| The Jiaozuo mining area is located in the typical North China coal field,and its working faces are greatly threatened by lower karst fractured aquifers,especially for coal mines threatened by coal and gas outburst at the same time.Lower gas-extraction roadways are usually constructed for these working faces and are likely to become water-conducting passageways during the mining process,which would greatly increase water inrush risks and had led to multiple accidents in the Jiaozuo mining area.On the other hand,several water inrush accidents occurred in recent years at working faces that had been reinforced by grouting,indicating more research needs to be done on water inrush mechanism and comprehensive evaluation for grouting.In this regard,mechanism of water inrush from floor for faces with lower gas-extraction roadways,mechanism of grouting reinforcement in floor,and risk assessment for water inrush at grout-reinforced faces were studied through various methods including case study,theoretical analysis,numerical modeling,in-situ measurement,in-door testing,and physical simulation.Innovative results obtained from this study are as follows:(1)To obtain the stress state in the surrounding rock of lower gas-extraction roadways that lie beneath and are parallel to the haulage roadway,the homogeneous semi-infinite elastomer model with uniform and linear loads was adopted for this plane strain problem.Considering the impacts of original rock stress,weight of fallen roof in the goaf,water pressure exerted by the confined aquifers,and mining-induced abutment pressure,the stress state at the lower gas-extraction roadway was calculated by the superimposition of far-field impacts given by the above-mentioned loads.(2)The water-resisting layer in the floor impacted by the lower gas-extraction roadway was analyzed through a 'thin plate' mechanical model.By analyzing the deflection of the water-resisting layer under water pressure,maximum deflection was found at the geometric center of the thin layer.Similar to the failure process of the roof,the 'O-X' type of breakage would occur for the water-resisting layer.The existence of plastic zone of the lower gas-extraction roadway would locally influence the strength and thickness of the water-resisting layer.If the gas-extraction roadway is constructed beneath the edge of the thin plate,its plastic zone is likely to contact the factures and cracks produced from the 'O-X' breakage of the water-resisting layer,thus leading to water inrush accidents.(3)Results from the numerical simulation by FLAC3D showed that the existence of the lower gas-extraction roadway increased the intensity of stress concentration in the coal wall by 12.8%and increased the failure depth in the floor by 16%;after cutting the roof,the scope of the plastic zone of the haulage roadway narrowed,and the intensity of stress concentration in the coal wall decreased by 6.8%;by filling the lower gas-extraction roadway,the intensity of stress concentration in the coal wall decreased by 9.2%,and the failure depth in the floor decreased by 6.9%.Results indicated that the existence of the lower gas-extraction roadway would increase the risk of water inrush from floor,but measures of cutting the roof and filling the lower gas-extraction roadway would be beneficial to the prevention of water hazards.(4)The maximum mining-induced failure depth in the floor at face 14141 of the Jiulishan coal mine estimated by empirical formula for cases with complete roof was 11.49,and observation in the floor borehole with direct current method showed that the maximum failure depth after cutting the roof was 9.82m,which was 14.53%less than the estimated one.The comparison showed that cutting the roof could effectively mitigate weighting intensity,reduce failure depth in the floor,and thus lower the risk of water hazards.(5)The in-door ultrasonic tests showed that the elastic modulus varied greatly for rock samples with different lithology.Among them,the elastic modulus was the highest in the limestone samples,moderate in the sandstone samples,and the lowest in the mudstone samples.After grouting was applied,the elastic modulus of the fractured rock masses increased by 104.1%for dry condition and by 102.9%for wet condition compared with the value measured before grouting,indicating that grouting could effectively enhance the continuity and integrity of the fractured rock masses and improve its mechanical properties.The test results macroscopically supported the'upgrade and downgrade' mechanical model for grouted fractured rock masses.(6)Based on the information of more than 2200 grouting boreholes in floor at 13 typical working faces in 5 main coal mines(4 of which suffered from inrush accidents)in the Jiaozuo mining area,a novel system of risk assessment for water inrush from floor at grout-reinforced working faces is proposed,which includes 2 impact types,7 impact factors,3 categories of factors based on their significance,8 analytic figures,and 2 methods for risk evaluation.Each part of the system is clarified as follows:'2 impact types' refers to 2 kinds of inrush-related factors for grout-reinforced working faces,which,regardless of the differences in mining-induced impact,are grouting condition factors and grouting effect factors.'7 impact factors' refers to 4 grouting condition factors,which are ?dangerous zone of water inrush at working faces,?relationship of water volume and pressure in grouting boreholes along strike,?water abundance and water-conducting properties of floor at working faces,?water-conducting property of floor in severely dangerous zones,and 3 grouting effect factors,which are ?difference in grouting amount between normal and fault zones,?relationship of pressure and cumulative grouting amount over time in grouting boreholes,and ?variation of pressure,water volume,and grouting amount(referred to as 'three indices').'3 categories of factors based on their significance' refers to significant factors,moderate factors,and implicit factors.5 significant factors include location of inrush points,area proportion of severely dangerous zone,type of water volume distribution in severely dangerous zones,grouting amount per area in normal zones,and comprehensive evaluation of the "three indices" in grouting boreholes;2 moderate factors include water source for working faces and water volume in grouting boreholes,and water abundance and water-conducting properties of floor at working faces;2 implicit factors include grouting coefficient in fault zones,and relationship of pressure and cumulative grouting amount over time.'8 analytic figures' refers to 8 kinds of analytic figures,which contain information on ?distribution of dangerous zones in floor,?variation of water volume vs.pressure along strike,?borehole quantity along depth,?cumulative times for moderate-to-large water volume in grouting boreholes along depth,?cumulative water volume in grouting boreholes along depth,?division of grouting area,?variation of pressure and cumulative grouting amount over time in grouting boreholes,and ?variation of pressure,water volume,and grouting amount over time in severely dangerous zones.'2 methods for risk evaluation' refers to 2 different methods for evaluating inrush risk from floor.One evaluates the risk according to water pressure,water volume,and grouting amount,and the other evaluates the risk by applying fuzzy mathematics to the analysis on the significant factors.(7)By taking the significant factors from the significance analysis as evaluation indices and determining the weight of each index using analytic hierarchy process,the risk of water inrush from floor at grout-reinforced working faces was evaluated based on fuzzy clustering theory.Results show that water inrush risk was low if the fuzzy clustering value(referred to as R)was less than 0.275;risk was moderate if R lay between 0.275 and 0.389;risk was high when R was more than 0.389.In the last case,extra measures should be taken to prevent the occurrence of water inrush.(8)The expressions on trace of grout diffusion and water pressure in the vertical direction were constructed for the case of grouting through a single inclined borehole,based on which 6 grouting modes were developed,such as grouting in open and closed fractured aquifers through a single inclined borehole,and grouting in complete rock masses through two boreholes.Engineering criteria were given considering water pressure,water volume,and grouting amount during the process of sequential grouting,and application of the criteria were also described.Finally,the sequential grouting model by borehole groups were established. |
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