ALCONBURY WESTON BRIDGE, CAMBRIDGESHIRE

Pump set up

PU injection, evidence of foam migration

Completed injection (before clean up)

Coring to facilitate strength testing

Core with evidence of PU penetration (showing yellow)

Strength testing

Alconbury Weston Flood Bridge is a 18C bridge, which carries North Road (C105) through the village of Alconbury Weston over a flood channel. The structure consists of five two-brick-ring arches and two 750mm concrete pipes either side of the brick arches. The arches vary in size, springing height and age. Following a Principal Inspection that had determined a reduced capacity of 7.5 tonnes, Atkins Ltd, on behalf of Cambridgeshire County Council, commissioned Tempo-PCE to increase the capacity of the structure to a 40 tonne GVW loading. The client was uneasy with the conventional methods of saddling/ backfilling by removal of and reinstatement of the road surface for this project, as this approach would have caused too great a burden on the local road network.

The alternative method of strengthening via a low-pressure injection into the backfill, was trialled in an onsite feasibility study by Tempo-PCE: Two alternative materials, a two-part polyurethane based grout and an acrylic and micro-fine cement mixture were used in the trial. The feasibility study allowed to design a tailor-made methodology and to confirm void ratios, as well as carrying out a post injection assessment. The Graham Cole Consultancy Ltd completed the trial assessment and post completion numerical analysis. A two-part, non-expansive PU grout was injected over a 500mm staggered injection grid. Post-completion cores revealed excellent saturation, re-bonded brick-arches, a consolidated backfill, well-bonded to the extrados, and the bridge achieved the desired 40 tonne loading.