| According to the geological condition characteristics,the engineering geological model of quicksand through fractures is simplified as that a sand layer exists on a thin bedrock.A fracture quicksand experimental system is designed to realize the experimental simulation of quicksand under single straight fracture condition.The experiments are conducted with dry sand and water and sand mixture under the different factors which contains sand particle diameter,fracture width,fracture aperture,fracture dip angle and hydraulic head.The transport state is observed and the images are captured during the conduction of the experiments.The mass flow rate and volume flow rate of dry and water and sand mixture are measured.The transport state under different factors are obtained.Due to the results of the experiments,the mass flow rate of dry sand and water and sand mixture in transport are analyzed by data fitting.Based on the characteristics of fracture network,the X-shaped intersection fractures and parallel crossed fracture network are investigated.The numerical simulations of dry sand transport and water and sand mixture transport through Xshaped intersection fractures and parallel crossed fracture network are conducted by using DEM and CFD and considering the factors containing fracture included angle,fracture aperture,gravitational included angle,flow rate of sand and hydraulic head.The transport characteristics and distribution proportion of sand during the dry sand transport and water and sand mixture under different factors and the effect of hydraulic head influencing the distribution proportion of sand and the distribution of dominant fractures in the fracture network are obtained.Finally,water and sand mixture transport through the fracture network in the water-conducting fractured zone are investigated,taking the Jieliangzi mine field in the Yibei coalfield as the prototype.The main conclusions are shown as follow.(1)Taking Jieliangzi mine field in Yibei coalfield as geological prototype,Jieliangzi mine field has a risk of quicksand induced by the connection between water-conducting fractured zone and watered sand layer according to the characteristics of geological conditions.Furthermore,the height of water-conducting fractured zone is calculated.The geological conditions and fracture network in the water-conducting fractured zone are simplified and a engineering geological model of quicksand through fractures is established.Moreover,the key fractures in the fracture network is simplified,resulting in that the fracture network is an assemblage formed by a series of single straight fractures.The transports of water and sand mixture in a single fracture,X-shaped fractures and fractures network are investigated respectively.(2)During the dry sand transport in a single straight fracture,the flow state of sand particles is in sparse state which is enhanced with the increment of moving distance.According to the characteristics of motion and mass flow rate with time,the transport of dry sand can be divided into three stages which are initial stage,stable stage and end stage.The mass flow rate in dry sand transport condition in stable stage is positive correlated with sand particle diameter,fracture width,fracture aperture and fracture dip angle.(3)During the water and sand mixture transport in a single straight fracture,the flow state of sand particles is in sparse state which is unchangeable with the increment of moving distance.According to the characteristics of motion and mass flow rate with time,the transport of water and sand mixture can be divided into four stages which are initial stage,stable stage,instable stage and end stage.The sand content constantly decreases during the transport,the sand content is high in the initial and stable in the stable stage,then it decreases gradually until the end of quicksand.The mass flow rate in water and sand mixture transport condition in stable stage is positive correlated with sand particle diameter,fracture width,fracture aperture,fracture dip angle and hydraulic head.(4)The starting condition of quicksand in a fracture under dry sand transport condition is that the fracture dip angle is greater than the smaller value of the response angle of sand and the friction angle between sand and fracture.Based on the state and loading features,the integrity starting condition and the locality starting condition in a fracture under water and sand mixture transport condition are obtained.According to the minimum energy method,a formula of mass flow rate of sand under dry sand transport condition in a single straight fracture is obtained,which shows the relationship between the mass flow rate of sand and fracture aperture is positive correlated by 1.5 power.(5)According to the experimental results of dry sand transport and water and sand mixture transport in a single straight fracture,empirical formulas of mass flow rate of dry sand,water and sand mixture,sand in water and sand mixture and water in water and sand mixture under dry sand transport condition or water and sand mixture transport condition through a single straight fracture are obtained.Moreover,the relationship between mass flow rate and fracture aperture is positive correlated by 1.5 power under dry sand transport condition in a single straight fracture.But the relationship between mass flow rate of water and sand mixture and fracture aperture does not fit the positive correlation by 1.5 power under water and sand mixture transport condition in a single straight fracture.However,the relationship between mass flow rate of the sand in water and sand mixture and fracture aperture is positive correlated by 1.5 power while the relationship between mass flow rate of the water in water and sand mixture and fracture aperture fit the positive correlation by cube,which means the water flow in water and sand mixture is satisfied with the cubic law.(6)Due to the transport of dry sand in X-shaped fractures,the distribution proportion of sand is obtained under different fracture included angle,fracture aperture,gravitational included angle,flow rate of sand in dry sand transport condition.The distribution proportion of sand of the fracture of which the dip direction is same to the original fracture is positive correlated with fracture included angle and fracture aperture.The fracture which have a smaller gravitational included angle is dominant in distribution proportion of sand while flow rate of sand does not influence distribution proportion of sand obviously.(7)Due to the transport of water and sand mixture in X-shaped fractures,the distribution proportion of sand is obtained under different fracture included angle,fracture aperture,gravitational included angle,flow rate of sand and hydraulic head in water and sand mixture transport condition.The enlargement of fracture included angle and flow rate of sand increase the distribution proportion of sand of the fracture of which the dip direction is same to the original fracture,but the effect of flow rate of sand is unobvious.The enlargement of fracture aperture increases the distribution proportion of sand of the fracture of which the dip direction is same to the original fracture after initial decrease.The gravitational included angle dose not affect the distribution proportion of sand.Furthermore,the fracture with high difference of hydraulic head is more possible to be dominant channel in X-shaped intersectional fractures.(8)Based on the numerical simulation results of water and sand mixture transport through a parallel crossed fracture network,the rules of hydraulic head influencing the distribution proportion of sand and the distribution of dominant fractures in the fracture network are obtained.With a condition of a low hydraulilc head,the dominant channels,which show serration,are concentrated in the bottom of sand inlet.With the increment of hydraulic head,the dominant channels gradually extended and strengthened outward.The incerement of hydraulic head makes the distance of sand transport further,the influence scope of sand streams larger and the sand concentrates in the far-end of the influence scope.(9)The simulation of quicksand through fractures induced by mining in the coal layer of the Jieliangzi mine field in Yibei coalfield shows that the dominant channels of sand stream in the fracture network in the water-conducting fractured zone are the approximately vertical fractures connecting the watered sand layer.After crossing the fracture network in the water-conducting fractured zone,the outlets of sand streams concentrate on the two sides of the water-conducting fractured zone.Therefore,the control of the development of water-conducting fractured zone is significant for the prevention of quicksand. |
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