Experimental Investigation On The Characteristics Of Water And Sand Flow Through Caving Rock Mass Due To Coalmining With Shallow Depth In Western China

Due to the large scale and high intensity mining in western China,quicksand or water-sand inrush is a common geological hazard in shallow coal mine under thin bedrock,which is a key issue to influence the safety and normal production in western Chinese coal mines.Halagou Coalmine in Shendong mining area was taken as the prototype,the characteristics of sand and water flow through caving zone are investigated systematically by using comprehensive methods of laboratory test,theoretical analysis and numerical simulation.The main conclusions can be drawn as follows:(1)Engineering geological model of the Halagou Coalmine was established.The geological and hydrogeological conditions and the influence of mining on hydrogeological structure of the Halagou Coalmine were studied and discussed in detail.Based on the geological conditions of Halagou Coalmine,the engineering geological model was generalized.It was found that the caving zone of Halagou Coalmine is not only the main pathway connected to the unconsolidated aquifers,but also provided the space for sand and water flowed from unconsolidated aquifers.(2)The quicksand test system was developed in the laboratory.The test device was composed of five parts:the water supply system,the model box for sand,the box using to simulate fissure or caving zone,the data capture system and the image acquisition system.This test system can be used to simulate the quicksand under the conditions of various factors in the laboratory.(3)The influence of various factors on the speed of sand flow through caving zone was studied in the laboratory,and the fitting equation between the speed of sand flow and the size of the broken mass and sand grains was proposed.Taking the broken mass in caving zone as the porous bed,the influence of the height of caving zone,the size of broken mass,the particle size of sand and the height of water head on the velocity of quicksand was investigated by using the quicksand test system.The results show that the speed of the sand flow decreases with an increase of height of caving zone,until the height of caving zone approaches a certain value.According to the test data,an equation which reflects the relationship between the speed of the sand flow and the diameter of the porous bed and sand grains is proposed.(4)According to the different migration characteristics of water and sand flow in the caving zone,the water and sand migration pattern can be divided into three different types:the seepage pattern,the driven pattern,and the clogging pattern.The characteristics of water and sand flow are related with the permeability of the sand layer,the water head and the void in the caving zone.Based on the different migration characteristics of water and sand flow in the caving zone,the water and sand migration pattern can be divided into three different types.The first type is the seepage pattern.Under the condition of high permeability of sand layer and low water head,the water pressure on the sand is relatively small during quicksand.When the water level was lowered under the surface of sand,the sand flow stopped,then the water flow stopped.The second type is the driven pattern.Due to high water head,the water pressure on the sand is relatively high during quicksand.In this situation,the height of the column of water and sand both dropped,while decline speed of sand column is faster than that of the water column.When the sand totally rushed into the caving zone,the sand flow stopped,however,the water flow still continually flowed into the caving zone.The third type is clogging pattern.This kind of quicksand happened in the condition of a low permeability of sand layer and a small pathway in the caving zone.The sand getting into the caving zone clogged the void in the caving zone,and changed the rock mass skeleton.Due to the low permeability,the sand column was not moved,and the speed of water was very slow,the water column could be thought constant.(5)An equivalent model of porous bed for caving zone was proposed.Test data indicated that the void of porous bed could be regarded as a tube bundle model,but not simplified as a single pipe model,which meant that the sand flow through porous bed can be regarded as parallel flow flowing through multiple pipes,and therefore,the relationship between the diameter of the porous medium and the number(D_p?N)and diameter of pipes(D)was given as_p=(1-0)).(6)The mechanism of quicksand was analyzed by using the discrete particle dynamic model and two-fluid model.Based on the force analysis of single sand particle,the equation of relative speed between water and sand flow was established by using the discrete particle dynamic model.In the two-fluid model,the sand with water was regarded as a kind of liquid,whose seepage law could be described by Forchheimer equation,and the water flow through porous bed could be described by Darcy law.The mathematical model between the speed of flow and the pressure gradient was proposed by using the seepage continuity equation of the porous bed.In addition,the reason of clogging phenomenon during quicksand process was also explained from microcosmic point of view.(7)Applied the random geometrical model to construct the numerical simulation model of quicksand,the panel 22402 of the Halagou Coalmine was taken as the prototype,a numerical model of quicksand through the broken rock mass of caving zone with random shape was constructed by using PFC.The simulation results show that the poorly graded rock masses in caving zone have a certain effect to block the sand flow;the caving zone with large voids or pores and good connectivity are the dominant pathways for sand migration.The velocity profiles of sand flow are spindle type,which means that the sand flow velocities along the centerline are larger than those near the wall.