Study On Apparent Physical Structure Of Tectonic Coal And Spallation Development Mechanism Of Coal And Gas Outburst

As the increasing of coal mine depth,the mining conditions are getting worse(such as geological condition,stress and gas condition,etc.),making coal and gas outburst disasters more serious.A considerable amount of outbursts in literatures have shown that geological movements are closely related to outbursts,and these geological areas are usually distributed with highly broken and soft tectonic coal.In this paper,using the related theories about rock mechanics,fracture mechanics,soil mechanics,fluid mechanics and adsorption theory,and based on the mathematical methods such as separation variables and linear programming,the physical structure changes of coal by geological movements are studied,and the influence of coal physical structure on microscopic pore characteristics,gas ccurrence/release ability and mechanical failure behavior are studied to analyze the outburst risk of tectonic coal.Then,based on the physical structure of tectonic coal,a similar simulation experiment of coal and gas outburst was carried out through the preparation of similar experimental materials,and the spallation development mechanism of coal and gas outburst was studied.The main conclusions are:1)Tectonic coal is a companion product produced by geological movements.It is a kind of reconstituted coal cemented by loose coal.This coal is mainly composed of tectonic fractures,and the joints are unclear or the joints lose their meaning.Influenced by the physical structure of tectonic coal,the tectonic coal has a characteristics of low strength and weak cohesion.The tectonic coal experiences a longer elastic deformation and yield deformation before the peak,while it shows a characteristics of damage expansion after the peak.So that,tectonic coal is extremely easy to be re-pulverized,and the macroscopic failure of tectonic coal cause its permeability increase after the peak to be much lower than that of intact coal.2)Using the microporosity test of coal samples with different particle sizes,the evolution of microscopic pores caused by failed/pulverized during the tectonic formation process are inverted,which indicates that the failed/pulverized of coal will change the small pores and mesopres firstly,and these pores increase with the increasing of the degree of pulverization.When the particle size of coal is pulverized to less than 0.074 mm,the micropores are also affected,resulting in a "sudden increase" of the pore volume and specific surface area.Therefore,the pore volume and specific surface area for tectonic coal are more than tens times higher than that of intact coal.Influenced by the physical structure of tectonic coal,the microscopic pores of tectonic coal are more developed,and the pore volume and specific surface area of the small pores,mesopores and macropores for tectonic coal are much higher than these of the intact coal.3)The pore characteristics of tectonic coal determine its gas occurrence and flow capacity.The tectonic coal has an extremely fast initial gas desorption capacity,which can release the adsorption gas in a short time,causing the desorption rate to decay rapidly,and the desorption amount is amlost saturated in 120 min.The failed/pulverized of coal leads to an increase in initial gas desorption rate of coal.When the coal sample is pulverized to less than 0.5 mm,the initial gas desorption rate will increase rapidly with the decrease of particle size,especially for the particle size is less than 0.074 mm.4)Based on the relationship between the geological movements and outburst,the changes of physical structure for tectonic coal causing by geological movements,and the intrinsic relationship between the physical structure of tectonic coal and its mechanical failure behavior,microscopic pore characteristics and gas release capacity,tectonic coal is shown as high outburst risk.Then,after the evaluation of the possibility for outburst occurs in intact coal,and the analysis of the role for tectonic coal in outburst,it is demonstrated that tectonic coal is a key factor for outburst.5)Based on the physical structure of tectonic coal,a similar simulation experiment of coal and gas outburst was carried out through the preparation of similar experimental materials.The results show that the outbursts are are continuous with a hemi spherical or semi conical of holes under the homogeneous coal seam conditions;the coal behind the outburst outburst hole is failed by coal spallation with spherical shell type;these spallations are arranged in layer by layer,and the thickness increases as the distance from outburst hole increases.It also shows that the average thickness of spallation decreases as gas pressure increases,while the increase of stress and decrease of coal strength lead to the increase of the width of spallation zone and the reduction of the thickness of spallation.However,under heterogeneous coal seam conditions,the soft coal area changes the distribution of outburst energy and energy consumption,and the soft coal area belongs to low energy consumption area.The soft coal area has a guiding effect on outburst,and once the sealing effect of this area is destroyed,the outburst occurs in this area firstly,resulting in a decrease of critical gas pressure threshold.When outburst developed to the edge of soft coal area,the outburst develops to the surroundin coal if the remaining energy is satisfied.6)Spallation is a failure mode of outburst coal,which is based on the physical structure of the tectonic coal.Coal spallation is a comprehensive product of multiple physical factors,including stress,gas and the coal strength.The formation process is related to stress transfer and mechanical behavior changes,gas migration and the evolution of the coal strength.The mechanical cause of spallation is the tensile failure of coal caused by the high gas pressure gradient near the exposed surface.The mechanical condition is that the gas pressure difference near the exposed surface is sufficient to overcome the tensile strength of coal.Based on the theory of spallation,it is considered that outburst develops layer by layer under the combined action of stress,gas and coal,and the spallation development mechanism of coal and gas outburst is put forward;then,it is used to describe the outburst stage and analyse the causes of typical outburs case.