Green mining technology of overburden isolated grout injection

Backfilling is a technology for reducing both mining damage and solid waste emission in coal mines. The o⁃ verburden isolated grout injection technology was proposed based on the accumulative effect of strata expansion induced by stress relief. By identifying the key strata controlling surface subsidence and overburden movement， this technology utilizes the new method of compaction grouting into the main injection horizon below a key stratum， which transfers the expansion volume of overburden strata into injectable zone. The injection ratio is significantly in⁃ creased，and a compaction zone that supports the overburden key strata and reduces subsidence is then generated，thus allowing the strata control and protection of surface buildings and infrastructures. The overburden isolated grouting uses fly ash，coal gangue，and coal slime as the injection material，allowing the reduction of coal waste by utilizing mining⁃ induced voids and fractures. The process and characteristics of slurring were proposed for different types of solid wastes. The transformation process of solid⁃grout⁃fill and the characteristics of grout flowing， bleeding， and consolidation were discussed for the injection material. The grout bleeds as soon as it flows into the overburden fracture from the borehole. Under the injection pressure，the grout is compacted continuously，and the water flows into the sur⁃ rounding rock layers. The saturation of the strata play a vital role in the seepage of bleeding into the underground long⁃ wall face. The grout is finally transformed into a compaction fill，which ensures the long⁃term stability of surface sub⁃ sidence control. In addition to subsidence control and solid waste emission reduction，the overburden isolated grou⁃ ting can also be used for mine quake reduction and outburst prevention. It has been successfully applied in some com⁃ plex geological and mining conditions such as non⁃pillar mining，adjacent gob mining and multi⁃seam mining. A cer⁃ tain thickness of bedrock in the overburden is needed for the application of this technology，which allows a key layer that can be determined as grouting horizon. This technology still needs to be further tested in mine quake reduc⁃ tion，prevention of outburst，and water conservation mining.