Wetting collapse in opencast coalmine backfill

Significant settlements of opencast coalmine backfill can occur if the partially saturated material is inundated by rising groundwater once mine pumping has ceased. In laboratory one-dimensional compression tests, the wetting collapse has been shown to increase almost linearly with overburden over the stress range studied and quadratically with compaction air voids. The results were used to predict the behaviour at the Sheffield and Rotherham City Airport site, and the predictions compared to field monitoring data. The modelling overpredicted the field settlements by 17%. It also revealed that the majority of the settlement may have been due to the collapse of relatively few, poorly compacted layers, and that the practice of specifying or recording only average air voids during compaction may be highly misleading.