Structure Of “Key Strata And Arch Structure In Unconsolidated Layers” And Its Application

It is known that mining-induced fractures of roof strata, subsidence, and environmental damage are correlated to longwall mining, which involves large-scale strata movement, which can lead to damage to mining equipment as well as personal injury. The key to comprehensively understanding the evolution of the movement of the overlying strata is to develop a mechanical model of the bearing structures in the overlying strata. The overlying strata consist of the bedrock and the unconsolidated layers. 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 fact, a large number of coal mines in eastern and northern China have unconsolidated layers in the overlying strata, and the average thickness of the unconsolidated layers is larger than 290 m. Given these geological conditions, the fractural characteristics of the overlying strata as determined while ignoring the effects of the ASUL do not match those determined through field measurements. More seriously, a number of coal mines in China are prone to significant mining-induced hazards. Therefore, a model for the bearing structure in the overlying strata needs to be established such that it takes into account the key stratum(KS) in the bedrock as well as the ASUL. With all this in mind, the theory of “Key Stratum-Arch Structure in Unconsolidated Layers” was proposed in this dissertation. This dissertation adopted a comprehensive methodology that incorporates theoretical analysis, simulation experiments and field trials to reveal the effects of the “Key Stratum-Arch Structure in Unconsolidated Layers” on fracture and failure of overlying strata.The bearing characteristics of KS and the morphological characteristics of the ASUL were revealed based on the mechanical model. The bearing structure in the bedrock is the KS, and the bearing structure in the unconsolidated layers is ASUL. The stress arch in the overlying strata during excavation is the pattern of the distribution of the maximum principal stress in the KS. The stress arch is not the bearing structure in the bedrock. The critical thickness of the unconsolidated layers for the formation of the ASUL can be derived from 1.2Lm-0.9Σh(Lm represents the mining width and Σh is the thickness of the bedrock strata).The mechanical model of the “Key Stratum-Arch Structure in Unconsolidated Layers” was established. The analytic solutions to the primary key stratum(PKS) and the stability criterion of the voussoir beam structure(VBS) were derived based on the model of the “Key Stratum-Arch Structure in Unconsolidated Layers”. The structure of the “Key Stratum-Arch Structure in Unconsolidated Layers” can restrict the overlying strata from moving downward. Further, it transfers the loading stress of the overlying strata to the periphery of the gob area during excavation; this can decrease the loading stress on the lower strata. With the periodic fracture of the PKS, the structure of the “Key Stratum-Arch Structure in Unconsolidated Layers” exhibited periodic failure and grew to the top face of the model; this occurs till the ASUL failed completely. Before the PKS first fracture, the fractural characteristics of the PKS is influenced by the evolution of the “Key Stratum-Arch Structure in Unconsolidated Layers”, while after the facture of the PKS, the fractural characteristics of the PKS and the stability of the VBS are both influenced by the model of the “Key Stratum-Arch Structure in Unconsolidated Layers”.The effects of the “Key Stratum-Arch Structure in Unconsolidated Layers” on fracture and failure of overlying strata were elucidated. In contrast to the results obtained using the previous model established based on the assumption that the effect of unconsolidated layers can be assumed to be similar to uniformly distributed loading, the fractural interval of the PKS will increase, and the VBS affected will become more stable. The loading reduction factor of the unconsolidated layers was modified based on the “Key Stratum-Arch Structure in Unconsolidated Layers”, which provided a basis of the calculation the equivalent loading of the unconsolidated layers in the process of the KS identification and the physical and numerical simulations. The results of the theoretical calculations were verified through field observations performed in longwall face 7130.