Local knowledge

The first phase of piling for a new apartment building in Norwich, eastern England has gone smoothly because of careful investigation and preparation. Ground Engineering's Max Soudain reports.

"Norwich can be a very difficult area to work in," says May Gurney Geotechnical and Environmental operations manager Neal Willis.

And the site of a £3M (Euros4.37M) sheltered housing development by client and developer McCarthy & Stone (Developments) close to Norwich's Roman Catholic cathedral, where May Gurney has just finished the first phase of piling, is no exception. The local contractor has much experience of the geological hazards lying beneath the city, particularly the chalk bedrock which has numerous solution features and was historically mined from the 11th century until the Second World War.

Construction of the Cathedral Church of St John the Baptist was beset by problems at the start - the chalk mines beneath the site meant construction was held up for two years between 1882 and 1884 before the first foundation stone could be laid by financier Henry Fitzalan Howard, the 15th Duke of Norfolk.

And it was the open and partially collapsed mine workings that caused initial headaches on the McCarthy & Stone site.

As the firm's construction director Steve Retter explains: "It's an awkward site. There are the old chalk workings, it is 50m from the Cathedral in one direction and 20m from a synagogue in another. We were also worried about the possibility of an underground passage used by monks running from the cathedral and across the site."

The site also sits above one of the main aquifers for the city so "the Environment Agency showed some interest at the start of the project," Retter says. In the end, the aquifer is deep enough to be unaffected by foundation work. "The danger was that the caverns would connect to the aquifer but they don't."

The steeply sloping site - ground level drops by between 7m to 8m from south to north - was previously occupied by a car showroom that McCarthy & Stone demolished at the beginning of 2005.

But construction of foundations for the single block of 51 sheltered apartments did not begin until the very end of last year. "It wasn't the easiest site to deal with when we first looked at it," Willis explains.

Site investigations had revealed the mine workings, so before piling could begin, May Gurney, hired as piling subcontractor under a Euros292,000 fixed price contract, probed all the pile positions to ensure that voids would not be encountered during the main foundation works. This work was carried out last summer.

Once any open or partially collapsed works had been located - the suspected monks' passage was never found - McCarthy & Stone brought in M&J Drilling to pump in grout to fill these voids under a Euros510,000 contract, starting in mid-November 2005.

Grout strength was 2.5N. "It was a question of getting the balance right between grout that was strong enough but that also allowed piles to be placed through it," says Willis. "From a piling point of view, grouting prevents installation problems but it also prevents upward migration of collapsed voids."

CFA piles were chosen for the project after consideration of steel tube piles and precast driven concrete piles. "But we worried about connections being lost during piling - we lost a couple of rods during probing through voids," Retter says.

One of the biggest issues for pile design was the depth to the chalk, which varies quite dramatically across the site. In the lower half of the site - which was piled first - the chalk was generally at about 3m depth, Willis says. But in places it was as much as 20m down.

The site is covered by 2m to 3m of fill and between 4m and 19m of Norwich Crag - a dense sand and gravel.

"All the piles are designed to bear onto the chalk," says Willis. "So there were concerns over having lots of piles with different lengths."

Piles design, with a factor of safety of 2, was carried out for a variety of lengths, with decisions made on site depending on the ground conditions encountered during piling. This was particularly useful when solution features were found on two or three pile positions, which meant lengthening the piles, Willis says.

The piles penetrate the chalk to varying depths, based on loadings and on the depth to rockhead, he explains. For example, where the chalk was at 2m depth, piles were 11m deep. The deepest pile on the first phase was 21m.

Before piling began, May Gurney installed two sacrificial test piles, 11.2m and 12.8m long. These were statically tested to 1234kN, twice the maximum working load - which varies from about 200kN up to over 600kN across the site.

May Gurney began the first phase of piling works in the northern and lower part of the site just after Christmas. Its Llamada P140 installed 120, 350mm diameter CFA piles beneath the positions of the new building's walls, typically at 1.05m spacing (ie three and a half the pile diameter). Work took about three weeks.

McCarthy & Stone is also main contractor for the scheme and was building the first two floors of the structure in this area were being built when EF went to site. This will bring the northern part of the building up to the level of the southern part.

May Gurney will return in April to install the remaining 114 piles. Work will then begin on the final three floors - when complete one section of the building will be three storeys high, the other five storeys high.

Retter expects the first residents to move into their new homes in Autumn this year. The year-long gap between demolition and construction was important, he says. "We spent about Euros220,000 but we had to be sure of our commitment [to develop the site]."

Willis agrees that preliminary work was essential to ease progress "Once [preliminary work] was done, piling was straightforward," he says, even though solution features in the chalk were hit during piling at two or three positions, which was to be expected.

The smooth progress of the project since the start of this year is testament to the importance of focused site investigation and ground preparation. "It shows the beauty of doing your homework," Retter says.