Protection Measures against Mine Subsidence Taken at a Building Site

With little mining information, old abandoned coal workings were grouted beneath a proposed building site. Mine stabilization was found necessary after an investigation of subsidence potential was performed. The investigation indicated that the 67 m deep abandoned mine, although old, still had not collapsed and consequently presented a significant risk of subsidence in the future. Further, based on the history of subsidence over old mines in the area, mine grouting was recommended. To stabilize the mine, about $\mathrm{11,468}{\mathrm{m}}^3$ of grout were pumped. The grouting was designed to account for the significant amount of rubble which existed in the mine. There were three grout mixes of different flowability characteristics specified. The 1,725 kPa grout was batched at a slump of 10 and 20–22 cm and at a efflux flow rate of 30–40 s. Permeation, squeeze (or intrusion), and compaction grouting was performed per the project specifications. Also, to economize on the amount of grouting necessary, subsidence resistant features were incorporated into the design of the building.