Analytical geometry model of narrow strip backfill slurry flow and roof-contact optimization

Roof-contact rate of backfilling greatly affects the overall quality of backfilling and the stability of goaf. The roof-contact rate of the backfill is insufficient, the lack roof-contact area formed at the top of the goaf will bring many safety hazards, such as roof collapse, sheet wall, surface settlement, harmful gas accumulation and other problems. In order to explore the causes and influencing factors of the under-contacted roof area and optimize the backfill and roof-contact process, this paper analyzed the expansion law of the backfill slurry based on the multi-point stratified backfill technology, and analyzed the narrow strip multi-point stratified backfilling process. The flow and expansion laws of the backfill slurry before and after roof-contact were quantitatively characterized. Meanwhile, based on the formation mechanisms, the enclosed area after the backfill slurry reaches the roof was classified into “Type I enclosed area”and “Type II enclosed area”, and a criterion for the non-roof-contact area was established. On the basis of the research on the analytical geometric model of backfill slurry expansion in narrow strips, an analytical model for multi-point and multi-layer backfilling in narrow strips was developed, and a near-horizontal narrow-strip backfilling simulation test was conducted to obtain the length of the enclosed area under different processes. The rational spacing of the material-feeding points and the roof-contact backfilling height significantly influence the improvement of the roof-contact rate of backfilling. For the backfilling enclosed area, a full-range exhaust/water-pipe method was proposed. This pipe is unidirectionally permeable to water and air but impermeable to the backfill slurry. A physical similarity test bench for backfilling was constructed. Multi-point and multi-layer backfilling tests and full-range drainage/gas tests were carried out. Meanwhile, geological radar was utilized to detect the distribution pattern of the non-roof-contact area in narrow strips of a backfilled mine, and a field test for optimizing roof-contact with full-range drainage/gas was carried out. The distribution pattern of the non-roof-contact area in the physical simulation test and the field test highly coincides with the results of the analytical model for multi-point and multi-layer backfilling in narrow strips. The full-range drainage/gas method can effectively enhance the roof-contact effect of narrow-strip backfilling.