Preservation orders

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.