Innovative use of adapted, continuous flight auger (CFA) piling plant, to form one of Manchester's deepest basement walls, has solved onerous settlement and waterproofing demands on a restricted city centre site.
The site is a tight, 80m long rectangular shape and will accomodate three, up to 13-storey hotel, office and apartment buildings.
West Properties' project team has the task of squeezing these three buildings, forming the £65M Origin development, into a 3200m2 site surrounded by streets, a canal and warehouse apartments.
Further compounding the challenges is the need for a 12m deep, basement to accommodate parking for 338 cars.
"Most of the surrounding roads, canal and shallow-founded apartment blocks lie less than 2m from our basement wall," explains Rob
Howarth, project manager for groundworks contractor Expanded Piling. "There were vibration, wall deflection and canal leakage issues to overcome."
The client's in-house construction arm, West Contracting, brought in Expanded Piling last September, early enough to have a significant influence on overall foundation design.
Forming what it claims are some of the deepest basement walls in central Manchester was to test the design team to the full. The brief was for an unsupported wall with a retained height of 12m and with piling founded some 8m into 10MPa strength Sherwood Sandstone.
Under its £972,000 design and construct contract, Expanded could offer high torque rotary, dual-use rigs capable of installing CFA piles.
CFA piling offered a cheaper – and one-third faster – programme compared with the rotary alternative. It also helped overcome boundary restrictions.
With two sides of the site bordering congested city centre streets, routed only 1m from basement piling, Manchester City Council had demanded a maximum wall deflection of 15mm over the full 17m pile length. Expanded's CFA rigs could offer a contiguous piled wall to a 1 in 200 verticality tolerance – three times the norm.
Where a third side of the basement piling ran similarly close to the Victorian brick lining of the channelled Rochdale Canal, the design brief was for negligible water seepage through the wall and into the site. Here the piling design switched to secant wall, installed by the same rigs and producing a near watertight barrier.
Key to this piling versatility has been two of the company's latest rigs – a Casagrande B300 and Soilmec CM120.
Both offer 24tm torque and can boost this boring strength with a simple yet innovative winch system.
The 290m long perimeter basement wall – with its 283 contiguous piles, plus 66 pile secant section by the canal – was completed in November. The Casagrande rig installed the 750mm diameter contiguous piles, with the Soilmec completing the 40m long secant section.