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A Study on Truss Bolt Mechanism in Controlling Stability of Underground Excavation and Cutter Roof Failure

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The truss bolt reinforcement system has been used in controlling the stability of underground excavations in severe ground conditions and cutter roof failure in layered rocks especially in coal mines. In spite of good application reports, working mechanism of this system is largely unknown and truss bolts are predominantly designed based on past experience and engineering judgement. In this study, the reinforcing effect of the truss bolt system on an underground excavation in layered rock is studied using non-linear finite element analysis. Different indicators are defined to evaluate the reinforcing effects of the truss bolt system. Using these indicators one can evaluate the effects of a reinforcing system on the deformation, loosened area, failure prevention, horizontal movement of the immediate layer, shear crack propagation and cutter roof failure of underground excavations. Effects of truss bolt on these indicators reveal the working mechanism of the truss bolt system. To illustrate the application of these indicators, a comparative study is conducted between three different truss bolt designs. It is shown that the design parameters of truss bolt systems, including tie-rod span, length, and angle of the bolts can have significant effects on the reinforcing capability of the system.
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ORIGINAL PAPER
A Study on Truss Bolt Mechanism in Controlling Stability
of Underground Excavation and Cutter Roof Failure
Behrooz Ghabraie Gang Ren
Kazem Ghabraie Yi Min Xie
Received: 14 September 2012 / Accepted: 28 January 2013 / Published online: 9 February 2013
ÓSpringer Science+Business Media Dordrecht 2013
Abstract The truss bolt reinforcement system has
been used in controlling the stability of underground
excavations in severe ground conditions and cutter
roof failure in layered rocks especially in coal mines.
In spite of good application reports, working mecha-
nism of this system is largely unknown and truss bolts
are predominantly designed based on past experience
and engineering judgement. In this study, the rein-
forcing effect of the truss bolt system on an under-
ground excavation in layered rock is studied using
non-linear finite element analysis. Different indicators
are defined to evaluate the reinforcing effects of the
truss bolt system. Using these indicators one can
evaluate the effects of a reinforcing system on the
deformation, loosened area, failure prevention, hori-
zontal movement of the immediate layer, shear crack
propagation and cutter roof failure of underground
excavations. Effects of truss bolt on these indicators
reveal the working mechanism of the truss bolt system.
To illustrate the application of these indicators, a
comparative study is conducted between three
different truss bolt designs. It is shown that the design
parameters of truss bolt systems, including tie-rod
span, length, and angle of the bolts can have significant
effects on the reinforcing capability of the system.
Keywords Truss bolt Reinforcement FEM
Stability indicators Underground excavation
Ground control
1 Introduction
Nowadays, rock bolt systems are being extensively
used in mining and civil engineering applications.
These systems are a dominant part of the New
Austrian Tunnelling Method (NATM) and can be
used as both temporary and permanent support (Brady
and Brown 2005; Karanam and Dasyapu 2005; Osgoui
and Oreste 2007; Maghous et al. 2012). The common
use of rock bolts is because of their flexibility, ease of
use and fast installation (Hoek and Brown 1980; Brady
and Brown 2005). However, in severe ground condi-
tions, roadway tunnels and highly stressed areas in
long-wall and room and pillar mining, especially in
response to cutter roof failure, conventional rock bolt
patterns could be inadequate and risky to use. In these
circumstances, Peng and Tang (1984) suggest using a
special configuration of rock bolts called Truss Bolt
systems.
Truss bolt, in its simplest form, consists of two
inclined members at two top corners and one
B. Ghabraie (&)G. Ren Y. M. Xie
School of Civil, Environmental and Chemical
Engineering, RMIT University, GPO Box 2476V,
Melbourne, VIC 3001, Australia
e-mail: behrooz.ghabraie@rmit.edu.au
K. Ghabraie
Faculty of Engineering and Surveying, University of
Southern Queensland, West Street, Toowoomba,
QLD 4350, Australia
123
Geotech Geol Eng (2013) 31:667–682
DOI 10.1007/s10706-013-9617-7
horizontal member on the roof. A common truss bolt
system, known as the Birmingham truss, consists of
two long cable bolts which are connected at the middle
of the roof. Horizontal tension is applied by means of a
turnbuckle at the connection point of the cables at the
roof and transferring a compression to the rock
(Gambrell and Crane 1986). According to several
researchers, end-anchored inclined members are pref-
erable (O’Grady and Fuller 1992; Cox 2003), how-
ever, there are not many resources available on
advantages and disadvantages of different types of
rock bolts. A schematic view of the Birmingham truss
is shown in Fig. 1.
One of the advantages of truss bolt systems is the
ability to control the cutter roof failure. Cutter roof is a
common type of failure in laminated rock formations
in flat roof excavations. In this type of failure, shear
cracks propagate from the corners of the roof and as
they reach the first bedding plane, a huge block
separates from the roof (Su and Peng 1987). Very good
responses of truss bolt have been reported in places
that systematic rock bolt failed to prevent cutter roof
(Stankus et al. 1996).
The successful applications of truss bolt have led
researchers to develop different truss bolt systems
which resulted in several patents (White 1969; Wahab
Khair 1984; Seegmiller and Reeves 1990). Alongside
with these developments, several researchers initiated
studies to understand the mechanism of the truss bolt
system and presented a number of practical design
schemes. A number of these works has been done by
means of photoelastic study during 1970s and 1980s
(Gambrell and Haynes 1970; Neall et al. 1977,1978;
Gambrell and Crane 1986). In design schemes for truss
bolt systems, Sheorey et al. (1973) statistically studied
the effects of position and thickness of blocking points
to find the optimum value of these parameters. Based
on several field investigations, Cox and Cox (1978)
proposed their design method by considering suspen-
sion and reinforcing effect of truss bolt system. Neall
et al. (1978) proposed a theoretical design approach
on the basis of beam building theory of reinforcement
systems and tabular overburden load. Wahab Khair
(1984) carried out lab experiments to understand the
effects of truss bolt on a simulated roof beam. Zhu and
Young (1999) proposed analytical based equations to
calculate the required minimum horizontal tension and
length of tie-rod for single and multiple truss bolt
systems. Most recently, Liu et al. (2005) published an
analytical based design procedure on the basis of a
number of simplifying assumptions. Further to these
studies, some field investigation and a small number of
numerical analyses are available in this field (Seegm-
iller and Reeves 1990; O’Grady and Fuller 1992;
Fig. 1 Schematic view of
truss bolt, tunnel and model
dimensions
668 Geotech Geol Eng (2013) 31:667–682
123
Stankus et al. 1996; Li et al. 1999; Liu et al. 2001;
Cox 2003; Ghabraie et al. 2012).
Despite these efforts in understanding the truss bolt
mechanism, the complicated effects of truss bolts on
load distribution around an underground excavation is
still largely unknown (Liu et al. 2005; Ghabraie et al.
2012). This lack of knowledge forces engineers to
consider large safety factors while using these
schemes.
Understanding the mechanism of truss bolt system
on reinforcing the rock around an underground
excavation is the most important and the first step in
obtaining a practical, reliable and easy to use design
scheme. This paper is focused on understanding the
mechanism of truss bolt systems on stability of
underground excavations and preventing cutter roof
failure. For this purpose, numerical modelling tech-
niques are used in order to capture the complicated
behaviour of truss bolt systems. Once a comprehen-
sive numerical model is established, one can repeat
numerous tests for varying input parameters at rela-
tively little extra cost.
In this paper, the finite element method (FEM) has
been used for numerical modelling, using ABAQUS
as the software package (ABAQUS 2010). An under-
ground excavation, containing bedding planes, several
rock layers and an installed truss bolt system has been
modelled. For the purpose of evaluating the effects of
truss bolt on stability of an underground excavation, a
number of stability indicators have been introduced.
Using these indicators, the effects of truss bolt system
on reinforcing an underground excavation and pre-
venting the cutter roof failure have been studied. Three
regular truss bolt pattern have been modelled to study
the effects of different parameters of the system. These
patterns have been chosen from several case studies in
the literature and adjusted to the dimensions of the
model in this study. Using the stability indicators and
studying the effects of each truss bolt pattern on the
stability of an underground excavation, mechanism
and effects of different design parameters have been
derived. Results showed that depending on the pattern
of truss bolt system, areas of reinforcing effect around
an excavation change dramatically. A long span truss
bolt with short inclined bolts results in reinforcing the
top side areas of the tunnel while a short span truss bolt
with long inclined bolts produce an arch shape
reinforced area above the roof. In conclusion, truss
bolt creates a trapezoid reinforced area above the roof
and between inclined bolts in which an arch shape area
is the major area of reinforcement.
2 Preliminary Understanding of Truss Bolt
Behaviour
Previous studies have pointed out that the effect of
reinforcement on the rock material is to apply the
confining pressure, suspend unstable blocks and
increase the strength properties of rock (Lang 1961;
Lang and Bischoff 1984; Huang et al. 2002;Li2006).
Among these, applying the confining pressure is the
most important effect which is the basis of the
systematic rock bolt patterns (Li 2006). The applied
compressive force tightens the rock fragments
together alongside with increasing the strength char-
acteristics of rock by increasing the mean stress and
decreasing the deviatoric stress. Any prestressed rock
bolt compresses and reinforces the rock in its vicinity.
In a systematic rock bolt pattern, the bolts are placed
close enough such that their reinforced area overlaps
and a compressed area is produced. This area acts like
a beam and carries the load to the sides of the
excavation (Lang and Bischoff 1982; Roy and Rajag-
opalan 1997;Li2006).
In truss bolt systems, the applied tension in the
middle of the tie-rod creates areas of compression
around the tunnel. The preliminary understanding of
the load distribution around truss bolt is shown in
Fig. 2. Results of the early photoelastic analysis and
physical modelling also confirmed the presence of a
compressive force which demolished the shear stress
at the middle of the roof (Gambrell and Haynes 1970;
Gambrell and Crane 1986). Also, the two inclined
members of the truss system are able to create a
compressive area above the abutments. Reinforcing
this area could be very effective in controlling the
horizontal movement of rock layers in the areas prone
to the cutter roof failure (Stankus et al. 1996).
3 Numerical Model
A typical underground excavation in a coal seam with
thickness of 2 m has been modelled. The tunnel is
assumed to be long enough to satisfy plain strain
assumptions. The model contains four bedding planes,
two above and two beneath the tunnel.
Geotech Geol Eng (2013) 31:667–682 669
123
Slipping or sticking behaviour of bedding planes
are governed by the Coulomb friction model
s¼lpð1Þ
In this equation, sis shear stress, lis the coefficient of
friction on the plane of weakness ðl¼tan /Þand pis
the contact pressure. In this model, no penetration is
allowed and pressure can be mobilized if two surfaces
are in contact. The responses of the model and the
bedding surfaces have been verified with the analytical
solutions proposed by Brady and Brown (2005).
An elastic-perfectly plastic material model has been
used to model the intact rock material and the Mohr-
Coulomb yield function has been adopted as the
failure criterion. The model is capable of capturing
separation and slipping along the bedding planes. This
material behaviour has been verified by the analytical
solution proposed by Hoek et al. (1998).
The pretensioned rock bolts (inclined bolts and hori-
zontal tie-rod) have been modelled by using pretensioned
one dimensional truss elements. Inclined bolts have been
anchored by tightening the end node of the rock bolt
element to the rock (no separation is allowed). By
increasing deformation in rock around the tunnel, because
of the relative displacement of two ends of the bolt
elements, the amount of stress in truss elements increases.
This extra load on the reinforcement system may exceed
the ultimate strength of bolts (Hoek et al. 1998). To
prevent this, the maximum allowable pretension is chosen
at 60 % of the ultimate tensile strength of the bolts.
Strength parameters of bolts are shown in Table 1.
Truss bolt patterns Three different typical truss bolt
patterns have been considered. These patterns are
chosen based on the proposed designs by several
researchers (Cox and Cox 1978; Liu et al. 2005;
Ghabraie et al. 2012). Design parameters in these
models have been adjusted to the dimensions of the
tunnel in this study. These parameters are shown in
Fig. 1and Table 2.
4 Stability Indicators
The behaviour of the rock after installing reinforce-
ment needs to be measured via defining some perfor-
mance indicators. For the scope of this study, these
Fig. 2 Compressive areas
around truss bolt
Table 1 Bolt strength properties
Bolt properties
Cross-sectional area 313 mm
2
Module of elasticity 200 Gpa
Ultimate tensile strength 1,670 Mpa
Mass per meter-cable 2.482 kg/m
Table 2 Three different truss bolt patterns (see Fig. 1)
Truss bolt patterns L(m) S(m) a(°)
Pattern 1 (Liu et al. 2005) 2 2.8 60
Pattern 2 (Cox and Cox 1978)2 2 45
Pattern 3 (Ghabraie et al. 2012) 3 1.6 60
670 Geotech Geol Eng (2013) 31:667–682
123
indicators should be able to evaluate the reinforcing
effect of the truss bolt system, roof deflection and
effects of truss bolt on preventing cutter roof failure.
4.1 Reinforced Arch
After excavating a tunnel, redistribution of the in-situ
stress forms a pressurized arch above the tunnel. This
arch is stable and can carry the load to the sides of the
tunnel. The rock material beneath this arch is consid-
ered as loosened material (Fig. 3). This phenomenon
can be observed in almost all types of coherent rock
formations (Li 2006) and is proved by experience as
well as numerical analysis (Bergman and Bjurstrom
1984; Huang et al. 2002). Position of this arch changes
drastically by changing the in-situ stress distribution.
High horizontal in-situ stress is favourable in forming
a closer natural arch to the roof, i.e. smaller loosened
area. It should be noted, however, that extensive
horizontal in-situ stress has negative effects on cutter
roof failure and also causes stability problems in
pillars.
Usually, the natural arch is positioned far above the
tunnel and the loosened area beneath it should be
stabilized (Li 2006). This can be achieved by either
removing or reinforcing the loosened rock. In coal
mines, however, where the shape of the tunnel is
normally governed by the shape of the coal layer,
removing the loosened rock is not an option and a
suitable reinforcement system should be designed
(Fig. 3).
Choosing parameters of the reinforcement systems
to carry the load of the loosened area, without
considering reinforcing effects of the system, nor-
mally results in overdesign parameters. The load of the
loosened area can be used as only to achieve an upper
limit (ultimate capacity) for the parameters of the
reinforcement system (Cox and Cox 1978). To have a
safe and economic design, the reinforcing effect of
truss bolt on the loosened rock area should be taken
into account. By applying a new load distribution
around the tunnel, truss bolt system reinforces the
loosened area and repositions the natural roof arch
which results in smaller loosened area (Ghabraie et al.
2012).
For specifying the position of the reinforced arch,
Huang et al. (2002) used the concept of invert stress
cone to find the natural arch position around an
underground excavation. In their model the thickness
of the arch has been governed by the direction of
principal stresses. According to Huang et al. (2002),
reinforced arch is the area in which principal stresses
are not in vertical or horizontal direction except on the
apex of the arch. Another approach to specify the
position of reinforced arch is to use the vertical
deformation of the rock above the roof. In this
approach, the reinforced arch is defined by the points
with the closest amount of vertical deformation to a
certain fraction of the maximum vertical displacement
of the tunnel roof. This fraction is the amount of
displacement which predicts the stable/unstable rock.
This condition can be expressed as (Ghabraie et al.
2012)
Fig. 3 Natural arch and loosened area
Fig. 4 Reinforced arch after installing truss bolt patterns
Geotech Geol Eng (2013) 31:667–682 671
123
jdiðndmaxÞj ¼ Minimum ð2Þ
where d
i
is the vertical displacement at points above
the roof in FE mesh, d-max is the maximum vertical
displacement on roof and nis a fraction between 0 and
1. In this approach, n9d
max
is a threshold (a certain
amount of displacement) which predicts the area of the
loosened rock. Areas with less deformation than this
threshold are considered to be stable and vice versa.
The fraction (n) can be chosen with respect to the
sensitivity of the tunnel to displacement and can be
different from case to case. In this study, n=50 %
has been chosen which implies that areas with less
than 50 % of the maximum displacement on the roof
are loosened area. The output of this method is a line
which connects all the points resulting from Eq. 2.It
should be noted that this approach does not necessarily
predict the actual area of loosened rock and is only
used to define a basis for comparing different designs.
Using n=50 %, the position of the reinforced arch
and area of the loosened rock for different truss bolt
patterns have been derived. These results are shown in
Fig. 4. It can be seen that truss bolt system repositions
the reinforced arch and reduces the area of loosened
rock around a tunnel under hydrostatic in-situ stress.
These results highlight the importance of the position
and the angle of the inclined bolts. The truss pattern
with short span and wide angled inclined bolts (pattern
3) shows the best result. One reason is that the major
area of the loosened rock is above the middle of the
roof and this pattern has better coverage on this area
compared to the other truss bolt patterns. On the other
hand, pattern 1, which has a bigger span, has a small
effect on the area above the middle of the roof but
shows a good response on the areas near the corners.
This is because in this pattern the inclined bolts are
closer to the corners of the roof.
4.2 Stress Safety Margin (SSM)
The Mohr-Coulomb failure criterion is frequently used
for modelling rock material (Jing 2003). In this
criterion, if the Mohr’s circle corresponding to the
stress condition at a point in rock material touches the
Mohr-Coulomb failure envelope, rock yields and
the elastic solution is no longer valid. By increasing
stress on the surrounding rock around an excavation,
more points will undergo failure and the tunnel would
collapse. The area beneath the failure envelope
represents elastic behaviour of rock with no failure
and can be considered as safe. The failure in Mohr-
Coulomb failure criterion is a function of two key
parameters: a) radius of Mohr’s circle (r
1
-r
3
)/2
and b) position of centre of the circle (r
1
?r
3
)/2.
Failure is happened by increasing radius of the circle
or/and decreasing the amount of r
1
?r
3
. Figure 5
shows two possible Mohr’s circles for these two paths
of failure. It can be seen that the possibility of failure
by decreasing radius of the circle is always more than
failure by decreasing the amount of r1þ
r3ðxc[xr=sin /Þ:Hence, the shortest distance to
failure is x
r
where x
r
equal to zero represents failure.
Now the stress safety margin can be defined based on
Fig. 5 Two possible paths
of failure in Mohr-Coulomb
failure model
672 Geotech Geol Eng (2013) 31:667–682
123
this parameter. The mathematical expression for x
r
can
be derived as (Ghabraie et al. 2008)
xr¼ccos /ð ÞðÞ þ r1þr3
2

sinð/Þ r1r3
2

ð3Þ
Using a dimensionless expression of this factor makes
it easier to compare the results of several models. This
will be achieved by the following equation
SSM ¼rþxr
rð4Þ
In this equation, SSM equal to one represents failure
and plastic behaviour of rock while SSM greater than
one means elastic behaviour of rock and safe Mohr’s
circle. Figures 6,7and 8show contours of SSM
difference before and after installing the three truss
bolt patterns around a tunnel under hydrostatic stress
distribution (SSM
before
-SSM
after
). By this defini-
tion, negative values represent areas in which truss
bolt has favourable effect. The green line in these
graphs shows the line in which truss bolt does not have
any significant effect on the value of SSM around the
tunnel. This line demonstrates the border of favourable
and unfavourable effects of truss bolt. It can be seen
that truss bolt effectively increases the value of SSM
around the roof and abutments of tunnel.
Comparing the three truss bolt patterns reveals that
short tie-rod, wide angle of inclination and long
inclined bolts (pattern 3) results in better effect on the
area above the roof but less favourable effect on the rib
area. On the other hand, in patterns 1 and 2, the most
effective areas around truss bolt are near inclined
bolts. This makes truss bolt patterns 1 and 2 capable of
reinforcing the area above the walls of the excavation
(rib area). The length of inclined bolts, in current
design schemes, is a function of the required load
carrying capacity of the reinforcement systems.
Inclined bolts should be long enough to ensure
sufficient length of anchorage in the safe area (behind
the rib line) to provide enough capacity to the truss
bolt system (Cox 2003; Liu et al. 2005). Figures 6,7
and 8show that the length of inclined bolts even
changes the load distribution around the truss bolt
where long inclined bolts (Fig. 8), in comparison with
short inclined bolts (Figs. 6and 7), are not able to
Fig. 6 Effect of pattern 1 on SSM Fig. 7 Effect of pattern 2 on SSM
Geotech Geol Eng (2013) 31:667–682 673
123
produce a highly reinforced area around inclined
members. On the other hand, failure in providing
enough length of anchorage results in failure of the
truss bolt system. Consequently, the required length of
anchorage to carry the applied load on truss bolt
system can be always used to find the lower limit for
the length of inclined bolts while this length can be
adjusted with respect to the required amount of
reinforcing effect near corners of the roof.
Figure 9shows a different illustration of effects of
pattern 3 on SSM around the tunnel. Contour lines in
this figure have been chosen to represent three
different areas, namely, major reinforced area (less
than -0.03), minor reinforced area (between -0.03
and 0) and unfavourable area (greater than 0). It can be
seen that the major reinforced area approximately fits
in an arch shape above the roof while the minor
reinforced area is more like a trapezoid area which is
located above the roof and between the inclined bolts.
In other patterns the major reinforced area can be seen
around the inclined members (Figs. 6and 7). How-
ever, load distribution around these patterns also
shows arch shape borders. The applied horizontal
tension at tie-rod can be well transferred to the rock at
blocking points and by lateral behaviour of inclined
bolts. This load produces an arch shape compressive
area above the roof. The reinforced areas in Figs. 6,7,
8and 9match the compressive areas of Fig. 2.
On the other hand, the horizontal tension in the tie-
rod places the area behind inclined bolts in tension.
This unfavourable area is mostly located on sides of
the tunnel and can cause stability problems, especially
when the side rock is relatively weak. In this case,
installing truss bolt can shear the side rock which
causes rock sliding in this area. Individual rock bolts
can be used to stabilise this area.
4.3 Cutter Roof
Cutter roof failure happens when shear cracks around
the corners of the roof propagate towards the imme-
diate roof layer and reach a plane of weakness,
resulting in separation of a massive unstable block (Su
and Peng 1987). This separation applies a huge load on
the reinforcement system that usually exceeds the load
carrying ability of regular systems and the whole block
Fig. 8 Effect of pattern 3 on SSM Fig. 9 Different reinforced areas around pattern 3
674 Geotech Geol Eng (2013) 31:667–682
123
drops into the excavated area. In some cases, re-
opening and stabilizing a site after cutter roof failure
has no efficient solution and the site would be
abandoned (Su and Peng 1987). Various researchers
had done field investigations and modellings to
understand the mechanism of cutter roof failure (Su
and Peng 1987; Altounyan and Taljaard 2001; Gadde
and Peng 2005; Coggan et al. 2012). In these works
the main controlling parameters for cutter roof failure
are mentioned as entry width, in-situ stress condition,
propagation of shear cracks, relative stiffness between
immediate roof layer and coal, geological anomalies,
separation of bedding, horizontal movement of rock
layers and gas pressure. The mechanism of truss bolt
on preventing cutter roof failure can be studied by
monitoring horizontal movement of the immediate
roof layer and shear crack propagation in models
under high horizontal or vertical in-situ stresses.
4.3.1 Slip on the First Bedding Plane
In numerical modelling, slip on the first bedding plane
can be determined by monitoring the relative dis-
placement of bedding surfaces. This parameter can be
interpreted as the relative horizontal movement of the
immediate rock layer.
Figures 10 and 11 show the relative horizontal
displacement between surfaces of the first bedding
plane before and after installing truss bolt on two
different in-situ stress distributions (high vertical
r
v
=2r
h
and high horizontal r
v
=1/2r
h
stresses).
These figures show that the truss bolt reduces the
amount of horizontal movement in the immediate rock
layer in both models.
A closer look at Fig. 10 reveals that, in high vertical
in-situ stress the major area of slip before installing
truss bolt is approximately located above the roof.
This slippage approaches zero near the rib area (radial
distance of 2 m). After installing different truss bolt
patterns, pattern 3 shows the best response which is
due to the location of the inclined bolts that pass
through the major area of the slip. By increasing the
length of the tie-rod, the effectiveness of truss bolt
reduces dramatically and pattern 1 shows small effect
on this factor.
In contrast, when the horizontal in-situ stress is
high, the slippage on the first bedding plane reaches a
peak above the roof and extends to almost 1.5 times of
the span of the opening (radial distance of 4 m) and
smoothly approaches zero after this distance (Fig. 11).
To prevent the cutter roof failure, horizontal displace-
ment, especially above and behind the rib area, need to
be controlled. Figure 11 shows that for the area above
the tunnel short span truss bolt has the best effect
(similar to results of high vertical in-situ stress,
Fig. 10). However, for the area around corners of the
roof (radial distance of 2 m) pattern 2 shows the best
results. In this area pattern 1 and 2 are more successful
than pattern 3 due to having inclined bolts passing
through this area. Also, angle of inclined bolts in
pattern 2 is another reason for effective application of
this pattern where 45°inclined bolts produce a larger
Fig. 10 Amount of slip on the first bedding plane (r
v
=2r
h
)Fig. 11 Amount of slip on the first bedding plane (r
v
=1/2r
h
)
Geotech Geol Eng (2013) 31:667–682 675
123
horizontal component than 60°for the same amount of
pretension. This component is in the opposite direction
to the horizontal in-situ stress and reduces the effect of
this stress.
4.3.2 Shear Crack Propagation
One of the main limitations of FEM is in modelling
fracture growth (Jing 2003). Capturing crack propa-
gation is only possible by employing relatively new
methods such as enriched FEM and generalized FEM
(Duarte et al. 2000; Deb and Das 2011). Using these
techniques in a comprehensive model of underground
excavation with complex geometry involves signifi-
cant computational costs. This problem becomes more
complicated when the model contains pretensioned
elements (rock bolts) and geological features such as
bedding planes.
Based on the Mohr-Coulomb failure criterion, shear
failure can happen under compressive stresses when
the maximum shear stress reaches the critical value
defined by the Mohr-Coulomb yield function. After
shear failure the rock behaviour could be assumed to
be plastic. This failure could thus be captured using an
elastic-plastic material model in FEA. Hence the
yielded areas resulted from elastic-plastic FEA, pro-
vided that the stresses are compressive, could be
assumed to represent the shear crack propagation.
However, if the failure occurs in tension, due to the
separation in material, the post failure behaviour could
not be captured appropriately using an elastic-plastic
FEA.
To monitor the effects of truss bolt on cutter roof,
progressive failure (shear crack propagation) around
the tunnel is modelled using a simplified interactive
approach. For this purpose, the model is solved with
elastic-plastic material model once and then the most
likely area to yield is found with respect to the Mohr-
Coulomb yield function and SSM factor (Eq. 4).
As discussed in Sect. 4.2 changes in radius of
Mohr’s circle is always smaller than the required
change in the amount of pressure to satisfy the failure
criterion (x
r
\x
c
). From Eq. 4, SSM equal to one
(x
r
=0) denotes failure (Fig. 5). Increasing load in
rock material results in changing the radius of Mohr’s
circle and causes an increase in the number of failure
points in rock. Modelling this progressive failure in
rock is possible by gradually increasing values of x
r
and finding the yielded points for the new stress
condition corresponding to the new x
r
. This approach
is essentially a linear extrapolation which helps us
estimate shear crack propagation.
The increase in the amount of x
r
can be defined
through several increments (I
n
) where
SSM 1¼Inð5Þ
In this equation SSM =1 represents yielding. By
replacing the definition of SSM in Eq. 5, different
increments can be derived as
In¼xr
rð6Þ
This equation identifies the locations where rock will
undergo shear failure at increment I
n
.I
n
equal to zero
interprets x
r
=0 which shows the area of the failure
under current loading condition. Increasing the
amount of I
n
shows propagation of yielded as loads
increase. It should be noted that the resulting yielded
areas for different increments do not necessarily mean
that these areas are yielded but shows the pattern of
potentially yielded area (shear cracked area) in
different time spans after excavation.
With respect to the definition of cutter roof by (Su
and Peng 1987), when shear cracks reach the plane of
weakness, cutter roof happens. Four different incre-
ments have been chosen to represent the shear cracks
just after excavation (I
n
=0) to cutter roof failure
(when shear cracks reach the plane of weakness). Two
different in-situ stress distributions have been mod-
elled. Results showed that when the horizontal in-situ
stress is high (r
v
=1/2r
h
) shear cracks tend to
propagate with a sharp angle to the roof of the
opening. Various markers in Fig. 12 show yielded
points for different increments. Different increments
are shown by different colours. The hypothetical lines
in this figure show the areas of yielded rock for
different increments. As it can be seen, at the final
increment (I
n
=0.015) shear cracks reach the plane of
weakness and the cutter roof happens. Similarly, using
the same method for a tunnel under high vertical in-
situ stress (r
v
=2r
h
), the pattern of shear crack
propagation can be obtained as shown in Fig. 13.
Comparing these two figures illustrates that the angle
of shear crack propagation and shape of the unstable
block is deeply related to the condition of the in-situ
stress. In high vertical in-situ stress, shear cracks
propagate at an approximately right angle to the roof
while in high horizontal in-situ stress this angle is less
676 Geotech Geol Eng (2013) 31:667–682
123
than 90°. Su and Peng (1987) on the basis of numerical
analysis, using FEA and safety factor, together with
field observations reported the same pattern of cutter
roof in high vertical and horizontal in-situ stress
conditions.
Figures 14,15,16,17,18 and 19 show results of
installing three different truss bolt patterns on two
identical tunnels under high horizontal and vertical in-
situ stresses. Comparing these results with Fig. 12 and
13 (pattern of shear cracks before installing truss bolt),
it can be concluded that truss bolt system reduces the
possibility of cutter roof by controlling shear crack
propagation. It appears that truss bolt system by
having inclined bolts near the area of initial shear
cracks (around the corners of the roof) prevents
continuous cracking and reduces the possibility of
cutter roof. It has been shown in Sect. 4.2 that, because
of the pretension force and induced compressive stress
around the inclined bolts, a reinforced area will be
created near the corners of the roof. In high vertical in-
situ stress, where inclined bolts are well located at the
area of shear crack propagation, the applied compres-
sive stress by inclined bolts prevents continues shear
crack propagation. In addition to this, investigating the
results of SSM factor around truss bolt system shows
another major reinforced area which is similar to an
arch shape between inclined bolts above the roof
(Fig. 9). Comparing patterns of shear cracks before
(Fig. 12) and after installing truss bolt (Figs. 14,15
and 16) in high horizontal in-situ stress shows that
truss bolt prevents propagation of cracks at areas near
blocking points and above the roof. In fact, this area is
identical to the produced reinforced arch area by truss
bolt.
Results of installing different truss bolt patterns on
preventing cutter roof illustrate that, depending on
design parameters of truss bolt and in-situ stress
distribution, effectiveness of the system on preventing
shear crack propagation varies. It can be seen that in high
vertical in-situ stress, pattern 2 shows the best applica-
tion. Inclined bolts in this pattern exactly pass through
the initial area of cracking and, by reinforcing this area,
Fig. 12 Pattern of shear crack propagation (r
v
=1/2r
h
)Fig. 13 Pattern of shear crack propagation (r
v
=2r
h
)
Geotech Geol Eng (2013) 31:667–682 677
123
this patternprevents further crack propagation (Fig. 18).
Figure 19 shows that pattern 3 is also able to reduce the
possibility of cutter roof in this in-situ stress condition.
On the otherhand, inclined bolts in pattern 1 are located
behind the area of initial cracking and even push the
crack propagation pattern slightly towards the middleof
the roof instead of controlling it (Fig. 17).
Comparing results of installing different truss bolts
on a tunnel under high horizontal in-situ stress shows
that patterns 2 and 3 prevent shear crack propagation
to reach the plane of weakness. Whilst pattern 1 does
not have any significant effect on preventing cutter
roof and shear cracks reach the plane of weakness
around the middle of the roof. This is probably
because of the position of inclined bolts in pattern 1
which, similar to Fig. 17 in high vertical in-situ stress,
is located behind the area of initial crack propagation.
As discussed in Sect. 4.2, pattern 3 by having long
inclined bolts and short tie-rod length produces a
stronger reinforced arch compared to other patterns.
This enables it to effectively control the shear crack
propagation above the roof and shows the best
response.
5 Discussion
The importance of a comprehensive consideration of
all the design parameters and site variables can be
concluded here. It has been shown that the shorter
length of inclined bolts produce better reinforced area
around the inclined bolts compared to longer bolts. If a
truss bolt system with short inclined bolts is located in
the right place to prevent crack propagation in high
vertical in-situ stress (by choosing suitable tie-rod
length), it can effectively prevent the cutter roof
failure. On the other hand, longer inclined bolts have
the advantage of adequate length of anchorage in
passive zone behind the rib line. The length of
anchorage is a key parameter to determine the capacity
of the system. If the applied load on truss bolt system
exceeds the capacity of truss bolt, the whole block
with truss bolt will fail.
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
3.9
Excavation
Horizontal distance from centre of the tunnel (m)
Vertical distance from centre of the tunnel (m)
Bedding
Bedding
Truss Bolt
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
Increment 0.05 O
X
X
X
X
XX
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
XX
X
X
X
Increment 0.045 X
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Increment 0.025 +
H
H
Increment 0 H
Fig. 14 Pattern of shear crack around pattern 1 (r
v
=1/2r
h
)
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
3.9
Excavation
Horizontal distance from centre of the tunnel (m)
Vertical distance from centre of the tunnel (m)
Bedding
Bedding
Truss Bolt
O
O
OOO
O
O
O
O
O
O
O
O
O
O
OO
O
O
O
Increment 0.05 O
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Increment 0.045 X
++
++
++
+
Increment 0.025 +
HH
H
Increment 0 H
Fig. 15 Pattern of shear crack around pattern 2 (r
v
=1/2r
h
)
678 Geotech Geol Eng (2013) 31:667–682
123
The length, position and angle of inclined bolts are
also important in controlling horizontal movement and
the area of the loosened rock. If inclined bolts pass
through the major area of slip (depending on the in-situ
stress distribution), the capacity of the truss bolt for
preventing horizontal movement increases signifi-
cantly. The area of slip varies with the in-situ stress
conditions. Results showed that medium length tie-rod
locates the inclined bolts at the best possible location
to prevent slip on the first bedding plane in high
horizontal in-situ stress. Further to the importance of
length of tie-rod in truss bolt, choosing an angle closer
to horizon would result in producing higher resisting
force against high horizontal in-situ stresses. It should
be mentioned that bolt angles less than 45°will result
in significant reduction in the capability of truss bolt to
control the area above the roof. Reinforcing this area
above the roof is vital to prevent cutter roof failure
when horizontal in-situ stress is high. In contrast, the
area of slip in high vertical in-situ stress is mainly
above the roof where short length tie-rod shows the
best response. Same as the latter case, capability of this
truss bolt pattern in controlling crack propagation
should be taken into account. Truss bolt with medium
length of tie-rod and 45°inclined bolts shows the best
response in controlling shear crack propagation in
high vertical in-situ stress.
Studying the effects of installing truss bolt on the
position of natural roof arch also shows that changing
the design parameters of truss bolt would result in
reinforcing different areas above the roof and corners
of the tunnel. These results match perfectly with
results of SSM factor where short span truss bolt with
wide angle inclined bolts are able to reinforce the area
above the roof. By increasing the length of tie-rod and
decreasing the length of inclined bolts, the main area
of reinforcing effect of truss bolt shifts from an area
above the middle of the roof to the area around
inclined bolts.
It has been shown that, impact of truss bolt system
changes with respect to the condition of the in-situ
stress distribution. There are many other geological
features that might have significant influence on the
practice of truss bolt systems, such as thickness of the
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
3.9
Excavation
Horizontal distance from centre of the tunnel (m)
Vertical distance from centre of the tunnel (m)
Bedding
Bedding
Truss Bolt
O
O
O
O
O
O
OO
Increment 0.05 O
X
X
XX
X
X
XX
X
X
X
X
X
X
X
X
X
X
X
X
XX
X
X
Increment 0.045 X
+
+
+
+
+
+
++
++
+
Increment 0.025 +
H
H
Increment 0 H
Fig. 16 Pattern of shear crack around pattern 3 (r
v
=1/2r
h
)
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
3.9
Excavation
Horizontal distance from centre of the tunnel (m)
Vertical distance from centre of the tunnel (m)
Bedding
Bedding
Truss Bolt
O
O
O
O
OO
O
O
OO
O
Increment 0.025 O
X
X
X
X
X
X
X
XX
X
X
X
X
X
X
X
X
X
Increment 0.02 X
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+
+
Increment 0.01 +
H
H
HH
H
H
H
H
H
H
H
H
H
H
H
H
Increment 0 H
Fig. 17 Pattern of shear crack around pattern 1 (r
v
=2r
h
)
Geotech Geol Eng (2013) 31:667–682 679
123
rock layers, strength parameters of rock, condition of
discontinuities, time factor, etc. (Neall et al. 1978).
Consequently, it can be concluded that obtaining an
optimum design for truss bolt systems entails consid-
eration of effects of each individual design parameter
alongside with comprehensive study of all of the
external geological and ground controlling parameters.
6 Conclusion
Truss bolt systems have proved effective in controlling
the stability of underground excavations in severe
ground conditions particularly in coal mines and layered
strata. Despite this, knowing the mechanism of truss bolt
systems on reinforcing underground excavations is
vital. The objective of this study was to understand the
mechanism of truss bolt by means of numerical
modelling. To evaluate and monitor the effects of truss
bolt on load distribution around the tunnel and under-
stand the mechanism of reinforcement, several stability
indicators have been introduced. These indicators cover
several features of a reinforcement system and are,
namely, area of the loosened rock above the roof, stress
safety margin, slip on the first bedding plane and shear
crack propagation. None of these indicators alone is able
to determine the stability of an underground excavation,
but together, they help to understand the effects and
mechanism of truss bolt system.
Results of employing these stability indicators
reveal that truss bolt systems stabilize underground
excavations in several ways such as repositioning the
natural reinforced arch and reducing the area of
loosened rock above the roof, creating a trapezoid
reinforced area in which an arch shape structure is the
major reinforced area, reducing horizontal movement
of rock layers, preventing shear crack propagation,
and decreasing the chance of cutter roof failure.
Results of studying several truss bolt patterns also
showed that changing the design parameters of the
truss bolt will change the effectiveness of the system in
facing different stability problems. Parameters such as
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
3.9
Excavation
Horizontal distance from centre of the tunnel (m)
Vertical distance from centre of the tunnel (m)
Bedding
Bedding
Truss Bolt
O
O
O
O
O
Increment 0.025 O
X
X
X
X
X
X
X
X
X
X
X
XX
X
Increment 0.02 X
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Increment 0.01 +
HHH
H
H
H
HH
H
H
H
Increment 0 H
Fig. 18 Pattern of shear crack around pattern 2 (r
v
=2r
h
)
00.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3
3.3
3.6
3.9
Excavation
Horizontal distance from centre of the tunnel (m)
Vertical distance from centre of the tunnel (m)
Bedding
Bedding
Truss Bolt
O
O
O
O
O
O
O
Increment 0.025 O
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Increment 0.02 X
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Increment 0.01 +
H
HHH
H
H
H
H
H
H
H
HH
H
H
Increment 0 H
Fig. 19 Pattern of shear crack around pattern 3 (r
v
=2r
h
)
680 Geotech Geol Eng (2013) 31:667–682
123
angle and length of the inclined bolts and the span of
the system or length of the tie-rod have been changed
and results have been studied. It has shown that to
reinforce the loosened area beneath the natural arch a
short span truss bolt with wide angle inclined bolts is
more appropriate while in high horizontal in-situ
stress, to prevent horizontal movement of the imme-
diate layer, a wider span and sharper angle of
inclination response better. In case of cutter roof
failure, to prevent shear crack propagation in high
vertical in-situ stress, a pattern with medium length of
tie-rod and inclined bolts and 45°inclined bolts results
in the best application whilst other patterns do not
show considerable improvement.
Results have showed that obtaining an optimum,
safe and efficient design of a truss bolt system is only
possible by considering all the design parameters, site
variables and the interacting effects of each parameter
on the other. This study has provided the necessary
understanding of the mechanism of truss bolt which is
an important step towards achieving a comprehensive
guideline to design a truss bolt pattern.
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... However, due to the poor supporting ability of the anchor mesh cable, the surrounding rock cannot be stable with the passage of time, although the surrounding rock displacement has been greatly controlled for the deep high-stress water-rich broken rock roadway. The arch truss structure is a new mechanical structure; the structure of the truss itself transforms the bending moment into the tension and compression axial force of the rod, and the arc form of the arch transforms the bending moment into the axial force, and the truss arch perfectly combines the two structures organically to achieve the best supporting effect (Yan et al., 2012;Ghabraie et al., 2013;Suchowerska et al., 2014;Yang et al., 2014;Wang et al., 2016;Yan et al., 2017;Wang et al., 2018a;Gao et al., 2018;Li et al., 2019;Su et al., 2019;Yan et al., 2020;Qiu et al., 2022). ...
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In the process of underground energy mining, the stability of roadway support is an important guarantee. In order to study the application of the anchor cable-truss support technology in water-rich soft rock roadways, the mechanical analysis of the anchor cable-truss structure is carried out, and the surrounding rock deformation of different supporting methods is numerically simulated under the consideration of the fluid–solid coupled interaction. We observed that the anchor cable (rod)-double arch truss coupling support can control the deformation of the surrounding rock and the expansion of the plastic zone well. The maximum vault subsidence of the roadway is 0.017 m, the horizontal convergence is 0.054 m, and the deformation of floor heave is 0.02 m, which are 3.8, 16.3, and 4% of the deformation under unsupported conditions, respectively; the roadway deformation is effectively controlled. The research results have certain guiding significance for the support design of the water-rich broken soft rock roadway.
... For example, Esterhuizen and Bajpayee [5] stated that the supporting pressures would have to be impractically high to prevent horizontal stress-related damage of the laminated roof rocks by observing the ground response curve in the FLAC model. Ghabraie et al. [20] studied the mechanism of truss bolt systems, showing that these support systems can prevent shear crack propagation by repositioning the natural reinforced arch and reducing the area of loosened rock above the roof. Bai and Tu [14] numerically validated the effect of confinement provided by roof skin support with a metal mesh to restrict progressive spalling in a laminated roof. ...
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The effect of discontinuities on the fracturing and mechanical behavior of shale has been extensively investigated on a laboratory scale in previous works. It is well agreed that the lamination properties, including discontinuity and lamina properties, affect the behavior of shale. However, it is still unclear how the lamination properties are affecting the stability of the shale roof in an underground coal mine entry. This paper investigated the effect of lamination properties using discrete element method on a mine-scale entry model as an extension to the previous work conducted on laboratory scale models (Q. Shi and B. Mishra, Discrete Element Modeling of Delamination in Laboratory Scale Laminated Rock, Mining, Metallurgy & Exploration, vol. 37, no. 5, pp. 1–14, Sep. 2020). The microparameters for both the laminas and discontinuities were calibrated with laboratory data. In the calibration, a numerical laminated Brazilian disc was created and tested for comparison with laboratory results. The effects of lamina thickness, discontinuity strength, and supporting pressure on the model’s roof strength and the stress distribution were also investigated. Numerical results showed that the lamination properties and supporting pressure contribute significantly to the stress distribution in the roof and its stability. The horizontal stress at a fixed depth in the roof increased with the lamina thickness, discontinuity strength, and supporting pressure. The laminated roof strength was found to increase with the increase of lamina thickness but never exceeds the strength of an intact roof comprising the only matrix.
... For example, Esterhuizen and Bajpayee [5] stated that the supporting pressures would have to be impractically high to prevent horizontal stress-related damage of the laminated roof rocks by observing the ground response curve in the FLAC model. Ghabraie et al. [20] studied the mechanism of truss bolt systems, showing that these support systems can prevent shear crack propagation by repositioning the natural reinforced arch and reducing the area of loosened rock above the roof. Bai and Tu [14] numerically validated the effect of confinement provided by roof skin support with a metal mesh to restrict progressive spalling in a laminated roof. ...
Article
The effect of discontinuities on the fracturing and mechanical behavior of shale has been extensively investigated on a laboratory scale in previous works. It is well agreed that the lamination properties, including discontinuity and lamina properties, affect the behavior of shale. However, it is still unclear how the lamination properties are affecting the stability of the shale roof in an underground coal mine entry. This paper investigated the effect of lamination properties using discrete element method on a mine-scale entry model as an extension to the previous work conducted on laboratory scale models (Q. Shi and B. Mishra, Discrete Element Modeling of Delamination in Laboratory Scale Laminated Rock, Mining, Metallurgy & Exploration, vol. 37, no. 5, pp. 1–14, Sep. 2020). The microparameters for both the laminas and discontinuities were calibrated with laboratory data. In the calibration, a numerical laminated Brazilian disc was created and tested for comparison with laboratory results. The effects of lamina thickness, discontinuity strength, and supporting pressure on the model’s roof strength and the stress distribution were also investigated. Numerical results showed that the lamination properties and supporting pressure contribute significantly to the stress distribution in the roof and its stability. The horizontal stress at a fixed depth in the roof increased with the lamina thickness, discontinuity strength, and supporting pressure. The laminated roof strength was found to increase with the increase of lamina thickness but never exceeds the strength of an intact roof comprising the only matrix.
... Gao [19] and Murphy [20] discussed the failure mechanics associated with the weak characteristics of bedding rocks and concluded that the horizontal stress is the main contributing factor in the deterioration of a weak roof and contributor to cutter roof failure. Ghabraie et al. [21] confirmed the reinforcing effect of the truss bolt system in an underground excavation in layered rock using non-linear finite element analysis. Xue [22] numerically studied the size effect of laminated rock with FLAC3D. ...
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Laminated rock can delaminate and fail under certain stress states. Delamination-caused roof fall and cutter roof failure is a common failure mechanism in roadways in Pittsburgh coal seam due to the existence of laminated shale roof. Two-dimensional numerical models were developed at laboratory scale based on the bonded particle method in order to simulate the delamination with particle flow code. In these models, an assembly of bonded particles and parallel weak planes represents the laminated rock. This model calibrated the micro-parameters of the bonded particle material and weak planes with laboratory data. The mechanism of fracturing and delamination in a laminated specimen was investigated using unconfined and confined compression test and unsupported roof compression test. The numerical results show that, under a constant horizontal stress, delamination occurs when the stress is much lower than its compressive strength. The confined compressive test indicates that delamination is restrained by applying confining stress since delamination-caused failure does not occur during any stage of the confined compressive test. Unsupported roof compression tests showed propagation of cutter roof failure with initiation of cracks and delamination at the intersections of the roof and ribs. An outer failed zone and inner failed zone form separately. The separated zones connect and form an opening in the roof. The presented numerical results provide additional insight into process of delamination.
... Numerical investigations by Gao et al. (2015) successfully illustrated the characteristics of roadway deformations because of mining-induced stresses. Ghabraie et al. (2013) reported that a wider span truss with sharper angle of incline bolt contribute to restraining horizontal movement of the immediate layer in high horizontal in situ stress. An analysis by Zhang et al. (2015) on the stress evolution of bolts and cables shed lights on effectively using these devices in roadways. ...
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To investigate the deformation and failure mechanism of the roadway under high in situ stresses at the 958 section in 2# mine in Jin Chuan, China, field and numerical investigations were conducted. Field investigations on the loosen ring of the roadway show that great roadway deformations and failure in surrounding rocks may result from poorly functioned bolts. Based on the old supporting system, a new supporting system characterized by the installation of steel pipe beams and the elongated bolts was proposed and applied in field. Field data indicate that the roof sag, rib convergence and floor heave significantly decrease. The numerical study illustrates that the properly functioned bolts and steel pipe beams are responsible for the decrease of roadway deformation. To further control the roadway deformation, two additional supporting systems were proposed and simulated. The increase of the bolt density contributes to reinforcement of the surrounding rocks and the floor heave in the first support system is under controlled. In the second support system, the installation of bolt on floor successfully restrains floor heaves accordingly.
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Selecting and designing the most suitable support systems are crucial for securing underground openings, limiting their deformation and ensuring their long-term stability. Indeed, the rock excavations imposed by the erection of deep tunnels generate various harmful effects such as stress perturbation, damage, fractures, rockbursts, convergence deformation, and so on. To combat such effects by helping the surrounding rocks of these structures to hold up, rock bolts are typically utilized as pioneer support systems. However, the latter must be efficient and sustainable to properly fulfil their vital roles. A thorough understanding of the existing rock bolt types or models and the relevant factors influencing their failure is highly required for appropriate selection, design and applications. It is observed that, despite numerous studies carried out, there is a lack of comprehensive reviews concerning the advances in such rock support systems. This paper provides an insight into the most pertinent rock bolt types or models and describes the potential factors influencing their failure. Additionally, it discusses the durability of rock bolts, which has a huge impact on the long-term stability of deep rock tunnels. Furthermore, the paper highlights some proposals for future trends.
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Anchoring mechanism and failure characteristics of composite soft rock with weak interface usually exhibit remarkable difference from those in single rock mass. In order to fully understand the reinforcement mechanism of composite soft roof in western mining area of China, a mechanical model of composite soft rock with weak interface and rock bolt which considering the transverse shear sliding between different rock layers was established firstly. The anchoring effect was quantified by a factor defined as anchoring effect coefficient and its evolution equation was further deduced based on the deformation relationship and homogenized distribution assumption of stress acting on composite structure. Meanwhile, the numerical simulation model of composite soft rock with shear joint was prompted by finite element method. Then detailed analysis were carried out for the deformation features, stress distribution and failure behavior of rock mass and rock bolt near the joint under transverse load. The theoretical result indicates that the anchoring effect of rock-bolt through weak joint changes with the working status of rock mass and closely relates with the physical and geometric parameters of rock mass and rock bolt. From the numerical results, the bending deformation of rock bolt accurately characterized by Doseresp model is mainly concentrated between two plastic hinges near the shear joint. The maximum tensile and compression stresses distribute in the plastic hinge. However, the maximum shear stress appears at the positions of joint surface. The failure zones of composite rock are produced firstly at the joint surface due to the reaction of rock bolt. The above results laid a theoretical and computational foundation for further study of anchorage failure in composite soft rock.
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Book
The safe and economical construction of tunnels, mines, and other subterranean works depends on the correct choice of support systems to ensure that the excavations are stable. These support systems should be matched to the characterstics of the rock mass and the excavation techniques adopted. Establishing the support requirements, designing support systems and installing these correctly are essential elements in safe underground construction. This is a comprehensive and practical work which also gives access to user-friendly computer programmes which enable the investigation and design of support techniques. Details on how to obtain this software are also included in the book.
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The condition at midspan produced by the roof truss appears to be superior, since it produced initial compressive stress parallel to the boundary of the opening whereas the roof bolt produced initial tensile stress parallel to the boundary. The primary reason for the installation of the more complicated and expensive roof truss is to obtain this prestressed compression parallel to the mine roof.
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An innovative concept in longwall tailgate support is discussed. This new concept utilizes a high-capacity roof-truss system to replace conventional wooden cribs. The first field test was conducted at the Bailey Mine and was very successful. Similar tests were installed in other mines and seams. An extensive instrumentation plan was designed and implemented in the truss test area. The test data demonstrated that the dynamic roof-truss system effectively controls the various abutment pressures caused by longwall mining. Thus, a stable tailgate entry is maintained.
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Truss usage for ground control at the Mid-Continent Resources Inc mines, Pitkin County, Colorado is the subject of this paper. The No 1 Mine uses an advancing longwall and is located approximately 143m below the No 2 Mine, a retreating longwall. Typical longwall gateroads are 6.1m wide, face lengths are 152m to 183m and mining depths range up to 610m or more. The mines are characterised as deep and gassy with high stresses, major abutment and seam interaction pressures, and a 13° dip. This paper describes these truss bolts and their application for roof support in problem areas. In addition, the use of Continuous Entry Trusses for secondary support in the rock tunnel belt and roadway is discussed. Roktruss usage as a support method in an arch rock opening is also described.
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Techniques used to safely control the roof in UK coal mines for some 10 years have been applied to a South African coal mine. This new approach known in the UK as advanced rockbolting technology, is based on applying four fundamental principles: ▶ Understanding the roof failure mechanisms ▶ Using an effective roof support system ▶ Designing this support using measurement ▶ Monitoring the performance of the system. The paper summarizes the approach in detail and describes how it is being applied by Anglo Coal, giving results obtained to date. Investigations at a number of South African coal mines, including stress measurements at two Anglo Coal mines, have confirmed that the mode of roof failure (lateral shearing due to horizontal stress) is the same as in other coalfields worldwide, and aspects of current world best practice, including the use of advanced technology rockbolting, are therefore relevant in South Africa. The stress measurements indicated a high level of stress field anisotropy and further investigation of stress conditions in South African coal mines is recommended. The most effective bolting system to resist shear failure is one with high bond strength and stiffness. Short encapsulation pull testing of existing South African systems confirmed that they have low bond strength and stiffness. An improved rockbolt system with the required performance, and features allowing rapid installation and installation quality and performance audit, has been developed and is currently under full-scale trial. Design by measurement and routine monitoring procedures including the use of a rotary telltale device are also under trial. It is anticipated that South African coal mines will be able to obtain significant safety and productivity benefits from the application of this technology.
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In the design of excavation supports, the rock arch may offer a better model of the voussior roof than the beam theory. The truss system provides an effective method of controlling roof in especially weak formations where the conventional supports simply do not work well. In this paper, a set of closed-form solutions for roof truss system design has been proposed based on the rock arching theory. The application and field observations made in a coal mine showed that it is simple but practical for the preliminary design of truss systems.
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The ground forces generated by a properly installed and tensioned mine roof truss assembly can provide permanent mine roof support, even in severe ground conditions. This can be accomplished either by direct suspension of the rock loads within the potential failure zone above the mine opening or by indirect reinforcement of the natural rock arch that tends to form within the immediate mine roof. Notable advancements in mine roof truss support systems hardware and design technology have occurred since the introduction of the in situ mine roof truss concept for permanent mine roof support in 1966. The systematic installation of mine roof truss support systems has proven to be an effective method of mine roof control in room-and-pillar mines under a variety of ground conditions. This is true even in areas otherwise considered unminable because of severe roof-control problems. Properly designed mine roof truss systems have also been used for permanent support of mine slopes and haulage tunnels.