Characteristics And Dynamic Evolution Law Of Sand Collapse Induced By Roof Caving During Mining Under Thin Bedrock In Typical Mines

Local Roof Caving under thick loose layer and thin bedrock is a kind of extreme mining situation,even if the bottom of loose layer is weak aquifer,water and sand may surge into the mining face.Roof caving is related to geological conditions,mining control and many other factors,and avoiding bad geological conditions,preventing and controlling caving and judging the development trend of water and sand surge after caving have important guiding significance for the prevention and control of this kind of mine geological disasters.The 16031 working face of Henan ZG1 Coal Mine was mined under the condition of large coal seam thickness,thick loose layer and thin bedrock,and local water gushing occurred several times in recent years.In 2019,there were sand burst casualties and flooded working face disasters.Based on the analysis of the region based on the engineering geological condition,hydrogeological condition,has carried on the analysis of characteristics of sand,and the integrated use of experimental research,theoretical analysis and numerical simulation methods and means,caving mining under thin bedrock through bedrock in the law of the collapse of loose aquifer of sand are studied,the following main conclusions:(1)Combined with the case of sand collapse in ZG1 mine,the basic characteristics of water and sand collapse in loose aquifer in this area are summarized,such as weak water abundance,great influence of periodic pressure,and controlled by bad geological body and local pumping.Thin bedrock were studied in detail-thick loose bed formation of geological characteristics,combined with the feature of roof water content in the study area,mine pressure regularity and roof strata occurrence characteristics,summarizes the collapse in the study area the sand is the main mechanism of the roof caving cause lead water fracture zones directly at the bottom of the conduction loose bed water sand,and broad distribution of clay layer in thin bedrock followed intense damage occurs after taking damage.(2)According to the characteristics of water inrush and sand inrush induced by mining in thin bedrock and its uncertainty and particularity in time and space,the types of water inrush and sand inrush are divided into burst and progressive inrush according to the length of time,and full and partial inrush according to the scale of water inrush and sand inrush.(3)The influence of thickness to span ratio,water and other factors on the instability and migration process of water-bearing sand layer after the formation of pumping channel is studied by model test.Under the same hydraulic recharge condition,the velocity and radius of sand collapse are significantly affected by the thickness to span ratio,that is,the larger the thickness to span ratio is,the less prone the sand body is to settlement and the more obvious the bottom arching effect is.When the thickness to span ratio is constant,the larger the head/sand thickness value is,the more fluid the sand body is and the weaker the shear strength is.On the contrary,due to the effect of capillary water,sand bodies with proper water content exhibit "false cohesion" and have a certain strength to resist failure.At the same time,through the analysis of stress response characteristics,it is found that under the influence of gravity and seepage force,the instantaneous response of stress is the most obvious in the direction of vertical and perpendicular to the inlet.(4)The numerical model of sand collapse was established by using discrete element software,and the whole process of sand instability,migration and stability after the formation of pumping channel under thin bedrock was simulated.According to the simulation results,the sand collapse process can be divided into three main stages,namely,the start-up stage,the stabilization stage and the attenuation stage.At the same time,the sand accumulation pattern also has an important influence on the internal stress distribution,grain migration characteristics and the rebalance process.(5)A geological model of stable structure with collapsing sand at the bottom of loose layer is established,the load transfer law of stable structure with collapsing sand is studied,and the judgment method of resting condition with collapsing sand is discussed.It is found that the soil arch phenomenon in thick loose layer is a continuous evolution process from the initial slip failure to the final equilibrium state.According to the geological model,the thick loose layer can be divided into shear failure zone and bending subsidence zone successively from bottom to top.The range and height of shear failure zone are affected by the amount of broken sand and the nature of soil.The paper has 65 diagrams,6 tables and 73 references.