A variable-section anchor rod and a method for testing the anchoring effect of layered rock under impact

Abstract

The present invention relates to a variable-section anchor rod and a method for testing the anchoring effect of layered rocks under impact, belonging to the field of rock dynamics test simulation technology. It is characterized in that the main body of the variable-section anchor rod is composed of three parts, namely a dredging drill bit, a hollow seamless steel pipe and threaded steel, wherein the diameters of the hollow seamless steel pipe and the threaded steel should conform to the distribution function of the axial force and shear stress of the anchor rod obtained by the force analysis of conventional rock anchor rods during anchoring. Its construction and testing methods are respectively carried out according to the characteristics of the variable-section anchor rod. The purpose of the present invention is to simulate the actual surrounding rock damage at the blasting construction site according to the experimental method, and to systematically study the destruction characteristics of layered rocks under different anchoring angles and anchor rod numbers, providing reference value for related projects.

Description

Technical field:

[0001] The invention belongs to the technical field of rock dynamics test simulation, and in particular relates to a test method for simulating the impact of layered rocks on variable-section anchor rods and their anchoring states. Background technology:

[0002] During actual blasting construction, due to the inhomogeneity of the rock, it often contains some cracks and joints, making its structure discontinuous and forming a layered rock structure. Layered composite rock structure is one of the rock structures commonly found in underground engineering. Unlike single-structure rock mass, layered composite rock is composed of multiple layers of rock with different properties. The rock properties of each component are different, and there are defects at the rock junctions. Its failure mechanism and mode are also significantly different from ordinary rock mass. Therefore, in actual engineering, to enhance the strength of the surrounding rock and reduce the occurrence of engineering accidents, surrounding rock reinforcement is a very necessary engineering procedure.

[0003] Among the measures for reinforcing surrounding rock, the anchor support method has been widely used in engineering due to its economy and practicality. As an auxiliary tool for stabilizing the surrounding rock, the anchor is essentially a tensile lever. Its function is to connect the unstable surrounding rock structure with the bedrock through the rod, so that multiple rock masses are combined into a stable composite structure. While ensuring the integrity of the entire project, the anchor greatly improves the shear and tensile resistance of the rock itself, and enhances the bearing capacity and stability of the entire rock mass, thereby achieving the purpose of consolidating the surrounding rock. However, during the construction process, conventional rock anchors need to be drilled first and then anchored. It is impossible to anchor while drilling, which is inefficient. Secondly, the overall force of conventional rock anchors is not the same everywhere during anchoring, while the overall material of conventional anchors is indeed the same everywhere, which causes material waste to a certain extent and increases costs. Therefore, a variable-section anchor is urgently needed to solve the above problems.

[0004] Due to the instability of layered rocks, the failure morphology of surrounding rocks under blasting impact often shows uncertainty. However, this uncertainty in the failure morphology of surrounding rocks under blasting impact is difficult to judge, and there is currently no good solution. Summary of the invention:

[0005] Purpose of the invention:

[0006] The present invention provides a method for testing the anchoring effect of layered rock under the action of variable-section anchor rods and blasting impact. Its purpose is to simulate the actual surrounding rock damage at the blasting construction site according to the experimental method, and systematically study the failure characteristics of layered rock under different anchoring angles and anchor rod numbers, providing reference value for related projects.

[0007] Technical solution:

[0008] A variable-section anchor bolt includes a tunnelable drill bit, characterized in that the main body of the variable-section anchor bolt is composed of three parts, the first part is the tunnelable drill bit, the second part is a hollow seamless steel pipe with a diameter larger than the tunnelable drill bit, both ends of the hollow seamless steel pipe are processed and provided with internal threads, one end is connected to the tunnelable drill bit, and the other end is connected to threaded steel with a diameter smaller than the hollow seamless steel pipe; the third part is threaded steel with external threads at both ends, one end is connected to the hollow seamless steel pipe, and the other end serves as the free end of the anchor bolt; the diameters of the hollow seamless steel pipe and the threaded steel should conform to the distribution function τ(x) of the axial force P(x) and shear stress of the anchor bolt obtained by force analysis of conventional rock anchor bolts during anchoring:

[0009]

[0010] Where D is the diameter of the anchor body, E is the composite elastic modulus of the anchor rod and anchoring agent, K is the shear stiffness of the anchor body, P is the axial load of the anchor body, τ(x) is the shear stress when the axial anchoring length is x, and p(x) is the axial force when the axial anchoring length is x.

[0011] 2. A construction method using the above variable cross-section anchor rod, characterized in that the construction process steps are as follows:

[0012] (1) Connecting a tunneling drill bit to one end of a large-diameter hollow seamless steel pipe, and connecting the other end of the large-diameter hollow seamless steel pipe to an anchor drilling rig to drill and anchor the rock mass;

[0013] (2) When the anchor drill reaches the required depth for construction, stop drilling and remove the anchor drill, grout the inside of the large diameter hollow seamless steel pipe, and screw on the connecting pipe sleeve after the grouting is completed;

[0014] (3) Combine the small diameter threaded steel bar with the large diameter hollow seamless steel pipe through the connecting pipe sleeve, and then perform integral grouting;

[0015] (4) Put the gasket on the free end of the anchor rod and tighten the bolt.

[0016] 3. A method for testing the anchoring effect of the variable cross-section anchor rod in layered rock under blasting impact, characterized in that the method is carried out in the following steps:

[0017] (1) Rock samples taken from actual projects are processed into 50mm×50mm standard cylindrical specimens;

[0018] (2) Perform equal thickness cutting and cut the sample into four independent pieces along the axial direction. Use a deep hole drill to drill holes at equal intervals along the radial direction of the sample. The multi-hole sample is equally spaced and processed into single, two, and three holes respectively. Holes are drilled at 0°, 30°, 60°, and 90° angles along the circumference. The hole diameter is 10 mm. Then, use silicate cement mortar to bond the four independent pieces of the sample together to form a layered rock structure.

[0019] (3) Anchor the specimens with different anchoring angles and numbers of anchor rods;

[0020] (4) Impacting the specimen using the Hopkinson system;

[0021] According to the anchoring force distribution and Hooke's law, the tensile deformation of the anchoring section under the elastic state can be obtained:

[0022]

[0023]

[0024] Where: D is the diameter of the anchor body; P is the axial load of the anchor body; E is the elastic modulus of the anchor body; ω(x) is the shear displacement deformation of the anchor section; u(x) is the axial displacement of the anchor body at coordinate x; G is the shear modulus of the anchor solid; e is a special mathematical constant in mathematics, an infinite non-repeating decimal, also known as the Euler number;

[0025] According to the propagation conditions of elastic waves at the interfaces of different media, the resultant forces on both sides of the interface must be equal. The resultant force between the interface and the incident rod end is: P1(t)=A0E(ε i +ε r ); the resultant force between the interface and the end of the transmission rod is: P2(t)=A0Eε t According to the principle of force balance of the specimen during the test, the average stress of the specimen is:

[0026]

[0027] Where: E is the elastic modulus; A and A0 are the cross-sectional areas of the specimen and the rod, respectively; ε r (t), ε t (t) is the reflected strain and transmitted strain on the elastic SHPB rod; ε i (t) is the strain pulse incident on the rod at time t; t is the time, which is the independent variable in the function;

[0028] This is used to simulate the blasting impact on the surrounding rock at the engineering site, conduct strength analysis on layered rocks under different anchoring conditions, and provide data for actual engineering construction.

[0029] In step (2), an anchoring angle of 60° is optimal.

[0030] In step (2), the best anchoring effect is achieved by using two anchor rods.

[0031] Advantages and effects:

[0032] Compared with the existing technology, this structure and test method are reasonable and have the following advantages:

[0033] 1. The rod body of the variable-section anchor rod consists of two parts with different diameters. It has higher tensile and shear resistance than ordinary rock anchor rods. At the same time, it can be anchored on demand in different rock formations, which can improve the anchoring effect while saving costs.

[0034] 2. Use rock cutting technology to slice the complete rock and re-bond it with silicate cement mortar to simulate the layered rock structure encountered in actual engineering and restore the layered characteristics of the layered rock in actual engineering.

[0035] 3. During the blasting construction process, the rock is subjected to repeated impact loads such as blasting shock and mechanical vibration, and is mostly in a high strain rate state. Therefore, the Hopkinson system is used to conduct impact experiments to simulate the state of layered rock under high strain rate, so as to explore the rock properties and provide a reference for actual engineering.

[0036] 4. Due to the complex working conditions during actual construction, the direction of multi-layer rock anchoring is often unable to be directionally controlled. It is often necessary to adjust the anchoring angle and the number of anchor rods according to the actual working conditions on site. This experimental method fully simulates the diversity of the surrounding rock structure on site and prepares rock specimens with different anchoring angles (including 0°, 30°, 60° and 90°) and different numbers of anchor rods (1-3) to fully simulate the construction site conditions. Description of the drawings:

[0037] Figure 1 This is a flow chart of the test method for the anchoring effect of layered rocks;

[0038] Figure 2 This is a diagram of rock processing forms;

[0039] Figure 3 This is the stress analysis diagram of ordinary rock anchor;

[0040] Figure 4 is the variable cross-section anchor diagram;

[0041] Figure 5 A variable cross-section anchor diagram was used for the experiment;

[0042] Figure 6 This is the specimen diagram after anchoring;

[0043] Figure 7 The stress-strain curves of anchored rocks at different anchoring angles;

[0044] Figure 8 The relationship diagram of dynamic compressive strength and elastic modulus of anchored rock at different anchoring angles is shown in Figure 2.

[0045] Figure 9 The stress-strain curves of anchored rocks with different numbers of anchor rods;

[0046] Figure 10 The relationship diagram of dynamic compressive strength and elastic modulus of anchored rock with different numbers of anchors is shown;

[0047] Figure 11 This is a schematic diagram of local anchor bolting in actual engineering;

[0048] Markings in the figure: 1. Layered rock slices, 2. Prefabricated holes, 3. Tunable drill bit, 4. Gasket, 5. Seamless steel pipe, 6. Threaded steel bar, 7. Nut and sleeve. Specific implementation method:

[0049] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods. The flow chart of the test method for the anchoring effect of layered rock under blasting impact is as follows: Figure 1 As shown, the specific process is as follows:

[0050] S1: Select the sample and process it. Select the complete rock mass without cracks and joints from the engineering site, and prepare the specimen according to the national standard "Standard for Engineering Rock Test Methods" (GB / T50266-2013). Use wire cutting to process the rock block into a cylindrical specimen with a diameter of 50 mm and a height of 50 mm. The ends and sides of the specimen should be carefully polished with sandpaper until the outer surface of the specimen is smooth and without protrusions. The specimen accuracy requirement is that the unevenness of the surface and circumference is less than 0.02 mm. The processed rock specimen is cut into equal thickness in the axial direction using a rock cutter and cut into four independent layered rock slices 1. Then, a deep hole bench drill is used to drill holes at equal intervals in the radial direction of the specimen (the hole spacing of the multi-hole specimen is equal), and they are processed into single hole, two holes and three holes, 0°, 30°, 60° and 90° holes 2. The hole angles are all relative to the diameter of the specimen slice. The hole diameter is 10 mm. The processing form is as follows Figure 2 As shown, the four independent pieces of the specimen were then bonded together using Portland cement mortar to form a layered rock structure.

[0051] S2: Anchor the specimen. Cement mortar was used as the anchoring agent, and a self-developed variable-section anchor rod was used to anchor the specimen through the drill hole. The specific implementation of the variable-section anchor rod is as follows.

[0052] like Figure 3 As shown in the figure, a conventional rock bolt is subjected to a pullout force P. Under this force, the bolt is pulled and displaced, creating resistance to the anchoring agent surrounding the bolt. This resistance and the adhesion of the anchor to the bolt transmit the pullout force to the anchor, which is then transferred to the formation through friction between the anchor and the surrounding rock. As the pullout force P gradually increases, the anchoring effect fails, and the anchored section enters the residual strength stage. Therefore, it can be concluded that the ultimate pullout resistance of the bolt is primarily controlled by the friction between the anchor and the surrounding rock.

[0053] In order to analyze the stress condition of the anchoring section, the diameter of the anchor body is set to D and the elastic modulus is set to E. m , the tensile elastic modulus of the anchor is E s , the bolt diameter is d, and the designed anchor length is l. For ease of analysis, the following assumptions are made: ① The surrounding rock mass and the anchor are made of linear elastic materials; ② The contact surface between the surrounding rock mass and the anchor follows the traditional Mohr-Coulomb criterion; and ③ The stress on the axial cross-section of the anchor is uniformly distributed. The distribution functions of the bolt's axial force P(x) and shear stress τ(x) are obtained as follows:

[0054]

[0055] It is further known that the diameter of the anchor rod plays an important role in the anchoring effect. The larger the diameter, the larger the range of the anchor rod and the better the control effect on the integrity of the rock mass. However, since rock is a natural material with large discreteness, it is necessary to flexibly anchor it according to the rock properties of different rock layers, that is, to improve the anchoring effect of the anchor rod and reduce the cost. Based on the above expectations for the anchoring effect, a variable-section anchor rod was independently developed. The variable-section anchor rod is as follows. Figure 4 As shown, the main body of the variable-section anchor rod is divided into three parts. The first part is a tunnelable drill bit 3, which is used to drill holes in the rock mass, eliminating the drilling process during the anchoring process and improving the anchoring efficiency. The second part is a large-diameter hollow seamless steel pipe 5, both ends of which are processed with internal threads. One end is connected to the tunnelable drill bit 3, and the other end is connected to the small-diameter threaded steel 6. This part mainly plays a role in resisting pullout in the anchor rod structure, and its diameter can be adjusted according to the construction process. The third part is a small-diameter threaded steel 6, both ends of which are processed with external threads. One end is connected to the large-diameter seamless steel pipe 5, and the other end serves as the free end of the anchor rod. This part mainly plays a role in resisting shear in the anchor rod structure, and its diameter can also be adjusted. Since the large-diameter hollow seamless steel pipe 5 and the small-diameter threaded steel 6 have different diameters at the connection point, resulting in an inability to connect, a connecting pipe sleeve 7 with internal and external threads is added at the connection point of the large-diameter seamless hollow steel pipe 5 to balance the uneven diameter and the inability to connect.

[0056] The construction process of variable cut anchor is as follows:

[0057] (1) Connect the excavation drill bit to one end of the large-diameter hollow seamless steel pipe 5, and connect the other end of the large-diameter hollow seamless steel pipe 5 to the anchor drilling rig to perform rock drilling and anchoring.

[0058] (2) When the anchor drill reaches the required depth for construction, the drilling is stopped and the anchor drill is removed, and grouting is performed inside the large-diameter hollow seamless steel pipe 5. After the grouting is completed, the connecting pipe sleeve 7 is screwed on.

[0059] (3) The small diameter threaded steel bar 6 is combined with the large diameter hollow seamless steel pipe through the connecting pipe sleeve, and then the whole is grouting is performed.

[0060] (4) Put the gasket on the free end of the anchor rod and tighten the bolt.

[0061] The variable-section anchor rod is different from the ordinary rock anchor rod. It can be anchored immediately while drilling in the rock mass, achieving the effect of "anchoring while drilling" and timely controlling the integrity of the rock mass. Because the rod body of the variable-section anchor rod is composed of two parts with different diameters, it has higher tensile and shear resistance than ordinary rock anchor rods, and can be anchored on demand for different rock formations, so it can improve the anchoring effect while saving costs. Due to the limitations of indoor test conditions, the variable-section anchor rod needs to be slightly modified before it can be used for indoor testing. In order to fix the position of the anchor rod by screwing a nut on the external thread during anchoring, the internal thread at one end of the large-diameter hollow seamless steel pipe 5 was changed to an external thread, and the other parts were not changed. After the modification, Figure 5 shown.

[0062] Connect the excavable drill bit 3 to one end of the large diameter hollow seamless steel pipe 5, and the other end of the large diameter hollow seamless steel pipe 5 to the drilling rig to perform rock drilling and anchoring. When the anchor drill rig drills to the required depth for construction, stop drilling and remove the anchor drill rig, and grout the inside of the large diameter hollow seamless steel pipe. After the grouting is completed, install the gasket 4 and screw on the nut sleeve 7. Combine the small diameter threaded steel bar 6 with the large diameter hollow seamless steel pipe through the connecting sleeve, and then perform overall grouting. Put the gasket on the free end of the anchor rod and tighten the bolt. Figure 6 This is the specimen diagram after anchoring.

[0063] According to different anchoring angles and anchoring numbers, three specimens of each category were prepared and numbered in the form of 0-1-1, where the first digit of the number represents the anchoring angle of the anchor rod, the second digit represents the number of anchor rods, and the third digit represents the order of the specimen. The impact air pressure was selected as 0.15 MPa. Among each group of three specimens, the group with the smallest data discreteness was selected for numerical analysis.

[0064] S3: Impact testing using the Hopkinson system. A split-type Hopkinson pressure bar test apparatus is used to simulate impact testing. The equipment consists of three main components: a bracket, a pressure bar body, and a data acquisition system. The bracket primarily supports and secures the pressure bar and ensures its levelness. To facilitate system installation and disassembly, it is constructed from aluminum alloy. The pressure bar body includes the barrel, impact bar (commonly known as the bullet), incident bar, projection bar, energy absorber, and corresponding pneumatic device. The data acquisition system consists of strain gauges attached to the bar, a Wheatstone bridge (strain gauge wiring bridge box), an ultra-dynamic strain gauge, and a high-speed acquisition system.

[0065] Attach strain gauges to the middle of the incident rod and the transmission rod, secure the strain gauge wires to the rods with tape, and connect the other ends of the wires to the data acquisition system. Check that the bullet tube, incident rod, and transmission rod are completely aligned. Adjust the device to ensure normal operation, close the combination valve, then open the nitrogen bottle switch, adjust the air pressure in the pressure chamber through the pressure valve, and adjust the acquisition system parameters.

[0066] Align the incident rod and the transmission rod and make them contact, then set the impact air pressure, open the air pressure valve to perform air flushing, close the valve after the air flushing is completed, and then adjust the dynamic strain collector according to the waveform diagram during the air flushing, set the instrument parameters, apply butter to both ends of the quartz sandstone sample to reduce the influence of the friction effect, clamp the sample between the incident rod and the transmission rod, and try to ensure that the centers of the three are aligned in a straight line.

[0067] Set the impact pressure to the preset value, turn on the switch to allow nitrogen to enter the pressure chamber, then quickly turn off the switch and click the impact button on the operating system. The bullet will quickly impact the incident rod. The data acquisition system collects data through the strain gauge on the rod, and then processes and analyzes the data. After the test, turn off all equipment and tighten the nitrogen bottle switch. Finally, take out the sample and export the data.

[0068] S4: Impact result analysis. The test results of the specimens after conventional uniaxial impact test of anchored rock with different anchoring angles are summarized in Table 1. According to Table 1, representative specimens of each group (numbered 90-1-3, 60-1-3, 30-1-1, 0-1-2) are selected for research and analysis. The representative specimens are those whose dynamic compressive strength values ​​are close to the average value of the dynamic compressive strength values ​​of the specimens in the group. Figure 7 Stress-strain curves of anchored rocks at different anchoring angles, Figure 8 The relationship between dynamic compressive strength and elastic modulus of anchored rock at different anchoring angles and anchoring angle. Table 1 is a statistical table of conventional uniaxial impact test results of anchored rock at different anchoring angles;

[0069] Table 1

[0070]

[0071] Depend on Figure 7 It can be seen that the peak stress of the 30° anchorage specimen (30-1-1) is 117.02 MPa, and the peak strain is 0.008, which are 15.9% and 2.2% higher than those of the 0° anchorage specimen (0-1-2); the peak stress of the 60° anchorage specimen (60-1-3) is 136.14 MPa, and the peak strain is 0.00566, which are 35.1% higher and 27.7% lower than those of the 0° anchorage specimen (0-1-2); the peak stress of the 90° anchorage specimen (90-1-3) is 122.39 MPa, and the peak strain is 0.00682, which are 22.1% higher and 8.7% lower than those of the 0° anchorage specimen (0-1-2). As the anchoring angle increases, the peak stress first increases and then decreases, while the peak strain first increases, then decreases, and then increases again. From the perspective of specimen stress, this suggests that when the anchoring angle is small, the angle between the anchor and the stress perpendicular direction is small, so the anchor cannot provide sufficient resistance. Consequently, the stress is borne by the layered rock flakes within the specimen. Since the layered rock flakes are not intact rock, their dynamic compressive strength is necessarily low, making them susceptible to failure. When the anchoring angle is large, the anchor provides greater resistance, making the specimen less susceptible to failure. From the perspective of specimen deformation, the presence of the anchor constrains the lateral deformation of the specimen, resulting in a longer duration of coordinated deformation between the specimen and the anchor, which slows the axial expansion of the internal cracks in the rock mass and reduces the axial strain of the specimen. When the anchoring angle is 60°, the axial strain of the specimen is minimized, and the lateral constraint of the anchor on the specimen is most pronounced, indicating that this anchoring angle has a more significant control effect on the deformation of the anchored rock.

[0072] Depend on Figure 8It can be seen that when the anchoring angle is less than 60°, the dynamic compressive strength of the specimen increases with increasing anchoring angle. However, when the anchoring angle is greater than 60°, the compressive strength of the specimen decreases with increasing anchoring angle. This may be due to the fact that a larger anchoring angle provides greater support within the specimen, resulting in a higher dynamic compressive strength. When the anchoring angle increases to 60°, the anchor's support weakens and the resulting resistance decreases, leading to a decrease in the dynamic compressive strength of the specimen. This suggests that a larger anchoring angle does not necessarily increase the dynamic compressive strength of the specimen. Therefore, there should be a limit to the selection of anchoring angles, ranging from 60° to 90°. The addition of anchors changes the specimen's original properties, leading to different specimens' ability to withstand the maximum failure load and deformation at different anchoring angles, manifested as different elastic moduli. The figure shows that the elastic modulus of the specimens initially increases and then decreases with increasing anchoring angle, with the magnitude of the increase increasing with increasing anchoring angle. When the anchoring angle is 60°, the slope of the elastic modulus of the sample increases dramatically, and the elastic modulus value is 24GPa, which is much larger than the other three anchoring angles. From this analysis, it can be seen that when the anchoring angle is 60°, the anchor rod can exert a better anchoring effect, which can control the integrity of the anchored rock and improve its bearing capacity.

[0073] The test results of the specimens after conventional uniaxial impact test on anchored rock with different numbers of anchor rods are summarized in Table 2. Representative specimens of each group (numbered 90-1-3, 90-2-2, and 90-3-2) were selected for research and plotted. Figure 9 Stress-strain curves of anchored rock with different numbers of anchor rods, Figure 10 The relationship between the dynamic compressive strength and elastic modulus of anchored rock with different numbers of anchor rods. Table 2 is a statistical table of the results of conventional uniaxial impact tests on anchored rock with different numbers of anchor rods.

[0074] Table 2

[0075]

[0076]

[0077] from Figure 9It can be concluded that the dynamic mechanical properties of the specimens differ. When the number of anchor rods increases from 1 (90-1-3) to 3 (90-3-2), the peak stress of the specimens increases first and then decreases with the increase in the number of anchor rods. In step 2, two sets of control tests were conducted: one set was a single anchor rod with different anchoring angles, and the other set was a comparative test with two anchor rods (90-2-2) anchored at the same angle. The peak stress of the specimens was 130.33 MPa, equivalent to 1.2 times the peak stress when three anchor rods were anchored. The peak stress of the specimens when one anchor rod was anchored was 122.39 MPa, equivalent to 1.1 times the peak stress when three anchor rods were anchored. From the above analysis, it can be seen that the peak stress of the anchor body increases the most when two anchor rods are anchored, and the number of anchor rods can affect the bearing capacity of the specimens. By observing the changing trends of the stress-strain curves of the specimens under three numbers of anchor rods, it was found that when two anchor rods were anchored, the stress of the specimen was the largest and the corresponding peak strain was the smallest, showing the mechanical properties of "high strength and low strain", indicating that the anchoring effect of two anchor rods was better. At this time, there was no group anchor effect of the anchor rods, and the anchoring spacing of the anchor rods was reasonably designed, which indirectly proves that the more anchor rods, the better the anchoring effect.

[0078] from Figure 10 The figure shows the relationship between the dynamic compressive strength and elastic modulus of anchored rock with different numbers of anchors. As shown in the figure, when the number of anchors increases from 1 to 3, the dynamic compressive strength of the specimens first increases and then decreases. The dynamic compressive strengths of the specimens with 1 and 3 anchors are 122.39 MPa and 111.68 MPa, respectively, representing 93% and 85% of the dynamic compressive strength of the specimens with 2 anchors. In summary, increasing the number of anchors effectively improves the dynamic compressive strength of the specimens, indicating that increasing the number of anchors does indeed improve the mechanical properties of the specimens. However, attention should also be paid to the spacing of the anchors. If too many anchors are arranged within a limited area, the abnormal behavior of the specimen with only three anchors will occur (reduced dynamic compressive strength and a cluster effect). Therefore, choosing a reasonable spacing allows the anchors to fully exert their anchoring function, better bond with the specimen, and effectively reduce the impact of dynamic loads on the specimen.

[0079] When the number of anchor rods increases from one to three, the elastic modulus of the specimen increases first and then decreases, consistent with the dynamic compressive strength trend. The elastic moduli of the specimen with one and three anchor rods are 18.2 GPa and 13.9 GPa, respectively, representing 83% and 64% of the elastic modulus with three anchor rods. This indicates that the elastic modulus of the specimen is highest when anchored with two anchor rods. A higher elastic modulus indicates a greater resistance to deformation and a lower resistance to deformation. Under impact loads, anchor rods effectively control the integrity of the specimen and improve its impact resistance.

[0080] According to the above, when the anchoring angle is 60 degrees or the number of anchoring roots is 2, the rock anchoring effect is the best. Therefore, when anchoring the layered surrounding rock mass, the anchoring should be differentiated according to the different bedding directions of the surrounding rock mass, so that the anchoring angle between the anchor rod and the surrounding rock mass is 60 degrees or the number of anchoring roots in a small range is two. Therefore, in actual engineering, the surrounding rock mass anchoring in a local range is as follows: Figure 11 As shown in the figure, the anchoring direction of the variable-section anchor rod forms an angle of 60 degrees with the rock bedding direction, which fully utilizes the material properties and anchoring properties of the variable-section anchor rod, enhances the stability and integrity of the surrounding rock mass to a certain extent, shares the high stress generated by the rock, and reduces engineering disasters.

Claims

1. A method for testing the anchoring effect of variable-section anchor rods in layered rock under blasting impact, characterized by: The method proceeds as follows: (1) Rock samples taken from actual projects are processed into 50mm×50mm standard cylindrical specimens; (2) Perform equal thickness cutting and cut the sample into four independent pieces along the axial direction. Use a deep hole drill to drill holes at equal intervals along the radial direction of the sample. The multi-hole sample is equally spaced and processed into single, two, and three holes respectively. Holes are drilled at 0°, 30°, 60°, and 90° angles along the circumference. The hole diameter is 10 mm. Then, use silicate cement mortar to bond the four independent pieces of the sample together to form a layered rock structure. (3) Anchor the specimens with different anchoring angles and numbers of anchor rods; (4) Impacting the specimen using the Hopkinson system; According to the anchoring force distribution and Hooke's law, the tensile deformation of the anchoring section under the elastic state can be obtained: Where: D is the diameter of the anchor body; P is the axial load of the anchor body; E is the elastic modulus of the anchor body; ω(x) is the shear displacement deformation of the anchor section; u(x) is the axial displacement of the anchor body at coordinate x; G is the shear modulus of the anchor body; e is a mathematical constant; According to the propagation conditions of elastic waves at the interfaces of different media, the resultant forces on both sides of the interface must be equal. The resultant force between the interface and the incident rod end is: P1(t)=A0E(ε i +ε r ); the resultant force between the interface and the end of the transmission rod is: P2(t)=A0Eε t According to the principle of force balance of the specimen during the test, the average stress of the specimen is: Where: E is the elastic modulus; A and A0 are the cross-sectional areas of the specimen and the rod, respectively; ε r (t), ε t (t) is the reflected strain and transmitted strain on the elastic SHPB rod; ε i (t) is the strain pulse incident on the rod at time t; t refers to time; This is used to simulate the blasting impact on the surrounding rock at the engineering site, conduct strength analysis on layered rocks under different anchoring conditions, and provide data for actual engineering construction; The variable-section anchor bolt comprises a tunnelable drill bit. The main body of the variable-section anchor bolt is composed of three parts. The first part is the tunnelable drill bit. The second part is a hollow seamless steel pipe with a diameter larger than the tunnelable drill bit. Both ends of the hollow seamless steel pipe are processed with internal threads, one end is connected to the tunnelable drill bit, and the other end is connected to threaded steel with a diameter smaller than the hollow seamless steel pipe. The third part is threaded steel with external threads at both ends. One end is connected to the hollow seamless steel pipe, and the other end serves as the free end of the anchor bolt. The diameters of the hollow seamless steel pipe and the threaded steel should conform to the distribution function P(x) of the axial force and the distribution function τ(x) of the anchor bolt obtained by the force analysis of conventional rock anchor bolts during anchoring: Where D is the diameter of the anchor body, E is the composite elastic modulus of the anchor rod and anchoring agent, K is the shear stiffness of the anchor body, P is the axial load of the anchor body, τ(x) is the shear stress when the axial anchoring length is x, and p(x) is the axial force when the axial anchoring length is x.

2. The method for testing the anchoring effect of variable-section anchor rods in layered rock under blasting impact according to claim 1, characterized in that: In step (2), under different anchoring angles, the anchoring angle of 60° is the best.

3. The method for testing the anchoring effect of variable-section anchor rods in layered rock under blasting impact according to claim 1, characterized in that: In step (2), under different anchor numbers, the anchoring effect of using two anchor rods is the best.

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