Bullivant's gothic church project: Focus on Piling and Ground Engineering

Efforts to correct subsidence at a Grade II listed church beside the River Humber are set to preserve 800 years of local ecclesiastical heritage. St Giles’ Church in the village of Marfleet near Hull was built in 1883 and is the third place of worship to stand on the site.

The current church was set to go the way of the previous two after severe cracks were noticed on the exterior, and its vestry began to pull away from the nave. Ground conditions change twice daily, with the incoming tide forcing water up through estuary deposits.

Remedial work carried out over 20 years ago to underpin the perimeter of the church with a mass concrete pour had only limited success. A more durable foundation was suggested and commissioned following site investigations, and the £450,000 works are supported by a restoration grant from the English Heritage Lottery Fund.

Foundation specialist Roger Bullivant was called in and has been working either side of Christmas to secure the perimeter walls and tie them into a new reinforced concrete floor slab. Its foundation design features the installation of 56 ‘jack down’ piles inside the church to provide support to the floor slab, and the insertion of ‘needle’ beams into the walls. Outside, a series of stitch piles will be driven beneath the buttresses to provide added support.

Thirty square pockets, extending to a depth of 400mm through the 500mm thick perimeter walls, were formed using hand held jack hammers. Reinforcing steel was then introduced to each pocket, which ties into ring beam reinforcement running round the inside of the walls. Two layers of equally spaced reinforcement were then installed throughout the interior of the church to provide added strength to the 500mm thick concrete floor.

Bullivant’s site supervisor Kevin McCabe says: “We had to be very delicate 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.”

Voids within the concrete slab for the 56 pile positions were formed by placing conical shaped, sacrificial polystyrene ‘formers’, secured within the two layers of equally spaced reinforcement. Groups of four holding down bolts were concreted to the raft reinforcement, in a square around each former to act as temporary anchors for the jack down rig.

Each pile is designed to accommodate a load of between 100kN and 185kN and will reach a depth of up to 15m, where layers of silt and sandy clay make way for a firm strata of clay and chalk. A leading section of 140mm diameter steel tube, measuring 2m in length, is inserted into the void and attached to the jack, whose pair of hydraulic rams force the pile section down through soft ground. Further sections are connected to the previous lengths of steel tube with a spigot joint. The process of completing one pile position takes around one hour.

Ready mixed concrete is then pumped into each pile position to complete the foundation and form a secure connection with the reinforced concrete floor slab.

The jack down system was chosen to install piles inside the church to avoid unsettling the fragile nature of the structure, says McCabe.

“This form of piling creates very little vibration and is an almost silent method of creating a foundation. We could not afford to drive the piles into the ground in a conventional manner because of the risk of causing further structural damage. We therefore believed this was the best method to adopt in this situation.”

Stitch piles are to be pre-drilled around the exterior of the church when the new concrete foundations inside and beneath the church reach full strength. At that point, the need to use vibration-less plant and equipment will have passed. A diamond core drill will be attached to a mini piling rig to create pilot holes at 1m intervals through the existing concrete trench, installed as part of earlier underpinning efforts. Each stitch pile will be driven to a recognised ‘set’ and, when concreted, will provide added support to the perimeter walls.

 “Three years ago, it was feared that the church may not survive much longer,” says project manager Richard Ellis, who is overseeing the restoration on behalf of the St Giles District Church Council. “But we can now look forward to the church reopening and standing for many generations to come.”