Effects of arch structure in unconsolidated layers on fracture and failure of overlying strata

Abstract

The conventional model for bearing structures was established based on the assumption that the effect of unconsolidated layers can be assumed to be similar to uniformly distributed loading, while ignoring the variations in the loading stress imposed on the strata owing to an arch structure in the unconsolidated layers (ASUL). In order to make up for this deficiency, in this study, a mechanical model of the arch structure in the unconsolidated layers was established, and the conditions for the formation of the arch structure in the unconsolidated layers were derived. The evolution of the arch structure in the unconsolidated layers during excavation was simulated using two-dimensional physical simulations. Mechanical models of the fractural characteristics of the key stratum (KS) and the voussoir beam structure (VBS) based on the arch structure in the unconsolidated layers were also developed. The simulation results showed that, when the thickness of the unconsolidated layers is larger than the critical thickness corresponding to the formation conditions, an arch structure in the unconsolidated layers can form. The critical thickness is affected by the mining width and the thickness of the bedrock strata. In the absence of the arch structure in the unconsolidated layers, the loading stress applied on the underlying strata can be assumed to correspond to uniformly distributed loading, as is the case in the traditional model. When affected by the arch structure in the unconsolidated layers, the loading stress of the overlying strata is transferred to the abutment of the arch structure in the unconsolidated layers. There is a stress-decreased zone under the arch structure in the unconsolidated layers, which can decrease the loading stress on the underlying strata. In longwall face (LW) 7130 of the Qidong coal mine, in an area affected by an arch structure in the unconsolidated layers, the fractural interval of the key stratum will increase and the key strata will be fractured successively upward. However, the sliding and rotational failures of the voussoir beam structure will not occur under the arch structure in the unconsolidated layers. In the absence of the arch structure in the unconsolidated layers, the fractural interval of the primary key stratum will decrease, resulting in the complete breakage of the overlying strata and the sliding of the voussoir beam structure. The results of the theoretical calculations were verified through field observations performed in longwall face 7130.