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Pressure-relief and permeability-increase technology of high liquid–solid coupling blast and its application
International Journal of Mining Science and Technology 24 (2014) 45–49
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Pressure-relief and permeability-increase technology of high liquid–solid coupling blast and its application Hao Zhiyong ⇑, Zhou Chao, Lin Baiquan, Pang Yuan, Li Ziwen State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Xuzhou 221008, China
a r t i c l e
i n f o
Article history: Received 20 May 2013 Received in revised form 10 June 2013 Accepted 10 July 2013 Available online 4 January 2014 Keywords: High stress High gas Low permeability coal seam Liquid–solid coupling blast
a b s t r a c t As for the coal seam with high stress, high gas and low permeability, a single technology cannot prevent the complex dynamic disasters. Because of this, the study proposes a new method of pressure-relief and permeability-increase technology of the high liquid–solid coupling blast. Through coal seam injection and charging structure change, the paper fully works out the dual functions of the water and explosion. Using the theoretical calculation, numerical simulation and physical experiments, we obtained that the initial blasting stress, displacement and overpressure of the liquid–solid coupling blast are much better than that of ordinary blasting. The technology has been used in the relative coal mine, and the application results show that the technique has effectively prevented the coal and gas outburst, which has a wide range of application. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction With the efficient intensification of coal production and the increase of mining depth, coal and gas outburst results in more and more serious and dangerous threat. Therefore, how to effectively prevent coal and gas outburst is significant to safety production of coal mine. With respect to the coal seam with high stress, high gas and low permeability, the technology of controlling this king of coal seam has been received small attention. At present, the main technologies in this aspect are deep-hole blasting, hydraulic cutting, water injection into coal seam, etc. Although those technologies have some effect in part, they cannot use the blast power and water energy, and cannot form large-scale fracture network, so they cannot radically solve the problem of pressure relief and permeability increase [1–10]. A single method cannot satisfy the requirement of preventing the complex dynamic disaster of deep coal seam outburst.
2. Pressure-relief and permeability-increase technology of high liquid–solid coupling blast There is natural stress in coal-rock mass, and this stress is fundamental power that leads to rock burst as well as coal-rock mass outburst [11–17]. The high liquid–solid coupling blast is the one that combines the drilling construction technology, coal seam injection technology and the water-pressure blasting technology, ⇑ Corresponding author. Tel.: +86 13852044546. E-mail address: [email protected] (Z. Hao).
It uses deep-hole injection to make the water and coal fully couple and then go on the water-pressure blasting, which can effectively weaken or break the coal-rock mass and bring water power and blast power’s superiority into full play using injecting water into coal seam and redesigning store-explosive structure (Fig. 2). This changes the common gas–solid blast into liquid–solid coupling one. The blast effect is improved by the strong energy transfer of water, which achieves the overall effect of pressure relief and permeability increase.
2.1. Analysis of initial energy of liquid–solid coupling blast in blast hole The liquid–solid coupling charging is different from the traditional one, in which water was injected into the blast hole and the explosion pressure delivered to the blast hole wall by water. The blast effect can be strengthened by blast wave caused by water-pressure blast, and secondary compression wave caused by bubble pulsation, and water jet. (1) Analysis of blast wave system and incident pressure Compared with air, the compressibility of water is much smaller than that of air, and water has a high efficiency of energy transfer, furthermore initial blast wave in water is greater than that in the air [18–20]. The wave generated in initial stage is complex. Fig. 1 shows the relationship of R and t (distance and time) in the initial stage. where OO1 is the detonation wavefront of explosive; ODD1 the blast wave underwater; O1CC1 the contact surface of blast product
2095-2686/$ - see front matter Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.12.008
46
Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49 Table 1 Blasting energy distribution in water (%). Energy distribution
Consumption of explosion energy
Energy left to next bubble pulsation
Energy used for blast wave formation Energy used for the first bubble pulsation Energy used for the second bubble pulsation
59.0
41.0
27.0
14.0
6.4
7.6
Fig. 1. Wave system in the explosion.
generally lower than 10–20% of that of the blast wave, but its effect time is longer than that of blast wave. Energy distribution is shown in Table 1. (1) Effect of water jet
Fig. 2. Role of water wedge.
and water; AO1B the rarefaction wave in the detonation products; and CD and CB1 the blast wave formed after the secondary blast wave reaches the contact surface. As shown in Fig. 1, when detonation wavefront reaches the contact surface of water, the blast wave forms in water, and it reflects the rarefaction wave into detonation products. The rarefaction wave converges into center. The refection occurs once the rarefaction wave arrives at the center, and CD blast wave forms in the water. CB1 blast wave is inside the detonation product. This process can continue to repeat, but the strength of blast wave is gradually small. When blast holes use the liquid–solid charging, the energy loss of blast wave in course of transfer is small and the blast wave can be transferred equably due to the incompressibility of water. The incident wave at the entrance to the hole wall should meet the energy and momentum conservations [21–24].
Ps ¼ qs Ds l
ð1Þ
where Ps is the incident pressure of blast wave underwater; qs the density of water; Ds the speed of detonation wave; and l the speed of the medium particle. The incident pressure of gas–solid coupling charge is as follows:
When the water pressure explodes, the blast wave affects on media and produces fracture in the media, and water and explosion gas seep into fractures, which makes fracture expand and extend. This effect can be regards as the splitting effect of the water jet. The energy carried by water is much higher than that by the gas. Therefore, the splitting effect of water wedge is greater than that of gas wedge. 2.2. Calculating the overpressure of blast wave in liquid–solid coupling coal seam According to the explosion similarity theory, the overpressure equation of blast wave in liquid–solid coupling coal seams can be expressed as:
DP m ¼
ai n X 1 Ai R i¼1
ð4Þ
where A is the constant, and it is determined by experiment; and ai the series which only consider only 3 or 4 terms. The maximum values of the other parameters in the blast wave, such as the maximum particle velocity vm, the maximum density qm, the maximum sound speed czm, the maximum wavefront speed D and the maximum temperature Tm and so on, can be expressed as a function of DPm .
v m ¼ v m ðRÞ
ð5Þ
ð2Þ
qm ¼ qm ðRÞ
ð6Þ
where q1 is the air speed; D1 the air blast wave speed at the entrance to the hole wall; P the incident pressure of air blast wave; and k the average heat insulation value of air gap, and k = 1.17–1.25. The ratio of incident pressure on the hole wall of blasting between the liquid–solid coupling charging and the gas–solid coupling charging was expressed in Eq. (3).
czm ¼ czm ðRÞ
ð7Þ
D ¼ DðRÞ
ð8Þ
T m ¼ T m ðRÞ
ð9Þ
P¼
4 q D2 kþ1 1 1
m¼
P s qs Ds l q Ds l ¼ ðk þ 1Þ ¼ A s 2 P 4q1 D21 q1 D1
ð3Þ
Because the density of water is far greater than that of the air, we can get the result that the underwater blast wave pressure which was generated by the liquid–solid coupling blast will increase exponentially. (1) Bubble pulsation and secondary compression wave The bubbles formed by the detonation product expand and shrink several times in water, which is called bubble pulsation phenomenon. The water will form a pressure wave in the process of bubble pulsation, and the secondary pressure wave has certain strength. The maximum pressure of secondary pressure wave is
If the state function (P0, q0, T0, czm) of media is consistent, we can obtain:
DPm ¼ P0 DPm ðR0 Þ
ð10Þ
qm ¼ q0 qm ðR0 Þ
ð11Þ
v m ¼ cz0 v m ðR0 Þ
ð12Þ
czm ¼ cz0 czm ðR0 Þ
ð13Þ
T m ¼ T 0 T m ðR0 Þ
ð14Þ
47
Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49
D ¼ cz0 DðR0 Þ
ð15Þ
The explosion in the coal-rock mass, the object shape around blast holes and relative position of the explosion center is the same, and that can be considered that the interaction between blast wave and object is the same. Explosion overpressure is equal to the particle velocity on the objects. Maximum value of overpressure of blast wave can be drawn.
DP ¼
0:76 23 2:25 13 6:5 105 P0 þ P0 þ R R2 R3
474.554 µs stress distribution
3975 µs stress distribution
(a) Stress distribution in air
Pa
ð16Þ
474.554 µs stress distribution
3975 µs stress distribution
Compared distance is
R ffiffiffiffiffiffi R¼p 3 W
ð17Þ
where R is the distance from blasting cartridge and W the explosive payload. No. (16) can be widely applied to the explosion in any threephase medium. Take P0, it needs to consider the state equation of media, so the P0 is not hydrostatic pressure of medium but the molecular pressure. Because the density of water is far greater than that of gas, molecular pressure of coal-rock mass in the gas–solid state is far smaller than that in the liquid–solid state, so the overpressure of the blast wave generated in the liquid–solid coal seam is far greater than that generated in the ordinary gas–solid state. For water, P0 = (10–12) 108 Pa. As for the coal-rock mass with water, the main problem is how to determine the value of P0. If injecting water into coal seam is moist, it can be approximated as P0 = (10–12) 108 Pa. In summary, after injecting water into blast holes and coal-rock mass, the properties of coal-rock mass and charging structure have been changed. Water has the small compressibility and high energy transfer efficiency. And it can cause that the stress peak value of initial blast wave produced by liquid–solid coupling blast and the overpressure produced in coal-rock mass is far greater than that produced in ordinary blast. The purpose of pressure relief and permeability increase was achieved. 3. Numerical simulation and physical experiments The liquid–solid coupling blast is achieved by changing the properties of coal-rock mass and charging structure through water injection. In order to better observe the blast effect after water injection, this study uses the numerical simulation and physical simulation experiment to verify the results.
Fig. 4. Stress simulation results (b) Stress distribution in water Fig. 4. Stress simulation results.
474.554 µs displacement variation
3975 µs displacement variation
(a) Displacement variation in air (a) Displacement variation in air
(a) Displacement variation in air 474.554 µs displacement variation
3975 µs displacement variation
(b) Displacement variation in water Fig. 5. Displacement simulation results.
2 m 1 m 1 m (length width height), of which the length of red explosive segment is 0.5 m, the length of purple air or water segment is 1.5 m, and the length of sealing segment is 0.5 m, is divided into 32,000 units. Fig. 3. shows the simulation results.
3.1. Numerical simulation of blast in hole Due to the limitations of computer resources, it needs to use the numerical simulation of finite volume instead of that of the infinite coal seam. In addition, it is necessary to prevent the boundary effect of stress wave. Because of this, the study uses the non-reflection boundary. Similarity ratio of this model is 1/8. This model
(1) Stress variation (2) Displacement variation As noted in Figs. 4 and 5, the variation of stress is consistent with that of displacement as time goes on. Meanwhile, it is also clear that the stresses and displacements at the same time are significantly different when the different buffer media blast. The stress and displacement of blasting in water medium is greater than that in the air medium.
3.2. Physical simulation of explosion overpressure of liquid–solid coupling coal-rock mass
Fig. 3. Physical model and stereogram of network division.
Testing the overpressure in the inward of coal-rock mass is carried out in coal, which density is 1.45–1.60 g/cm3. Because coal contains gas, three-phase state of coal that contains water and a small amount of gas is made for better simulating experimental results. Explosive is buried to a sufficient depth to avoid the
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Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49
From the experimental curve, overpressure empirical equation is as follows:
DP m ¼ A 1
a1 1 R
105
Pa
ð17Þ
where A1 and a1 are the constants, and is given in Table 2. 4. Engineering application No. 12 Mine of Pingmei Group has high gas and pressure, so there is danger of coal-and-gas outburst and rock in this coal mine. Using cross-layer drilling is to carry out the liquid–solid coupling blast, making the coal-rock mass to relief pressure and increase permeability in advance, so that the energy of coal-rock mass is released early.
Fig. 6. Relation between DP and R.
4.1. Construction scheme Table 2 Coal density after water injection. Coal density after water injection (g/cm3)
A1
a1
q = 1.60–1.70 q = 1.52–1.60 q = 1.45–1.50
15.0 7.5 2.5
2.80 3.00 3.50
First of all, borehole in the high roadway of 14th coal seam is drilled through the top of wind roadway of No. 31010 working face to the bottom of 16th and 17th coal seams. Injecting water into coal seam and cross-layer pressure are used to control blast. A pressure-relief surface is formed along roadway trend between high roadway of 14th coal seam and machine wind roadway of 15th coal seam, and it protects the relief pressure produced by the roadway of 15th coal seam. The water injection hole and blast hole are ahead of the heading face about 100 m. Fig. 7 shows the sketch map of blast hole and water injection hole.
influence of the free surface. Fig. 6 shows the relation between DP and R. As evident in Fig. 6, curves from 1 to 4 are the experimental curve of the coal with water decrease and gas increase gradually, and curve 5 is the experimental curve of the gas-contained coal without water. The result shows that: as water in coal-rock mass grows, maximum overpressure DP produced by blast increases but slightly decreases. Overpressure of coal without water is lower 100 times than that with water.
4.2. Analysis of test results (1) Variation of coal seam permeability is shown in Fig. 8. For ordinary coal seam, the permeability coefficient is 0.0707 102 m2/MPa2 d on average. After blasting of ordinary
(a) Profile map of blast hole and water injection hole
75 mm
1m
8m
(b) Sketch map of blast hole and water injection hole Fig. 7. Liquid–solid coupling blasting schematic diagram.
75 mm
Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49
49
Permeability coefficient (m2 /MPa2 ·d)
(4) Field application shows that the parameters and the indexes changes obviously after using this measures, which demonstrates the better effect of pressure relief and permeability increase and reaches the goal of preventing coal-and-gas outburst.
Acknowledgments Financial supports for this work, provided by the National Eleventh Five-Year scientific and Technological Support Plan Subject of China (No. 2007BAK29B01), the National Natural Science Foundation (No. 50534090), the National Key Basic Research Development Program of China (No. 2011CB201205), State Key Laboratory of Coal Resources and Mine Safety of China University of Mining Technology of China (No. SKLCRSM08X03), and the Youth Science and Technology Fund of China University of Mining and Technology (No.JGY101605), are gratefully acknowledged.
Fig. 8. Comparison of permeability coefficient of coal seam.
References
5
10
15
20
(d)
25
30
35
40
45
Fig. 9. Variation of the deviation value d.
control, the average permeability coefficient is 0.51233 102 m2/ MPa2 d, which increases 7.3 times. After liquid–solid coupling blasting, the coefficient is 1.60325 102 m2/MPa2 d, which increases 22.7 times, but the maximum 36.2 times. This shows that there is a wider range of fracture network in coal-rock mass after using the liquid–solid coupling blast, so the permeability of coal seam improves a lot. (2) Fig. 9 shows the variation of deviation value d. As noted in Fig. 9, the ratio above the predictive index testing the risk greatly decreases, and there is a little variation, which obviously controls outburst, such as serious jet orifice. 5. Conclusions (1) This study combines injecting water into coal seam with water pressure blast, and proposes the technology of liquid–solid coupling blast. Besides, it is the first time to use the technology for preventing rock burst and coal-andgas outburst. The problem of pressure relief and permeability increase is solved, which provides a new way for coal seam with high gas, high stress, and low permeability. (2) The research uses LS-DYNA software to simulate the variation of stress and displacement of coal and rock in the process of liquid–solid coupling blasting and ordinary blasting. Following conclusion is verified: at the same time, the stress and displacement of blasting in water medium are greater than that in the air medium blasting. (3) The calculation and experience equations of overpressure are implied by physical simulation of overpressure in liquid–solid coupling coal-rock mass, and it concludes that: as water increases, maximum value DP of overpressure produced by blast grows but slightly decreases. The overpressure without water is lower 100 times than that with water.
[1] Yu BF, Wang YA. The technical manual of gas disaster prevention and use. Beijing: Coal Industry Press; 2010. [2] Lin BQ, Cui HX. Theory and technology of mine gas control. Xuzhou: China University of Mining and Technology Press; 2010. [3] Zhang TG. Mine gas control technology. Beijing: Coal Industry Press; 2001. [4] Cheng Y, Wang H, Wang L, Ma X. Principle and engineering application of pressure relief gas drainage in low permeability outburst coal seam. Min Sci Technol 2009;19(3):342–5. [5] Liang ZG, Zhang WB. China’s decade of rock burst disaster. Fuxin Min J 1990; 9(4):l–8. [6] Zhang MT. The rock burst instability theory and numerical model. Rock Mech Eng 1987;6(3):l97–204. [7] Li XZ, Lin BQ, Zhai C, Ni GH, Li ZW. Relaxation study of cement based grouting material using nuclear magnetic resonance. Int J Min Sci Technol 2012;22(6): 821–4. [8] Hao ZY, Lin BQ, Gao YB, Cheng YY. Establishment and application of drilling sealing model in the spherical grouting mode based on the loosing-circle theory. Int J Min Sci Technol 2012;22(6):895–8. [9] Lin BQ, Zhang JG, Shen CM, Zhang QZ, Sun C. Technology and application of pressure relief and permeability increase by jointly drilling and slotting coal. Int J Min Sci Technol 2012;22(4):545–51. [10] Shen CM, Lin BQ, Zhang QZ, Yang W, Zhang LJ. Induced drill-spray during hydraulic slotting of a coal seam and its influence on gas extraction. Int J Min Sci Technol 2012;22(6):785–91. [11] Wang HT, Fan XG, Jia JQ, Hu GZ, Yuan ZG. Effects of key strata on protection when exploiting a steep-incline underlying a protecting stratum. J China Univ Min Technol 2011;40(01):23–8. [12] Hu GZ, Wang HT, Fan XG, Li XH, Deng Y, Shen YH. Gas pressure investigation on protection region of up-protective layer of pitching oblique mining. J China Univ Min Technol 2008;37(03):328–32. [13] Xie HP. Rock and concrete damage mechanics. Xuzhou: China University of Mining and Technology Press; 2010. [14] Xia MF. Statistical mesoscopic damage mechanics and damage evolution induced catastrophe. Adv Mech 1995;25(l):l–40. [15] Cai SJ, Zhang LH, Zhou WL. The II rock burst disaster prediction of Sham Tseng hard rock and mineral. China Saf Sci Technol 2005;1(5):17–21. [16] Xu LS, Wang LS, Li YL. Rock burst mechanism and judgment. Rock Soil Mech 2002;23(3):300–3. [17] Zhao B, Li Q, Li XY. Deep mine exploitation pressure bump rule and influential factors. Coal Eng 2005;11:52–4. [18] Yan SL, Xu Y. Numerical simulation study of the coupling of water blasting of rock breaking mechanism. Underground Space Eng 2005;1(6):921–4. [19] Huang WY, Yan SL, Liu ZG, Wu HB, Liu J, Chen QY. Research and application of water gel explosive grain on coal mine gas extraction in coal seam deep hole blasting. J China Coal Soc 2012;37(03):472–6. [20] Chen SH. Study on charging structure and its use of deep hole water pressure blasting. J China Coal Soc 2000;25(12):112–6. [21] Zong Q, Luo Q. Experimental study on distribution character of blasting stress when boreholes with water-couple charge. J Exp Mech 2006;21(03):393–7. [22] Zong Q, Li YC, Xu Y. Preliminary discussion on shock pressure on hole wall when water-couple charge blasting in the hole. J Hydrodyn 2004;19(05):610–5. [23] Zhang HS, Hu XH. Engineering blasting technology. Beijing: Coal Industry Press; 2001. [24] Zhang Q. Water coupling of blast mechanism and parameter calculation. Xinjiang non-Ferrous Metals 1997;4:21–3.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Pressure-relief and permeability-increase technology of high liquid–solid coupling blast and its application Hao Zhiyong ⇑, Zhou Chao, Lin Baiquan, Pang Yuan, Li Ziwen State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Xuzhou 221008, China
a r t i c l e
i n f o
Article history: Received 20 May 2013 Received in revised form 10 June 2013 Accepted 10 July 2013 Available online 4 January 2014 Keywords: High stress High gas Low permeability coal seam Liquid–solid coupling blast
a b s t r a c t As for the coal seam with high stress, high gas and low permeability, a single technology cannot prevent the complex dynamic disasters. Because of this, the study proposes a new method of pressure-relief and permeability-increase technology of the high liquid–solid coupling blast. Through coal seam injection and charging structure change, the paper fully works out the dual functions of the water and explosion. Using the theoretical calculation, numerical simulation and physical experiments, we obtained that the initial blasting stress, displacement and overpressure of the liquid–solid coupling blast are much better than that of ordinary blasting. The technology has been used in the relative coal mine, and the application results show that the technique has effectively prevented the coal and gas outburst, which has a wide range of application. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction With the efficient intensification of coal production and the increase of mining depth, coal and gas outburst results in more and more serious and dangerous threat. Therefore, how to effectively prevent coal and gas outburst is significant to safety production of coal mine. With respect to the coal seam with high stress, high gas and low permeability, the technology of controlling this king of coal seam has been received small attention. At present, the main technologies in this aspect are deep-hole blasting, hydraulic cutting, water injection into coal seam, etc. Although those technologies have some effect in part, they cannot use the blast power and water energy, and cannot form large-scale fracture network, so they cannot radically solve the problem of pressure relief and permeability increase [1–10]. A single method cannot satisfy the requirement of preventing the complex dynamic disaster of deep coal seam outburst.
2. Pressure-relief and permeability-increase technology of high liquid–solid coupling blast There is natural stress in coal-rock mass, and this stress is fundamental power that leads to rock burst as well as coal-rock mass outburst [11–17]. The high liquid–solid coupling blast is the one that combines the drilling construction technology, coal seam injection technology and the water-pressure blasting technology, ⇑ Corresponding author. Tel.: +86 13852044546. E-mail address: [email protected] (Z. Hao).
It uses deep-hole injection to make the water and coal fully couple and then go on the water-pressure blasting, which can effectively weaken or break the coal-rock mass and bring water power and blast power’s superiority into full play using injecting water into coal seam and redesigning store-explosive structure (Fig. 2). This changes the common gas–solid blast into liquid–solid coupling one. The blast effect is improved by the strong energy transfer of water, which achieves the overall effect of pressure relief and permeability increase.
2.1. Analysis of initial energy of liquid–solid coupling blast in blast hole The liquid–solid coupling charging is different from the traditional one, in which water was injected into the blast hole and the explosion pressure delivered to the blast hole wall by water. The blast effect can be strengthened by blast wave caused by water-pressure blast, and secondary compression wave caused by bubble pulsation, and water jet. (1) Analysis of blast wave system and incident pressure Compared with air, the compressibility of water is much smaller than that of air, and water has a high efficiency of energy transfer, furthermore initial blast wave in water is greater than that in the air [18–20]. The wave generated in initial stage is complex. Fig. 1 shows the relationship of R and t (distance and time) in the initial stage. where OO1 is the detonation wavefront of explosive; ODD1 the blast wave underwater; O1CC1 the contact surface of blast product
2095-2686/$ - see front matter Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2013.12.008
46
Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49 Table 1 Blasting energy distribution in water (%). Energy distribution
Consumption of explosion energy
Energy left to next bubble pulsation
Energy used for blast wave formation Energy used for the first bubble pulsation Energy used for the second bubble pulsation
59.0
41.0
27.0
14.0
6.4
7.6
Fig. 1. Wave system in the explosion.
generally lower than 10–20% of that of the blast wave, but its effect time is longer than that of blast wave. Energy distribution is shown in Table 1. (1) Effect of water jet
Fig. 2. Role of water wedge.
and water; AO1B the rarefaction wave in the detonation products; and CD and CB1 the blast wave formed after the secondary blast wave reaches the contact surface. As shown in Fig. 1, when detonation wavefront reaches the contact surface of water, the blast wave forms in water, and it reflects the rarefaction wave into detonation products. The rarefaction wave converges into center. The refection occurs once the rarefaction wave arrives at the center, and CD blast wave forms in the water. CB1 blast wave is inside the detonation product. This process can continue to repeat, but the strength of blast wave is gradually small. When blast holes use the liquid–solid charging, the energy loss of blast wave in course of transfer is small and the blast wave can be transferred equably due to the incompressibility of water. The incident wave at the entrance to the hole wall should meet the energy and momentum conservations [21–24].
Ps ¼ qs Ds l
ð1Þ
where Ps is the incident pressure of blast wave underwater; qs the density of water; Ds the speed of detonation wave; and l the speed of the medium particle. The incident pressure of gas–solid coupling charge is as follows:
When the water pressure explodes, the blast wave affects on media and produces fracture in the media, and water and explosion gas seep into fractures, which makes fracture expand and extend. This effect can be regards as the splitting effect of the water jet. The energy carried by water is much higher than that by the gas. Therefore, the splitting effect of water wedge is greater than that of gas wedge. 2.2. Calculating the overpressure of blast wave in liquid–solid coupling coal seam According to the explosion similarity theory, the overpressure equation of blast wave in liquid–solid coupling coal seams can be expressed as:
DP m ¼
ai n X 1 Ai R i¼1
ð4Þ
where A is the constant, and it is determined by experiment; and ai the series which only consider only 3 or 4 terms. The maximum values of the other parameters in the blast wave, such as the maximum particle velocity vm, the maximum density qm, the maximum sound speed czm, the maximum wavefront speed D and the maximum temperature Tm and so on, can be expressed as a function of DPm .
v m ¼ v m ðRÞ
ð5Þ
ð2Þ
qm ¼ qm ðRÞ
ð6Þ
where q1 is the air speed; D1 the air blast wave speed at the entrance to the hole wall; P the incident pressure of air blast wave; and k the average heat insulation value of air gap, and k = 1.17–1.25. The ratio of incident pressure on the hole wall of blasting between the liquid–solid coupling charging and the gas–solid coupling charging was expressed in Eq. (3).
czm ¼ czm ðRÞ
ð7Þ
D ¼ DðRÞ
ð8Þ
T m ¼ T m ðRÞ
ð9Þ
P¼
4 q D2 kþ1 1 1
m¼
P s qs Ds l q Ds l ¼ ðk þ 1Þ ¼ A s 2 P 4q1 D21 q1 D1
ð3Þ
Because the density of water is far greater than that of the air, we can get the result that the underwater blast wave pressure which was generated by the liquid–solid coupling blast will increase exponentially. (1) Bubble pulsation and secondary compression wave The bubbles formed by the detonation product expand and shrink several times in water, which is called bubble pulsation phenomenon. The water will form a pressure wave in the process of bubble pulsation, and the secondary pressure wave has certain strength. The maximum pressure of secondary pressure wave is
If the state function (P0, q0, T0, czm) of media is consistent, we can obtain:
DPm ¼ P0 DPm ðR0 Þ
ð10Þ
qm ¼ q0 qm ðR0 Þ
ð11Þ
v m ¼ cz0 v m ðR0 Þ
ð12Þ
czm ¼ cz0 czm ðR0 Þ
ð13Þ
T m ¼ T 0 T m ðR0 Þ
ð14Þ
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Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49
D ¼ cz0 DðR0 Þ
ð15Þ
The explosion in the coal-rock mass, the object shape around blast holes and relative position of the explosion center is the same, and that can be considered that the interaction between blast wave and object is the same. Explosion overpressure is equal to the particle velocity on the objects. Maximum value of overpressure of blast wave can be drawn.
DP ¼
0:76 23 2:25 13 6:5 105 P0 þ P0 þ R R2 R3
474.554 µs stress distribution
3975 µs stress distribution
(a) Stress distribution in air
Pa
ð16Þ
474.554 µs stress distribution
3975 µs stress distribution
Compared distance is
R ffiffiffiffiffiffi R¼p 3 W
ð17Þ
where R is the distance from blasting cartridge and W the explosive payload. No. (16) can be widely applied to the explosion in any threephase medium. Take P0, it needs to consider the state equation of media, so the P0 is not hydrostatic pressure of medium but the molecular pressure. Because the density of water is far greater than that of gas, molecular pressure of coal-rock mass in the gas–solid state is far smaller than that in the liquid–solid state, so the overpressure of the blast wave generated in the liquid–solid coal seam is far greater than that generated in the ordinary gas–solid state. For water, P0 = (10–12) 108 Pa. As for the coal-rock mass with water, the main problem is how to determine the value of P0. If injecting water into coal seam is moist, it can be approximated as P0 = (10–12) 108 Pa. In summary, after injecting water into blast holes and coal-rock mass, the properties of coal-rock mass and charging structure have been changed. Water has the small compressibility and high energy transfer efficiency. And it can cause that the stress peak value of initial blast wave produced by liquid–solid coupling blast and the overpressure produced in coal-rock mass is far greater than that produced in ordinary blast. The purpose of pressure relief and permeability increase was achieved. 3. Numerical simulation and physical experiments The liquid–solid coupling blast is achieved by changing the properties of coal-rock mass and charging structure through water injection. In order to better observe the blast effect after water injection, this study uses the numerical simulation and physical simulation experiment to verify the results.
Fig. 4. Stress simulation results (b) Stress distribution in water Fig. 4. Stress simulation results.
474.554 µs displacement variation
3975 µs displacement variation
(a) Displacement variation in air (a) Displacement variation in air
(a) Displacement variation in air 474.554 µs displacement variation
3975 µs displacement variation
(b) Displacement variation in water Fig. 5. Displacement simulation results.
2 m 1 m 1 m (length width height), of which the length of red explosive segment is 0.5 m, the length of purple air or water segment is 1.5 m, and the length of sealing segment is 0.5 m, is divided into 32,000 units. Fig. 3. shows the simulation results.
3.1. Numerical simulation of blast in hole Due to the limitations of computer resources, it needs to use the numerical simulation of finite volume instead of that of the infinite coal seam. In addition, it is necessary to prevent the boundary effect of stress wave. Because of this, the study uses the non-reflection boundary. Similarity ratio of this model is 1/8. This model
(1) Stress variation (2) Displacement variation As noted in Figs. 4 and 5, the variation of stress is consistent with that of displacement as time goes on. Meanwhile, it is also clear that the stresses and displacements at the same time are significantly different when the different buffer media blast. The stress and displacement of blasting in water medium is greater than that in the air medium.
3.2. Physical simulation of explosion overpressure of liquid–solid coupling coal-rock mass
Fig. 3. Physical model and stereogram of network division.
Testing the overpressure in the inward of coal-rock mass is carried out in coal, which density is 1.45–1.60 g/cm3. Because coal contains gas, three-phase state of coal that contains water and a small amount of gas is made for better simulating experimental results. Explosive is buried to a sufficient depth to avoid the
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Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49
From the experimental curve, overpressure empirical equation is as follows:
DP m ¼ A 1
a1 1 R
105
Pa
ð17Þ
where A1 and a1 are the constants, and is given in Table 2. 4. Engineering application No. 12 Mine of Pingmei Group has high gas and pressure, so there is danger of coal-and-gas outburst and rock in this coal mine. Using cross-layer drilling is to carry out the liquid–solid coupling blast, making the coal-rock mass to relief pressure and increase permeability in advance, so that the energy of coal-rock mass is released early.
Fig. 6. Relation between DP and R.
4.1. Construction scheme Table 2 Coal density after water injection. Coal density after water injection (g/cm3)
A1
a1
q = 1.60–1.70 q = 1.52–1.60 q = 1.45–1.50
15.0 7.5 2.5
2.80 3.00 3.50
First of all, borehole in the high roadway of 14th coal seam is drilled through the top of wind roadway of No. 31010 working face to the bottom of 16th and 17th coal seams. Injecting water into coal seam and cross-layer pressure are used to control blast. A pressure-relief surface is formed along roadway trend between high roadway of 14th coal seam and machine wind roadway of 15th coal seam, and it protects the relief pressure produced by the roadway of 15th coal seam. The water injection hole and blast hole are ahead of the heading face about 100 m. Fig. 7 shows the sketch map of blast hole and water injection hole.
influence of the free surface. Fig. 6 shows the relation between DP and R. As evident in Fig. 6, curves from 1 to 4 are the experimental curve of the coal with water decrease and gas increase gradually, and curve 5 is the experimental curve of the gas-contained coal without water. The result shows that: as water in coal-rock mass grows, maximum overpressure DP produced by blast increases but slightly decreases. Overpressure of coal without water is lower 100 times than that with water.
4.2. Analysis of test results (1) Variation of coal seam permeability is shown in Fig. 8. For ordinary coal seam, the permeability coefficient is 0.0707 102 m2/MPa2 d on average. After blasting of ordinary
(a) Profile map of blast hole and water injection hole
75 mm
1m
8m
(b) Sketch map of blast hole and water injection hole Fig. 7. Liquid–solid coupling blasting schematic diagram.
75 mm
Z. Hao et al. / International Journal of Mining Science and Technology 24 (2014) 45–49
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Permeability coefficient (m2 /MPa2 ·d)
(4) Field application shows that the parameters and the indexes changes obviously after using this measures, which demonstrates the better effect of pressure relief and permeability increase and reaches the goal of preventing coal-and-gas outburst.
Acknowledgments Financial supports for this work, provided by the National Eleventh Five-Year scientific and Technological Support Plan Subject of China (No. 2007BAK29B01), the National Natural Science Foundation (No. 50534090), the National Key Basic Research Development Program of China (No. 2011CB201205), State Key Laboratory of Coal Resources and Mine Safety of China University of Mining Technology of China (No. SKLCRSM08X03), and the Youth Science and Technology Fund of China University of Mining and Technology (No.JGY101605), are gratefully acknowledged.
Fig. 8. Comparison of permeability coefficient of coal seam.
References
5
10
15
20
(d)
25
30
35
40
45
Fig. 9. Variation of the deviation value d.
control, the average permeability coefficient is 0.51233 102 m2/ MPa2 d, which increases 7.3 times. After liquid–solid coupling blasting, the coefficient is 1.60325 102 m2/MPa2 d, which increases 22.7 times, but the maximum 36.2 times. This shows that there is a wider range of fracture network in coal-rock mass after using the liquid–solid coupling blast, so the permeability of coal seam improves a lot. (2) Fig. 9 shows the variation of deviation value d. As noted in Fig. 9, the ratio above the predictive index testing the risk greatly decreases, and there is a little variation, which obviously controls outburst, such as serious jet orifice. 5. Conclusions (1) This study combines injecting water into coal seam with water pressure blast, and proposes the technology of liquid–solid coupling blast. Besides, it is the first time to use the technology for preventing rock burst and coal-andgas outburst. The problem of pressure relief and permeability increase is solved, which provides a new way for coal seam with high gas, high stress, and low permeability. (2) The research uses LS-DYNA software to simulate the variation of stress and displacement of coal and rock in the process of liquid–solid coupling blasting and ordinary blasting. Following conclusion is verified: at the same time, the stress and displacement of blasting in water medium are greater than that in the air medium blasting. (3) The calculation and experience equations of overpressure are implied by physical simulation of overpressure in liquid–solid coupling coal-rock mass, and it concludes that: as water increases, maximum value DP of overpressure produced by blast grows but slightly decreases. The overpressure without water is lower 100 times than that with water.
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