Ground-up

Preservation orders

Old meets new at Liverpool’s Half Tide Dock. From the 18th century the docks helped to generate the city’s industrial and commercial strength. In the 21st century, City Loft’s apartment development reflects the docks’ central role in Liverpool’s regeneration and the demand for ‘city living’.

Below ground, considering how new would meet old was of critical importance, because the old dock wall, made from sandstone blocks with a granite capping beam, is a listed structure. Any horizontal loads exerted during piling or once the building was built could have been disastrous for the heritage wall.

Piling contractor Bachy Soletanche provided a solution which involved sheathing the piles next to the wall in a steel casing, effectively debonding the pile from the ground around it until it reached a depth lower than the base of the old dock wall. ‘It’s not a solution which is used very often,’ says Bachy Soletanche’s contracts manager Steve Mallinson. ‘The only other one I’ve seen it on was for a job next to a tunnel in London.’

The nine-storey building, which has 121 flats and includes underground parking, will be supported by 84 piles, all 750mm in diameter. Most of them are conventional CFA (continuous flight auger) piles and 17 are within five metres of the harbour wall. Bachy Soletanche had to submit the designs and method statement for the £180,000 piling job to the Mersey Dock and Harbour Company for approval.

After four metres of made ground, the piles hit Sherwood Sandstone which extends down for 30 metres. This is hard ground, so a big piling rig was necessary. ‘We had to ensure that it could dig down to 14m through the rock,’ explains Mallinson. ‘A small piling rig would never have got the depth.’ As is usual, main contractor Pochin Contractors had dug out obstructions within the top four metres of ground before Bachy Soletanche came to site.

With CFA piles, the spoil comes up to the surface as the auger digs down, and the concrete is delivered through a hollow tube down the middle of the auger once the right depth is reached. To install the special piles the contractor used a large diameter auger (LDA), drilling down 0.5 metres at a time, then extracting the auger to dump the spoil and going down a further 0.5 metres. This required a high torque rig. ‘It’s a mighty piece of kit,’ says Mallinson. Through the made ground the process requires a temporary steel casing to support the hole – once in the sandstone it’s self-supporting.

Once the hole is dug, a permanent steel liner goes down followed by the reinforcement cage and finally the concrete. Only the bottom two metres of the 14 metre-deep pile are without the casing, so the vertical loads transfer to the rock.

LDA piles are much more expensive than CFA piles because they take so much longer. Bachy Soletanche managed four to five LDA piles a day compared with 10-12 CFA piles. The whole piling contract took four weeks to complete.

Main contractor Pochin is now constructing the block which is due to be complete before the end of the year.

Piles and piles of piles

Piling contractor Aarsleff has installed a whopping 4,000 precast concrete piles over the past four years at Cardiff’s £75m Capital Business Park. The reason is that developer JR Smart sublets entire specialist contracts on negotiated fixed price terms rather than going out to tender.

‘I prefer to have people on my site who I can trust,’ says JR Smart chairman John Smart (pictured right). ‘You can go out to tender and get something very cheap and you end up with a lot of problems. I don’t want to be messing around with claims. If there’s a problem I want someone I know I can deal with.’

Aarsleff first worked for Smart 10 years ago on a project at Spring Meadow, Cardiff.

Smart says he pays all subcontracts within seven days and without any retentions. ‘We play fair with them and they play fair with us.’

The lastest phase of the park involves the erection of steel portal framed industrial units. Aarsleff installed 250mm and 200mm square section Centrum continuously reinforced piles, in lengths varying between eight and 16 metres.

The larger section piles support a ring beam around the buildings’ perimeters while the smaller piles will support a cast in-situ concrete floor.

Around 30,000m2 of industrial space is already complete or under construction. The 75 acre site will have 100,000m2 of development when finished.

Knot what the doctor ordered

Before contractor Stradform could get started on a £2.5m office job in Bristol, it had to get rid of an unwelcome visitor: Japanese Knotweed.

Now almost complete, the three-block development, called Brazabon Park and built for developer Terrace Hill, occupies a former British Aerospace site. Once home to the firm’s canteen, the site was dormant for 30 years. Japanese Knotweed, originally imported by wealthy Victorians to fill up their vast gardens, moved in.

Japanese Knotweed is tenacious stuff and can penetrate tarmac and unreinforced concrete. ‘It’s pretty powerful, if it sees a chink of light it will grow towards it,’ admits Patrick Leydon, managing director of ground remediation specialist Leydon Kirby Associates, which was brought in to deal with the problem.

The other problem is that it does not require pollination to procreate. Instead, a tiny part of its underground stems, known as rhizomes, is all that’s needed to produce a new plant.

There are several ways to get rid of it. If you’ve got a few years, use herbicides. If you’ve got lots of money, dig and dump, because soil containing Knotweed is treated as contaminated land and costs £30-£50 a tonne to dispose of in landfill.

At Brazabon Park, Leydon Kirby dug up the weeds and sifted through the soil to remove the rhizomes, which were then burnt. The contractor then buried the sifted soil at depth: below two metres to make sure the Knotweed didn’t rise again, and on parts of the site outside the new buildings’ footprints.

For Stradform contracts director Nigel Jones, the weed was one of several potential problems in the ground. There was also a possibility of finding Second World War structures and Roman remains. The contractor employed an archeological consultant on a watching brief. Happily, no remains were found.

Meanwhile Leydon Kirby, which set up its specialist Japanese Knotweed unit three years ago, is working on several sites around the country. You could say it’s a growing business…