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Don't spare the horses

FOUNDATIONS: Construction of a secant pile wall for a new community arts centre at Tally Ho Corner in North Finchley, London is nearly finished. Max Soudain reports.

The Tally Ho pub in North Finchley was once an important stop for stagecoaches travelling north from London, where horses were changed after the very steep climb from the city.

Buses have replaced the horses but the recently renovated pub is still going strong and Tally Ho Corner remains a focal point for transport. For years it was the home of a large north London bus depot.

Now the depot and the Gaumont Cinema that once stood in the middle of this busy junction have been demolished to make way for a community arts centre.

Barnet council's £44M Artsdepot project includes a 400-seat auditorium, a studio theatre, galleries and exhibition space, a multi-storey block of luxury apartments, shops and a leisure centre with swimming pool.

Tally Ho's transport heritage is not forgotten, as a new bus depot and an improved public transport interchange are included in the project.

Barnet council is carrying out the redevelopment in partnership with Wilcon Homes and Chiltern Developments. Plans have been on the table for some time - the council first considered an outline scheme in 1996. The project has attracted some controversy, including objections from local environmental groups, but the council approved the project in 1998 and planning permission was granted in 2000.

Carillion, the original main contractor, pulled out of the project in January 2001. It was replaced by MJ Gleeson, which began work in November last year.

Norwich-based foundation contractor May Gurney Technical Services is in the middle of piling works for a perimeter secant pile wall for the project's two level basement.

May Gurney Technical Services engineering manager John Chick says construction of the 276m long wall was not straightforward.'As secant walls go it is at the tougher end of the spectrum, ' he says.

The site is underlain by 0.5m-1m of fill overlying 8-9m of very stiff glacial clays, 3m or more of water bearing gravels and London Clay below.

'If there had just been clay beneath the site, we could have used a contiguous piled wall but the clay here has thin [between 50mm and 200mm thick] sand layers and we were worried about groundwater ingress to the basement, ' Chick explains.

'The sand layers influenced the entire approach of the job. There was a chance of the layers causing groundwater inflow but the flows were unpredictable. If there was a leak it would be difficult to locate.'

The solution was to build a hard/soft secant wall, but he says the job was still potentially very difficult because the wall has to reach about 13m depth so that it is well into the London Clay to cut off any groundwater flow. This meant there were big issues with verticality of the piles, to ensure there were no gaps.

Chick says this was achieved by using 'very large' soft piles in the wall.'Verticality was easier to control with the bigger soft piles. And this was our way of getting as much sealant into the ground as possible.

'We were lucky the site was big enough to give us space for the soft piles, ' he adds. The 900mm diameter, up to 14m long soft piles are being installed at 900mm centres, so they are just touching, and were made using a soft concrete rather than cement bentonite mix.

In contrast, the hard piles are only 600mm diameter but are up to 20.8m deep. They are also installed at 900mm centres and are reinforced with cages comprising 14, T25 bars.

'If we had used smaller softer piles, then we would have to install them closer together, which would have meant more hard piles and more reinforcement and concrete, with no cost savings, ' Chick says.

May Gurney is using a guide wall to install the 307 soft and 307 hard piles. However, the wall's design meant that when it came to installing the hard piles, only a small part of their circumference was left to guide the auger in. A mobile steel frame that restrains the auger had to be specially designed. This sits in the top of the guide wall and the rig's foot is placed on it to hold everything in place.

Two rigs are being used on the job. A large Llamada rig is installing the bigger soft piles, while May Gurney's new Woltman rig (GE news last month) is installing the hard piles. Chick is very impressed with the Woltman rig, as it is ideal for city projects where space is at a premium. The Woltman is compact and powerful and can be set up and working in a very short time, he says.

The scheme originally called for a large number of bearing piles across the site. Design consultant Walsh Associates decided to remove these and give the secant wall a load bearing function.'Line loads vary from 315kN/m to 900kN/m, with the largest loads where the flats will be, ' Chick says.

Wall performance during excavation will be monitored by eight inclinometers fitted to the pile cages, two to each side of the basement.

The wall will be cut down by between 2m and 3m to accommodate a large pile capping beam.

The Elliot pile breaking method will be used to bring the wall down to the required cut-off level.

'Although the method is not ideal for secant walls, it does help the breaking process, so we are fitting foam to the tops of cages, ' Chick says.

Basement excavation will then begin. The first stage will be to dig down to 4m and cast a waling beam around the perimeter. Excavation will continue for a further 4m in the centre of the basement, with a 4m high berm left around the edges. Raking shoring will follow to allow the berm to be removed and the basement slab to be cast.

Chick says this stage is by no means easy. 'The base of the excavation is just 0.5m above the top of the water bearing gravel layer on top of the London Clay. This means there is a potential for heave and even blowing of the excavation floor.'

Walsh Associates came up with an ingenious solution. The basement slab varies in thickness across the site. Excavation will progress towards the end where the apartment block (and the heaviest loads) will be. Casting of the slab will then begin at this end and it will be thickest here. Because the earliest exposed areas will have had time to heave the slab here will be thinner.

The excavation is being dewatered using six sumps through the clay and into the gravel. These will continue to work until the slab is cast. 'The problem is one of pressure rather than flow, ' Chick says.

May Gurney's 10-week contract began in mid December and is due to finish this month.

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