Overburden Failure In Thin Bedrock And Characteristics Of Mixed Water And Sand Flow Induced By Mining

The mixed water and sand flow inrush is a common type of geological hazards in underground coal mines adjacent to unconsolidated aquifers, which should be regarded as an independent type of mining hazard due to its special mechanism, influencing factors and treatment methods.This paper focuses on the overburden failure in the thin bedrock and the characteristics of the mixed water and sand flow induced by coal mining, choosing Taiping coalmine as a geological prototype. The mine geology, structural formation, hydrogeology, engineering geology and characteristics of the thin bedrocks were analyzed. The meaning of the thin bedrock was clearly defined. Based on the geological prototype, an engineering geological model can be taken to explore the rupture pattern and the failure features when mining under the thin bedrock. Three engineering geological modes of the mixed water and sand flow transfer induced by coal mining under the thin bedrock and thick unconsolidated aquifers were concluded and summarized. A test model of the mixed water and sand flow transfer and inrush was designed and used to simulate the startup, transfer and inrush process of the mixed water and sand flow in the overburden fractured channel. The transfer characteristics and dynamic mechanism of the mixed water and sand flow induced by mining were researched. The main achievements of this paper are as follows.(1) The failure features induced by mining of the thin bedrocks with the various lithological structures were investigated. The main geological characteristics and safety mining problem of the study area were researched and summarized in detail. The overburden failure and development characteristics of"three zones"in the study area were obtained through model tests and numerical simulations. The engineering geomechanics model and scaled model test results revealed the failure law in the thin bedrock with different lithological structures. The distribution of stress and strain was obtained through the finite element numerical simulation. The overburden movement and the periodic roof pressure of overburden were analyzed by the discrete element model. The water flowing fractured zone easily transfixes bedrocks under the condition of the thin bedrock and directly sweeps the Quaternary bottom aquifers. It would cause the water and sand inrush into underground workings.(2) The test model of the mixed water and sand flow transfer and inrush was designed and manufactured. The model can be used to simulate the startup, transfer and inrush process of the mixed water and sand flow in the overburden fractured channel. The water-rock stress coupling of the cracked surrounding rocks and the mixed water and sand flow was then conveniently researched.(3) The quantitative relationships among fractured channel width, particle size, outlet size and the mixed water and sand flow transfer in the overburden fractured channel were researched by the test model. Geological information of the mixed water and sand flow transfer for the various compositions of the flow, water pressure and the channel feature was quantitatively studied. The water pressure variation curves in the different positions were gained from different model tests. The relations between the channel width and the velocity of the mixed water and sand flow in the fractured channel were analyzed, showing that the velocity of the same mixed water and sand flow decreased with the increase of the fractured channel width. In the condition of same fractured channel width, the velocity of the mixed water and sand flow increased with the decrease of the particle size. The relationship between time and sand production volume of the mixed water and sand flow channel overflows was obtained from observational results, namely the sand content of gushing material decreased with time prolonged. Just at the very beginning the sand content of gushing material was large. With the test time and the process of seepage deformation and failure, the sand content of gushing material decreased gradually. Finally, the collapse doline formed and the sand content of gushing material became almost zero.(4) The characteristics of the mixed water and sand flow in the various types of fractured channels were researched. Three stages of the mixed water and sand flow transfer and three types of the mixed water and sand inrush were brought forward. Based on the caving zone and water flowing fractured zone channel of overburden failure by mining, the transfer channel sidewalls of the mixed water and sand flow were divided into three types; the straight and smooth with no filling, rough and stepped with no filling and filling with soft mud substance. The model tests were carried out through the three types. The variation characteristics of water pressure in different position of the fractured channel were revealed by considering the variations of the width, inclination angle, and sidewall types of the fractured channel under the water pressure of 0.05MPa and 0.1MPa. The results show that the water and sand inrush took place instantly in the mined area. Based on the characteristics of water pressure variations, the transfer process of the mixed water and sand flow in the fractured channel was divided into three interdependent stages: the increasing stage, stability stage and outburst stage. According to the characteristics of the water pressure variations during the mixed water and sand flow inrush, the mixed water and sand flow outburst was divided into three basic types: the direct injection outburst type, the saltatory outburst type and the gentle slope outburst type. Therefore, the transfer characteristics and dynamic mechanism of the mixed water and sand flow were explored.