Review and prospects of shear test of bolted rock joints-深地科学

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Review and prospects of shear test of bolted rock joints

Abstract

Rock bolting is a critical approach in geotechnical engineering for supporting weak rocks. However, weak structural planes, such as rock joints and faults, can easily cause shear deformation of bolts, leading to large shear deformation failure. So, the shear mechanical properties of bolt anchorage are a crucial aspect of engineering stability. Laboratory tests and numerical simulation methods have been adopted earlier to study the shear deformation characteristics, shear instability modes, and shear mechanical models of the bolt. This article summarizes the indoor shear test equipment for bolt anchorage. Then, the research results of the influence of test variables such as rock strength, joint surface roughness, and bolt type on the anchorage shear performance were analyzed. The numerical simulation method for bolt anchorage was also presented, and the advantages and limitations of the numerical methods were analyzed. Finally, based on summarizing and analyzing the relevant research results, the research prospects of new material research and development, indoor test, and numerical simulation of the anchor bolt were put forward, which can serve as a reference for improving the theoretical system of anchor bolt shear resistance and engineering support design.

Highlights

•
The anchorage shear test equipment, test method, and numerical simulation of bolted rock joints were summarized.
•
The shortcomings and limitations of the current research of the anchorage shear test were analyzed.
•
The research prospects of new support material research and development, indoor test, and numerical simulation of the anchor bolt were put forward.

1 INTRODUCTION

Rock stability is crucial in geotechnical engineering. Rock bolting is widely used in support engineering because of its relatively convenient construction, good reinforcement effect, low cost, safety, and reliability (Chen et al., 2017; Yang et al., 2020; Zheng et al., 2021). The rock mass is affected by frequent geological tectonic movement and complex stress fields, and many cracks, joints, faults, and other structural planes are produced. The integrity and continuity of rock mass are seriously damaged by these structural planes, and the rock mass is prone to large deformation and failure phenomena, such as dislocation and separation along weak structural planes, leading to the failure of the anchorage support system (Cui et al., 2022; Gong et al., 2023; He et al., 2005; Li et al., 2016). Field engineering and laboratory tests have shown that bolt and anchor cable failure is often presented as a tension-shear composite failure mode in which the unstable rock mass slips along the structural plane (Li, 2010a; Ma et al., 2019; Wu, Jiang, Wang, et al., 2019). For example, the detachment slip of the surrounding rock along the joint plane, the shear dislocation of the active fault along the fault plane, and the sliding collapse of the landslide along the sliding plane can easily lead to the shear failure of the support materials. Therefore, studying the shear resistance of bolts is critical to guide the anchorage engineering practice.

The shear test on bolted rock joints was first carried out by Bjurstrom (1974) to study the shear performance of the rock bolt. Subsequently, many researchers conducted extensive studies on key test variables, such as bolting angle, rock joint roughness, rock strength, bolt type, and grouting strength, and corresponding results were obtained for anchorage shear resistance, mechanical mechanism, deformation behavior, and shear strength prediction. Liu et al. (2018) studied the research progress in bolting mechanism and theories of fully grouted bolts in jointed rock masses. Zhou et al. (2024) carried out a comprehensive review of experimental studies on the shear behavior of bolted rock joints with varying rock joint and bolt parameters and normal stress. Chen and Xiao (2024) systematically summarized the analytical model and experimental method of anchor bolt anchorage under shear load. The above research results have summarized the research field of anchorage shear resistance of bolt, but the anchorage shear test equipment, simulation method, and further research direction still need to be improved. In this article, the laboratory shear test equipment, test method, and numerical simulation of bolted rock joints were summarized. The shortcomings and limitations of the current research were analyzed, and the research prospects were proposed, which can provide a reference for improving the anchoring support theory and on-site support design.

2 OVERVIEW OF SHEAR RESISTANCE OF BOLTED ROCK JOINTS

The shear resistance of the bolt refers to the restraint on the shear deformation and relative displacement of surrounding rock along the bolt in any direction except axial direction, which mainly includes preventing the surrounding rock from slipping along the weak plane, preventing new shear failure plane in surrounding rock, and limiting the relative rotation of block in surrounding rock. The shear force of bolt anchorage rock joints consists of the shear stress generated by the bolt at the joint cross-section and oblique section and the normal stress generated at the interface between the bolt and anchor agent on the bonding surface or between the anchor agent and rock mass.

The shear deformation characteristics and shear force–shear displacement curves obtained after the interaction between bolt and rock joints are the key to studying the shear performance of bolted rock joints. As shown in Figure 1, the shear load of the bolt locally produces S-shaped deformation near the joint plane. The rock mass on both sides of the joint surface forms a “tensile stress zone” and a “compressive stress zone,” manifested in the separation of the rock mass and the grouting body in the tensile stress zone and the crushing phenomenon of the rock mass and the grouting body in the compressive stress zone due to the extrusion action of the bolt during tensile shear deformation.

Details are in the caption following the image — Figure 1
Open in figure viewer PowerPoint

Local shear deformation of bolted rock joints. (a) Indoor test diagram and (b) schematic diagram (Ge & Liu, 1988; He et al., 2023).

Figure 2 shows the shear force–shear displacement characteristic curves obtained by the shear test of unbolted and bolted rock joints (He et al., 2023). The shear load of unbolted rock joints increases with the shear displacement until it exceeds the bond strength of cement grout on the joint surface, debonding the rock joints and the cement grout, leading to a sharp decline in the shear force instantaneously (section AB). Then, it mainly relies on the physical friction between the two sides of the joint to resist the horizontal shear (section CD), and its failure has a distinct brittle failure characteristic. However, when a bolt anchors the rock joints, the bolt can exert shear resistance. After the cement bonding effect of the joint surface fails, the jointed rock mass can still maintain high shear strength (section FH) under large shear displacement due to the reinforcement effect of the bolt. Unlike the brittle failure characteristics of unbolted jointed rock mass, the bolted rock joints have a prominent ductile failure, which is significant to the stability of the jointed rock mass.

Summarily, the reinforcement of rock joints by bolts is mainly reflected in two aspects.

(1)
The intrinsic shear resistance of the bolt plays a certain role in restraining the shear dislocation of rock joints, that is, the lateral “pin” effect.
(2)
After shear dislocation occurs in the surrounding rock, additional axial stress is generated through axial deformation of the bolt, increasing the normal stress of the joint plane and then increasing the shear strength of the joint plane, that is, the axial “restraint” effect. The bolt can effectively improve the shear strength of the rock joints and restrain the dislocation of the rock mass.

3 LABORATORY TESTS

3.1 Test equipment

The shear tests of bolted rock joints are primarily used to study the shear mechanical properties and resistance mechanism of the bolt. The shear test types mainly include single and double rock joint shear tests (Zhang, Xu, et al., 2022). The principle of the two test methods is the same. Both test methods make shear dislocation deformation of jointed rock mass samples by applied shear loads, and the anchorage shear performance of the bolt is studied by monitoring the data of shear force and shear displacement. Figure 3a shows the schematic diagram of the single joint shear test. The sample mainly comprises two rock masses, and the operation is simple. The anchorage shear test can be conducted under varying normal stresses and anchoring angles. However, the single shear test generally has a low shear load and small shear displacement, so it can only complete the small-scale shear test, and the failure displacement of the bolt may not be achieved. In addition, the shear friction between the joint surfaces cannot be eliminated, and the pure shear state of the bolt cannot be attained. Figure 3b illustrates the double-joint shear test. The double-joint shear sample comprises three rocks with two symmetric shear planes; the engineering scale bolt can be carried out, and the prestress can be applied at both ends of the bolt. In addition, the shear friction of the joint surface can be eliminated by setting gaps between rock samples, but applying normal stress in the double shear test is challenging. Based on these two anchoring shear test principles, many bolt shear test equipment has been continuously developed and improved, and representative relevant shear equipment is presented as follows.

3.1.1 Single shear test device

Table 1 is a summary of the single shear test device. The principle of the single joint plane shear test is the same as that of the conventional direct shear test, so the conventional direct shear device can be used to study the shear performance of bolted rock joints (He et al., 2022, 2023; Liu & Li, 2018; Zhang & Liu, 2014). With further development, the design of single shear test factors and variables of bolted joints becomes more complex and difficult. Jiang et al. (2004) and Thirukumaran and Indraratna (2016) found that the normal stress on deep rock mass changes constantly with shear displacement, which is more consistent with the boundary condition of constant normal stiffness (CNS) than with constant normal loading (CNL). Therefore, MIS-233-1-55-03 shear equipment capable of carrying out constant normal stiffness loading has been developed. A comparative study was conducted on shear tests of bolted joints under CNL and CNS boundary conditions (Jiang et al., 2021). To meet the shear test requirements under conditions of medium and high strain rates, a large-scale rock mass dynamic performance test system was developed by Qi et al. (2020), and the bolted shear tests under conditions of different loading rates were conducted (Luo et al., 2023).

Table 1. Summary of the single shear test device.

No.	Equipment parameter	Functional evaluation	Reference
1	RMT-301 rock mechanics loading test system Maximum normal load: 1500 kN Maximum tangential load: 500 kN Sample size: 100–200 mm cube Maximum stroke of vertical and horizontal: 50 mm	It can meet the basic test requirements of small-scale anchor bolt shear under quasi-static conditions, cannot apply prestress, and has small shear displacement.	Liu and Li (2018)
2	JAW-600 rock shear seepage testing machine Maximum vertical load: 600 kN Maximum horizontal load: 600 kN Maximum stroke: 25 mm Sample size: 200 mm × 100 mm × 100 mm Loading speed: 0.01–100.00 mm/min		Wang et al. (2018); Zhang et al. (2022)
3	Large-scale rock mass dynamic performance test system Maximum vertical load: 500 kN Maximum horizontal load: 1000 kN Maximum sample size: 1000 mm × 200 mm Loading speed: 0.001–1000 mm/s	Can satisfy the anchorage shear test loading of small-scale bolts under quasi-static and medium and high strain rate conditions.	Qi et al. (2020)
4	MIS-233-1-55-03 shear instrument Maximum vertical load: 200 kN Maximum horizontal load: 200 kN Sample size: 200 mm × 100 mm × 100 mm	Can satisfy the anchorage shear test load of the small-scale bolt under constant normal stiffness and constant normal strength.	Jiang et al. (2004)
5	SINTEF/NTNU anchor test device Maximum axial load: 500 kN Maximum tangential load: 600 kN Sample size: two cubes with a side length of 0.95 m	It has the functions of full-size bolt independent drawing, independent shear and tensile shear coupling test, but the friction between the sample and the equipment frame is large during loading, and the normal stress cannot be applied.	Chen (2014); Stjern (1995)
6	Central South University bolt tensile shear comprehensive test equipment Maximum axial load: 500 kN Maximum tangential load: 600 kN Sample size: two cubes with a side length of 1 m	The roller design reduces the friction between the sample and the equipment frame, can meet the basic test requirements of full-size anchor shear under quasi-static conditions, and cannot apply normal stress.	Chen et al. (2020)
7	Indian Institute of Technology large-scale direct shear test system Maximum vertical load: 1500 kN Maximum horizontal load: 2000 kN Sample size: two 75 cm × 75 cm × 90 cm blocks	Large group anchor shear tests can be carried out, and prestress cannot be applied.	Srivastava et al. (2015)
8	MISSR single shear device Maximum shear load: 1200 kN A single cylindrical specimen has a diameter of 25 cm and a length of 1.8 m.	A single shear test of bolt and cable with a maximum anchor length of 3.6 m can be carried out, and prestress cannot be applied.	Aziz et al. (2018)
9	The State Key Laboratory of Efficient Coal Mining and Clean Utilization Testing equipment for comprehensive mechanical properties of rock bolt Maximum axial load: 610 kN Maximum tangential load: 1100 kN Sample size: The diameter of the specimen is 12 cm, and the length is 1 m	It has the functions of independent stretching, torsion, bending, shearing, and impact, but it does not have the function of pure shear strength test of prestressed anchor bolts.	Kang et al. (2020)

Due to the limitations of sample size, equipment load, and shear displacement, conventional direct shear equipment can only carry out the anchorage shear test of small-scale bolts. To address this issue, the test device dedicated to the shear performance of full-size bolts has been improved and developed. The SINTEF/NTNU bolt test device developed by SINTEF in 1995 in Trondheim, Norway, and the Rock Mechanics Laboratory of the Norwegian University of Science and Technology is the most representative full-size tensile-shear comprehensive test device, which has the functions of full-size bolt pulling, shearing, and tensile shear coupling test. Nevertheless, the friction between the sample and the equipment frame is significant. In 2020, a new bolt tension and shear comprehensive test device was designed by Central South University in China based on the SINTEF/NTNU bolt test device to reduce the friction between concrete specimens and rigid frame, but the device could not apply normal stresses (Chen et al., 2020). In 2015, the servo-controlled large-size direct shear test device was developed by the Indian Institute of Technology to carry out the group anchor test and large-scale anchorage shear test. The device could increase the loading performance and the size of the shear box. The shear force applied by the servo-controlled hydraulic jack could reach 2000 kN, and the maximum normal force could reach 1500 kN, satisfying the anchoring shear test under the condition of a single anchor or group anchor of a full-size bolt (Srivastava & Singh, 2015). A single shear device named “MISSR” was developed by Megabolt Company in Australia in 2018 to test the shear performance of a longer-length bolt or anchor cable. The device comprised two 90 cm cylindrical specimens connected by a 1200 kN press, and the single shear test on bolts and cables with anchor lengths up to 3.6 m could be carried out (Aziz et al., 2018). In 2020, the State Key Laboratory of Efficient Coal Mining and Clean Utilization in China successfully developed a new comprehensive mechanical properties test device. The device has independent functions of stretching, twisting, bending, shearing, and impact, and each functional module can be combined. The device can test the longest anchorage length of 1.2 m. However, a high strain rate test cannot be performed on it (Kang et al., 2020).

3.1.2 Double shear test device

Table 2 is the summary table of the double shear test device. Aziz et al. (2016) developed a double shear device for small-size bolts, which adopted a universal testing machine as the shear load-applying unit. However, the size range of the device for testing bolts was minimal, and it was not suitable for testing large-size bolts and higher loads. Craig and Aziz (2010) scaled up the initial device, developed the MKII double shear device, and replaced the small universal testing machine with a large press because of the small size of the test piece and the limited output shear load of the small universal testing machine. However, it was difficult to test the pure shear strength of prestressed anchor bolts due to the high friction caused by the close contact between specimens. Aziz et al. (2016) further improved the MKII double shear device by equipping it with an independent axial rigid frame to provide transverse support and avoid close contact between concrete specimens during the prestress shear process of the bolt, thus blocking the transfer of prestress between specimens and eliminating the friction generated by the shear surface. Imperial College London (Grasselli, 2005), China General Institute of Coal Research (Li, 2019), and China University of Mining and Technology-Beijing (Tong et al., 2023) have also developed and produced double shear test devices, whose loading function, test principle, size, and load are similar to the MKII double shear device. Li, Hagan, et al. (2019) changed the central shear load unit of the conventional double shear device from a press to an impact hammer to develop a double impact shear device with a maximum theoretical impact energy of 5.439 kJ (185 kg × 9.8 m/s2 × 3 m) to realize the double shear test of anchor rod under impact load. With a maximum impact speed of 7.67 m/s, the device can test up to 0.9 m anchorage length bolts; however, compared with the loading requirement of higher strain rate, it cannot be achieved at present.

Table 2. Summary of the double shear test device.

No.	Equipment parameter	Functional evaluation	Reference
1	British Imperial College bolt double shear device The size of a single concrete specimen is 60 cm × 60 cm × 100 cm.	Not suitable for large-size bolts and high test loads.	Grasselli (2005)
2	Wollongong University MKII double shear device Maximum length of specimen: 105 cm Specimen size at both ends: 30 cm × 30 cm × 30 cm Middle specimen size: 30 cm × 30 cm × 45 cm	The shear surface produces a higher friction force when the specimens are in close contact after prestressing.	Craig and Aziz (2010)
3	Wollongong University small bolt double shear device Maximum length of specimen: 60 cm Specimen size at both ends: 15 cm × 15 cm × 15 cm Middle specimen size: 15 cm × 15 cm × 45 cm	Not suitable for large-size bolts and high test loads.	Aziz et al. (2016)
4	Wollongong University MKIII double shear device Maximum length of specimen: 105 cm Specimen size at both ends: 30 cm × 30 cm × 30 cm Middle specimen size: 30 cm × 30 cm × 45 cm	An independent axial rigid frame and transverse support are set up to eliminate the friction generated by the shear surface during prestressed loading.	Aziz et al. (2016)
5	China University of Mining and Technology-Beijing bolt double shear device Maximum shear load: 6000 kN Maximum shear displacement: 250 mm Maximum tensile load: 1200 kN Maximum tensile displacement: 200 mm Single concrete specimen size: 300 mm × 300 mm × 300 mm	The double shear test on full-size bolts can consider the combined loading of prestress and axial force of the bolt, and the normal stress cannot be applied.	Tong et al. (2023)
6	Bolt impact double shear device Maximum theoretical impact energy: 5.439 kJ (185 kg × 9.8 m/s2 × 3 m) Maximum impact speed: 7.67 m/s Single concrete specimen size: 30 cm × 30 cm × 20 cm	Impact loads can be applied.	Li, Hagan, et al. (2019)
7	HiBay laboratory, Faculty of Engineering and Information Sciences, University of Wollongong, dynamic shear device Maximum theoretical impact energy: 23.2 kJ (592 kg × 9.8 m/s2 × 4 m) Maximum impact speed: 8.85 m/s Single concrete specimen size: 30 cm × 30 cm × 20 cm	Impact loads can be applied.	Khaleghparast et al. (2023)

Overall, the shear test device of anchored rock joints is gradually improved in terms of loading function, meeting the test requirements of greater shear load, shear displacement, and sample volume. So the shear test for rock mass with anchored joints can be well completed under conventional test variable conditions. However, the collaborative coupling function of multiple loading conditions still needs to be further improved, which cannot meet the loading requirements of complex situations. For example, the application of normal stress on the sample often causes the unloading of the prestress of the bolt, so the two cannot be applied simultaneously, and the intelligent operation and refinement level of the device must be further improved. Also, most of the devices can only complete the shear loading under a quasi-static or low strain rate state, and hence, the anchoring punch shear test devices of high strain rate and ultrahigh strain rate still need to be further developed.

3.2 Test variable analysis

Many factors affect the shear properties of bolted rock joints, such as the geometrical and mechanical parameters of rock mass, bolt, and structural plane. Based on the indoor anchorage shear test, researchers have carried out a lot of research on the test variables (Table 3), such as the strength of rock specimen, the type of bolt, the anchoring angle, and the roughness of the joint surface.

Table 3. Shear test statistics of bolted rock joints.

Experiment type	Sample size (mm) (length × width × height)	Host rock type	Bolt type/Bolt diameter	Main content	Reference
SS	/	Granite	Full-length bonded bolt	Bolting angles	Bjurstrom (1974)
SS	/	Gypsum, basalt	Rebar bolts	Bolting angles	Dight (1982)
SS	/	Limestone, schist	Resin-grouted bolt	Surrounding rock strength, bolting angles	Ansell (2005); Haas (1976)
SS	150 × 150 × 130 220 × 200 × 150	Granite, concrete, sandstone	Rebar bolts/8, 10 mm	Strength of surrounding rock, diameter of anchor bolt	Spang and Egger (1990)
SS	300 × 400 × 800	Concrete, granite	Rebar bolts/8, 12 mm	Bolt strength, bolt diameter, strength of surrounding rock	Ferrero (1995)
DS	600 × 150 × 150	Concrete	AX, AXR, JAB bolt/24 mm	Concrete strength, prestress	Ansell (2005); Aziz et al. (2003)
DS	180 × 60 × 100	Concrete	Swellex bolt/16, 20 mm	Bolting angles, bolt types	Grasselli (2005)
DS	600 × 150 × 150	Concrete	Rebar bolts/24.7 mm	Bolt profile, surrounding rock strength	Jalalifar et al. (2006)
SS	950 × 950 × 950	Concrete	Rebar bolts, D-Bolts/20 mm	Anchoring angle, bolt type, strength of surrounding rock	Chen and Li (2015); Chen (2014)
SS	750 × 750 × 900	Concrete	Fully grouted passive bolts/10 mm	Normal stress, bolt number	Srivastava and Singh (2015)
DS	150 × 150 × 450 300 × 300 × 1050	Concrete	Fiber glass bolts, rock bolts, cable bolts	Bolt type, bolt diameter, prestress	Li et al. (2016)
SS	100 × 200 × 90	Rock-like materials	S45C steel/3 mm	Joint roughness	Wu et al. (2017)
DS	250 in diameter, 900 in length	Concrete	Fully encapsulated cable bolts/30 mm	Cable diameter, cable geometry, prestress	Aziz et al. (2018)
SS	150 × 150 × 150	Concrete	HRB400/4 mm	Joint roughness	Chen et al. (2018)
DS	200 × 100 × 100	Rock-like materials	Anchor bolt/10 mm	Joint roughness, bolt elongation	Wang et al. (2018)
SS	100 × 200 × 100	Rock-like material	Rebar bolts/6 mm	Cyclic loading, joint roughness	Wu, Jiang, Wang et al. (2019)
SS	400 × 400 × 400	Concrete	8, 12 mm	Bolting angles, grout strengths, shear loads, bolt diameter	Li and Liu (2019)
DS	300 × 300 × 200	Concrete	Rock Bolts/8, 16 mm	Dynamic loading	Li, Hagan, et al. (2019)
SS	100 × 100 × 100	Rock-like material	HRB400 rebar/6 mm	CNL and CNS conditions, bolt inclination angle	Cui et al. (2020)
DS	900 × 150 × 150	Concrete	HRB500 steel, CRM700 steel/13 mm	Dynamic loading, Pretension	Shan, Bao, et al. (2021)
SS	100 × 100 × 200	Rock-like material	thread bar/6 mm	Non-persistent joints	Zhang, Jiang, et al. (2022)
SS	150 × 150 × 120	Rock-like material	#11 wire/3 mm	Bolt number, grouting state of the joint, bolt inclination angle	Lin et al. (2022)
SS	150 × 150 × 150	Concrete	HRB400/10 mm	Bolting angles, bolt number, bolt spacing	Liu et al. (2022)
DS	100 × 100 × 100	Rock-like materials	China DIN 975 steel/10 mm	Prestress, connectivity rates of joints	Ding et al. (2022)
SS	150 × 150 × 150	Granite, marble, sandstone	NPR steel, Q235 steel/8 mm	Surrounding rock strength, bolt type, normal stress	He et al. (2022)
SS	700 × 700 × 1400	Concrete	1860 anchor cable/15.2 mm	Prestress	Wang et al. (2023)
DS	300 × 150 × 150	Concrete	Ribbed bolt, smooth bolt/18 mm	Tendon type, pretension, drop height	Khaleghparast et al. (2023)
SS	150 × 150 × 150	Marble	NPR steel, Q235 steel, #45 steel/8 mm	Bolt type, normal stress	He et al. (2023)
SS	150 × 150 × 150	Granite	NPR steel, HRB400e steel/8 mm	Shear rates	Luo et al. (2023)
DS	300 × 300 × 300	Concrete	Anchor cable/21.6 mm	C-Shaped Tube, Preload	Shan et al. (2021, 2022); Tong et al. (2023)
SS	200 × 100 × 100	Rock-like materials	Carbon steel/6 mm	Bolted mode	Jiang et al. (2024)
SS	100 × 100 × 50	Red sandstone, black sandstone, green sandstone, white sandstone, coal	8.8-grade carbon steel/6 mm	Anisotropic rock joint	Jiang et al. (2024)
SS	200 × 100 × 100	Rock-Like Materials	6061–T6 aluminum rod/6 mm	JRC, anchorage depth	Han et al. (2024)

3.2.1 Strength of rock specimen

The mechanical properties of rock mass vary significantly in geotechnical engineering under different working conditions, considerably impacting the performance of bolt support. Representative samples of soft and hard rock jointed rock have been used to conduct laboratory tests to study the shear strength contribution and deformation and instability characteristics of bolts under different rock strength conditions. Spang and Egger (1990) used red sandstone, concrete, and granite to experimentally study the influence of surrounding rock strength on the shear performance of anchorage and showed that the shear contribution of the bolt in soft rock is greater than that in hard rock under the same conditions. Ferrero (1995) showed that the higher the strength and stiffness of the surrounding rock, the more beneficial the play of the shear effect of the bolt. Jalalifar et al. (2006) studied the load transfer mechanism and mechanical properties of rock mass with varying strengths under different prestressed forces through indoor double shear tests and confirmed that rock mass strength and prestressed magnitude greatly influence the shear resistance of joint surfaces, shear displacement, and failure characteristics. Zhang and Liu (2012, 2014) also validated that rock mass strength significantly influences the yield form of the bolt. Bolts generally produce bending yield in soft rocks, whereas tensile shear yield occurs in hard rocks. Chen (2014) found that the yielding speed of the bolt is faster in hard rock. Liu and Li (2018) used white sandstone, marble, and granite as the representatives of soft, medium-hard, and hard rocks, respectively, to conduct anchorage shear tests and proved that the higher the strength of rock mass, the greater the improvement in the shear resistance to the rock joints, and the better the reinforcing effect. He et al. (2023) experimentally investigated the influence of surrounding rock strength on the anchorage shear resistance of 2G-NPR bolt and found that the failure displacement and shear absorption energy effect of the bolt decreased with the increased strength of the surrounding rock (Figure 4). Under the lower strength of the surrounding rock, the bolt would yield at the plastic hinge and then produce tensile bending failure. As the strength of the surrounding rock increases, the bolt is transformed into tension shear failure (Figure 5).

The research studies show that the strength of surrounding rocks significantly influences the shear contribution and the deformation and failure characteristics of the bolts. The shear strength of the joint of the high-strength surrounding rock is better than that of the low-strength surrounding rock, but the rock bolts often exhibit better deformation in the low-strength surrounding rocks. Tensile-shear yield and tensile-shear failure mainly occur when the high-strength surrounding rock is anchored by a bolt, while tensile-bend yield and tensile-bend failure often occur in the low-strength surrounding rock. However, there is no unified understanding of the critical quantitative criteria of tensile shear and tensile bend modes. In addition, the strength of the surrounding rock and the mechanical properties of the anchor bolt should be relative. The same kind of bolt may show tensile bending or tensile shear deformation and failure characteristics under varying strengths of rock mass. Also, the same strength rock mass may cause different deformation and failure characteristics for bolts having different mechanical properties. Therefore, the synergistic relationship between the strength of the surrounding rock and the shear performance of the anchor bolt should be fully considered in the study of the strength of the surrounding rock, and the coupling evaluation criteria of the strength of the surrounding rock and the performance of the bolt should be established.

3.2.2 Joint surface roughness

The dilatancy effect of varying roughness and the wear, climb, and nibbling of the joint surface during shearing affect the anchorage shear resistance of the bolt. Wu et al. (2017) showed that the bolt has a good inhibition effect on the dilatancy of joint planes through a comparative study of shear tests of rock mass with and without anchorage. Chen et al. (2018) studied the influence of joint surface roughness on the deformation characteristics of bolts and demonstrated that the length of the transverse shear deformation section of the bolt gradually increased with the increased joint surface roughness (Figure 6). Zhang (2020) argued that with the increased joint roughness, the peak shear strength and residual shear strength of bolted joints increase to varying degrees. Wang et al. (2021) explored the influence of JRC values on the deformation mechanical properties of the bolt and found that the dilatation effect of rough joint surface can increase the axial deformation ability and axial force of the bolt and change the failure mode of the bolt from tensile shear to tensile bending.

The tests were primarily carried out to study the shear performance of anchored rough-jointed rock mass with the same wall strength. In practical engineering, the soft interbedded geological structures are widely distributed, and researchers call this kind of joint with different wall strengths a heterotropic joint or soft and hard interbedded joint. Shear tests on anisotropic joints show that the shear properties of the anisotropic structural plane are more similar to those of the side of the rock mass with relatively weak strength (Mouzannar et al., 2017; Renaud et al., 2021). Jia (2021) carried out anchorage shear tests on jointed rock mass with varying wall strengths and showed that the side with lower strength has a greater degree of failure. Also, the wall strength ratio affects the characteristics of anchorage shear curves and crack development. Gao et al. (2023) believe that the shear strength of anchored jointed rock mass increases with weak sidewall strength.

The literature review presented earlier shows that the dilatancy effect not only directly affects the shear strength of the joint surface but also affects the mechanical properties of anchored joints because the dilatancy angle changes the relative displacement of the joint surface during the shear process. The larger the dilatancy angle, the smaller the deflection of the bolt relative to its axial deformation, that is, the weaker the “pin effect” of the bolt and the greater the effect of the tensile performance of the bolt. However, at present, the evaluation of the shear strength of the anchor rough joint is usually only qualitative, and the quantitative expression of the shear strength and roughness morphology of the anchor roughness joint and the parameters of the bolt still need to be further studied.

3.2.3 Bolt type

Various bolts have been successfully developed for engineering construction and resource exploitation, considering the supporting effect, economic benefit, engineering conditions, and other factors. The double shear test of a full-length bonded bolt and expansion hollow bolt was carried out by Grasselli et al. (1999, 2005). When the shear displacement was small, the contribution of the full-length bonded bolt to the shear strength of the rock joint reached 75% of its ultimate tensile strength, whereas that of the expansion hollow bolt was only 20%. When the bolt fails, the contribution of the full-length bonded bolt to the shear strength of the joint surface can reach 85%–95% of its ultimate tensile strength, whereas the expansion hollow bolt is only 65%–75%. Chen and Li (2015) comprehensively studied the mechanical behavior of full-length bonded bolts and D-type energy-absorbing bolts under tensile shear load, and the results showed that D-type bolt has a broader range of plastic deformation segments and substantial deformation and damage resistance. Li et al. (2016) evaluated the shear performance of rock mass anchored by glass fiber bolt, steel rebar bolt, and ordinary anchor cable by double shear test. It was shown that steel rebar bolts made the most significant contribution to the shear strength, while glass fiber bolts used for temporary support exhibited the least effect. A tension and compression composite energy absorbing bolt (TCC bolt) was developed by Wu, Jiang, et al. (2019), which demonstrated a large deformation performance through structural design, while the anchorage shear test results showed significantly improved shear performance than ordinary bolts. Zhang, Jiang, et al. (2022) conducted a comparative study on the shear performance of a new energy-absorbing bolt and an ordinary rigid bolt under joint cracks of different shapes. The results showed that the shear performance of a new type of bolt was greatly affected by the crack angle and crack shape in the rock mass. The new bolt had a better inhibition effect on shear deformation and dilatation of the jointed rock mass than that of an ordinary bolt. Zhang, Wang, et al. (2022) compared the shear properties of basalt fiber reinforced polymer (BFRP) and traditional reinforcement anchorage joints and found that the shear stiffness of BFRP bolt at the elastic stage was lower than that of conventional reinforcement bolts, yet the peak shear displacement was larger and the residual shear strength was higher. He et al. (2023) conducted indoor shear tests on rock mass anchored by 2G-NPR steel under different normal stress conditions. Q235 steel and #45 steel, two representative common anchor steels, were set as the control group (Figure 7). 2G-NPR steel did not exhibit fracture failure under large transverse shear deformation, whereas Q235 steel and #45 steel had fracture failure under small deformations. The equivalent cohesion, internal friction angle, and shear strength of the rock joints showed the greatest improvement of 2G-NPR steel and had super energy absorption characteristics.

The research studies presented above show that the bolt material properties can significantly affect the shear resistance of the anchorage joint. Also, developing high-strength and high-ductility bolts can substantially improve the shear strength and shear displacement of rock joints. However, the shear resistance properties and mechanical mechanism of the new bolt under complex conditions still need to be further studied. In addition, since research on the shear performance of different types of bolts is often carried out using various test methods under different test conditions, it is impossible to make a quantitative comparison between the shear performance of bolts. Therefore, a unified shear test method and quantitative evaluation criteria for the anchorage joints of bolts should be established.

3.2.4 Anchorage angle

The anchoring angle is the angle between the bolt layout angle and the joint plane. In engineering practice, the anchoring angle is the angle between the bolt and the horizontal plane. Bjurstrom (1974) performed shear tests on bolted granite jointed rock mass, demonstrating that the anchoring angle significantly influences the strength of the “pin effect” exerted by the bolts. When the anchoring angle is below 35°, the bolts mainly suffer tensile failure, while the transverse shear effect is negligible. Dight (1982) showed that the inclined bolt arrangement could attain higher joint plane shear stiffness than the vertical arrangement. Haas (1976) and Ansell (2005) also found the same phenomena and conclusions through shear tests on limestone and schist anchorage at varying angles. Ge and Liu (1988) analyzed the influence of anchoring angles on the shear performance of concrete test blocks with through joints. The results showed that the bolt had a good toughening and cracking arresting effect; also, the shear performance was enhanced the most when the anchoring angle was 60°; 45°–75° was the optimal installation angle. Liu et al. (2017) and Li (2019) examined the influence of anchoring angle under different joint friction angles, confirming that the bending deformation angle and transverse deformation length of the bolt under tensile-shear condition increase with the anchoring angle. Also, when the anchoring angle is smaller than the joint friction angle, the “pin effect” of the bolt can be ignored, and the transverse shear resistance is zero. Li et al. (2021) verified that the anchorage angle and shear direction significantly affect the shear resistance of the bolt. In actual engineering, the installation angle of the bolt should be below 90°, and there is an optimal installation angle. Liu et al. (2022) conducted the double shear test to compare the shear strength under different anchoring angles, and the results showed that the shear strength of the anchoring joint was the largest when the anchoring angle was 60°. In addition, the anchoring angle also had a certain influence on the failure mode of the bolt. When the anchoring angle is small, the bolt exhibits tension shear failure; when the anchoring angle is large, the bolt has tension bending failure.

Based on the previous findings, it is concluded that the anchorage angle significantly influences the shear resistance, especially the “pin effect.” Also, the deformation and failure characteristics of the bolt (tensile-shear failure, tensile-bending failure, and tensile failure) are closely related to the anchoring angle. There is an optimal anchoring angle for bolt anchoring. Still, there is no unified understanding of the quantification criteria of the optimal anchoring angle, which is generally believed to be between 30° and 60° (Figures 8 and 9).

3.2.5 Other test parameters

The anchoring type can be classified as prestressed active support and non-prestressed passive support based on whether the prestressing is applied or not, and full-length anchorage and end anchorage according to its anchoring length. Based on double shear tests, Jalalifar et al. (2006) found that the strength of rock mass and prestress greatly influence the shear resistance of the joint surface, shear displacement, and the failure characteristics. Wang et al. (2016) confirmed that prestressed anchorage significantly improved the shear resistance and shear stiffness of the initial section of the shear-displacement curve and made the final section of the curve exhibit plastic strengthening. Zhang (2010) proved that prestress has a certain effect on the tangential stiffness and peak shear strength of anchored joints through shear tests with or without prestress. Teng et al. (2017, 2018) used 45# steel and preset concrete blocks to conduct shear tests, proving that the full-length anchorage specimen can withstand greater shear force, and the bolt resistance increases faster. Chen (2019) conducted a study on the mechanical properties of rock samples with discontinuous joints and proved that full-length anchoring can significantly improve the mechanical properties of anchoring, and its peak strength and elastic modulus both increase.

Researchers have also studied the effects of load type, bolt diameter, grouting strength, and joint penetration. Spang and Egger (1990) experimentally investigated the influence of bolt diameters on the shear properties of anchorage and showed that the contribution of the bolt to the shear properties of the shear joint plane is proportional to the cross-sectional area of the bolt. Wu, Jiang, Gong et al. (2019) studied the failure characteristics of bolts and the reinforcement effect of jointed rock mass under cyclic shear load, indicating that the shear effect of bolts is greatly affected by cyclic displacement. Cui et al. (2020) carried out shear tests under CNL and CNS boundary conditions and studied the influence of different normal boundary conditions on the shear performance of anchored jointed rock mass. The joint surface failure phenomenon is more severe in the shear process of anchored rock mass under CNS conditions. Zhang, Jiang et al. (2022) studied the anchorage shear properties of non-penetrating joints and found that the joint penetration degree could significantly affect the shear strength and shear displacement of anchorage. The anchorage action of the bolt could also substantially reduce the shear damage of rock mass. With the decrease in joint penetration, bolt resistance to shear deformation decreased.

4 NUMERICAL SIMULATION

With the continuous progress and development of computer technology, numerical simulation methods in geotechnical engineering have gained much attention because of their advantages of low cost, short cycle, extensive data, and simple operation. Shear numerical modeling of bolted rock joints mainly includes the solid bolt, rock mass, grouting body, and their contact relation. Researchers have adopted a variety of simulation methods to carry out relevant research based on numerical simulation to supplement and enrich the anchorage shear test data, mainly including finite difference method (FDM), finite element method (FEM), and discrete element method (DEM). Table 4 summarizes the numerical simulation methods of bolted rock joints in recent years and analyzes the contributions.

Table 4. Summary of numerical simulation of bolt anchorage shear.

Numerical method	Software system	Bolt simulation method	Major work or contribution	References
Discrete element method (DEM)	PFC2D	Rigid body spring method	The failure characteristics of rock joint surfaces at different anchoring angles under shear load were revealed.	He et al. (2019)
	PFC2D	Rigid body spring method	A new viscous contact model was proposed to realize the fracture response of rock, grouting material, and bolt-grouting interface during shear.	Saadat and Taheri (2020)
	PFC2D	Rigid body spring method	The influence of rock mesoscopic parameters on the simulation effect of anchorage shear resistance was revealed.	Chen et al. (2022)
	PFC3D	Rigid body spring method	A three-dimensional model was established to study the effect of prestress on crack growth in the anchoring system.	Chen (2019)
	PFC2D	Parallel bonding model	The mechanical characteristics of the PFC software bolt under small deformation conditions were realized using a parallel bond model.	Li et al. (2023)
	3DEC	Virtual element, elastoplastic constitutive	The influences of rock mass strength, anchoring angle, bolt diameter, and prestress on the shear properties of anchorage are studied.	Zhang (2010)
	DDA	Beam model, no solid element	The formation and failure analysis of the plastic hinge of the bolt in jointed rock mass was described, but the action of grouting could not be considered.	Nie et al. (2020)
Finite difference method (FDM)	FLAC3D	Cable element	The effects of joint surface roughness, anchoring angle, and other factors on the shear properties were studied, but the shear mechanics model of the bolt element was not considered.	Guo (2019)
	FLAC3D	Pile element	The problem that the bolt element cannot characterize the shear action was solved.	Lin et al. (2014)
	FLAC3D	Improved pile element	The mechanical model of the pile element was modified and optimized, characterizing the tensile shear and fracture failure of the bolt.	Jiang et al. (2022)
	FLAC3D	Improved pile element	A new numerical model was developed to analyze the tension-shear coupling fracture behavior of rock bolts.	Sun et al. (2022)
Finite element method (FEM)	ZSOIL_3D	Solid element modeling, elastic mechanics	The characteristics of anchorage shear deformation of bolts at the elastic deformation stage were studied.	Grasselli (2005)
	ANSYS	Solid element modeling, elastic mechanics	A three-dimensional model of rock bolt reinforcement for discontinuous rock mass was established.	Song (2002)
	Abaqus	Solid element modeling, elastic mechanics	The interaction mechanism between bolt-slurry and surrounding rock during shear was studied.	Jalalifar et al. (2006)
	Abaqus	Solid element modeling, elastic-plastic mechanics	The shear behavior of anchorage force on flat and undulating structural planes was studied.	Zhao (2013)
	Abaqus	Solid element modeling, elastic-plastic mechanics	The structural behavior of fully grouted cable bolts under impulsive loading was studied.	Tahmasebinia et al. (2018)
	Abaqus	Solid element modeling, tensile mechanics parameters	Considering the interface damage effect between rock and grouting body, the progressive failure characteristics of the anchorage shear system were studied.	Zhao et al. (2022)

4.1 Discrete element method (DEM)

The discrete element method (such as PFC and 3DEC) was first proposed by P.A. Cundall in 1971 and has unique advantages in solving discontinuous media problems. The object is discretized into spheres and other microscopic elements as the basic calculation unit in this method. The geometric shape of the element depends on the spatial position and occurrence of the weak plane in the rock mass. Newton's law of motion is applied to describe the motion process of each block and the force transfer between the basic elements, and the mutual translation, rotation, and deformation between the basic elements can be considered.

Zhang (2010) established a numerical model of anchorage shear using the block discrete element software 3DEC. Considering the prestress and shear performance of the bolt, the influence rule of four parameters, that is, rock mass strength, anchoring angle, bolt diameter, and prestressing magnitude, was studied. However, the bolt was only considered as the stress node in this method, and no solid element of the bolt was established. He et al. (2019) used the particle flow numerical simulation method (PFC2D) to develop numerical models of anchorage joints at different anchorage angles, revealing the failure characteristics of joint surfaces at different anchorage angles under shear load. Chen (2019) used PFC3D software to simulate the mechanical behavior and crack growth of anchored discontinuous jointed rock mass and proved that the prestress of the bolt has a significant influence on the growth rate of the number of micro-cracks during the loading process. Saadat and Taheri (2020) proposed a new viscous contact model based on the PFC2D simulation system to realize the fracture response of rock, grouting material, and bolt-grouting interface during shear. This method can better predict and describe the anchorage shear strength and shear mechanical behavior. Chen et al. (2022) explored macroscopic shear behavior and microscopic failure characteristics of the joint anchored by a single bolt under varying joint types. It was shown that a triangular compression reinforcement zone would appear around the bolt, where micro-cracks were highly developed and rock fractures were severe. Still, the bolt reinforcement could effectively enhance the shear resistance of rock joints. Li et al. (2023) developed the mechanical constitutive relationship for bolts for the first time using the parallel bond model and realized the mechanical behavior simulation under small shear deformation of the bolt. However, large deformation characteristics and fracture criteria of the bolt could not be considered.

Research studies have confirmed the great applicability and advantages of DEM in exploring the meso-mechanical level of shear resistance of anchorage joints, especially in the development, expansion, and failure of surrounding rock cracks during shear resistance. However, the bolt simulation in this method is mainly realized through cluster elements, that is, the bolt is simulated as an undeformable rigid body, which cannot describe the mechanical behavior of the bolt. Therefore, while using this method to simulate the anchorage shear mechanical behavior, the next step should be to explore the discrete element mechanical constitutive model of the bolt, which can describe the deformation mechanical behavior.

4.2 Finite difference method (FDM)

FDM can replace the continuous fixed solution region with a network composed of finite discrete points, approximate the continuous variable function on the continuous fixed solution region to the discrete variable function defined on the grid, and replace the derivative of each place with the finite difference approximation formula to realize the solution of the problem of the partial equation to solve the algebraic equation. Li et al. (2022) used FLAC numerical software to simulate a variety of energy-absorbing bolts by defining the energy absorption criteria and fracture criteria of bolts, which solves the problem of large deformation of bolt support that cannot be accurately simulated by traditional numerical simulation methods. Guo (2019) established a FLAC3D numerical model of shear test under different joint surface roughness and anchoring angles. The simulation shows that the optimal anchoring angle of the bolt is related to the internal friction angle and dilatation angle of the joint surface. When the sum of the two is <35°, the optimal anchoring angle can be the sum of the two; when the sum is larger, the optimal anchoring angle is around 60°. Li, Hagan, et al. (2019) adopted this method to study the influence of preload, surrounding rock strength, bolt diameter, bolt strength, and joint surface roughness on shear performance. FLAC numerical simulation software with its cable unit was used, considering the cementation between the interface of bolt and mortar, so it was convenient to realize the numerical analysis of anchoring engineering. However, the cable unit did not consider the shear mechanical constitutive of the bolt and could not reflect the shear and flexural effects of its anchoring support; only the tensile mechanical behavior of bolt material could be simulated. Lin et al. (2014) simplified the process by using the embedded rock mass model of pile element built into the software, which could reflect the tensile and shear effects of bolt reinforcement but still could not accurately characterize the yield strength, tensile strength, and fracture criteria of bolt (Figure 10b). Based on the theoretical criterion of bolt failure, Jiang et al. (2022, 2023) revised the pile element mechanical model in FLAC3D program (Figure 10c), corroborated its accuracy, and studied the influence of normal stress and joint roughness coefficient on bolt anchoring performance under shear load. Further, the distribution and evolution characteristics of bolt axial force and shear resistance were revealed (Figure 11).

4.3 Finite element method (FEM)

FEM is based on the variational principle, the weighted margin method, and the principle of discrete processing of partition interpolation. The continuous field function is transformed into the field function at finite discrete points to solve mathematical physics problems. FEM has significant advantages for the fine simulation of bolt materials. Jalalifar et al. (2006) used FEM to simulate the double shear test to analyze the ring thickness of anchorage agents and the interaction mechanism between bolt, slurry, and surrounding rock. It was shown that the increase in the strength of the surrounding rock and the preload of the bolt significantly increased the shear strength of the rock joints. However, the numerical model of the bolt only considered the elastic stage and did not consider the plastic deformation. Using Abaqus numerical software, Zhao (2013) established the anchorage mechanical model of flat and undulating structural planes, and simplified treatment of the mechanical properties of the bolt was carried out by using the elastoplastic mechanical constitutive method (Figure 12a). The micromechanical behavior of the anchorage structural plane was studied, and the influence of prestressed anchor cables on the anchorage of the structural plane was analyzed. Zhao et al. (2022) used Abaqus to comprehensively consider the interface and damage effects of the anchoring system and simulated the mechanical damage and stiffness degradation in the shear process by the bolt damage model (Figure 12b). Thus, the simulation of the coupling and progressive failure behavior of the rock, bond layer, and bolt was realized. In addition, based on the finite element dynamic display method, a three-dimensional numerical model of anchoring under impact load was established to reveal the neck collapse characteristics under impact load and the energy evolution of the anchoring system. However, the bolt parameters were mainly obtained by the tensile test, and the mechanical constitutive and fracture criteria of anchoring under different tensile shear angles were not considered. Song (2002, 2003) used ANSYS numerical analysis software to establish a three-dimensional model of discontinuous rock bolt reinforcement and assessed the lateral restraint effect of the bolt in the elastic stage. It was confirmed that the surrounding rock produces relative dislocation along the joint surface under the action of external shear load, and the local large deformation of the bolt exerts a restraining effect on the surrounding rock, improving the shear strength of jointed rock mass. Studies have shown that FEM has a certain rationality and accuracy in simulating the shear behavior of anchorage joints. However, based on the continuum theory, FEM still has certain limitations on fracture development and shear fracture of rock mass. In addition, considering the mechanical constitutive and fracture criteria of the bolt under varying tensile shear angles, the simulation of the deformation and failure behavior of the bolt is still a dire issue to be addressed.

5 PROSPECTS AND APPLICABILITY

5.1 Development and application of high-strength and high-toughness bolt/cable materials

Currently, all bolts in the project are mainly processed from steel exhibiting Poisson's ratio effect (PR), that is, small deformation materials that are broken due to neck deformation during tension and cannot adapt to the large deformation characteristics of disaster rock mass, resulting in functional and engineering failure. With the continuous development of anchoring technology, the supporting structure should meet the strength coupling and stiffness coupling; that is, the bolt should have sufficient ductility to adapt to the deformation of rock mass and adequate strength to prevent deformation. Thus, new bolt types with good ductility and high support strength have been developed, such as energy-absorbing bolts, for example, 1G-NPR structural bolt (He et al., 2014), Conebolt (Ansell, 2005; Jager, 1992; Ortlepp, 1992), D-bolt (Li, 2010), Garford bolt (Gunton et al., 1972), and Durabar bolt (Ortlepp et al., 2001, 2002) have been successfully developed and have shown excellent performance in support engineering. These bolts improve the mechanical properties of large deformation and high strength through the complex structural combination or geometric structure design of ordinary materials, which still does not change the essence of Poisson's ratio materials but increases the installation complexity due to unique structural settings, while the cost is also high. In addition, the test results show that the anchorage shear performance of the bolt has the characteristics of local deformation and failure. Therefore, making necessary changes and breakthroughs from the material nature of the bolt is the key to the research of geotechnical engineering support structure.

He et al. (2021) integrated the design concept of the 1G-NPR structural bolt, that is, the bolt does not exhibit necking during tension, into the smelting of molten steel. Through the change of steel trace elements and smelting process, the multi-scale multi-grid interface design and other methods were adopted. Finally, in 2018, a new 2G-NPR bolt material with extraordinary mechanical properties was successfully developed with a significantly reduced Poisson ratio (no necking in tension), the disappearance of the yield platform, and strain value >20%. Unlike the ordinary PR bolt material, the new material has no necking during the static tensile process and exhibits uniform large deformation, no yield platform, and high strength. The yield strength range is 600–1050 MPa, and the elongation range is 30%–68%, realizing the negative Poisson's ratio effect of the material itself. The issue of small deformation of rock and soil support material is thus solved (Gu et al., 2021). As shown in Figure 13a, He et al. (2022, 2023) conducted a large number of anchorage shear tests on 2G-NPR steel and ordinary steel, elucidating that 2G-NPR steel significantly improved the shear strength of jointed rock mass better than ordinary steel. Figure 13b compares shear and energy absorption effects under varying normal stress conditions. 2G-NPR steel exhibits exceptional shear strength and energy absorption (toughness). Limited by the shear displacement of shearing equipment, 2G-NPR steel has no shear breakage, and its shear and energy absorption effect are far superior to those of ordinary steel. However, the new material mechanical test under complex tensile shear state must be carried out further, and the mechanical theory of anchorage shear deformation should be studied.

5.2 Development of high strain rate shear test system for bolt anchoring

The typical dynamic failure of engineering rock mass is characterized by “nonlinear, large deformation, high energy, high speed” and “high intensity, high stress, high strength,” which are challenging to control by existing technologies. Traditional supporting theories, materials, and systems are also controversial in terms of the applicability of rock mass in dynamic engineering. Thus, new materials, theories, and systems, considering nonlinear dynamics characteristics, are needed to solve the theoretical and technical problems and prevent and control engineering dynamic failures. Reasonable and effective engineering stability analysis methods for prevention and control under dynamic load are urgent needs of engineering rock stability and rock dynamics research. Engineering prevention and control materials include bolts, anchor cables, anchor nets, and concrete. For example, in rockfall protection, active protective nets and passive protective structures are often used. Still, the impact of high drops or large volumes of rolling stones cannot be fully resisted. The tunnel support is mainly reinforced by spray anchor or prestressed bolt technology. Rock bolting can significantly improve protection. However, the results of many blasting tests and engineering measurements indicate that it cannot help attain an excellent dynamic control effect because the nonlinear behavior of rock mass and supporting materials is more prominent under dynamic loading. The characteristics of rock failure, as well as large-scale and high energy levels, are more prominent. The strength and deformation properties of supporting materials often show the attributes of weakening performance with the increased strain rate, ultimately leading to engineering failure. At present, the indoor anchorage shear tests mainly focus on quasi-static loading. A small amount of moderate strain rate dynamic shear tests is carried out (Figure 14). Therefore, developing indoor anchorage shear test equipment under medium and high strain rates and studying the impact of shear mechanical properties of the bolt under different dynamics conditions is imperative to provide a basis for establishing nonlinear dynamic prevention and control theory and technical system of engineering rock mass.

5.3 3D finite numerical simulation method for bolt anchoring

The shear process of anchoring joints is a complex nonlinear process involving the deformation and failure of the bolt, jointed rock mass, and grouting body, which is manifested by the nonlinearity of material constitutive relationship and the large deformation of the anchoring system under shear load, that is, geometric nonlinearity. Considering the coupled effect of jointed rock mass, bolt, and grouting body, studying the 3D finite numerical simulation method, which comprehensively considers the elastic-plastic mechanical behavior of rock mass, the tensile-shear constitutive fracture criterion of the bolt, and the damage and deterioration of grouting body, is of paramount significance. At present, static tensile mechanical parameters are often used to assign the mechanical parameters of bolts in the anchoring numerical simulation. Using tensile mechanical behavior to characterize the mechanical behavior of tensile shear at different angles causes large simulation errors and fails to truly reflect the shear mechanical behavior of bolt anchoring.

To address this problem, the authors have developed a set of tensile-shear test equipment for the bolt that can meet different tensile-shear angles. The mechanical properties of 2G-NPR steel were tested to obtain the force-displacement curve under tensile-shear conditions (Figure 15). Combined with the finite element numerical model and the steel fracture model, a 2G-NPR steel tensile-shear fracture criterion was established, which could accurately characterize the conditions of varying tensile-shear angles (Figure 16). On this basis, a three-dimensional numerical model of bolt anchoring was established, and the simulation results could precisely reflect the shear deformation and failure characteristics of 2G-NPR bolts under different surrounding rock strength conditions (Figure 17). However, this method does not take into account the fracture development and failure criteria of surrounding rock, which is a further research work in the follow-up of 3D finite numerical modeling.

6 CONCLUSIONS

This article reviews and summarizes the research status of shear laboratory tests and numerical simulations of bolted rock joints. The following conclusions are drawn from the summarized findings.

1.
The laboratory shear tests of rock joints anchored by bolt can be classified as single shear and double shear tests. With continuous research, development, and improvement, the functions of the two test devices gradually become perfect. However, the collaborative coupling functions of various loading conditions still need to be further improved, which cannot meet the loading requirements of actual complex working conditions. The intelligent operation and refinement level of the devices still need to be further enhanced. In addition, the research and development of a shear test system of rock mass anchored by the bolt under a high-stress rate is an urgent problem in equipment research and development.
2.
There are many factors affecting the shear properties of rock mass anchored by bolts, such as the strength of rock specimen, the roughness of the joint surface, the type of bolt, and the anchoring angle, which have significant effects on the shear deformation instability mode and mechanical properties of the bolt. However, there is no unified understanding of the critical quantitative criteria of the influence of surrounding rock strength on the tensile shear and tensile bending mode of the bolt, and there is no unified understanding of the quantitative criteria of the optimal anchoring angle. In addition, the new high-strength and high-toughness anchor bolt/cable material significantly improves the shear strength and shear displacement of jointed rock mass. The mechanical tests on the new high-strength and high-toughness anchor bolt/cable material under complex tensile and shear conditions must be further conducted, and the relevant anchorage shear theories need to be refined, which can lay a theoretical and experimental foundation for the engineering application of the new bolt material.
3.
The numerical simulation method of rock mass shear resistance of bolt anchoring joints can simulate the fracture development and failure characteristics of surrounding rock as well as the shear deformation characteristics of the bolt in the elastic section. However, the elastic and rigid body models of bolt elements are often simplified, or only the tensile mechanical constitutive relationship is considered, which cannot truly reflect the mechanical state of the bolt under varying tensile shear angles. Therefore, studying mechanical constitutive and fracture criteria under different tensile shear angles is the key to realizing 3D finite numerical simulation of anchorage joints.

ACKNOWLEDGMENTS

This study has been supported by the Postdoctoral Fellowship Program of CPSF under Grant Number GZB20240556 and the National Natural Science Foundation of China under Grant Number 42377154.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

Biography

Zhigang Tao, a doctoral supervisor at China University of Mining and Technology-Beijing, was selected for the National Youth Talents Program in 2021 and ranked among the top 2% of the world's leading scientists on the Annual Scientific Impact Ranking in 2024. Dr. Tao is dedicated to foundational research on the mechanisms of large deformation in engineering rock masses and methods for disaster prevention and control. His research has been applied in over 20 national-level major construction projects, yielding remarkable economic and social benefits and earning widespread recognition from renowned experts and scholars worldwide. His achievements include the China Patent Gold Award, the Special Prize for Scientific and Technological Progress from the Chinese Society for Rock Mechanics and Engineering, among over 20 provincial and ministerial scientific research awards. As the first author or corresponding author, he has published more than 120 papers in SCI/EI-indexed journals. Dr. Tao holds 6 international invention patents and 51 Chinese invention patents and has published 8 academic books. Furthermore, Dr. Tao has received the Outstanding Young Scientist Award from the International Association for Geohazards and Disaster Reduction in 2024, the Young Award from the China Communications and Transportation Association in 2022, and the China Industry-University-Research Cooperation Innovation Award in 2019 and was recognized as an Outstanding Young Energy Scientist and Technologist in China in 2018.