May Gurney Geotechnical is undertaking a £321,000 seven week piling project for Durkan for a new apartment block in west London.
The site itself is extremely tight: it is enclosed on two sides by existing buildings, Kensal Road runs the length of the front of the site and there is a canal along the back.
The piling fell into two packages. The first involved sinking a contiguous bored pile retaining wall for a basement, consisting of 287 CFA piles, each 450mm in diameter and varying in depth from 9m to 17.5m. The shorter piles - those between 9m and 13m - are designed for retained height, while the 17.5m deep piles are needed to support columns with loads of up to 635kN.
May Gurney built one section of the wall from 26 bored piles, 600mm in diameter, using its low headroom rotary rig. This was due to height restrictions caused by an overhang in an adjacent building. Larger diameter piles were used because of a surcharge from the building's foundations.
The second phase of piling was for 91 rotary bored piles, all 600mm in diameter and sunk to between 14.7m and 23.7m in depth for load bearing purposes within the basement area. They were designed to a safety factor of 2.0, and have full length reinforcement cages consisting of six T32mm main steel bars to accommodate compression and tension loads.
Rotary bored piling was used rather than CFA due to the restrictive size of the site and because the pile caps needed to be cast at basement level, 5m to 6m below platform level.
Other options would have been to excavate to basement level and then crane a CFA rig into the hole, or to install CFA piles from platform level and then excavate around them. This would have cost more due to concrete wastage, pile cutting and a more difficult and time consuming excavation.
May Gurney solved the problem by suggesting rotary piles, allowing concreting close to the required cut off level, and reducing the cost of piling.