M6 Junction 26, Orrell Interchange

Soil Engineering undertook major consolidation works to shallow abandoned workings in the Smith Coal Seam beneath an existing motorway link road, as part of a scheme to improve the pinch point at Junction 26 of the M6 motorway.

The works involved drilling a grid of rotary percussive holes across the treatment area and injecting the workings with a cementitious grout in order to render the ground stable and enable the safe upgrading of the road.

The carriageway was required to remain live throughout the duration of the works, with the exception of night time and weekend closures. The majority of the drilling and grouting was undertaken from the verges at the side of the carriageway, with a large number of inclined holes calculated to intercept the seam at the correct sub-surface location.

Soil Engineering completed the works successfully in a very challenging environment, using 2no rotary percussive drilling rigs and a large-scale bulk grout mixing set-up. Boreholes were drilled using water flush, in line with Coal Authority permit conditions, due to the proximity of housing, and to minimise dust generation in the vicinity of the road network. The grout mixing compound was established remotely from the treatment area on the eastern roundabout of the motorway junction and grout was pumped to the treatment area via a network of underground ducts, designed to minimise disruption to the flow of traffic.

Initial drilling indicated that ground conditions were not as modelled, requiring a significant increase to the original scope of works. In order to minimise disruption to road users Soil Engineering were able to accommodate these requirements by undertaking night time and weekend works, avoiding peak time lane closure.

A total of 1,135no holes (11,570 linear metres) were drilled and 4,370 tonnes of PFA/cement grout were injected into the workings. Pressure testing in the treated seam at the end of the works provided assurance that effective consolidation was achieved.

The works were completed in less than 12 weeks, utilising in-house drilling and grouting plant. Strict adherence to the stringent health, safety and environmental requirements of the contract were maintained at all times throughout the works.

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