Method for preparing natural structural surface of dangerous rock and testing shear strength

Abstract

The invention provides a method for preparing a natural structural plane of a dangerous rock and testing shear strength, which comprises the following steps of: judging the saturated compressive strength of the rock through a point load test to determine whether the rock is a hard rock; constructing a three-dimensional coordinate system by adopting a drilling television method, calculating an included angle between a normal vector of a structural plane and a normal vector of a rock wall surface, and drilling and coring according to the angle; the rock sample is poured into an Acm * Acm * Acm test piece, the structural surface contact area is kept to be Bcm * Bcm, then maintenance is conducted, A is not smaller than 15, and B is not smaller than 10; carrying out CT scanning three-dimensional reconstruction on the maintained rock sample to calculate a structural surface combination complete area and an opening area so as to obtain a correction coefficient; carrying out a direct shear test to obtain a cohesive force and an internal friction angle; and performing correction according to the correction coefficient to obtain the shear strength parameter of the natural structural plane.

Description

Technical Field

[0001] This invention relates to the field of rock mechanics testing technology, specifically to a method for preparing natural structural surfaces of unstable rocks and testing their shear strength. Background Technology

[0002] Due to prolonged geological processes, rock masses in their natural environment are dissected by natural structural planes such as joints, faults, and fissures, forming unstable rock formations prone to slippage or collapse. These crisscrossing and complex natural structural planes control the mechanical behavior of the rock mass. Historically, many engineering failures and instabilities have been caused by shear deformation along these structural planes. Therefore, the shear strength of rock mass structural planes is crucial to rock mass stability, and its analysis and evaluation are of great significance for rock mass stability analysis.

[0003] The most common method for studying the shear mechanics characteristics of rock mass fracture surfaces is the direct shear test. However, existing direct shear tests on rock mass fracture surfaces mostly use pre-fabricated fracture surfaces on intact rock or serrated fracture surface specimens formed by artificially cast concrete. While these pre-fabricated rock fracture surface specimens and artificially cast concrete fracture surfaces can reflect the shear mechanics characteristics of fracture surfaces to some extent, they still differ significantly from naturally formed rock mass fracture surfaces and cannot reflect the properties of fracture surfaces formed under natural environmental conditions and long-term geological evolution.

[0004] For natural structural planes in rock masses, the following technical problems exist: (1) Difficulty in sampling: The dangerous rock mass is often located in a steep or unstable position, and direct sampling poses a safety risk; the natural structural surface is easily disturbed during the sampling process, making it difficult to maintain its original state.

[0005] (2) Difficulty in selecting representative surfaces: The structural surfaces are highly heterogeneous, making it difficult to select representative structural surfaces for testing.

[0006] (3) Difficulty in determining the attitude: Under the constraints of on-site conditions, it is difficult to accurately determine the attitude of structural surfaces, such as strike, dip, and dip angle.

[0007] (4) Significant sampling disturbance: In the traditional drilling sampling process, the dynamic disturbance of the drill bit vibration and the weakening effect of the drilling rig water supply will cause the naturally closed structural surface to open locally, which will significantly affect the accuracy of the indoor direct shear test results.

[0008] (5) Lack of correction methods: Existing technologies lack effective methods for quantitatively assessing and correcting the intensity loss caused by sampling disturbances.

[0009] Therefore, there is an urgent need for an experimental testing method that can accurately obtain the shear strength of the natural structural surface of dangerous rocks. Summary of the Invention

[0010] This invention proposes a method for preparing natural structural surfaces of dangerous rocks and testing their shear strength, solving the technical problems of difficult sampling of natural structural surfaces, significant sampling disturbance, and large deviations in indoor direct shear test results.

[0011] To address the aforementioned technical problems, this invention provides a method for preparing natural structural surfaces of unstable rocks and testing their shear strength, comprising the following steps: Step S1: Determine the saturated compressive strength of the rock through point load tests. To determine whether it is hard rock; Step S2: Construct a three-dimensional coordinate system using the borehole television method, and calculate the angle between the normal vector of the structural surface and the normal vector of the rock wall surface. ,according to Angled core drilling; Step S3: Calculate the structural planes and the complete area of ​​the rock sample using 3D reconstruction via CT scanning. and opening area The correction coefficient is obtained. ; Step S4: Cast the rock sample into an Acm×Acm×Acm specimen, with the contact area of ​​the structural surface maintained at Bcm×Bcm, and then cure it, where A is not less than 15 and B is not less than 10. Step S5: Conduct direct shear tests to obtain cohesion. and internal friction angle ; Step S6: Obtain the shear strength parameters of the natural structural surface according to the correction coefficient. , .

[0012] Preferably, the saturated compressive strength of the rock in step S1 The calculation formula is: =22.82 ,in The load strength index of the rock sample point; when A pressure greater than 30 MPa indicates hard rock.

[0013] Preferably, in step S2, the structural surface normal vector The calculation method includes: taking three points A, B, and C on the intersection line of the structural plane and the borehole, and calculating the direction vector. and Structural surface normal vector ; Rock wall normal vector The calculation method includes: taking three points E, F, and G on the line where the rock face intersects the borehole, and calculating the direction vector. , Rock wall normal vector Angle between two planes The calculation formula is: .

[0014] Preferably, the length of the core sample containing the structural surface obtained in step S2 is not less than B cm.

[0015] Preferably, in step S4, 525 grade cement, standard sand and water are used to pour the rock sample according to a preset ratio.

[0016] Preferably, the total area of ​​the structural surfaces in step S3 ,in To combine the areas of the complete part, The area of ​​the opened portion.

[0017] Preferably, in step S3, CT non-destructive identification technology is used to perform CT scanning on the rock sample, and 3D reconstruction software is used to perform 3D reconstruction on images of different CT layers to obtain the actual bonding of the structural surfaces.

[0018] Compared with the prior art, the beneficial effects of the present invention include at least the following: (1) Scientifically determine the sampling location: The rock type is determined by point load test, avoiding the use of inappropriate sampling methods in soft rocks, thus improving the sampling success rate and safety.

[0019] (2) Accurate calculation of drilling angle: By using the drilling television method combined with the three-dimensional coordinate system calculation, the angle between the structural surface and the rock wall can be accurately determined, guiding the drilling rig to drill and core at the optimal angle, minimizing damage to the structural surface.

[0020] (3) Quantitative assessment of sampling disturbance: Using CT non-destructive identification technology to reconstruct the sample in three dimensions, it is possible to intuitively and quantitatively identify the opening of the structural surface during the sampling process, providing a scientific basis for strength parameter correction.

[0021] (4) Method for establishing correction coefficients: A correction coefficient for shear strength based on structural integrity was proposed for the first time. This coefficient has a clear physical meaning, is easy to calculate, and can effectively correct the intensity loss caused by sampling disturbance.

[0022] (5) Improve the accuracy of test results: The shear strength parameters obtained by correction are closer to the true state of the natural structural surface. Compared with traditional methods, the accuracy and reliability of test results are significantly improved, providing more reliable parameter basis for the evaluation of the stability of dangerous rocks. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the method flow according to an embodiment of the present invention; Figure 2This is a columnar diagram of the drilling of the structural surface portion according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the borehole wall with structural surface and a three-dimensional rectangular coordinate system according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the on-site drilling operation according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the sample casting and curing process according to an embodiment of the present invention; Figure 6 This is a schematic diagram of a rock sample CT scan according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the three-dimensional reconstruction of a rock sample according to an embodiment of the present invention. Detailed Implementation

[0024] The technical solutions of 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 protection scope of the present invention.

[0025] This invention provides a method for preparing natural structural planes of unstable rocks and testing their shear strength, comprising the following steps: Step S1: Determine the saturated compressive strength of the rock through point load tests. To determine whether it is hard rock.

[0026] Specifically, based on the natural exposure area of ​​the structural surface, random lithological sampling was conducted by drilling into the upper and lower hanging walls of the structural surface to obtain preliminary rock samples. Field point load tests were then carried out to obtain the point load strength index of the rock samples. According to the engineering rock mass classification standard, the rock saturated compressive strength is converted to the standard. =22.82 .

[0027] The rocks are classified according to the engineering rock mass classification standard as shown in Table 1.

[0028] Table 1: Correspondence with rock hardness in Rocks with a pressure greater than 30 MPa are classified as hard rocks. Rocks with a pressure ≤30MPa are classified as soft rocks. If the rock is classified as hard rock, this method can be used for sampling; if the rock is soft, water drilling is not suitable, and it is necessary to select a new sampling location on the structural surface or use other sampling methods.

[0029] Step S2: Construct a three-dimensional coordinate system using the borehole television method, and calculate the angle between the normal vector of the structural surface and the normal vector of the rock wall surface. ,according to Angled drilling for core extraction.

[0030] Specifically, firstly, boreholes are drilled in the area where structural surface sampling is to be carried out. Using borehole television, a camera probe is inserted along the borehole to scan the inner wall of the borehole. The scan is then imaged using an imaging device, and a three-dimensional Cartesian coordinate system is constructed using the drill string cylinder as the surface. Three points A, B, and C are selected on the intersection line of the structural surface and the borehole, and two direction vectors are calculated. , The normal vector of the structure surface is obtained through the cross product of vectors. Similarly, take three points E, F, and G on the line where the borehole sampling rock wall intersects with the borehole, and calculate the two direction vectors. , The normal vector of the rock wall surface is obtained. Through the formula Calculate the angle between the two planes Then, a drilling rig is used for sampling, with the drilling direction perpendicular to the rock face. Drilling should be performed at an angle. The length of the core sample obtained by the drilling rig, including the structural surface, should not be less than 10cm, which is the minimum length requirement for a single complete sample.

[0031] Step S3: Calculate the structural planes and the complete area of ​​the rock sample using 3D reconstruction via CT scanning. and opening area The correction coefficient is obtained. .

[0032] Specifically, the structural surfaces are in a naturally closed state before sampling, and the total area of ​​the structural surfaces before sampling is denoted as . During sampling, the natural closed structural surfaces may partially open due to the dynamic disturbance caused by drill bit vibration and the weakening effect of water supply from the drilling rig. Therefore, CT non-destructive testing was used to perform CT scans on the rock samples to obtain CT images. Based on the images from different CT layers, 3D reconstruction software (such as VGStudio MAX) was used to reconstruct the rock samples in three dimensions to evaluate the actual bonding of the natural structural surfaces after experiencing sampling dynamic disturbance and water weakening. The area of ​​the still intact bonding portion was recorded as [the area of ​​the remaining intact bonding portion]. The area of ​​the opened portion is denoted as Satisfying the relation Based on the change in the total area, the shear strength correction factor is calculated. This correction factor reflects the degree of impact of sampling disturbance on the structural integrity. The larger the value, the more severe the disturbance, and the larger the correction coefficient λ.

[0033] Step S4: Cast the rock sample into an Acm×Acm×Acm specimen, with the contact area of ​​the structural surface maintained at Bcm×Bcm, and cure it. Where A is not less than 15 and B is not less than 10, in this embodiment, A=15 and B=10 are used for treatment.

[0034] Specifically, after the samples were transported back to the laboratory, they were cast into 15cm × 15cm × 15cm cubic specimens using 525 grade cement, standard sand, and water according to a pre-set mix ratio. During casting, the effective contact area of ​​the structural surfaces remained at 10cm × 10cm to ensure that the structural surfaces were centered within the specimen. After casting, the specimens were placed under standard curing conditions and cured for 28 days.

[0035] Step S5: Conduct direct shear tests to obtain cohesion. and internal friction angle .

[0036] Specifically, structural surface specimens obtained by sampling and processing in the same area will undergo direct shear tests according to the provisions of the "Code for Testing Rocks in Hydraulic and Hydropower Engineering SL / T 264-2020". Based on the test data, the shear strength parameters of the structural surface will be obtained: cohesion. and internal friction angle .

[0037] Step S6: Obtain the shear strength parameters of the natural structural surface based on the correction factor. , .

[0038] Specifically, based on the shear strength parameters of the natural structural plane obtained from the direct shear test... and and the correction coefficients calculated in step S4 This yields corrected values ​​for the shear strength parameters of natural structural surfaces, taking into account the strength loss from sampling operations. The method for correcting the internal friction angle is as follows: first calculate... Then calculate the corrected internal friction angle. The method for correcting cohesion is as follows: Through the above modifications, structural shear strength parameters closer to the natural state are obtained. and It can be directly used for the stability analysis and evaluation of dangerous rocks.

[0039] Example 1 This embodiment uses the sampling and testing of the natural structural surface of a dangerous rock mass in a certain area as an example to illustrate the implementation process and technical effects of the method of the present invention in detail.

[0040] Step 1: Determine the sampling location Based on the on-site geological survey, random lithological sampling was conducted by drilling at the hanging wall and footwall locations of the naturally exposed structural surface of the unstable rock mass. A 100mm diameter drill bit was used for sampling, and preliminary rock samples were obtained. The rock samples were identified as limestone.

[0041] A rock point load tester was used to conduct on-site point load tests. A total of 4 rock samples were tested, and the test results are shown in Table 2.

[0042] Table 2: Results of Rock Point Load Strength Test The formula for calculating the rock point load strength index is: ; ; Where F and m are correction coefficients. The uncorrected point load strength of the rock is taken as 0.45. According to the engineering rock mass classification standard, the formula for calculating the saturated compressive strength of rock is: =22.82 .

[0043] As shown in Table 2, the saturated compressive strength of the four rock samples calculated through point load tests... The pressure at these rock samples is between 30 and 60 MPa, classifying them as relatively hard rock within the hard rock category according to engineering rock mass classification standards. Therefore, the rock at this sampling point meets the conditions for water-cooled drilling, and this method can be used for further sampling.

[0044] Step 2: Determine the drilling angle and perform core drilling. At the determined sampling location, a 100mm diameter drill barrel was used for on-site drilling. After drilling was completed, a borehole television system was used to insert a camera probe into the borehole for a 360-degree scan. The borehole wall image was then transmitted to a ground imaging device via a signal acquisition system.

[0045] Based on the borehole wall images obtained from borehole television, such as Figure 2 As shown, the distribution of structural surfaces on the borehole wall can be clearly identified. A three-dimensional Cartesian coordinate system is established using the drill string cylinder as the surface, as follows: Figure 3 As shown. Three clearly identifiable points A, B, and C are selected on the intersection line of the structural plane and the borehole, and their three-dimensional coordinates are measured. Two direction vectors are obtained through coordinate calculation: , and structural surface normal vector Similarly, three points E, F, and G are selected on the line where the borehole sampling rock wall intersects with the borehole. Their three-dimensional coordinates are measured, and two direction vectors are calculated: , and .

[0046] Based on the two normal vectors and Through formula Calculate the angle between the two planes. In this embodiment, the angle between the normal vector of the structural plane and the normal vector of the rock wall plane is calculated. =27°.

[0047] Based on the calculation results, adjust the drilling rig angle so that the drilling direction forms a 27° angle with the rock wall surface for core drilling. Figure 4 As shown. Water drilling was used during the drilling process to control the drilling speed and minimize disturbance to the structural surface. The core sample containing the structural surface was 12cm in length, meeting the requirement of not less than 10cm.

[0048] Step 4: CT scan to identify the opening status of natural structural surface specimens After 28 days of curing, the specimens were removed from the standard curing room and subjected to CT scanning experiments using CT scanning equipment.

[0049] After the scan is completed, a series of CT tomographic images are obtained, such as Figure 6 As shown, CT images can clearly identify the location of structural planes, the distribution of pores within the structural planes, and the contact between the structural planes and the surrounding rock.

[0050] DICOM format images obtained from CT scans are imported into the 3D reconstruction software VGStudio MAX 3.0 for 3D reconstruction. Through operations such as thresholding, surface reconstruction, and pore extraction, 3D reconstructed images of the structural surfaces are obtained, such as... Figure 7 As shown in the image, the red areas represent the morphology of structural surfaces, while the blue areas represent the porosity distribution in the remaining locations of the rock.

[0051] The area measurement function of the software was used to quantitatively analyze the bonding of the structural surfaces. In this embodiment, the measurement results are shown in Table 3.

[0052] Table 3: Statistics of Disturbance Area Based on the measurement results, the total area of ​​the structural surface =100cm², where the area of ​​the combined complete part is... =93.332cm², area of ​​the opened portion =6.668cm², which satisfies .

[0053] Calculate the shear strength correction factor: =1+6.668 / 100=1.067.

[0054] This correction factor indicates that the sampling process caused approximately 6.7% of the structural surface area to open, necessitating a corresponding correction to the direct shear test results.

[0055] Step 4: Indoor sample preparation and standard sample production The retrieved rock samples were processed indoors. First, the rock samples were polished to make the end faces of the upper and lower plates of the structural surface flat. Then, a mixture of 525 grade ordinary Portland cement, standard sand, and water was prepared, and the rock samples were poured into a pre-prepared 15cm×15cm×15cm cube mold.

[0056] During casting, carefully control the position of the structural surface within the specimen, ensuring an effective contact area of ​​10cm x 10cm and that the structural surface is centered within the specimen. After casting, place the specimen in a standard curing room for 28 days. Figure 5 As shown.

[0057] Step 5: Direct Shear Test of Structural Surface After the CT scan was completed, the specimen was placed in a direct shear test apparatus for structural surface direct shear tests. The test results are shown in Table 4.

[0058] Table 4: Results of direct shear tests on structural surfaces The shear strength parameters of the structural surface are obtained: cohesion. =0.281MPa, internal friction angle =32.46°.

[0059] Step Six: Correction of Shear Parameters of Structural Surfaces Based on the correction coefficient obtained in step four =1.067 and the shear strength parameter obtained in step five =0.281MPa =32.46°, corrected as follows: (1) Correction for internal friction angle: First, calculate: =tan(32.46°)=0.636 Then calculate the corrected internal friction angle: =arctan(1.067×0.636)=arctan(0.679)=34.16° (2) Cohesion correction: =1.067 × 0.281 = 0.300 MPa Therefore, considering the impact of sampling disturbance, the shear strength parameters of the natural structural plane of the unstable rock mass are: cohesion c = 0.300 MPa, and internal friction angle φ = 34.16°. These parameters can be directly used for the stability analysis and evaluation of the unstable rock mass.

[0060] If the results of indoor direct shear tests are used directly for rockfall stability analysis without correction, the shear strength of the structural surfaces will be underestimated, resulting in a lower calculated safety factor and potentially unnecessary reinforcement investment. The method of this invention uses CT scans to quantitatively identify sampling disturbances and establish correction coefficients, effectively improving the accuracy of the test results.

[0061] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described; only preferred embodiments of the present invention are illustrated. The descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. As long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification.

[0062] It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of this invention. Therefore, the scope of protection of this invention should be determined by the appended claims.

Claims

1. A method of testing the preparation and shear strength of natural structural planes of dangerous rock, characterised in that: The method comprises the following steps: Step S1: Determine whether the rock is hard rock by point load test to determine whether it is hard rock; Step S2: Construct a three-dimensional coordinate system using the borehole television method, and calculate the angle between the normal vector of the structural surface and the normal vector of the rock wall surface. ,according to Angled core drilling; Step S3: Calculate the complete area of structural plane joint by CT scanning three-dimensional reconstruction and the opening area , get the correction coefficient ; Step S4: pouring the rock sample into an Acm x Acm x Acm test piece, keeping the contact area of the structural plane as Bcm x Bcm, and then curing, wherein A is not less than 15, and B is not less than 10; Step S5: Conducting direct shear test to obtain cohesion and internal friction angle ; Step S6: Correcting the natural structural plane shear strength parameter according to the correction coefficient, ​​ 2. A method of testing the preparation and shear strength of a dangerous rock natural structure plane according to claim 1, characterized in that: The formula for calculating the rock saturation compressive strength in step S1 is = 22.82 , wherein is the rock sample point load strength index; when is greater than 30 MPa, the rock is judged as hard rock.​ 3. A method of testing the preparation and shear strength of a dangerous rock natural structure plane according to claim 1, characterized in that: The normal vector of the structural plane in step S2 The calculation method includes: taking three points A, B, C on the intersection line of the structural plane and the borehole, calculating the direction vectors and , and the normal vector of the structural plane ; Rock wall surface normal vector The calculation method comprises the following steps: taking three points E, F and G on the intersection line of the rock wall surface and the borehole, calculating the direction vectors , , and the rock wall surface normal vector ; and the calculation formula of the included angle between two planes is .​ 4. The method of claim 1, wherein the method is characterized by: The length of the structural plane sample obtained by the core drilling machine in step S2 is not less than Bcm.

5. The method of claim 1, wherein the method is characterized by: In step S4, the rock sample is poured by using 525 labeled cement, standard sand and water in a preset ratio.

6. The method of claim 1, wherein the method is characterized by: Total area of structural plane in step S3 wherein is the area of the complete part, is the area of the opening part.

7. The method of claim 1, wherein: the method further comprises: determining a shear strength of the rock mass structure surface. In step S3, the rock sample is scanned by using the CT nondestructive identification technology, three-dimensional reconstruction software is used to perform three-dimensional reconstruction on different CT layer images, and the actual combination of the structural plane is obtained.

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