Principle and practice of hydraulic softening top-cutting and pressure relief technology in weakly cemented strata

Abstract

Extremely thick and hard roofs are difficult to break in the mining of a working face, and the large area of the suspended roof easily induces a strong ground pressure or dynamic impact disasters. The roof control of a coal mining face in a mine in western China was taken as a case study. The mineral composition, microstructure, and hydrophysical properties of the hard roof overlying the coal seam were analyzed. The characteristics of the weak-cementation strata that are prone to mud and collapse when encountering water were targeted to investigate the hydraulic softening roof-cutting and pressure relief technology. It was found that the clay mineral composition in the roof plate accounts for 60.6%. After 24 h of natural immersion, the rock strength decreased by approximately 10.3%–49%, and further immersion caused disintegration. By arranging high and low double-row water injection softening drilling holes in the cutting hole and roadway of the working face, the strength of roof rock strata in the target area was reduced, and the initial weighting step distance and weighting strength of the working face were reduced. The hydraulic softening roof-cutting pressure relief technology effectively regulated the weighting step distance of the hard roof and the peak weighting of the working face.

Full text

TYPE Original Research
PUBLISHED 27 February 2024
DOI 10.3389/feart.2024.1367933
OPEN ACCESS
EDITED BY
Bin Gong,
Brunel University London, United Kingdom
REVIEWED BY
Jun Liu,
Sichuan University, China
Tianran Ma,
China University of Mining and Technology,
China
*CORRESPONDENCE
Liyuan Liu,
liuliyuan@ustb.edu.cn
RECEIVED 09 January 2024
ACCEPTED 15 February 2024
PUBLISHED 27 February 2024
CITATION
Wang T, Xu G, Liu L, Bai C, Ye W and Sun L
(2024), Principle and practice of hydraulic
softening top-cutting and pressure relief
technology in weakly cemented strata.
Front. Earth Sci. 12:1367933.
doi: 10.3389/feart.2024.1367933
COPYRIGHT
© 2024 Wang, Xu, Liu, Bai, Ye and Sun. This is
an open-access article distributed under the
terms of the Creative Commons Attribution
License (CC BY). The use, distribution or
reproduction in other forums is permitted,
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No use, distribution or reproduction is
permitted which does not comply with
these terms.
Principle and practice of
hydraulic softening top-cutting
and pressure relief technology in
weakly cemented strata
Tao Wang1, Guoyao Xu1, Liyuan Liu1*, Chaoqiang Bai 1,
Weiwei Ye1and Lihui Sun2
1Beijing Key Laboratory of Urban Underground Space Engineering, School of Civil and Resources
Engineering, University of Science and Technology Beijing, Beijing, China,2School of Mining and
Geomatics Engineering, Hebei University of Engineering, Handan, China
Extremely thick and hard roofs are dicult to break in the mining of a working
face, and the large area of the suspended roof easily induces a strong ground
pressure or dynamic impact disasters. The roof control of a coal mining face in
a mine in western China was taken as a case study. The mineral composition,
microstructure, and hydrophysical properties of the hard roof overlying the coal
seam were analyzed. The characteristics of the weak-cementation strata that
are prone to mud and collapse when encountering water were targeted to
investigate the hydraulic softening roof-cutting and pressure relief technology.
It was found that the clay mineral composition in the roof plate accounts
for 60.6%. After 24h of natural immersion, the rock strength decreased by
approximately 10.3%–49%, and further immersion caused disintegration. By
arranging high and low double-row water injection softening drilling holes in
the cutting hole and roadway of the working face, the strength of roof rock
strata in the target area was reduced, and the initial weighting step distance and
weighting strength of the working face were reduced. The hydraulic softening
roof-cutting pressure relief technology eectively regulated the weighting step
distance of the hard roof and the peak weighting of the working face.
KEYWORDS
weakly consolidated formation, giant thick roof plate, hydraulic softening, pressure
relief technique, roof-cutting
1 Introduction
e large-scale exploitation of coal resources in western China has resulted in certain
phenomena such as a strong rock pressure and dynamic damage (Mirenkov, 2020;
Yangetal., 2021). A major factor aecting the working face weighting and dynamic impact
disasters is the properties of the overlying roof rock above the coal seam (Alehossein
and Poulsen, 2010). e giant thick roof plate tends to accumulate energy, and its
rupture process and subsidence can increase the pressure on the working face, which
would damage the supporting structure within the coal–rock mass. In severe cases, the
accumulated energy may result in the uncontrollable deformation of the surrounding
rock or cause impact dynamic disasters such as the crushing of the supporting structure
or a coal outburst (Heetal., 2012;Rozenbaum and Demekhin, 2014;Liuetal., 2023).
Large hanging roof areas are easily formed in the hard roof during mining, and the roof
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FIGURE 1
Layout of working face.
failure oen releases a signicant amount of elastic energy, thereby
triggering impact dynamic disasters (Pavlovaetal., 2019;Xuetal.,
2021). erefore, relevant techniques are crucial for the prediction
and prevention of dynamic impact disasters to ensure deep coal
mining safety (Guetal., 2022;Liuetal., 2023;Wangetal., 2023).
In the coal mining practice in western China, it is dicult to
understand the rock pressure characteristics of several working faces
in newly commissioned mines or in the initial mining stages of the
mining areas (panel areas). e diculty is due to the relatively
hard and intact overburden structure. e structural hardness
is manifested through the excessive weighting step of the initial
pressure and the weighting timing unpredictability (Lianetal.,
2023), resulting in a high rock pressure on the working face
(Danilievetal., 2022). ese uncertainties pose certain risks to the
mine production. erefore, the control of hard-roof plate weighting
has been extensively investigated, leading to the development
of many practical solution technologies. For instance, deep hole
blasting can be used to reduce the weighting step (Wangetal., 2013;
Chenetal., 2022;Kanetal., 2022) or hydraulic fracturing of the
roof (Moghadasietal., 2019;Zhengetal., 2021;Liu, 2022). Similarly,
slotting in the middle of the roof can prevent the roof from falling
suddenly while supporting cutting, and mined-out areas or areas
with a detached roof can be lled to eliminate the dynamic pressure
impact caused by roof collapses (Changetal., 2021;Sunetal., 2021).
Additionally, mining protective layers can be installed to reduce the
roof pressure (Zhangetal., 2020;Chengetal., 2021;Leietal., 2022),
among other methods.
e geological formations in western China are mainly
composed of Jurassic and Cretaceous strata. ese formations have
relatively recent diagenetic ages and certain special properties,
such as weak cementation, easy weathering, and ease of becoming
muddy and sandy when in contact with water. Consequently, mining
operations in these formations face signicant challenges (Liuetal.,
2020;Liuetal., 2021;Liuetal., 2022;Asifetal., 2022;Liuetal.,
2022;Lietal., 2022;Liuetal., 2023). is diculty necessitates in-
depth research on utilizing the geological characteristics of western
China to implement targeted measures for preserving the weak-
roof rock layer and regulating the working face weighting (Xuetal.,
2018;Wangetal., 2023a;Wangetal., 2023b;Wangetal., 2023c).
is study investigated the physical, mechanical, and hydrophysical
properties of weakly cemented strata, using a coal mining face in
a mine located in western China as a case study. e principles and
implementation plans of the hydraulic soening for roof cutting and
pressure relief techniques were examined, providing a reference for
preventing and controlling high rock pressure and impact dynamic
disasters according to local conditions in western China.
2 Property of the western strata
2.1 Overview of the working face
e coal mine is located in the western region of China and
extracts coal from the Jurassic coal seam. e mining face is adjacent
to the mined-out area on the west side and is bordered by solid coal
on the east side. Figure1 shows that the length of the mining face is
250m, and it extends to a total distance of 2,603m.
e mining face for the coal three to one seam is located in
a stable coal layer. e thickness of the coal seam in the working
face ranges from 1.1m to 6.9m. e coal seam is buried at an
embedding depth of 527.6–571.7m, with an average coal thickness
of 5.18m and a dip angle of 1–3°. During the initial mining and
rst release period, the mining height should be controlled at 5.5m.
e immediate roof of the coal seam consists of Jurassic sandy
mudstone with a thickness of 2.6–22.0m, which averages 12.5m.
e basic top comprises medium-to-ne sandstone, with a thickness
of 4.2–16.3m, averaging 10.3m. e geological columnar diagram
of the working face is shown in Figure2.
2.2 Mechanical properties of roof plate
rock
Based on the geological borehole histogram, rock samples of the
relevant layers ofthe co al seam roofwere obtained using core drilling
and processed in the laboratory to create standard specimens.
irty-six standard specimens were prepared, representing three
lithologies: the coal three to one, sandy mudstone of the immediate
roof, and medium-to-ne sandstone of the basic top. Owing to the
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FIGURE 2
Geological borehole histogram.
inuence of the rock strata structure, more sandstone samples were
obtained during the sample preparation.
e impact of the roof plate on the working face risk
under natural conditions was investigated through pressure tests
conducted on the specimens using a pressure testing machine.
e tests provided the physical and mechanical parameters of
the roof plate in their natural state, yielding an average uniaxial
compressive strength of approximately 5.27MPa for the coal
samples. e average uniaxial compressive strength of the medium-
to-ne sandstone and sandy mudstone samples was approximately
65.65 and 66.75MPa, respectively. e specimens had a uniform
compressive strength distribution, and the test results are shown in
Table1. e roof plate in their natural state exhibited a high strength
and thickness, which had a signicant impact on the pressure
exerted on the working face.
2.3 Rock mineral composition and
microstructural characterization
To explore methods for weakening the strength of the roof plate,
an assay analysis was conducted on the roof plate rock, considering
the weak cementation properties of the Jurassic formations. e
results revealed that the main constituents of the rock were quartz,
feldspar, and clay minerals, with clay minerals accounting for
60.6% of the composition. Notably, the content of high-expansive
montmorillonite within the clay minerals peaked at 82%. is value
indicates that the rock formation belongs to extremely swelling so
rock, which tends to react and expand when in contact with water.
To analyze the internal material distribution and structural
morphology of the rocks, microscopic observations of the rock
samples were performed using a scanning electron microscope
(SEM). e SEM images of the sandy mudstone and medium-to-ne
sandstone are shown in Figures3,4, respectively. Microscopically,
both rock types exhibit a fragmented particle distribution. e shaly
sand shows well-developed internal microssures and noticeable
linear fractures. e cementitious material in the medium-to-ne
sandstone appears as occulent aggregates, and the internal pores
are well developed. e microscopic features indicate that the
rocks possess a loose structure, making them prone to breaking
into fragments or dispersoids when exposed to external factors
and subjected to damage. Additionally, the pore structure exhibits
good water absorption and permeability, leading to a high degree
of soening and extensive fracturing of the rocks under the
inuence of water.
2.4 Hydrophysical properties of the roof
rock
e eects of the water-immersion soening of roof rocks
and the mechanism of rock disintegration under prolonged water
immersion conditions were studied through soaking experiments
conducted on the sandy mudstone and medium-to-ne sandstone
of the roof plate. e variations in the uniaxial compressive strength
of the rock samples under dierent water content conditions
were analyzed to assess the soening eect of water immersion.
Additionally, the failure patterns of the rocks aer long-term water
immersion were observed to summarize the mechanism of water-
soaking disintegration.
2.4.1 Softening eect of water immersion
Laboratory experiments were conducted to compare the
strength of the samples aer water immersion. e water
content of the rocks was articially controlled, and Figure5
illustrates the uniaxial compressive strength of the samples for
dierent water contents. e results showed that aer 24h
of natural water immersion, the water content of the sandy
mudstone reached 3%, reducing the strength by approximately
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TABLE 1 Physical and mechanical parameters of the dierent rocks.
Rock sample Volume-
weight/KN/m3Compressive
strength/MPa
Tensile
strength/MPa
Elastic
modulus/GPa
Poisson’s ratio
Coal 13.45 5.27 1.25 2.27 0.17
Medium-to-ne
sandstone
26.96 65.65 5.66 18.68 0.19
Sandy Mudstone 26.22 66.75 4.21 10.21 0.22
FIGURE 3
Microscopic characterization of sandy mudstone: (A) 500 times; (B) 1,000 times.
FIGURE 4
Microscopic characteristics of medium-to-ne sandstone: (A) 500 times; (B) 1,000 times.
10.3%–49%. For the medium-to-ne sandstone, the water
content reached 2.4% aer 24h of water immersion, reducing
the strength by approximately 26%. e soening eect of the
sandstone under water immersion was more pronounced, with a
signicant increase in water absorption rate and a large decrease
in strength.
Weakly cemented rock contains a higher content of cohesive
clay minerals, which easily react with water. Subsequent experiments
demonstrated that aer the rocks reached water saturation under
prolonged water immersion, their soening continued over time
at a stabilized water absorption rate, and the strength continued
to decrease.
2.4.2 Eect of water disintegration of rocks
Long-term immersion experiments were conducted on the
specimens to observe the state of the rocks during water immersion.
Figure6 shows images comparing the states of the medium
sandstone during immersion. At the initial immersion stage, the
rock samples exhibited signicant water absorption accompanied
by bubble generation. Partially soluble matters on the rock surface
were lost, and cracks were formed, gradually developing into
ssures. Subsequently, the water–rock interaction proceeded slowly,
causing the cementitious materials to dissolve and the rock
particles to disintegrate gradually. Eventually, the sandstone became
muddy overall.
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FIGURE 5
Dierent rocks’ strength versus water content.
Figure7 shows images of the sandy mudstone states during
immersion. e disintegration damage of the sandy mudstone
diered from that of the medium sandstone. During immersion, the
sandy mudstone disintegration progressed more slowly, primarily
undergoing water absorption and rock swelling. Subsequently, joints
or microcracks expand and form cracks. e clay minerals within
the rock are distributed in a band-like pattern that intersects in
multiple directions. e ultimate failure of the rock is characterized
by the formation of fragmented blocks accompanied by debris, as
well as an overall fragmented disintegration.
e above experiments indicate that the water stability of weakly
cemented rocks under these geological conditions can be classied
into two categories. In the rst category, the cementitious material
of the rocks dissolves upon contact with water, causing the rock
particles to disintegrate into fragments. An example of this category
is the Jurassic medium sandstone. In the second category, the dense-
lithology rocks possess well-developed joints and internal fractures.
When exposed to water, these rocks undergo water absorption and
swelling, causing the cracks to expand and propagate. Eventually, the
rocks disintegrate into fragments. An example of this category is the
Jurassic sandy mudstone.
2.4.3 Analysis of rock softening and
disintegration mechanism
e soening eect of water on rocks is mainly attributed to
adsorption and absorption, hydration, wedging, and dissolution.
e rock disintegration described above is a failure phenomenon
that occurs under continuous soening. is process can be
summarized as the generation of secondary surface pores, pore
connectivity leading to crack formation, generation of internal
secondary pores, and internal ssure fragmentation through
connectivity.
According to the soening process eects, the generation of
secondary surface pores in rocks can be attributed to the following
reasons, and the results are shown in Figure8:
(1) Some minerals are water-soluble or can chemically react with
water, forming secondary pores in their original positions.
(2) e dissolution of carbonate cement in rocks weakens the
interconnection between rock particles, resulting in the loss of
cementitious material and the formation of secondary pores.
(3) Clay minerals absorb water and swell, and the water molecules
enter the crystal lattice of the clay minerals, causing secondary
pores to expand and form along the stratication plane.
An analysis of the hydrophysical properties of the roof
plate shows that the rock exhibits good soening eects when
exposed to water, and prolonged water immersion conditions
cause disintegration. In engineering rock formations, this
phenomenon is manifested as crack and joint expansion. ese
manifestations can enlarge the soened zone and enhance
the soening eect, indicating that the roof plate possesses
hydrophysical conditions necessary for inducing water injection
soening.
3 Hydraulic softening roof cutting
program
3.1 Necessity of cutting the top of the
working face for pressure relief
According to geological exploration data and mining exposure
data, the area within a range of 50–55m above the coal three to
one seam contains sequential layers of sandy mudstone (8.3m),
coarse sandstone (7.5m), gravel layer (26–28m), coarse sandstone
(1.8m), and ne-grained sandstone (7.4m). A thick layer of
coarse sandstone with a thickness ranging from 154 to 165m
exists above the 55m mark. e rock pressure of the completed
mining face indicates that the initial weighting step of the old
roof is relatively large during the initial mining stage when
inuenced by the hard roof plate above the coal seam. e
initial weighting step ranges from 31.6 to 47.8m, and the support
resistance weighting is between 36.8 and 42.0MPa, as presented in
Table2. Excessive initial weighting steps hinder roof management
in the working face and pose signicant safety hazards to
production operations.
Owing to the slow cracking process, as well as the reversal
and sinking of the massive sandstone layer with a total thickness
exceeding 150m above the coal three to one seam, the roof plate
of the two adjacent mined-out areas become interconnected aer
the working face mining, forming a unied subsidence zone. During
the mining process, the roof of the goaf, which is formed aer three
adjacent working faces are mined, may experience a phenomenon
known as dynamic pressure. erefore, it is necessary to regulate the
roof plate weighting in the initial mining stage of this working face
to eectively reduce the weighting step and control the weighting
intensity. An excessive initial weighting step of the working face
results in unfavorable conditions for the roof, posing signicant
safety hazards to the mining.
3.2 Hydraulic softening cut top pressure
relief program design
Because the Jurassic strata is prone to soening and
disintegration when it encounters water, the initial weighting
step of the immediate roof and the old roof can be signicantly
reduced by pre-soening the roof plate of the working face using an
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FIGURE 6
Hydrophysical characteristics of sandstone during dierent immersion times: (A) Pre-laboratory; (B) Soak for 3days; (C) Soak for 28days.
FIGURE 7
Hydrophysical characteristics of sandy mudstone during dierent immersion times: (A) Pre-laboratory; (B) Soak for 5h; (C) Soak for 30days.
FIGURE 8
Disintegration process of rocks in contact with water.
underground high-pressure water supply. is approach provides
sucient time for the cracking, reversal, and sinking of the
massive sandstone layer above the working face while reducing
the volume of subsided blocks in the massive sandstone layer.
As a result, the instantaneous dynamic eects on the roof of
the working face are minimized, ensuring the safe mining of the
working face.
According to the columnar display of boreholes, the roof
plate gravel layer in the local area belonging to the cut hole is
approximately 40m away from the roof plate of the coal three to one,
with a thickness exceeding 150m of the water-bearing sandstone
layer above the gravel layer. Scientic exploration data show that
the upper boundary of the gravel layer is generally consistent with
the boundary of the water-conducting fracture zone. Soening and
cutting the roof plate of the gravel layer and the underlying rock
strata are crucial for controlling the rock pressure on the roof of
the working face, as well as the degree of instantaneous dynamic
manifestation of the massive sandstone layer. Considering the roof
plate structure, the following water injection soening scheme is
implemented:
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TABLE 2 Statistics of incoming weighting in the initial mining phase of
the working face.
Working
surface
number
Initial
weighting
step/m
Weighting
working
resistance of
the
support/MPa
1 31.6 36.8
2 47.6 42.0
3 47.8 41.5
TABLE 3 Hydraulic softening top-cutting pressure relief
borehole design.
Position Drill
hole
number
Drilling
length/m
Overhead
angle/°
Total
footage/m
Cutting
eye
High-
position
01–15
60 54
1,320
Low-
position
01–12
35 44
Belt side
groove
Belt 1–7 47 75 329
(1) Cutting eye: Weakening treatments are conducted on
the roof within a forward horizontal distance of 30m
and an upward distance of 48m from the cutting eye.
e treatments are conducted using 12 low-position
boreholes and 15 high-position boreholes. e design
of the high- and low-position boreholes ensures the
coverage of the immediate roof, basic top, and hard rock
overlying layers.
(2) Track side groove (on the mined-out area side): e roof of the
track side groove is inuenced by the adjacent mined-out area,
and most of the area has been fractured. No water-injection
soening boreholes are constructed in the track side groove.
(3) Belt side groove (on the solid coal side): For the roof
of the belt side groove, seven boreholes are arranged
at intervals of 7m to soen the roof within a vertical
distance of 45m and a horizontal distance of 12m above
the coal seam.
e drilling parameters are presented in Table3 and visually
depicted in Figure9 as follows:
Aer completing the borehole construction, the AB glue sealing
technique is used to seal the borehole. e sealing length is 3m,
and aer sealing, a valve is installed at the borehole opening.
Subsequently, a high-pressure water pump is connected to maintain
an injection water pressure of 5.0MPa, ensuring that the borehole is
lled with water and exerts a certain outward diusion pressure. e
determination of the appropriate water injection volume directly
aects the water injection eect. e water injection operation
lasts for 10 days.
3.3 Numerical simulation of
water-injection softening eect
3.3.1 Numerical calculation model
To investigate the eect of hydraulic soening roof cutting
and pressure relief, a numerical simulation of the evolution of the
stress eld aer roof cutting in the working face was performed
using Flac3D soware. e Mohr–Coulomb elastoplastic model
was used for the constitutive behavior. e model dimensions
were established according to the actual dimensions of the original
structure, with a size of 800m × 837m × 270m. Displacement
constraints were applied to the bottom and lateral boundaries,
and a load of 8.75MPa was applied to the top to simulate the
overburden pressure. e structural positions in the model are
illustrated in Figure10.
3.3.2 Stress eld distribution in the working face
area
In the working face, water injection boreholes are arranged in
the cutting eye and tape side groove to soen the roof plate by
injecting water inward. Long-term pressurized water injection can
enhance the soening eect on the coal-rock mass, weakening the
strength of the strata within a 40m range above the coal seam and
promoting the expansion of borehole cracks, which is benecial for
roof pressure management. Figure11 shows the distribution of the
stress eld in the water injection area. It can be observed that the rock
mass strength is reduced in the central region of the water injection
borehole, and the bearing stress is transferred to the deeper part of
the rock mass. e vertical stress values decrease simultaneously,
with some areas reducing by up to 3.5MPa, indicating the presence
of stress concentration regions in the roof above the boreholes. As
the working face advances, the stress concentration regions further
expand, and the hard roof plate fractures.
3.3.3 Evolution of stress eld in the advancement
process of the working face
By analyzing the evolution of the stress eld and plastic zone
during the mining process aer arranging the water-injection
soening boreholes, we can assess the eectiveness of hydraulic
soening in fracturing the roof plate and avoiding large-scale roof
hanging, thereby controlling rock pressure accumulation.
To analyze the inuence of hydraulic soening boreholes on
the stress eld of surrounding coal–rock mass, the vertical stress
evolution was analyzed along the direction oft hehydraulic soening
boreholes, with the cutting eye coal wall being the origin. e vertical
stress around the boreholes remained high at a mining depth of
10m. At this stage, the roof could still transmit the pressure from
the upper part of the mined-out area to the coal wall aer the cutting
eye, and the soening eect had not yet manifested. When mining
reached a depth of 20m, the low-stress regions around the boreholes
spread and connected with one other, forming stress reduction zones
in the lower rock layers along the borehole prole. Hence, the stress
transfer of the immediate roof and part of the basic top were cut o.
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FIGURE 9
Arrangement of water-injection softening and pressure relief drill holes: (A) Borehole layout plan; (B) Cutting eye drilling prole; (C) Belt side
drilling prole.
FIGURE 10
Numerical model working face: (A) 3D model; (B) Drill hole distribution.
At a mining depth of 30m, the stress reduction zone expanded to the
entire height of the boreholes. e strength of the rock layers around
the borehole prole decreased, and the roof transitioned to a state of
support into two sides. e results are shown in Figure12.
Aer the excavation of the working face, the stress in the coal
seam is transmitted to the surrounding areas. To analyze the stress
state in the roof region, the stress eld evolution prole was analyzed,
using the mid-section of the working face as the origin. As the
working face is excavated, a pressure arch is formed, and the stress
is concentrated in front of the coal wall and behind the mined-
out area, forming stress arch feet. As the working face advances,
the concentration of stress on the arch feet increases. At a mining
depth of 30m, the stress arch foot in front of the coal wall reaches
23MPa, and the results are shown in Figure13. e hydraulic
soening weakens the strength of the roof, and the roof pressure is
transmitted to the sides. e supporting pressure in front of the coal
wall increases, fracturing the roof in the high-stress area in front of
the coal wall and collapsing the roof above the mined-out area along
the borehole prole.
At a mining depth of 30m, the mined-out area expands, and
the hanging area of the roof increases. e mining eld stress causes
cracks to propagate in the roof plate. Aer hydraulic soening, the
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Wang etal. 10.3389/feart.2024.1367933
FIGURE 11
Stress eld distribution in the water-injection softening area of the working face: (A) Zone of inuence of vertical stress on the oblique prole of the
injection borehole; (B) Zone of inuence of vertical stress in the vertical prole of the injection borehole.
FIGURE 12
Vertical stress evolution in oblique prole of working face cut-hole drilling: (A) Recovery 10m; (B) Recovery 20m; (C) Recovery 30m.
plastic zone in the roof plate extends and exhibits a destructive
distribution at a depth of 10m above the roof, indicating the
occurrence of fracture. At a mining depth of 20m, the plastic zone
in the roof plate expands, but it becomes evident at a depth of
40m. is indicates that the roof of the working face undergoes
fracture at approximately 30m of mining depth. e results
are shown in Figure14.
4 Roof-cutting decompression eect
test
Aer implementing the hydraulic soening roof-cutting
program as described above, the stress of the supports in the
working face was analyzed during the initial mining phase to
examine the rock pressure characteristics. e statistics analyses
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Wang etal. 10.3389/feart.2024.1367933
FIGURE 13
Vertical stress evolution in the middle section of the working face: (A) Recovery 20m; (B) Recovery 30m.
FIGURE 14
Evolution of the plastic zone of the roof plate 30m back in the working face: (A) 10m above the roof plate; (B) 20m above the roof plate; (C) 40m
above the roof plate.
were performed for eight consecutive days. e distribution curve
of support resistance in Figure15shows that as the footage increases,
high-support resistance occurs in the middle and lower parts of the
working face. On the sixth day, when the coal mining machine
advanced to 22.4m, the initial weighting occurred in the upper
roof. e stress generated when the initial weighting occurs is
shown in Figure16. e maximum support pressure was 43.85MPa
(130# support), the minimum was 18.9MPa (25# support), and the
average was 31.9MPa.
With the initial weighting of the basic top, the coal strength
near the coal wall reaches its ultimate failure strength, generating
abnormal noises and causing extensive spalling of the piece. ese
phenomena can be utilized to forecast the initial weightage of a
working face in inuencing an index. e high stress in the coal
gradually transfers toward the front of the coal wall. en, the roof
in the mined-out area collapses, revealing large blocks of white
sandstone gangue. During the initial weighting of the working face, a
large amount of water is not encountered in the roof, indicating that
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Wang etal. 10.3389/feart.2024.1367933
FIGURE 15
Distribution curve of brace resistance in the working face at the initial mining stage.
FIGURE 16
Bracket stress curve at the initial pressure of the basic top.
the water injection and soening zone in the roof is controllable, and
there is no leakage of water injection.
5 Conclusion
(1) e clay mineral composition in the roof plate accounts for
60.6%, causing the loose internal structure of the rock and easy
soening and disintegration when water is encountered. Aer
natural water immersion for 24h, the strength decreases by
approximately 10.3%–49%.
(2) Weak-cementation strata are prone to disintegration when
encountered with water. e action of water leads to the
formation of secondary pores in the rock, which then connect
to form cracks. ese cracks continue to expand and generate
new pores until the intact rock is fragmented. is indicates
the hydraulic conditions required for implementing water
injection soening measures in the roof.
(3) By arranging high and low double-row water-injection
soening boreholes in the cutting eye and roadway of the
working face, the strength of the roof plate in the target area
is reduced. e initial weighting step of the working face
is 22.4m, with a maximum support pressure of 43.85MPa.
e hydraulic soening and top-cutting unloading technique
eectively controls the hard roof weighting step and the peak
weighting value in the working face.
Data availability statement
e original contributions presented in the study are included in
the article/Supplementary material, further inquiries can be directed
to the corresponding author.
Author contributions
TW: Conceptualization, Supervision, Validation, Formal
Analysis, Methodology, Project administration, Resources,
Writing–original dra. GX: Conceptualization, Formal Analysis,
Methodology, Validation, Writing–original dra, Data curation,
Soware. LL: Conceptualization, Soware,Validation, Funding
acquisition, Supervision, Writing–review and editing. CB:
Methodology, Soware, Supervision, Validation, Writing–review
and editing. WY: Investigation, Methodology, Project
administration, Supervision, Validation, Writing–review and
editing. LS: Conceptualization, Data curation, Investigation, Project
administration, Supervision, Writing–review and editing.
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Wang etal. 10.3389/feart.2024.1367933
Funding
e author(s) declare nancial support was received for the
research, authorship, and/or publication of this article. is work
was funded by National Natural Science Foundation of China (Grant
Nos. 52004015, 51874014, and 52311530070), the fellowship of
China National Postdoctoral Program for Innovative Talents (Grant
No. BX2021033), the fellowship of China Postdoctoral Science
Foundation (Grant Nos. 2021M700389 and 2023T0025), and the
Fundamental Research Funds for the Central Universities of China
(Grant No. FRF-IDRY-20-003).
Acknowledgments
ese supports are gratefully acknowledged.
Conict of interest
e authors declare that the research was conducted in the
absence of any commercial or nancial relationships that could be
construed as a potential conict of interest.
Publisher’s note
All claims expressed in this article are solely those of the
authors and do not necessarily represent those of their aliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or claim
that may be made by its manufacturer, is not guaranteed or endorsed
by the publisher.
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