Comparative Study of Transverse Shear Characteristics of Shear-Yielding Bolts and Traditional Bolts Based on Numerical Simulations and Direct Shear Tests

Abstract

The shear-yielding bolt is a new type of anchoring structure, and its working mechanism in layered rocks is not yet well understood. To investigate its transverse shear characteristics, this paper takes the shear-yielding bolt as the research subject and uses different anchoring states of bolts as variables. A comparative study of shear-yielding bolts and traditional bolts is conducted using the Abaqus numerical simulation software and large-scale direct shear tests. The results show that (1) low-modulus material allows a slight displacement between the structural surface layers, which exerts the friction strength between rock mass layers and avoids stress concentration on the bolt. The shear-yielding bolts reach their peak shear stress in the case of greater displacement, averagely increased by 40% compared to traditional anchor bolts. (2) An increase in the moisture content has less influence on the shear-yielding bolt owing to the material properties. When the moisture content of the structural surface rises from 12% to 20%, for cases where the shear-yielding bolts are used, the peak shear stress decreases by 0.12 kPa, which only accounts for 12% of the original strength. (3) There is an optimum thickness of the low-modulus material in the shear-yielding bolt, considering its effect of releasing shear and the bonding effect between it and the bolt. According to the test results and numerical analysis, the optimum thickness is 15 mm. The results of this research provide a reference and basis for future study and engineering applications of shear-yielding bolts.

1. Introduction

In recent years, roadbed disasters and slope instabilities have frequently occurred due to factors such as climate change and human activities. Geotechnical ground anchoring technology has been increasingly applied in engineering practice. Improving the anchoring structure, extending the service life of bolts, and improving slope disaster prevention and control have become urgent issues to address [1,2,3,4].

Anchorage structure is one of the important structures in engineering. In addition to its application in houses and bridges, it is also widely used in ground anchor support engineering [5,6,7,8,9]. Ground anchor support engineering has a long history. Since 1872, steel rebar bolts have been used in ground anchoring projects. In 1911, rock bolts were first used in American coal mines to support mine tunnels, marking the beginning of the rapid development of ground anchoring technology. Early studies on bolts mainly focused on three areas: experimental research on anchoring structures, numerical simulations of anchoring structures, and theoretical modeling of anchoring structures.

Experimental studies on anchoring structures primarily explored the axial load transfer mechanism and shear resistance of anchoring structures [10,11,12]. For example, Zhang et al. [13] carried out laboratory shear tests to study the shear characteristics of jointed rocks reinforced with basalt fiber-reinforced polymer (BFRP) bars, which are gradually being used as bolts in underground engineering. The result showed that the BFRP-bolted specimens have different shear properties compared to traditional bolts using steel bars, which can be summarized as lower shear stiffness, larger peak shear displacement, and higher residual shear strength. Numerical simulation studies on bolts have mostly been based on field and laboratory experiments to validate experimental accuracy [14,15]. In recent years, it has been used more in safety evaluation systems and hopefully will be used in real-time monitoring and early warning systems. Based on the Visual Studio 2019 development environment and the .NET Framework 4.0 platform, the C# advanced programming language was used by Meng et al. [16] to develop the safety evaluation system for bolt–mesh–cable supports in the deep roadway along goaf. Existing theoretical models of bolts primarily analyze the overall shear strength of structural surfaces, leading to the emergence of the suspension theory, support theory, composite beam theory, anchoring theory, and dowel theory. They more accurately describe the relationship between the anchoring bolt and the anchored body [17,18]. In recent years, new theoretical models have constantly been proposed, and more and more factors are considered to deal with the shortcomings of traditional theories [19,20,21]. Earlier studies on anchoring structures covered a broad range of topics and primarily examined factors affecting the load-bearing capacity of bolts. Such studies have shown that factors such as the material, diameter, length, anchoring angle, anchoring grout, and surrounding rock strength have various levels of effect on the anchoring performance. Theoretical anchoring models were then developed based on theoretical analyses [22,23,24,25,26], and some studies also improved the above parameters [27,28,29,30].

In recent years, because traditional anchoring structures are ineffective and have a short service life, researchers have shifted their focus to studying new types of anchoring structures [31,32,33,34]. As research progressed, improving bolt service life and anchoring efficiency became the focus of new anchoring structure studies [35]. However, in many physical model experiments on bolts, the presence of rock interlayer structural surfaces was often overlooked. In practical engineering, the cracks and soil particles in the interlayer structure of rock formations affect the natural shear strength of interlayer rock masses [36,37,38]. The shear deformation caused by interlayer dislocation is often the cause of bolt failure, which is also the focus of current research [39,40].

A shear-yielding bolt is a new type of anchor rod. It considers the interlayer dislocation that commonly exists in practical engineering but is always neglected in bolt design. Owing to the effect of low-modulus material inserted inside, the shear-yielding bolt enables the structure to release shear and fully utilizes the natural shear strength of the interlayer. This paper takes shear-yielding bolts as the research object and investigates its failure mechanism in layered rock, using numerical simulations and large-scale direct shear tests, and the effect of the distinctive low-modulus material is discussed. The research aims to provide references and guidance for future studies and engineering applications of shear-yielding bolts.

2. Introduction to Traditional Bolts and Shear-Yielding Bolts

In slope ground anchoring design, traditional bolts are mainly composed of three parts: the bolt head, the free section, and the anchor section, which are shown in Figure 1. The bolt head is where a tensioner applies prestress to the bolt. The anchor head, composed of an anchor, nut, pedestal, and cross beam, is located outside of the rock mass, facilitating the application of prestress to the entire bolt. The free section serves as the load transfer channel for the entire bolt. The free section is located near the ground and is typically wrapped in a plastic sleeve in the layered rock mass to prevent corrosion. The retaining structure is used to further maintain stability. The anchoring section is where the bolt is stably connected to the bedrock. The cement mortar stabilizes the anchoring section to the bedrock, forming a solid bolt. The three sections, namely, the anchor head, free section, and anchoring section, connect the bolt to the rock mass, reducing the sizes of the cracks in the weak interlayer of the rock mass and making the particles in the rock mass tighter, thereby enhancing the overall integrity of the rock mass. However, due to the poor shear resistance of traditional bolt materials, they are easily damaged by shear forces, interlayer dislocation is usually neglected in their design, and the shear forces acting on the bolts are not considered. However, in reality, interlayer dislocation in rock formations is prevalent and often leads to combined tension and shear stress on the bolt, causing failures in slope anchoring.

The shear-yielding bolt is a new type of bolt that fully utilizes the natural shear strength of interlayer structural surfaces. Structurally similar to a traditional bolt, it consists of four sections: the free section, the anchoring section, the anchorage segment, and the tensioning segment, which are shown in Figure 2. The anchoring section of the shear-yielding bolt is located beneath the potential slip surface of the layered rock and is tightly connected to the stable bedrock. Its cross-section is circular, and between the end of the bolt and the stop-grouting plug is filled with grout. The anchoring bar provides tensile strength, and the grout acts as an adhesive that secures the anchoring bar within the bedrock, thereby providing high-strength anchoring. The anchorage segment is primarily composed of an anchor chock and an anchor plate, which help secure the entire bolt. The tensioning segment, composed of anchor fixtures and an anchoring bar, is the part of the anchoring tendons exposed outside of the rock mass, and it facilitates the application of prestress by the tensioner. The main difference between the shear-yielding bolt and a traditional bolt lies in the free section. The free section of the shear-yielding bolt is composed of an anchoring bar and low-modulus materials and is located above the potential slip surface. These low-modulus materials have a high compressibility, great toughness, and low water permeability, which are filled between the anchor chock and stop-grouting plug. As a filler, the low-modulus material wraps around the anchoring bar to a certain thickness, allowing for deformation under external loads without being easily damaged. This design enables the structure to release shear and fully utilizes the natural shear strength of the interlayer. Therefore, the shear-yielding bolt performs better and has a longer service life than traditional bolts.

3. Experimental Plan

3.1. Research Objectives

• Using large-scale laboratory direct shear tests and numerical simulation analysis, the load transfer mechanism and failure mechanism of shear-yielding bolts were studied, and their mechanisms of operation were investigated.
• Comparative analysis of the shear strength of layered rock structural surfaces under traditional bolt and shear-yielding bolt conditions was conducted to verify the superior anchoring performance of shear-yielding bolts.
• The factors influencing the anchoring performance of shear-yielding bolts were studied, and the working behavior of the bolts was determined.

3.2. Direct Shear Test Plan and Procedure

Based on the research objectives, large-scale laboratory direct shear test equipment was used to conduct direct shear tests on layered rocks under three different conditions: no anchoring (N), anchoring using traditional bolts (T), and anchoring using shear-yielding bolts (S). A total of five groups and 19 tests were conducted, and the performance of the bolt bar of the free section under the structural surface is simulated to show the difference between the traditional bolt and the shear-yielding bolt.

3.2.1. Test Materials

The bolt bars were composed of HPB300 plain steel rebar. Its ultimate tensile strength is 420 MPa, yield strength is 300 MPa, and diameter is 8 mm.

The rock mass comprised upper and lower test blocks with total dimensions of 200 × 200 × 200 mm. The mortar used to make concrete blocks was created by mixing tap water, P.O32.5 Portland cement, river sand, and gravel with particle sizes of 6~10 mm. The mixing ratio of water–cement–sand–gravel was 0.5:1:1.5:1.5. After the concrete reaches a certain strength, the surface of the test block is roughened to simulate a real structural surface. When the mortar is completely solidified, the tests of the concrete blocks are carried out. The results are as follows: The simulated rock mass reaches its density of 1950 kg/m³, Poisson’s ratio is 0.3, compressive strength is 12.6 MPa, and elastic modulus is 30 GPa.

The low-modulus material between the bar and the surrounding rock was polyurethane foam material, which was inserted after the installation of the upper and lower concrete blocks. The elastic modulus of the material is affected by many factors, such as temperature. It was hard to make the real elastic modulus of the material inserted in the block consistent with the result of the individual test of the material. Thus, the elastic modulus would be determined through numerical simulation according to the test results.

The structural surface simulation material was based on typical engineering detrital soil and was prepared with moisture contents of 12%, 16%, and 20%. The relevant parameters are shown in

Table 1. Content of each grain size range of structural surface simulated material.

Grain Size (mm)	<0.075	0.075–0.25	0.25–0.5	0.5–1	1–2
Content (%)	16.03	18.18	12.61	30.02	23.16

and

Table 2. Basic parameters of structural surface simulated materials.

Median Diameter d50 (mm)	Constrained Grain Size d60 (mm)	Dry Density ρd (g/cm3)	Porosity n
0.48	0.64	1.85	0.4

. The material is evenly spread between the rough surface of two concrete test blocks and compacted, and the average thickness is 4 mm.

Using the above materials, PVC pipes of different diameters were embedded to make traditional anchoring bolts and shear-yielding anchoring bolts for drilling. The relevant test parameters are presented in

Table 3. Direct shear test scheme.

Number	Test Type	Moisture Content of Structural Surface w (%)	Diameter of Low-Modulus Material d (mm)	Normal Stress σ (MPa)
1	N	16	0	0.25, 0.5, 0.75, 1
		12, 20	0	0.5
2	T	16	0	0.25, 0.5, 0.75, 1
3	S	16	15	0.25, 0.5, 0.75, 1
4	S	16	12, 15, 18	0.5
5	S	12, 20	15	1

. The schematic presentation of the specimen is shown in Figure 3.

3.2.2. Test Equipment

The large-scale direct shear test instrument, shown in Figure 4, was capable of generating a maximum vertical force of 5000 kN and maximum horizontal force of 2500 kN. It can collect data between 4 and 100% of the full range at the lowest frequency of 5 Hz, and the control error is less than 1%. The maximum piston stroke is 500 mm. During the test, the normal pressure and horizontal thrust are applied to the specimen to promote the shear failure of the specimen. The change in force and displacement during the failure of the specimen can be recorded on the computer at the control end by the sensor on the instrument.

3.2.3. Test Procedure

The normal pressure was applied at a rate of 4 kN/min. After the normal pressure was fully applied, horizontal loading was applied to the sample at a rate of 3 mm/min until the test was completed [41]. Figure 5 illustrates the experimental procedure.

3.3. Numerical Simulation Model and Parameter Determination

The contact surface in geotechnical engineering, including the structure surface of rock mass, is simulated by Goodman thickness-free element. The thickness of the contact surface in Abaqus is not considered. Therefore, Goodman believes that the parameters of the model should be used to consider the friction characteristics on the contact surface.

Using the finite element software Abaqus 6.14, a joint shear calculation model was established, which is shown in Figure 6. Solid elements were used to simulate the rebar and low-modulus materials. A linear hexahedron 8-noded element is used to simulate rock mass, steel bar, and low-elastic-modulus material. Due to the symmetry of the calculation model, only half of the model was selected for analysis.

The concrete was assumed to be a linear elastic material with an elastic modulus of 30 GPa and a Poisson’s ratio of 0.3. The rebar was assumed to be an elastic plastic material with a yield strength of 300 MPa and to satisfy the von Mises yield criterion. Based on the test results, the elastic modulus of the low-modulus material used in direct shear test was assumed and then used in numerical simulation. The simulation and test results were compared, and the assumed parameter was adjusted, until the difference between the simulation and the test results reached an acceptable range. The low-modulus material’s elastic modulus was determined to be 600 MPa through this procedure.

The size of the rock mass unit was 1 cm. The low-elastic-modulus material and the steel bar were divided into 4 parts in the horizontal direction. The vertical size was the same as the rock mass unit, with a total of 5280 units. The given normal pressure was applied on the top of the specimen, and the displacement boundary was given on the side of the specimen. It allows only tangential displacement and prohibits normal displacement. The interfacial shear stress was calculated by the reaction force of the boundary node.

It was assumed that the bond between the rebar and the low-modulus material was strong and that the low-modulus material and the concrete were in contact. The normal direction is hard contact, and the friction coefficient is 0.6. The upper and lower concrete blocks were in contact with each other. The shear stress and relative shear displacement between them followed a hyperbolic relationship.

The associated constitutive model used in the study is as below [42]. The material parameters in this model like friction angle and nonlinear indexes can consider the friction effect caused by simulated material between the structural surfaces, as well as the relationship between shear strength and displacement. These parameters are determined according to the test results, as shown in

Table 4. Constitutive parameters of contact surface.

Nonlinear Index K1	Nonlinear Index K2	Nonlinear Index n	Nonlinear Index Rf	Friction Angle of Structure Surface δ (°)	Atmospheric Pressure Pa (kPa)	Weight of Water γw (kN/m3)
2.3×105	2.3×105	0.02	1	22.6	100	9.8

.

$$\begin{array}{l}{k}_{s1}={\left(1-R_f{\displaystyle \frac{{\tau }_1}{{\sigma }_{\mathrm{n}}\tan \delta }}\right)}^2{K}_1{\gamma }_{\mathrm{w}}{\left({\displaystyle \frac{{\sigma }_{\mathrm{n}}}{p_{\mathrm{a}}}}\right)}^n\\ k_{s2}={\left(1-R_f{\displaystyle \frac{{\tau }_2}{{\sigma }_{\mathrm{n}}\tan \delta }}\right)}^2{K}_2{\gamma }_{\mathrm{w}}{\left({\displaystyle \frac{{\sigma }_{\mathrm{n}}}{p_{\mathrm{a}}}}\right)}^n\end{array}$$

(1)

4. Results Analysis

4.1. Shear Deformation Characteristics of Structural Surfaces Under Different Anchoring Methods

Layered rock masses contain complex structural surfaces that, due to their discontinuity and irregularity, cause instability in the entire rock mass. Under external loads, rock masses are prone to dislocation along these structural surfaces. The mechanical behavior of layered rock varies depending on the anchoring method. To investigate the impacts of different anchoring methods on the shear strength of layered rock structural surfaces, three sets of direct shear tests were conducted under the following conditions: no anchoring (N), traditional bolt anchoring (T), and shear-yielding bolt anchoring (S). Additionally, numerical simulations were performed using the Abaqus software for comparison. The results are presented below. The shear force–displacement curves are shown in Figure 7, and the peak shear force and displacement values are presented in

Table 5. Direct shear test results of peak shear force and displacement values under different anchoring conditions and normal pressures.

Normal Pressure σ (MPa)	Anchoring Conditions	Average Shear Stiffness G (MPa/mm)	Peak Displacement νmax (mm)	Peak Shear Stress τmax (MPa)
	N	0.015	9.65	0.14
0.25	T	0.057	8.37	0.48
	S	0.044	16.50	0.73
	N	0.024	10.15	0.24
0.5	T	0.085	8.26	0.71
	S	0.058	16.20	0.94
	N	0.031	10.31	0.32
0.75	T	0.098	8.82	0.87
	S	0.072	16.60	1.20
	N	0.036	10.62	0.38
1	T	0.120	8.70	1.05
	S	0.101	14.40	1.42

.

For the direct shear tests under a normal pressure of 1 MPa and the no anchoring condition, the peak shear stress of the structural surface is 0.38 MPa, the peak displacement is 10.62 mm, and the average shear stiffness is 0.036 MPa/mm. For the traditional bolt anchoring condition, the peak shear stress is 1.05 MPa, the peak displacement is 8.70 mm, and the average shear stiffness is 0.120 MPa/mm. The peak shear stress of traditional bolts is 2.76 times that of the no anchoring condition. For the shear-yielding bolt anchoring condition, the peak shear stress is 1.42 MPa, the peak displacement is 14.40 mm, and the average shear stiffness is 0.101 MPa/mm. The peak shear stress of shear-yielding bolts is 1.35 times that of traditional bolts. Similarly, under the normal pressure of 0.25 MPa, 0.5 MPa, and 0.75 MPa, the peak shear stress of bolts is 1.52, 1.32, and 1.37 times that of traditional bolts. On average, the peak shear stress increases by 40%. The shear strength of the structural surface in the layered rock is significantly improved by the anchoring.

The numerical simulation results, according to Figure 7, are largely consistent with the test results. When the normal stress is high, the agreement between the two is better. This indicates that the contact surface parameters can be obtained through inversion in the numerical simulation, which reasonably describe the mechanical behavior of rock joints under shear conditions. Such parameters can effectively reflect the shear mechanical characteristics of jointed rock masses under different anchoring conditions.

When comparing traditional bolts and shear-yielding bolts, the numerical simulation and test results exhibit consistent trends with nonlinear strengthening and softening phases. When the normal stress is low, there is some deviation between the numerical results and the test results, but as the normal stress increases, the deviation decreases. This is because the model parameters were calibrated under high normal stress, and the computational model does not exactly match the experimental conditions.

In the shear-yielding bolt case, differences exist between the computed curve and the experimental data. The computed curve exhibits a hyperbolic nonlinear increase, while the experimental curve continues to increase after the first peak and eventually stabilizes. This is likely due to the contact relationship between the concrete block and the low-modulus material, which leads to unavoidable penetration phenomena during the computation. Additionally, the low-modulus material undergoes significant deformation during shearing, leading to mesh distortion and errors in the calculated results.

By analyzing the variations in the shear stress under shear-yielding anchoring from the direct shear test results, it can be seen that under different anchoring conditions, the shear forces and the shapes of the curves vary significantly, but the overall trends are consistent, which can be seen in Figure 7 and concluded as the four stages that the shear-yielding anchoring structure undergoes during shear loading:

• Elastic Material Stress Stage: In the initial stage of shearing, the rock mass begins to undergo shear deformation under external forces. The low-modulus material outside the bolt is first subjected to compression from the rock mass. Due to its material properties, the low-modulus material undergoes significant deformation without breaking, causing minor dislocation between the structural surfaces and generating friction. As the shear stress increases, the friction also increases. When the shear force exceeds the friction between the structural surfaces, the structure breaks the equilibrium state and starts to move slowly, and the shear stress remains constant. In this stage, the bolt is not yet resisting the shear force, so the shear force is governed by the Mohr–Coulomb criterion.
• Bolt Stress Stage: As the shear displacement continues to increase, the shear force is transmitted from the rock mass to the low-modulus material and then to the bolt. The bolt begins to resist the shear force, causing a rapid increase in the shear stress. In this stage, the bolt is in an elastic stress state, and the shear stiffness reaches its peak. The shear stress and displacement exhibit a nonlinear relationship.
• Bolt Yield Strengthening Stage: As the shear displacement increases further, the bolt begins to yield and undergoes irreversible deformation. The shear stiffness gradually decreases, and the concrete surrounding the bolt starts to fracture. In this stage, the shear stress exhibits minor fluctuations, and the structural surface exhibits ductile strengthening characteristics. The shear stress continues to increase, but the rate of increase decreases as the shear stiffness decreases.
• Bolt Shear Failure Stage: As the shear displacement continues to increase, the shear stress reaches its peak, and the bolt fails due to deformation. The shear stress stabilizes and eventually stops increasing.

4.2. Analysis of Factors Affecting the Shear Resistance of Shear-Yielding Anchoring Structures

The shear force–displacement curves for different moisture contents and various low-modulus material thicknesses are shown in Figure 8. The peak values are listed in

Table 6. Numerical simulation results of peak shear force values for different moisture contents of the structural surface.

Moisture Content w (%)	Peak Shear Stress τmax (MPa)
	No Anchoring	Shear-Yielding Bolt Anchoring
12	0.31	0.98
16	0.24	0.94
20	0.16	0.87

and

Table 7. Numerical simulation results of peak displacement and shear force values for different low-modulus material thicknesses.

Diameter of Low-Modulus Material and Anchoring Tendon d (mm)	Peak Displacement νmax (mm)	Peak Shear Stress τmax (MPa)
12	14.50	1.22
15	16.46	1.42
18	16.58	1.35

.

As can be seen from the direct shear test results, the moisture content of the structural surface has a certain effect on the shear strength of the surface. For the no anchoring condition, the higher the moisture content is, the lower the shear strength of the surface is, which is consistent with general natural laws, as shown in Figure 8a. The shear strength of a layered rock in its natural state is primarily provided by the interactions between the soil particles and the rock on the structural surface. For a soil body, many factors including its mineral composition, physical properties, and soil structure will significantly influence its shear strength. As the moisture content increases, the shear strength of the structural surface decreases. Water can lubricate large soil particles, reducing the frictional resistance. For small clay particles, an increase in the moisture content thickens the bound water film, reducing cohesion. An increase in the water content alters the physical properties and structure of the soil particles and lowers the overall shear strength.

For shear-yielding bolted structural surfaces, the low-modulus material between the structural surfaces ensures a tighter connection, as shown in Figure 8b. The additional axial pressure applied during anchoring increases the density of the material of the structural surface, making the soil structure more stable. Therefore, the moisture content has less influence on the bolted structural surface.

As shown in Figure 8c, the anchoring performance of the shear-yielding bolts initially increases and then slightly decreases as the thickness of the low-modulus material increases. The reason for this is that the thickness of the low-modulus material positively influences the anchoring performance of the shear-yielding bolt to a certain extent. A thicker low-modulus material allows for greater shear-yielding displacement, maximizing the interaction between the structural surfaces. Additionally, a thicker material provides more cushioning, dispersing the originally concentrated stress and extending the service life of the bolt. However, as the thickness continues to increase, the bond between the rebar and the low-modulus material deteriorates, and the compressive strength of the material becomes limited. In the later stages of shearing, the low-modulus material breaks under compression, reducing the anchoring performance.

The numerical simulation results for various thickness conditions are consistent with the test results, and the shear mechanical properties of the structural surface under different moisture conditions are reasonably described. The effect of the thickness of the low-modulus material is also well-reflected in the numerical model. In addition, due to the strong anchoring effectiveness of the shear-yielding bolt, the moisture content on the joint surface has less influence in the numerical simulation. Many scholars have studied the interface between the bolt and the grout [43,44]. It not only shows different failure modes, but also shows the influence of different boundary effects. Our research only shows the influence of low-modulus material thickness in the process of shear failure and only shows a failure mode of bolt shear failure. The influence of temperature, acidic and alkaline environment, and other factors on the low-modulus material is not enough, which needs further study in practical engineering.

4.3. Shear Stress Analysis of Structural Surfaces Under Different Anchoring Methods

In order to better show the difference between the traditional anchor rod and the new anchor rod, the middle values of different moisture contents and different normal stresses in the test are chosen to carry out detailed discussion. The corresponding moisture content is 16%, and the normal stress is 0.5 MPa. When simulating the condition without anchoring, the lower stress level can better show the stress distribution. The corresponding moisture content is 20%, and the normal stress is 0.5 MPa. Using the Abaqus software, a joint shear calculation model was established to analyze the stress distribution during the shearing process of the joint. The results are shown in Figure 9, Figure 10 and Figure 11.

As can be seen from Figure 9, in the no anchoring case, the shear stress is approximately symmetrically distributed, with the maximum shear stress located in the middle of the joint and lower shear stresses on both sides of the joint.

As can be seen from Figure 10, under the conventional bolt reinforcement, the joint surface and the bolt bear the shear action together. Due to the low strength of the joint surface, the shear stress is mainly borne by the bolt, and there is obvious stress concentration on the shear surface of the bolt. The plastic strain is mainly distributed near the shear surface, and the bolt experiences considerable localized deformation at the shear surface.

As can be seen from Figure 11, in the shear-yielding bolt case, the low-modulus material surrounding the rebar provides an elastic cushioning effect during the shearing process, distributing the concentrated stress more evenly. Through comparison with the traditional bolt anchoring case, it can be seen that the stress concentration and localized deformation of the shear-yielding bolt are significantly lower. The low-modulus material absorbs much of the shear stress, and during the deformation process, it evenly transmits the shear stress to the bolt, thereby indirectly enhancing the shear resistance of the bolt and extending its service life.

In summary, the shear strength of the structural surface under shear-yielding bolt anchoring is further enhanced compared to that with traditional bolt anchoring. This improvement is due to the shear-yielding characteristic of the new type of bolt. In the initial stage of shearing, the rock mass first interacts with the low-modulus material. Because of the material’s elastic properties, the interlayer can undergo a certain amount of displacement without directly acting on the bolt, allowing the natural shear strength of the interlayer structural surface to take effect, thus improving the overall shear resistance. After this stage, the bolt begins to take on the shear force, similar to the process observed with traditional bolts. Additionally, the low-modulus material tightly wraps around the bolt, transmitting the shear force from the concrete block to the low-modulus material, which then transmits the force to the bolt. Throughout this process, the low-modulus material provides an elastic buffering effect, which distributes the concentrated stress more evenly. This indirectly enhances the shear resistance of the bolt and extends its service life.

5. Conclusions

Large-scale direct shear test and numerical simulation using Abaqus are carried out to study the difference between the traditional bolt and shear-yielding bolt in their shear performances and stress distributions in the shear process. After using numerical simulation to define the elastic modulus of the low-modulus material inserted between the bolt and the rock mass, the effect of the low-modulus material is discussed, and such effect is compared under different thicknesses of the material. The findings can be concluded as follows:

• The main difference between shear-yielding bolts and traditional bolts lies in the presence of a layer of low-modulus material wrapped around the outside of the shear-yielding bolt. The low-modulus material has a low elastic modulus and easily deforms under compression. Due to the presence of this low-modulus material, when the structural surface undergoes shear slip in layered rock bolted with a shear-yielding bolt, the structural surface generates frictional resistance to sliding, allowing the natural shear strength of the interlayer to take effect and improving the overall shear performance. The elastic properties of the low-modulus material exert a certain unloading effect on the pressure transmitted from the rock mass, ensuring that the pressure is more evenly distributed onto the rebar of the bolt, avoiding excessive stress concentration, and extending the service life of the bolt.
• Due to low-modulus material’s effect of releasing shear and fully utilizing the natural shear strength of the interlayer, the shear resistance of the shear-yielding bolt is superior to that of ordinary traditional bolts. From the direct shear test results, it can be seen that by allowing for greater peak displacement, the shear-yielding bolts reach its peak shear stress as 0.73 MPa, 0.94 MPa, 1.20 MPa, and 1.42 MPa under the normal pressure of 0.25 MPa, 0.50 MPa, 0.75 MPa, and 1.0 MPa. Compared with the peak shear stress of traditional anchor bolt, it is increased by 40% on average.
• Within a certain range, an increase in the moisture content of the structural surface suppresses the shear strength of the structural surface. The higher the moisture content is, the lower the density and cohesion of the soil are, which indirectly alters the interaction forces between the structural surfaces and reduces the overall shear strength. However, for the shear-yielding bolt, due to the material properties, the moisture content on the structural surface has less influence. When the moisture content of the structural surface rises from 12% to 20%, for cases where there is no anchoring measure, the peak shear stress decreases by 0.15 kPa, almost half. But for cases where the shear-yielding bolts are used, the peak shear stress decreases by 0.12 kPa, which only accounts for 12% of the original strength.
• The thickness of the low-modulus material in the shear-yielding bolt affects its anchoring ability. Specifically, as the thickness of the low-modulus material increases, the anchoring capacity of the shear-yielding bolt initially improves; however, beyond a certain point, the capacity no longer improves and even decreases because the bonding between the rebar and the material weakens. The optimum thickness of the low-modulus material is 15 mm according to the test results and numerical analysis. Thus, the influence of various factors on the bonding effect between the low-modulus material and the bolt and the rock mass remains to be studied.

However, how various factors in practical engineering, including temperature, higher moisture content, acidic or alkaline environment, and actual inserting state, affect the behavior of the low-modulus material is still unclear. Considering that it is the distinctive design making the shear-yielding bolt superior to the traditional bolt, the study of the shear-yielding bolt needs to be further verified in practical engineering.