Mining-induced permeability evolution in weakly cemented strata under stress-damage-seepage coupling
The permeability evolution of mining-induced overburden in weakly cemented strata under stress-damage-seepage coupling is an important factor affecting water-conserved mining.Considering the conditions of weakly cemented strata in the 21103 coalface of Ili No.4 Coal Mine in Xinjiang,the Juras-sic weakly cemented mudstone and sandstone were selected for analysis in this study.First,the permea-bility evolution of weakly cemented rock during the full stress-strain process was analyzed.On this basis,the influence mechanism of pore structure and mineral composition of weakly cemented rock on its per-meability was clarified,and the stress-damage-seepage coupling model of weakly cemented rock was es-tablished.Finally,the permeability evolution of weakly cemented overburden was studied.The results disclose that within a confining pressure of 5 MPa,the initial permeability of weakly cemented rock is high,and the permeability decreases significantly in the compaction stage,with a maximum decrease of 91.5％,which is directly related to its high initial porosity and large pore size.At the same time,the maximum permeability and residual permeability of weakly cemented rock increase slightly compared with the initial permeability,the maximum ratios being only 4.51 times and 4.17 times,respectively.This is attributed to the closure of partial water-conducting fractures caused by the expansion and argilli-zation of water-sensitive minerals such as kaolinite and montmorillonite in the weakly cemented rock.Based on the assumptions of compressible fusion and fracture of weakly cemented rock matrix,the dam-age variable D was defined and the stress-damage-seepage coupling model of weakly cemented rock was established,which effectively characterized the permeability characteristics,high initial permeability and small increase in residual permeability,of weakly cemented rock.At the engineering scale,the weakly cemented strata in the goaf are rapidly compacted and closed after coalface mining,resulting in the emergence of a permeability recovery zone.The permeability of this area is merely 1.1-1.2 times that of the initial strata.When the continuous advancing length of the coalface exceeds the coalface length,the permeability increase zone of weakly cemented overburden evolves from being"saddle-shaped"to being"double-V-shaped",and is concentrated within the range of 25-30 m in front of the open-off cut and behind the coalface.The research results can provide guidance for water-conserved mining in weakly cemented strata.
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