| The shaft is the throat of the coal mine,and the stability of the shaft is related to the safety and economic benefit of the coal mine.Since 1987,the damage of shaft in Huang Huai area has occurred,which has brought great threats to the safety production of coal mines.After years of research and practice,it is found that the water hydrophobicity at the bottom of the deep unconsolidated alluvium is the fundamental cause of the shaft failure.In order to ensure the long-term safety and stability of the shaft,based on the failure mechanism of the shaft,a method of stabilizing water level by water injection was put forward to prevent the shaft failure and be applied to the coal mine site.However,during the process of water injection,it was found that the flow rate of water injection decreased and the effect of water injection decreased.The underlying reason lies in the lack of understanding of mechanism of the water injection method to prevent the shaft failure.In this paper,taking the method of preventing shaft failure by water injection in Jining No.3 Coal Mine as an example,based on the analysis of the engineering properties of consolidated alluvium and the deformation mechanism of deep clay after the water loss,through theoretical analysis and field test,the coupling relationships between the variation of water level,the deformation of stratum and wall strain of the shaft in the water injection process were studied,and The engineering criteria for the effectiveness of water injection were put forward,and the reasonable water injection flow rate and injection efficiency coefficient were determined.At the same time,the macro and meso-scopic mechanisms of fine particles movement in the process of water injection were studied by similar simulation and numerical simulation.The feasibility of the eccentric water injection was explored,the technical control measures for increasing the flow rate were put forward,and predictive models for non-mining fracture of shaft were established.The feasibility of the application of water injection method in large drawdown conditions was also discussed,and the technical approach to control shaft failure under conditions of deep drainage in Zhuxianzhuang Coal Mine was analyzed.The main research contents and conclusions of the paper are as follows:?1?Through the analysis of the physical and mechanical properties of 79 deep-buried clay in several mining areas in Huanghuai Region,it is concluded that the density of deep-buried clay generally increases with the depth,and the clay liquidity index decreases with the depth.It shows that with the increase of the depth,clay changes from plastic condition to hard plastic or semi-solid conditions,and clay fluidity deteriorates.Through the analysis of volumetric flasks and thermogravimetric experience,it was concluded that the content of adsorbed bound water is greater than that of moisture,and all the water is adsorbed water.Therefore,when the aquifer was hydrophobic,the clay layer was not deformed,and the temperature limits of the forms of bound water in clay is divided.The XRD test shows that the minerals of deep-buried clay are mainly kaolinite,illite and quartz,and some clay is mixed with montmorillonite and feldspar.This shows that deep-buried clay has the characteristics of water absorption and expansion,and the process of releasing and absorbing water from clay and sand in the process of water injection was analyzed from the microscopic point of view.Also,the microscopic mechanism of water injection to prevent shaft failure was expounded.?2?Through the analysis of the geological conditions near the shaft of the Jining No.3 Coal Mine,it is found that there are relatively stable clay at the bottom of the alluvium in the industrial site.The caly has the characteristics of small bottom thickness,low permeability coefficient,and poor connectivity,which is beneficial to the implementation of the water injection method.At the same time,a comprehensive analysis of the applicability of the water injection method for the Jining No.3 Coal Mine considering timeliness,economy,and other aspects shows that the formation near the shaft of the Jining No.3 Coal Mine is injectable,and it is feasible to use the water injection method to prevent shaft failure.?3?According to the knowledge of groundwater dynamics,the distribution formula of water heads for steady flow injection in group well was deduced,and the theoretical basis for feasibility of water injection method was established.Based on the knowledge of soil mechanics,the equations of formation swelling and vertical additional stress in the shaft wall were deduced,and the mechanism of water injection to prevent shaft failure was revealed from the perspective of macro-mechanics.?Distribution formula of water head of water injection in group well:????Formation swelling in the process of water injection:????The amount of additional stress release on the shaft wall from water level is restored to the water level before the injection:????4?NM2dc numerical simulation software was used to study the intrinsic mechanism of shaft force and formation deformation under hydrophobic and water injection conditions.It was concluded that the water level decreased and the shaft wall produced additional vertical downward stress.As the water level rose,the additional stress on the shaft wall significantly reduced.During the process of water injection,the strata uplifted,and the vertical compressive stress of the shaft wall reduced.The vertical displacement of the strata in the water injection site changes most obviously,and the water injection circularly spreads to the surroundings.As the distance from water injection increases,the amount of strata uplift gradually decreases.The simulation results showed that the water injection method could effectively prevent shaft failure.?5?Through short-term industrial tests and long-term water-injection measurements,it is concluded that short-term water injection has achieved expected results of the rising of water level and the decrease of strata swelling and shaft wall compressive microstrain.There was good coupling among water level,stratum microstrain and shaft wall microstrain.In the long-term water injection process,the effect of water injection weakened,which had not reached the goal of stabilizing the water level and reducing the vertical pressure stress of the shaft wall.In the process of short-term water injection,the amount of microstrain in the formation and the change in water level are in line with the function of y=aebx as a whole,where y is the amount of microstrain in the formation,x is the amount of change in the water level,a and b are constants.The relation between the amount of microstrain in the shaft wall and formation microstrain is z=k1y2+b1y+c1,where z is the amount of microstrain in the shaft wall and y is the amount of microstrain in the formation,and k1,b1,and c1 are constants.In the process of long-term water injection,the flow rate of water injection gradually decreased.Due to the uneven injection in the stratum,the coupling relationship between local water level,formation microstrain,and shaft wall microstrain was very different.There was no unified functional relationship.?6?The effects of short-term and long-term water-injection were evaluated quantitatively.Short-term water injection reversed the trend of increased compressive microstrain in the shaft wall.Although long-term water injection did not change the trend of increased compressive microstrain in the shaft wall,it effectively alleviated the increasing rate of compressive microstrain in the shaft wall.Based on the experience of water injection during industrial tests period,the engineering criteria of the water injection validity index k and reasonable criteria for water efficiency were proposed.?Engineering criteria for water injection effectiveness index k:The formula for k is: k =?y-b?/x,where k is the validity index,i.e.the slope of the linear fitting of wall microstrain and the water injection time,y is the microstrain of the shaft wall,x is from the start of water injection to a certain time?month?,b is a constant.The engineering criteria for k is as follow:0?k?35,effect of water injection is ideal and the shaft stability can be maintained for a long timek?0,effect of water injection is not good and it is difficult to maintain the stability of the shaft for a long time?Determination of reasonable water injection efficiency value:??20(m3·h-1·Mpa-1)Q?5(m3·h-1)?7?An experimental device for the mechanism of fine particles moving to form dense circle in water-bearing sand layer during water injection was developed?patent application number: 201721548417.7?,and the characteristics of migration of fine particles such as clay powder,kaolin,and bentonite during water injection were studied.The results showed that by using clay powder as fine particles,the flow rate of the effluent gradually decreases with the water injection time,which is consistent with the results of on-site water injection.The kaolin has high solubility and is easily lost with the water flow,and the bentonite has relatively strong cementation.The water flow is slow,and the water flow starts slowly.The effluent flow at the stage increases with time,which is inconsistent with the on-site water injection results.?8?Similarity simulations were used to study the macroscopic mechanism of the formation of dense particles in the water-bearing sand layer during the water injection process.The more fine particles,the longer the water effluent was,and the smaller was the amount of water output per hour at the beginning of the water discharge.The greater the pressure was,the less time was required for effluent,and the greater was the amount of flow per hour at the beginning of the effluent.However,the effluent declined more quickly afterwards.At the same time,the PFC5.0 numerical simulation software was used to analyze the variation of mesoscopic parameters such as water pressure distribution,particle contact force chain,coordination number,porosity,and particle movement tendency.It was concluded that the parameters reflecting the migration mechanism of fine particles in the process of water injection was consistent in nature.That was,with the progress of water injection,fine particles gradually migrated toward the outlet.With the gradual formation of the water flow channel,the pressure loss of water injection reduced.The water pressure at each point in the model reaches the particle start-up power,then the particles began to migrate.The water pressure near the inlet was large,and the particles moved fast while the water pressure far from the inlet was small.The speed was relatively slow,and a relatively dense structure was gradually formed in the migration channel.As the water flow channel was further formed,the water pressure loss gradually decreased.At the same time,fine particles continued to migrate toward the outlet,and finally densely accumulate at the outlet end to form a dense structure.?9?FLAC3D numerical simulation software was used to study the effect of eccentric water injection on stress in the shaft wall by setting different injection pressure?or flow?and different distance between water injection hole and shaft.From the analysis of two aspects of the variation of vertical pressure stress and the unequal stress of the shaft,it was concluded that low pressure?0.3MPa?was adopted under the ideal condition,and the water injection at the distance from the shaft 50 m had no obvious adverse effect on the stress of the sahft.Considering that the long-term injection rate gradually decreases,it is suggested that the distance from the wellbore to the water will increase appropriately with the increase of water injection pressure when the injection pressure is not less than 0.3MPa.According to the reason of the gradual decrease of water injection flow rate,technical measures such as well washing,shaking pressurization and make-up water injection holes were proposed considering the mechanism of the migration of fine particles in sand.Well washing is in the axial direction of the water injection hole by using liquid CO2 gasification and artificial blowout,so the fine particles can be blown out of the wellhead,which achieve the purpose of cleaning up the sludge.Oscillation pressure is applied in the radial direction of the water-permeable flower tube to produce similar sound waves and increase the instantaneous energy of the water,so it partially eliminated the blocking phenomenon and increased the water injection flow rate.If the well washing or oscillating pressurized water injection was not effective,the number of water injection holes should be increased to increase the flow rate of water injection.?10?For the shaft that has not been ruptured,seven factors are selected as the discriminating factors including surface subsidence velocity,cumulative surface subsidence,main compression zone depth,shaft net diameter,shaft thickness factor,construction method,shaft construction quality,and shaft tower factors,which were used to establish Fisher's discriminant and fuzzy clustering analysis model.Based on the discriminant model,it is comprehensively evaluated that the probability of damage to the main,auxiliary,and west wind shafts is very high under the conditions of deep dropping of the levels in the Zhuxianzhuang Coal Mine.The possibility of damage to the three shafts in descending order is as follows: auxiliary shaft>main shaft> ventilation shaft.For the repeated broken shaft,considering the shaft treatment method,8 factors were selected to establish Fisher discriminant and fuzzy clustering analysis model,which included shaft diameter,unconsolidated alluvium thickness,water level drop,compression rate of pressure relief slot,fracture rating,service life rate,treatment methods and compression rate of alluvium.The Fisher model regression rate for training samples is 100%,and the fuzzy cluster regression judgment rate is 95%.Based on the two discriminant models,the stability status of the main,auxiliary,and ventilation shaft in the Jining No.3 Coal Mine in June 2016 was comprehensively evaluated,and the shafts did not fail in June 2016.?11?In response to the problem of the drop of the level in the aquifer at the bottom of alluvium that affected the stability of the shaft,a scheme,combining with the results of previous GMS numerical simulations,for different dropping rates of the level in the fourth aquifer was set up by taking Zhuxianzhuang Coal Mine with "L" shaped curtain closure drainage of fifth aquifer caused with water level dropped in fourth aquifer as example.The NM2 dc numerical simulation software was used to simulate shaft force and formation deformation at fast,medium,and low dropping velocity.Results showed that the maximum compressive stress in the shaft wall at fast,medium,and low dropping velocity is larger than the strength of the shaft wall,and the possibility of damage to the shaft wall is high.At present,the shafts under working conditions with natural dredging that a hydrophobic velocity of 0.2m/month have a low possibility of damage in the next two years.However,the stress on the shaft wall has increasing tendency,and the possibility of damage is still high in the long term.On this basis,the water injection method was discussed under the conditions of large-scale precipitation in the Zhuxianzhuang Coal Mine.The advantages and disadvantages of technical measures such as water injection method,open pressure relief slot and ground grouting reinforcement were compared.The water injection method is not suitable to use in the case of large-scale precipitation.In contrast,the ground grouting reinforcement technology is the most reasonable method. |
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