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Ground stabilisation: Mine shaft threatens Oldham water mains

Midlands-based M&J Drilling recently undertook a second phase of ground stabilisation work at an abandoned mine shaft in Oldham, located close to two operational pumped water mains.

Ground stabilisation

Tight fit: Work under way at Oldham

If the pressurised mains had been damaged or fractured, it could have led to flooding and inconvenience to utility owner United Utilities’ (UU) customers. The water mains were thought to be at risk after ground settlement and a localised surface depression were observed.

Initial probing by M&J Drilling in early 2013 linked the surface depression to consolidation of material overlying the abandoned mine.

Stabilisation work at the site at Rhodes Bank, Oldham began in late November 2013, under the supervision of UU’s engineering department, with the UU Network Construction Alliance acting as the CDM co-ordinator, and was completed in December.

The works were designed by M&J Drilling following discussions with UU engineers, and were phased to minimise the risk of movement caused by loss of ground support. The water mains are ductile iron pipes of nominal 600mm and 450mm diameter at depths of between 800mm and 1m.

Prior to deep drilling within the original shaft column, a 5m deep grout plug was formed over a 14m square area centred on the shaft; 36t of neat cement grout was injected in stages by use of inclined drilling, and then working ­gradually away towards and beyond the shaft.

Having bound up and strengthened the superficial deposits by consolidation grouting, a large lattice steel platform was moved into position and decked out with timbers ready to allow a Klemm 806 rig to set up and drill within the body of the shaft.

The shaft was believed to have been sunk to a considerable depth, and the design of the stabilisation works required the in-filled column to be drilled and pressure grouted to a maximum of 150m below ground.

Three holes were drilled to depths of 150m, 100m and 75m in alternate positions within the shaft to confirm the quality of the grouting work.

On completion, 243t of 10:1 pulverised fuel ash/cement grout was injected into the shaft column. The second and third re-drills accepted only 3.85t and 1.65t respectively.

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