壓縮包內(nèi)含有CAD圖紙和說(shuō)明書,均可直接下載獲得文件，所見所得，電腦查看更方便。Q 197216396 或 11970985
英文原文
Stress evolution with time and space during mining of a coal seam
Wei Yanga,b, Bai-quan Lina,b, Yong-an Quc, Zi-wen Lia,b, Cheng Zhaia,b, Li-li Jiad, Wu-qiang Zhaoc
a Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
b The State Key Laboratory of Coal Resources and Mine safety, China University of Mining & Technology Xuzhou, Jiangsu 221116, China
c Hancheng Mining Co. Ltd., Shanxi coal, Hancheng, Shanxi 715400, China
d College of Literature, Law & Politics, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China
Abstract
Mining of the upper protective coal seam is widely practiced in China for coal mine safety, but relief gas may present a new risk of blasting. To control the relief gas effectively, a strain-soften model was built by FLAC3D software to investigate the stress evolution during the process of mining the upper protective coal seam. The results show that the abutment stress changes rapidly within 10 m in front of the coal face, and the maximum abutment stress is approximately twice the original when the coal seam is mined 20–30 m. The abutment stress should break the rock mass and cause the gas to flow easily. In the stable mining period, the change trends of the x -stress and z -stress are different, and these should also pre-break the rock mass. The stress distributions of the rock mass at different distances under the protective coal seam are different, especially near the coal face, which should greatly affect the gas flow when the space of the protective and protected coal seams change over a large range. The relief angle also changes over a large range, increasing to a maximum approximately 30 m behind the coal face, and it decreases gradually when it is far away from the protective coal seam. The results are helpful for designing the coal face of protected coal seams and borehole layouts to control the relief gas.
1. Introduction
China is a country rich in coal resources but poor in oil and gas. For many years, coal has supplied more than 70% of China’s energy. In 2010, coal production reached approximately 3.2 billion tons, and it currently supports the rapid growth of China’s GDP. However, more than 95% of China’s coal mines are underground, and 17.6% of key state-owned coal mines are coal and gas outburst mines, which seriously threaten coal mine safety. In 2008, for example, 25 accidents caused 120 deaths. To effectively control coal and gas outburst accidents, the State Administration of Work Safety unveiled the ‘‘Provisions for the prevention of coal and gas outburst’’ in August 2009, which stresses regional gas control mainly by mining protective coal seams and by methane pre-drainage to eliminate the risk of outburst from a large region. Additionally, the mining of protective coal seams should be preferentially selected if possible [1].After the protective coal seam is mined, the stress of the protected coal seam decreases and the permeability increases, which causes the methane to be released continuously, and the outburst danger is eliminated [2 , 3]. However, during the process of mining the protective coal seam, the relieved gas from the current and adjacent seams may flow into the coal face, causing the methane to overrun. The relieved gas flow and the protected range are determined by the stress distribution [4]. The stress distribution of mining fields and roadways has been extensively studied for many years, but most of these studies only address some static states [5 ,6]. Because mining is a dynamic process, the stress field evolves dynamically with time and space, which causes the gas flow and the protected range to change dynami-cally [7]. Therefore, the stress distribution during the process of mining a protective coal seam must be studied from a dynamic viewpoint while considering the stress evolution with time and space for gas relief control and coal face arrangement. However, this question is not addressed in the current literature, so it is necessary to study the issue thoroughly.
The geological condition of a coal mine is non-uniform and complex, which makes it a challenge to study the stress distribu-tion and evolution with time and space with only field or laboratory experiments. Professional geotechnical numerical ana-lysis software could be a good choice for this situation.
2. Establishment of a numerical model
FLAC3D is one of the most important numerical software tools in current rock mechanics calculations. This tool is particularly suitable for solving nonlinear large deformation problems in geotechnical mechanical engineering and is widely used in the field of mining engineering, among other fields [8 ,9]. The software contains eleven kinds of material constitutive models, as follows: one empty element model, three elastic material models and seven plastic models. Mining a coal seam will cause rock mass deformation and will decrease the rock strength; thus, the rock mass is assumed to be an elastic-plastic material, and it presents strain-softening characteristics after it is damaged. Therefore, the strain-softening model was chosen. The coal strength parameters are shown in Table 1 according to the typical values of the Hancheng Mining Co. Ltd.
The strain is both elastic and plastic. The stress initially changes linearly with the elastic strain, but a strain-softened character will appear after the rock mass is damaged beyond the yield point[8]. Thus, the friction angle, cohesion, dilation angle and tensile strength soften as the plastic strain increases. Many researchers use a linear softening model [10 – 12], but a different relationship was proposed because the strength decreases much faster in the beginning, as shown in Fig. 1 [13] .
The model was built according to the geological conditions of the Xia Yukou coal mine of the Hancheng Mining Co. Ltd., where the long wall mining method is widely used. The length, width and height of the model are 320, 260 and 120 m, respectively, with 520,800 zones and 582,144 grids. The #3 coal seam has the highest outburst danger, so the #2 coal seam was mined first as the protective coal seam to eliminate the outburst risk of the #3 coal seam. In Hancheng, the ground stress gradient is approxi-mately 0.025 MPa/m. Considering that the model top is approxi-mately 400 m below the ground surface, a compressive stress of 10 MPa was loaded on the top of the model as the in-situ stress. The protective coal seam is approximately 470 m below the ground surface, and the in-situ stress was approximately 12 MPa in the protective coal seam. At the same time, the rolling boundaries were loaded on the other sides. Fig. 2 shows the integrated histogram and grid method of the model [14] .
The model was built according to the 21,206-long wall work-ing face in the protective coal seam. The mined length of the protective coal seam is 160 m, extending from –80 to 80 m in the x-direction; the width is 96 m, extending from –48 to 48 m in the y-direction; and the mining height is 1 m. The software iterated 250 steps for each 2-m length mined.
3. Results and discussion
3.1. Evolution of abutment stress in front of the coal face
During the mining process, stress will increase in front of the coal face ( Fig. 3 ). This stress is called the abutment stress, and it can compress and break the coal and rock mass ahead of the coal face, generating cracks and affecting the gas flow in the coal seam [15] . Therefore, it is necessary to study the dynamic evolution regularity of abutment stress during the mining process for gas control.
The abutment stress evolution regularity can be studied by the stress distribution at different mining lengths. Fig. 4 shows 16 abutment stress distribution curves ahead the coal face when the protective coal seam is mined 10, 20, 30, y, and 160 m, respec-tively. Note that vertical coordinates presents the vertical abut-ment stress in front of the coal face (z-stress), negative values indicate compression. For example, a curve of 10 m presents the abutment stress distribution in front of the coal face when the coal seam is mined 10 m, and it intersects the horizontal ordinate at 70 m, which means the coal face is at 70 m.
Fig. 4 shows that the stress changes complexly and rapidly within 10 m in front of the coal face. The stress at the coal face is approximately 0 MPa, and it rapidly increases to a maximum value of approximately 20–24 MPa (1.67–2 times the original) approximately 5 m in front of the coal face. The stress then decreases rapidly to the original stress approximately 10 m in front of the coal face. The severe stress change will break the coal and rock in front of the coal face and generate numerous cracks[16 –19], which will increase the permeability. The adsorption gas will begin to release gradually, especially within 5 m in front of the coal face, so the high quality gas can be extracted from the range to control the gas of the current coal seam.
Fig. 4 also shows that the peak value of the abutment stress changes regularly during the mining process: the peak value increases gradually at the beginning of the mining process, and it reaches the maximum at approximately 24 MPa when the coal seam is mined 20 m, which is approximately twice the original stress; later, the peak value gradually decreases and stabilizes at approximately 21 MPa, with a slight fluctuation when the coal seam is mined 60 m, which is approximately 1.75 times the original stress. Thus, the coal and rock are more easily broken at the beginning than later, and the adsorption gas should be more easily released at the beginning than later. The gas drainage borehole must be laid out in advance to prevent the gas from flowing into the coal face, thus causing the gas to overrun.
It can be seen from Fig. 4 that the abutment stress distribution is quite different before and after 50 m, and the abutment stress distribution curve in front of the coal face does not significant change after the coal seam is mined 50 m. Fig. 5 shows the stress distribu-tion curves when the coal seam is mined 20 and 80 m, respectively. Note thatx-stress represents horizontal stress along the x-direction, y-stress represents horizontal stress along the y-direction, and z-stress represents the vertical stress along the z-direction.
At the beginning of the mining process, when the protective coal seam is mined 20 m, the stresses increase to the peak value at approximately 5 m in front of the coal face. The peak value of the z -stress is approximately 24 MPa, approximately twice the original stress, and the stresses remain the same before the peak value.
When the abutment stress distribution is stable and the protective coal seam is mined 80 m, the y-stress and z-stress increase to the peak value at approximately 5 m in front of the coal face, but the maximum z-stress is only approximately 21 MPa, which is 1.75 times the original stress. The x-stress and z-stress before the peak value do not stay the same; rather, they change in the opposite trend: the z-stress increases while the x-stress decreases.
Researches show that the high stress gradient and the high stress asymmetry should break the rock much more easily [18] . In the early mining period, the z-stress is comparatively large at approximately 5 m in front of the coal face, which increases the asymmetry of three-dimensional stresses. Consequently, the coal is much more easily broken, and the gas gradually becomes active and may flow into the workspace. In the stable mining stage, the high stress gradient and asymmetry should also break the rock mass, but the different stress change trend may pre-break the rock, which may cause the gas to release much earlier. Thus, gas drainage boreholes should be set there to drain the gas earlier.
3.2. Stress distribution along the mining direction under the protective coal seam
After the upper protective coal seam is mined, the stress will decrease gradually, which will affect the release and flow of the gas. Therefore, it is necessary to analyze the stress evolution regularities of different distances below the protective coal seam, as shown in Fig. 6. It can be seen from Fig. 6 that there is a large difference both ahead of and behind the coal face, whereas in the other parts, the curves nearly overlap each other and show only minor differences.
The stresses also behave differently in front of the coal face. The abutment stress can concentrate stress near the protective coal seam, but when far away, there is no stress concentration. For example, an obvious stress concentration appears in front of the coal face 2.55 m under the protective coal seam, whereas there is no stress concentration 19 m under the protective coal seam.
Behind the coal face, the distance away from the protective coal seam will delay the stress decrease to the minimum: the closer to the protective coal seam, the earlier the stress decreases to the minimum, whereas the farther away it is, the later it decreases. For example, the stress of the rock mass 2.55 m under the protective coal seam drops to a minimum at approximately 0 MPa slightly behind the coal face, while the stress of the rock mass 19 m under the protective coal seam drops to a minimum at approximately 1.5 MPa and 20 m behind the coal face, which suggests that the stress gradient of the rock mass near the protective coal seam is greater than that far away.
Therefore, during the mining process, the rock mass near the protective coal seam experiences a sharp stress concentration and a sharp stress drop, whereas the rock mass far from the coal seam experiences a much slower stress drop. Thus, there will be much more damage to the nearby rock mass than that far away.
Fig. 7 shows the three-dimensional stress distributions at different distances below the protective coal seam.Fig. 7 shows that the stresses all decrease to a very low level 7, 11 and 14.7 m below the protective coal seam. However, only thez-stress is very low 26 m below the protective coal seam, whereas the x-stress and y-stress still maintain high levels. Thus, it can be concluded that the stress of all directions of the rock mass near the protective coal seam are all very low, but when far away from the coal seam, the z-stress is very low, but the horizontal x –stress and y-stress are still very high. Thus, the cracks near the protective coal seam are more severe than those far away.
3.3. Evolution regularities of relief angle
The relief angle, as shown in Fig. 8 , is one of the major parameters for describing the relief effect of mining a protective coal seam. The relief area presents the range where the outburst danger is eliminated. Relief angles a and b can be expressed as a =arctan (a 1/b 1) and b = arctan ( a 2/b 2) . Here a =b because the coal seam angle is assumed to be 0o.
The greater the relief angle, the larger the relief area. In China, constant relief angle has been used to design the coal face, but the stress will change dynamically during the mining process, which should also cause the relief angle to change dynamically.
3.3.1. Relief angle evolution in the gob
Stress in the gob is different, and it should result in a different relief angle. The z-stresses at different distances behind the coal face 14.7 m below the protective coal seam are shown in Fig. 10. The data extraction method is shown in Fig. 9. Fig.10 shows that the z-stress presents a symmetrical distribution in the gob. The stress changes significantly at different distances into the gob, and it drops to its lowest point between 18 and 30 m behind the coal face. The relief angle can be calculated by the following critical stress relief value [20 –22].
σzc≤(cosα2+λcosβ2)γН (1)
where σzcis the stress perpendicular to the coal seam when the coal seam angle is 0o , σzc is the z-stress, a is the coal seam angle, λis the lateral pressure coefficient, γ is the bulk density and H is the initial depth of the outburst.
According to the conditions of the Hancheng Mining Co. Ltd. the following are taken: γ=ρg=25000kg/m2s2，Н=320m，α=0，then σzc≤25000×320=8MPa。Therefore, when the stress decreases to 8 MPa, a gas outburst will not occur. Therefore, 8 MPa is taken as the critical value, the cross point of the critical value line and the stress curves are critical point, the distance between the critical point and the border is recorded as b1.mBecause the stress curves are 14.7 m below the protective coal seam, that is to say a1=14.7 m, the relief angle a can be calculated as α=arctan（a1/b1）=α=arctan（14.7/b1）, after which the relief angles in different positions can be obtained as in Fig. 11.
Fig. 11 shows that during the process of mining the upper protective coal seam, the relief angle changes within the range of 70–81oin different positions of the gob. The relief angle increases rapidly within 30 m behind the coal face, and it reaches its maximum of 80.54 1 30 m behind the coal face. From that point on, the relief angle decreases gradually, and will stabilize at approximately 71o and approximately 80 m from the coal face in the gob.
3.3.2. Relief angle evolution when far away from the protective coal seam
Under the protective coal seam, the stress near the seam is very low, but it will increase when far away. The data of the z-stresses at different distances from the protective coal seam were extracted as shown inFig. 12 and its distribution is shown in Fig. 13 . Fig. 13 shows that the z -stress is also symmetric. The same critical stress relief value and the same method were used to calculate the relief angle, and the relief angles of different distances below the protective coal seam are shown in Fig. 14.
It can be seen that under the protective coal seam, the relief angle changes within the range of 88.88–65.771 when the distance to the protective coal seam is between 2.55–28.89 m. The relief angle is 88.881 when the distance to the protective coal seam is 2.55 m, and the relief angle drops to 65.77 1 when the distance is increased to 28.89 m. Therefore, a constant relief angle cannot be used to design the coal face when the distance between the protective and protected coal seam changes over a large range. Therefore, the relief angles must be tested by field measurements to conduct the coal face design.
4. Summary and conclusions
The following conclusions can be made from the analysis:
(1) The abutment stress changes drastically within 10 m in front of the coal face, the stress peak value is approximately 1.67–2 times the original stress, and the maximum peak value appears when the coal seam is mined 20–30 m. The abutment stress usually keeps a high stress gradient and should break the rock mass, allowing the gas to flow easily into the broken rock mass. Therefore, the gas drainage boreholes should not be abandoned 10 m before the coal face. In the stable mining process, the coal seam is mined more than 50 m, and the change trends of the z -stress and x -stress are opposite approximately 10 m ahead of the coal face, which will also help to break the rock mass before the stress peak. Gas may easily flow into the broken rock mass, so boreholes should be laid out beforehand to drain the gas.
(2) The stress distribution is different when the distance below the protective coal seam is different, which will cause the gas flow characteristics to be significantly different below the protective coal seam. When the space between the protective and protected coal seams varies over a large range, the flow character of the relief gas from the protective coal seam is different. For example, the coal seam space in Hancheng Mining Co. Ltd. varies by 2-28 m, so the relief gas must be extracted according to the different stress distributions.
(3) The relief angles can reflect the stress relief range of the protected coal seam and can affect the relief