Wetting collapse in opencast coalmine backfill

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