sensors
Review
A Review of Rock Bolt Monitoring Using
Smart Sensors
Gangbing Song 1,† , Weijie Li 1,† , Bo Wang 2, * and Siu Chun Michael Ho 1
1
2
*
†
Department of Mechanical Engineering, University of Houston, Houston, TX 77004, USA;
[email protected] (G.S.); [email protected] (W.L.); [email protected] (S.H.M.H.)
Key Laboratory of Transportation Tunnel Engineering, Ministry of Education,
Southwest Jiaotong University, Chengdu 610031, China
Correspondence: [email protected]; Tel.: +86-139-0817-7290
These authors contributed equally to this work.
Academic Editor: Vittorio M. N. Passaro
Received: 25 January 2017; Accepted: 6 March 2017; Published: 5 April 2017
Abstract: Rock bolts have been widely used as rock reinforcing members in underground coal mine
roadways and tunnels. Failures of rock bolts occur as a result of overloading, corrosion, seismic burst
and bad grouting, leading to catastrophic economic and personnel losses. Monitoring the health
condition of the rock bolts plays an important role in ensuring the safe operation of underground
mines. This work presents a brief introduction on the types of rock bolts followed by a comprehensive
review of rock bolt monitoring using smart sensors. Smart sensors that are used to assess rock
bolt integrity are reviewed to provide a firm perception of the application of smart sensors for
enhanced performance and reliability of rock bolts. The most widely used smart sensors for rock
bolt monitoring are the piezoelectric sensors and the fiber optic sensors. The methodologies and
principles of these smart sensors are reviewed from the point of view of rock bolt integrity monitoring.
The applications of smart sensors in monitoring the critical status of rock bolts, such as the axial
force, corrosion occurrence, grout quality and resin delamination, are highlighted. In addition,
several prototypes or commercially available smart rock bolt devices are also introduced.
Keywords: rock bolts; smart sensors; piezoelectric sensors; fiber optic sensors; non-destructive testing
1. Introduction
Millions of rock bolts are being installed worldwide every year and the consumption of rock
bolts continues to increase. In 2011, the global usage of rock bolts was in excess of 500 million bolts
and the US accounted for 95 million of those bolts. Rock bolts are rock reinforcing members used to
reinforce the unstable rock strata by which subsequent deformation can be resisted. As an important
component of the supporting systems, rock bolts have been extensively used to support and stabilize
engineering structures, such as slopes, retaining walls, tunnels, bridge abutments, dam foundations,
and underground excavations [1]. Rock bolts can be divided into three major types based on their
anchoring mechanisms [2]. The first type is the mechanically anchored rock bolt that can be anchored
by a slit and wedge mechanism or an expansion shell. The second type is the friction anchored rock
bolt, such as split-set and swellex, which are anchored by friction between the bolt and the surrounding
rock. The third type is the partially or fully grouted rock bolts that are anchored by cement or resin.
The anchorage strength of the partially and fully grouted rock bolts is dependent on the grout strength,
the shear resistance at the bolt–grout and grout–rock interfaces.
Accidents due to roof falls (i.e., collapse of the roof) are commonly faced problems in the
underground coal mining industry. Statistical data show that roof falls have historically been
responsible for approximately 50% of all underground fatalities in US, with nearly 45,000 coal miners
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killed due to roof falls in the entire history of coal mining [3]. There were 77 fatal accidents and
239 serious accidents caused by roof and side falls in India for the period from 2004 to 2006 [4].
In South Africa, 1087 ground fall accidents were reported in 2006 with 85 people were killed in
these accidents [5]. The roof falls can cause injuries, disabilities or fatalities for workers while also
detrimentally affecting the mining company due to downtime, interruptions in the mining operations,
and equipment breakdown [6]. All of these large roof collapses are closely related to the failure of
the roof bolting system. Recent decades have seen a substantial reduction in the number of roof
fall accidents due to the worldwide adoption of rock bolts. Rock bolts play an important role in
maintaining the integrity of the mining tunnels and reducing roof fall accidents. Attention is therefore
focused on the condition monitoring of rock bolts in service.
There are various problems associated with the integrity of rock bolts. The loading status,
anchorage quality, and the bolt integrity are of most concern. Exposed to the corrosive groundwater,
one of the primary causes of rock bolt failure is rusting or corrosion. Studies showed that in the
United Kingdom, most of the rock bolt failures were found to be initiated by pitting corrosion, while in
Australia, the fractures of rock bolts were attributed to a combination of bending and stress corrosion
cracking [7]. The corrosion of rock bolts can be mitigated by filling the gap between the rock bolt and
the rock with cement or grout. For resin grouted rock bolts, their efficiency largely depends on the
grout quality which is affected by many factors. Poor grout quality due to over-spinning, incomplete
mixing and partially encapsulated bolts is a common issue. For example, it had been reported that
approximately half of the fully grouted bolts were not as effective as they were supposed to be for
the period from 1996 to 1998 [5,8]. The installation quality and the grouting of resin anchored bolts is
therefore considered as a critical issue by the underground mining industry.
Various methods have been developed to determine the integrity of the rock bolt and a
conventional approach to address this issue is the pull-out test. However, it is a time-consuming
and destructive process. Therefore, the need to develop non-destructive test methods that can be
used to determine the integrity of the rock bolt in situ has never been this urgent. Advances in
smart sensing materials have provided various potential sensing methods; piezoelectric materials
and fiber optic sensors are two of the most widely adopted sensing techniques. In this paper,
the methodologies of these smart sensors are critically reviewed from the point of view of rock bolt
monitoring. The developments and applications of the smart sensors in monitoring the critical status
of rock bolts, such as the axial load, corrosion occurrence, grout inadequacy and resin delamination,
are highlighted. Additionally, several prototypes or commercially available smart rock bolt devices are
also introduced.
2. Basics and Types of Rock Bolts
Rock bolts can be divided into four major types according to the coupling mechanism between
the rock bolt and the rock mass, and their functions [9]:
•
•
•
•
Mechanically anchored rock bolts;
Friction-anchored rock bolts;
Fully grouted rock bolts;
Instrumented rock bolts.
Each bolt has its own advantages and purpose. A detailed overview of the bolts is provided in
the following section.
2.1. Mechanically Anchored Rock Bolts
Mechanically anchored rock bolts are the oldest way of reinforcing underground mining tunnels
due to their fast rate of installation. They can be further divided into two types: the slit/wedge
rock bolt, and the expansion shell anchor. An example of the expansion shell rock bolt is shown in
Figure 1. The main components are a bolt shaft, a faceplate, a conical expansion shell and a wedge.
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into the drilled hole on the rock mass [10]. Rotating the end of the bolt will force the shell to expand
against
the rockshell
walland
of the
thereby
increasing
force.
The
force ofisthis
type
The expansion
thehole,
wedge
are screwed
ontothe
theanchor
bolt shaft
and
theanchoring
entire assembly
inserted
of
rock
mainly
friction
and interlocking
forces
between
the expansion
shell
and
into
thebolt
drilled
holecomes
on thefrom
rock the
mass
[10]. Rotating
the end of
the bolt
will force
the shell to
expand
the
wall
of
the
borehole.
The
anchoring
force
of
this
type
of
bolt
is
relatively
low
because
the
effective
against the rock wall of the hole, thereby increasing the anchor force. The anchoring force of this type
anchoring
the from
expansion
shell and
is very
limited.forces
The between
bolt is generally
tensioned
to
of rock boltlength
mainly of
comes
the friction
interlocking
the expansion
shell and
approximately
70% of the
bearing
capacity
of of
thebolt
bolt
after being
installed
The
the wall of the borehole.
Theultimate
anchoring
force of
this type
is relatively
low
becauseinitially.
the effective
pretension
gives of
a reserving
capacity
in case
ofbolt
additional
loadtensioned
induced by
displacement
anchoring length
the expansion
shellof
is the
verybolt
limited.
The
is generally
to approximately
of
theofrock
mass. bearing capacity of the bolt after being installed initially. The pretension gives a
70%
the ultimate
reserving capacity of the bolt in case of additional load induced by displacement of the rock mass.
faceplate drilled for tubes
breather tube
bolt shaft
anchor nut
wedge
tape
expansion shell
grout injection tube
Figure 1. Components of an expansion shell type mechanically anchored rock bolt with provision
Figure
1. Components of an expansion shell type mechanically anchored rock bolt with provision for
for grouting.
grouting.
Mechanically anchored rock bolts perform better in hard rocks but are not very effective in
Mechanically anchored rock bolts perform better in hard rocks but are not very effective in soft
soft rocks due to the large deformation and breakage of the rock in contact with the wedge grips.
rocks due to the large deformation and breakage of the rock in contact with the wedge grips. This
This type of bolt should be used in rock strata that has a uniaxial compressive strength of more
type of bolt should be used in rock strata that has a uniaxial compressive strength of more than
than 50 MPa [11]. The mechanically anchored rock bolts have several advantages in terms of low
50 MPa [11]. The mechanically anchored rock bolts have several advantages in terms of low cost, and
cost, and rapid installation. However, their effectiveness can be reduced in corrosive environments,
rapid installation. However, their effectiveness can be reduced in corrosive environments, where the
where the corrosion of the rock bolt is the primary cause of rock bolt failure. In such case, grouting of
corrosion of the rock bolt is the primary cause of rock bolt failure. In such case, grouting of the rock
the rock bolts is used to protect the rock bolts from the corrosive environments. Another disadvantage
bolts is used to protect the rock bolts from the corrosive environments. Another disadvantage of this
of this type of rock bolt is that they will lose their load bearing capacity completely if any slip occurs
type of rock bolt is that they will lose their load bearing capacity completely if any slip occurs or the
or the bolt breaks. While this is not a problem for other types of rock bolts. Therefore, they are mainly
bolt breaks. While this is not a problem for other types of rock bolts. Therefore, they are mainly used
used for temporary reinforcement or in combination with later grouting or corrosion protection.
for temporary reinforcement or in combination with later grouting or corrosion protection.
2.2. Friction Anchored Bolts
2.2. Friction Anchored Bolts
Friction anchored rock bolts work by the frictional resistance generated from the bolt/rock
Friction anchored rock bolts work by the frictional resistance generated from the bolt/rock
interface along the whole bolt length [10]. Since the load transfer mechanism is dependent on the
interface along the whole bolt length [10]. Since the load transfer mechanism is dependent on the
friction force, this type of bolt does not require any important equipment such as the mechanical
friction force, this type of bolt does not require any important equipment such as the mechanical
interlocking device or grouting to transfer the load to the reinforcement. Based on the mode of
interlocking device or grouting to transfer the load to the reinforcement. Based on the mode of
operation, the frictional bolts can be grouped into two types: Swellex and Split Set.
operation, the frictional bolts can be grouped into two types: Swellex and Split Set.
The Swellex bolt, which was developed and marketed by Atlas Copco, consists of a tube which is
The Swellex bolt, which was developed and marketed by Atlas Copco, consists of a tube which
folded during the manufacturing process to create a smaller diameter unit. The installation procedure
is folded during the manufacturing process to create a smaller diameter unit. The installation
of Swellex bolts is shown in Figure 2. When the bolt is set in place in the borehole, the folded
procedure of Swellex bolts is shown in Figure 2. When the bolt is set in place in the borehole, the
bolt is expanded by hydraulic pressure of approximately 30 MPa, which produces a tight frictional
folded bolt is expanded by hydraulic pressure of approximately 30 MPa, which produces a tight
contact and mechanical interlocking at the bolt/rock interface [12]. Immediate support is obtained
frictional contact and mechanical interlocking at the bolt/rock interface [12]. Immediate support is
after installation.
obtained after installation.
The Split Set rock bolts were originally developed by James Scott and were commercialized by
The Split Set rock bolts were originally developed by James Scott and were commercialized by
Ingersoll-Rand Company [12]. The Split Set rock bolts rely solely on friction between the outer surface
Ingersoll-Rand Company [12]. The Split Set rock bolts rely solely on friction between the outer surface
of the bolt and the rock mass [13]. The Split Set rock bolts consist of a slotted, hollow, high-strength
of the bolt and the rock mass [13]. The Split Set rock bolts consist of a slotted, hollow, high-strength
steel tube and a faceplate. The bolt, with greater diameter than that of the borehole, is pushed into the
steel tube and a faceplate. The bolt, with greater diameter than that of the borehole, is pushed into
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the slightly undersized hole. Therefore, a radial spring force is created by the compression of the
slightly
undersized
hole.and
Therefore,
a radial spring
force
is created
by interface
the compression
of the C-shaped
C-shaped
steel tube
a tight frictional
contact
at the
bolt/rock
is generated.
steel tube and a tight frictional contact at the bolt/rock interface is generated.
Drill the hole
Insert Swellex bolt
Swellex
Expand the bolt
until preset pressure
Installed Swellex bolt
Figure
2. The
installation
of aof
Swellex
frictional
anchored
rockrock
bolt.bolt.
Figure
2. The
installation
a Swellex
frictional
anchored
These
frictional
anchored
rockhave
bolts
the advantages
of rapid
simplicity,
rapid
speed of
These
frictional
anchored
rock bolts
thehave
advantages
of simplicity,
speed of
installation
immediate
supportimportant
[14]. Another
important
property
this
typethey
of bolt
thattothey
andinstallation
immediateand
support
[14]. Another
property
of this
type of of
bolt
is that
areisable
are able to large
accommodate
large rockHowever,
deformation.
their
load bearing
is [15].
low in
accommodate
rock deformation.
theirHowever,
load bearing
capacity
is low capacity
in general
general
[15].
On
the
other
hand,
corrosion
of
these
types
of
rock
bolts
is
a
major
issue
since
the
bolts
On the other hand, corrosion of these types of rock bolts is a major issue since the bolts are directly
are directly
exposed toenvironments.
the aggressiveTherefore,
environments.
Therefore,
they
be used
in long-term
exposed
to the aggressive
they cannot
be used
incannot
long-term
applications
in
applications
in
corrosive
environments.
corrosive environments.
2.3.2.3.
Fully
Grouted
Rock
Bolts
Fully
Grouted
Rock
Bolts
TheThe
working
principle
of fully
grouted
rock
bolts
is that
the the
whole
length
of the
boltbolt
is bonded
working
principle
of fully
grouted
rock
bolts
is that
whole
length
of the
is bonded
continuously
to the
rock
mass
through
resin
or cement.
Therefore,
it can
be classified
intointo
twotwo
types:
continuously
to the
rock
mass
through
resin
or cement.
Therefore,
it can
be classified
types:
resin-grouted
rockrock
boltsbolts
and cement-grouted
rock bolts
of type
bolt normally
consists of
three of
resin-grouted
and cement-grouted
rock[16].
boltsThis
[16].type
This
of bolt normally
consists
components:
a reinforcing
bar, an anchorage
faceplate,faceplate,
and the bonding
surface
ofsurface
the
three components:
a reinforcing
bar, an anchorage
and the material.
bonding The
material.
The
reinforcing
bar is generally
a deformed
ribbed profile
to improve
theimprove
bondingthe
efficiency
between
of the reinforcing
bar is in
generally
in a or
deformed
or ribbed
profile to
bonding
efficiency
the between
rebar andthe
therebar
bonding
agent.
Fully agent.
grouted
boltsgrouted
can be advantageously
used in long-term
and
and the
bonding
Fully
bolts can be advantageously
used in
longsystematic
applications
as
grouting
agent
protects
the
bolt
from
the
corrosive
underground
waters.
term and systematic applications as grouting agent protects the bolt from the corrosive underground
The fully resin-grouted bolts are the most popular type of rock bolting system. A resin cartridge
waters.
containsThe
a certain
amount of resin
and
catalyst
separate
compartments.
shownAinresin
Figure
3a,
fully resin-grouted
bolts
area the
mostin
popular
type
of rock boltingAs
system.
cartridge
the contains
cartridges
are firstamount
set in place
in and
the borehole
then thecompartments.
bolt shaft is rotated
into in
theFigure
resin 3a,
a certain
of resin
a catalystand
in separate
As shown
cartridges.
The rotation
theinbolt
breaks
plastic and
sheath
of the
the bolt
cartridges
then into
mixes
the cartridges
are firstofset
place
in thethe
borehole
then
shaft isand
rotated
thethe
resin
resin
and
catalyst
together
[12].
Two
different
resin
cartridges
are
used
together:
the
fast-setting
resin
cartridges. The rotation of the bolt breaks the plastic sheath of the cartridges and then mixes the resin
cartridge
which is
placed further
inside
the borehole
and the slow-setting
one whichthe
is placed
behind
and catalyst
together
[12]. Two
different
resin cartridges
are used together:
fast-setting
resin
the cartridge
fast-setting
one.isThe
former
provides
setting
before
tensioning of
bolt,iswhich
which
placed
further
inside instant
the borehole
and
the slow-setting
onethe
which
placedtakes
behind
only
two
to three minutes.
latterprovides
sets the bolt
in place
in just
30 min.
The fast
of which
the resin
the
fast-setting
one. TheThe
former
instant
setting
before
tensioning
of setting
the bolt,
takes
bonding
agent
the fastThe
installation
compared
to theincement-grouted
only two
to ensures
three minutes.
latter setsasthe
bolt in place
just 30 min. Therock
fast bolts.
settingHowever,
of the resin
in weak
argillaceous
rocks the
or in
highly
fractured
rocks, goodtobonding
between therock
bolt bolts.
and the
rock
bonding
agent ensures
fast
installation
as compared
the cement-grouted
However,
mass
difficult
to achieve.
In or
such
cases, cement
grout
is considered
despite
it taking
a longer
timerock
in is
weak
argillaceous
rocks
in highly
fractured
rocks,
good bonding
between
the bolt
and the
to set.
mass is difficult to achieve. In such cases, cement grout is considered despite it taking a longer time
to set.
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(a)
(a)
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rebar
rebar
slow-setting resin
cartridge
slow-setting resin
cartridge
faceplate
faceplate
(b)
(b)
fast-setting resin
cartridge
fast-setting resin
cartridge
rebar
rebar
faceplate
faceplate
cement grout
cement grout
Figure 3. Fully grouted rock bolt: (a) fully resin-grouted rock bolt and (b) fully cement-grouted
Figure 3. Fully grouted rock bolt: (a) fully resin-grouted rock bolt and (b) fully cement-grouted rock
rock bolt.
Figure
bolt. 3. Fully grouted rock bolt: (a) fully resin-grouted rock bolt and (b) fully cement-grouted rock
bolt.
Groutingwith
withcement
cement is
traditional
wayway
to provide
bonding
along along
the fullthe
Grouting
is an
aninexpensive
inexpensiveand
and
traditional
to provide
bonding
Grouting
with
cement
is in
anFigure
inexpensive
and
traditional
way
to set
provide
bonding
along
thegrout
full is
length
of the
bolt.
As
shown
3b, when
bolt the
is set
in place,
the
cement
grout
is pumped
full
length
of the
bolt.
As shown
in Figure
3b,the
when
bolt
is
in place,
the
cement
length
of
the
bolt.
As
shown
in
Figure
3b,
when
the
bolt
is
set
in
place,
the
cement
grout
is
pumped
into
the
borehole
through
the
hole
in
the
rebar
center
or
a
plastic
tube
alongside
the
rebar.
pumped into the borehole through the hole in the rebar center or a plastic tube alongside theThe
rebar.
into
the boreholerock
through
in primary
the
center
afor
plastic
alongside
rebar.
The to
cement-grouted
boltbolt
isthe
always
the
choice
fororrock
reinforcement
when itthe
comes
weak
The
cement-grouted
rock
ishole
always
therebar
primary
choice
rocktube
reinforcement
when
ittocomes
cement-grouted
rock
boltPortland
is always
the
primary
choice
forapplications.
rock reinforcement
when
it comes
to weak
or fractured
strata.
Portland
cement
is adequate
for most
The fully
cement-grouted
rock
weak
or fractured
strata.
cement
is adequate
for
most
applications.
The
fully
cement-grouted
or
fractured
strata.
Portland
cement
is
adequate
for
most
applications.
The
fully
cement-grouted
rock
bolts
have
the
disadvantages
of
cement
shrinkage
which
reduces
the
bonding
strength,
and
longer
rock bolts have the disadvantages of cement shrinkage which reduces the bonding strength, and longer
bolts
have
the
disadvantages
of cement shrinkage which reduces the bonding strength, and longer
setting
time
thecement
cement[16,17].
[16,17].
setting
time
ofofthe
setting time of the cement [16,17].
2.4.
InstrumentedRock
RockBolts
Bolts
2.4.
Instrumented
2.4. Instrumented Rock Bolts
Instrumentedrock
rockbolts
boltsare
areaaspecial
special type
type of
strain
gages
within
Instrumented
of bolts
boltsthat
thatare
areinstrumented
instrumentedwith
with
strain
gages
within
Instrumented
are a special
typeforce
of bolts
instrumented
with
strain
the bolt,
which arerock
usedbolts
to monitor
the axial
andthat
theare
strain
distribution
along
thegages
lengthwithin
the
the bolt, which are used to monitor the axial force and the strain distribution along the length ofof
the
bolt.
the
bolt,
are used
to monitor
the axial forcerock
and the
distribution
the length
of the
bolt.
As which
illustrated
in Figure
4, an instrumented
boltstrain
generally
consistsalong
of a bolt
shaft, several
As illustrated in Figure 4, an instrumented rock bolt generally consists of a bolt shaft, several spaced
bolt.
As strain
illustrated
Figure
an instrumented
rock
bolt generally
consists
ofthe
a bolt
shaft,
several
spaced
gagesinalong
the4,axis
of the bolt, and
a portable
readout
unit for
strain
gages.
The
strain gages along the axis of the bolt, and a portable readout unit for the strain gages. The installation
spaced
strainofgages
along the rock
axis of
the bolt,
a portable
readout
unit for rock
the strain
The
installation
instrumented
bolts
is noand
different
to the
conventional
bolts.gages.
It is very
of instrumented rock bolts is no different to the conventional rock bolts. It is very important to ensure
installation
instrumented
rock
bolts
is no
to the the
conventional
rock ofbolts.
It isbolt
very
important toofensure
that all the
strain
gages
aredifferent
located within
loaded portion
the rock
in
that
all the strain
gages
areall
located
within
theare
loaded
portion
of the
the loaded
rock bolt
in order
to obtain
accurate
important
to ensure
that
the strain
gages
located
within
portion
of the
rock bolt
in
order to obtain
accurate
loading
results.
loading
results.
order to
obtain accurate loading results.
Figure 4. The illustration of an instrumented rock bolt.
Figure 4. The illustration of an instrumented rock bolt.
Figure 4. The illustration of an instrumented rock bolt.
The instrumented rock bolts play an important role in providing useful warning information to
The
bolts
an important
important
roleinA
inproviding
providing
useful
warning
information
Theinstrumented
instrumented
rock
bolts play
an
role
useful
warning
to to
mining
workers
on therock
impending
collapse
of the strata.
proper
arrangement
of theinformation
instrumented
mining
collapseof
ofthe
thestrata.
strata.AAproper
properarrangement
arrangement
instrumented
miningworkers
workerson
onthe
theimpending
impending collapse
of of
thethe
instrumented
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rock bolts will give the loading capacity distribution of the strata. Based on such information, a wider
bolting system can be adopted while maintaining the safety requirements and thus reducing the
equipment cost significantly.
3. Related Smart Sensors
3.1. Piezoelectric Sensors
Piezoelectricity is the phenomenon that a piezoelectric material is able to convert mechanical
energy into electrical energy and vice versa. In a direct manner, electric charges are generated when
the piezoelectric material is mechanically stressed. Conversely, the geometry of the piezoelectric
material will deform according to the applied electrical field. The direct effect of the piezoelectric
material can be utilized in dynamic sensing applications. Furthermore, the converse effect can be
used in actuation [18]. There are a few materials that possess the properties of piezoelectricity,
including piezoelectric ceramics (Lead Zirconate Titanate, also known as PZT), piezoelectric polymers
(Polyvinylidene Fluoride, denoted as PVDF) and piezoelectric ceramic/polymer composites, which are
frequently used as piezoelectric actuators and sensors for structural health monitoring of various
structures [19,20].
According to compact matrix notation [21], the coupled electromechanical constitutive equations
of a linear piezoelectric material can be expressed as
"
D
S
#
"
=
εT d
dt s E
#"
E
T
#
(1)
where D and E are the electric displacement (3 × 1) (C/m2 ) and electric field (3 × 1) (V/m) respectively.
S and T are the mechanical strain (6 × 1) and stress (6 × 1) (N/m2 ), d, ε T and s E are the piezoelectric
strain constant (3 × 6) (C/N or m/V), dielectric permittivity (3 × 3) (Farad/m) and compliance
constant (6 × 6) (m2 /N), respectively. When the mechanical stress/strain are in the x-direction and the
electrical field/displacement are in the z-direction, the constitutive equations can be simplified as
D3 = ε T33 E3 + d31 T1
E T +d E
S1 = s11
1
31 3
(2)
Various non-destructive testing (NDT) techniques based on piezoelectricity have been developed
over the last decades, such as Lamb wave [22,23], electro-mechanical impedance (EMI) [24–26],
guided ultrasonic waves [27], and acoustic emission (AE) [28,29]. Lamb waves are a form of elastic
perturbation that can propagate in a solid plate with free boundaries, which can be excited by
piezoelectric patches driven at a particular frequency. The time of flight, propagation velocity,
amplitude and phase of Lamb waves can be utilized to locate, detect and quantify a damage in
a plate structure. The electro-mechanical impedance technique measures the changes in the frequency
response of the electrical impedance of the PZT due to the changes in the mechanical impedance of
the host structure. Compared to a baseline measurement in pristine state, the peak frequency shift
and amplitude change of the impedance spectra indicate the damage severity of the host structure.
The guided ultrasonic waves technique exploits the waveguide geometry of the tested structure,
such as planar and tubular structures. The guided ultrasonic waves are excited in this type of structure,
which is then reflected from defects such as crack, delamination and corrosion. The location of
the defects can be determined from the arrival time of the reflected waves. Unlike Lamb wave,
electro-mechanical impedance and guided ultrasonic wave techniques, which belong to active NDT
methods, the acoustic emission method is a passive technique. The acoustic emission technique relies
on the stress waves generated from the rapid release of energy from a structure due to damage-related
deformation. The descriptive parameters of the AE waves offer rich damage-related information,
which can be used to characterize the location, source, and evolution of the damage. Among all
Sensors 2017, 17, 776
7 of 24
these piezoelectric-based NDT methods, the guided ultrasonic waves are more suited for condition
monitoring of rock bolts since rock bolts act as a waveguide by nature.
Sensors 2017, 17, 776
Guided Ultrasonic Waves in a Cylinder
7 of 24
Guided Ultrasonic
Waves
in ain
Cylinder
Guided
waves occur
only
plate-like or waveguide-like structures, such as plates and bars.
Their energy
is
concentrated
near
the
boundaries.
Since they are
confinedsuch
in the
waveguide,
guided
Guided waves occur only in plate-like
or waveguide-like
structures,
as plates
and bars.
waves
are energy
able toispropagate
over
long
Usually,
theare
guided
waves
arewaveguide,
excited using
a short
Their
concentrated
near
the distance.
boundaries.
Since they
confined
in the
guided
waves
are able to In
propagate
over
distance.
Usually,
the guided waves
are excited
usingvelocity
a short is a
duration
toneburst.
general,
thelong
guided
waves
are dispersive,
and their
traveling
duration
toneburst.Also,
In general,
the modes
guided exist
waves
dispersive,
and their traveling velocity is a
function
of frequency.
multiple
forare
a specific
waveguide.
function
of
frequency.
Also,
multiple
modes
exist
for
a
specific
waveguide.
The geometry of a steel bar embedded in solid medium is presented in Figure 5. There are three
The geometry of a steel bar embedded in solid medium is presented in Figure 5. There are three
modes in this cylindrical steel bar: the longitudinal modes (0, m), torsional modes T(0, m), and flexural
modes in this cylindrical steel bar: the longitudinal modes (0, ), torsional modes T(0, ), and flexural
modes F(n, m). Here, n and m are the circumferential order and modulus, respectively. The selection of
modes ( , ). Here, and are the circumferential order and modulus, respectively. The selection
a suitable
mode for
the application
of ultrasonic
guided
waveswaves
in NDT
is very
this selection
of a suitable
mode
for the application
of ultrasonic
guided
in NDT
is important;
very important;
this
can be
made
by
examining
the
dispersion
curves.
The
details
of
the
procedure
are
described
selection can be made by examining the dispersion curves. The details of the procedure are describedin the
following
[30,31]. In
the guided
wave NDT,
the arrival
time,time,
amplitude,
and
velocity
in thereferences
following references
[30,31].
In the guided
wave NDT,
the arrival
amplitude,
and
velocityof the
received
waves
canwaves
be used
defects/damage
of the
of the
received
canto
becharacterize
used to characterize
defects/damage
of waveguide.
the waveguide.
Figure 5. Geometry of a steel bar in solid medium.
Figure
5. Geometry of a steel bar in solid medium.
3.2. Fiber Optic Sensors
3.2. Fiber Optic Sensors
Fiber optic sensors are a category of sensors that use the optical fiber or the light traveling
Fiber
optic
a category
of sensors
that use the
optical
the light
traveling
through
through
thesensors
fiber asarethe
sensing element.
Properties
of the
lightfiber
willorchange
due
to external
the fiber
as the sensing
element.
Properties
of the lightand
willelectric
change
due to experienced
external perturbations
perturbations
such as
strain, pressure,
temperature
currents
by the fiber such
as strain,
and electric
currents
by the
fiber optic.
An optical
optic.pressure,
An opticaltemperature
fiber sensor system
typically
consistsexperienced
of a light source,
a receiver,
an optical
fiber as fiber
thesystem
sensingtypically
element, consists
a modulator,
signal processing
unit.
fiberfiber
opticassensors
convertelement,
or
sensor
of a and
lighta source,
a receiver,
an The
optical
the sensing
encode
these
external
perturbations
into
corresponding
variations
in
the
optical
properties
of
the
a modulator, and a signal processing unit. The fiber optic sensors convert or encode these external
transmitted
light,
such as intensity,
phase,
wavelength,
frequency of
and
Such
perturbations
into
corresponding
variations
in the
optical properties
thepolarization
transmitted[32].
light,
such as
variations are then demodulated by a dedicated demodulation system. Fiber optic sensors have
intensity, phase, wavelength, frequency and polarization [32]. Such variations are then demodulated
several distinctive advantages over conventional electrical sensors which include small size, high
by a dedicated demodulation system. Fiber optic sensors have several distinctive advantages over
sensitivity, corrosion resistance, immunity to electromagnetic interference, and ability to multiplex.
conventional
electrical sensors which include small size, high sensitivity, corrosion resistance, immunity
Therefore, fiber optic sensors have been widely used in monitoring various engineering structures.
to electromagnetic
interference,
andoptic
ability
to multiplex.
Therefore,
fiber optic
sensors
have been
Among the various
types of fiber
sensors,
the fiber Bragg
grating (FBG)
sensors
and Brillouin
widely
usedtime
in monitoring
various engineering
structures.
Among
fiberofoptic
optical
domain reflectometer
(BOTDR) sensors
have shown
the the
mostvarious
promisetypes
in theoffield
sensors,
the fiber
Bragg
grating (FBG) sensors and Brillouin optical time domain reflectometer (BOTDR)
structural
health
monitoring.
sensors have shown the most promise in the field of structural health monitoring.
3.2.1. FBG Sensors
3.2.1. FBGASensors
FBG is produced by inscribing periodic and permanent modifications in the core refractive
index
the opticalby
fiber
axis [33]. periodic
When a broadband
light is launched
throughinthe
the
A
FBGalong
is produced
inscribing
and permanent
modifications
thegratings,
core refractive
reflected Bragg wavelength follows the form
index along the optical fiber axis [33]. When a broadband light is launched through the gratings,
=2
(3)
the reflected Bragg wavelength follows the form
is the Bragg wavelength,
is the effective refractive index of FBG and
is the grating
where
period. The grating period, and thereforeλ Bthe
= Bragg
2ne f f Λwavelength, are linear to both strain and (3)
temperature. The relation between the relative Bragg wavelength shift and the axial strain and the
where
λ B is the Bragg
wavelength,
ne f f is
effective refractive index of FBG and Λ is the grating
temperature
variation
Δ is expressed
as the
follow
period. The grating period, and therefore the Bragg wavelength, are linear to both strain and
Sensors 2017, 17, 776
8 of 24
temperature. The relation between the relative Bragg wavelength shift and the axial strain ε and
the temperature
Sensors
2017, 17, 776 variation ∆T is expressed as follow
8 of 24
Δ
∆λ B
= Cε ε + CT ∆T
=λ B +
(4)
(4)
where Cε and CT are
where
arestrain
strainand
andtemperature
temperaturesensitivity
sensitivitycoefficients,
coefficients,respectively.
respectively.
As illustrated
when
a broadband
lightlight
passes
through
opticaloptical
fiber grating,
the portion
As
illustrated in
inFigure
Figure6, 6,
when
a broadband
passes
through
fiber grating,
the
of
the
spectrum
which
equals
to
the
Bragg
wavelength
of
the
FBG
is
reflected,
and
others
can
portion of the spectrum which equals to the Bragg wavelength of the FBG is reflected, and
can
pass through
through the
theFBG
FBGwith
withsmall
smallattenuation.
attenuation.In In
this
way,
interrogation
of FBG
sensors
pass
this
way,
thethe
interrogation
of FBG
sensors
cancan
be
be easily
multiplexed
using
the wavelength
division
multiplexed
(WDM)
technique
to realize
easily
multiplexed
using
the wavelength
division
multiplexed
(WDM)
technique
to realize
a quasi-a
quasi-distribution
network
distribution
sensorsensor
network
[34]. [34].
Figure 6. Functioning principle of the FBG sensor.
Figure 6. Functioning principle of the FBG sensor.
3.2.2. BOTDR Distributed Sensors
3.2.2. BOTDR Distributed Sensors
Brillouin scattering is a phenomenon in which a light wave transmitted in an optical fiber is
Brillouin
scattering
is awith
phenomenon
in which
a light
wave transmitted
in an optical
fiber
is
scattered
by the
interaction
acoustic waves
[35,36].
The frequency
of the scattered
Brillouin
light
scattered
by
the
interaction
with
acoustic
waves
[35,36].
The
frequency
of
the
scattered
Brillouin
light
is dependent on the strain and temperature of the optical fiber, which will shift from the frequency of
is
dependent
on the
strain
and temperature
of the
optical
will shift
from the frequency
the
incident light.
The
Brillouin
frequency shift
v B is
givenfiber,
by thewhich
following
equation
of the incident light. The Brillouin frequency shift
is given by the following equation
2v = 2nv A
(5)
(5)
= B
λ
where n isisthe
thethe
sound
wave
velocity
andand
λ is the
of incident
light.
where
therefractive
refractiveindex,
index,v A is is
sound
wave
velocity
is wavelength
the wavelength
of incident
The Brillouin
frequency
shift isshift
linearly
proportional
to the change
strainoforstrain
temperature,
which is
light.
The Brillouin
frequency
is linearly
proportional
to the of
change
or temperature,
expressed
as
follows
which is expressed as follows
v B (ε, T ) = v B0 (ε 0 , T0 ) + Cε (ε − ε 0 ) + CT ( T − T0 )
(6)
(6)
, =frequency
+ at strain
−
−
, shift
where v B (ε, T ) is the Brillouin
ε+and temperature
T; Cε and CT are the strain
and temperature coefficients, respectively; T0 and ε 0 are the initial strain and initial temperature that
,
is the Brillouin frequency shift at strain and temperature ;
and
are the
where
correspond to the reference Brillouin frequency v B0 . Figure 7a shows the Brillouin scattering intensity
strain and temperature coefficients, respectively;
and
are the initial strain and initial
distribution at different frequencies along the optical fiber. Figure 7b shows the frequency shift of the
temperature that correspond to the reference Brillouin frequency
. Figure 7a shows the Brillouin
Brillouin back scattering light at a specific location due to its corresponding strain, Figure 7c shows
scattering intensity distribution at different frequencies along the optical fiber. Figure 7b shows the
frequency shift of the Brillouin back scattering light at a specific location due to its corresponding
strain, Figure 7c shows the Brillouin scattering intensity spectrum at a specific frequency along the
optical fiber. BOTDR is very suitable for long-distance distributed strain sensing with a sensitivity of
5 με [37]. However, the spatial resolution is relatively low, about 1 m, and therefore this technique
is not suitable for structural monitoring applications that require dense distribution of sensors.
Sensors 2017, 17, 776
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the Brillouin scattering intensity spectrum at a specific frequency along the optical fiber. BOTDR is
very suitable for long-distance distributed strain sensing with a sensitivity of 5 µε [37]. However,
the spatial resolution is relatively low, about 1 m, and therefore this technique is not suitable for
Sensors 2017,
17, 776
9 of 24
structural
monitoring
applications that require dense distribution of sensors.
Figure
7. Principle
of the
Brillouin
Scattering-basedfiber
fiberoptic
opticmeasuring
measuring technique:
technique: (a)
Figure
7. Principle
of the
Brillouin
Scattering-based
(a)The
TheBrillouin
Brillouin
scattering
spectrum
an optical
fiber;
The Brillouin
scattering
at afrequency;
specific
scattering
spectrum
alongalong
an optical
fiber; (b)
The(b)
Brillouin
scattering
spectrumspectrum
at a specific
frequency;
(c)scattering
The Brillouin
scattering
at a specific location.
(c) The
Brillouin
spectrum
at aspectrum
specific location.
4. Rock
Monitoring
Using
Smart
Sensors
4. Rock
BoltBolt
Monitoring
Using
Smart
Sensors
Monitoring
4.1. 4.1.
LoadLoad
Monitoring
In order
to ensure
proper
functioningofofrock
rockbolts
boltsand
andthat
that the
the rock
rock bolts
In order
to ensure
thethe
proper
functioning
bolts are
arenot
notsubjected
subjected
to
excessive
loading,
it
is
important
to
monitor
the
load
experienced
by
the
rock
bolts.
to excessive loading, it is important to monitor the load experienced by the rock bolts. The
Thethree
three
quantities of primary significance are the applied load at the rock bolts, the bolt head displacement,
quantities of primary significance are the applied load at the rock bolts, the bolt head displacement,
and the load distribution along the anchored length [1].
and the load distribution along the anchored length [1].
4.1.1. Traditional Load Monitoring Methods
4.1.1.
Traditional Load Monitoring Methods
One traditional way to measure the rock bolt load was the use of electrical-resistance strain
One traditional way to measure the rock bolt load was the use of electrical-resistance strain
gages, which perform relatively well for short-term load measurement. However, the long-term
gages, which perform relatively well for short-term load measurement. However, the long-term
survivability of electrical-resistance strain gages was still an issue and the gages are prone to damage
survivability of electrical-resistance strain gages was still an issue and the gages are prone to damage
during and after installation. To compensate for the shortcoming of electrical-resistance strain gages,
during
and after installation. To compensate for the shortcoming of electrical-resistance strain gages,
the vibrating-wire strain gages were developed and commercialized [1,38]. The operating principle
the of
vibrating-wire
and commercialized
operating
principle
vibrating-wirestrain
straingages
gageswere
reliesdeveloped
on the phenomenon
in which the[1,38].
naturalThe
vibration
frequency
of
of vibrating-wire
strain
gages
relies
on
the
phenomenon
in
which
the
natural
vibration
frequency
a wire changes with its tension. The vibrating-wire strain gages have good stability, long-term
of areliability
wire changes
with
its tension.
The
vibrating-wire
strain gages
have
good stability,
long-term
and high
accuracy
and can
either
be surface-mounted
on or be
embedded
into the structure.
reliability
and high
and canthe
either
be surface-mounted
on or be
embedded
the structure.
Benmokrane
et accuracy
al. investigated
performance
of vibrating-wire
strain
gages into
instrumented
in
Benmokrane
et
al.
investigated
the
performance
of
vibrating-wire
strain
gages
instrumented
in
grouted
grouted anchors from the perspectives of the load transfer mechanism, the debonding process, creep
anchors
from and
the perspectives
of the load [1].
transfer
mechanism,
the debondingstrain
process,
creep
behavior,
long-term performance
Compared
to the vibrating-wire
gages,
thebehavior,
metalstrainperformance
gages have larger
stretch capacity.
Mitri and
Marwan
a rock bolt strain
load
andbased
long-term
[1]. Compared
to theTherefore,
vibrating-wire
strain
gages,devised
the metal-based
cellhave
by placing
a metal
strain gage
insideMitri
the bolt
via adevised
small drilled
holeload
in its
center,
and
gages
larger stretch
capacity.
Therefore,
andhead
Marwan
a rock bolt
cell
by placing
implemented
such
techniques
practical
[39,40].
Mitri
later introduced
an improved
a metal
strain gage
inside
the boltinhead
via a application
small drilled
hole in
its center,
and implemented
such
rock bolt load cell based on the concept of the coupler load cell, which has improved performance in
terms of measuring the ultimate strength of the rock bolt, shipping, and installation [38]. Figure 8
shows the coupler load cell concept and the photo of the load cell.
Sensors 2017, 17, 776
10 of 24
techniques in practical application [39,40]. Mitri later introduced an improved rock bolt load cell based
on the concept of the coupler load cell, which has improved performance in terms of measuring the
ultimate strength of the rock bolt, shipping, and installation [38]. Figure 8 shows the coupler load cell
concept
and
photo of the load cell.
Sensors
2017,the
17, 776
10 of 24
Sensors 2017, 17, 776
10 of 24
(a)
(b)
Figure 8. (a) The coupler load cell design concept; (b) photo of the coupler load cell [38].
Figure 8. (a) The coupler
(a) load cell design concept; (b) photo of the
(b)coupler load cell [38].
4.1.2. Load Monitoring
on FBG
Figure 8. (a) Based
The coupler
loadSensors
cell design concept; (b) photo of the coupler load cell [38].
4.1.2. Load Monitoring Based on FBG Sensors
Most of the rock bolts on the market are made of steel, but a small portion of rock bolts are made
4.1.2.
Load
Based
FBGfiber
Sensors
of
composite
materials
ason
glass
polymer
carbon
Most
of
theMonitoring
rock
boltssuch
on the
market
arereinforced
made of steel,
but(GFRP)
a smalland
portion
offiber
rockreinforced
bolts are made
polymer
(CFRP).
Embedding
the
fiber
optic
sensors
inside
the
composite
materials
is
very
effective
of composite
fiberare
reinforced
polymer
(GFRP)
andofcarbon
fiber
Mostmaterials
of the rocksuch
bolts as
on glass
the market
made of steel,
but a small
portion
rock bolts
arereinforced
made
in
transferring
strain
and
protecting
the
fragile
optical
fiber
from
mechanical
damage.
Frank
et effective
al.
of composite
materials such
asfiber
glassoptic
fiber reinforced
polymer
and materials
carbon fiber
polymer
(CFRP). Embedding
the
sensors inside
the (GFRP)
composite
is reinforced
very
fabricated
GFRP
rock
bolts
instrumented
with
FBG
sensors
by
embedding
the
FBG
sensors
in
the
polymer (CFRP).
the fiber
sensors
inside
thefrom
composite
materials
is very effective
in transferring
strain Embedding
and protecting
theoptic
fragile
optical
fiber
mechanical
damage.
Frank et al.
GFRP
rock bolts strain
duringand
theprotecting
pultrusionthe
process
[41].
The fiber
tensile
tests
indicated damage.
that the embedded
in
transferring
fragile
optical
from
mechanical
Frank
et al.
fabricated GFRP rock bolts instrumented with FBG sensors by embedding the FBG sensors
in the
GFRP
FBG
sensorsGFRP
were rock
able to
survive
a high strain
1.5%.
The GFRP
rock boltsthe
equipped
with in
FBG
fabricated
bolts
instrumented
with of
FBG
sensors
by embedding
FBG sensors
rock bolts during the pultrusion process [41]. The tensile tests indicated that the embedded FBG the
sensors
sensors
were bolts
installed
in athe
tunnel
near Sargans,
for long-term
load monitoring
[42,43].
GFRP rock
during
pultrusion
processSwitzerland
[41]. The tensile
tests indicated
that the embedded
wereThe
able to survivescheme
a highofstrain
of 1.5%. The GFRP
rock bolts
bolts in
equipped
with
sensors were
the FBG-equipped
rock
a tunnel
and FBG
the one
FBGinstallation
sensors were able to
survive
a high strainGFRP
of 1.5%.
The GFRP
rock
bolts equipped
withyear
FBG
installed
in a tunnel
near
Sargans,
Switzerland
for Switzerland
long-term
load
monitoring
[42,43].
installation
measurements
shown
TheSargans,
collected
strain information
along the
rock
boltsThe
reveal
the
sensors were are
installed
inina Figure
tunnel 9.
near
for long-term
load
monitoring
[42,43].
scheme
themovement
FBG-equipped
GFRP
rock bolts
in a GFRP
tunnelrock
and
the one
year
measurements
shown
long-term
of theof
rock
and allow
operators
to bolts
take
the
precaution.
Theofinstallation
scheme
themasses
FBG-equipped
in necessary
a tunnel
and the oneare
year
in Figure
9. The collected
strain
information
along thestrain
rock information
bolts revealalong
the long-term
movement
of the
measurements
are shown
in Figure
9. The collected
the rock bolts
reveal the
rock masses
and
allow operators
to masses
take the
necessary
precaution.
long-term
movement
of the rock
and
allow operators
to take the necessary precaution.
(a)
(b)
Figure 9. (a) Installation of GFRP rock bolts equipped with FBG sensors; (b) one year observation of
rock movement in the (a)
tunnel with the three installed GFRP sensors: wire(b)A51 (diamonds) and
rockbolts
(squares) and
C74 (dots)
Figure 9.C612
(a) Installation
of GFRP
rock [43].
bolts equipped with FBG sensors; (b) one year observation of
Figure 9. (a) Installation of GFRP rock bolts equipped with FBG sensors; (b) one year observation of
rock movement in the tunnel with the three installed GFRP sensors: wire A51 (diamonds) and
rockFor
movement
in the tunnel
with the three
installed GFRP sensors: wire A51 (diamonds) and rockbolts
steel rock
however,
strain
rockbolts
C612bolts,
(squares)
and C74the
(dots)
[43].can go up to 20%. A direct mounting of FBG sensors
along
bolt shaft
the sensor under loading condition. Schroeck et al. devised a
C612 the
(squares)
and can
C74easily
(dots)damage
[43].
specialFor
arrangement
the however,
FBG sensors
ordercan
to measure
[44]. Inoftheir
steel rock of
bolts,
theinstrain
go up tolarge
20%.relative
A directstrain
mounting
FBGdesign,
sensors
they
found
a neutral
lineeasily
on thedamage
cylinder
along
which
the elongation
and contraction
effects a
along
the bolt
shaft can
theshell,
sensor
under
loading
condition. Schroeck
et al. devised
special arrangement of the FBG sensors in order to measure large relative strain [44]. In their design,
they found a neutral line on the cylinder shell, along which the elongation and contraction effects
Sensors 2017, 17, 776
11 of 24
For steel rock bolts, however, the strain can go up to 20%. A direct mounting of FBG sensors
along the bolt shaft can easily damage the sensor under loading condition. Schroeck et al. devised a
special
arrangement
of the FBG sensors in order to measure large relative strain [44]. In their design,
Sensors 2017,
17, 776
11 of 24
Sensorsfound
2017, 17,
11 of 24
they
a 776
neutral line on the cylinder shell, along which the elongation and contraction effects
cancel out each other.
other. A FBG
FBG sensor
sensor bonded on the rock bolt
bolt in
in aa small
small inclination
inclination from the
the neutral
neutral
cancel
out
each
other.
A
FBG
sensor
bonded
on
the
rock
bolt
in
a
small
inclination
from
the
neutral
angle
angle will give a predictable
predictable ratio
ratio to the elongation
elongation of the rock
rock bolt.
bolt. Three FBG sensors were installed
angle
a predictable
ratio to the elongation
the
rock
bolt. Three
FBG
sensors
were
◦ will
◦ give 40
◦
at
direction
ofof
the
bolt
to yield
strain
transfer
ratios
of −installed
15%,
0%
at 17
17°,, 29
29° and
and 40°totothe
thecircumferential
circumferential
direction
of
the
bolt
to yield
strain
transfer
ratios
of
−15%,
at
17°,
29°
and
40°
to
the
circumferential
direction
of
the
bolt
to
yield
strain
transfer
ratios
of
−15%,
and
+15%
withwith
respect
to thetostrain
of theof
bolt,
The Bragg
change and
applied
0% and
+15%
respect
the strain
therespectively.
bolt, respectively.
Thewavelength
Bragg wavelength
change
and
0% and
+15%the
with
respect
tocompressed
the strain ofsensor
the bolt,
The(29
Bragg
wavelength
change(40
and
◦)
strain
versus
load
for
(17◦respectively.
), neutral
sensor
and
elongated
applied
strain versus
thethe
load
for the compressed
sensor
(17°),
neutral◦ )sensor
(29°) andsensor
elongated
applied
strain
versus
the
load
for
the
compressed
sensor
(17°),
neutral
sensor
(29°)
and
elongated
are
illustrated
Figure 10.inInFigure
this inclination
it ismethod,
feasibleittoismeasure
themeasure
large elongation
sensor
(40°) areinillustrated
10. In thismethod,
inclination
feasible to
the large
sensor
(40°)
are
illustrated
in
Figure
10.
In
this
inclination
method,
it
is
feasible
to
measure
the large
of
steel rockofbolts
breaking
thebreaking
FBG sensors.
Moerman
al. applied
FBG sensors
on
elongation
steelwithout
rock bolts
without
the FBG
sensors.etMoerman
et the
al. applied
the FBG
elongation
of steel rock
bolts without
breaking
the FBG
sensors.
Moerman
et to
al.measure
applied the
the force
FBG
the
loadon
cell,
waswhich
placed
between
bearing
plate
and anchorage
plate
sensors
thewhich
load cell,
was
placedthe
between
the
bearing
plate and anchorage
plate to measure
sensors
on the load
cell, which
was
placed
between
the bearing
and
platethe
to measure
in
ground
Each
load
cell
was
equipped
with plate
three
FBGanchorage
sensors,
and
cell
thethe
force
in theanchor
ground[45].
anchor
[45].
Each
load
cell
was equipped
with three
FBG sensors,
andload
the load
the
force
in
the
ground
anchor
[45].
Each
load
cell
was
equipped
with
three
FBG
sensors,
and
the
load
was
calibrated
to have
linearlinear
response
and an
accuracy
of 3.5of
kN.
load cells
cell was
calibrated
to have
response
and
an accuracy
3.5The
kN.FBG-equipped
The FBG-equipped
loadwere
cells
cell was calibrated
to have
linear response
and
anmonitoring.
accuracy ofFigure
3.5 kN.
The
FBG-equipped
load cells
installed
in
a
quay
wall
for
long-term
anchor
force
11
shows
the
FBG-instrumented
were installed in a quay wall for long-term anchor force monitoring. Figure 11 shows the FBGwerecell.
installed
in12ashows
quay the
wallanchor
for long-term
anchor
force monitoring.
Figure 11 shows the FBGload
Figure
forces
a 15-month
period.
instrumented
load
cell. Figure
12 shows
the over
anchor
forces over
a 15-month period.
instrumented load cell. Figure 12 shows the anchor forces over a 15-month period.
(a)
(a)
(b)
(b)
(c)
(c)
Figure 10. Bragg
wavelength
change
and
applied
strain
vs.vs.
load
for (a) (a)
compressed
sensor
(17°),
(b)
◦ );
Figure
Braggwavelength
wavelengthchange
changeand
andapplied
appliedstrain
strainvs.
load
compressed
sensor
(17(b)
Figure 10.
10. Bragg
load
forfor
(a) compressed
sensor
(17°),
neutral
sensor
(29°)
and
(c)
elongated
sensor
(40°)
[44].
◦
◦
(b)
neutral
sensor
(29and
) and
(c) elongated
sensor
(40
) [44].
neutral
sensor
(29°)
(c) elongated
sensor
(40°)
[44].
(a)
(a)
(b)
(b)
Figure 11. (a) The FBG instrumented load cell; (b) the load cell is placed in the quay wall [45].
Figure
11. (a)
load cell;
cell; (b)
(b) the
the load
load cell
cell is
is placed
placed in
in the
the quay
quay wall
wall [45].
[45].
Figure 11.
(a) The
The FBG
FBG instrumented
instrumented load
Ho et al. instrumented the FBG sensor on the load bearing plate to measure the rock bolt axial
Ho et al. instrumented the FBG sensor on the load bearing plate to measure the rock bolt axial
load Ho
[46].etFigure
13 shows the
anchor
instrumented
with the
and
al. instrumented
theload
FBGmeasuring
sensor on the
loadplate
bearing
plate to measure
theFBG
rocksensor
bolt axial
load [46]. Figure 13 shows the load measuring anchor plate instrumented with the FBG sensor and
the
representative
testing
results
are
shown
in
Figure
14.
Such
a
method
ensured
that
the
FBG
sensor
load [46]. Figure 13 shows the load measuring anchor plate instrumented with the FBG sensor and
the representative testing results are shown in Figure 14. Such a method ensured that the FBG sensor
wasrepresentative
not over-strained.
Other
applications
sensors
in load
monitoring
include
monitoring
the
the
testing
results
are shownofinFBG
Figure
14. Such
a method
ensured
that the
FBG sensor
was not over-strained. Other applications of FBG sensors in load monitoring include monitoring the
axialnot
stress
of rock boltsOther
during
the subway
construction
the testing
model
[47] and
was
over-strained.
applications
of tunnel
FBG sensors
in loadusing
monitoring
include
monitoring
axial stress of rock bolts during the subway tunnel construction using the testing model [47] and
investigating
the
load
transfer
mechanism
of
the
ground
anchor
and
tensile
force
distribution
the axial stress of rock bolts during the subway tunnel construction using the testing model [47]along
and
investigating the load transfer mechanism of the ground anchor and tensile force distribution along
the tendon [48].
investigating
the load transfer mechanism of the ground anchor and tensile force distribution along
the tendon [48].
the tendon [48].
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776
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12
12
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Figure 12. Overview of the monitoring results during a period of more than 1 year after installation
Figure
monitoring
results
during
period
more
than
11 year
after
installation
Figure
than
1 year
after
installation
of
Figure 12.
12. Overview
Overview of
of the
the monitoring
monitoringresults
resultsduring
duringaaaperiod
periodofof
ofmore
more
than
year
after
installation
of the ground anchors [45].
of
the
ground
anchors
[45].
the
ground
anchors
[45].
of the
ground
anchors
[45].
Figure 13. The load measuring anchor plate using an FBG sensor. Note: the circle indicates the location
Figure
Figure
13. The
Theload
loadmeasuring
measuringanchor
anchor plate
plate using
using an
anFBG
FBGsensor.
sensor. Note:
Note: the
thecircle
circleindicates
indicates the
the location
location
Figure 13.
13.
The
load
measuring
anchor
plate
using
an
FBG
sensor.
Note:
the
circle
indicates
the
location
of the FBG strain sensor [46].
of
the
of
the
FBG
strain
sensor
[46].
of the FBG strain sensor [46].
(a)
(a)
(a)
(b)
(b)
(b)
Figure 14.
14. FBG
test in
in which
which the
the plate
plate was
was cyclically
cyclically loaded
loaded
Figure
FBG signal
signal for
for the
the nondestructive
nondestructive compression
compression test
Figure
Figure 14.
14. FBG
FBG signal
signal for
for the
the nondestructive
nondestructive compression
compression test
test in
in which
which the
the plate
plate was
was cyclically
cyclically loaded
loaded
from 00 to
to 75
75 kN:
kN: (a)
(a) Strain;
Strain; (b)
(b) Force
Force [46].
[46].
from
from
from 00 to
to 75
75 kN:
kN: (a)
(a) Strain;
Strain; (b)
(b) Force
Force [46].
[46].
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4.1.3. Load
Load Monitoring
Monitoring Based
Based on
on Distributed
Distributed Brillouin
Brillouin Sensing
Sensing
4.1.3.
Unlike the
the quasi-distributed
quasi-distributed FBG
FBG sensors,
sensors, the
the optical
optical fiber
fiber Brillouin
Brillouin scattering
scattering sensing
sensing technique
technique
Unlike
is
truly
distributed,
which
can
be
advantageously
used
to
measure
the
stress
distribution
along the
the
is truly distributed, which can be advantageously used to measure the stress distribution along
rock bolt.
examined
thethe
Brillouin
optical
timetime
domain
analysis
(BOTDA)
optical
fiber
rock
bolt. Iten
Itenand
andPuzrin
Puzrin
examined
Brillouin
optical
domain
analysis
(BOTDA)
optical
distributed
sensing
technique
in
assessing
the
stress
distribution
along
ground
anchors
[49].
Three
fiber distributed sensing technique in assessing the stress distribution along ground anchors [49].
different
methods
of fiber
opticoptic
sensor
integration
into
steel
tendon—external
Three
different
methods
of fiber
sensor
integration
into
steel
tendon—externallongitudinal
longitudinal trench,
trench,
internal
integration,
and
helix
integration—were
investigated.
Among
these
methods,
trench
and
internal integration, and helix integration—were investigated. Among these methods, trench and
internal
integration
methods
were
more
suited
for
distributed
stress
monitoring,
and
therefore
they
internal integration methods were more suited for distributed stress monitoring, and therefore
were were
applied
in a wall
an of
excavation
pit forpit
field
The load
measured
by this
they
applied
in a of
wall
an excavation
fortesting.
field testing.
Thedistribution
load distribution
measured
technique
along
the
tendon
for
different
load
steps
is
shown
in
Figure
15.
The
load
distribution
by this technique along the tendon for different load steps is shown in Figure 15. Theunder
load
different pullout
forces
was clearly
observed
the Brillouin
distributed
technique.
More
recently,
distribution
under
different
pullout
forces using
was clearly
observed
using the
Brillouin
distributed
Moffat et al.More
experimentally
tested et
theal.BOTDR
techniquetested
for rock
loading
conditionfor
monitoring
technique.
recently, Moffat
experimentally
thebolt
BOTDR
technique
rock bolt
[50].
loading condition monitoring [50].
Figure 15. Load distribution measured by the Brillouin scattering sensing technique at different load
steps [49].
4.2. Corrosion
Corrosion Monitoring
Monitoring
4.2.
Corrosion is
is aa process
process involving
involving the
the physical
physical alternation
alternation of
of aa material
material from
from aa chemical
chemical reaction
reaction
Corrosion
in the
the corrosive
corrosive environments
environments which
which often
often leads
leads to
to the
the reduction
reduction of
of mechanical
mechanical properties
properties of
of the
the
in
material. Steel
very likely
likely to
to be
be
material.
Steel rock
rock bolts
bolts are
are particularly
particularly susceptible
susceptible to
to corrosion
corrosion since
since they
they are
are very
exposed
to
the
corrosive
underground
water.
The
rate
of
steel
rock
bolt
corrosion
is
dependent
on
exposed to the corrosive underground water. The rate of steel rock bolt corrosion is dependent on
the ground
ground water
water composition,
composition, flow
the
flow rate,
rate, water
water pH
pH value,
value, temperature,
temperature, humidity,
humidity, surface
surface condition,
condition,
presence
of
corrosion
inhibitors,
applied
stresses,
and
any
hydrogen
sulphide
concentrations
presence of corrosion inhibitors, applied stresses, and any hydrogen sulphide concentrations [51,52].
[51,52].
Stress corrosion
corrosion cracking
cracking (SCC)
(SCC) and
and pitting
pitting corrosion
corrosion of
of rock
rock bolts
bolts constituted
constituted the
the major
major cause
cause of
of
Stress
prematurefailure
failureofofrock
rock
bolts
in corrosive
environments,
where
SCC accounted
forof63%
of the
premature
bolts
in corrosive
environments,
where
SCC accounted
for 63%
the broken
broken
bolts
and
localized
pitting
corrosion
represented
30%
[53].
SCC
results
from
the
combined
bolts and localized pitting corrosion represented 30% [53]. SCC results from the combined effects
effects
high tensile
stress
and corrosive
environment
the susceptible
will in
result
of
highof
tensile
stress and
corrosive
environment
on the on
susceptible
metals,metals,
which which
will result
the
in the progressive
development
and growth
ofcracks
brittle on
cracks
on the surface
of the[54].
metal
[54]. Localized
progressive
development
and growth
of brittle
the surface
of the metal
Localized
pitting
pitting corrosion
is marked
by the development
of defined
sharplyholes,
defined
or ‘pits’,
which
result
from
corrosion
is marked
by the development
of sharply
or holes,
‘pits’, which
result
from
localized
localized
loss accelerated
by the of
presence
a small
a large[52].
cathode
Rock bolt
metal
lossmetal
accelerated
by the presence
a small of
anode
andanode
a largeand
cathode
Rock [52].
bolt corrosion
corrosion
reduces
the
load
bearing
capacity
and
life
expectancy
of
ground
support
systems,
reduces the load bearing capacity and life expectancy of ground support systems, and thereforeand
the
therefore theand
monitoring
and determination
of rock bolt
is an urgent task.
monitoring
determination
of rock bolt corrosion
is ancorrosion
urgent task.
Electrochemical-based corrosion
corrosion measuring
measuring methods,
methods, such
such as
as open
open circuit
circuit potential
potential (OCP)
(OCP) and
and
Electrochemical-based
electrochemical
impedance
spectroscopy,
are
the
traditional
ways
to
measure
corrosion
of
metals.
electrochemical impedance spectroscopy, are the traditional ways to measure corrosion of metals.
Spearing et
et al.
al. [55]
[55] used
to determine
potential of
of rock
Spearing
used the
the OCP
OCP measurement
measurement technique
technique to
determine the
the corrosion
corrosion potential
rock
bolts. As shown in Figure 16, the results showed that a more negative OCP value can indicate the
possibility of the presence of corrosion. However, such a method only provides qualitative results.
Sensors 2017, 17, 776
14 of 24
bolts. As shown in Figure 16, the results showed that a more negative OCP value can indicate the
Sensors 2017, 17,
776 presence of corrosion. However, such a method only provides qualitative results.
14 of 24
possibility
of the
Figure 16. Tefel plot for a test steel sample [55].
Figure 16. Tefel plot for a test steel sample [55].
In order
order to
to detect
detect SCC,
SCC, Craig
Craig et
et al.
al. [53]
[53] utilized
utilized the
the ultrasonic
ultrasonic crack detector on rock
rock bolts
bolts taken
taken
from aa mine
mine site.
site. A peak will appear in the received signals if there are any
any signal
signal reflections
reflections from
from
cracks or the
the end
end of
of the
the bolt.
bolt. The load tests indicated that the bolts with more severe SCC
SCC presented
presented
stronger reflection
reflection signals.
signals.
stronger
On the
On
thatthat
voluminous
corrosion
products
will exert
expansive
stress on the
surrounding
theaccount
account
voluminous
corrosion
products
will
exert expansive
stress
on the
rock
mass, Wei
et mass,
al. [56]
proposed
method toa monitor
rock
bolt corrosion
in an early
by
surrounding
rock
Wei
et al. [56]a proposed
method to
monitor
rock bolt corrosion
instage
an early
stage by measuring
the corrosion-induced
strain
using a fiber-optic
Michelson
interferometer.
Two
measuring
the corrosion-induced
strain using
a fiber-optic
Michelson
interferometer.
Two schemes
of
winding
on the
rock
werebolt
tested:
one
directly
fiber on
rock
schemes
of optical
windingfiber
optical
fiber
on bolt
the rock
were
tested:
onewound
directlythe
wound
thethe
fiber
onbolt
the
and
with one,
a mortar
placed
between
fiber and
rock
rockanother
bolt andone,
another
withcushion,
a mortarwas
cushion,
was
placedthe
between
the the
fiber
andbolt.
the The
rocklatter
bolt.
scheme
showed
more showed
uniform strain
underdevelopment
accelerated corrosion
as shown
The latter
scheme
more development
uniform strain
under experiments,
accelerated corrosion
in
Figure 17. as
The
test results
demonstrated
effectiveness
of thethe
proposed
sensing
technique
in
experiments,
shown
in Figure
17. The testthe
results
demonstrated
effectiveness
of the
proposed
sensing technique
in corrosion
monitoring
bolt stage.
corrosion in an early stage.
monitoring
rock bolt
in rock
an early
Figure 17.
17. The
The results
results from
from the
the sensor
sensor in
in R–C–O–C
R–C–O–C configuration.
configuration. (a)
(a) The
The uncompensated
uncompensated results
results
Figure
without
the
reference
fiber;
(b)
The
results
after
being
compensated
by
the
reference
sensing
without the reference fiber; (b) The results after being compensated by the reference sensing fiber.fiber.
[56].
[56].
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15
4.3. Grout Quality Testing
4.3. Grout Quality Testing
The grouted rock bolts, either concrete grouted or resin grouted, account for the majority of the
rockThe
bolts.
For grouted
rockeither
bolts, concrete
the groutgrouted
qualityor
determines
the anchorage
strength
of theof
bolts.
grouted
rock bolts,
resin grouted,
account for
the majority
the
The bolts.
assessment
and control
of thethe
grout
quality
is adetermines
challengingthe
issue.
The very
traditional
to
rock
For grouted
rock bolts,
grout
quality
anchorage
strength
of theway
bolts.
determine
the grout
qualityofisthe
by grout
the pull-out
the over-coring
test,
which
are considered
The
assessment
and control
qualitytest
is a and
challenging
issue. The
very
traditional
way to
destructivethe
and
time-consuming
[8,57].
Whentest
ultrasonic
are introduced
of
determine
grout
quality is by the
pull-out
and the waves
over-coring
test, whichin
arestructures
considered
slenderness and
suchtime-consuming
as rock bolts, guided
excited. The
useare
of guided
ultrasonic
waves for
destructive
[8,57].waves
Whenare
ultrasonic
waves
introduced
in structures
of
nondestructive
is emerging
as a new
researchThe
initiative
in rock ultrasonic
bolt testing
and for
has
slenderness
suchmonitoring
as rock bolts,
guided waves
are excited.
use of guided
waves
attracted substantial
research
interestasover
theresearch
last two
decades
Beard
and
[31]
nondestructive
monitoring
is emerging
a new
initiative
in [31,58–61].
rock bolt testing
and
hasLowe
attracted
pioneered the
application
the guided
waves
in the non-destructive
of rock bolts.
substantial
research
interestofover
the last ultrasonic
two decades
[31,58–61].
Beard and Lowetesting
[31] pioneered
the
Due to the dispersive
nature
of the ultrasonic
they systematically
selection
ofthe
the
application
of the guided
ultrasonic
waves inwaves,
the non-destructive
testingstudied
of rockthe
bolts.
Due to
suitable test
frequencies
and identified
that
thesystematically
fundamental longitudinal
L(0, 1) mode
more
dispersive
nature
of the ultrasonic
waves,
they
studied the selection
of thewas
suitable
suited
for rock and
boltidentified
testing. Later
on, fundamental
two researchlongitudinal
groups, oneL(0,
from
Dalhousie
University
test
frequencies
that the
1) mode
was more
suited and
for
another
fromLater
Korea
were
actively
to this research
fieldand
in recent
years.
rock
bolt one
testing.
on,University,
two research
groups,
onedevoted
from Dalhousie
University
another
one from
researchwere
group
at Dalhousie
University
successively
the effects of grout strength
KoreaThe
University,
actively
devoted
to this research
field inexamined
recent years.
[58,60],
groutedgroup
length [62],
and missing
grout [61]successively
on the behavior
of the ultrasonic
guided
Thethe
research
at Dalhousie
University
examined
the effects
of waves
grout
in rock bolts.
The
of the
guided
ultrasonic
waves
rock bolt
testing
consisted
of a
strength
[58,60],
theinstrumentation
grouted length [62],
and
missing
grout [61]
on thefor
behavior
of the
ultrasonic
guided
function
generator
andinstrumentation
data acquisition
piezoelectric
transducers
asbolt
the testing
transmitter
and
waves
in rock
bolts. The
of device;
the guided
ultrasonic waves
for rock
consisted
receiver;
a signal
amplifier
and acquisition
personal computer.
An exampletransducers
of the equipment
setup for
of
a function
generator
and data
device; piezoelectric
as the transmitter
transmission-through
shown in
Figure
18a. In
order to avoid
the complexity
of multiplesetup
modesfor
at
and
receiver; a signal is
amplifier
and
personal
computer.
An example
of the equipment
higher frequencies and
simplify
data interpretation,
L(0, the
1) mode
of low of
frequencies
less than
transmission-through
is to
shown
in Figure
18a. In order tothe
avoid
complexity
multiple modes
at
100 kHz
were usually
The
group
velocity andthe
attenuation
of the
guided
ultrasonic
higher
frequencies
and selected.
to simplify
data
interpretation,
L(0, 1) mode
of low
frequencies
lesswaves
than
were
the
most
important
characteristics
to describe
the grout of
quality.
Theyultrasonic
have concluded
that
100
kHz
were
usually
selected.
The group velocity
and attenuation
the guided
waves were
higher
quality,
either longer
curing time
of thequality.
grout or
higher
strength
the
the
mostgrout
important
characteristics
to describe
the grout
They
havecompressive
concluded that
higher of
grout
grout, resulted
in lower
group
in grouted
bolts
and higher
attenuation
the waves.
quality,
either longer
curing
timevelocity
of the grout
or higher
compressive
strength
of theof
grout,
resultedThe
in
representative
results
are
shown
in
Figure
19.
With
the
obtained
information
on
the
attenuation
and
lower group velocity in grouted bolts and higher attenuation of the waves. The representative results
group
velocity
of the19.
guided
waves,
grout quality
the defective
bolts
can thus
be
are
shown
in Figure
With ultrasonic
the obtained
information
on theand
attenuation
and rock
group
velocity
of the
detected.
guided
ultrasonic waves, grout quality and the defective rock bolts can thus be detected.
Thetransmission-through
transmission-throughsetup
setupwas
wasgenerally
generallynot
notpractical
practicalin
inthe
thefield
fieldsince
sinceonly
onlyone
oneend
endof
ofthe
the
The
boltisisexposed
exposedfor
fortesting.
testing.
Therefore,
they
conducted
similar
using
the transmission-echo-offbolt
Therefore,
they
conducted
similar
teststests
using
the transmission-echo-off-path
path setup,
as shown
in Figure
18bThe
[63].
Thesetup
new was
setup
was shown
to be
to receive
meaningful
setup,
as shown
in Figure
18b [63].
new
shown
to be able
to able
receive
meaningful
results
results
to determine
the attenuation
andvelocity.
group velocity.
to
determine
the attenuation
and group
(a)
(b)
Figure 18. Equipment setup for the rock bolt test: (a) transmission-through setup [62], (b)
Figure 18.
Equipment setup for the rock bolt test: (a) transmission-through setup [62];
transmission-echo-off-path
setup
[63].
(b) transmission-echo-off-path
setup
[63].
The research group at Korea University also successively conducted several investigations on
The research group at Korea University also successively conducted several investigations on
the non-destructive evaluation of rock bolt integrity using guided ultrasonic waves, particularly on
the non-destructive evaluation of rock bolt integrity using guided ultrasonic waves, particularly on
the non-grouted ratio or defect ratio of the rock bolt [64–67]. A typical experimental setup of the rock
the non-grouted ratio or defect ratio of the rock bolt [64–67]. A typical experimental setup of the
bolt integrity test is shown in Figure 20. Experiments with different defect ratios for grouted non-
Sensors 2017, 17, 776
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rock Sensors
bolt integrity
2017, 17, 776test is shown in Figure 20. Experiments with different defect ratios for16grouted
of 24
non-embedded
rock
were
Sensors 2017, 17,
776 bolts, and for rock bolts embedded into a concrete block and rock mass
16 of 24
embedded
rock
bolts,
and
for
rock
bolts
embedded
into
a
concrete
block
and
rock
mass
were
carried
carried out. They analyzed the signals in the frequency domain using the Fourier transform and in
embedded
rockdomain
bolts,
for rock
embedded
into
a concrete
block and
rock mass
carried
out.
They analyzed
the and
signals
inwavelet
thebolts
frequency
domain
using
Fourier
transform
andwere
in
the
timethe time-frequency
using
transform
based
onthe
a Gabor
wavelet.
From
the
frequency
out. Theydomain
analyzed
the signals
in transform
the frequency
domain
using the
Fourier
transform
and in the
timefrequency
using
wavelet
based
on
a
Gabor
wavelet.
From
the
frequency
domain
domain analysis, they found that a portion of high frequency content increases with the increase of the
frequency
domain
based oncontent
a Gabor
wavelet.with
From
frequency
domain
analysis,
they
foundusing
that awavelet
portion transform
of high frequency
increases
thethe
increase
of the
defect
defect ratio. The group velocity and phase velocity of the waves were calculated from the peak values
analysis,
they
found
that
a
portion
of
high
frequency
content
increases
with
the
increase
of
the
defect
ratio. The group velocity and phase velocity of the waves were calculated from the peak values in the
in the ratio.
time-frequency
domain
analysis.
Similar
to
findings
from the from
researchpeak
group
at Dalhousie
The groupdomain
velocity
and
phaseSimilar
velocity
thethe
waves
were
calculated
values
in the
time-frequency
analysis.
toofthe
findings
from
the researchthe
group at
Dalhousie
University,
this
research
group
concluded
that
the
velocities
of
the
guided
ultrasonic
waves
increased
time-frequency
domain
analysis.
Similar
to
the
findings
from
the
research
group
at
Dalhousie
University, this research group concluded that the velocities of the guided ultrasonic waves increased
withwith
the
increase
in research
the
defect
ratio
ininallallthree
conditions,
namely
non-embedded
rock
bolts,
University,
this
group
concluded
that
velocities of
the guided
ultrasonic waves
increased
the increase
in the
defect
ratio
threethe
conditions,
namely
non-embedded
rock
bolts,
rockrock
boltsbolts
embedded
in concrete
columns,
and
rock
bolts
embedded
in
a
rock
mass.
The
representative
withembedded
the increase
in
the
defect
ratio
in
all
three
conditions,
namely
non-embedded
rock
bolts,
rock
in concrete columns, and rock bolts embedded in a rock mass. The representative
bolts
embedded
in
concrete
results
are shown
in Figure
21.21.columns, and rock bolts embedded in a rock mass. The representative
results
are shown
in Figure
results are shown in Figure 21.
(a)
(a)
(b)
(b)
Figure 19. (a) Variation of amplitude ratio with grout compressive strength at different wave
FigureFigure
19. (a)
of of
amplitude
compressivestrength
strength
different
wave
19. Variation
(a) Variation
amplituderatio
ratiowith
with grout
grout compressive
at at
different
wave
frequencies; (b) Variation of average group velocity with grout compressive strength [60].
frequencies;
(b) Variation
of average
group
velocitywith
withgrout
grout compressive
compressive strength
[60].
frequencies;
(b) Variation
of average
group
velocity
strength
[60].
Figure
20.20.
Schematic
diagram
bolt
integritysystem
system
[66].
Figure
20.Schematic
Schematic
diagramof
ofthe
therock
rock bolt
bolt integrity
[66].
Figure
diagram
of
the
rock
[66].
Thereflection
reflection
method,
whichboth
boththe
thepiezoelectric
piezoelectric transmitter
transmitter
andand
receiver
were
installed
at
method,
which
both
the
receiver
werewere
installed
at
The The
reflection
method,
ininin
which
piezoelectric
transmitter
receiver
installed
thefree
freeend
endofofthe
thebolt,
bolt,was
wasalso
alsoexamined
examined for
for practical
practical application
application [67]. It is worth
the
worth noting
noting that
that for
for
at the free end of the bolt, was also examined for practical application [67]. It is worth noting that
thecase
caseofofrock
rockbolts
boltsembedded
embeddedin
inaarock
rockmass,
mass,the
thepiezoelectric
piezoelectric transducer could
the
could not
not generate
generatestrong
strong
for the case of rock bolts embedded in a rock mass, the piezoelectric transducer could not generate
guidedwaves.
waves.The
Thehammer
hammerimpact
impact with
with aa center
center punch
punch method
method was adopted
guided
adopted to
to produce
produce strong
strong
strongreflected
guided waves.
The hammer
impact
with a center
method wasmethod
adopted to
produce strong
guidedwaves.
waves.
Therefore,
suggested
that punch
the hammer
hammer
reflected guided
Therefore,
ititisissuggested
that
the
impact method is
is more
more practical
practical
reflected
guidedthe
waves.
Therefore,
itfield
is suggested
that the hammer impact method is more practical to
generate
the
guided
wavesininfield
tests.
totogenerate
guided
waves
tests.
generate the guided waves in field tests.
Sensors
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17 17
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Figure 21. Velocity of guided ultrasonic waves versus the defect ratio (DR) for non-embedded rock
Figure
Figure21.
21. Velocity
Velocityof
ofguided
guidedultrasonic
ultrasonicwaves
waves versus
versus the
thedefect
defect ratio
ratio (DR)
(DR) for
fornon-embedded
non-embedded rock
rock
bolts, rock bolts embedded in concrete and rock bolts embedded in rock [65]. Note: Velocity of free
bolts,
inin
concrete
and
rock
bolts
embedded
in rock
[65].[65].
Note:Note:
Velocity
of freeofsteel
bolts,rock
rockbolts
boltsembedded
embedded
concrete
and
rock
bolts
embedded
in rock
Velocity
free
steel bar VS = 4955 m/s; velocity of cement mortar VM = 3336 m/s; velocity of concrete block VC = 3290
bar
V
=
4955
m/s;
velocity
of
cement
mortar
V
=
3336
m/s;
velocity
of
concrete
block
V
=
3290
m/s;
S
M
C
steel bar VS = 4955 m/s; velocity of cement mortar VM = 3336 m/s; velocity of concrete block VC = 3290
m/s; velocity of rock mass VR 2700 m/s.
velocity
of rock
VR ≥ V2700
m/s.m/s.
m/s; velocity
ofmass
rock mass
R 2700
4.4. Delamination Monitoring for Grouted Rock Bolts
4.4.
4.4. Delamination
Delamination Monitoring
Monitoring for
for Grouted
GroutedRock
RockBolts
Bolts
Apart from the grout quality monitoring and control, the amount of delamination between the
Apart
the grout
grout quality
qualitymonitoring
monitoringand
andcontrol,
control,the
the
amount
delamination
between
Apart from
from the
amount
of of
delamination
between
the
rock bolt and the grout is critical in evaluating the anchorage strength of the rock bolt system. The
the
grout
is critical
in evaluating
the anchorage
strength
of rock
the rock
system.
rockrock
boltbolt
andand
the the
grout
is critical
in evaluating
the anchorage
strength
of the
bolt bolt
system.
The
delamination between the bolt and the grout means a weak bonding condition. He et al. [68]
The
delamination
between
groutmeans
meansa aweak
weakbonding
bonding condition.
condition. He
He et
et al.
delamination
between
the the
boltbolt
andand
thethe
grout
al. [68]
[68]
investigated the propagation of guided ultrasonic waves in a rock bolt system theoretically and
investigated
investigated the
the propagation
propagation of
of guided
guided ultrasonic
ultrasonic waves
waves in
in aa rock
rock bolt
bolt system
system theoretically
theoretically and
and
experimentally. First off, they calculated the high-frequency theoretical wave structures for a steel
experimentally.
experimentally. First
First off,
off, they
they calculated
calculated the
the high-frequency
high-frequency theoretical
theoretical wave
wave structures
structures for
for aa steel
steel
rock embedded in concrete using the Semi-Analytical Finite Element Method and identified that low
rock
embedded
in
concrete
using
the
Semi-Analytical
Finite
Element
Method
and
identified
that
rock embedded in concrete using the Semi-Analytical Finite Element Method and identified that low
low
frequency waves were suitable for length measurement and delamination detection since these
frequency
suitable
for for
length
measurement
and delamination
detection
since these
frequencywaves
waveswere
were
suitable
length
measurement
and delamination
detection
sincewaves
these
waves were susceptible to leakage at the interface. In the experimental studies, six specimens with
were
susceptible
to leakage
at the interface.
In the experimental
studies, six
specimens
with different
waves
were susceptible
to leakage
at the interface.
In the experimental
studies,
six specimens
with
different delamination lengths, such as 0%, 25%, 33%, 50%, 70% and 100%, were prepared. They
delamination
lengths,
such
as
0%,
25%,
33%,
50%,
70%
and
100%,
were
prepared.
They
adopted
different delamination lengths, such as 0%, 25%, 33%, 50%, 70% and 100%, were prepared.
They
adopted a pulse-echo arrangement using the tone-burst system with piezoelectric transducers. Both
aadopted
pulse-echo
arrangement
using theusing
tone-burst
system system
with piezoelectric
transducers.
Both high
a pulse-echo
arrangement
the tone-burst
with piezoelectric
transducers.
Both
high frequency modes (above 2 MHz) and low frequency modes (below 1.6 MHz) were used to excite
frequency
modes
(above
2
MHz)
and
low
frequency
modes
(below
1.6
MHz)
were
used
to
excite
the
high frequency modes (above 2 MHz) and low frequency modes (below 1.6 MHz) were used to excite
the guided waves. Results showed that the high frequency modes were not sensitive to the presence
guided
waves.
Results
showed
thatthat
the the
highhigh
frequency
modes
were
notnot
sensitive
to the
presence
of
the guided
waves.
Results
showed
frequency
modes
were
sensitive
to the
presence
of delamination. However, the low frequency modes were more sensitive to the delamination
delamination.
However,
the low
modesmodes
were more
to the delamination
damage,
of delamination.
However,
the frequency
low frequency
weresensitive
more sensitive
to the delamination
damage, as shown in Figure 22. A gain of approximately 6 dB correlated to one-third delamination.
as
shown as
in shown
Figure 22.
A gain22.
of A
approximately
6 dB correlated
to one-third
delamination.
damage,
in Figure
gain of approximately
6 dB correlated
to one-third
delamination.
(a)
(a)
(b)
(b)
Figure
22. (a)
Typical
waveformsatatlow
low frequency
modes:
MHz excitation;
1.17excitation.
MHz
Figure
Typical
waveforms
modes:
0.97 0.97
MHz
(b) 1.17(b)
MHz
Figure22.
22.(a)(a)
Typical
waveforms at frequency
low frequency
modes:
0.97excitation;
MHz excitation;
(b) 1.17
MHz
excitation.
Pink
for
no
delamination
and
blue
for
one-third
delamination
[68].
Pink for no delamination and blue for one-third delamination [68].
excitation. Pink for no delamination and blue for one-third delamination [68].
The relationship between attenuation and the delamination percentage is shown in Figure 23. It
is clear that the wave amplitude increases with the increase of delamination percentage. The fitted
slopes suggested that low frequencies are more sensitive to delaminated damage of the grouted rock
bolts. The study concluded that guided waves with low frequency modes were efficient for
Sensors 2017, 17, 776
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estimating the amount of delamination between the rock bolt and the grout.
Interfacial debonding and delamination issues are also encountered in many other engineering
structures
such as composites,
and concrete-encased
steel structures.
Zeng is
et shown
al. [69] in
showed
The relationship
between attenuation
and the delamination
percentage
Figurethat
23. Itthe
is
activethat
sensing
approach
based
on thewith
shear
ofofpiezoelectric
effective
in
clear
the wave
amplitude
increases
the mode
increase
delaminationtransducers
percentage. was
The fitted
slopes
detecting the
debonding
between
concrete
and steel damage
in a concrete-encased
suggested
thatinterfacial
low frequencies
are more
sensitive
to delaminated
of the grouted composite
rock bolts.
structure.
It
is
believed
that
the
active
sensing
method
using
shear
waves
willfor
beestimating
promisingthe
in
The study concluded that guided waves with low frequency modes were efficient
detectingofthe
delamination
of grouted
rockbolt
bolts.
amount
delamination
between
the rock
and the grout.
Figure 23. The attenuation versus the delamination percentage [68].
4.5. Rock
Bolt Devices
with NDT
Interfacial
debonding
and Capability
delamination issues are also encountered in many other engineering
structures
such
composites,
and concrete-encased
steelthe
structures.
Zeng etthat
al. [69]
that
Several
rockasbolt
testing system
were developed over
last two decades
wereshowed
specialized
the
active
sensing
approach
based
on
the
shear
mode
of
piezoelectric
transducers
was
effective
for non-destructive testing of rock bolts. Some of them are commercially available and some are
in
detecting prototyping
the interfacial
debonding
undergoing
and
testing. between concrete and steel in a concrete-encased composite
structure.
It is believedconcept
that the active
sensing method
using shearin
waves
will beThe
promising
in detecting
The Boltometer
was proposed
by Geodynamik
Sweden.
Boltometer
uses a
the
delamination
of
grouted
rock
bolts.
piezoelectric transducer to transmit compressional and quasi-flexural waves into the rock bolt and
receives the echoes reflected from the discontinuities in the bolt and the grout [5]. The Boltometer is
4.5. Rock Bolt Devices with NDT Capability
able to indicate bad grout that generates a distinct echo. If the grout quality is good, the energy will
Several rock
system
werelittle
developed
over the last two decades that were specialized
be dissipated
intobolt
the testing
rock mass,
leaving
or no echoes.
for non-destructive
testing ofintegrity
rock bolts.
Some
of them are
commercially
available
some are
The ground anchorage
testing
(GRANIT)
system
was developed
at and
University
of
undergoing
and
testing.
Aberdeen inprototyping
Scotland [70].
The
system induces low frequency vibrations to the rock bolt by an impact
The
Boltometer
concept
wassignals
proposed
byaccelerometer.
Geodynamik The
in Sweden.
Thesignals
Boltometer
uses a
device
and
receives the
vibration
by an
acceleration
are complex
piezoelectric
transducer
to
transmit
compressional
and
quasi-flexural
waves
into
the
rock
bolt
and
by nature. Therefore, neural networks were used to interpret the relationship between bolt condition
receives
echoessignals.
reflected
from
thefor
discontinuities
in the
bolt andtothe
grout [5].existing
The Boltometer
is
and the the
response
The
need
training neural
networks
recognize
conditions
able
to indicate
grout
that generates
distinct for
echo.
If the
grout
is good,
energy
will
be
means
that the bad
system
cannot
be easilyaapplied
new
rock
boltquality
conditions
thatthe
have
never
been
dissipated
intobefore.
the rock mass, leaving little or no echoes.
characterized
The Smart
ground
anchorage
integrity testing
was developed
University
of
Rockbolt
was proposed
at Lulea(GRANIT)
Universitysystem
of Technology
in Swedenat[71].
The Smart
Aberdeen
Scotland rock
[70].bolt
The
system with
induces
low sensor,
frequency
vibrations processing
to the rockmodule
bolt byand
an
Rockbolt isin
a standard
equipped
a strain
accelerometer,
impact
and receives
the vibration
an accelerometer.
The in
acceleration
are
wirelessdevice
communication
module.
A photosignals
of the by
Smart
Rockbolt is shown
Figure 24. signals
Integrated
complex
nature.of
Therefore,
neuraltechnology,
networks were
interpret the
relationship
between bolt
with the by
Internet
Things (IoT)
the used
SmarttoRockbolt
is able
to continuously
and
condition
and the
response
Thevibration
need for of
training
neural
to Rockbolt
recognize isexisting
simultaneously
monitor
the signals.
strain and
the rock
bolt.networks
The Smart
under
conditions
means
cannot available.
be easily applied for new rock bolt conditions that have never
development
andthat
not the
yet system
commercially
been characterized before.
The Smart Rockbolt was proposed at Lulea University of Technology in Sweden [71]. The Smart
Rockbolt is a standard rock bolt equipped with a strain sensor, accelerometer, processing module and
wireless communication module. A photo of the Smart Rockbolt is shown in Figure 24. Integrated with
the Internet of Things (IoT) technology, the Smart Rockbolt is able to continuously and simultaneously
monitor the strain and vibration of the rock bolt. The Smart Rockbolt is under development and not
yet commercially available.
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Figure 24. Photo of the Smart Rockbolt [72].
Figure 24. Photo of the Smart Rockbolt [72].
Figure 24. Photo of the Smart Rockbolt [72].
A group at AGH University of Science and Technology in Poland devised a new Rock Bolt Tester
Figure 24. Photo of the Smart Rockbolt [72].
(RBT)
rock bolt testing using
guided ultrasonic
waves [73].in
The
RBT is adevised
portable instrument
thatBolt Tester
A group
atfor
AGH
Technology
Poland
new
Rock
A group
at
AGHUniversity
Universityof
ofScience
Science and
and Technology
in Poland
devised a anew
Rock
Bolt Tester
consists of specially designed analog electronics for generating and receiving the guided waves and
(RBT)
for
rock
bolt
testing
using
guided
ultrasonic
waves
[73].
The
RBT
is
a
portable
instrument
(RBT)
rock
bolt
testing
using
guided
ultrasonic
waves
[73].
The
RBT
is
a
portable
instrument
that
A for
group
at
AGH
University
of
Science
and
Technology
in
Poland
devised
a
new
Rock
Bolt
an embedded digital computer for signal processing, operator communication and data storage. The Tester
that
consists
of
specially
designed
analog
electronics
foringenerating
and
receiving
the
guided
waves
consists
of
specially
designed
analog
electronics
generating
and
receiving
the guided
wavesthat
and
the
block
diagram
of
the
RBT arefor
shown
FigureThe
25.
The
RBT
used
to determine
(RBT)
forphotograph
rock
bolt and
testing
using
guided
ultrasonic
waves
[73].
RBT
is awas
portable
instrument
the
rock
bolt
condition,
especially
the
grouting
condition.
and
an
embedded
digital
computer
for
signal
processing,
operator
communication
and
data
storage.
an
embedded
digital
computer
for
signal
processing,
operator
communication
and
data
storage.
The
consists of specially designed analog electronics for generating and receiving the guided waves and
photograph
the block
diagram
of the of
RBT
areRBT
shown
Figure
25.Figure
The RBT
used
to was
determine
The
photograph
and
the
block
diagram
the
arein
shown
in
25.was
The
RBT
used
an
embeddedand
digital
computer
for signal
processing,
operator
communication
and
data
storage.
The to
the rock the
boltand
condition,
especially
the
grouting
photograph
the
block
diagramespecially
of the
RBTthe
arecondition.
shown in condition.
Figure 25. The RBT was used to determine
determine
rock
bolt
condition,
grouting
the rock bolt condition, especially the grouting condition.
Figure 25. The portable, battery-supplied Rock Bolt Tester (RBT) instrument (left), and its simplified
block diagram (right) [73].
More recently, Wang et al. proposed the Smart Washer for looseness of the rock bolt based on
the electro-mechanical impedance method [74]. As shown in Figure 26, the Smart Washer was
Figure
The
battery-supplied
Rock
Bolt Tester
instrument
(left),
and
itsits
simplified
Figure
25.25.
The
portable,
battery-supplied
Rock
Bolt
Tester
(RBT)
instrument
(left),
and
simplified
fabricated
byportable,
sandwiching
a piezoceramic
transducer
into (RBT)
two metal
rings. The
Smart
Washer
was
block
diagram
(right)
[73].
Figure
25.toThe
portable,
battery-supplied
Rock
Tester
(RBT)
instrument
(left), and its
simplified
used
monitor
the
looseness
of the rock
boltBolt
based
on the
variations
of the impedance
signatures.
block
diagram
(right)
[73].
The
normalized
block
diagram
(right)root
[73].mean square deviation (RMSD) index was developed to evaluate the
degradation
of the
bolt pre-stress.
27a
shows the
Figure
More
recently,level
Wang
etrock
al. proposed
theFigure
Smart
Washer
forexperimental
looseness setup
of theand
rock
bolt27b
based on
Moreshows
recently,
Wang
et al.
proposedtothe
Smart
Washer
for looseness
of the
rock
bolt based
the
RMSD
indices
corresponding
different
loading
conditions.
As
can
be
seen,
the
RMSD
the electro-mechanical
impedance
method
AsWasher
shownfor
in looseness
Figure 26,ofthe
Washer
More recently, Wang
et al. proposed
the[74].
Smart
the Smart
rock bolt
basedwas
on
on the electro-mechanical
impedance
methodlevel
[74].
Asrock
shown
indices can effectively
indicate the looseness
of the
bolt. in Figure 26, the Smart Washer was
fabricated
by sandwiching
a piezoceramic
into two
TheSmart
SmartWasher
Washer was
was
the
electro-mechanical
impedance
methodtransducer
[74]. As shown
in metal
Figurerings.
26, the
fabricated
by sandwiching
a piezoceramic
transducer
into
two
metal of
rings.
The Smartsignatures.
Washer was
used to monitor
the looseness
of the rock bolt
based on
thetwo
variations
theThe
impedance
fabricated
by sandwiching
a piezoceramic
transducer
into
metal rings.
Smart Washer was
used
monitor
looseness
the
bolt based
based
onthe
theindex
variations
ofthe
the
impedance
signatures.
Thetoto
normalized
root
mean of
square
deviation
(RMSD)
was ofdeveloped
to evaluate
the
used
monitorthe
the
looseness
of
the rock
rock
bolt
on
variations
impedance
signatures.
The
normalized
root
mean
square
deviation
(RMSD)
index
was
developed
to
evaluate
the
degradation
degradation
level
of
the
rock
bolt
pre-stress.
Figure
27a
shows
the
experimental
setup
and
Figure
27b
The normalized root mean square deviation (RMSD) index was developed to evaluate the
level
of the
bolt
pre-stress.
Figure
27atoshows
the
experimental
setup
Figure
shows
therock
RMSD
indices
corresponding
different
conditions.
Asand
can
be seen,
theshows
RMSD
degradation
level
of
the rock
bolt
pre-stress.
Figure
27aloading
shows the
experimental
setup
and27b
Figure
27bthe
RMSD
indices
corresponding
to
different
loading
conditions.
As
can
be
seen,
the
RMSD
indices
indicesthe
canRMSD
effectively
indicate
the looseness
level of loading
the rock conditions.
bolt.
shows
indices
corresponding
to different
As can be seen, the RMSDcan
effectively
indicate
the looseness
level
of the rock
indices can
effectively
indicate the
looseness
level bolt.
of the rock bolt.
(a)
(b)
Figure 26. (a) The design of the smart washer; (b) Photograph of the smart washer [74].
(a)
(b)
(a)
(b)
Figure 26. (a) The design of the smart washer; (b) Photograph of the smart washer [74].
Figure 26. (a) The design of the smart washer; (b) Photograph of the smart washer [74].
Figure 26. (a) The design of the smart washer; (b) Photograph of the smart washer [74].
Sensors 2017, 17, 776
20 of 24
20 of 24
Pre-tension looseness index
Sensors 2017, 17, 776
te
Pre-
(a)
nsio
n (M
pa )
(b)
Figure27.
27.(a)
(a)The
Theexperimental
experimentalsetup;
setup;(b)
(b)the
thenormalized
normalizedRMSD-based
RMSD-basedrock
rockbolt
boltlooseness
loosenessindices
indicesof
of
Figure
three
repeating
experiments
[74].
three repeating experiments [74].
5. Future Trends
5. Future Trends
From the above mentioned literature, rock bolt technology has taken a direction towards
From the above mentioned literature, rock bolt technology has taken a direction towards
instrumentation with sensors. While rock bolts have been shown to greatly help stabilize rock
instrumentation with sensors. While rock bolts have been shown to greatly help stabilize rock
formations and increase safety, knowledge of the bolt condition will be essential for the continued
formations and increase safety, knowledge of the bolt condition will be essential for the continued
function of rock bolt systems over time. Depending on the parameter to be monitored, different types
function of rock bolt systems over time. Depending on the parameter to be monitored, different types
of sensors are used and installed at different locations of the rock bolt system. The interfaces in the
of sensors are used and installed at different locations of the rock bolt system. The interfaces in the
rock bolt system and the loads experienced by the bolt are key areas that need monitoring.
rock bolt system and the loads experienced by the bolt are key areas that need monitoring. Specifically,
Specifically, the fiber optic sensors, and shear mode smart aggregates can be applied to assess the
the fiber optic sensors, and shear mode smart aggregates can be applied to assess the interface condition
interface condition between the rock bolt and the grout, and the interface condition between the grout
between the rock bolt and the grout, and the interface condition between the grout and the rock mass.
and the rock mass. The piezoelectric-based active sensing technique can also be used to monitor the
The piezoelectric-based active sensing technique can also be used to monitor the bolt load, and bolt
bolt load, and bolt looseness [75,76]. Thus, the trend observed from the body of literature is that fiber
looseness [75,76]. Thus, the trend observed from the body of literature is that fiber optic sensors and
optic sensors and piezoelectric transducers have taken a front seat in the innovation of rock bolt
piezoelectric transducers have taken a front seat in the innovation of rock bolt monitoring systems.
monitoring systems.
A challenge in implementing rock bolt monitoring systems is robustness against the typically
A challenge in implementing rock bolt monitoring systems is robustness against the typically
harsh and remote environment. With the cost of electronics becoming cheaper, different types of
harsh and remote environment. With the cost of electronics becoming cheaper, different types of
sensors, sensing interrogation systems, and communication systems will become more readily available
sensors, sensing interrogation systems, and communication systems will become more readily
for rock bolt monitoring and will overcome more and more of the challenge. Fiber optic sensors are
available for rock bolt monitoring and will overcome more and more of the challenge. Fiber optic
useful in monitoring within areas with high electromagnetic interference and high temperatures.
sensors are useful in monitoring within areas with high electromagnetic interference and high
On the other hand, piezoelectric transducers can provide critical information that may not be gathered
temperatures. On the other hand, piezoelectric transducers can provide critical information that may
by fiber optic sensors. However, piezoelectric transducers require proper protection and obtaining
not be gathered by fiber optic sensors. However, piezoelectric transducers require proper protection
signals from sensors can be problematic due to cabling issues. In the future, the introduction of radio
and obtaining signals from sensors can be problematic due to cabling issues. In the future, the
frequency identification (RFID) may be used to obtain the overall information of the rock bolts installed
introduction of radio frequency identification (RFID) may be used to obtain the overall information
in critical locations without the need for excessive cabling.
of the rock bolts installed in critical locations without the need for excessive cabling.
6. Conclusions
6. Conclusions
In this review paper, the basic principles of commonly used smart sensors used in rock bolt
In thisare
review
paper,
the basic
principles
of commonly
used
smart sensors
used in rock
bolt
monitoring
briefly
reviewed,
followed
by a description
of the
state-of-the-art
applications
of these
monitoring
reviewed,
followed
by a description
the state-of-the-art
applications
of these
smart
sensorsare
in briefly
monitoring
the health
condition
of rock bolts.ofThese
smart sensors are
the piezoceramic
smart
sensors
in
monitoring
the
health
condition
of
rock
bolts.
These
smart
sensors
are the
sensors and fiber optic sensors. Several aspects of rock bolt monitoring using these smart sensors
piezoceramic
sensors
and
fiber
optic
sensors.
Several
aspects
of
rock
bolt
monitoring
using
are critically reviewed, including axial load monitoring, corrosion monitoring, grout monitoringthese
and
smart
sensors
are
critically
reviewed,
including
axial
load
monitoring,
corrosion
monitoring,
grout
delamination monitoring. Lastly, the smart rock bolt concept was introduced in rock bolt monitoring
monitoring
and delamination
monitoring.
Lastly, the
rock bolt
wasof
introduced
in rock
and
several smart
rock bolt devices
were reviewed.
In smart
conclusion,
the concept
application
smart sensors
in
bolt bolt
monitoring
and is
several
smart
rock bolt
were
reviewed.
In conclusion,
theinapplication
of
rock
monitoring
becoming
mature
anddevices
they have
replaced
conventional
sensors
various real
smart
sensors
in
rock
bolt
monitoring
is
becoming
mature
and
they
have
replaced
conventional
world applications. With the capability of smart sensors, in-line and real-time condition monitoring of
sensors
various
real world
applications.
With
the
capability
smart sensors,
in-line and real-time
rock
boltsincan
be achieved,
which
will provide
more
critical
safetyofinformation
on underground
mining.
condition monitoring of rock bolts can be achieved, which will provide more critical safety
information on underground mining.
Sensors 2017, 17, 776
21 of 24
Acknowledgments: This work was partially supported by the Major State Basic Research Development
Program of China (973 Program, grant number 2015CB057704), National Natural Science Foundation of China
(Nos. 51378434, 51578456) and China Scholarship Council (No. 201306060086).
Author Contributions: G.S. developed the concept, and made critical revision to the paper; W.L. collected the
literatures and wrote the paper; B.W. collected the literatures and drew some of the figures; S.H. helped write
the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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