
Cowan says the site restrictions and difficulties with getting plant on site meant that all piles for the project were designed to a safety factor of three.
The benefit of having such safe parameters means that no load-bearing tests were necessary. "Load tests take time and cost money," says Cowan. "It was decided that for this job it would be more time and cost-efficient to have slightly longer piles."
Piling work on the project was due to be complete by the end of last month and the new facilities are planned to open in November 2008.