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SAVINGS SCHEME

UNDERPINNING - Underpinning a bank in the west of England has called for special plant to meet the challenge of stabilising a structure with 400 years of hidden history.

Hidden behind the Georgian facade of a high street bank in Hereford, Roger Bullivant's Underpinning & Minipiling division has been securing a building with a long history.

The cellar and a two storey masonry and timber framed structure, enclosed within its three storey steel frame, are what remains of the Swan & Falcon, a 16th century inn and coach house. Close inspection had revealed the structure has been slowly sinking - partly as a result of the site's medieval use - and needed support to prevent a major collapse.

Although public areas of the bank are open and roomy, works access was more restricted.

The cellar is reached via the bank's relatively modern basement, down another flight of stairs and through a passage too tight to stand in.

Plant for the £70,000, contract won from main contractor Graybuild, had to be assembled in the cellar and concrete pumped through the bank's back door and down a hole in the floor.

The cellar is 5.5m wide and 10m long with a vaulted ceiling. Four chambers formed by cross walls were added after the original structure was built.

Structural engineer Stewart & Harris associate director Andy Pearson says: 'A desk top archaeological investigation revealed that the Swan & Falcon was built close to a medieval rampart and ditch which had been filled in as Hereford's city walls took shape.' 'Window sample boreholes through the cellar floor showed the fill is a mixture of peat, decayed vegetation, fragments of leather and other evidence of medieval life. Inclined probes confirmed the outer walls are on dense sands with the exception of one corner which is over the line of the ditch.' Cores were also drilled through the cellar's vaulted ceiling. The floor above, says Pearson, appears to have been built up over time with new concrete screed to level the floor on more than one occasion as the central section of the cellar has gradually settled.

Bullivant's contracts supervisor Byron Lambert says: 'Maximum headroom was less than 2m and the smallest clearance above jacking points was no more than 1.2m. We had to build a one-off hydraulic rig for the job, powered by a three phase 2.2kW electric motor.' 'The machine stands about 800mm high and can drive sections between 600mm long and as short as they can be cut due to the sensitivity of the hydraulic controls. Pile casings were driven in sections down to 50mm long where the headroom was at its lowest, ' he says.

Bullivant built a 350mm deep reinforced concrete raft across the entire floor of the cellar.

The slab is tied into the cellar's outer walls by 350mm diameter needles, which were diamond cored and then cast with the raft insitu.

Some 39 steel cased piles were jacked down with the slab as kent- ledge before being grouted, to underpin the concrete raft and the whole cellar and structure above it. Steel pots were cast into the raft at each pile location with threaded anchor points for holding down the jacking plant.

Site workers also drove steel pile sections, 250mm in diameter, into dense sands 3m below the concrete slab and grouted them to form support for the new slab. It is hoped that the end result is a sound structure from the sandstone up to the timber and masonry building above, via the piles and slab tied into the cellar's outer walls.

But construction of the new reinforced concrete slab temporarily accelerated the settlement. The raft was isolated from the cross walls to prevent its kent- ledge action from lifting the internal part of the structure. As a side effect, Bullivant was tightening temporary props beneath the vaulting as piling got under way.

Lambert says: 'We noticed some widening of cracks in the internal walls after the slab was cast, probably due to the weight of the slab on the fill below.' There has been no additional movement since piling was completed, which suggests the design has worked. The raft and the cellar's outer walls are now founded on the dense sands and the slab is being connected to the cross walls with dry packing.

Main work on the overall £150,000 restoration contract was completed in December.

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