A high-fidelity replication method for fractured rock mass samples for direct shear test

Abstract

The present application relates to the technical field of energy exploitation, and discloses a high-fidelity replication method for a fractured rock mass sample for direct shear test, which replicates the fractured rock mass sample by using a customized numerical control 3D carving device combined with a two-step carving method. The customized numerical control 3D carving device distributes the Z-axis movement from the carving spindle to the workbench that bears the rock sample, and the relative action direction of the cutting tool and the rock sample is that the workbench actively rushes up to the cutting tool. The two-step carving method is to mill the protective shoulder first and then carve the fracture surface. Through the structural optimization design of the customized numerical control 3D carving device, the inertia effect of the Z-axis in the carving process is effectively reduced, the excessive vibration of the cutting tool and the spindle is inhibited, the structural rigidity of the device is improved, and the non-expected micro-cracks and surface damage of the rock mass in the carving process are reduced. Through the two-step carving method, the stress concentration and fragmentation problem of the edge of the fracture surface are effectively reduced, and the geometric precision of the replication of the fracture surface is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of energy extraction technology, specifically to a high-fidelity replication method for fractured rock mass samples used in direct shear tests. Background Technology

[0002] Fractures in rock masses, acting as weak structural planes and preferential fluid flow channels in geological structures, directly determine the shear friction behavior along the fractures and the flow characteristics of fluids within them. Therefore, in geotechnical engineering, hydraulic engineering, and geological resource extraction and storage, the mechanical and hydraulic properties of fractured rock masses are core bases for engineering design and safety assessment. Direct shear tests are a key experimental method for revealing the mechanical and hydraulic properties of fractured rock masses. However, conducting a series of such tests requires preparing rock mass samples with identical fracture surface morphology to ensure the uniformity of experimental variables and enable effective comparison and analysis of test results. However, under natural geological conditions, it is impossible to obtain rock samples with the same fracture surface morphology. On the one hand, natural rock masses have inherent heterogeneity, and there is no natural fracture surface with completely consistent geometry and strength. On the other hand, shear tests will cause irreversible damage to the fracture surface morphology. A single sample can only complete one test. However, the study of rock mass mechanical properties requires multiple sets of tests under different normal stress conditions, which puts forward clear requirements for the repeatability of the samples. The above problems seriously limit the repeatability of direct shear tests on fractured rock masses and make it difficult to accurately distinguish the influence of fracture surface roughness on key mechanical and hydraulic responses such as rock mass shear strength and shear dilatation.

[0003] To address the issue of non-reproducible natural rock mass samples, existing technologies commonly employ rock-like materials to replicate fracture surface rock mass samples with identical morphological characteristics through methods such as silicone mold casting, casting, mechanical cutting, 3D printing, or 3D carving. While silicone mold casting and mechanical cutting methods can achieve a certain degree of geometric replication, they suffer from significant technical drawbacks: similar materials cannot fully simulate the brittle fracture and damage characteristics of natural rock masses; even when the strength of similar materials is designed to approximate that of natural rock through formulation, their strength is generally still lower, leading to material property failure. Mechanical cutting can only produce regular sawtooth or wavy fracture contours, failing to reproduce the random and complex morphology of natural fracture surfaces, resulting in an oversimplification of the fracture surface morphology. Furthermore, the mechanical response of replicated samples prepared by these methods deviates significantly from the actual fracture mechanical response of natural rock, making it difficult to guarantee the authenticity of experimental results.

[0004] Compared to other methods, 3D engraving technology achieves a better balance between realism and experimental controllability by precisely controlling the geometry of the fracture surface while preserving the inherent lithology of the surrounding rock. This has made it an increasingly widely used replication method in experimental research on the shear properties of fractured rock masses. However, existing 3D engraving technologies still face many technical challenges that urgently need to be addressed.

[0005] Firstly, most existing engravings use general-purpose CNC engraving machines. These machines are not designed for engraving complex rock fissure surfaces. When engraving highly brittle, high-strength, and structurally complex rock fissure surfaces, they suffer from insufficient geometric accuracy and mechanical reliability. The Z-axis motion of a general-purpose CNC engraving machine is completed by the spindle. The high-frequency, short-distance Z-axis motion is prone to generating significant inertial effects, causing the cutting tool to overshoot downwards, resulting in excessive material removal. At the same time, excessive vibration of the spindle and the tool in the Z-axis direction can easily cause unexpected microcracks, surface damage, and edge fragmentation in the rock mass. This problem is particularly prominent in the carving of highly brittle, high-strength rocks or rocks with anisotropy such as bedding and foliation. In addition, the axis travel range of general-purpose CNC engraving machines is much larger than the working requirements of laboratory-scale rock mass samples. The excessively long beam span increases the effective lever arm of the cutting reaction force and reduces the structural rigidity of the equipment, making it difficult to maintain stable processing conditions in high-resolution engraving of hard rock surfaces. This means that existing 3D engraving technology can only be adapted to relatively uniform, medium-strength rock masses, and has obvious limitations in terms of rock type adaptability.

[0006] Secondly, existing 3D carving methods do not consider the stress concentration problem at the edge of rock samples. They directly carve the rough fracture surface geometry to the edge of the rock sample, which leads to premature damage and failure caused by stress concentration at the edge of the sample during direct shear tests. This results in the failure mode of the rock mass not being shear failure of the fracture surface, but local failure of the rock, which seriously affects the mechanical reliability of the shear test results.

[0007] Third, existing engraving techniques are insufficient to achieve high-fidelity replication of fractured rock masses. The three-dimensional morphology of the fracture surface of the replicated sample is not consistent with the original fracture surface. The surface roughness characteristics of replicated samples of the same lithology are significantly different. They cannot exhibit repeatable mechanical responses in shear tests, making it difficult to provide standard samples for serial shear tests of fractured rock masses. The comparability and reference value of the test data are greatly reduced.

[0008] In summary, there is an urgent need for a replication method that can achieve high-fidelity replication of fractured rock mass samples, adapt to various lithologies, and ensure the reliability of shear test results. This would solve the problems of low geometric replication accuracy, narrow lithology adaptation range, insufficient reliability of test results, and poor sample repeatability in existing technologies, and meet the research needs of serialized direct shear tests on fractured rock masses. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a high-fidelity replication method for fractured rock mass samples used in direct shear tests, thus solving the problems mentioned in the background section.

[0010] To achieve the above objectives, the present invention provides the following technical solution: a high-fidelity replication method for fractured rock mass specimens used in direct shear tests, employing a customized CNC 3D engraving device combined with a two-step engraving method to replicate the fractured rock mass specimen. The customized CNC 3D engraving device distributes the Z-axis motion from the engraving spindle to the worktable supporting the rock specimen, and the relative action direction between the cutting tool and the rock specimen is such that the worktable actively meets the cutting tool upwards. The two-step engraving method involves first milling the protective shoulder and then engraving the fracture surface, specifically including the following steps: Step 1: Collect and process the three-dimensional topographic point cloud data of the fracture surface of the target rock mass; Step 2: Based on the processed point cloud data, design the carving tool paths for the protective shoulder and the crack surface respectively and convert them into machine-executable code; Step 3: In the customized CNC 3D carving device, first mill the protective shoulder around the fracture surface on the rock sample, and then carve a three-dimensional rough fracture surface that is consistent with the fracture surface of the target rock mass in the central area enclosed by the protective shoulder. Step 4: After carving, obtain a replica specimen of the fractured rock mass suitable for direct shear testing.

[0011] Preferably, the customized CNC 3D carving device has a compact structure and includes a fully enclosed processing chamber. The dimensions of the processing chamber and its internal X, Y, and Z axes are adapted and adjusted according to the conventional dimensions of fractured rock mass samples at laboratory scale. The positioning accuracy of the customized CNC 3D carving device is 0.01 mm, and the maximum feed speed of the coordinated three-axis movement of the carving spindle and the worktable is 3000 mm / min.

[0012] Preferably, the engraving spindle of the customized CNC 3D engraving device is a water-cooled spindle driven by a precision servo motor, and the engraving spindle is compatible with cutting tools with tool holder diameters of 2.3mm, 3mm, 4mm and 6mm.

[0013] Preferably, in step one, a 3D digital scanner with a resolution of 0.05mm is used to collect three-dimensional morphological data of the fracture surface of the target rock mass, generate a high-resolution point cloud, and then the original point cloud data is subjected to noise reduction processing to obtain a clean point cloud of the fracture surface morphology.

[0014] Preferably, the size of the rock sample is larger than the size of the target rough fracture surface, a flat-end milling cutter is used to mill the protective shoulder, and a conical ball cutter is used to carve the three-dimensional rough fracture surface. The conical ball cutter is used to carve with fine step distance and zero machining allowance.

[0015] Preferably, the protective shoulder is a flat shoulder surrounding the fracture surface. The width of the protective shoulder is greater than the shear displacement length of the direct shear test plan. The top surface of the protective shoulder is lower than the height of the stress edge of the shear box and the top surface of the protective shoulder is lower than the plane of the lowest point of the fracture surface. The distance between the protective shoulder and the lowest point of the fracture surface is ≥5mm.

[0016] Preferably, when the size of the target rough fracture surface is 60×60mm, the size of the rock sample is on the order of 100×100mm, the width of the protective shoulder is 20mm, and the distance between the protective shoulder and the lowest point of the fracture surface is 5mm.

[0017] Preferably, the method is applicable to the replication of fracture surfaces of one or more rock mass samples from sandstone, granite, limestone, and gneiss, and is particularly suitable for anisotropic rock masses with complex structures, such as high brittleness, high strength, or with bedding / foliation intersecting with fracture surfaces.

[0018] Preferably, the protective shoulder milled in step three is formed by directly milling from the parent rock of the rock sample, which preserves the continuity and mechanical compatibility of the rock material and makes the transfer of shear load from the shear box to the fracture surface more uniform.

[0019] Preferably, the fractured rock mass specimen replicated by the method has good consistency between the three-dimensional morphology of the fracture surface and the surface contour of the fracture surface of the target rock mass. The replicated specimens of the same lithology have high consistency in the 3D directional roughness index in each shear direction, and exhibit repeatable mechanical response in the direct shear test under constant normal load.

[0020] This invention provides a high-fidelity replication method for fractured rock mass samples used in direct shear tests. It offers the following advantages: 1. This invention, through the optimized structural design of a customized CNC 3D carving device, effectively reduces the inertial effect of the Z-axis during carving, suppresses excessive vibration of the cutting tool and spindle, and improves the structural rigidity of the device. This reduces unexpected micro-cracks, surface damage, and edge fragmentation of the rock mass during carving, significantly improves the geometric accuracy and mechanical reliability of fracture surface replication, and allows high-precision 3D carving technology to be adapted to more types of rock masses, breaking through the limitations of existing carving technology in terms of rock type adaptation.

[0021] 2. The two-step carving method adopted in this invention effectively reduces the stress concentration problem at the edge of the rock mass sample during direct shear test by milling a flat protective shoulder integral with the parent rock around the fracture surface. This avoids premature failure caused by the edge of the sample, makes the shear load transfer from the shear box to the fracture surface more uniform, ensures the mechanical integrity of the sample, and makes the failure mode of the shear test more closely resemble the real working condition of fracture surface shear, significantly improving the mechanical reliability of the test results.

[0022] 3. The replication method of the present invention achieves high-fidelity reproduction of the three-dimensional rough morphology of the fracture surface of the target rock mass. The fracture surface morphology of the replicated sample has good consistency with the original fracture surface. Replicated samples of the same lithology also have highly consistent surface roughness characteristics and can exhibit repeatable mechanical response in shear tests. This solves the problem that the geometric and mechanical properties of rock mass samples are difficult to replicate after replication in the prior art. It provides repeatable standard samples for serial shear tests of fractured rock masses and improves the comparability and reference value of test data. Attached Figure Description

[0023] Figure 1 This is a diagram of the CNC engraving device of the present invention; Figure 2 This is a conceptual diagram of the two-step carving method for fractured rock masses according to the present invention; Figure 3 This is a flowchart illustrating the process of replicating fractured rock mass samples according to the present invention. Figure 4 This is a schematic diagram comparing and verifying the surface contours between the original crack surface and the etched crack surface of the present invention. Figure 5 This is a schematic diagram illustrating the comparison and verification of directional roughness between different samples of the same lithology in this invention. Figure 6 This is a schematic diagram of the direct shear test verification under constant normal load (CNL) of the present invention. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] Please see the appendix Figure 1 - Appendix Figure 6This invention provides a high-fidelity replication method for fractured rock mass specimens used in direct shear tests. This method focuses on replicating a 60×60mm three-dimensional rough fracture surface as the core test target. A standard laboratory-scale rock mass specimen of 100×100×50mm is selected as the processing substrate. The specimen lithology includes sandstone, granite, limestone, and anisotropic gneiss with intersecting bedding and fracture surfaces—all typical rock mass types commonly found in geotechnical engineering and energy development. Based on the shear displacement design requirements of direct shear tests, the width of the protective shoulder is set to 20mm. Following the design principle that the distance between the protective shoulder and the lowest point of the fracture surface is ≥5mm, the distance between them is determined to be 5mm, thus ensuring that the fracture surface is the sole primary bearing interface during shearing. This embodiment achieves high-fidelity replication of fractured rock mass samples through five core steps: customized CNC 3D engraving device debugging, fracture surface 3D morphology data acquisition and processing, two-step engraving tool path design, two-step engraving processing, and multi-dimensional verification of the replicated sample. The specific operation details and process parameters of each step are as follows: I. Customized CNC 3D carving device debugging The customized CNC 3D engraving device used in this embodiment is a high-precision, compact device specifically designed for replicating fracture surfaces of rock masses at laboratory scale. The entire device features a fully enclosed processing chamber structure, effectively preventing external interference and ensuring safe operation. For processing 100×100×50mm rock mass samples, the effective working dimensions of the X, Y, and Z axes within the processing chamber are customized to 120mm (X-axis, length), 110mm (Y-axis, width), and 80mm (Z-axis, thickness). This compact design reduces the effective bending moment arm of the cutting reaction force, significantly improving the overall structural rigidity of the device and avoiding the insufficient processing stability issues caused by the long axis stroke of general-purpose CNC engraving machines.

[0026] The device is equipped with a high-performance water-cooled spindle driven by a precision servo motor. The maximum spindle speed can reach 24,000 r / min, which can meet the cutting and processing requirements of brittle and high-strength hard rocks. The water-cooling heat dissipation structure can ensure the stability of the spindle under high load for a long time and avoid positioning deviations caused by spindle overheating. The overall positioning accuracy of the device reaches 0.01 mm. The maximum feed speed of the carving spindle in the XY plane is 3,000 mm / min, and the maximum movement speed of the worktable carrying the rock sample along the Z axis is synchronously 3,000 mm / min, realizing coordinated synchronous movement of the three axes and accurately tracking the height and irregular morphological changes of the fracture surface.

[0027] Based on the process requirements of the two-step engraving method, a flat-end mill with a shank diameter of 3mm and a conical ball end mill with a shank diameter of 2.3mm are selected for milling the protective shoulder and high-precision engraving of the crack surface, respectively. After the tools are installed, the device is tested by no-load operation and positioning accuracy calibration. The X, Y, and Z three-axis single-axis motion and three-axis linkage tests are performed in sequence to check the tool running trajectory, the positioning accuracy of the worktable and the rotation stability of the spindle, to ensure that the operation of each component of the device is smooth and the positioning is accurate, thus meeting the process requirements of high-precision engraving.

[0028] II. Acquisition and Processing of Three-Dimensional Morphology Data of Fractured Surfaces Natural joint surfaces collected from the engineering site were selected as the target fracture surfaces for replication. These fracture surfaces possess the random roughness morphology characteristics of natural rock fractures, meeting the research requirements of direct shear tests. The rock mass sample with the target fracture surface was fixed on the motorized turntable of a 3D digital scanner. The relative position of the scanner and the sample was adjusted, and the scanner's scanning resolution was set to 0.05 mm. This resolution can accurately capture the micro-roughness characteristics and macro-contour changes of the fracture surface. The turntable and scanner were started to perform a 360° full-angle scan of the target fracture surface. Through multi-view point cloud stitching technology, high-resolution three-dimensional morphological point cloud data covering the entire fracture surface was generated, completely recording the geometric morphological information of the fracture surface.

[0029] The raw point cloud data is imported into professional point cloud processing software for multi-step data optimization: First, noise reduction is performed, using filtering algorithms to remove noise points and isolated points caused by environmental interference and equipment errors during the scanning process; then, smoothing is performed to weaken local burrs in the point cloud data and ensure the naturalness of the fracture surface morphology; finally, data simplification is performed, eliminating redundant point cloud data while retaining key morphological features of the fracture surface, reducing the computational load for subsequent toolpath design. The processed data yields a clean point cloud with a clear fracture surface morphology, ensuring the accuracy and effectiveness of the point cloud data and providing a reliable geometric model foundation for subsequent digital engraving.

[0030] III. Two-Step Engraving Tool Path Design The processed clean point cloud data is imported into professional CNC machining software. Based on the two-step engraving method of this invention, the engraving path is designed by region and tool to ensure that the tool path closely matches the morphology of the target crack surface and meets the structural design requirements of the protective shoulder. The specific design process is as follows: Protective shoulder toolpath design: The 60×60mm area at the center of the sample is designated as the core machining area for the fracture surface, and the surrounding annular area is designated as the protective shoulder machining area. The protective shoulder width is 20mm, consistent with the preset design parameters. A flat-end milling cutter corresponding to the machining module is selected, and a roughing toolpath is designed. The milling depth is set according to the principle that "the protective shoulder plane is 5mm lower than the top surface of the shear box and 5mm lower than the lowest point of the fracture surface," ensuring that the protective shoulder can effectively avoid edge stress concentration during the shearing test. The toolpath adopts a circular feed method to ensure the milling smoothness of the protective shoulder plane while improving machining efficiency.

[0031] Tool path design for the fracture surface: Within the 60×60mm fracture surface machining area at the center of the sample, the finishing module corresponding to the conical ball engraving tool is selected to design a tool path that fits the three-dimensional shape of the target fracture surface; the engraving parameters are set to fine step distance + zero machining allowance, so that the tool movement trajectory completely replicates the geometric features of the clean point cloud and accurately restores the rough shape of the target fracture surface; for the undulating areas of the fracture surface, the Z-axis movement trajectory of the tool is automatically adjusted to ensure that the engraving depth is consistent with the height of the target fracture surface.

[0032] After the toolpath design is completed, the path is simulated and tested to check for tool interference and smooth tool path. Once confirmed to be correct, the toolpaths for the protective shoulder and the crack surface are converted into G-codes that can be recognized by the customized CNC 3D engraving device and saved as independent processing files for easy access during the engraving process.

[0033] IV. Two-step carving process The prepared 100×100×50mm standard rock mass samples (3 sets each of sandstone, granite, limestone, and gneiss, to meet the requirements of parallel testing) were clamped and processed sequentially. The specific operation is as follows: First, the rock mass samples were fixed on the worktable of the customized CNC 3D carving device using a special clamp. A multi-point positioning clamping method was used to ensure that the samples were firmly clamped without loosening, and that the processing reference surface of the samples was kept horizontal with the worktable plane to ensure processing accuracy. After clamping, the tool was set to determine the initial processing position of the tool and to complete the calibration of the processing coordinates.

[0034] The customized CNC 3D engraving device is activated, and the first engraving step, protective shoulder milling, is performed. The G-code for the protective shoulder toolpath is invoked, and the device drives a flat-end milling cutter to move along a preset path, milling the outer area of ​​the sample to form a flat, continuous annular protective shoulder. The width, height, and relative position of the protective shoulder to the fracture surface processing area strictly conform to the design parameters. After the protective shoulder milling is completed, the sample does not need to be disassembled, and the tool does not need to be re-set. The second engraving step, high-precision fracture surface engraving, is then performed directly. The G-code for the fracture surface toolpath is invoked, and the device switches to a conical ball engraving cutter processing mode. The worktable actively moves upward along the Z-axis to meet the cutting tool, replacing the traditional downward impact cutting method of the spindle, effectively reducing the Z-axis inertial effect and suppressing excessive vibration of the tool and spindle. During the engraving process, the three axes move in a coordinated and synchronous manner, and the tool precisely cuts along the preset trajectory, replicating a three-dimensional rough morphology completely consistent with the target fracture surface in the central area of ​​the sample, completing the engraving of the entire fractured rock mass sample.

[0035] The entire carving process is completed in a fully enclosed processing chamber, effectively avoiding the impact of rock chips on processing accuracy. At the same time, for high-strength and complex rock masses such as granite and gneiss, the spindle speed and feed rate are adjusted to ensure the stability of the cutting process and avoid unexpected micro-cracks, surface damage and edge fragmentation in the rock mass.

[0036] V. Verification of Replica Samples To verify the replication effect of the method of this invention, a systematic verification was conducted on all the fractured rock mass replication samples after carving, covering three dimensions: geometric accuracy, surface roughness, and mechanical reliability. The verification process strictly followed the rock mechanics testing specifications, and the verification results corresponded to the attached figures. Figure 4 Appendix Figure 5 Appendix Figure 6 The specific verification content and results are as follows: Geometric accuracy verification: Sandstone replica samples and original parent samples were selected as verification objects. A 3D digital scanner was used to scan the fracture surfaces of both samples. From the point cloud data obtained from the scans, 2D profile contours of the fracture surfaces were extracted along two perpendicular shear directions, 90° and 270° (see attached). Figure 4 The comparison results show that the 2D profile of the replicated sample fracture surface is highly consistent with the profile of the original parent sample in terms of height undulation, peak and valley positions and values, with no obvious deviation. This proves that the method of the present invention can accurately replicate the geometric morphology of the target fracture surface, and the geometric fidelity meets the experimental requirements.

[0037] Surface roughness verification: Replica samples of three lithologies—gneiss, granite, and limestone (three parallel samples for each lithology)—were selected as verification objects. The 3D directional roughness index θ3D was used to quantitatively evaluate the surface roughness of the fracture surface of each sample. This index comprehensively reflects the maximum apparent dip angle and distribution characteristics of micro-protrusions on the fracture surface and is a key indicator for evaluating fracture surface roughness. The θ3D values ​​of each sample under different shear azimuth angles were calculated and compared (see attached). Figure 5 The results showed that the three replicated samples with the same lithology exhibited strong consistency in 3D directional roughness indices at various shear azimuth angles, with no significant dispersion. This demonstrates that the replication repeatability of the method of the present invention is excellent and can be effectively adapted to complex rock masses with anisotropy, such as gneiss. The replication accuracy is not affected by lithological characteristics.

[0038] Mechanical reliability verification: Two sets of granite replica specimens with identical fracture surface morphology were selected as verification objects. Direct shear tests under constant normal load (CNL) were conducted using an RMT-150 rock testing machine. The test parameters were set as follows: normal load 6 MPa, shear displacement rate 0.01 mm / s, consistent with engineering practice and conventional rock mechanics testing parameters. The shear force-shear displacement curves and shear dilatation changes of the two sets of specimens were recorded (see attached). Figure 6 The results show that the shear force-shear displacement curves of the two groups of samples are highly consistent in terms of overall trend, peak shear force, shear stiffness, and shear dilatation. Furthermore, no premature damage or local failure caused by edge stress concentration occurred during the test, and the rock mass failure mode was shear failure at the fracture surface, which is completely consistent with the experimental expectations. These results demonstrate that the fractured rock mass samples replicated by the method of this invention have repeatable mechanical responses, and their mechanical reliability meets the research requirements of direct shear tests, effectively ensuring the authenticity and comparability of the experimental results.

[0039] Through the above steps, this embodiment successfully achieved a high-fidelity replication of a 60×60mm three-dimensional rough fracture surface on rock samples of four typical lithologies: sandstone, granite, limestone, and gneiss. The replicated samples all met the requirements for direct shear tests: the geometric morphology was highly consistent with the original fracture surface, the surface roughness had good repeatability, the mechanical response was stable and reproducible, and there was no failure problem caused by edge stress concentration in the shear test.

[0040] The instruction manual is attached Figure 1 In the text, the serial numbers represent the following meanings: 1. Carving tool; 2. Carving spindle; 3. Rock sample; 4. Worktable; 5. Movable beam (Z-axis); 6. Movable beam (X-axis); 7. Movable beam (Y-axis); 8. XY plane; 9. Z-axis; 10. Tool - rock cutting direction.

[0041] Instruction manual attached Figure 2 In the text, the serial numbers represent the following meanings: 1. Rock sample; 2. Shear box; 3. Shearing direction; 4. Stress concentration zone; 5. Flat end mill; 6. Conical ball end mill; 7. First step carving - Protective shoulder toolpath; 8. Second step carving - Crack surface toolpath; 9. Protective shoulder plane; 10. Lowest point plane of crack surface; 11. Top surface of shear box; 12. Protective shoulder width.

[0042] Instruction manual attached Figure 3 In the text, the serial numbers represent the following meanings: 1. Rock sample; 2. Turntable; 3. 3D digital scanner; 4. Three-dimensional topographic point cloud data of fracture surface; 5. Rock sample; 6. Protective shoulder tool path; 7. Fracturing surface tool path; 8. Carving knife; 9. Spindle; 10. Replicating fractured rock mass (sandstone); 11. Replicating fractured rock mass (granite); 12. Replicating fractured rock mass (gneiss); (a) Acquisition of three-dimensional topographic data of fracture surface; (b) Processing of three-dimensional topographic point cloud data of fracture surface; (c) Two-step carving tool path editing; (d) Two-step carving process; (e) Replicating rock mass sample.

[0043] Instruction manual attached Figure 4 In the text, the serial numbers represent the following meanings: 1. Original fracture surface 2D profile; 2. Carved fracture surface 2D profile; 3. Shear direction; (a) Original sandstone specimen 2D profile (90° direction); (b) Original sandstone specimen 2D profile (270° direction); (c) Carved sandstone specimen 2D profile (90° direction); (b) Carved sandstone specimen 2D profile (270° direction).

[0044] Instruction manual attached Figure 5 In the text, the serial numbers represent the following meanings: 1. Gneiss sample 1; 2. Gneiss sample 2; 3. Gneiss sample 3; 4. Granite sample 1; 5. Granite sample 2; 6. Granite sample 3; 7. Limestone sample 1; 8. Limestone sample 2; 9. Limestone sample 3; (a) Gneiss sample; (b) Granite sample; (e) Limestone sample.

[0045] The above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-fidelity replication method for fractured rock mass specimens used in direct shear tests, characterized in that, A customized CNC 3D carving device combined with a two-step carving method was used to replicate a fractured rock sample. The customized CNC 3D carving device distributed the Z-axis motion from the carving spindle to the worktable supporting the rock sample, and the relative action direction between the cutting tool and the rock sample was such that the worktable actively met the cutting tool upwards. The two-step carving method involved first milling the protective shoulder and then carving the fracture surface, specifically including the following steps: Step 1: Collect and process the three-dimensional topographic point cloud data of the fracture surface of the target rock mass; Step 2: Based on the processed point cloud data, design the carving tool paths for the protective shoulder and the crack surface respectively and convert them into machine-executable code; Step 3: In the customized CNC 3D carving device, first mill the protective shoulder around the fracture surface on the rock sample, and then carve a three-dimensional rough fracture surface that is consistent with the fracture surface of the target rock mass in the central area enclosed by the protective shoulder. Step 4: After carving, obtain a replica specimen of the fractured rock mass suitable for direct shear testing.

2. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 1, characterized in that, The customized CNC 3D carving device has a compact structure and includes a fully enclosed processing chamber. The dimensions of the processing chamber and its internal X, Y, and Z axes are adapted and adjusted according to the conventional dimensions of fractured rock mass samples at laboratory scale. The positioning accuracy of the customized CNC 3D carving device is 0.01 mm, and the maximum feed speed of the coordinated three-axis movement of the carving spindle and the worktable is 3000 mm / min.

3. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 2, characterized in that, The engraving spindle of the customized CNC 3D engraving device is a water-cooled spindle driven by a precision servo motor, and the engraving spindle is compatible with cutting tools with tool holder diameters of 2.3mm, 3mm, 4mm and 6mm.

4. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 1, characterized in that, In step one, a 3D digital scanner with a resolution of 0.05 mm is used to collect three-dimensional morphological data of the fracture surface of the target rock mass, generate high-resolution point cloud, and then the original point cloud data is denoised to obtain a clean point cloud of the fracture surface morphology.

5. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 1, characterized in that, The size of the rock sample is larger than the size of the target rough fracture surface. A flat-end milling cutter is used to mill the protective shoulder, and a conical ball engraving cutter is used to carve the three-dimensional rough fracture surface. The conical ball engraving cutter is used to carve with fine step distance and zero machining allowance.

6. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 5, characterized in that, The protective shoulder is a flat shoulder surrounding the fracture surface. The width of the protective shoulder is greater than the shear displacement length of the direct shear test plan. The top surface of the protective shoulder is lower than the height of the stress edge of the shear box, and the top surface of the protective shoulder is lower than the plane of the lowest point of the fracture surface. The distance between the protective shoulder and the lowest point of the fracture surface is ≥5mm.

7. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 1, characterized in that, The method is applicable to the replication of fracture surfaces of one or more rock mass samples, including sandstone, granite, limestone, and gneiss, and is especially suitable for anisotropic rock masses with complex structures, such as high brittleness, high strength, or with intersecting bedding / foliation and fracture surfaces.

8. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to claim 1, characterized in that, The protective shoulder milled in step three is formed directly from the parent rock of the rock sample, which preserves the continuity and mechanical compatibility of the rock material and makes the shear load transfer from the shear box to the fracture surface more uniform.

9. The high-fidelity replication method for fractured rock mass specimens used in direct shear tests according to any one of claims 1-8, characterized in that, The fractured rock mass specimen replicated by the method has good consistency with the surface contour of the fractured surface of the target rock mass in terms of three-dimensional morphology. The replicated specimens of the same lithology have high consistency in 3D directional roughness index in each shear direction, and exhibit repeatable mechanical response in direct shear test under constant normal load.

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