Study On Mechanism Of Water And Sand Inrush In Thin Bedrock Working Face Based On Stability Of Combined Structure

In the Shendong mining region,the coal seams exist shallow burried,thick,thin bedrock,thick loose layer and rich water at the bottom of loose layer,and the fully mechanized mining or fully mechanized caving top coal technology is generally used for mining.In this case,the degree of overburden fractures increase greatly.The penetrating channel between the stope and the loose aquifer caused by mining will lead to the water-sand mixtures inflowing into the stope,resulting in the occurrence of water and sand inrush which has seriously affected the normal production and seriously threatenting the safety of underground workers and production equipment.The special geological mining conditions combined with thin bedrock and thick loose aquifer determine the particularity of the law of overlying strata movement.In this thesis,the water and sand inrush disaster of the working face caused by mining in shallow bedrock of the Shendong mining region are studied.By analyzing a large number of on-site disaster characteristics and mine pressure measurement data,the law of water and sand inrush disasters is summarized.Through the laboratory experimental study of geotechnical samples,the physical and mechanical properties of typical overburden and loose sands are obtained.By analyzing the bearing characteristics of weak rock strata,the composite structure of main control strata and its controlled weak rock strata is put forward,and the instability criterion of the "main control layer-weak layer" combination structure is derived.The characteristics of overburden failure corresponding to different load-to-load ratios were obtained by using similar material simulation experiments,and the "main control layer-weak layer" composite structure model was verified.Independently developed the burst water and sand start-up tester,designed and implemented 25 sets of burst water and sand start-up experiments based on the principle of orthogonal experiment,realized the indoor simulation of water and sand inrush disaster and obtained the characteristics of water and sand inrush pregnancy disaster.Based on the theories of hydrodynamics,statics and strength criterion,the mechanism of water and sand inrush in loose aquifer was analyzed.For the first time,the burst element of sand body which loses its restraint on the side after cutting the roof is put forward,and the stability mechanical model of lateral triangular element on the free surface of sand is established.The criterion for starting water and sand inrush disaster is given.The range and minimum amount of water burst in the water-bearing loose layer are solved by using the theory of ellipsoid ore drawing.Finally,according to the mechanism of burst water and sand in shallow and thin bedrock working face,a disaster prevention and control technology system of water and sand inrush strengthened by accelerating working face speed and coordinating working face top cutting prevention,water release and grouting at the bottom of loose layer is put forward.The research content of this thesis belongs to the problem of safe mining under special geological mining conditions.The research results have certain theoretical significance for the deepening and enrichment of special mining theory,and have strong application value in promoting efficient and safe mining of coal resources.The main achievements of this thesis are as follows:1)According to a large number of on-site disasters of water and sand inrush in the Shendong Mining region,the characteristics of water and sand inrush disaster are summarized and analyzed,which are mainly shown as follows:(1)Cutting of roof before or after roof frame during disaster occurrence.(2)Water and sand mixture influx into stope during disaster occurrence.It is concluded that overburden cutting and water-sand factor are two key factors to induce water and sand inrush.2)By analyzing the bearing characteristics of the weak strata,a "main control strata-weak strata" composite structure is proposed,which coordinates the bearing capacity of the main control strata and the controlled weak strata.According to the special occurrence characteristics of the Shendong minng region,a "single revolving rock block" structure corresponding to the basic roof is proposed.The overlying strata at any level of the thin bedrock working face are divided into a "simply supported beam" composite bearing structure and a "single revolving rock block-simple support"composite bearing structure.The overall overburden structure skeleton of the first coal seam in the Shendong mining region is divided into four types:1?k group "simply supported beam-simply supported beam" composite bearing structure,"simply supported beam-simply supported beam" composite bearing structure,"single revolving rock block-simply supported beam" composite bearing structure and "broken roof" structure.The stability criterion of the combined bearing structure of the top in the overburden skeletal model is proposed,that is,and the multiple layers of the overlying rock are obtained.The formulas for calculating the support load after the overburden structure loses its self-stabilization ability are obtained,that are H.Based on the formulas,three types of roof cutting under thin bedrock thick loose aquifer are put forward,namely,pre-support static pressure type,pre-support dynamic pressure type and post-support free shearing type,and the collapse after overburden rock cutting is established.The width model of the inrush channel and the related calculation methods are analyzed.3)Based on the characteristics of overburden in surge face,the idea of similar material simulation is put forward to determine the basic roof position of stope by the movement and integrity of overburden controlled by rock pressure,and good simulation results are obtained.When the thickness of loose stratum is 40m and the base load ratio is greater than 0.625,the overlying strata of stope continue to cut through to the surface;when the thickness of loose stratum is 40m and the base load ratio is greater than 0.75,during the initial weighting period,the overlying strata show the composite bearing structure of lower caving and middle-upper simple-supported beam,and during the period of cyclic weighting,the overlying strata show the single-return of lower caving and upper-middle-simple-supported beam.The correctness of the combined bearing structure proposed in this paper is verified by the combined bearing structure of rock block and simply supported beam.4)The test equipment for water and sand start-up conditions is developed independently,which can be used for the laboratory simulation of the whole process of water and sand inrush disaster.According to the influencing factors of water and sand inrush,the water-sand start experiment under different experimental conditions is designed by orthogonal experiment.The laboratory simulation of water and sand inrush is started under the condition of bedrock fissure,and the water and sand inrush disaster is obtained.The stage characteristics of sand have obtained the disaster characteristics of the pore water pressure drop in the loose layer before water and sand inrush starts,which provides a basis for on-site monitoring and early warning.5)Based on the theories of soil mechanics,fluid mechanics,groundwater dynamics and so on,the stability mechanics model of lateral triangular element of free surface of overburden cut backsand is established,and the starting criterion of overburden cut water and sand inrush disaster is given Based on the theories of ellipsoid ore drawing,the blowout range and minimum blowout quantity of water-bearing loose layer are solved (?).6)There are two main conditions for the disaster caused by water and sand inrush in shallow buried thin bedrock face:(1)overburden failure caused by coal seam mining and structural instability lead to overlying strata movement and formation of gushing channel;(2)after the formation of the gushing channel,the sand body of the water-bearing loose layer suffers shear failure and breaks into the stope along the channel.Combining with the relevant parameters,collapse chung start-up criterion is proposed:to decrease the size of the channel ? roof cutting technology,lower water head height of Hw cutting prevention technology,strengthening the loose bed soil column at the bottom of the shear strength of grouting technique at the bottom of the loose bed water and sand inrush prevention and control technology.In the typical working face of shendong mining area,the field engineering application has been carried out,and the good prevention and control effect has been obtained,and the correctness of the theoretical results has been verified.