Risk of undermining old buildings during attempts to give them added stability has led groundwork specialist Roger Bullivant to develop a delicate method of underpinning.
Known as jack down piling, this little known technique allows steel cases to be installed in soft ground without causing vibrations that can otherwise occur when driving precast piles.
A jack down pile is typically created by forcing sections of hollow steel casing into soft ground with a hydraulic rig working off a rigid platform. When complete, the casing is filled with concrete. Piles can be used to support a new floor slab, which in turn can be tied into the walls of an unstable building to provide additional structural support.
The jack down technique was used by Bullivant this spring to underpin a Grade II listed church near Hull. The church had been slowly sinking into saturated estuary deposits beside the River Humber. Large cracks had appeared in the exterior and the vestry had begun to pull away from the nave.
Bullivant was commissioned by the English Heritage Lottery Fund to carry out £450,000 worth of remedial works, which included the installation of 56 jack down piles inside the church to support a new floor slab. The reinforced concrete slab was tied into the walls using a series of needle beams inserted at 1m intervals. Outside, a series of stitch piles were driven beneath the church buttresses to provide extra support.
Work at the church began with the installation of a dual grid of reinforcing steel throughout the interior. A ring beam was also placed around the inside of the walls.
Sacrificial polystyrene formers were secured to the reinforcement to identify the 56 jack piling positions before concrete was poured to create a 500mm thick floor. Groups of four holding down bolts were connected to the raft reinforcement in a square around each former to act as temporary anchors for the jack down rig.
The piles were designed to accommodate loads of between 100kN and 185kN and reached a depth of up to 15m, the point where layers of silt and sand give way to a firm strata of clay and chalk.
A leading section of 2m long, 140mm diameter steel tube was inserted into the void and attached to the jack, whose twin hydraulic rams forced the pile through the soft ground. More sections were connected to the the tube with a spigot joint. Completing one pile position took around an hour.
Thirty square pockets formed by Bullivant in the perimeter walls were designed to receive reinforcing steel needle beams that tie into the ring beam. Each 400mm socket was formed using hand held jack hammers.
"We had to be very careful when breaking out the walls of the church. Many of the blocks are loose and the mortar was in a poor state. The last thing we wanted to do was to further compromise the stability of the structure," says McCabe.
Stitch piles were later pre-drilled around the exterior of the church when the new concrete foundations inside and beneath the church had reached full strength.
At that point, the need to use vibration-free plant and equipment had passed. A diamond core drill attached to a mini piling rig created pilot holes at 1m intervals through an existing concrete trench, installed as part of an earlier underpinning effort. Each stitch pile was driven to a recognised set and provided added support to the perimeter walls.