Study on the Instability Activation Mechanism and Deformation Law of Surrounding Rock Affected by Water Immersion in Goafs

Large-scale goafs are left after coal seam mining. Due to the low-lying terrain, the goaf will be filled and soaked by groundwater, which may lead to instability of the remaining coal pillars in the goaf and cause uneven settlement of the overlying rock. Consequently, there may be overlying rock movement and surface subsidence, which endangers the safety of the building (structure) above the goaf. Considering the strip goaf of Dai Zhuang coal pillar as an example, this study investigated the evolution of instability and deformation of surrounding rocks affected by water immersion using the similar material simulation test method. The results of the study reveal that under the effect of prolonged water immersion in the goaf, the damage to the coal pillar in the strip underwent a stagewise evolution process of several instances of creep damage at the edge of coal pillar followed by overall destabilization damage, and the overburden movement revealed stage characteristics of small step subsidence several times followed by sudden large subsidence. Furthermore, based on Wilson’s coal pillar instability theory, the instability mechanism of the strip coal pillar under the action of water immersion was found to be triggered by the reduced strength of the coal pillar from the effect of water immersion, the continuous creep damage to the strip coal pillar from outside to inside, and the continuous shortening of the elastic zone of the coal pillar until its bearing capacity was lower than the load it was carrying. The research results are expected to serve as theoretical guidance for the study of coal pillar stability and the development and utilization of surface construction above goafs.