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1、Beijing, PRC 100083jiangyd yixin_zhaochinazhujieAbstractThe numerical approach to translatory coal bumps analysis is used to examine the effects of joint and coal seam stiffness on the outburst velocity, deformation magnitude of the opening wall and plastic zone length in the coal seam. A series of

2、tests are performed by varying the joint dip angles, spacing normal to joint tracks, joint block size and coal seam stiffness ratio to the surrounding rock. Results of joint and coal seam stiffness effects analysis indicate that joint dip angle, interval spacing, block size and coal seam stiffness r

3、atio have profound influences on coal bumps. The increasing of interval spacing and joint block size can generate larger roadway deformation, lower roadway deformation velocity and longer plastic region in the coal seam. Higher stiffness ratio can also result in larger deformation, higher deformatio

4、n velocity of opening wall. Coal bumps conditions deteriorate as the dip angle rotated to 90 degree or the elastic zones disappear in the whole active zone of coal seam for different spacing distributions. Keywords: Coal bumps, numerical simulation, joint, translatory 1、INTRODUCTIONUnderground coal

5、bumps is one of the catastrophic mine failures result from sudden releases of energy. With the enormous amounts of released energy, coal bumps may cast several tons of coal mas openings horizontally, which always result in destruction and collapse of roadways, damage of facility even death and injur

6、y to the miners. The phenomenon of coal bumps can be found in various kin roadway shape. Coal bumps throws out large quantity of coal from the wall and then the roadway will be closed over hundreds of meters. The research is financially supported by Research Fund for the Doctoral Program of Higher E

7、ducation under grant No. 20030290001.2、MECHANISM OF COAL BUMPS Generally speaking, coal bumps can be classified （Rice, 1935） into two types: One is pressure bumps which are caused when strong and brittle coal pillars or portions of pillars and loaded beyond their load-carrying capacity resulting in

8、sudden and violent failure. The other is shock bumps, which are attributed to rupturing of strong strata above the coal seam. 34In fact, most coal bumps occurred like translatory rock bursts （Lippmann, 1989）, which result in coal seam projecting dynamically into the excavation and the excavation can

9、 be closed over hundreds of meters. The qualitative description of the mechanism of coal bumps is shown in Figure 1.At pre-excavation state, the primitive overburden pressure q acting on the coal seam can be assumed to be at static friction elastic state, shown in Figure 2（a）. However, after the roa

10、dway is mined out, the primitive vertical pressure exerted on the seam over the width of the excavation should be transferred onto the seam adjacent to the excavation 5.The vertical pressure distribution after excavation is shown in Figure 2（b）. Perturbations in the original litho static stress fiel

11、d caused by the excavation can, therefore, result in coal/joint failure on the seam horizons. Thus, the coal seam can be divided into three distinct zones: the pressure relief zone A, maximum static equilibrium zone B and the primitive overburden pressure zone C.The mining induced seismic, detonatio

12、ns or other activities may cause waves and unstable crack propagation in the coal seam and surrounding rock. This will cause decrease of normal stress and increase of shearing stress to the interfaces between the coal seam and the adjacent overlying and underlying rock layers. The effects may conver

13、t the sticking friction into a sliding friction. Moreover, at the active zones, the coal seam near the excavation will be an active plastic region. Only when the whole active zones are transferred to be one active plastic region at some condition, the coal bumps are more likely to occur. Once the co

14、al bumps occur, coal at A zone and portion of B zone will translatory eject to the roadways. 3、NUMERICAL SIMULATION TECHNIQUE 3.1 Numerical Modeling Numerical modeling using UDEC version 3.0 （ITASCA, 1996） has been carried out for Tangshan coal mine in order to investigate stability of 12 coal seam

15、on 14th level. Numerical models were developed for analyzing the effects of three main parameters of joint on coal bumps. The geometry of the two-dimensional 40m40m model studied is shown in Figure 3. According to the mining depth and overlying strata situation, the original litho static stress of 2

16、1 Mpa is imposed on the top boundary of the mechanical model. Moreover, the tectonic stress is 25 Mpa loaded on the left and right boundary horizontally.3.2 Geo-Material Properties In current numerical modeling, the Mohr-Coulomb model was adopted. For all cases, the mechanical properties for coal se

17、am and both the roof and floor rock are listed as the following, coal seam: elastic modulus 5.9 Gpa, passions ratio 0.34; the roof and floor rock: elastic modulus 36 Gpa and passions ratio 0.28. 4、ANALYSIS OF THE RESULTSWhen joints with straight joint traces are developed parallel to one another the

18、y form a set. The simulation focuses on analyzing the multiple joint set and coal seam stiffness ratio K （E/E） effects on coal bumps.The modeling was performed in two stages. Firstly, the effects of joint dip angle and joint spacing d were simulated by fixing the stiffness ratio K. Secondly, the eff

19、ect of K was evaluated4.1 Joint effects on coal bumps 4.1.1 Joint dip angle effects Firstly, the effect of joint dip angle is simulated by changing from 0o to 90o in increments of 15o as counter clockwise. It is found that in all cases the coal bumps conditions deteriorate when the joint dip angle i

20、s rotated to the 90 degree. This means that at the same joint spacing condition the steeper joints make the coal seam grow easier unstable or outburst. It can be explained that the slender layers parallel to the wall of roadway in coal seam will easily buckle or rupture as compressed by the surround

21、ing rocks or influenced by seismic events. The wall deformation and velocity toward the roadway are recorded by monitoring middle point of opening wall. With the dip angle of joint increasing, the deformation and the average deformation velocity of the opening wall increase. The distribution of simu

22、lation results is almost symmetry to=90o. The alterations of spacing distribution could not change the effect tendency of joint dip angle on the deformation and deformation velocity of roadway. The joint dip angle effects on both deformation and velocity at 1.0 m joint spacing are shown in Figure 4（

23、a）, Figure 4（b） and Figure4（c） respectively. 4.1.2 Joint spacing effects The simulation was performed by adopting joint spacing d with various values from 0.25 to 5.0 m in the dip angle of 45 degree. It is found that larger joint spacing can give rise to larger deformation of the roadway and lower o

24、utburst velocity of the coal seam, as shown in Figure 5. Moreover, the larger joint spacing leads to longer active plastic region in the coal seam.4.1.3 Joint block size effects Randomly sized polygonal blocks were created in the coal seam and the average edge length of block specified. The simulati

25、on results indicate that the size of joint blocks can affect deformation of roadway greatly. Bigger joint blocks leads to larger convergence deformation of roadway. This is because intact coal seam or larger coal blocks can store more strain energy easily, which will result in deformation of roadway

26、 and longer unstable process of roadway after excavation. However, to small blocks, more strain energy was consumed by joint sets, which leads to smaller deformation of road faster deformation velocity. 4.2 Stiffness ratio effects Considering the relative stiffness of coal seam and surrounding rock,

27、 the simulation was performed by changing the stiffness ratio K （E/E from 1 to 15 in increments of 2.5. Moreover, there is a joint set consisting of multiple joints, which has=45o and d=1.0m, in the coal seam. It is assumed in the present study that the reduced “competence” of the coal seam is direc

28、tly associated with reduced stiffness coal. The translatory bumps seem to occur only where the rock in the roof and floor layers adjacent to the seam are about 10 times stiffer and stronger than the coal （Lip man, 1989）. The simulation results also show that whether the joint spacing is large or sma

29、ll, the deformation of the roadway and the initial deformation velocity of the roadway wall after excavation increase along with the increasing of stiffness ratio, as shown in Figure 7.5 CONCLUSIONS Result of joint and stiffness ratio effects analysis indicate that joint dip angle, interval spacing,

30、 join block size and stiffness of coal seam and surrounding rock have profound influences on coal bumps. The increasing of interval spacing can generate larger deformation, lower initial outburst velocity of the opening wall and longer active plastic region in the coal seam. Higher stiffness ratio c

31、an also result in larger roadway deformation, higher initial outburst velocity of coal seam. Coal bumps conditions deteriorate as the dip angle rotated to 90 degree or the elastic zones disappear in the whole active zone of coal seam for different spacing distributions. The simulation gives the proo

32、f to the techniques of preventing coal bumps, such as relief blasting or relief drilling which can cause many small fractures/joints in the coal seam adjacent to excavation. This study also indicates that joint spacing, joint dip angles, joint block size and the stiffness ratio play an important role on coal bumps or coal seam translatory deformation to the roadway. 中文翻译数值模拟煤层节点和刚度影响煤层突出