Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface

Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Rock Mechanics and Geotechnical Engineering journal homepage: www.jrmge.cn

Full Length Article

Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface Chuanqing Zhang a, b, Guojian Cui a, b, *, Xiangrong Chen c, Hui Zhou a, b, Liang Deng d a

State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, 430071, China University of Chinese Academy of Sciences, Beijing, 100049, China c PowerChina Huadong Engineering Corporation Limited, Hangzhou, 310014, China d College of Civil Engineering and Architecture, China Three Gorges University, Yichang, 443002, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 May 2019 Received in revised form 19 September 2019 Accepted 8 October 2019 Available online xxx

Shearing behavior and failure mechanism of bolt-grout interface are of great significance for load transfer capacity and design of rock bolting system. In this paper, direct shear tests on bolt-grout interfaces under constant normal load (CNL) conditions were conducted to investigate the effects of bolt profile (i.e. rib spacing and rib height) and grout mixture on the bolt-grout interface in terms of mechanical behaviors and failure modes. Test results showed that the peak shear strength and the deformation capacity of the bolt-grout interface are highly dependent on the bolt profile and grout mixture, suggesting that bolt performances can be optimized, which were unfortunately ignored in the previous studies. A new interface failure mode, i.e. ‘sheared-crush’ mode, was proposed, which was characterized by progressive crush failure of the grout asperities between steel ribs during shearing. It was shown that the interface failure mode mainly depends on the normal stress level and rib spacing, compared with the rib height and grout mixture for the range of tested parameters in this study. Ó 2020 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Keywords: Bolt-grout interface Direct shear test Shear behavior Failure mode Bolt profiles

1. Introduction Rock bolting system is one of widely used support systems in geotechnical engineering, as it can effectively improve the mechanical properties of rock masses and reduce the deformation, support time and construction cost (Cai et al., 2004; Blanco Martín et al., 2011; Kang, 2014; Ma et al., 2014; Li et al., 2017). The rock bolting system is basically composed of bolt rod, internal and external fixtures, and surrounding rocks. The bolt may be coupled with rock mass via a medium or a mechanical action. According to the coupling mechanisms for the internal fixture, the rock bolting systems can be generally divided into three types: continuously mechanically coupled (CMC), continuously frictionally coupled (CFC), or discretely mechanically and frictionally coupled (DMFC) (Windsor, 1997). Amongst them, the CMC fully grouted rock bolts are the most widely used type in civil and mining engineering,

* Corresponding author. E-mail address: [email protected] (G. Cui). Peer review under responsibility of Institute of Rock and Soil Mechanics, Chinese Academy of Sciences.

which plays a decisive role in prevention and mitigation of engineering disasters. The support effect of fully grouted rock bolts mainly depends on the load transfer capacity of the bolting system, which is influenced by the mechanical properties of bolt rod, boltgrout interface, grout-rock interface, grouting, and rock mass. It has been widely acknowledged that bolt-grout interface is usually the weakest component in the rock bolting system (Yazici and Kaiser, 1992; Moosavi et al., 2005; Thenevin et al., 2017; Chen et al., 2018). Therefore, the shear behavior and failure mechanism of the bolt-grout interface are the key to understanding the load transfer mechanism and support effect of rock bolting system. To date, a large number of laboratory and in situ pull- and pushout tests have been conducted under both constant confining pressure and constant radial stiffness conditions to characterize the shearing behaviors of bolt-grout interface and failure mechanism (Hyett and Bawdent, 1995; Kilic et al., 2002, 2003; Moosavi et al., 2005; Aziz et al., 2008; Li, 2012; Cao et al., 2014a; Chen et al., 2016; Vlachopoulos et al., 2018). The results demonstrated that the shear behavior of the bolt-grout interface is related to the profile, diameter and length of rock bolt, in addition to the mechanical properties of grouting materials and the boundary conditions. For example, Kilic et al. (2002, 2003) found that increasing

https://doi.org/10.1016/j.jrmge.2019.10.004 1674-7755 Ó 2020 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BYNC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

the curing time of grout mixture and/or embedment length of a bolt could increase its pull-out capacity. Aziz and Webb (2003) reported that the bonding capacity of the bolt increases with increasing profile spacing. Therefore, it is preferable that the optimization of bolt profiles and grout mixture can improve the mechanical performance of rock bolting system. A comparison of the test results obtained from pull- and pushout tests showed similar interface failure modes, i.e. parallel shear failure mode and dilational slip failure mode (Cao et al., 2014b). However, the shear strength of bolt-grout interface obtained from the push-out test is approximately 10% greater than that obtained from pull-out test due to the Poisson’s effect of bolt rod (Jalalifar, 2006; Cao et al., 2013a). Although pull- or push-out tests have been widely employed, there are still some disadvantages. For example, as the bolt is encapsulated in steel tubes or surrounding rocks, it is rather difficult to evaluate the interface failure process in pull- or push-out tests. In addition, the radial displacement and stress of the boltgrout interface are experimentally difficult to be directly measured. Nevertheless, good knowledge of radial deformation behavior is helpful for understanding of rock bolting system. Bolt-grout interface is the typical discontinuities with different joint wall compressive strengths (DDJCS) and therefore can be considered as a special case of rock joints (Ghazvinian et al., 2010). As such, direct shear test has been used to investigate the shear behavior of the bolt-grout interface. In the direct shear test, the bolt-grout interface can be simplified as twodimensional (2D) interface shear model by unfolding the axisymmetric cylinder interface along the axial direction of rock bolt (Grasselli et al., 2002; Gu et al., 2003; Khosravi et al., 2018; Shang et al., 2018). The interface shear model consists of the upper grout specimen and the lower steel plate. Johnston and Lam (1989) and Lam and Johnston (1989) experimentally and analytically described the shear behavior of regular triangular concrete/rock joints and then evaluated the relevancy and accuracy of these expressions using laboratory test, under the conditions of constant normal stiffness (CNS) and constant normal load (CNL). Gu et al. (2003) conducted the sandstone-concrete interface shear tests and noted that wear debris is only distributed over one side of the concrete asperity, and the length of the wear debris is nearly equal to the shear displacement. Ghazvinian et al. (2012) proposed that the failure mode of asperity is related to the ratio of uniaxial compressive strength (UCS) of the lower and upper specimens. Their results indicated that the asperity degradation occurs on both sides of the joint, while at sc max/sc min < 1.56, asperity degradation only occurs on the weaker side of the joint. Compared with the commonly tested discontinuities with different joint wall compressive strengths, the mechanical properties of bolt and grout specimens differ greatly because the uniaxial compressive strength ratio scr and elastic modulus ratio Er are larger than 10. Aziz (2002) assessed the performance of rock bolt with regard to load transfer mechanisms in the laboratory under

the CNS conditions, suggesting that the difference between stress profiles under various loading cycles was negligible at low normal stress values, and the maximum dilation occurred at a shear displacement of about 60% of the bolt rib spacing. Cao (2013) demonstrated that the failure mode in the CNS direct shear tests coincided with that in the pull- and push-out tests. Yokota et al. (2018) assessed the impact of rib angle, strength of the grouting material, and the normal stress on the interface shear behavior in the rock bolting system. Regarding the optimization of bolt profile by direct shear test methods, Zhang et al. (2018) proposed that the rib spacing can affect both the mechanical performance and failure mode, but the rib spacing considered is relatively small compared with that tested in the pull-out test. Shang et al. (2018) investigated the progressive debonding process of the mortar-bolt interface and subsequent mortar rupture (due to mechanical interlocking) under the CNL conditions using the discrete element method (PFC2D). However, a systematic study on the optimization of bolt profile using direct shear test is rarely reported. For this, the study attempts to experimentally investigate the shear behavior and failure mode of the bolt-grout interface. In order to evaluate the influence of rib spacing, rib height, and grout mixtures, a series of laboratory direct shear tests were carried out. The research results can be helpful for understanding of bolting mechanism and optimizing rock bolt support design parameters. 2. Specimen preparation and direct shear test 2.1. Steel plate The steel plates were used in this study to simulate the bolt rod. The surface profile of standardized steel plate for the direct shear test is based on a type of threadbar which is commonly used in China, as shown in Fig. 1a and the simplified 2D surface profile of the steel plate is depicted in Fig. 1b (Cao et al., 2013b). In Fig. 1b, the parameters a, b, L, h and q are the rib top width, rib base width, rib spacing, rib height, and rib angle, respectively. The specification of threadbar is listed in Table 1. In addition, the rib face angle, which is defined as the angle of transverse rib relative to the axis of rebar bolt, is set as 90
. The previous pull-out test results indicated that rib spacing and rib height are the two main factors accounting for optimizing bolt profiles. In this instance, seven steel plates with different rib spacings and rib heights were designed and manufactured to investigate their effects. Table 1 also summarizes the profile of all steel plates (case Nos. 01e07) and Table 2 lists the mechanical properties of the steel plate. The mechanical properties of typical threadbars are also tabulated in Table 2 (Li, 2017). The comparative result revealed that the mechanical properties of the steel plate and typical threadbars are almost the same and therefore the steel plate can simulate threadbar well. The length of the steel plate is set to be 150 mm for encapsulating an adequate number of ribs, because the maximum rib spacing is 40 mm, and both width and height of steel plate are 50 mm.

L

a h

θ b

50 mm

2

150 mm (a)

(b)

Fig. 1. The rib profiles of (a) threadbar and (b) simplified two-dimensional steel plate (Cao et al., 2013b).

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx Table 1 Summary of the profiles of threadbar and steel plates. Case No.

Rib top width, Rib base Rib spacing, Rib height, Rib angle, L (mm) h (mm) a (mm) width, q (
) b (mm)

01 (Threadbar) 02 03 04 05 06 07

2.5 2.5 2.5 2.5 3 2 1.5

4.5 4.5 4.5 4.5 4.5 4.5 4.5

10 20 30 40 10 10 10

1.2 1.2 1.2 1.2 0.9 1.5 1.8

50 50 50 50 50 50 50

Table 2 Mechanical properties of the steel plate and typical threadbars (Li, 2017). Material

Tensile Shank Elastic strength (MPa) elongation (%) modulus (GPa)

Steel plate 630 Threadbars 600e660 (Li, 2017)

14 16e19

Poisson’s ratio

Hardness (HB)

196e206 0.24e0.28 207 190e210

2.2. Grout specimen preparation and test scheme Shear resistance of bolt-grout interface comprises the following three components: adhesion, friction, and mechanical interlocking between the bolt and grout. The effect of adhesion in the pull-out resistance is minor, because the failure tends to occur in the grout specimen even if a significant adhesion exists between the grout and bolt, suggesting that adhesion of bolt-grout interface can be ignored (Windsor and Thompson, 1993; Satola, 2007; Ho et al., 2019). Therefore, the unbonded bolt-grout specimens were prepared in this study. The bolt-grout specimens for direct shear tests consist of the well-matched upper and lower halves of the specimens, i.e. grout specimen and steel plate. The mixture ratio of the grout specimen has an important influence on the shear behavior of the bolt-grout interface. The grout specimens are mostly made of cement (32.5R Portland cement used in this study), sand with a particle size less than 2 mm, and tap water. As such, the water and sand contents can affect the interface shear behavior. The effect of the water content has been intensively studied. According to the results reported by Hyett et al. (1992), the optimum water-cement ratio is about 0.4. However, the effect of sand content has rarely been investigated. To understand the effect of sand content on the interface behavior, three groups of grout specimens with a cement-water ratio of 1:0.4 and different cement-sand ratios (i.e. 1:0, 1:1 and 1:2) were prepared. The maximum cement-sand ratio is limited to 1:2 due to grout fluidity, which decreases as sand content increases. In addition, the early strength agent of calcium formate was also added to the grout mixture (3% of the cement by weight). When preparing the grout specimen, the steel plate was first placed inside the mold, and then the grout mixture was poured into the mold, and finally vibrated for 1 min. All specimens were removed from the mold in 24 h and then cured in a concrete curing box at temperature of (20  1)
C and humidity of no less than 95% for 3 d. The prepared grout specimens are shown in Fig. 2. Cubic specimens with dimensions of 50 mm  50 mm  100 mm and 100 mm  100 mm  100 mm were also prepared from the same casting batch of bolt-grout specimens to obtain the physicomechanical properties of the grout materials. The uniaxial compression tests (specimen size of 50 mm  50 mm  100 mm) and direct shear tests (specimen size of 100 mm  100 mm  100 mm) under different normal stresses were conducted. Uniaxial compression test

3

was repeated at least three times, the direct shear test was repeated at least twice, and the average results were used, as tabulated in Table 3. In this context, a total of 47 direct shear tests were performed to study the influences of bolt profile (i.e. rib spacing and rib height) and grout mixture on the shear behavior of bolt-grout interface under the CNL conditions. The detailed test scheme is listed in Table 4. This stress range is basically consistent with that in Aziz (2002). Repeated tests were also carried out to verify the reproducibility and accuracy of the test results. 2.3. Direct shear tests Bolt-grout interface shear tests were conducted using a rock joint shear test apparatus RJST-616, as shown in Fig. 3 (Zhang et al., 2019). It consists of a vertical piston and a horizontal piston, which can apply normal and shear loads with load capacities of 200 kN and 300 kN, respectively. Both load and displacement control modes can be realized based on a feedback hydraulic servocontrolled system. The load and displacement were measured automatically by a data acquisition system with a sampling frequency of 10 Hz and were displayed in the computer in real-time. The accuracies of displacement sensors and load cells are 0.001 mm and 0.2% full span, respectively. Two U-shaped shear boxes (100 mm  100 mm  47.5 mm) were used to hold the specimen and placed on the linear guide for reducing the tangential resistance. By using U-shaped shear boxes, surface crack propagation evolution process can be observed. Prior to the shear test, the matching steel plate and grout specimen were placed inside the shear boxes. Then the pre-set normal stress was applied at a constant load rate of 0.1 kN/s on the interface. After that, constant normal stress was maintained, and shear stress was applied at a constant shear velocity of 0.005 mm/s until the test was terminated. 3. Test results 3.1. Shear behavior of the standardized bolt-grout interface Fig. 4 shows the shear test results of the standardized bolt-grout interface under different normal stresses. As shown in Fig. 4a, the peak shear strength, residual shear strength, and shear stiffness increased with rising normal stress. On the contrary, the dilation capacity decreased with increase of normal stress (Fig. 4b). The shear

Fig. 2. Steel plate and grout specimen.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

4

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Table 3 Physico-mechanical properties of the grout specimens. Cement-sand-water ratio Density (g/cm3) UCS (MPa) Elastic modulus (GPa) Poisson’s ratio Cohesion (MPa) Internal friction angle (
) Residual friction angle (
) 1:0:0.4 1:1:0.4 1:2:0.4

1.9 2.18 2.21

29.84 29.64 26.36

5.71 7.54 7.62

Table 4 Schemes for bolt-grout interface shear test. Cement-sand ratio Rib spacing (mm) Rib height (mm) Normal stress (MPa) 1:1 1:1 1:0, 1:2

10, 20, 30, 40 10 10

1.2 0.9, 1.2, 1.5, 1.8 1.2

0.1, 0.25, 0.5, 1, 2, 4, 6 0.5, 2, 6 0.25, 0.5, 1, 2, 6

Fig. 3. Direct shear test equipment RJST-616 (Zhang et al., 2019).

stress curves exhibited clearly softening behavior. At the post-peak stage, the peak shear stress first dropped rapidly to residual shear stress and then kept nearly constant with further shear displacement. It can be noted that the degree of brittleness decreased with increase of normal stress, because normal stress would strengthen the interface resistance. For normal displacement curves, it is clear that these curves underwent three stages with respect to the shear displacement, as shown in Fig. 4b. Similar observations were also reported by Meng et al. (2018) in the rock joint shear tests. The changing trend of the first two stages for all dilation curves is the same, i.e. changing from compression to dilation. The third stage of the normal displacement depended on the normal stress level. When normal stress was less than 1 MPa, the normal displacement almost kept constant at the third stage. However, it turned to compression again under normal stress of more than 1 MPa. This scenario can be explained by the change of the failure mode of the bolt-grout interface and would be discussed later.

0.17 0.25 0.22

4.46 4.79 5.12

47.23 48.81 40.43

29.7 34.62 28.83

To better understand the interface shear behavior during the whole shear process, the shear stress and dilation angle versus shear displacement curves under normal stresses of 0.25 MPa and 6 MPa are presented in Fig. 5. Under normal stress of 0.25 MPa, dilational slip failure occurred. At the initial stage of shearing, the bolt-grout interface was compressed, and the shear stress increased approximately linearly until the dilational slip failure load was reached (point 1) where dilational slip occurred. The shear stress of point 1 corresponding to dilational slip failure is 0.853 MPa, accounting for 52% of the peak shear strength which is consistent with the theoretical result derived by Cao et al. (2014b) and Ren et al. (2010). After point 1, the dilation angle and shear stress increased simultaneously with increasing shear displacement until peak shear stress point 2 was reached. Then, a sudden stress drop to point 3 was observed, and the dilation angle slightly increased with increased shear displacement. The maximum dilation angle was obtained at point 3, followed by reductions in the dilation angle and the shear stress until residual shear stress point 4. Finally, the residual shear stress and dilation angle kept constant at the third stage of the shear stress curve. Under normal stress of 6 MPa, the asperities of the grout specimen are cut off completely and sheared-off failure of the bolt-grout interface is a dominating interface failure mode. As can be seen in Fig. 5, it is noticed that the trends of the shear stress curves and dilation angle curves in the first two stages under both low and high normal stress conditions are extremely similar. However, the dilation angle was smaller under high normal stress condition, which caused more damage in the grout specimens than that under low normal stress condition. In addition, the maximum dilation angle was reached at the peak shear stress point. At the third stage of dilation angle curve, the dilation angle became negative, meaning that the bolt-grout interface was re-compressed because the sheared-off plane became flatter resulting from interface friction slip effect as shearing progressed. Based on the results obtained from the direct shear tests, the effect of normal stress on the mechanical parameters of the boltgrout interface at peak shear stress point is plotted in Fig. 6. The shear strength sp increased nonlinearly with increase of normal

Fig. 4. Shear test results of the standardized bolt-grout interface: (a) Shear stress versus shear displacement curves, and (b) Normal displacement versus shear displacement curves.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

5

Fig. 5. Shear stress versus shear displacement curves and dilation angle versus shear displacement curves under different normal stresses.

stress, and the increasing rate was observed to decrease gradually with increase of normal stress. The results are in agreement with that of Moosavi et al. (2005) using the constant radial pressure pullout tests. In contrast, the peak friction coefficient ðs=sÞp decreased monotonously with increase of normal stress where the friction coefficient is defined as the ratio of the shear stress to the normal stress. The decrease in ðs=sÞp gradually diminished as normal stress increased. The dilation angle also showed the same change trend as that of ðs=sÞp , because the apparent friction angle (arctanðs=sÞp ) is equal to the sum of the dilation angle and the basic friction angle. In this instance, the basic friction angle can generally be considered as constant. This implies that the contribution of the dilation effect to the apparent friction coefficient reduced when normal stress was increased, which caused a reduction in the increasing rate of shear strength. In addition, the peak shear displacement sp decreased first and then increased slowly with the increasing normal stress. The reduction in sp may mainly result from the deformation generated in the normal loading stage. As the applied normal stress increased, the elastic deformation and the closure deformation of aperture increased before shear tests and decreased at the initial stage of the shear test, and thus sp decreased. The subsequent increase in sp is related to the increase in the peak shear strength, because the elastic deformation and closure deformation of aperture approximately disappeared at the initial stage of the shear test and shear stiffness is nearly equal to each other. The peak normal displacement np decreased from 0.18 mm to 0.02 mm when normal stress increased from 0.1 MPa to 1 MPa and then kept almost constant when normal stress was greater than 1 MPa. This phenomenon is associated with interface failure modes. The initial reduction in np is due to more asperities being cut off at the pre-peak stage as normal stress increased (Fig. 15). After 1 MPa normal stress, the dilational slip almost disappeared and np kept constant.

Fig. 7 illustrates the effects of rib spacing on the shear stress and normal displacement versus shear displacement curves under normal stress of 0.25 MPa. In this figure, the peak shear strength decreased with increase of rib spacing under the CNL conditions. Besides, the residual shear strength of bolt-grout interface with different rib spacings is almost the same. The degree of brittleness is also related to rib spacing, being weakened with increase of rib spacing. This is because of more asperity damage being involved in the shearing process of the bolt-grout interface with smaller rib spacing. The normal displacement curves show a similar change trend. The maximum normal displacement is slightly lower than the rib height. Therefore, dilational slip failure is a dominating failure mode for these tests. According to the experimental results of Zhang et al. (2018), the interface failure mode depended on applied normal stress level and rib spacing. To reveal the difference in the shear behavior of boltgrout interface with different rib spacings under different normal stress conditions, a comparison of shear behavior of the bolt-grout interface with rib spacings of 20 mm and 40 mm at normal stresses of 0.5 MPa, 2 MPa and 6 MPa is illustrated in Fig. 8. The shear stiffness generally increased as normal stress increased. A comparison of Figs. 7 and 8 indicate that the effect of rib spacing on the degree of brittleness is more pronounced under high normal stress conditions. For steel plate with rib spacing of 40 mm under normal stress of 6 MPa, the interface showed an approximately plastic shear behavior after the peak.

3.2. Effect of rib spacing on the interface shear behavior To study the effect of rib spacing on the shear behavior of the bolt-grout interface, four groups of direct shear tests were conducted under normal stresses of 0.1e6 MPa for four rib spacing levels, i.e. 10 mm, 20 mm, 30 mm and 40 mm. Typical shear test results of the bolt-grout interface with different rib spacings are shown in Figs. 7 and 8, and listed in Table 5. The results revealed that the rib spacing dramatically influenced the shear behavior of the bolt-grout interface.

Fig. 6. Effect of normal stress on the mechanical parameters of the bolt-grout interface at the peak strength point.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

6

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Fig. 7. Curves of (a) shear stress and (b) normal displacement versus shear displacement of the bolt-grout interface with different rib spacings under 0.25 MPa normal stress.

Fig. 8. Results of shear tests on the bolt-grout interface with rib spacings of 20 mm and 40 mm at normal stresses of 0.5 MPa, 2 MPa and 6 MPa: (a) Shear stress versus shear displacement curves, and (b) normal displacement versus shear displacement curves.

Table 6 summarizes the calculated peak dilation angles obtained from normal displacement curves for the bolt-grout interface with different rib spacings. As normal stress increased, the maximum normal displacement and the peak dilation angle both decreased rapidly. Besides, the increasing rib spacing also led to a reduction in the peak dilation angle. The reduction in the dilation angle suggested that the slip plane became flatter for the boltgrout interface with larger rib spacing when dilational slip failure occurred. Fig. 9a illustrates the relationship between the peak shear strength and the normal stress of the bolt-grout interface with different rib spacings. Under the same rib spacing condition, the peak shear strength increased nonlinearly with increasing normal stress. The peak shear strength gradually decreased as the rib spacing increased when normal stress was between 0.1 MPa and 4 MPa for the selected bolt profiles. However, when normal stress was 6 MPa, the peak shear strength first increased at rib spacings of 10e20 mm and then decreased at rib spacings of 20e40 mm. The residual shear strength envelopes of boltgrout interface with different rib spacings are plotted in Fig. 9b. The linear MohreCoulomb criterion was then used to obtain residual strength envelopes as there is a good linear correlation between residual shear strength and normal stress. Meanwhile, the MohreCoulomb parameters can be obtained from the fitting curves. On the basis of the fitting results, it can be noted that only friction strength existed in the bolt-grout interface at the residual shear stage (fitting cohesion is approximately close to 0). The

residual strength and the residual friction coefficient increased with rib spacing. When rib spacing increased from 10 mm to 40 mm, the residual friction coefficient increased by 16.3% from 0.768 to 0.893. Fig. 9c depicts the relationship between the peak normal displacement and the normal stress of the bolt-grout interface with different rib spacings. The peak normal displacement showed a trend of an initial decrease, then increase, and finally stability or further decrease with increasing normal stress. When normal stress was less than 1 MPa, the peak normal displacement decreased at the rib spacings of 10e30 mm and increased at the rib spacings of 30e40 mm. As normal stress increased from 1 MPa to 6 MPa, the peak normal displacement increased at the rib spacings of 10e20 mm and then decreased at the rib spacings of 20e40 mm. It is clear that the peak normal displacement of bolt-grout interface with rib spacing of 20e30 mm is nearly the same. 3.3. Effect of rib height on the interface shear behavior Fig. 10 shows the effects of rib height on the shear stress and normal displacement of the bolt-grout interface under normal stress of 2 MPa. The test results revealed that the peak shear strength increased as rib height increased. Similar behavior has also been noted in discrete element method (DEM) simulations by Shang et al. (2018) that the shear strength increased when the rib height was increased. However, the rib height has no significant

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

7

Fig. 9. Relationships of (a) peak shear strength, (b) residual shear strength, and (c) peak normal displacement with normal stress for various rib spacings.

Fig. 10. Results of shear tests on the bolt-grout interface with different rib heights at a normal stress of 2 MPa: (a) Shear stress versus shear displacement curves, and (b) Normal displacement versus shear displacement curves.

influence on the peak dilation angle, as shown in Fig. 10b, and it slightly influences the peak normal displacement. The peak normal displacements of the bolt-grout interface with rib spacings of 0.9 mm, 1.2 mm, 1.5 mm and 1.8 mm under normal stress of 6 MPa were 0.078 mm, 0.055 mm, 0.057 mm and 0.058 mm, respectively. Fig. 11 shows the relationship between the peak shear strength and rib height under normal stresses of 0.5 MPa, 2 MPa and 6 MPa. It can be observed that there is a marked linear correlation between the peak shear strength and rib height for the range of rib heights. The slope of the fitting curves decreased with the increasing normal stress, which revealed that high normal stress reduced the effect of rib height on the interface shear strength.

3.4. Effect of grout mixture on the interface shear behavior The shear stress versus shear displacement relationships of the bolt-grout interface with different grout mixtures under normal stresses of 0.25 MPa, 1 MPa and 6 MPa are shown in Fig. 12. The results indicated that sand contents in the grout specimen would result in an enhancement of the degree of brittleness at the postpeak shear stage as compared to the grout specimen without sands. This is due to the fact that sand, as a kind of inert filler fine aggregate in grout specimen, can increase the cohesion of grout material, and higher cohesion would result in more loss of shear stress after the peak point (Table 3). The peak shear displacement under different sand contents is almost the same. However, the

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

8

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Fig. 11. Variations of peak shear strength with rib height under normal stresses of 0.5 MPa, 2 MPa and 6 MPa.

Fig. 12. Shear stress versus shear displacement curves of the bolt-grout interface with different grout mixtures at normal stresses of 0.25 MPa, 1 MPa and 6 MPa.

shear behavior is different for the interface with different sand contents when shear stress approached the peak. For grout specimen without sands, a rapid reduction in the shear stiffness is observed, while the reduction in the shear stiffness is small for the grout specimen with sands. This is the cause of the grout deformability. Generally, the grout specimen with lower elastic modulus can generate larger deformation when the stress reached its peak.

The difference in the residual shear strengths of the bolt-grout interface with different sand contents is small. Fig. 13 shows the effects of the sand content and normal stress on the normal displacement. This figure indicated that sand contents affected both the shape of normal displacement curves and the peak normal displacement. It is clear that the interfaces first underwent compression, then dilation, and finally stability or compression for all tests except for bolt-grout interface with a cement-sand ratio of 1:0 under normal stress of 6 MPa which underwent complete compression behavior during the shearing process. Meanwhile, the dilation at the second stage is insignificant for the bolt-grout interface with a cement-sand ratio of 1:1 and 1:2 under normal stress of 6 MPa. This is due to the flatter sheared-off plane being generated along the base under higher normal stress and the difference in the compression capacity of grout specimens with different sand contents. The compression capacity diminished with increasing cement-sand ratio. Fig. 13b depicts the variation of the peak normal displacement of the bolt-grout interface with the normal stress. The peak normal displacement increased with increasing sand content when normal stress was less than 2 MPa. However, it increased first and then decreased with increasing sand content when normal stress was 6 MPa. The peak normal displacement and the peak shear strength showed a similar trend with the sand content (Fig. 14a). Furthermore, the peak normal displacement of the bolt-grout interface with cement-sand ratios of 1:1 and 1:2 was larger than that with a cement-sand ratio of 1:0 due to the effect of sands. The peak and residual shear strengths and corresponding normal stress of the bolt-grout interface with different grout mixtures are shown in Fig. 14. As indicated in this figure, the peak shear strength increased as sand content increased when normal stress was less than or equal to 1 MPa; while it first increased and then decreased with increasing sand content when normal stress was greater than 1 MPa. In addition, the peak shear strength of the boltgrout interface is always the smallest at the cement-sand ratio of 1:0. The shear strength is mainly controlled by cohesion at a low normal stress level. As the sand content increases, it can induce higher cohesion. Nevertheless, at a high normal stress level, the shear strength is controlled by both the friction resistance and cohesion where friction coefficient first increased and then decreased as sand content increased. The maximum friction coefficient was obtained when the cement-sand ratio was 1:1. At the residual stage, the shear resistance is almost dominant in the interface friction. The change trend of residual shear strength with

Fig. 13. Normal displacement characteristics of the bolt-grout interface with different grout mixtures: (a) Normal displacement versus shear displacement curves of bolt-grout interface with different cement-sand ratios at normal stresses of 0.25 MPa, 1 MPa and 6 MPa; and (b) Variation of peak normal displacement with normal stress.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

9

Fig. 14. Peak and residual shear strength envelopes of steel-grout interface with different grout mixtures: (a) Peak shear strength, and (b) residual shear strength.

Fig. 15. Typical interface failure modes of the bolt-grout interface: (a) Dilational slip failure (L ¼ 20 mm, s ¼ 0.5 MPa), (b) Sheared-off failure (L ¼ 10 mm, s ¼ 2 MPa), and (c) Sheared-crush failure (L ¼ 30 mm, s ¼ 6 MPa).

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

10

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

sand content matched that of the residual friction angle of intact grout specimens, which first increased and then decreased with increasing sand content and the minimum residual shear strength was obtained when the cement-sand ratio was 1:2 due to nonexisting cohesion at this stage. 3.5. Interface failure mode The failure mode of the bolt-grout interface is related to the interface shear behavior and failure mechanism. Similar to the failure mode of the bolt-grout interface identified in pull-out tests, sheared-off failure (parallel shear failure), dilational slip failure, and combined failure were also observed in direct shear tests. Interestingly, a new interface failure mode, i.e. ‘sheared-crush’, was also identified in this study (Fig. 15). Fig. 16 shows the effects of normal stress and rib spacing on the interface failure mode. It can be seen that normal stress has a major influence on interface failure mode. For standardized steel plate, under low normal stress level of 0.5 MPa, dilational slip failure is the dominant. In addition, some broken fragments resulting from sheared-off asperities were observed in the gouges adjacent to the loading end (Fig. 16a). As normal stress increased, shear-off failure gradually became the dominant failure mode (Fig. 15b). The rib spacing can also significantly affect the interface failure mode. The effect of rib spacing on the interface failure modes is mainly reflected in the transformation of the interface failure modes under the same normal stress. For example, under normal stress of 2 MPa, the sheared-off failure is a dominant failure mode for bolt-grout interface with 10 mm rib spacing; by contrast, the dilational slip failure is the dominant when rib spacing is 20 mm and above (Fig. 16a, b, d). Under normal stress of 6 MPa, the grout asperity tended to be cut off along the base. In addition to the sheared-off failure mode, sheared-crush was also observed. Schematic diagrams of three types of failure modes are illustrated in Table 7. For sheared-crush failure mode, the grout asperities were not completely sheared off. Instead, the approximately intact grout asperities in front of steel ribs were continuously compressed during shearing accompanied with some tensile cracks resulting from compression effect, as shown in Fig. 15c. The difference in the interface failure modes is attributed to the fact that lower stress interactions in between the ribs existed for the specimens with larger rib spacing. In addition to the interface failure, the spalling failure also occurs outside of grout specimen under high normal stress condition (i.e. 6 MPa) due to the lack of lateral restraint. Fig. 17 shows the typical pictures of the spalling rupture outside the grout specimen. Similar observations were also reported by Meng et al. (2018) with rock joint shear tests. It can be seen that the spalling failure resulted from the tip of ribs. Larger rib spacing can result in more serious surface damage, because tensile cracks can be generated easily under continuous compressive load. Considering that the spalling failure does not exist in the pull-out test, this phenomenon is not analyzed in detail in this context. Compared with sheared-off and sheared-crush failures, the effect of dilation is more pronounced for dilational slip failure. In this case, the failure mode can be identified by dilation curves as well. When the maximum normal displacement is approximately equal to the rib height, the dilational slip failure occurred. When the maximum normal displacement is close to 0, the interface failure mode is sheared-off or sheared-crush failure, depending on rib spacing. When the maximum normal displacement is in between, the combined failure mode occurred. According to Fig. 4b and Table 3, it can be predicted that dilational slip failure mode occurred when normal stress is less than 1 MPa and the shear-off failure occurred when normal stress is larger than 2 MPa for bolt-

grout interface with 10 mm rib spacing. The surface profiles of the bolt-grout interface after shear tests verified the above conclusions (see Fig. 16). Based on dilation curves, it can be concluded that the influence of rib spacing on the interface failure mode is more pronounced than that of rib height and grout mixtures in the range of rib spacing, rib height and grout material tested in this study. This matches the observed failure phenomenon in our tests. 4. Discussion 4.1. Effect of rib face angle In the present study, the rib face angle of all steel plates was simplified as 90
. Because the rib of most rock bolts is not perpendicular to the axial direction of the bolt; therefore, the actual rib face angle is generally less than 90
. According to the below equation (Grasselli et al., 2002), the apparent rib angle is determined by rib angle and rib face angle, and it decreased with the decrease of rib face angle when the rib angle is fixed. The apparent rib angle specifies the contribution of inclined rib to the shear behavior of bolt-grout interface along the shear direction. *

tanq ¼ tan q sin a *

where a is the rib face angle, and q is the apparent rib angle. The lateral displacement is accompanied by the dilation of boltgrout interface with inclined rib during the shearing process, and thus it can further decrease the normal displacement (Khosravi et al., 2018). Therefore, the shear strength and dilation of boltgrout interface with a rib face angle of 90
is the upper limit. It should be emphasized that the influence of an apparent rib angle gradually diminished or even disappeared when the critical apparent rib angle, which refers to as the maximum angle where the slip along the rib surface occurs, was exceeded in the CNL tests. To better understand the combined influence of rib face angle and rib angle on the shear behavior of bolt-grout interface, direct shear tests of bolt-grout interface with different rib face angles and rib angles should be taken into account in the future. 4.2. Optimization of bolt profiles The previous pull- and push-out test results indicated that the shear strength is associated with the rib spacing of rock bolts. The shear strength tended to increase first and then decrease with increasing rib spacing. For example, Aziz et al. (2008) concluded that the loading capacity of the bolt increased when rib spacing increased from 12.5 mm to 37.5 mm and decreased when it further increased to 50 mm. However, the shear strength almost decreased monotonously with increasing rib spacing, as reported in this study. This behavior can be attributed to the difference of shear behaviors of the bolt-grout interface under the CNL and CNS boundary conditions. Under the CNL conditions, the normal stress keeps constant during the shearing process; while it would vary under the CNS conditions according to the input normal stiffness on the bolt-grout interface. For pull-out test, the radial displacement of the bolt-grout interface is restricted by surrounding cylindrical rock mass or steel tube, which leads to the variation of applied radial force on the interface. The boundary conditions should be simplified as CNS conditions rather than CNL boundary conditions in the laboratory. To better understand the effect of rib spacing on the shear behavior of bolt-grout interface, CNS direct shear tests need to be taken into account in future study. On the basis of the dilation curves displayed in Figs. 4, 7 and 8, it can be found that the normal displacement coincides with each other when normal stress is less than 0.5 MPa or greater than

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

11

Table 5 Laboratory test results of the bolt-grout interface with different rib spacings. Normal L ¼ 10 mm L ¼ 20 mm L ¼ 30 mm L ¼ 40 mm stress (MPa) Shear Peak normal Shear Peak normal Shear Peak normal Shear Peak normal strength (MPa) displacement (mm) strength (MPa) displacement (mm) strength (MPa) displacement (mm) strength (MPa) displacement (mm) 0.1 0.25 0.5 1 2 4 6

1.18 1.64 2.37 3.39 4.4 6.01 6.92

0.18 0.07 0.07 0.02 0.04 0.03 0.04

0.95 1.27 1.59 2.5 3.61 5.52 7.3

0.11 0.07 0.05 0.02 0.08 0.09 0.05

Table 6 Peak dilation angle of bolt-grout interface with different rib spacings. Normal stress (MPa)

0.1 0.25 0.5 1 2 4 6

Peak dilation angle (
) L ¼ 10 mm

L ¼ 20 mm

L ¼ 30 mm

L ¼ 40 mm

40.79 46.34 32.51 26.05 16.22 6.69 6.42

34.33 38.93 28.32 21.57 16.61 10.43 2.55

29.65 31.08 20.63 14.15 8.2 5.56 1.27

30.37 30.62 20.06 16.62 7.2 3.82 0.94

4 MPa; while the normal displacement tends to slightly decrease with increase of rib spacing at rib spacing of 20e40 mm and are much larger than that of bolt-grout interface with rib spacing of 10 mm when normal stress is between 0.5 MPa and 4 MPa. Therefore, the actual normal stress applied on the bolt-grout interface would increase with shear displacement under the CNS

0.69 1.03 1.31 1.75 2.85 4.26 6.13

0.05 0.05 0.03 0.06 0.04 0.04 0.01

0.63 0.89 1.21 2.02 2.62 4.11 5.66

0.15 0.06 0.09 0.04 0.03 0.05 0

conditions resulting from dilation. Meanwhile, the peak normal stress would show first increasing and then decreasing trend with increase of rib spacing. According to the actual normal stress applied on the interface and strength envelopes of the bolt-grout interface under the CNL conditions, it can be predicted that the shear strength would first increase and then decrease as rib spacing increases under the CNS conditions. In addition, the shear behavior of bolt-grout interface at the post-peak region can be improved significantly with increasing rib spacing under the CNS conditions. The bolt with larger rib spacing would maintain the peak shear stress to a larger shear displacement, which means that it can absorb more deformation energy to avoid premature failure of the bolting system as compared with the bolt with 10 mm rib spacing. Therefore, it is suggested that using rock bolt with larger rib spacing in rock engineering, especially in soft rock engineering, is preferable. As for the variation of peak shear strength with rib height, they behaved in a similar fashion under both CNL and CNS conditions due

Fig. 16. Effects of normal stress and rib spacing on the interface failure mode: (a) Dilational slip failure (L ¼ 10 mm, s ¼ 0.5 MPa), (b) Dilational slip failure (L ¼ 20 mm, s ¼ 2 MPa), (c) Sheared-off failure (L ¼ 20 mm, s ¼ 6 MPa), and (d) Dilational slip failure (L ¼ 30 mm, s ¼ 2 MPa).

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

12

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Table 7 Schematic diagrams of three types of failure modes. Failure mode

Initial state

Failure process

Grout asperities after failure

Dilational slip

Sheared-off

Sheared-crush

Fig. 17. Shear rupture outside the grout specimens: (a) s ¼ 6 MPa, L ¼ 20 mm; and (b) s ¼ 6 MPa, L ¼ 30 m

to the small difference in the normal displacement curves under the CNL conditions (Fig. 10). Thereby, the bolt with larger rib height is recommended when higher load-bearing capacity is demanded. 4.3. Limitations of this study The rock bolt may be subjected to quasi-static or dynamic loading. An appropriate evaluation of the effects of shear velocity on the interfacial shear behaviors and failure modes is essential. Many researchers have reported the shear rate-dependent test results in rock joints (Atapour and Moosavi, 2014; Tang and Wong, 2016; Wang et al., 2016; Meng et al., 2019). With respect to the variation of shear strength with shear velocity, there are still no consistent conclusions. As for failure modes, the asperities tended to be sheared off easier with increase of shear velocity due to inadequate time for adjusting deformation (Meng et al., 2019). Therefore, both shear behavior and interface failure modes change with shear rate. On the other hand, the cyclic loading affects the shear behavior and failure modes as well (Mirzaghorbanali et al., 2014). However, we did not compare the difference of bolt-grout interface at different shear rates and cyclic numbers. The direct shear tests of the bolt-grout interface with different shear rates and cyclic numbers will be conducted in the future. The rib of rock bolt is simplified as isosceles trapezoid when designing steel plate, which ignored the effects of rib shape. More realistic steel plate is indispensable in future experiments for better revealing the shear behavior and failure mechanism of the boltgrout interface, which can be produced by laser scanning and printing or by die making. In addition, the effects of other parameters, such as rib angle, rib face angle, and rib width, also need to be considered in the shear tests further. 5. Conclusions In the present study, a series of bolt-grout interface direct shear tests was conducted to investigate the effects of bolt profile (i.e. rib

spacing and rib height) and grout mixture on the shear behavior and failure mode under the CNL conditions. The following conclusions can be drawn: (1) Both the shear stress curves and normal displacement curves exhibited multi-stage characteristics during shearing process. (2) Peak-shear strength of the bolt-grout interface is associated with the bolt profiles and grout mixtures. It decreased with increase of rib spacing when normal stress was between 0.1 MPa and 4 MPa. However, it first increased at rib spacings of 10e20 mm and then decreased at rib spacings of 20e40 mm under normal stress of 6 MPa. Higher rib height can lead to greater peak shear strength. In addition, the shear strength showed an increasing trend with increasing sand content when normal stress was less than 1 MPa; while it first increased and then decreased with increasing sand content when normal stress was greater than 1 MPa. (3) The peak normal displacement of the bolt-grout interface is affected by rib spacing, rib height, and sand content, depending on the normal stress applied. (4) The shear behavior of the bolt-grout interface in the postpeak deformation region is mainly governed by rib spacing. Increasing the rib spacing can reduce the degree of brittleness, especially under high normal stress conditions where shear stress remains nearly constant after the peak. (5) Three interface failure modes were observed in the CNL shear tests. In addition to the conventional sheared-off failure mode and dilational slip failure mode observed in the pull-out tests, a new interface failure mode named sheared-crush failure was observed when rib spacing was greater than 20 mm under normal stress of 6 MPa. The interface failure mode mainly depends on the normal stress level and rib spacing as compared to the rib height and grout mixtures.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Declaration of Competing Interest We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. Acknowledgments This work is supported by the Key Projects of the Yalong River Joint Fund of the National Natural Science Foundation of China (Grant No. U1865203), the National Natural Science Foundation of China (Grant No. 51279201) and Special project of the National Natural Science Foundation of China (Grant No. 41941018). The partial support from the Youth Innovation Promotion Association, Chinese Academy of Sciences is gratefully acknowledged. List of symbols

scr Er a b L h

q a q* s sp ðs=sÞp sp np

Ratio of joint wall compressive strengths Ratio of joint wall elastic modulus Rib top width of the bolt Rib base width of the bolt Rib spacing of the bolt Rib height of the bolt Rib angle of the bolt Rib face angle of the bolt Apparent rib angle Initial normal stress on the bolt-grout interface Peak shear stress Peak friction coefficient Peak shear displacement Peak normal displacement

References Atapour H, Moosavi M. The influence of shearing velocity on shear behavior of artificial joints. Rock Mechanics and Rock Engineering 2014;47(5):1745e61. Aziz N. A new technique to determine the load transfer capacity of resin anchored bolts. In: Proceedings of the coal operators’ conference, Wollongong; 2002. p. 176e84. Aziz N, Webb B. Study of load transfer capacity of bolts using short encapsulation push test. In: Proceedings of coal operators’ conference, Wollongong; 2003. p. 72e80. Aziz N, Sinclair S, Aziz N, Jalalifar H, Remennikov A. Optimisation of the bolt profile configuration for load transfer enhancement. In: Proceedings of the coal operators’ conference, Wollongong; 2008. p. 125e31. Blanco Martín L, Tijani M, Hadj-Hassen F. A new analytical solution to the mechanical behaviour of fully grouted rockbolts subjected to pull-out tests. Construction and Building Materials 2011;25(2):749e55. Cai Y, Esaki T, Jiang Y. A rock bolt and rock mass interaction model. International Journal of Rock Mechanics and Mining Sciences 2004;41(7):1055e67. Cao C. Bolt profile configuration and load transfer capacity optimisation. PhD Thesis. University of Wollongong; 2013. Cao C, Nemcik J, Aziz N, Ren T. Analytical study of steel bolt profile and its influence on bolt load transfer. International Journal of Rock Mechanics and Mining Sciences 2013a;60:188e95. Cao C, Ren T, Cook C. Calculation of the effect of Poisson’s ratio in laboratory push and pull testing of resin-encapsulated bolts. International Journal of Rock Mechanics and Mining Sciences 2013b;64:175e80. Cao C, Ren T, Chris C. Introducing aggregate into grouting material and its influence on load transfer of the rock bolting system. International Journal of Mining Science and Technology 2014a;24(3):325e8. Cao C, Ren T, Cook C, Cao Y. Analytical approach in optimising selection of rebar bolts in preventing rock bolting failure. International Journal of Rock Mechanics and Mining Sciences 2014b;72:16e25. Chen J, Hagan PC, Saydam S. Load transfer behavior of fully grouted cable bolts reinforced in weak rocks under tensile loading conditions. Geotechnical Testing Journal 2016;39(2):252e63. Chen J, Hagan PC, Saydam S. A new laboratory short encapsulation pull test for investigating load transfer behavior of fully grouted cable bolts. Geotechnical Testing Journal 2018;41(3):435e47.

13

Ghazvinian AH, Taghichian A, Hashemi M, Mar’Ashi SA. The shear behavior of bedding planes of weakness between two different rock types with high strength difference. Rock Mechanics and Rock Engineering 2010;43(1):69e 87. Ghazvinian AH, Azinfar MJ, Geranmayeh Vaneghi R. Importance of tensile strength on the shear behavior of discontinuities. Rock Mechanics and Rock Engineering 2012;45(3):349e59. Grasselli G, Wirth J, Egger P. Quantitative three-dimensional description of a rough surface and parameter evolution with shearing. International Journal of Rock Mechanics and Mining Sciences 2002;39(6):789e800. Gu XF, Seidel JP, Haberfield CM. Direct shear test of sandstone-concrete joints. International Journal of Geomechanics 2003;3(1):21e33. Hyett AJ, Bawden WF, Reichert RD. The effect of rock mass confinement on the bond strength of fully grouted cable bolts. International Journal of Rock Mechanics and Mining Sciences 1992;29(5):503e24. Hyett AJ, Bawdent WF. A constitutive law for bond failure of fully-grouted cable bolts using a modified Hoek cell. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 1995;32(1):11e36. Ho DA, Bost M, Rajot JP. Numerical study of the bolt-grout interface for fully grouted rockbolt under different confining conditions. International Journal of Rock Mechanics and Mining Sciences 2019;119:168e79. Jalalifar H. A new approach in determining the load transfer mechanism in fully grouted bolts. PhD Thesis. University of Wollongong; 2006. Johnston IW, Lam TSK. Shear behavior of regular triangular concrete/rock joints Analysis. Journal of Geotechnical and Geoenvironmental Engineering 1989;115(5):711e27. Kang H. Support technologies for deep and complex roadways in underground coal mines: a review. International Journal of Coal Science & Technology 2014;1(3): 261e77. Khosravi A, Sadaghiani MH, Khosravi M, Meehan CL. The effect of asperity inclination and orientation on the shear behavior of rock joints. Geotechnical Testing Journal 2018;36(3):404e17. Kilic A, Yasar E, Celik AG. Effect of grout properties on the pull-out load capacity of fully grouted rock bolt. Tunnelling and Underground Space Technology 2002;17(4):355e62. Kilic A, Yasar E, Atis CD. Effect of bar shape on the pull-out capacity of fully-grouted rockbolts. Tunnelling and Underground Space Technology 2003;18(1):1e6. Lam TS, Johnston IW. Shear behavior of regular triangular concrete/rock joints Evaluation. Journal of Geotechnical and Geoenvironmental Engineering 1989;115(5):728e40. Li CC. Performance of D-bolts under static loading. Rock Mechanics and Rock Engineering 2012;45(2):183e92. Li CC. Chapter 2: typical rockbolts. In: Li CC, editor. Rockbolting. Butterworth-Heinemann; 2017. p. 7e35. Li D, Masoumi H, Saydam S, Hagan PC. A constitutive model for load-displacement performance of modified cable bolts. Tunnelling and Underground Space Technology 2017;68:95e105. Ma S, Nemcik J, Aziz N, Zhang Z. Analytical model for rock bolts reaching free end slip. Construction and Building Materials 2014;57:30e7. Meng F, Zhou H, Wang Z, Zhang C, Li S, Zhang L, Kong L. Characteristics of asperity damage and its influence on the shear behavior of granite joints. Rock Mechanics and Rock Engineering 2018;51(2):429e49. Meng F, Ngai L, Wong Y, Zhou H, Yu J, Cheng G. Shear rate effects on the postpeak shear behaviour and acoustic emission characteristics of artificially split granite joints. Rock Mechanics and Rock Engineering 2019;52(7):1e 20. Mirzaghorbanali A, Nemcik J, Aziz N. Effects of shear rate on cyclic loading shear behaviour of rock joints under constant normal stiffness conditions. Rock Mechanics and Rock Engineering 2014;47(5):1931e8. Moosavi M, Jafari A, Khosravi A. Bond of cement grouted reinforcing bars under constant radial pressure. Cement and Concrete Composites 2005;27(1):103e 9. Ren F, Yang Z, Chen J, Chen W. An analytical analysis of the full-range behaviour of grouted rockbolts based on a tri-linear bond-slip model. Construction and Building Materials 2010;24(3):361e70. Satola I. The axial load-displacement behavior of steel strands used in rock reinforcement. PhD Thesis. Helsinki University of Technology; 2007. Shang J, Yokota Y, Zhao Z, Dang W. DEM simulation of mortar-bolt interface behaviour subjected to shearing. Construction and Building Materials 2018;185: 120e37. Tang ZC, Wong LNY. Influences of normal loading rate and shear velocity on the shear behavior of artificial rock joints. Rock Mechanics and Rock Engineering 2016;49(6):2165e72. Thenevin I, Blanco-Martín L, Hadj-Hassen F, Schleifer J, Lubosik Z, Wrana A. Laboratory pull-out tests on fully grouted rock bolts and cable bolts: results and lessons learned. Journal of Rock Mechanics and Geotechnical Engineering 2017;9(5):843e55. Vlachopoulos N, Cruz D, Forbes B. Utilizing a novel fiber optic technology to capture the axial responses of fully grouted rock bolts. Journal of Rock Mechanics and Geotechnical Engineering 2018;10(2):222e35. Wang G, Zhang X, Jiang Y, Wu X, Wang S. Rate-dependent mechanical behavior of rough rock joints. International Journal of Rock Mechanics and Mining Sciences 2016;83:231e40.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004

14

C. Zhang et al. / Journal of Rock Mechanics and Geotechnical Engineering xxx (xxxx) xxx

Windsor CR. Rock reinforcement systems. International Journal of Rock Mechanics and Mining Sciences 1997;34(6):919e51. Windsor CR, Thompson AG. Rock reinforcement e technology, testing, design and evaluation. In: Proceedings of the excavation, support and monitoring; 1993. p. 451e84. Yazici S, Kaiser PK. Bond strength of grouted cable bolts. International Journal of Rock Mechanics and Mining Sciences 1992;29(3):279e92. Yokota Y, Zhao Z, Nie W, Date K, Iwano K, Okada Y. Experimental and numerical study on the interface behaviour between the rock bolt and bond material. Rock Mechanics and Rock Engineering 2018;52(3):1e11. Zhang C, Cui G, Deng L, Zhou H, Lu JJ, Dai F. Laboratory investigation on shear behaviors of boltegrout interface subjected to constant normal stiffness. Rock Mechanics and Rock Engineering 2019. https://doi.org/10.1007/s00603-01901983-6. Zhang C, Cui G, Zhou H, Liu L, Liu Z, Lu J, et al. Experimental study on shear and deformation characteristics of the rod-grout interface. Chinese Journal of Rock Mechanics and Engineering 2018;37(4):820e8 (in Chinese).

Dr. Chuanqing Zhang is a professor at Institute of Rock and Soil Mechanics, Chinese Academy of Sciences (CAS). He is a member of editorial board of Rock and Soil Mechanics, and a member of Chinese Society for Rock Mechanics and Engineering (CSRME) and International Society for Rock Mechanics and Rock Engineering (ISRM). He is one of the first group members and outstanding members of Youth Innovation Promotion Association, CAS, and is the winner of the 9th Youth Science and Technology Gold Award of CSRME. His research interests focus on the rock mechanics theory, testing and prevention technology related to high stress disasters in deep underground engineering. In recent years, Dr. Zhang has chaired one of the Key Projects of the Yalong River Joint Fund of the National Natural Science Foundation of China, hosted and participated in 11 projects funded by national, provincial and ministerial projects, and managed more than 20 national major engineering research projects. The research results have been successfully applied to many large hydropower stations in China.

Please cite this article as: Zhang C et al., Effects of bolt profile and grout mixture on shearing behaviors of bolt-grout interface, Journal of Rock Mechanics and Geotechnical Engineering, https://doi.org/10.1016/j.jrmge.2019.10.004