Crusader Avenue, Glasgow – Mine Shaft Stabilisation

Project Summary

Following on from intrusive ground investigation works undertaken at Crusader Avenue, Soil Engineering were instructed to stabilise the two abandoned mine shafts found at the site by means of bulk infill grouting.

Soil Engineering Solution

Grouting was arranged from two silos with premixed 6:1 pfa : cement. Drilling and grouting was completed using two Boart Longyear 420 rotary drilling rigs. One placed on to each of the platforms. Boreholes were sunk vertically into each shaft to confirm their depths.

Eighteen boreholes (1033 m) were drilled into or around the northern shaft, proving it to be up to 145m in depth. Five holes were treated (although four of these proved a base (ledge) at some 96m) with the injection of 487 Tonnes of grout. The grout mix was placed into the shaft through the casing and pressurised in successive 3 metre lifts to ground level.

Twenty two boreholes were drilled into or around the southern shaft, proving it to be up to 147m in depth. Eight holes were grouted with some 878 Tonnes of grout (including 47 tonnes with a gelling additive and three holes were filled or partly filled with stone. The gelling additive was tried after some 300 tonnes of grout was found not to be filling the shaft, but probably running off into workings. An investigation borehole was drilled to 42 m to allow a Sonar survey sonde to be deployed into the hole. This revealed a water filled void between 42m and 21m depth. A borehole was drilled into this cavity with a cable tool rig, 212 tonnes of 40 mm stone was conveyored into the shaft over a 3 day period. The casings were withdrawn and a new grouting hole drilled and successfully injected. Two subsequent holes were then drilled and grouted to complete the treatment of this southern shaft.

On one occasion operations were suspended when a void opened up adjacent to the northern platform, the ground funnelling in beside the, then exposed, shaft cap underneath it. This was monitored and subsequently in-filled with pea gravel down a chute off the side of the platform. This operation was repeated a few times during the early drilling.

The works were completed with a long reach excavator employed to work its way across the site raking and hammering the ground to check for any near surface cavities which may have existed and endangered the personnel and plant undertaking the next phase of treatment. None were found and the site surface was re-graded, vegetation removed and a run-off catch ditch excavated around the bottom edge of the site, so that run-off from the subsequent phase of drilling and grouting work could be contained.

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