Influences on Pressure Releasing by Blasting Breaking Hard Roof

Transcription

1 The 8th Russian-Chinese Symposium. Coal in the st Century: Mining, Processing an Safety Influences on Pressure Releasing by Blasting Breaking Har Roof Wei Zhang, Yunliang Tan, Weiyao Guo, Shitan Gu, Dianrui Mu, Shanchao Hu Shanong University of Science an Technology Qingao 90, China ifferent mining conitions. The charge structure of blasting hole influence the effect of blasting pressure releasing obviously, but the research on the charge structure influencing blasting effect was not perfect, an the esign process of the blasting parameters has a few weaknesses. Base on the 308 working face, the influence of charge structures an blasting parameters on the pressure releasing effect was analyze through theoretical analysis an numerical simulation. Combine with the construction conitions in mine, the blasting parameters an construction technology were esigne. The effect of rock burst prevention was obvious. Abstract In orer to optimize an upgrae the pressure releasing effect of blasting breaking har roof, influence factors of it was analyze by theoretical analysis, numerical simulation an fiel practice. Firstly, the impact on charge structure (charge uncoupling coefficient K, charge axial coefficient le an charge raius rb) was analyze by inexes of fracture zone raius an crush zone raius. Seconly, the evolution tren of pressure releasing effect of ifferent blasting parameters (angle of blasting hole, length of blasting hole an explosive charge) was stuie by numerical simulation. Results show that blasting effect improves obviously with increase of charge raius an explosive charge. Reucing charge uncoupling coefficient K uner the premise of using uncoupling charge ways an taking axial continuous charge way are beneficial for improving blasting effect. When the angle of blasting hole is an it penetrates har roof rock, pressure releasing effect is better. Combine with the construction equipment an materials in a mine, the charge structure an blasting parameters were esigne, an a goo application effect was got II. ENGINEERING BACKGROUND The main coal seam, which was 3.9m in thickness an has simple structures, was 3#coal seam in 308 working face. The.m height of coal seam below roof was soft an broken. The Platts harness of coal seam was f = ~3. The mining epth was 30m an the slope length was 0m. Generally, the 308 working face was a monoclinal structure. The ip angle of the coal seam was ~0, an the average value was. The immeiate roof was.0m mustone, an the basic roof was.83m fine-graine sanstone, which consists of felspar, quartz an mica an the Platts harness was f = ~ 8. The immeiate an basic floors were siltstone an sanstone, whose thickness were.3m an.0m, respectively. During the preliminary mining process, one rock burst occurre ue to the caving of basic roof, thus blasting to break basic roof for pressure relief in 308 roaway is essential Keywors Har Roof; Blasting Breaking Roof; Pressure Releasing Effect; Charge Structure; Blasting Parameters I. INTRODUCTION Rock burst can easily cause large losses as one of the major isasters in coal mine []. Har roof is one of the important factors because large area will hang when the roof has strong lithology an goo integrity after coal seam mining an it is not easily to cave naturally in short term. As the longwall face avances, the roof will fracture in front of coal wall, which can easily lea to rock burst [-]. Improving the roof conition is one of the important means to reuce the times an the intensity of rock burst []. A large number of engineering applications show that eep hole blasting breaking roof is one of the effective measures to improve roof conitions an prevent the rock burst cause by har roof [-8].Scholars analyze stress istribution rule of surrouning rock before an after pressure releasing through numerical simulation, which provies goo references for the stuy of blasting pressure releasing of rock burst prevention [9,0]. The mechanism of eep hole blasting breaking roof was iscusse an its practice for rock burst prevention an control was carrie by Qi[]. Zhao [] an Wei [3] analyze the blasting effect uner ifferent blasting parameters (the angle of blasting hole an charge length) an 0. The authors - Publishe by Atlantis Press The conference was fune by RFBR accoring to the research project III. THE TECHNOLOGY PRINCIPLE OF BLASTING BREAKING ROOF TO RELIEF PRESSURE The shock wave, etonation gas an strong isturbance cause by blasting can lea to coal an rock amage. Therefore, mechanical properties of coal an rock are weakene. Its ability of accumulating high stress an storing high elastic eformation energy, an the stress graient of the axial irection are reuce. The stress conition an energy transfer conition of rock burst occurrence is mitigate [], in other wors, the occurrence probability of rock burst was reuce. The principle of blasting breaking roof is making full use of shock compression effects of the shock wave an the crack effect of stress wave, leaing to the roof fracture an continuity amage. Accoring to the amage characteristics 3

2 of rock after blasting, it can be ivie into crush zone, fracture zone an vibration zone, as shown in fig.. blast hole charge crushing zone fracture zone vibration zone Fig.. blasting amage effect The rock fracture starts at the blasting of charge containing in the hole, an the gas from burst impacts on the hole wall instantaneously. Pressure of blast pulse close to the hole greatly excees the strength of the rock, which looks like the nature of flui. The area is crushe by the energy of pressure pulses, which is calle the crush zone. The raius of crush zone is R c, which can be expresse as formula () [8]. R c 0DvnK leb 8 c When the crush zone forme, the shock wave attenuates to stress wave an spreas aroun the hole. The fissure zone is forme in coal connecte to crush zone, which can easily lea to fracture amage calle fracture zone. Fracture zone is the major functional area uring the blasting process. The raius of fracture zone R p can be represente by formula (). R p r c 0Dv nk e l B t 8 c In the area outsie of the fracture zone, the stress wave turns into soun wave or strain wave, which is less than the compressive strength of the rock. It comes into the rock without amage along the raial, until attenuates to less than the tensile strength of the rock. In aition, if the free surface is close to the blasting center, it will cause stress concentration of the hoop leaing to raial fracture, an extening to the free surface. This area is calle vibration zone, an the raius of vibration area R s can be expresse as follows (3) [] : R s. ~.0 3 b r b () () q (3) In type (), (), (3) : α β is shock wave attenuation inex an stress wave attenuation inex respectively,,, ρ 0 is the ensity of explosive, kg/m 3 ; D v is etonation velocity of explosive in coal seam, m/s;k is raial charge uncoupling coefficient, K = b / c ; The b an c are blast hole raius an charge raius; l e is the charge axial coefficient, l l l l.where, l c is the length of charge e c b s in blast hole. l b is the length of blast hole. l s is the sealing length in hole, l e = shows that without blank column in axial. B b b b,where, b is the lateral stress coefficient, b, is the ynamic poisson's ratio of rock; n is pressure increase coefficient of explosive prouct crashing to hole wall, generally taken n=0; is the expansion aiabatic exponent of etonation proucts, which generally is set to be 3; σ c is the uniaxial compressive strength of rock, MPa; σ t is uniaxial tensile strength of rock, MPa; r b is charge raius. q is charge explosive in unit volume of rock, kg /m 3. IV. INFLUENTIAL FACTORS OF PRESSURE RELEASING EFFECT OF BLASTING BREAKING ROOF A. The influence of charge structure on the pressure releasing effect Charge The mechanism of blasting breaking roof shows that the size of fracture zone after blasting is the main factor affecting blasting effect, an it is also the founation for etermining the parameters of blasting technology []. Therefore, the influence of charge structure on blasting effect was iscusse by analyzing the istribution of crush zone an fracture zone uner the ifferent charge structure conitions. Assuming that rock meium istributes uniformly. The pressure increasing coefficient n of explosive prouct crashing the hole wall is 0. Expansion aiabatic exponent of the etonation proucts is 3. Rock uniaxial compressive strength σ c is 8.3MPa, while the tensile strength σ t is.3mpa. Rock ynamic poisson's ratio is 0.0, thus b is 0. an B is.. The shock wave attenuation inex an stress wave attenuation inex were. an. respectively. The ρ 0 is. 0 3 kg/m 3 (SMⅡ- water-gel explosive is φ3 30mm, an the weight is 300g/volume). Explosive velocity is 000m/s. The relationships between blasting effect with charge uncoupling coefficient K an charge axial coefficient l e were stuie. Consiering the construction factors an the iameter of plastic hose, charge uncoupling coefficient K was selecte as.,.,.,.8,.0,.,.. Charge axial coefficient was l e =. The change rules of raius of fracture zone an crush zone are shown in figure. When charging axial coefficient l e is, 0.9, 0.8, 0., 0., 0. respectively an charge uncoupling coefficient is K=., change rules of raius of fracture zone an crush zone are shown in figure 3. Figs. an 3 show that the raiuses of fracture zone an crush zone increase with the increase of the charge raius, but the charge raius is limite by bit raius an charge raius, etc. Obviously, the parameters shoul be selecte accoring to the equipment in coal mine.

3 fracture zone raius Rp/m 3 r b =0.0 r b =0.0 r b =0.0 r b =0.0 r b =0.0 r b =0.0 3 crushing zone raius Rc/m fracture zone raius Rp/m r b =0.0 r b =0.0 r b =0.0 r b =0.0 r b =0.0 r b = crashing zone raius Rc/m Charge uncoupling coefficient K Fig Blasting effect with charge uncoupling coefficient K The uncoupling charge ways, which the hole ha a certain istance between charge volume an hole wall to ecrease compression shock wave action time an prolong stress wave action time, was selecte. Explosion pressure was buffere an explosive power was istribute more uniformly[]. However, increasing the uncoupling charge coefficient woul reuce the energy refraction into the rock[], leaing to less energy use for rock break. The figure shows that raiuses of fracture zone an crush zone ecrease with the increase of charge uncoupling coefficient. Therefore, when the uncoupling charge way is use, the charge coupling coefficient still nees to be reuce. Zong gave the impeance matching relations of the uncoupling charge [] K Zm Ze () Where, Z m is the wave impeance of rock, while Z e is the wave impeance of explosive, an K is the charge uncoupling coefficient. Fig. 3 shows that the raius of fracture zone an crush zone increase graually with the increase of charge axial coefficient. The continuous charge way is goo for expaning the scope of fracture zone an increasing the pressure relief effect, but continuous charge woul cause too much energy consumption in crush zone compresse at hole wall, which means it is not goo for saving cost. The homogeneous iscontinuous charge ways at the blasting cartrige coul make better use of blasting energy, an the function of stress wave an etonation gas is better, so the rock breaks more uniformly. The impeance matching relations of axial charging was formula () []. Z charging axial coefficient le Fig 3 Blasting effect with charge axial coefficient l c ls ls ( le )( le ) l l b b m Ze () Where, l e is the charge axial coefficient. l b is the length of blasting hole. l s is the sealing length of blasting hole. B. Simulation Stuies of Influence of Blasting Parameters on Pressure Releasing Effect The No.308 working face was chosen as the research object to establish the numerical moel by FLAC 3D, the moel size was 0m 0m m. The unit number of calculation moel was 08, the moel bounary conitions were as follows: the horizontal isplacement constraint was applie all aroun, the three irections at the bottom was isplacement limite, an the upper bounary was free. Accoring to the 30m mining epth,.mpa weight of overburen stress was applie at the top. The elastic moel was use in the moel builing an Moore Coulomb constitutive moel was use in the excavation. Rock mechanical parameters were liste in table. In the simulation process of blasting breaking roof, the effect of blasting was simulate by reucing the strength of working face an roaway [0], the influence of angles an lengths of blasting holes were researche by weakening ifferent areas. An the influence of ifferent charges on blasting effect was stuie through selecting ifferent 0.0 lithology height(m) TABLE Physical An Mechanical Parameters Of Rock Mass volumeweight moulus friction shear internal volume moulus K(GPa) (kg/m³) G(GPa) angle( ) Cohesion (MPa) tensile strength (MPa) mile-fine sanstone meium mustone mustone #coal seam meium mustone siltstone # coal seam meium mustone siltstone

4 SZZ(MPa) -.30~- -~-. -.~-0-0~-. -.~- -~-. -.~-0-0~-. -.~-. Fig.. Stress istribution state before blasting attenuation coefficient r (Figs. an 3 show that the strength in the range of.m aroun the hole was weakene). ) Influence of Blasting Hole Angle on Pressure Releasing Effect: The state of stress istribution of roaway surrouning rock was simulate when the length of blasting hole was m, the weakening egree was 30%, an angles of the blasting hole were 90, an 0 respectively. Fig. gives the stress istribution at the 0m position in front of the working face before blasting, an Fig. gives the stress istribution of ifferent angles of the blasting hole after blasting. Fig. shows that the excavation of the roaway an the working face leas to stress concentrations at both two sie walls of the roaway, an the stress of the coal wall is larger. The maximum concentration stress is.3mpa (the stress concentration factor is.3) before blasting breaking roof, an the concentration area is large, an the length from the stress concentration area to the roaway wall is about m. As shown in Fig., after the implementation of blasting breaking roof in roaway, the stress concentration is improve an the istance from the roaway wall increases. When the angle of blasting hole is 90, the maximum stress is.9mpa, just reuce by.%, but the istance from roaway wall to the peak stress area is about m,. When the blast hole angle is, the maximum stress is.mpa. The istance between the coal wall an the peak stress area is about m. When blasting hole angle is 0, the maximum stress is.mpa an between the coal wall an the peak stress area is about m. Besies, when the blasting hole angle are 90 an 0, the roof an the two coal walls are loose an broken, which can lea to support ifficulties. In conclusion, if the blasting hole length an weakening egree are same, the stress concentration is the smallest an the istance between the coal wall an the peak stress area is the largest when angle of blasting hole is, which means the angle can get best blasting effect. ) Influence of Blasting Hole Length on Pressure Releasing Effect The state of stress istribution of roaway surrouning rock was stuie when the angle of blasting hole is, the the maximum stress value in roaway surrouning rock/mpa weakening egree was 30%, while the blasting hole length were m, 9m, m, an 3m, respectively. The influence of blasting hole length on the blasting effect was stuie, as shown in Fig.. the maximum stress value in roaway surrouning rock/mpa the angle of blasting hole an coal seam( ) Fig.. Stress istribution with ifferent blasting hole angle blasting hole length/m the istance from stress concentration area to roaway wall/m Fig.. Stress istribution with ifferent blasting hole length Fig. shows that the stress concentration of the surrouning rock ecreases an the istance between the coal wall an the peak stress area increases graually with the increase of blasting hole length. When the blasting hole length increases from m to 9m, the stress concentration of surrouning rock ecreases obviously, but when the blasting hole length increases from 9m to 3m, the stress concentration an the istance between the coal wall an the peak stress area have little changes. When the blasting hole length increases to 3m, the peak stress reuces 0.0MPa an the istance increases 0.8m, because the blasting hole has gone through the har rock. Above all, it shows that the blasting hole length influences blasting effect when the angle of the blasting hole angle an the weakening egree are same. When the blasting hole reaches the stress concentration area, there is no obvious improvement of the blasting effect, but the blasting effect can be improve again when the blasting hole goes across the har rock. One obvious result can be got is that when the blasting the istance from stress concentration area to roaway wall/m

5 hole goes through the har rock, the pressure releasing effect is the most obvious. 3) Influence of Explosive Charge on Pressure Releasing Effect Different explosive charge has various effects on the weakening egree of rock strength. The influence of ifferent explosive charges on the blasting effect was stuie when the weakening egree was 0%, 30%, an 0% respectively, as shown in Fig., while the angle an the length of the blasting hole keeps same, which were an 3m, respectively Fig. shows that the stress concentration of the surrouning rock ecreases an the istance between the coal wall an the peak stress area increases graually with the increase of blasting weakening egree. When the blasting weakening egree is 0%, the peak stress ecreases from.3mpa to.9mpa, an the istance between the coal wall an the peak stress area increases from m to.m. When the blasting weakening egree reaches 0%, the peak stress is.88mpa, ecreasing.3mpa. The istance between the coal wall an the peak stress area is 8.m, increasing.m in epth. Obviously, the blasting effect improves significantly when the explosive charge increases. the maximum stress value in roaway surrouning rock/mpa % 30% 0% the weakening egree at blasting area r 9 8 the istance from stress concentration area to roaway wall/m Fig.. Stress istribution state with ifferent egree of weakening V. FIELD APPLICATION A. Blasting Parameters an Construction Technology Construction equipment an materials: The ZLJ-0 mine roaway rilling rig is use for hole rilling, the iameter of the rilling pipe is mm an the iameter of the iamon core is mm. The emulsion explosive of coal mine permissibility, whose specification is Φ3 30mm, an the weight of each explosive is 0.3kg.Cement cartrige is use to seal hole, whose specification is Φ3 mm. Polyethylene plastic pipe was use in the cartrige, whose specification is Φ 3mm (wall thickness). MSB-00 type exploer is use to etonate. Coal mine permitte millisecon elay electric etonators is use an per hole. Parameter esign: The parameters of charge structure an blasting parameters is esigne an liste in table,an the Blasting hole layout is shown in figure 8. tail entry Fig.8. Cross-section rawn of blasting borehole construction in roof Construction technology: One blasting hole was constructe at the interval of m in the two roaways from the cut roaway. Positive continuous charge structure without interval was use. The iagram of charge structure is shown in Fig. 9. Detonators were connecte in parallel, an cartriges were connecte in series, an one to six rilling holes were blaste one time. 3m 0m cement hole sealing section 8 9 cement sealling hole length 0m charging length.8m 3.PVC tube. seconary emulsion explosive in mine 3.millisecon elay electric etonator.cement sealing hole agent.etonator crural line.power cor.rilling hole wall 8.clay stemming 9.Non-slip agnail B. Application Effect.8m #coal Fine sanstone charging section mustone 0m blasting hole 308working face Fig.9. Hole charge structure iagram haulage entry Microseismic monitoring metho, rilling cuttings metho an electromagnetic raiation monitoring metho were use uring the mining process of No.308 working face for rock burst monitoring. During the two months of the early prouction, there were 83 times of microseismic events, one of which ha vibration sense an the releasing energy was 9. 0 J, but no abnormal pressure showe. The microseismic events were liste in table 3: TABLE. Parameters Of Charge Structure An Blasting Parameters hole iameter mm length of explosive charge.8m charge raius mm length of cement hole sealing 0m charge uncoupling coefficient. charge axial coefficient 0.8 angle of blasting hole length of blasting hole 3m spacing of blasting hole m explosive charge kg m m m m

6 TABLE 3. Microseismic Events Of 308 Working Face Energy level (J) Times The proportion E<0 J % E 0 J 3.% summation 83 The microseismic events whose energy was less than 0 J accounting for 9.8% of the total in the No.308 working face, which inicates that most of the microseismic events release small energy. Overall, the probability of rock burst in No.308 working face was small, an the blasting effect was goo. VI. CONCLUSION The theoretical analysis, numerical simulation an fiel application were use to stuy the influencing factors of the pressure releasing effect of blasting breaking roof, an the main conclusions are as follows: ) Crush zone, fracture zone an vibration zone are forme aroun the blasting hole after the blasting to break har roof. The mechanical properties of the coal an rock are weakene ue to the amage of coal in the crush zone an the fracture zone. Thus the coal loses the ability to accumulate high stress an store high elastic eformation energy. Obviously the raiuses of crush zone an fracture zone can be use as evaluation inexes of blasting effect. ) Charge uncoupling coefficient K, charge axial coefficient l e, an charge raius r b has obvious influences on the blasting effect. The pressure releasing effect increases with the increase of charge raius. On the premise of using uncoupling charge way, reucing the charge uncoupling coefficient is goo for improving the pressure relief effect. Continuous charge ways are beneficial for expaning the scope of fracture zone an increasing the pressure relief effect. The best impeance matching relationship of uncoupling charge an axial charge are also given. 3) Numerical simulation results show that the stress concentration of surrouning rock is the minimum an the istance between the coal wall an the peak stress area is the maximum when the angle of the blasting hole is. When the blasting hole goes through the har rock, the effect of breaking roof to relief pressure is better. The larger the blasting weakening egree is, an the more the explosive charge is. () Base on the research results, an the equipment an materials of one coal mine, the parameters of charge structure an blasting parameters were esigne: the charge uncoupling coefficient is., the charging axial coefficient is 0.8, the charge raius is 0.0m, the m blasting spacing is m, the angle of blasting hole is, the length of the blasting hole is 3m an explosive charge is kg. The fiel monitoring results show that the application effect is goo. ACKNOWLEDGMENT The research escribe in this paper was financially supporte by National Natural Science Founation of China (No.3, No.3, No.30 an No.33), Shanong Province Science an Technology Development Plan Item (No.0GSF000), Tai shan Scholar Engineering Construction Fun of Shanong Province of China, Qingao Postoctoral Applie Research Project (No. 098), China Postoctoral Science Founation Fune Project (No.0M90). 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