Experimental study on acoustic emission characteristics of coal rock at different confining pressure

This paper conducts experimental investigation on acoustic emission(AE) characteristics of coal rock at uniaxial and triaxial compression by MTS815 rock mechanics test system. The results show that the variation of AE has tendency effect at uniaxial compression and confining pressure experiments. We indicate that AE signal decreases significantly in the elastic stage, and the relatively quiet period of AE before the specimens failure is not obvious under confining pressure. The AE amplitude distribution with time and its envelope line can indirectly reflect the trend of stress. The AE events peak stress and elastic modulus decrease with the temperature increase, and the value of m which reflects the cumulative distribution of amplitude increases as the confining pressure rises. The results also show that the AE frequency distributes in three frequency domains, and the proportions of dominant frequency significantly increase while the specimens are broken.     

The influence of initial internal structure of coal sample on gas flow characteristic

Three kinds of coal samples with different components are chosen as study objects.Density calculation and SEM are adopted to study their internal structures,and a self made triaxial gas seepage instrument is used to study their internal gas flow characteristics.The results show that internal structures of coal samples made of different particle size coal powders under identical external conditions are various,and nonlinearity is presented on the influence caused by fabric.Gas flow characteristic is complex in complete stress strain process,generally it firstly increases and then decreases,initial gas flow velocity is lower than the one after the failure of coal sample,and obvious hysteretic nature of gas flow characteristic is showed.The influence of gas flow characteristic caused by initial internal structure of coal sample is nonlinear and the stress sensitivity is high on the influence of gas flow characteristic caused by initial internal structure.The more complex the initial internal structure of the coal sample is,the lower the axial tress sensitivity is,and the higher the confining pressure sensitivity is.     

Experiments on triaxial mechanical properties of soft coal containing gas

Taking briquette specimens composed by soft coal with outbursting trends as research object, systematic research of the influence on triaxial mechanical properties of soft coal containing gas caused by confining pressure and gas pressure is done. The results show that confining pressure can significantly affect mechanical properties of soft coal containing gas, the compression phase of soft coal containing gas is obvious and the triaxial strength is increased gradually as confining pressure is increased; the influence on elastic modulus and shape of stress-strain curve is not obvious. The triaxial strength of soft coal containing gas shows decrease trend as gas pressure is increased; the influence on strength of soft coal containing gas caused by gas pressure is obvious when the confining pressure is smaller, but it is not obvious when the confining pressure is bigger. the influence caused by gas pressure on elastic modulus and deformation of yield step is not obvious; gas pressure has not only obvious mechanical effects,but also have chemical action on soft coal containing gas; stress-strain curve of soft coal containing gas can be expressed by conic, the influence caused by gas pressure can be considered as equivalent confining pressure effects.     

RESEARCH ON THE MECHANISM OF GAS OUTBURST INITATION IN MINES

The mechanism of coal-gas outburst iniration is comprehensively analysed. The direct courses of initating coal-gas outburst are the sudden loss of confining stresses and the gas pressure gradient in coal seam. The effects of working face's advancing rate, geological conditions and gas pressure gradient as well as mechanical & physical characteristics of coal on outburst initation are expounded by means of mathematic analyses about gas seepage which takes place in coal seams. The results are of certain practical significance and reference value to further quantitive study of outburst and its predication.     

Multi-field coupling effect on coalbed methane seepage characteristics and numerical simulation

This paper studies the seepage of coal samples under the influence of each single factor, the axial stress, confining pressure, gas pressure, and temperature by experiments. It uses Ansys12.0 to numerically simulate the law of coalbed methane seepage, coal deformation, pore pressure, the distribution of flow field under the influence of each single field and multi-field coupled. The numerical simulation show that the result of numerical simulation of gas seepage law in the coal under each single fields is basically the same as the experimental result. The curve of gas seepage and average effective stress under the condition of multi-field coupled is negative exponential relationship. Stress influenced the deformation of the specimen is greater than the seepage field. The axial stress on the deformation of the specimen is greater than confining presses The influence on the flow field by confining pressure is greater than axial stress, the influence of axial stress on the pore pressure is greater than confining pressure, pore-pressure under the action of multi-field is bigger than pro-pressure under each single field. To study the seepage law of coalbed methane, it should consider the influence of gas-solid-heat at the same time.     

Experimental Analysis on Crack Initiation Stress and Rupture Energy of Marble Failure

Based on the marble conventional triaxial test data, the crack initiation stress and rupture energy under different confining pressures are analyzed. Results showed that the crack initiation position is gradually close to the peak strength point with the increase of the confining pressure. The location of the crack initiation occurs in 50% ~70% peak strength. The rupture energy and confining pressure are in positive linear relationship as well as the rupture energy and peak axial stress. And sudden increase of AE energy count corresponds with the crack initiation stress. AE energy count and cumulative energy increase quickly after the crack initiation position, which increase significantly in the crack initiation point. The crack initiation point of theoretical calculation is in good agreement with acoustic emission monitoring. Mogi-Coulomb criterion is in keeping with experimental results.     

Study on Adsorption Characteristics of Coal to Gas in Geophysical Field

It is very important to mine safe exploitation, gas drawing and predict of coal and gas outburst that adsorption characteristics of coal to gas in geophysical field are studied. Adsorption and desorption characteristics of coal to gas in geophysical field have been reviewed in detail, then it is obtained that adsorption characteristics of coal to gas in electromagnetic field should be further studied . Adsorption and desorption characteristics of coal to methane in alternating electric field (AEF) have been studied mainly. Adsorption characteristics of three coal samples in AEF have been studied by means of Volume Method. The result shows that chemical properties and matter constituent of coal surface are no changed , adsorption and desorption of the coal samples in AEF well accord with Langmuir equation and two constants empirical formula, and because coal potential energy is increased and coal temperature is raised caused by Joule heat effect , adsorption ability of coal is decreased, and the desorption process is slowed down in AEF.     

Technique of Classification of Coal and Gas Outburst Mines

managing the coal mines according their dangerous grade determined. can decrease the engineering quanity and lessen the sightless characteristic. It has marked benefit for community and great economic benefit. The dangerous grade of outburst mines are determined Outburst coal mines are classified into mines with serious outburst hazard, mines with middle outburst hazard and mines with ordinary outburst hazard. Twelve classification indicators are used in comprehensively evaluating the dangerous grade of outburst mines. The dangerous grade of outburst mines are determined according to the principle of the most subjection with fuzzy comprehensive evaluation method, on the basis of the classification indicators and its quantification. By the model founded, some coal mines are studied and the research results are accordant with the fact.     

不同自燃性煤氧化阶段的表征差异

对3种不同自燃倾向性煤样进行低温氧化实验，利用CO体积分数与煤体温度间变化的计算模型，求解出活化能和煤氧化过程发生转变的特征温度，同时结合热重-差示扫描量热（TG-DSC，theremogravimetric analysis-differential scanning calorimetry）实验结果，分析了不同自燃性煤氧化特性和活化能的低温表征规律。结果表明：1）低温氧化阶段，CO生成量、耗氧量和耗氧速率随着煤自燃倾向性增强而增大；不同煤样在实验过程中出现同样的CO生成量和耗氧速率急剧上升的温度拐点，且煤的自燃性越强，该拐点温度越低，同时CO体积分数的变化具有明显的阶段性。2）不同自燃性煤氧化阶段活化能变化规律存在显著差异，当各煤样的温度到达活性温度时，活化能快速减少，且活化能变化点对应于煤氧化过程发生转变的特征温度点。3）根据煤特征温度和活化能的变化规律，把煤低温氧化进程分为4个阶段，分别为表面氧化、氧化自热、加速氧化和深度氧化。     

THE EFFECT OF STRESS ON PERMEABILITY OE COAL AND POROUS ROCK AND FRACTURED ROCK

In this paper, the permeability of coal, porous rock (sandstone), fractured rock(limestone) varifies with different confining pressure is discussed. Exponential functions between permeability and confining pressure are concluded. As a primary exploratiotion of permeability, it is significant to study coal and rock seepage characteristics and methane emission prediction. It is also useful in mine methane drainage system design.     

Experimental analysis of matrix shrinkage’s influence on mechanical permeability of gas filled coal

Based on the coal seam of Zhao zhuang coal mine and Si he coal mine belong to Jin cheng anthracite Mining Group in Shanxi province, by using the self developed three axial servo controlled seepage equipment for hot fluid solid coupling of coal containing methane, parallel experiments for permeability of helium and methane under different gas pressures are carried out to quantitatively study the influence of Klinkenberg effects on the permeability of coal. Meanwhile, the problem of permeability decreasing caused by fluid pressure decreasing and gas desorption is eliminated by controlling the effective stress. The results show thatthe Klinkenberg effect is obvious under low gas pressure. With the increase of gas pressure, the permeability of coal decreases at first and then increases. The inflection points of gas pressure for Klinkenberg effect are about 0.9 MPa both for coal samples in Zhao zhuang coal mine and Si he coal mine. When the effective stress is constant, with the increase of gas pressure, the permeability increment caused by Klinkenberg effect decreases, while permeability increment caused by coal matrix shrinkage increases. Coal matrix shrinkage has a close relationship with the mechanical properties of coal rock mass. For the coal sample with a poor mechanical property, coal matrix shrinkage is more obvious.     

Damage Analysis on Coal and Rocks and the Method of the Checking Damage by the Deviator Stress

The characteristic of the original damage and the breaking of the coal and rocks under the loading has been discussed by a lot of microscopic structure analysis and by whole stress strain testing on the coal and rocks. The non homogeneity causes the stress locating in a small spot. It is the essential cause of the damage developing and causing the structure breaking of the coal rocks. The failure of brittle fracturing, rubbing later on and granulating should be a major pattern in the outburst of coal and gas. The method of the examining damage by the deviator stress, is proposed. It can be used to analyse the characteristic of the coal and rocks.     

Test on engineering properties of unsaturated undisturbed loess Q2

A series of triaxial shear tests with controlled constant confining pressure are conducted to study the failure modes, effects of deformation, strength and yield properties of unsaturated undisturbed loess Q2, with improved SJ-1A triaxial shear equipment. The test results show that the failure mode of unsaturated undisturbed loess Q2 is shear failure with the rupture angle between 55 and 61 degree; Shear dilatancy is shown under low confining pressure, and shear shrinkage is shown under high confining pressure; The stress-strain curves of undisturbed loess Q2 will change from perfectly plastic to weak strain softening as the confining pressure increases; A effective method is suggested to identify the field stress of undisturbed loess Q2 under triaxial shear condition with the curves of - q/p ; when the confining pressure increases, the deviatoric yield stress qy has a logarithmic relationship with net mean yield stress py .     

Mechanism and application of protection againstoutburst for abrasive water jet slotting technology

Aiming at the status quote of long time for methane extraction and slow speed for laneway excavating in coal seams of Liziya south 2JHJ well, abrasive water jet slotting technology is adopted to improve the methane desorption and increase excavating speed in semi-coal laneway. Through analyzing damage model and protection against outburst mechanism, the process of Coal-Rock Mass being cut and fractured is studied. Through lab test on abrasive water jet cutting coal-bed gangue, a new abrasive water jet cutting device is successfully designed and made and all parameters are determined. It is showed that abrasive water jet can cut through a coal-bed gangue with thickness 60~80 mm and uniaxial compressive strength 62 MPa. After cutting coal seams by abrasive water jet, the surface area for methane is enlarged and then coal seams pressure fell rapidly. Therefore the permeability and desorption of coal seams are greatly increased. The results are as follows: the average productivity of single-hole was enhanced by 2.83 times.     

Relationship between effective volumetric stress and permeability of gas-filled coal

Considering the damage effect of porous media under triaxial compression, the calculation formula of effective stress is modified. It is verified that the formula can very well describe complete stress-strain process of gas-filled coal under triaxial compression. The permeability experiments under triaxial compression show that the relationship between the effective volumetric stress and permeability of gas-filled coal samples is negative exponent before the peak stress. It is still exponentially after the peak point, but the coefficient changes. The permeability of the gas-filled coal samples changes in reverse direction with the effective volumetric stress, that is, the effective volumetric stress increases when the permeability decreases, and vice versa.     

Behavior of Loose Sand-structure Interface Undergoing High Frequency Cycle Shearing

The behavior of loose sand-structure interface under the high frequency cycle shearing is analyzed by using Particle Discrete Element Method (DEM).The changes of sand box porosities of DEM model and sand volume strains are examined in detail.The results show that sand volumetric shrinkage due to shearing is not obvious in spite of the larger loose sand particle friction coefficients and particle stiffness.The particle stiffness ratio has a little effect on the mechanical properties of sands.In addition,the decrease in volume of sand under shearing is more obvious with the increase of the confining pressures.Moreover,the decrease in volume of loose sand under shearing is greater with the increase of the high frequency cycle shear amplitude and the shear frequency.     

Experimental study of coalbed methane seepage in fuxin basin

Taking raw coal samples collected from the wangying colliery in Fuxin basin as the research object, the experiment of coalbed methane seepage law is conducted by the triaxial osmoscope and the relationship between effective stress and coal permeability is presented. The experiment results indicate that the coal permeability is of stress sensitivity and tended to decline in exponent with the increment of effective stress, which is consistent with the former research results. The seepage flow curves from experiments demonstrate the nonlinear characteristics under different confining pressures and moisture contents, and it accurately reflecte the effect of coal body deformation on the coalbed methane seepage flow in the experimental process. Therefore considering the influence of effective stress on the coal body deformation, a new motion equation for the nonlinear percolation characteristic of coalbed methane is established. Moreover, the motion equation is correlated well with the experimental data, and the fitting correlation coefficients are above 99.6%.     

Experiment of the gas effect on coal mechanical properties under zero effective confirming pressure

Considering the seal and storage of gas, mechanical index of coal containing gas is usually obtained by normal three-axis experiment. While, according to national standard, mechanical index of coal is usually tested by uniaxial experiment. By utilizing the gas sealing system, an analogic uniaxial experiment method is proposed. Through designed experiments, the effect of gas on mechanical properties and mechanical response of the coal sample when effective confirming pressure is 0 is studied too. The experimental results show that pore gas changes the mechanical response of complete deformation stage, but the trend of change gradually weakens as the gas pressure increases, and it means that non-mechanical effects caused by absorbed gas play a leading role gradually.     

A visco elasto plastic creep model of outburstprone coal under triaxial compression

Using self made creep seepage experimental apparatus, triaxial creep tests on outburst prone coal are carried out with coal samples taken from Songzao Coal Mine. It is found that the attenuation creep property is shown in the conditions of less than its long term strength load, while non attenuation creep property for the greater conditions. By introducing a visco elasto plastic body which can describe the deformation property of non Newtonian fluids, the visco elasto plastic creep model of outburst prone coal is put forward combined with a Poyting Thomson model. The model parameters were fitted out with Matlab programmer and validation is employed. It is found that the test results were in good agreement with those by the proposed model.     

小麦堆体变模量的测定与实验研究

利用应变控制式三轴仪对小麦的体积压缩和体变模量进行了实验测定,并探讨了围压和水分对体变模量的影响。结果表明:小麦堆的体积随着围压的增大而减小;小麦堆(水分为13.7%、16.4%、18%)的体变模量随着围压(5 kPa～30 kPa)的增大而增大,它们的体变模量的范围分别为173.5 kPa～259.3 kPa、164 kPa～240 kPa、151 kPa～229.4kPa;小麦堆体变模量随着水分的增大而减小,当小麦堆围压为30 kPa,水分为13.7%～18%时,它的体变模量变化范围为223.5 kPa～270 kPa。