P. LI et al.: TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT OF HYDRAULIC PUNCHING ...
89–96
TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT
OF HYDRAULIC PUNCHING AND DEEP-HOLE PRE-SPLITTING
BLASTING IN A "THREE-SOFT" COAL SEAM
IZBOLJ[ANJE PREPUSTNOSTI TREH MEHKIH PLASTI
PREMOGA Z ZDRU@EVANJEM TEHNOLOGIJ HIDRAVLI^NEGA
PREBIJANJA IN MINIRANJA GLOBOKIH VRTIN
Pu Li
1,2
, Xiaodong Zhang
1,3
, Hui Li
4*
1
School of Energy Science and Engineering, Henan Polytechnic University, Henan Province, Jiaozuo 454000, China
2
Zhengzhou Coal Industry (Group) Corporation Ltd, No. 188 Zhongyuan West Road, Zhongyuan District, Zhengzhou City,
Zhengzhou 450042, China
3
Collaborative Innovation Center of Coal-Bed Methane and Shale Gas for Central Plains Economic Region, Henan Province,
Jiaozuo 454000, China
4
School of Safety Science and Engineering, Henan Polytechnic University, Henan Province, Jiaozuo 454000, China
Prejem rokopisa – received: 2020-05-08; sprejem za objavo – accepted for publication: 2020-10-26
doi:10.17222/mit.2020.075
Aiming at the problems of poor gas permeability, serious difficulty in gas extraction and low pre-extraction efficiency of the
"three-soft" coal seam in the Zhengzhou mining area, a co-penetration technology of hydraulic punching and deep-hole
pre-splitting blasting is put forward, and the mechanism of the co-penetration of hydraulic punching and deep-hole pre-cracking
blasting is analyzed. Based on the theory of explosive dynamics and fracture mechanics, a numerical simulation method was
adopted to study the stress distribution and propagation law of blast cracks in coal for different spaces between a blast hole and
control extraction hole, and the reasonable space between the two blast holes is determined to be 4 m. Through a field test of the
Zhengzhou Coal Group GC Coal Mine, No. 188 Zhongyuan West Road, Zhongyuan District, Zhengzhou City, it is shown that
the penetration effect after pre-cracking blasting is significant, the average and maximum purity of the gas extraction in the adja-
cent four groups after blasting reach 0.065 m
3
/min and 0.11 m
3
/min, respectively, which is 2.5 and 5.5 times more than before
blasting, and the permeability of the coal body near the blast hole reaches 2.47 m
2
/(Mpa2·d), which is 466 times higher than that
of the original coal body and 5.5 times higher than after the hydraulic-punching measures.
Keywords: "three-soft" coal seam, hydraulic punching, deep-hole pre-splitting explosion, unloading permeability improvement,
gas extraction
Zaradi te`av s slabo plinsko prepustnostjo, te`avami z ekstrakcijo plina in slabo u~inkovitostjo predekstrakcije plina treh mehkih
plasti premoga na rudarskem obmo~ju Zhengzhou, Kitajska so avtorji tega ~lanka analizirali tehnologijo kombiniranega
hidravli~nega prebijanja (punktiranja) in cepljenja plasti med miniranjem globokih vrtin. Analizirali so mehanizem dodatne
penetracije med hidravli~nim prebijanjem in nastajanja razpok z miniranjem globokih vrtin. Na osnovi dinamike eksplozij in
lomne mehanike, so {tudirali, s pomo~jo metod numeri~ne simulacije, porazdelitev napetosti in napredovanje razpok v premogu
zaradi eksplozij pri razli~nih razdaljah med vrtinami za namestitev eksploziva, ko so le-te prilagojene vrtinam za ekstrakcijo.
Ugotovili so, da je primerna razdalja med dvema vrtinama za namestitev eksploziva 4 m. Na preizkusnem polju rudnika
Zhengzhou Coal Group – GC Coal, Zhongyuan, Zhengzhou na Kitajskem so ugotovili, da je u~inek penetracije (prodiranja) po
miniranju zelo pomemben, povpre~na in maksimalna u~inkovitost ekstrakcije plina v sosednjih {tirih (4) skupinah je bila 0,065
m
3
/min oz. 0,11 m
3
/min, kar je 2,5-krat in 5,5-krat ve~ kot pred miniranjem in plinska prepustnost premoga poleg vrtin za
eksploziv je bila 2,47 m
2
/(MPa
2
·d), kar je 466-krat ve~, kot jo ima originalna premogova plast in 5,5-krat ve~ja prepustnost, kot
so jo izmerili po hidravli~nem punktiranju.
Klju~ne besede: tri mehke plasti premoga, hidravli~no punktiranje, priprava globokih vrtin pred miniranjem, izbolj{anje
neobremenjene prepustnosti, ekstrakcija plina
1 INTRODUCTION
The Zhengzhou mining area mainly includes the
Shanxi Group No. 21 coal seam, which is a typical "three
soft" coal-seam area in China with a low porosity, low
permeability and high adsorption characteristics.
1
The
safety of the mine and the production-replacement ratio
are seriously affected because of the poor coal-seam per-
meability and difficult gas extraction.
2
Coal-mine gas di-
sasters are tackled seriously at home and abroad. In order
to improve gas drainage and coal-seam permeability, re-
searchers from various countries put forward many
methods, such as hydraulic flushing, hydraulic cutting,
hydraulic fracturing, etc. By studying the applicability of
different methods under various conditions of coal seams
and comprehensively analyzing their advantages and dis-
advantages better technical methods can be selected and
created. However, these methods have shortcomings.
In order to improve the permeability of the coal seam
and realize an efficient gas extraction, the Zhengzhou
Group carried out a lot of experimental researches on the
underground coal mine. Practice shows that the hydraulic
Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96 89
UDK 622.012.2:622.24:620.193.29:622.232.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(1)89(2021)
*Corresponding author's e-mail:
lihui0391@aliyun.com (Hui Li)

punching of the Rock Lane bottom plate is a more effec-
tive method of unloading the pressure and increasing the
extraction for the "three soft" coal seam, but there are
still several problems. Firstly, due to the soft-coal body
of the No. 21 coal seam, the challenges of hole drilling,
collapsing and plugging are serious.
3–6
After intense hy-
draulic punching, the holes formed in the coal seam lead
to a local stress concentration and an uneven pressure-re-
lief effect, causing hidden danger for roadway supports
in the later stage.
7
Secondly, due to the poor permeability
of coal seam 21, the pre-pumping effect after hydraulic
punching is still not ideal, and the gas concentration in
the borehole attenuates rapidly. Generally, the gas con-
centration in 3–8 days after drilling is lower than 10 %
andevenlowerthan5%insome holes. With respect to
the problems of anti-protrusion measures for the pre-
pumping area of the "three soft" coal seam drilled with
hydraulic punching in the Zhengzhou mining area, it be-
came urgent to find whether it is possible to take second-
ary anti-reflection measures to further improve the per-
meability of the coal body between pre-drainage
boreholes, without changing the original "hole-pipe-
seal-pumping" structure of the pre-pumping borehole of
the penetration layer, in order to prolong the effective life
span of the pre-pumping gas after the implementation of
hydraulic measures, improve the gas pre-pumping effect
and shorten the regional extinction-outburst period.
In recent years, deep-hole pre-splitting blasting, a
technology for an effective improvemement of the per-
meability of a coal seam, has been widely used in the
management of gas disasters in coal mines.
8
At present,
deep-hole pre-splitting blasting is mainly used with local
anti-outburst technical measures; however, it is rarely
studied, at home or abroad, as a technical measure to
strengthen gas drainage and improve the drainage effect
in a soft coal seam with low permeability. Based on this,
the above authors put forward a penetration technology
integrating hydraulic punching and deep-hole pre-split-
ting blasting, analyzing the mechanism of co-penetration
of hydraulic punching and deep-hole pre-cracking blast-
ing, the influences of different ways of blast-hole spac-
ing on the blasting effect of deep-hole pre-cracking with
a numerical simulation and determining the optimum
spacing of blast drilling.
9
An engineering experiment
was carried out in the coal face of the 25031 Santa Yard
of the GC coal mine in the Zhengzhou mining area, and
the results showed that the permeability of the coal seam
and the gas-pumping flow were effectively improved af-
ter the implementation of our synergistic penetration
technology; a new way of exploring the gas treatment of
a "three soft" low-permeability coal seam was started.
10,11
2 HYDRAULIC PUNCHING AND DEEP-HOLE
PRE-SPLITTING BLASTING CO-PENETRATION
MECHANISM
The co-penetration technique of hydraulic punching
and deep-hole pre-splitting blasting consists of drilling
into the penetration layer of the construction of the Rock
Lane bottom slab, and implementing the medium and
high pressure.
12
We use hydraulic punching for drilling,
which ends after a period of continuous pumping, and a
pre-splitting blast hole arranged between hydraulically
punched boreholes so that the hydraulic punching for
drilling and blast drilling form a cross-arrangement. Hy-
draulic punching forms a continuous plastic zone in the
coal body. In particular, punching holes play a role in
providing a free surface and a moving space for blast
drilling; as a result, blasting after the induction of holes
not only forms explosive fissures, but also promotes the
displacement of coal bodies so as to achieve a regional
pressure relief and permeability improvement.
2.1 Induction effect of hydraulic punching on blasting
Under the condition of a free surface, the stress wave
produced by an explosive detonation propagates to the
interior of the medium in the form of elastic waves. And
when a stress wave propagates to the vicinity of a hy-
draulically punched hole, it immediately reflects back.
Due to the tensile stress caused by reflected waves and
weak discontinuous waves before and after, the range of
cracks at the edge of hydraulically punched holes are
further expanded and, at the same time, the radial cracks
generated in the direction of the line connecting the blast
holes are interpenetrated to form a penetration fracture
surface of the blast holes and hydraulically punched
holes. Since the fractures in the direction of the hydrauli-
cally punched holes occur earlier than those in the other
directions, the fractures in this direction restrict the gen-
eration and development of the fractures in the other di-
rections. So, in a sense, hydraulically punched holes are
induced by radial fissures.
2.2 Pressure relief and anti-reflection effect
Under the action of blasting, blasting cavities are pro-
duced in the coal and rock mass, and internal cracks de-
velop. The blast cavity is equivalent to an enlarged blast
hole, releasing the ground stress and the high gas pres-
sure around the blast hole so that the stress of the sur-
rounding rock shifts to both sides of the borehole and re-
duces the high gas pressure around it. The coal and rock
mass destroyed by the blast cavity formed by compres-
sion crushing reduces the potential energy contained in
the coal rock mass and gas balance system, thus reducing
the outburst risk. The most obvious function of a blasting
fracture is to provide the circulation channel for the
high-pressure gas. At the same time, each fracture also
releases the pressure of the adjacent coal body, and under
P. LI et al.: TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT OF HYDRAULIC PUNCHING ...
90 Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96

the action of seepage diffusion, the adsorbed gas is con-
verted into free gas, thereby increasing the permeability
of the coal body.
3 NUMERICAL SIMULATION OF REASONABLE
BLAST-HOLE SPACING
3.1 Numerical models and parameters
In order to study the propagation and attenuation law
of the explosion stress wave in the coal and rock mass
and the influence of different ways of blast-hole spacing
on the blasting effect, in accordance with the field work-
ing conditions, a numerical calculation model is estab-
lished using finite-element-analysis software ANSYA/
LS-DYNA, as shown in Figure 1, and the dimensions in
the model are presented in cm.
The upper and lower sides of the model are non-re-
flective boundary conditions, while the front and rear
boundaries of the model are loaded displacement con-
straints in the UZ direction. The sides of the model are
blast holes and the middle is the control pumping hole.
The calculation is not done when the simulation is
performed. The blast holes are 1.5 m away from the
boundary. Diameters of the blast holes and control ex-
traction holes are 94 mm, and the diameters of the medi-
cine rolls are 75 mm.
The JWL state equation in ANSYS/LS-DYNA can
accurately describe the work process of the expansion of
detonation products. From the equation, the detonation
pressure acting on the detonation body at any time can
be obtained, and the detonation pressure at any time is:
PA
RV
eB
RV
e
E
V
RV RV
=−
⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ +−
⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ +
−−
11
12
0 12
  (1)
In the formula, A and B are explosive characteristic
parameters, Gpa; R
1
, R
2
,  are dimensionless explosive
characteristic parameters; P is the pressure expressed in
kg/ m
2
; E
0
is the internal energy of a detonation product,
GPa; V is the relative volume of a detonation product,
m
3
.
The mechanical-physical parameters of the coal seam
and the basic parameters of the explosives in the simula-
tion calculation are shown in Tables 1 and 2.
3.2 Simulation-results analysis
3.2.1 Analysis of the stress changes to the coal bodies
In the blasting process, the stress waves produced by
blasting focus on the blast holes and spread in concentric
circles to the surrounding coal bodies (Figures 2 and 3).
The stress waves produced around two blast holes meet
at a certain distance after the propagation, resulting in
the superposition effect of the stress waves. Because of
different distances between the two blast holes, the ef-
fects of the stress waves produced by the two blast holes,
superimposed on each other, are also different. In Model
1, the stress waves meet at 2436.6 μs (Figure 2c). Then
the stress waves produced by the two blast holes are su-
perimposed on each other and spread continuously
around them (Figure 2d). In Model 2, since the distance
between the two blast holes is longer, the stress waves
meet at 2934.1 μs (Figure 3d) and then spread toward
each other, being superimposed.
In order to clearly analyze the variation law of the ef-
fective stress of the blasting-stress wave on a coal parti-
cle, a representative observation unit is selected on the
lines connecting two blast holes to depict the curves of
P. LI et al.: TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT OF HYDRAULIC PUNCHING ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96 91
Table 1: Parameters on mechanical-physical of coal seam
Bulk weight /(gcm
–3
)
Elastic modulus
/GPa
Poisson’s ratio
Layer thickness of
primary rock /m
Compressive
strength /MPa
Porosity /%
1.4 2.62 0.2 4.5 7.32 6.4
Table 2: Basic parameters of explosives
P /(Kgm
–2
)
Blast speed
v/(ms
–1
)
JWL state equation parameters
A (GPa) B (GPa) R
1
R
2
 R
cj
/GPa E
0
(GPa)
1620 6920 347 3.22 4.15 0.95 0.3 27 7.0
Figure 1: Numerical simulation model of deep-hole pre-splitting
blasting: 1) simulation model with blast-hole spacing of 3 m, 2) simu-
lation model with blast-hole spacing of 4 m

the effective stress changing with time, as shown in Fig-
ure 4.
When the blast-hole spacing is 3 m, the observation
units (A, B, C) are set in the sequence 0.5, 1, 1.6 m from
the left-hand blast hole
When the blast-hole spacing is 5 m, the observation
units (A, B, C, D) are set in the sequence 0.5, 1, 1.5, 1.9
m from the left-hand blast-hole
As can be seen from Figure 4, the detonation stress
at a distance of 0.5 m from the blast hole increases rap-
idly after blasting and the maximum stress values reach
140 MPa and 107 MPa, respectively, which is much
larger than the tensile compressive strength of the coal
body. The compression-crushing zone is formed here,
and then the stress wave continues to propagate outward,
while the stress value rapidly attenuates. At a distance of
1 m between the blast holes, the maximum effective
stress values are 58 MPa and 60 MPa, respectively, and
the fractures continue to expand in this range. At dis-
tances of 1.6 m and 1.9 m between the blast holes, it is
near the control extraction hole. It can be seen from the
point curves in Figures 4a C and 4b D that the stresses
of the two blast holes are superimposed to form a stress
concentration, and the stress after the superposition
P. LI et al.: TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT OF HYDRAULIC PUNCHING ...
92 Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96
Figure 3: Stress cloud map of the coal body at the blast-hole spacing of 4 m: a) t = 562.24 μs, b) t = 895.44 μs, c) t = 2134.73 μs,
d) t = 2934.13 μs
Figure 2: Stress cloud map of the coal body at the blast-hole spacing of 3 m: a) t = 427.13μs, b) t = 967.48 μs, c) t = 2436.67 μs, d) t = 2839.41 μs
Figure 4: Effective stress distribution curves of each observation unit
with a different blast-hole spacing

reaches 118 MPa and 85 MPa, respectively, which indi-
cates that the effective stress at the midpoint is larger for
the shorter hole spacing.
3.2.2 Analysis of a coal-crack propagation
As can be seen from Figures 5 and 6, the interaction
between the two models in the early stage of the crack
propagation is very small and the crack growth is within
the control range of the respective detonation gas. Due to
the superposition of the stress waves and the drive of the
detonation gas, the cracks perpendicular to the propaga-
tion direction of the stress wave begin to appear in
Model 1 at 769.3 μs, and the coal fragmentation near the
blast hole is accelerated. In Model 2, the cracks perpen-
dicular to the stress-wave-propagation direction appear at
913.9 μs. In Model 1, the cracks generated by the two
blast holes run through each other and are fully broken,
enhancing the permeability of the coal body and having
an obvious anti-reflection effect at 2667.2 μs. In Model
2, due to the large distance between the two blast holes,
the superposition effect of the two blast holes also de-
creases; as a result, the cracks produced by the two blast
holes are unlikely to meet at an early stage. They meet
and merge at 3026.3 μs.
Based on the repeated numerical simulation and com-
parative analysis, it is concluded that when the spacing
between the two blast holes is 3 m, the coal body is fully
broken after blasting, the crack propagation is more rapid
and the crack density is the largest, forming fractured
nets that cross each other and provide enough channels
for the gas migration into the coal body.
4 FIELD TEST
In order to investigate the penetration effect of
deep-hole pre-cracking blasting through hydraulic
punching under the condition of a controlled pumping
hole, a field engineering test was carried out on the un-
covered coal-mine face of Shimen, the substitute lane
yard of the coal mine of Zhengzhou Coal Group, 25031,
No. 188 Zhongyuan West Road, Zhongyuan District,
Zhengzhou City. The coal-seam thickness of the uncov-
ered coal area is 4.5 m and the gas content is 7.36–10.37
m
3
/t. At the 25031 lower-bottom pumping-lane construc-
tion, upward perforation drilling of the pre-pumped stone
of the coal-seam area was carried out for the coal-bed
gas. According to the test result for the effective radius
of gas extraction after hydraulic punching in the GC coal
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Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96 93
Figure 6: Generation and propagation of cracks at different times when the spacing of blast holes is 4 m: a) t = 913.91 μs, b) t = 1493.36 μs,
c) t = 2357.92 μs, d) t = 3026.33 μs
Figure 5: Generation and propagation of cracks at different times when the spacing of blast holes is 3 m: a) t = 769.33 μs, b) t = 1048.57 μs,
c) t = 1765.43 μs, d) t = 2667.26 μs

mine, that is, when the coal discharge quantity for a
coal-hole section is at least 1.5 t/m, the effective pump-
ing radius of hydraulic punching is 3.5 m and the range
of the hydraulic-punching coal leakage is 0.47–0.69 m.
At the same time, combined with the results for the sec-
tion 2.2 blasting numerical simulation, the hydraulically
punched holes in the test area are arranged according to a
mesh arrangement of 6×6 m. The coal-drainage range of
the hydraulically punched holes is 0.5 m and the distance
between a blast hole and a hydraulically punched hole is
2 m, that is, the distance between two blast holes is 4 m.
A total of 49 hydraulically punched boreholes and 12
blast boreholes were arranged in the test area, as shown
in Figure 7. All the hydraulically punched and blast
holes are 94 mm in diameter.
4.1 Construction process of the synergistic penetra-
tion-enhancement technology
The implementation of punching and blasting syner-
gistic penetration-enhancement technology is divided
into two stages: hydraulic punching and blasting. The
steps are as follows:
• Firstly, drilling is used to cross the holes according to
design requirements.
• Hydraulic punching is carried out on a crossing hole
with a large dip angle.
Drilling adopts a low water pressure and the
large-flow mode for punching holes, and controls the
single-hole-leakage coal volume. Large-dip-angle drill-
ing adopts a low water pressure and the large-flow-mode
punching to control the single-hole-leakage coal volume.
Plain holes adopt a high water pressure, the small-flow-
mode punching or an extended punching time to improve
the single-hole coal leakage. The amount of the coal-
hole leakage per meter should not be lower than 1.5 t,
and the amount of the single-hole coal should not exceed
30 % of the designed coal leakage.
3) At the end of punching, all the boreholes are con-
nected to the extraction pipeline for a week, and the
blasting-drilling construction is carried out according to
the design. Using the method of loading 1–2 medicine
rolls each time, the length of a single-hole charge is
6–10 m and the length of the bottom-hole rock section is
not below 3 m. The pre-cracking blasting-sealing equip-
ment is used for sealing, and detonating is performed af-
ter the sealing is completed. First, the holes with Nos.
1–6 are blasted, and the construction blasting of the
holes with Nos. 7–12 is carried out after an inspection.
4.2 Effect assessment of the synergistic penetration-en-
hancement technology
4.2.1 Gas-extraction concentration and a flow analysis
At the end of the blasting operation, the changes in
the pumping concentration and flow rate for each orifice
plate with time are obtained by testing and monitoring.
As an example, Figure 8 shows the variation in the gas
concentration and flow with time before and after the
blasting of 1#–4# orifice plates after blasting holes with
Nos. 1–6.
As can be clearly seen from the figure above, after
the detonation of 1–6 blast holes, the gas concentration
and flow rate for 1#-4# orifice plates increase. The gas
concentration of 2# orifice plate averagely increases 3.6
times in the 7 days after blasting compared with the pre-
P. LI et al.: TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT OF HYDRAULIC PUNCHING ...
94 Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96
Figure 7: Diagram of the borehole layout in the test area

vious state, and the concentration of extraction becomes
stable at 24.3–35.4 %. The extraction concentrations of
1# orifice plate and 3# orifice plate increase 2.83 and 2.7
times, respectively, compared with the concentration be-
fore blasting. As the drilling construction has a signifi-
cant effect on the coal-rock loosening, 4# orifice plate
exhibits a higher gas concentration within the 7 days af-
ter blasting. And it also averagely increased 1.9 times
compared with the previous state. Finally, the gas con-
centration of an orifice plate goes up and down at 20 %.
The gas purity of 2# orifice plate peaks on the 5th day
after the detonation, which is an increase of 5.5 times of
the average gas purity before the blasting. Under the ac-
tion of blasting, the gas purity of 1# orifice plate and 3#
orifice plate is 3.5 and 4.5 times higher than before. The
gas purity of 4# orifice plate is much better than that of
the other plates before blasting, and it is stable at the
original level after the blasting; on the 5th day it reaches
an increase of 2.3 times. There is no significant change
in the gas-extraction concentration and extraction purity
of 5# orifice plate after blasting.
4.2.2 Coal-permeability variation
Based on the above-mentioned punching and syner-
gistic gas-absorption data, the gas-permeability coeffi-
cients of the No. 21 coal seam before and after hydraulic
punching and blasting were determined with the radial
unstable flow method (Table 3).
It can be seen from Table 3 that the original gas-per-
meability coefficient of the coal seam is
0.0053 m
2
/(MPa
2
d). After the hydraulic-punching mea-
sures, it increases to 0.45 m
2
/(MPa
2
d), which is an in-
crease of 85 times. After the blasting measures are taken,
it increases to 2.47m
2
/(MPa
2
d), which is an increase of
466 times compared with the original coal body and
5.5 times compared with the hydraulic-punching mea-
sures.
5 DISCUSSION
In order to clearly analyze the variation law of the ef-
fective stress of the blasting wave on coal particles, rep-
resentative observation units are selected on the blast-
ing-hole connecting lines to depict the curves of the
effective-stress changing with time, as shown in Fig-
ure 4. As can be seen from Figures 5 and 6, the interac-
tion between the two models in the early stage of the
crack-propagation phase is very small, and the crack
growth is within the control range of the respective deto-
nation gas. It is concluded that when the spacing be-
tween two blast holes is 3 m, the coal body is most fully
P. LI et al.: TECHNOLOGY OF COUPLED PERMEABILITY ENHANCEMENT OF HYDRAULIC PUNCHING ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96 95
Figure 8: Variation curves for the purity and concentration of gas drainage from 1#–4# hole plate before and after blasting
Table 3: Comparative analysis of hydraulic punching and deep-hole pre-cracking effect before and after blasting
Parameter Before punching After punching After collaborative blasting
Coal-seam permeability coefficient
 /(m
2
(MPad)
–1
)
0.0053 0.45 2.47

broken after blasting, the crack propagation is more rapid
and the crack density is the largest.
6 CONCLUSIONS
1) In this research, aiming at the problems of
"three-soft" coal seams with poor permeability and low
gas drainage, taking the Zhengzhou coal mine as an ex-
ample, we found many disadvantages of the hydrau-
lic-punching technology when used alone. Practice
shows us that the pre-splitting blasting technology can
improve these problems significantly and can help us im-
prove the coal-seam permeability and gas drainage.
Through the application of a numerical-simulation analy-
sis, combining hydraulic punching and blasting, the ex-
perimental data demonstrated the feasibility and out-
standing role of the combination of the two processes,
allowing us to realize the theoretical analysis of the prac-
ticality and mechanism of the technology.
2) Hydraulic punching of holes has an obvious induc-
tion-promoting effect on the development of deep-hole
bursting ruptures, while blasting can use the free surface
provided by a hydraulically punched hole so that the gap
network between the two holes is greatly connected, im-
proving the permeability of the coal seam.
3) By comparing and analyzing the numerical-simu-
lation results, it becomes clear that the detonation shock
wave generated by the explosion can cause crack expan-
sion around the coal body, realizing the effect of coal-
seam penetration enhancement. When the effective stress
wave is transmitted to the control extraction hole, the
stress wave forms a stress concentration at the extraction
hole. The effective-stress value is greater than the tensile
compressive strength of the coal rock mass around the
pumping holes, promoting the formation of more cracks
in the coal rock mass around the pumping holes, and the
control extraction holes obviously play a role in the frac-
ture expansion in the coal rock mass. If the control ex-
traction hole is not punched, it is determined that the dis-
tance between the blast hole and the control extraction
hole is 1.5 m. In short, the blasting effect is better when
the distance between the two blast holes is 3 m.
4) Field tests show that after hydraulic penetration
and deep-hole pre-splitting blasting, the synergistic
anti-reflection technology is implemented, the coal-seam
penetration improvement and pressure relief are realized,
and the effect of gas extraction is apparent. After blast-
ing, the gas permeability of the coal body is 466 times
larger than that of the primitive coal body and 5.5 times
larger than that of the coal body after hydraulic punch-
ing, while the average pure amount and maximum flow
of gas extraction are 2.5 times and 5.5 times larger than
before blasting, respectively.
5) As the research on the hydraulic punching technol-
ogy and blasting technology is scarce at home and
abroad, the research on this kind of technology, included
in this paper, has an academic value and guiding value in
the world, which requires a technical method to solve the
problems of poor permeability and low gas extraction
from "three-soft" coal seams as well as the theoretical
basis for the application of this kind of technology.
The experiment from this research is mainly based on
the Zhengzhou coal mine. The experimental data and re-
sults have certain limitations while the theoretical results
lack universal applicability. In the future, the research
scope should be broadened and sampling methods
should be selected appropriately. Then, the experimental
results should be compared and analyzed. In addition,
the pre-splitting blasting technology for "three-soft" coal
seams is difficult to operate due to the softness of coal
seams, so relevant research should be carried out to im-
prove the technical method.
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96 Materiali in tehnologije / Materials and technology 55 (2021) 1, 89–96