A Study of Soft Rock Roadway Coupling Support in Xiajing Coal Mine

Abstract

Highly stressed soft rock subject to expansion deformation is particularly difficult to control. Field observations and experiments analysis show that an exhaust air roadway, level 100 of the Xiajing Coal Mine in Guangxi, China, is located in an intumescing rock with high in situ stress. Then we designed a bolt-mesh-anchor coupling support for the roadway repair and reinforcement, which consists of a first coupling support with bolt-mesh and secondary reinforcement with anchor cables. Field monitoring results show that bolt-mesh -cable coupling support has guaranteed the stabilization of the roadway and reduced the deformation of the surrounding rock considerably.

Full text

Procedia Engineering 84 ( 2014 ) 812 – 817
1877-7058
© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer
-review under responsibility of scientific committee of Beijing Institute of Technology
doi: 10.1016/j.proeng.2014.10.500
ScienceDirect
Available online at www.sciencedirect.com
“2014 ISSST”, 2014 International Symposium on Safety Science and Technology
A study of soft rock roadway coupling support in Xiajing Coal Mine
LI Xuefeng
*
, CHENG Guihai, LI Xiaoquan, ZHANG Ruichong
College of Resources and Metallurgy, Guangxi University, Nanning 530004, China
Abstract
Highly stressed soft rock subject to expansion deformation is particularly difficult to control. Field observations and experiments
analysis
show that an exhaust air roadway, level 100 of the Xiajing Coal Mine in Guangxi, China, is located in an intumescing
ro
ck
with high in situ stress. Then we designed a bolt-mesh-anchor coupling support for the roadway repair and reinforcement,
which consists of a first coupling support with bolt-mesh and secondary reinforcement with anchor cables. Field monitoring
results sho
w that bolt
-mesh -cable coupling support has guaranteed the stabilization of the roadway and reduced the deformation
of
the surrounding
rock considerably.
©
2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of scientific committee of Beijing Institute of Technology.
Keywords: soft rock; highly stress; deformation mechanism; coupling support; bolt-mesh-anchor support
1. Introduction
As the depth of underground mining increasing, failure problems in the soft rock get more serious. Roadways
s
urrounded by soft rock
at great
depth show the typical deformation characteristics of high stress, large
deformatio
ns, and support difficulties
[1-3].
Conv
entional supports cannot effectively control deformation and failure and this has brought great
difficulties to coal mine safet
y
[4,5]. Much research work has been conducted on the mechanical characteristics
of the sur
rounding rock and stability
control techniques of mining tunnels at great depth. Concepts of
“changi
ng axis theory”, “combine
d support technology”, “anchor-arc board support measures”, “excavation
dam
aged zone”, “primary and secondary loading
zones”, as well as other suppor
ting theories and techniques
were proposed[2,6-8].
* Corresponding author: Tel:+86 13768373489.
Email address: li
xfcshncn@126.com
© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(
http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer
-review under responsibility of scientific committee of Beijing Institute of Technology

813
Li Xuefeng et al. / Procedia Engineering 84 ( 2014 ) 812 – 817
It was found that a bolt-mesh-anchor support technique could effectively control deformation of tunnels at great
depth[9]. But if the coupling effect between this bolt-mesh-anchor support and deformation of the surrounding rock
w
as not realized, deformations of the bolt
-mesh support and tensile failure of the anchors will appear.
Therefore, the key issue of a successful bolt-mesh-anchor support is to harmonize deformations between support
and rock
mass
[10]. Research on roadway deformation and the coupling support in
deep, soft rock is particularly
important. This has a profound significance to achieve sustainable economic and efficient coal mine development.
In this paper a return air roadway in the Xiajing Coal Mine located in deep soft rock at level 100 is discussed. A
bolt-mesh-anchor coupling support as the main truss supporting technology is studied.
2. Geological characteristics of Xiajing Coal Mine
2.1. Lithology
Xiajing Coal Mine level 100 (at a buried depth of 600 m) return air roadway is located in the Devonian and
C
arboniferous strata and has a lithology consisting of sandstone, quartz sandstone
,mudstone, and a sandstones
in
terbedded with mudstone. A geologic histogram
of the roadway is shown in Fig. 1.
Vjkempguu*o+
Vqvcn
vjkempguu*o+
30:2
2092
2092
3042 30;2
2062 4052
209: 502:
2052 505:
20:2 603:
20:2 60;:
30;7 80;5
2092 9085
3052 :0;5
4052 33045
33032 47055
2042 47075
3082 49035
2044 49057
Eqnwopct
ugevkqp
Nkvjqnqi{
Owfuvqpg
4%"Eqcn"ugco
Swctv|"ucpfuvqpg
sandstones
interbedded with
mudstone
Owfuvqpg
ucpfuvqpg
Owfuvqpg
3%"Eqcn"ugco
Owfuvqpg
Ucpfuvqpg
Swctv|"ucpfuvqpg
vjkem/dgffgf
owfuvqpg
2%"Eqcn"ugco
wrrgt"unkeg
Owfuvqpg
2%"Eqcn"ugco
nqygt"unkeg
3086 340:9
2065 3505
20;4 36045
Fig. 1. Geologic histogram of the roadway.

814 Li Xuefeng et al. / Procedia Engineering 84 ( 2014 ) 812 – 817
2.2. Lithologic analysis
Samples were taken from the air roadway at level 100, level 150 and level 200, the total amount is 28 specimens.
They were analyzed with electron microscopy and X-ray diffraction, the results being shown here in Table 1.
The material contains clay minerals that swell. The cla
y minerals are mainly illite, kaolinite, and
amorphous clay.
This has a great impact on the rock strength. The swelling propensity upon water absorption of these minerals is
considerable and the clay minerals
has a relative content of 40%. In the mixed layers it is more than 60–65%. The
rock
surrounding roadway is a soft rock that swells.
Table 1. X-ray diffraction results: mineral type and content (%).
Amorphous clay
illite
kaolinite
gypsum
Quartz
Pyrite
Calcite
ankerite
siderite
total
mean
3.82
12.23
23.36
1.06
51.68
2.27
1.03
2.99
1.58
100.0
2.3. Mechanical properties of rock analysis
28 specimens of rock and coal were taken at level 100, level 150 and level 200. They were tested with rock
me
chanics
testing machine and rock point load instrument system, the results being shown here in Table 2.
Table 2. Mechanical properties of rock.
No
sample
Density
(t/m
3
)
Compressive
strength
(MPa)
Tensile
strength
(MPa)
Elasticity
(GPa)
Poisson’s
ratio
Bonding
strength
(M
PA)
Friction
angle(°)
1
sand stone (1)
2.63
98.7
15.1
46.19
0.29
5.8
69.3
2
sand stone (2)
2.64
97.4
12.7
54.02
0.29
7.3
69.0
3
quartz sandstone
2.66
182
13.2
56.67
0.29
7.3
70.1
4
mudstone (1)
2.46
31.3
1.2
—
—
8.9
31
5
mudstone (2)
2.64
28.6
1.1
—
—
8.1
31
6
coal
1.35
9.6
0.4
—
—
3.1
24
2.4. Stress field analysis
Xiajing Coal Mine is located in the south-west margin of Jiangnan anteclise, eastern edge of Bai Dan anticline.
T
he main geological structure is monoclinic, strata inclines
in an easterly direction with angle of 15°. In the southern
area of
the
mine there are a group of fold belt from north- east to south-west.
Stress in the rock is influenced by these geological structures. In-situ stress measurement results suggest that the
m
aximum stress in this region is a horizontal stress
. Based on measured values, the linear regression equation
bet
ween the maximum horizontal principal stress
or vertical stress and depth of the measuring point was generated,
as
follow.
165.50298.0σ
hmax
-? H
7611.00291.0σ
v
/? H
where
hmax
σ
is the maximum horizontal principal stress,
v
σ
is the vertical principal stress, H is the depth of the
measuring point
.
T
he maximum principal stress is 20–21 MPa at an azimuth of 206º.

815
Li Xuefeng et al. / Procedia Engineering 84 ( 2014 ) 812 – 817
3. An analysis of the deformation mechanism
At the start of the roadway excavation, an ordinary bolt-mesh support was applied. The deformation of the
sur
rounding rock mass was large and
accidents of roof falls occurred frequently.
Some damage photos are given below. As shown in Fig. 2, it is clear that: the roof of the
roadway has suffered
extensive damage and
localized failure was particularly serious. The
wall of the roadway was damaged seriously.
The wall of the roadway is in a special geological formation
where the expansion of soft rock by tectonic stress and
w
ater erosion
has occurred. The lower strength of these materials allows the coal wall to undergo a great amount of
drum
deformation.
Shrinkage of the roadway cross section is g
enerally 30% and can sometimes reach 60%
in
Xiajing Coal Mine, which seriously affects safety during production.
S
pot observations show that the deformation mechanism in the 100 level roadway consists of more than one type
and in
cludes mechanical deformation as well as expansion of the highly stressed composite soft rock.
Fig. 2. Damage of the roadway under early forms of support.
4. Coupling support design test
4.1. Principles of coupling support
The deformation of the air roadway required the use of a bolt-mesh-anchor coupling support technique for
roadw
ay repair and reinforcement. It consists of a first coupling support
with bolt-mesh and secondary
rein
forcement with anchor
cables, which are introduced timely at key positions[2, 11].
A bolt-mesh-anchor coupling support is meant to harmonize deformations by the coupling among bolt-mesh-
anchor support and the coupling between support and rock mass[12]. This way, rock strength may be improved by
th
e retaining structures and by coupling between components of them. The roadway is stabilized by a self
-
supporting anchor network involving soft layer coupled support trusses, bottom bolting, and lag anchor bracing.
Bol
t
-mesh-tray coupling trusses enhance the strength of the surrounding rock mass. Lag pre-stressed grouting
anchors prov
ide a suspension for stabilizing the tunnel. The
bottom bolting controls floor heave in the roadway by
cutti
ng off lateral slip
-lines. Overall stability is achiev
ed by homogenizing the loads through integrated support
design.
4.2. Support materials and timbering parameters
The timbering parameters of lane were determined according
to timbering engineering empirical formula.
(1) Anchors bolts: Bolts is made in left – handed thread steel, with a total length of 2400 mm andФ 22 mm in
dia
meter. It is anchored at the ends with two pieces of Z2350
resins .Preloading force in each bolt is more than

816 Li Xuefeng et al. / Procedia Engineering 84 ( 2014 ) 812 – 817
80KN. Compound steel tray is 100mm×100mm×10mm in size. The distance between rows is 800⁄800 mm in a
three flower arrangement.
(2) Anchor cables: Cables 21.6 mm in diameter and 7400 mm long with an exposed length less than 300 mm are
u
sed. It is anchored at the ends with three pieces of Z2350
resins, the pre-stressing force of cable is 98kN.
Compound steel tray is 300mm×300mm×10mm in size. The distance between rows is 1600×1600 mm in a 2×3
la
yout, the interval between cables is 800
-1200 mm.
(3) Metal netting: A 4.5 mm diameter steel welded mesh 3200×1000 mm in size with a mesh pitch 100×100 mm
is
used in the roof of roadway, and 2500×1000 mm in the wall .
(4) W - shaped steel strip: Type BHW-250-3-3000-3 steel strip is used in the roof of roadway, Type BHW-250-3-
2300-3 in the wall.
(5
) Corner anchors: Seamless tubing Ф32
mm in diameter are used with steel bars and grouting at a row distance
of
800 mm.
(6
) Concrete: The spray thickness of
the initially applied concrete is
40 mm. The results showed that the rock was
stable toward deformation and the steel frame was re
-sprayed. One to two months after installing the permanent
bracin
g a spray layer was applied to cover the frame and to provide an outer protective layer
60 mm thick. This layer
co
nsisted of C
20 grade concrete.
(7
) Bottom arches: Poured co
ncrete was placed 1
50 mm thick. The permanent design formed a high floor from
pouring concrete of a C
20 grade.
The design is shown in Fig. 3.
φ 54⁄4622oo
ugconguu"uvgcn"vwdg
Φ 44o⁄9622oo"cpejqt"ecdng
φ 44⁄4622oo"dqnv
"":22oo⁄:22oo
""""""3422oo⁄3822oo
Fig. 3. Design of a bolt-mesh-anchor coupling support.
4.3. Discussion of test results
After using the scheme of bolt-mesh-anchor coupling support, the field monitoring shows that the contraction of
th
e two sidewalls, the roof subsidence, and the floor
heave were efficiently controlled. Within three months

817
Li Xuefeng et al. / Procedia Engineering 84 ( 2014 ) 812 – 817
monitoring period, the minimum roof subsidence speed was no more than 0.2 mm/d and averaging 1.12 mm/d. The
maximum relative convergence of the side walls was 80 mm. The maximum loading of every anchor rod run up to
40-49 KN in two sidewalls, the minimum loading is 20-24 KN.
5. Conclusions
The deformation characteristics of surrou
nding rock of broken and soft roadway are complicated and related to
lithology, physical
- mechanical properties of rock and f
ield stress etc. A field investigation and experiments
analysis were
used to determine the cause of damage to the roadway. A bolt-mesh-anchor coupling support was
design
ed, which used bolt
-mesh, spray, anchor cables and corner coupled truss supports, and implemented for the
rep
air and reinforcement of the return air roadway in
level 100.
The field monitoring shows that the designed coupling support had efficiently controlled the deformation of the
su
rrounding rock in this soft seam. This design can be used for similar roadways
support.
Acknowledgments
This study is supported by the Natural Science Foun
dation of Guangxi (No 2011GXNSFA018020
).
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