applied
sciences
Article
The Experimental Study of the Temperature Effect on
the Interfacial Properties of Fully Grouted Rock Bolt
Fuhai Li 1,2,† , Xiaojuan Quan 3,† , Yi Jia 1 , Bo Wang 3, *, Guibin Zhang 1 and Siyin Chen 1
1
2
3
*
†
Department of Building Materials, School of Civil Engineering, Southwest Jiaotong University,
Chengdu 610031, China; [email protected] (F.L.); [email protected] (Y.J.);
[email protected] (G.Z.); [email protected] (S.C.)
Key Laboratory of High-speed Railway Engineering, Ministry of Education, Southwest Jiaotong University,
Chengdu 610031, China
Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, Southwest Jiaotong University,
Chengdu 610031, China; [email protected]
Correspondence: [email protected]; Tel.: +86-139-0817-7290
These authors contribute equally to this paper.
Academic Editor: Alkiviadis Paipetis
Received: 9 January 2017; Accepted: 24 March 2017; Published: 27 March 2017
Abstract: This study analyzes the phenomenon of performance deterioration in fully grouted rock
bolts in tunnels with a dry, hot environment and high geothermal activity with a focus on temperature
effects on interfacial bond performance. Three groups of fully grouted rock bolt specimens were
designed based on similar mechanical principles. They were produced and maintained at 20 ◦ C,
35 ◦ C, and 50 ◦ C. Through the indoor gradual loading tensile test of specimens, variations of
axial force and shear stress between the rock bolt and mortar adhesive interface were obtained
under different environmental temperatures. Distribution of the axial force and shear stress on the
anchorage section were found under different tensile forces. Results showed that, with an increase in
specimen environmental temperature, maximum shear stress of the rock bolt section became smaller,
while shear stress distribution along the rock bolt segment became more uniform. In addition,
the axial force value at the same position along the pull end was greater, while axial stress along the
anchorage’s length decayed faster. With an increase in tensile force under different temperatures,
the axial force and maximum shear stress of rock bolt specimens along the anchorage section has
a corresponding increase.
Keywords: tunneling engineering; fully grouted rock bolt; interfacial bond strength; shear stress;
axial force distribution
1. Introduction
As an important support method in tunnel engineering, rock bolt supports are widely used in
design and construction. When rock bolts are applied in a geothermally active tunnel, a series of
physical and chemical reactions can arise affecting the anchorage system. These reactions may lead
to bond behavior degradation and structural damage to the anchorage body, thus compromising the
durability and safety of the entire tunnel structure. Therefore, there is a need to study the effects of
temperature on bond behavior of the anchorage system’s interface. Anchorage performance in the
system mainly depends on interfacial bond strength between the rock bolt and grouting body, and on
the strength of the rock bolt rod body. In this context, the complex nature of interfacial bond strength
is therefore the focus of this study of the anchoring system.
In the exploration of stress characteristics of the rock bolt’s interface under the action of the
tensile load, it is generally accepted at present that frictional resistance of the anchoring section is not
Appl. Sci. 2017, 7, 327; doi:10.3390/app7040327
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Appl. Sci. 2017, 7, 327
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evenly distributed along the length of the rock bolt. Moreover, maximum shear stress appears near the
orifice. Frictional resistance appears to have nonlinearly decreasing distribution along the rock bolt rod
body [1]. However, there is no unified conclusion about the specific distribution of shear stress. In 1970,
Philips [2] proposed a formula for the distribution of friction resistance along the length of a rock
bolt according to the power function. According to the formula, frictional resistance at the external
end of the anchorage body was the maximum frictional resistance. After many experiments, it was
found that this feature did not agree with field measurements. Based on the displacement solution of
Mindlin, supposing that the adhesive material and that the rock and soil mass were the same kind
of elastic material, You [3] derived an elastic solution for shear stress distribution of an anchorage
body. Zhang and Tang [4] established a hyperbolic function model for the load transfer of bolts and
derived a calculation formula for the distribution of frictional resistance and shear displacement along
the length of a rock bolt. On the supposition that a whole anchorage system is in an elastic condition,
and considering the axial stress distribution on the rock bolt, Zhong et al. [5] derived a calculation
formula for the shear stress along the length distribution of a rock bolt. In order to better reflect the
nonlinear and softening characteristics of the relationship between shear stress and shear displacement
of an anchorage interface, Zhang and Yin [6] established a composite exponential-hyperbola nonlinear
shear slip model. Based on that model, interfacial mechanical characteristics for the whole system,
from the elastic stage to the rock and soil mass interface slip of the anchored section, were analyzed.
Huang [7] established a shear slip model of a double exponential curve of an anchorage interface by
analyzing the characteristics of the relationship between shear stress and the shear displacement of the
load transfer of the anchored section. Delhomme et al. and others [8–11] carried out many theoretical
analyses and experimental studies on the mechanical principles and anchorage mechanism of the rock
bolt. Yao et al. [12] derived the rock bolt interface shear stress produced by the deformation of the
surrounding rock, taking the displacement of rock as the functional parameter, thereby allowing the
location of the neutral point and axial force distribution of a fully grouted rock bolt to be determined.
Based on the damage characteristics of rock and soil under the action of shear stress and the load
transfer mechanism of rock bolts in rock and soil mass, Zou et al. and others [13–15] established
a differential equation for the load transfer of a rock bolt when the rock and soil mass was damaged,
and derived an analytical solution of the distribution of axial force, shear displacement, and side
friction along the length of a rock bolt.
Though numerous studies have been conducted on interfacial bond performance of a rock bolt,
little research focuses on the interfacial properties of fully grouted rock bolts under high temperature.
Results have indicated that the failure mechanism of fully grouted rock bolts under high temperature
and the load transfer mechanism of the interface are very complicated. Consequently, studying
the distribution of the axial force and shear stress of the interface of fully grouted rock bolts is of
great theoretical and practical significance for understanding the mechanism of anchorage to the
surrounding rock. In this paper, through a tensile test of fully grouted rock bolts produced under
different environmental temperatures, the distribution and size of axial force and shear stress of the
rock bolt’s interface are analyzed. Ideally, this will lead to the improved application of fully grouted
rock bolts in support engineering for tunnels with high geothermal activity, and provide data and
theoretical support for the formulation of relevant standards [16,17].
2. The Mechanical Principle of the Interfacial Bond between the Rock Bolt and the Grouting Body
It is generally believed that the bond-slip is divided into three stages according to the failure
process of tensile test. These stages are the elastic stage, the plastic slip stage, and the residual shear
stage [5]. Under the action of axial tensile force, when shear stress on the surface between the rock
bolt and the grouting body is smaller than the shear strength on the interface, the tensile force at this
time is less than the ultimate tensile force under an elastic state. Under a complete elastic state, there
is no relative displacement between interfaces. When the tensile force increases continuously, once
the shear stress on the surface between the rock bolt and the grouting body is greater than the shear
Appl. Sci. 2017, 7, 327 Appl. Sci. 2017, 7, 327 Appl. Sci. 2017, 7, 327 3 of 11
3 of 11 3 of 11 plane. The extrusion plastic zone will expand continuously and form a squeezing wedge accumulation strength on the interface, the extrusion plastic zone will develop in the front of the deformation plane.
plane. The extrusion plastic zone will expand continuously and form a squeezing wedge accumulation and a new extrusion slip surface. When the extrusion slip surface is crushed, the interface will slip The
extrusion plastic zone will expand continuously and form a squeezing wedge accumulation and a
and a new extrusion slip surface. When the extrusion slip surface is crushed, the interface will slip and come into the plastic flow state. With a further increase in tensile force, shear stress decreases new extrusion slip surface. When the extrusion slip surface is crushed, the interface will slip and come
and come into the plastic flow state. With a further increase in tensile force, shear stress decreases significantly and transverse fracture and failure happen on the anchorage body. Finally, the anchorage into the plastic flow state. With a further increase in tensile force, shear stress decreases significantly
significantly and transverse fracture and failure happen on the anchorage body. Finally, the anchorage body is then separated from the hole wall and enters the residual shear stage [18]. Shear stress on the and transverse fracture and failure happen on the anchorage body. Finally, the anchorage body is then
body is then separated from the hole wall and enters the residual shear stage [18]. Shear stress on the interface of the anchorage body increases with the increase in tensile force. The variations are shown separated from the hole wall and enters the residual shear stage [18]. Shear stress on the interface of
interface of the anchorage body increases with the increase in tensile force. The variations are shown in Figure 1. the anchorage body increases with the increase in tensile force. The variations are shown in Figure 1.
in Figure 1. τ
τ
0
0
τ
τ
τ
τ
(a) (a) S
S
0
0
S
S
(b)
(b)
0
0
(c) (c) S
S
Figure 1. Shear stress variation law along with the tensile force: (a) elastic stage; (b) plastic slip stage; Figure 1. Shear stress variation law along with the tensile force: (a) elastic stage; (b) plastic slip stage; (c) residual shear stage. Figure 1. Shear stress variation law along with the tensile force: (a) elastic stage; (b) plastic slip stage;
(c) residual shear stage. (c) residual shear stage.
Considering shear strength between the rock bolt and grouting body and supposing that the Considering shear strength between the rock bolt and supposing anchorage body material conforms to Hooke’s law, along grouting body and the direction of the rock bolt that the length, Considering
shear strength
between
the rock
bolt
and grouting
bodyof and
supposing
that
the
anchorage body material conforms to Hooke’s law, along the direction the rock bolt length, a micro rock bolt segment whose length is dz is taken at the place where the distance from the pull anchorage
body
material
conforms
to
Hooke’s
law,
along
the
direction
of
the
rock
bolt
length,
a
micro
a micro rock bolt segment whose length is dz is taken at the place where the distance from the pull end is z, as shown in Figure 2. On the supposition that axial displacement u( z ) is produced under rock bolt segment whose length is dz is taken at the place where the distance from is produced under the pull end is z, as
end is z, as shown in Figure 2. On the supposition that axial displacement u( z )
the action of axial force , according to the static equilibrium relationship and the principle of the P( z )supposition
shown in Figure 2. On the
that axial displacement u(z) is produced under the action of
the action of axial force P( z ) , according to the static equilibrium relationship and the principle of the axial transfer force P(zmethod, to the
static equilibrium
relationship
and the principle
of the
load transfer
load it can be concluded that the shear displacement of z at any position on the ) , according
load transfer it can that
be concluded that the shear of z on
at the
any position on the method,
it canmethod, be concluded
the shear displacement
ofdisplacement z at any position
interface
between
interface between the anchorage body and the rock bolt equals the value of the axial displacement of interface between the anchorage body and the rock bolt equals the value of the axial displacement of the anchorage body and the rock bolt equals the value of the axial displacement of the rock bolt under
the rock bolt under the action of tensile force (this does not consider the deformation of the rock bolt the rock bolt under the action of tensile force (this does not consider the deformation of the rock bolt the action of tensile force (this does not consider the deformation of the rock bolt itself) [5].
itself) [5]. itself) [5]. dd
U(z)
U(z)
P(z)
P(z)
τ
τ
(z)
(z)
Micro segment
of anchor
Micro
segment
of anchor
U(z+dz)
U(z+dz)
P(z)+dP(z)
P(z)+dP(z)
τ
τ
dz
(z)
(z)
dz
Figure 2. Schematic diagram of the interaction between rock bolts and the grouting body. Figure 2. Schematic diagram of the interaction between rock bolts and the grouting body.
Figure 2. Schematic diagram of the interaction between rock bolts and the grouting body. When the calculated position of z on the interface between the rock bolt and the grouting body When the calculated position of z on the interface between the rock bolt and the grouting body
When the calculated position of z on the interface between the rock bolt and the grouting body is at the position of j on the strain gauge, combined with Equation (1) and based on the variation in is
at
the
position of j on the strain gauge, combined with Equation (1) and based on the variation in
is at the position of j on the strain gauge, combined with Equation (1) and based on the variation in strain value of the strain gauge at each point, which is arranged along the axial direction of the rock strain value of the strain gauge at each point, which is arranged along the axial direction of the rock
strain value of the strain gauge at each point, which is arranged along the axial direction of the rock bolt, Equation (2) can be obtained. Equation (2) is used to calculate the axial force at point j. bolt, Equation
(2)
bebe obtained.
Equation
(2) is(2) used
calculate
the axial force
at point
j.at According
bolt, Equation (2) can
can obtained. Equation is to
used to calculate axial force point j. According to the strain value of two adjacent points and Equation (3), the average value of shear According to the strain value of two adjacent points and Equation (3), the average value of shear Appl. Sci. 2017, 7, 327
4 of 11
to the strain value of two adjacent points and Equation (3), the average value of shear stress on the
interface between the rock bolt and the grouting body of two adjacent points can be obtained [19].
ε (z) =
du(z)
dz
=−
P(z)
(1)
AE
P( j) = ε ( j) EA
(2)
[ε ( j) + ε ( j+1) ] EA
(3)
πd∆z
where P( j) is the calculated value of the axial force at the point j; τ(i) is the average value of shear stress
between point j and point j + 1. ε ( j) and ε ( j+1) are the strain values measured by the strain gauge at the
point j and point j + 1; E is the elastic modulus of the rock bolt. A and d are the cross-sectional area and
diameter of the rock bolt rod, respectively; ∆z is the distance between adjacent strain gauges.
τ(i) =
3. Test Schemes
3.1. Test Introduction
In this test, an ordinary hollow thread rock bolt was used. Cement mortar with a water/cement
ratio of 0.45 and a cement/sand ratio of 1.00 was used as the grouting material. Three anchorage body
specimens whose anchorage section lengths were 100 cm were made. Strain gauges were attached
to the surface of the rock bolt along the axial direction. The anchorage body specimens were then
maintained in a constant temperature box at temperatures of 20 ± 0.2 ◦ C, 35 ± 0.2 ◦ C, and 50 ± 0.2 ◦ C,
respectively. Indoor tensile tests were then carried out for all specimens after 28 days of environmental.
During the procedure of step loading, the strain value at the mark position on the interface under each
step loading was recorded. The first step loading of the test was 5 kN. Afterwards, each level of load
was 10 kN. The tests were stopped when the load reached 110 kN.
3.2. Test Materials and Device
(1) Test materials
Rock bolt: Rock bolts complying with standard TB/T 3209-2008, “Technical condition of hollow
rock bolt” were provided by Hangzhou Tuqiang Engineering Materials Co., Ltd. (Hangzhou, China).
The types of the rock bolt were φ25 × 5. In order to measure the tensile properties of the rock bolts and
study the effects of temperature stress on performance, according to the requirements of the Chinese
national standard, “Metallic Materials Tensile Testing at Ambient Temperature (GB/T 228-2002),” these
rock bolts were sampled, marked, and stretched. Test results for the mechanical performance of these
rock bolts are shown in Table 1.
Table 1. Mechanical parameters of rock bolts.
Parameters
Maximum Tensile Force
(kN)
Cross Sectional Area
(mm2 )
Ultimate Tensile Strength
(MPa)
Elongation
(%)
Test values
161.5
314.2
514
19.47
Cement: The type of the cement was P·O 42.5 Lafarge ordinary Portland cement produced
by Dujiangyan City, Sichuan Province. The density of the cement was 3.1 g/cm3 , the specific
surface area was 328 m2 /kg, and the water requirement of normal consistency was 26.8%. The main
parameters are shown in Table 2. Detection results showed that all indicators met the Chinese national
standard requirements.
Steel pipe: Seamless steel pipes of type DN75 were used. The wall thickness was 5 mm and the
material type was Q235.
Appl. Sci. 2017, 7, 327 5 of 11 Steel pipe: Seamless steel pipes of type DN75 were used. The wall thickness was 5 mm and the Appl. Sci.
2017, 7, 327
5 of 11
material type was Q235. Resistance strain gauge: Type BFH120‐3AA gauges with a resistance value of 119% ± 0.2% were Resistance strain gauge: Type BFH120-3AA gauges with a resistance value of 119% ± 0.2% were
used. The sensitivity coefficient and sensitive gate size were 2.05% ± 0.23% and 6.9 mm × 3.9 mm, used.
The sensitivity coefficient and sensitive gate size were 2.05% ± 0.23% and 6.9 mm × 3.9 mm,
respectively. It is a kind of self adhesive strain gauge. respectively.
is a kind
of selfCyanoacrylate adhesive strainadhesive gauge. 502 produced by a Beijing chemical plant Strain It
gauge binder: Strain
gauge
binder:
Cyanoacrylate
adhesive
502 produced
byKangda a Beijing
chemical
plant
(Beijing, China) and South 704 silicone rubber produced by Liyang Chemical Co., Ltd. (Beijing,
China)
and
South
704
silicone
rubber
produced
by
Liyang
Kangda
Chemical
Co.,
Ltd.
(Liyang, China) were used. The binder could endure temperatures from −50 °C up to +250 °C, ◦ C,
(Liyang,
China)
used. The binder “Single could endure
temperatures
from −
50 ◦ C up to +250
conforming to were
Q/320481KD‐001‐2010, Pack Room Temperature Environmental Silicone conforming
to Q/320481KD-001-2010, “Single Pack Room Temperature Environmental Silicone Rubber
Rubber Executive Standard.” Executive Standard.”
Table 2. Material parameters of cement. Table 2. Material parameters of cement.
Initial Setting Final Setting Fineness InitialTime (s) Setting
FinalTime (s) Setting
Project Project
Fineness
Test results National standards Test results
National standards
3.9 3.9≤10 ≤10
Time (s)
171 171>45 >45
Time (s)
259 <600 259
<600
Stability
Stability
Qualified
/ Qualified
/
Compressive Compressive
Strength (MPa) Strength
(MPa)
3 d 28 d
3d
2849 d
30 ≥17 ≥42.5 30
49
≥17
≥42.5
(2) Test instrument (2) Test
instrument
Static strain gauge: The static strain gauge was a model JC‐4A produced by the Beijing Chuanger Building Test Technology Development Co., Ltd. (Beijing, China) and was connected to a Static strain gauge: The static strain gauge was a model JC-4A produced by the Beijing Chuanger
PC through the CAN bus. Building
Test Technology Development Co., Ltd. (Beijing, China) and was connected to a PC through
Rock bolt shank tension meter: The model YML‐30B rock bolt shank tension meter was the CAN
bus.
produced by Shanghai ShenRui Testing Equipment Co., Ltd. (Shanghai, China). Rock bolt shank tension meter: The model YML-30B rock bolt shank tension meter was produced
by Shanghai ShenRui Testing Equipment Co., Ltd. (Shanghai, China).
3.3. Strain Gauge Layout and Test Procedure 3.3. Strain
Gauge Layout and Test Procedure
Considering the distribution of shear stress along the length of the rock bolt under axial load, strain gauges were mostly arranged around tensile end of ofthe The distance Considering
the distribution
of shear
stressthe along
the length
therock rock bolt bolt rod. under
axial
load,
between strain gauges changed from 20 cm to 10 cm. Longitudinal strain gauges were attached on strain
gauges were mostly arranged around the tensile end of the rock bolt rod. The distance between
the surface the rock bolt the length direction each strain gauge a number. strain
gauges of changed
from
20 rod cm toalong 10 cm.
Longitudinal
strain and gauges
were
attached
onhad the surface
of The specific layout strain gauge and
is shown in Figure Three specimens produced under the
rock
bolt rod
alongof thethe length
direction
each strain
gauge3. had
a number.
The specific
layout
of
environmental temperatures of 20 35 specimens
°C, and 50 °C, labeled D3, and D5, respectively, the
strain gauge is
shown in Figure
3. °C, Three
produced
underD2, environmental
temperatures
ofwere tested. 20 ◦ C, 35 ◦ C, and 50 ◦ C, labeled D2, D3, and D5, respectively, were tested.
Marking the strain gauge
number at the end of wire
N1 measuring
point
N2 measuring
point
N3 measuring
point
N4 measuring N5 measuring
point
point
Anchoring section
N6 measuring
point
Figure 3. Strain gauge layout (unit: mm). Figure
3. Strain gauge layout (unit: mm).
The specific steps of this are as follows: ① Firstly, the surface of the measuring point is 1 Firstly,
The
specific
steps
of this
testtest are as
follows:
the surface
of the measuring
point is ground
ground with a grinding wheel and scrubbed with acetone, and the coordinate line of strain gauge is with
a grinding wheel and scrubbed with acetone, and the coordinate line of strain gauge is drawn.
drawn. Secondly, the bottom of the strain gauge and the surface of the measuring point are brushed Secondly,
the bottom of the strain gauge and the surface of the measuring point are brushed with
with a thin layer of binder. Strain gauges are attached to the surface of the rock bolt along the axial a thin layer of binder. Strain gauges are attached to the surface of the rock bolt along the axial direction
direction according to design requirements. The bubbles and excess binder are squeezed out until according
to design requirements. The bubbles and excess binder are squeezed out until the strain
the strain gauge is tightly bonded to the rock bolt. Thereafter, protection and sealing treatment are gauge
is tightly bonded to the rock bolt. Thereafter, protection and sealing treatment are carried out.
2 The steel pipes, which are 100 cm long,
A thin layer of the silica gel is attached to the strain gauge. Appl. Sci. 2017, 7, 327 Appl. Sci. 2017, 7, 327 Appl. Sci. 2017, 7, 327 6 of 11 6 of 11 6 of 11 carried out. A thin layer of the silica gel is attached to the strain gauge. carried out. A thin layer of the silica gel is attached to the strain gauge. ② ② The steel pipes, which are The steel pipes, which are carried out. A thin layer of the silica gel is attached to the strain gauge. ② The steel pipes, which are 100 cm long, are used as the forming die. Rock bolts with strain gauges are inserted into the middle 100 cm long, are used as the forming die. Rock bolts with strain gauges are inserted into the middle Appl.
Sci. 2017, 7, 327
6 of 11
100 cm long, are used as the forming die. Rock bolts with strain gauges are inserted into the middle of 5 cm in the lower portion. ③ The of the steel pipes, leaving a reserved length at the end gap of the steel pipes, leaving a reserved length at the end of 5 cm in the lower portion. ③ The gap of the steel pipes, leaving a reserved length at the end of 5 cm in the lower portion. ③ The gap between the steel pipe and the rock bolt is filled with configured cement mortar. The thickness of the between the steel pipe and the rock bolt is filled with configured cement mortar. The thickness of the are
used as the forming die. Rock bolts with strain gauges are inserted into the middle of the steel
between the steel pipe and the rock bolt is filled with configured cement mortar. The thickness of the cement mortar surrounding the bolt is 25 mm, as shown in Figure 4 and a hammer is used to tap on cement mortar surrounding the bolt is 25 mm, as shown in Figure 4 and a hammer is used to tap on 3 The gap between the steel
pipes,
leaving a reserved length at the end of 5 cm in the lower portion. cement mortar surrounding the bolt is 25 mm, as shown in Figure 4 and a hammer is used to tap on ④ Shrinkage curves of cement the bottom of steel pipe to make sure the concrete was condensed. ④ Shrinkage curves of cement the bottom of steel pipe to make sure the concrete was condensed. pipe
and the rock bolt is filled with configured cement mortar. The thickness
of the cement mortar
④ Shrinkage curves of cement the bottom of steel pipe to make sure the concrete was condensed. mortar environment previously obtained. The shrinkage mainly mortar in in a a high high temperature environment have been been previously obtained. The shrinkage mainly surrounding
the temperature bolt
is 25 mm,
as shown inhave Figure
4 and
a hammer
is used to
tap
on the bottom
of
mortar in a high temperature environment have been previously obtained. The shrinkage mainly occurs at early 3 3 days. Then, the rate down and steady. occurs at an an early age, age, within within days. Then, the shrinkage shrinkage rate slows slows down and becomes becomes steady. 4 Shrinkage
steel
pipe
to
make
sure
the
concrete
was
condensed.
curves
of
cement
mortar
in
a
high
occurs at an early age, within 3 days. Then, the shrinkage rate slows down and becomes steady. in environment as Thus, specimens in a a standard standard environmental environmental environment for 1 day day as Thus, molded molded specimens are are maintained maintained temperature
environment
been previously
shrinkage mainly
occurs atfor an1 early
age,
in a obtained.
standard The
environmental environment for 1 day as Thus, molded specimens have
are maintained shown Figure 5. under respective temperatures for within
3in days.
Then,
the
shrinkage
rate slowsare down
and becomes
steady.
Thus, molded
specimens
shown in Figure 5. ⑤
⑤ Molded Molded specimens specimens are maintained maintained under respective temperatures for shown in Figure 5. ⑤ Molded specimens are maintained under respective temperatures for 5 Molded
are
maintained
in a standard environmental environment for 1 day as shown in Figure 5. 28 days. ⑥ 28 days. ⑥ Tensile tests are carried out on all specimens as shown in Figures 6 and 7. When the load Tensile tests are carried out on all specimens as shown in Figures 6 and 7. When the load 28 days. ⑥ are
Tensile tests are carried out on all specimens as shown in Figures 6 and 7. When the load 6 Tensile tests are carried out
specimens
maintained
under
respective
temperatures
for
28
days.
is applied, the steel baffle and steel pipe form a reaction frame depending on the friction between is applied, the steel baffle and steel pipe form a reaction frame depending on the friction between is applied, the steel baffle and steel pipe form a reaction frame depending on the friction between on
all specimens
as shown in FiguresThe 6 and 7. When
thetransferred load is applied,a the
steel baffle
and steel and pipe
steel steel pipe pipe and and grouting grouting material. material. The load load will will be be transferred to to a rock rock bolt. bolt. Test Test data data and steel apipe and frame
grouting material. load will be transferred to a grouting
rock bolt. Test data and form
reaction
depending
on The the friction
between
steel pipe and
material.
The load
observation are recorded. observation are recorded. observation are recorded. will
be transferred to a rock bolt. Test data and observation are recorded.
Rock
Rockbolt
bolt
Rock bolt
Grouting
Groutingbody
body
Grouting body
Steel
Steelpipe
pipe
Steel pipe
Figure
4. Section of rock bolts (unit: mm).
Figure 4. Section of rock bolts (unit: mm). Figure 4. Section of rock bolts (unit: mm). Figure 4. Section of rock bolts (unit: mm). Figure 5. Forming diagram of rock bolt specimens. Figure 5. Forming diagram of rock bolt specimens. Figure
5. Forming diagram of rock bolt specimens.
Figure 5. Forming diagram of rock bolt specimens. Figure 6. Tensile test of rock bolts. Figure 6. Tensile test of rock bolts. Figure 6. Tensile test of rock bolts. Figure
6. Tensile test of rock bolts.
Appl. Sci. 2017, 7, 327
Appl. Sci. 2017, 7, 327 7 of 11
7 of 11 Appl. Sci. 2017, 7, 327 7 of 11 Steel pipe
Grouting
body
Steel pipe
Rock bolt
Grouting body
Rock bolt
Grouting body
Grouting
Steel
pipebody
Steel baffle
Steel pipe
Steel baffle
Figure 7. Schematic diagram of rock bolts with steel baffle (unit: mm). Figure 7. Schematic diagram of rock bolts with steel baffle (unit: mm).
Figure 7. Schematic diagram of rock bolts with steel baffle (unit: mm). 4. Results and Discussion 4.
Results and Discussion
4. Results and Discussion 4.1.
Distribution of Internal Force under Different Loads
4.1. Distribution of Internal Force under Different Loads 4.1. Distribution of Internal Force under Different Loads According
(2)(2) andand (3), (3), the axial
forceforce and average
shear stress
the bonding
According toto Equations
Equations the axial and average shear ofstress of the interface
bonding According to Equations (2) and (3), the axial force and average shear stress of the bonding of
the anchorage body specimen can be obtained after processing test data. Through the action of
interface of the anchorage body specimen can be obtained after processing test data. Through the interface of the anchorage body specimen can be obtained after processing test data. Through the tensile
force, the distribution curves of the axial force and shear stress along the length of the anchorage
action of tensile force, the distribution curves of the axial force and shear stress along the length of action of tensile force, the distribution curves of the axial force and shear stress along the length of body
under environmental temperature of 20 ◦ C, 35 ◦ C, and 50 ◦ C were obtained and are shown in
the anchorage body under environmental temperature of 20 °C, 35 °C, and 50 °C were obtained and the anchorage body under environmental temperature of 20 °C, 35 °C, and 50 °C were obtained and Figure
8.
are shown in Figure 8. are shown in Figure 8. 40
40
20
20
2
1
0
10
(a) 20 °C (a) 20 °C 5kN
5kN
30kN
30kN
70kN
70kN
100kN
3
100kN
10kN
10kN
40kN
40kN
80kN
80kN
110kN
90kN
110kN
2
1
20
20
30
30
40
50
60
70
40
50 depth
60 (cm)
70
Anchoring
Anchoring depth (cm)
(d) 20 °C (d) 20 °C 80
80
10 20
20
10
90
90
20kN
20kN
50kN
50kN
90kN90kN
40 40
20 20
30
30
40
40
50
50
60
60
0 0 10 1020 2030 3040 4050 5060 60
70 70
80
70
70 80
80 9090 100
100
Anchoring
depth
(cm)(cm)
Anchoring
depth
Anchoring depth
Anchoring
depth (cm)
(cm)
(b) 35 °C
(b) 35
°C
3.5
3.5
20kN
20kN
50kN
50kN
90kN
10kN
10kN
40kN
40kN
80kN80kN
0 0
00
5kN
5kN
30kN
30kN
70kN
70kN
60 60
00
Anchoring
depth
(cm)
Anchoring
depth
(cm)
0
10
80 80
2020
4
Shear stress (MPa)
Shear stress (MPa)
3
100
4040
0
10
100
10
20 20
30 3040 4050 5060 6070 70 80 80 90 90 100
0
100
20kN
20kN
50kN
50kN
90kN
90kN
6060
0
0
4
8080
10kN
10kN
40kN
40kN
80kN
80kN
Axial force (kN)
60
5kN
5kN
30kN
30kN
70kN
70kN
100kN
100kN
100
100
Axial force (kN)
80
60
120
120
20kN
20kN
50kN
50kN
90kN
90kN
3.0
3.0
2.5
2.5
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0 20
20
5kN
5kN
30kN
30kN
70kN
70kN
100kN
10kN
10kN
40kN
40kN
80kN
80kN
40
50
60
70
40
50 depth
60 (cm)
70
Anchoring
Anchoring depth (cm)
(e) 35 °C
(e) 35 °C
100
90 100
(c) 50 °C (c) 50 °C 3.5
20kN
20kN
50kN
50kN
90kN
3.5
80
80
5kN
10kN
20kN
5kN
10kN
20kN
30kN
40kN
50kN
30kN
40kN
50kN
3.0
70kN
80kN
90kN
70kN
80kN
90kN
2.5
2.5
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
10
20
30
40
50
60
70
80
90
0.0
10
20
30
40 depth
50 (cm)
60
70
80
90
Anchoring
3.0
90kN
100kN
30
30
90
80
Shear force (MPa)
Shear force (MPa)
Axial force (kN)
Axial force (kN)
80
10kN
10kN
40kN
40kN
80kN
80kN
110kN
110kN
Axial force (kN)
5kN
5kN
30kN
30kN
70kN
70kN
100kN
100kN
Axial force (kN)
120
100 100
Shear stress (MPa)
Shear stress (MPa)
120
90
90
Anchoring depth (cm)
(f) 50 °C (f) 50 °C Figure 8. Axial force and shear stress distribution diagrams under different loads: (a,d) for Figure
8.8. Axial
force
andand shear
stressstress distribution
diagrams
under different
loads: (a,d)
for Specimen
Figure Axial force shear distribution diagrams under different loads: (a,d) for Specimen D2; (b,e) for Specimen D3; (c,f) for Specimen D5. D2;
(b,e) for Specimen D3; (c,f) for Specimen D5.
Specimen D2; (b,e) for Specimen D3; (c,f) for Specimen D5. As can be seen from Figure 8a, the axial force on the interface of Specimen D2 decreases As can
can bebe seen from Figure 8a, axial
the axial force the interface of Specimen D2 gradually
decreases As
seen
from
Figure
8a, the
force on
the on interface
of Specimen
D2 decreases
gradually from the pull end of the anchorage section to the non‐pull end under different loads. With gradually from the pull end of the anchorage section to the non‐pull end under different loads. With from
the pull end of the anchorage section to the non-pull end under different loads. With an increase
an increase in tensile force, the shape of the curve rises slightly upward. Moreover, the maximum an increase in tensile force, the shape of the curve rises slightly upward. Moreover, the maximum in
tensile
force, the shape of the curve rises slightly upward. Moreover, the maximum shear stress at
shear stress at the pull end of the anchorage section increases correspondingly and tends to zero as it shear stress at the pull end of the anchorage section increases correspondingly and tends to zero as it the
pull end of the anchorage section increases correspondingly and tends to zero as it comes close to
comes close to the end (Figure 8d). On the supposition that it has entered the stage of elastic‐plastic reinforcement, the shear stress reaches its peak value near the end of the anchorage section and the comes close to the end (Figure 8d). On the supposition that it has entered the stage of elastic‐plastic the
end (Figure 8d). On the supposition that it has entered the stage of elastic-plastic reinforcement,
peak value moves continuously parallel along the length of the anchorage body. reinforcement, the shear stress reaches its peak value near the end of the anchorage section and the the
shear stress reaches its peak value near the end of the anchorage section and the peak value moves
During the test procedure of Specimen D3, the strain gauge, which was 10 cm from the pull end peak value moves continuously parallel along the length of the anchorage body. continuously
parallel along the length of the anchorage body.
of the anchorage section, was damaged. The axial force value at a distance of 10 cm from the pull end During the test procedure of Specimen D3, the strain gauge, which was 10 cm from the pull end During
the test procedure of Specimen D3, the strain gauge, which was 10 cm from the pull end
of the anchorage section and the shear stress distribution value at a distance from the pull end of less of the anchorage section, was damaged. The axial force value at a distance of 10 cm from the pull end of
the
anchorage section, was damaged. The axial force value at a distance of 10 cm from the pull end
than 25 cm were missing. When tensile force was increased to 110 kN, the strain gauges near the pull of the anchorage section and the shear stress distribution value at a distance from the pull end of less of
the anchorage section and the shear stress distribution value at a distance from the pull end of less
than 25 cm were missing. When tensile force was increased to 110 kN, the strain gauges near the pull Appl. Sci. 2017, 7, 327
8 of 11
than 25 cm were missing. When tensile force was increased to 110 kN, the strain gauges near the pull
end of anchorage section had already failed. As can been seen from Figure 8b,e, the variation in axial
force and shear stress on the interface of the rock bolt specimen along the length of the anchorage body
is similar to that of the 20 ◦ C specimen (D2). Axial force and shear stress all gradually decreased from
8 of 11 the pullAppl. Sci. 2017, 7, 327 end to the opposite
end of the anchorage section until they became zero.
In the
case of Specimen D5, the average shear stress value of the anchorage section at the top
end of anchorage section had already failed. As can been seen from Figure 8b,e, the variation in axial 10 cm could
not be obtained due to failure of the strain gauge. When the tensile force was increased
force and shear stress on the interface of the rock bolt specimen along the length of the anchorage body similar to the 20 °C specimen and shear stress all gradually to 90 kN, theis reading
ofthat the of strain
gauge
near the(D2). pullAxial end force of the
anchorage
body
was abnormal.
decreased from the pull end to the opposite end of the anchorage section until they became zero.
Figure 8c,f show that the variation in shear stress and axial force along the length of the anchorage
In the case of Specimen D5, the average shear stress value of the anchorage section at the top ◦
◦
body is almost the same at 20 C and 35 C. Axial force and shear stress all gradually decreased
from
10 cm could not be obtained due to failure of the strain gauge. When the tensile force was increased the pullto 90 kN, the reading of the strain gauge near the pull end of the anchorage body was abnormal. end to the non-pull end until they became zero.
Figure 8c,f show that the variation in shear stress and axial force along the length of the anchorage 4.2. Effect
of Temperature on the Axial Force and Shear Stress on the Interface of Specimens
body is almost the same at 20 °C and 35 °C. Axial force and shear stress all gradually decreased from the pull end to the non‐pull end until they became zero. Analysis
of temperature effects on the distribution of axial force and shear stress of the anchorage
section 4.2. Effect of Temperature on the Axial Force and Shear Stress on the Interface of Specimens was achieved by changing the physical and mechanical properties of anchorage material.
The ratio of elastic modulus in forming the medium of anchorage system has a large impact on the
Analysis of temperature effects on the distribution of axial force and shear stress of the value and the distribution form of axial force and shear stress of the rock bolt. When the value and the
anchorage section was achieved by changing the physical and mechanical properties of anchorage distribution
of axial
and
shear
stress of
anchorage
body of under
the action
of has higha temperature
material. The force
ratio of elastic modulus in the
forming the medium anchorage system large were analyzed,
the
data
of
four
representative
groups
whose
tensile
force
were
10
kN,
30 kN, 70 kN,
impact on the value and the distribution form of axial force and shear stress of the rock bolt. When the value and the distribution of axial force and shear stress of the anchorage body under the action and 90 kN
were chosen, and relevant analyses were carried out.
of high temperature were analyzed, the data of four representative groups whose tensile force were (1) Effect
of temperature on axial force
10 kN, 30 kN, 70 kN, and 90 kN were chosen, and relevant analyses were carried out. (1) Figure
Effect of temperature on axial force From
9, it can be seen that, under the same tensile force, with an increase in environmental
temperature,From the axial
force
theseen length
of under the anchorage
decreases
more
law
Figure 9, it along
can be that, the same body
tensile force, with an slowly.
increase Decay
in along the
length of the anchorage body is the slowest at an environmental temperature of 50 ◦ C,
environmental temperature, the axial force along the length of the anchorage body decreases more slowly. Decay law of With
the anchorage body is the slowest the
at an environmental followed
by axial
force
at along 35 ◦ Cthe andlength 20 ◦ C.
an increase
in temperature,
axial
force at the same
temperature of 50 °C, followed by axial force at 35 °C and 20 °C. With an increase in temperature, the place from the pull end of the anchorage body becomes larger.
axial force at the same place from the pull end of the anchorage body becomes larger. 35
12
20℃
35℃
50℃
Axial force (kN)
Axial force (kN)
8
6
4
2
0
20℃
20
15
10
5
0
10
20
30
40
50
60
70
80
90
0
100
0
10
20
30
50
60
70
80
90
100
(b)
20℃
35℃
100
50℃
Axial force (kN)
70
40
Anchoring depth (cm)
(a) 80
Axial force (kN)
50℃
25
Anchoring depth (cm)
60
50
40
30
20
20℃
35℃
50℃
80
60
40
20
10
0
35℃
30
10
0
10
20
30
40
50
60
70
Anchoring depth (cm)
(c) 80
90
100
0
0
10
20
30
40
50
60
70
80
90
100
Anchoring depth (cm)
(d)
Figure 9. The distribution diagrams of axial force under different tensile force. (a) Tensile force is Figure 9.
The distribution diagrams of axial force under different tensile force. (a) Tensile force is 10 kN.
10 kN. (b) Tensile force is 30 kN. (c) Tensile force is 70 kN. (d) Tensile force is 90 kN. (b) Tensile force is 30 kN. (c) Tensile force is 70 kN. (d) Tensile force is 90 kN.
Appl. Sci. 2017, 7, 327
Appl. Sci. 2017, 7, 327 9 of 11
9 of 11 Effect of temperature on shear stress (2) (2)Effect
of temperature on shear stress
From Figure 10, it be
can be that,
seen under
that, the
under the same force,
tensile force, with an inincrease in From
Figure
10, it
can
seen
same
tensile
with
an increase
temperature,
temperature, the decay law of shear stress along the length of anchorage body becomes slower. the decay law of shear stress along the length of anchorage body becomes slower. At the point near
At the point near the pull end of the anchorage body, with an increase in temperature, the maximum the pull end of the anchorage body, with an increase in temperature, the maximum value of shear
value of shear stress becomes smaller which means the interface bond strength between rock bolt stress becomes smaller which means the interface bond strength between rock bolt and grouting body
and grouting body decreases gradually. With an increase in temperature, when the depth exceeds a decreases gradually. With an increase in temperature, when the depth exceeds a certain value along
certain value along the length of the anchorage body, the shear stress at the same point increases the length of the anchorage body, the shear stress at the same point increases gradually, which means
gradually, which means the distribution of the shear stress along the length of the anchorage body the distribution of the shear stress along the length of the anchorage body becomes more uniform.
becomes more uniform. 0.40
20℃
35℃
1.4
50℃
0.30
0.25
0.20
0.15
0.10
0.8
0.6
0.4
20
30
40
50
60
70
80
0.0
10
90
20
30
40
3.2
20℃
60
70
80
90
(b)
35℃
4.0
50℃
20℃
3.5
Shear stress (MPa)
2.8
50
Anchoring depth (cm)
(a) Shear stress (MPa)
50℃
1.0
Anchoring depth (cm)
2.4
2.0
1.6
1.2
0.8
0.4
0.0
10
35℃
0.2
0.05
0.00
10
20℃
1.2
Shear stress (MPa)
Shear stress (MPa)
0.35
35℃
50℃
3.0
2.5
2.0
1.5
1.0
0.5
20
30
40
50
60
70
Anchoring depth (cm)
(c) 80
90
0.0
10
20
30
40
50
60
70
Anchoring depth (cm)
80
90
(d)
Figure 10. The distribution diagrams of shear stress under different Tensile force. (a) Tensile force is Figure 10. The distribution diagrams of shear stress under different Tensile force. (a) Tensile force is
10 kN. (b) Tensile force is 30 kN. (c) Tensile force is 70 kN. (d) Tensile force is 90 kN. 10 kN. (b) Tensile force is 30 kN. (c) Tensile force is 70 kN. (d) Tensile force is 90 kN.
It can be seen from the above result that the axial force and shear stress all gradually decreased It can be seen from the above result that the axial force and shear stress all gradually decreased
from the pull end to the non‐pull end until they became zero. High temperature has little influence from
the pull end to the non-pull end until they became zero. High temperature has little influence on
on the fully grouted rock bolt used in tunnel support construction with high geothermal activities. the grouted
surrounding is a kind of nonabsorbent rock, the with
surrounding rock will prevent the theIf fully
rock rock bolt used
in tunnel
support construction
high geothermal
activities.
If the
evaporation of water in the process of cement hydration. If the surrounding rock is absorbent rock, surrounding
rock is a kind of nonabsorbent rock, the surrounding rock will prevent the evaporation
evaporate, which will hydration.
affect the performance of the anchorage system. Thus, is very of water water will in the
process of
cement
If the surrounding
rock is absorbent
rock,it water
will
important to take measures to avoid the evaporation of water during the strength growth process in evaporate, which will affect the performance of the anchorage system. Thus, it is very important
to future anchor construction. take measures to avoid the evaporation of water during the strength growth process in future
anchorage system is a heterogeneous material. The stress distribution is not to be linear, anchor The construction.
with the maximum load at the rock bolt head and zero stress not at the end. The point of zero stress The anchorage system is a heterogeneous material. The stress distribution is not to be linear, with
called the critical point. The critical length is different because of a different diameter, the theis maximum
load at the rock bolt head and zero stress not at the end. The point of zero stress is called
roughness of the rock bolt, and a different strength of grouting material. Therefore, it is suggested the critical point. The critical length is different because of a different diameter, the roughness of the
that the design length of the rock bolt be longer than the critical length. rock bolt, and a different strength of grouting material. Therefore, it is suggested that the design length
of the
rock bolt be longer than the critical length.
5. Conclusions In this paper, the size and distribution of axial force and shear stress on the interface of fully 5. Conclusions
grouted rock bolts produced under different environmental temperatures are studied systematically, In this paper, the size and distribution of axial force and shear stress on the interface of fully
with the following conclusions obtained: grouted rock bolts produced under different environmental temperatures are studied systematically,
with the following conclusions obtained:
Appl. Sci. 2017, 7, 327
(1)
(2)
(3)
10 of 11
With an increase in environmental temperature, the maximum shear stress on the interface
between rock bolt and grouting body becomes smaller, while the distribution of shear stress along
the length of the anchorage section becomes relatively uniform. A high temperature has little
influence on the fully grouted rock bolt used in tunnel support construction with high geothermal
activities. It is very important to avoid the evaporation of water during the strength growth
process in future anchor construction.
With an increase in tensile force, the axial force and maximum shear stress on the interface
between the rock bolt and the grouting body along the anchorage section increase correspondingly.
Axial force and shear stress all gradually decreased from the pull end to the non-pull end until
they became zero. The design length of the rock bolt should be longer than the critical length.
The performance of the anchorage system is also related to other factors except high geothermal
The deformation of the surrounding rock will produce a large tensile force on the rock bolt.
The coupling of high temperature and load will lead to further deterioration of the anchorage
system. In addition, the roughness of the surrounding rock will affect the ultimate pull-out force
of rock bolts. These are very important research areas for the future.
Acknowledgments: The authors are very grateful for the funding from projects 51308471, 51378434, and 51578456
supported by the National Natural Science Foundation of China. Their thanks also go to the Fundamental
Research Funds for the Central University with Grant No. 2682015ZD13. The first author wishes to thank the Key
Laboratory of High-speed Railway Engineering, Ministry of Education, Southwest Jiaotong University, People’s
Republic of China, for its support.
Author Contributions: Fuha Li and Xiaojuan Quan conceived and put forward the research ideas. Bo Wang, Yi Jia
and Siyin Chen designed the test project. Fuhai Li and Guibin Zhang carried out the research and wrote the paper.
Conflicts of Interest: The authors declare no conflicts of interest.
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