Client: Asda Superstores

Main Contractor: Midas Construction

Consulting Engineer: Clarke Bond

Quantum Contract Value: Approximately £800,000

Quantum were contracted initially to undertake gas monitoring within three shafts in the car park area. The three shafts were close together and had been capped in the early 1980s. To enable gas monitoring, we had to core drill 100mm diameter holes through the reinforced concrete caps. The cored holes revealed that the shaft fill in all three shafts had slumped. Although the slump was only about 1 metre in one shaft, the others had slumped to a considerable depth.

Clarke Bond instructed a camera survey to be carried out on the shaft linings, through the cored holes. This revealed that the lining to each shaft was not in good condition and potentially unstable.

A number of solutions were discussed, primarily with the safety of personnel in mind. It was finally agreed that two shafts would be filled with Benefil. This ensured that no-one need enter those shafts.

The third was stabilised by forming a plug at a level below rockhead, and constructing a reinforced concrete lining to the shaft using sprayed concrete. This work was undertaken from a manriding basket suspended form a crane. The sequence of treatment of the lining was from the top down to ensure that the operatives were working within a safe zone. This shaft was left open to allow drainage of the surrounding area, thus accommodating rises and falls of water levels in the shaft.

The existing RC caps were demolished using hydro-demolition techniques, also from crane baskets and MEWPs.

The shafts were quite near to a live rail line and prior to removing the existing caps (making the shafts more unstable), a 4m deep band of sands and gravels was drilled and grouted around the shafts to reduce the zone of instability to an acceptable area away from the railway line.

One large reinforced concrete cap was constructed to cover all three shafts.

The works were completed on programme and the store opened on the due date.