Measuring device and measuring method

Abstract

To efficiently measure stress of a ground and a rock base with a simple configuration.SOLUTION: A measurement device includes: a sensor unit 11A including a plurality of load cells 20; an insertion rod 16 for inserting the sensor unit 11A; and a stress calculation unit 30 for calculating main stress based on stress values detected by the plurality of load cells 20. The plurality of load cells 20 are arranged so as to detect stress values in arbitrary six directions different from each other. The stress calculation unit 30 calculates the main stress based on the stress values in the six directions detected by the plurality of load cells 20.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a measurement device and a measurement method, and in particular to a technique suitable for measuring the initial geological pressure (initial stress) of the ground or rock mass. [Background technology]

[0002] When constructing a cavity in rock mass during tunnel construction or the like, it is necessary to evaluate the impact of cavity formation on the surrounding rock mass. The impact of cavity formation is evaluated, for example, by measuring the displacement and initial stress of the rock mass. Known conventional techniques for measuring initial stress include the conical hole strain method and hydraulic fracturing method (see, for example, Patent Documents 1 and 2).

[0003] The conical hole bottom strain measurement method involves first drilling an insertion hole in the ground or rock mass to be measured using a drilling device, and then creating a conical hole at the bottom of the insertion hole using a conical bit. After the conical hole is created, the bottom is observed using a borehole camera. If no cracks or other defects are found, the bottom of the conical hole is polished and cleaned. After the conical hole is cleaned, a strain cell equipped with multiple strain gauges on its surface is fixed to the bottom of the conical hole with adhesive or other means. Once the adhesive has hardened, the rock mass around the strain cell is overcored to measure the released strain. After measuring the released strain, the core is retrieved, and the Young's modulus and Poisson's ratio are measured through a laboratory test (uniaxial cyclic loading test). Using the measurements from the laboratory test, the strain sensitivity coefficient is calculated based on numerical analysis such as the boundary element method or finite element method, assuming that the ground or rock mass to be measured is an isotropic, homogeneous, linear elastic body. Then, the initial stress is calculated from the strain sensitivity coefficient thus obtained and the measured value of the released strain by the least squares method.

[0004] On the other hand, hydraulic fracturing involves drilling a borehole in the ground or other structure to be measured using a drilling device. Once the borehole is drilled, a predetermined section of the borehole is plugged with a packer. A high-pressure pump is used to apply water pressure to the section plugged with the packer, causing a crack, and the initial stress is calculated based on the relationship between the water pressure and the crack. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-069937 [Patent Document 2] Japanese Patent Application Publication No. 10-220160 Summary of the Invention [Problem to be solved by the invention]

[0006] In the conical-hole strain measurement method, strain gauges are fixed to the bottom of the conical hole to measure the released strain. This means that the deformation of the ground or rock mass within the area where the strain gauges are fixed is measured. Therefore, the method is highly susceptible to the influence of local heterogeneity, such as minute cracks, and the presence or absence of cracks must be carefully confirmed by observing the bottom of the hole using a borehole camera. Furthermore, after overcoring, the core must be retrieved and subjected to laboratory testing to measure the Poisson's ratio and Young's modulus. This requires a large number of steps for measurement and laboratory testing, and there are issues such as the considerable effort, time, and labor required to determine the initial stress.

[0007] Furthermore, the conical hole strain measurement method assumes that the soil or rock mass being measured is an isotropic, homogeneous, linear elastic body, and calculates the initial stress through inverse analysis of the measured values. However, soil and rock mass contain many anisotropic and heterogeneous properties, such as uneven distribution of minerals and microcracks, as well as inelastic deformation behavior. For this reason, calculation methods based on inverse analysis, which assume an isotropic, homogeneous, linear elastic body, may not be able to fully guarantee the accuracy of initial stress calculations, depending on the measurement results of release strain, etc.

[0008] On the other hand, hydraulic fracturing can evaluate the initial stress component in the orthogonal plane of the borehole, but cannot evaluate the initial stress component in the borehole axis direction. This requires drilling multiple boreholes at different angles, which increases the cost and labor required for measurement. Another issue with hydraulic fracturing is that the entire equipment becomes large, requiring packers and high-pressure pumps.

[0009] The technology disclosed herein has been made in consideration of the above circumstances, and aims to provide a measurement device and a measurement method that can efficiently measure stress in the ground or rock mass with a simple configuration. [Means for solving the problem]

[0010] The measurement device of the present disclosure includes: a sensor unit including a plurality of load cells that are inserted into a hole drilled in the ground or bedrock and fixed to the surface of the hole; an insertion rod for inserting the sensor unit into the hole and fixing the plurality of load cells to the surface of the hole; a stress calculation unit that calculates a principal stress inside the ground or the rock mass based on stress values ​​detected by the plurality of load cells, The plurality of load cells are arranged to detect stress values ​​in any six directions different from one another, and the stress calculation unit calculates the principal stress based on the stress values ​​in the six directions detected by the plurality of load cells.

[0011] In another aspect of the measurement device of the present disclosure, The plurality of load cells Assuming that the three mutually orthogonal axes are the X-axis, Y-axis, and Z-axis, and the positive directions along the X-axis, Y-axis, and Z-axis are respectively the +X-axis direction, +Y-axis direction, and +Z-axis direction, and the negative directions along the X-axis, Y-axis, and Z-axis are respectively the -X-axis direction, -Y-axis direction, and -Z-axis direction, a first load cell for detecting a stress value in a +X-axis direction or a -X-axis direction perpendicular to the YZ plane; a second load cell for detecting a stress value in the +Y-axis direction or the -Y-axis direction perpendicular to the ZX plane; a third load cell for detecting a stress value in a +Z-axis direction or a −Z-axis direction perpendicular to the XY plane; a fourth load cell that detects a stress value in a direction extending from the intersection of the three axes on the XY plane at an angle of +45 degrees or -135 degrees with respect to the X axis; a fifth load cell that detects a stress value in a direction extending from the intersection of the three axes on the ZX plane at an angle of +45 degrees or -135 degrees with respect to the Z axis; It is preferable to have a sixth load cell that detects a stress value in a direction extending from the intersection of the three axes on the YZ plane at an angle of +45 degrees or −135 degrees with respect to the Y axis.

[0012] Another aspect of the measurement device of the present disclosure is Further provided is a hemispherical bit for drilling a hemispherical hole at the bottom of the hole, The sensor unit a hemispherical or spherical shell attached to the tip of the insertion rod and having a diameter smaller than that of the hemispherical bit; the plurality of load cells are arranged on the surface of the spherical shell so that the center of the spherical shell coincides with the intersection of the three axes; It is preferable that the plurality of load cells are fixed to the surface of the hemispherical hole by inserting the spherical shell into the hemispherical hole using the insertion rod.

[0013] In another aspect of the measurement device of the present disclosure, The sensor unit a main rod attached to the tip of the insertion rod; a first support rod whose tip is an intersection of the three axes, extending from the intersection in the +X-axis direction or the −X-axis direction, and having the first load cell attached to its tip; a second support rod extending from the intersection in the +Y-axis direction or the −Y-axis direction and having the second load cell attached to its tip; a third support rod extending from the intersection in the +Z-axis direction or the −Z-axis direction and having the third load cell attached to its tip; a fourth support rod extending from the intersection point on the XY plane at an angle of +45 degrees or -135 degrees with respect to the X axis, and having the fourth load cell attached to its tip; a fifth support rod extending from the intersection point on the ZX plane at an angle of +45 degrees or -135 degrees with respect to the Z axis, and having the fifth load cell attached to its tip; It is preferable to have a sixth support rod that extends from the intersection point on the YZ plane at an angle of +45 degrees or -135 degrees with respect to the Y axis, and has the sixth load cell attached to its tip.

[0014] In another aspect of the measurement device of the present disclosure, The plurality of load cells are preferably compression-tension type load cells.

[0015] Another aspect of the measurement device of the present disclosure is Further provided is a cylindrical bit for over-coring the ground or rock around the sensor unit, It is preferable that the calculation unit calculates the initial stress generated in the ground or the bedrock based on the stress values ​​detected by the plurality of load cells when overcoring is performed with the cylindrical bit.

[0016] The measurement method of the present disclosure is a method for measuring an initial stress using the measurement device including the cylindrical bit, Drilling the hole in the ground or rock using a drilling device; inserting the sensor unit into the hole using the insertion rod to fix the plurality of load cells to a surface of the hole; a step of overcoring the ground or rock around the hole in which the sensor unit is installed using the cylindrical bit; and a step of calculating the initial stress generated in the ground or the bedrock based on the stress values ​​detected by the plurality of load cells when the overcoring is performed. [Effects of the Invention]

[0017] According to the measurement device and measurement method of the present disclosure, stress in the ground or rock mass can be measured efficiently with a simple configuration. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic overall configuration diagram showing a measurement device according to a first embodiment. [Figure 2] FIG. 2 is a schematic perspective view showing a sensor unit of the measurement device according to the first embodiment. [Figure 3] FIG. 2 is a schematic cross-sectional view showing a part of a sensor unit according to the first embodiment. [Figure 4] 3A and 3B are schematic diagrams illustrating an example of installation of a sensor unit according to the first embodiment. [Figure 5] 10A and 10B are schematic diagrams illustrating another example of installation of the sensor unit according to the first embodiment. [Figure 6] FIG. 1 is a diagram showing a schematic example of a site where initial stress is measured. [Figure 7] FIG. 3 is a flowchart illustrating a method for measuring an initial stress using the measurement device of the first embodiment. [Figure 8] 8 is a schematic diagram for explaining some of the steps in FIG. 7. FIG. [Figure 9] FIG. 4 is a schematic diagram illustrating another example of the measurement method according to the first embodiment. [Figure 10] FIG. 10 is a schematic overall configuration diagram showing a measurement device according to a second embodiment. [Figure 11] FIG. 10 is a schematic perspective view showing a sensor unit of a measurement device according to a second embodiment. [Figure 12] FIG. 10 is a flowchart illustrating a method for measuring an initial stress using the measurement device of the second embodiment. [Figure 13] 13 is a schematic diagram for explaining some of the steps in FIG. 12. FIG. [Figure 14] FIG. 2 is a schematic perspective view showing a guide mechanism provided on an insertion rod of the measurement device according to the present embodiment. [Figure 15] 10A and 10B are diagrams illustrating stress on an arbitrary surface in a calculation procedure of a measurement device according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, a measurement device and a measurement method according to the present embodiment will be described with reference to the accompanying drawings. The same components are denoted by the same reference numerals, and their names and functions are also the same. Therefore, detailed description thereof will not be repeated.

[0020] [First embodiment] FIG. 1 is a schematic overall configuration diagram showing a measurement apparatus 10A according to the first embodiment.

[0021] (Overall composition) As shown in FIG. 1, the measurement device 10A of the first embodiment includes a sensor unit 11A, an insertion rod 16 for inserting the sensor unit 11A into an insertion hole drilled in the ground or bedrock, a hemispherical bit 17 for drilling a hemispherical hole in which the sensor unit 11A is to be installed at the bottom of the insertion hole, a cylindrical bit 18 for overcoring the ground or bedrock around the sensor unit 11A, and a calculation processing device 30 for calculating the principal stresses (σ1, σ2, σ3) in three directions based on the electrical signals input from the sensor unit 11A.

[0022] The sensor unit 11A includes a plurality of load cells, specifically six load cells 20 X ,20 Y ,20 Z ,20 XY45 ,20 ZX45 ,20 YZ45 These load cells 20 are arranged in a predetermined direction. X ,20 Y ,20 Z ,20 XY45 ,20 ZX45 ,20 YZ45 The details of the arrangement position of the load cell 20 will be described later with reference to FIG. X ,20 Y ,20 Z ,20 XY45 ,20 ZX45 ,20 YZ45These may be collectively referred to simply as "load cell 20."

[0023] The load cell 20 is a small tension-compression load cell that is smaller than the diameter of the insertion hole (e.g., 86 to 100 mm). The load cell 20 is configured by housing a flexure element and a strain gauge attached to the flexure element, both of which are not shown, inside a case 21. A load button 22 connected to the flexure element is disposed on the outside of the case 21, and the flexure element is deformed when force is applied to this load button 22. The load cell 20 outputs an electrical signal proportional to the amount of deformation of the flexure element to the calculation processing device 30 as the electrical resistance of the strain gauge changes in proportion to the amount of deformation of the flexure element.

[0024] The insertion rod 16 is formed in a long, approximately cylindrical shape, and the sensor unit 11A is detachably attached to its tip. This insertion rod 16 is used in the preparation step described below when inserting the sensor unit 11A into the insertion hole and fixing it to the surface of the hemispherical hole. The axial length of the insertion rod 16 is not particularly limited, but it is preferably formed to a length that allows the sensor unit 11A to reach the hemispherical hole. The material from which the insertion rod 16 is formed is also not particularly limited, but it can be formed from, for example, stainless steel, aluminum alloy, etc.

[0025] In this embodiment, the insertion rod 16 is provided with a guide mechanism 16A (see FIG. 14) that aligns the axis of the insertion rod 16 substantially with the axis of the insertion hole, thereby facilitating installation of the sensor unit 11A in the hemispherical hole. Preferably, two guide mechanisms 16A are provided at a predetermined interval in the axial direction of the insertion rod 16. The guide mechanism 16A includes a plurality of elastically deformable plate members 16B (three in the illustrated example), and each plate member 16B is attached to the insertion rod 16 so as to be curved and convex in a substantially arc shape radially outward. When viewed in the axial direction of the insertion rod 16, the plate members 16B are preferably arranged at equal circumferential intervals (120-degree intervals) on the outer circumferential surface of the insertion rod 16.

[0026] When the insertion rod 16 is inserted into the insertion hole, each plate member 16B comes into contact with the wall surface of the insertion hole and elastically deforms, applying a radially inward reaction force to the insertion rod 16, making it easy to hold the insertion rod 16 approximately in the center of the insertion hole. Furthermore, because each plate member 16B supports the insertion rod 16 while sliding against the wall surface of the insertion hole, the insertion rod 16 can be easily inserted into the insertion hole without being hindered by unevenness on the wall surface or pebbles remaining in the insertion hole.

[0027] The number of plate members 16B is not limited to three as in the illustrated example, and may be, for example, four. In the case of four, the plate members 16B may be arranged at 90-degree intervals in the circumferential direction. Furthermore, the number of guide mechanisms 16A is not limited to two as in the illustrated example, and may be three or more depending on the length of the insertion rod 16.

[0028] The hemispherical bit 17 is attached to the drilling rod of the drilling device. The outer diameter of the hemispherical bit 17 is smaller than the inner diameter of the cylindrical bit 18. The inner diameter of the hemispherical bit 17 is larger than the outer diameter of the sensor unit 11A. This hemispherical bit 17 is used when drilling a hemispherical hole in the bottom of the insertion hole in the preparation step described below.

[0029] The cylindrical bit 18 is attached to the drilling rod of a drilling device such as a drill jumbo or a boring machine. The outer diameter of the cylindrical bit 18 is smaller than the inner diameter of the insertion hole to be drilled by the drilling device so that it can be easily inserted into the insertion hole. The inner diameter of the cylindrical bit 18 is larger than the outer diameter of the sensor unit 11A so that it does not interfere with the sensor unit 11A when overcoring. This cylindrical bit 18 is used when overcoring the rock or ground around the sensor unit 11A in the measurement process described below.

[0030] The arithmetic processing device 30 is equipped with a processing unit such as a CPU, a storage unit such as a RAM or ROM, an input / output interface, an auxiliary storage device, etc., and is configured as an information processing device such as a personal computer or a server. The arithmetic processing device 30 functions as a device equipped with a six-direction stress value acquisition unit 31 and a principal stress calculation unit 32, as a result of the CPU executing a program stored in the ROM. In addition, a display unit 40 such as a display, and an input unit 50 such as a keyboard and a mouse are connected to the arithmetic processing device 30.

[0031] The six-direction stress value acquisition unit 31 acquires stress values ​​in six directions occurring inside the ground or rock mass based on the electrical signals input from each of the six load cells 20. Here, assuming that the three mutually orthogonal axes are the X-axis, Y-axis, and Z-axis, the six directions are the X-axis direction, the Y-axis direction, the Z-axis direction, the +XY45 degree direction extending on the XY plane at an angle of +45 degrees from the center O, the +ZX45 degree direction extending on the ZX plane at an angle of +45 degrees from the center O, and the +YZ45 degree direction extending on the YZ plane at an angle of +45 degrees from the center O. Hereinafter, the stress values ​​in these six directions will be referred to as σ X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 The six-direction stress values ​​σ acquired by the six-direction stress value acquisition unit 31 are called X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 is transmitted to the principal stress calculation unit 32.

[0032] The principal stress calculation unit 32 calculates the stress values ​​σ in six directions. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 Based on this, the principal stresses σ1, σ2, and σ3 generated inside the ground or rock mass are calculated. Here, in the three-dimensional stress state, the independent stress components are the normal stress σ X ,σ Y ,σ ZThe invariants J1, J2, and J3 are the stress invariants. The invariants J1, J2, and J3 are expressed as invariant formulas (J1=σ X +σ Y +σ Z ,J2=-(σ Y σ Z +σ Z σ X +σ X σ Y )+τ YZ 2 +τ ZX 2 +τ XY 2 ,J3=σ X σ Y σ Z -σ X τ YZ 2 -σ Y τ ZX 2 -σ Z τ XY 2 +2τ YZ τ ZX τ XY ) and the shear stress τ in the invariant equation XY ,τ ZX ,τ YZ By using Mohr's stress circle in the XY, ZX and YZ planes, the stress values ​​in six directions σ X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 Using the formula (τ XY =1 / 2 (2σ XY45 -(σ X +σ Y )),τ ZX =1 / 2 (2σ ZX45 -(σ X +σ Z )),τ YZ =1 / 2 (2σ YZ45 -(σ Y +σ Z ))) respectively.

[0033] The principal stress calculation unit 32 first calculates the stress values ​​σ in six directions transmitted from the six-direction stress value acquisition unit 31. X,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 Based on the shear stress τ XY ,τ ZX ,τ YZ , and the calculated shear stress τ XY ,τ ZX ,τ YZ and normal stress value σ X ,σ Y ,σ Z The invariants J1, J2, and J3 are calculated by substituting the above invariant formulas. Then, the equation (σ 3 -J1σ 2 The principal stresses σ1, σ2, and σ3 in the three directions are calculated by solving the equation (-J2σ-J3=0). The principal stresses σ1, σ2, and σ3 calculated by the principal stress calculation unit 32 are displayed on the display unit 40 in response to the operation of the input unit 50, or are output as a report from a printer (not shown) or the like.

[0034] (sensor part) FIG. 2 is a schematic perspective view showing a sensor unit 11A of the measurement device according to the first embodiment.

[0035] As shown in Fig. 2, the sensor unit 11A of the first embodiment has a substantially hemispherical spherical shell 12 to which a plurality of load cells 20 are attached. The spherical shell 12 has a sphere center O connected to the tip of an insertion rod 16. The spherical shell 12 has a spherical diameter smaller than that of the hemispherical bit 17 (see Fig. 1). The shape of the spherical shell 12 is not limited to a hemispherical shape, and it can also be formed into a spherical shape.

[0036] Multiple load cells 20 are arranged on the spherical surface of the spherical shell 12 with their load buttons 22 protruding outward from the spherical surface. When the spherical shell 12 is inserted into a hemispherical hole drilled with a hemispherical bit 17 (see Figure 1), the load buttons 22 of each load cell 20 come into contact with the inner surface of the hemispherical hole.

[0037] In this embodiment, each load cell 20 is arranged in the spherical shell 12 so that the intersection of the three axes (X-axis, Y-axis, and Z-axis) coincides with the spherical center O of the spherical shell 12 and the Z-axis coincides with the axial direction of the insertion rod 16.

[0038] Specifically, the third load cell 20 Z The first load cell 20 is provided on the spherical surface of the spherical shell 12 at a portion where the extension line (Z axis) of the axis of the insertion rod 16 intersects with the spherical shell 12. X The second load cell 20 is provided at a portion of the spherical surface of the spherical shell 12 where the X axis intersects with the spherical shell 12. Y is provided at a portion of the spherical surface of the spherical shell 12 where the Y axis intersects with the spherical shell 12.

[0039] 4th load cell XY45 The first load cell 20 is located on the spherical surface of the spherical shell 12. X and the second load cell 20 Y The fifth load cell 20 is provided at the intermediate portion between the XY plane and the spherical shell 12, i.e., at the portion where the +XY45 line, which extends at an angle of +45 degrees with respect to the X axis, intersects with the spherical shell 12. ZX45 The first load cell 20 is located on the spherical surface of the spherical shell 12. X and the third load cell 20 Z The sixth load cell is provided at the intermediate portion between the ZX plane and the spherical shell 12, i.e., at the portion where the +ZX45 line, which extends at an angle of +45 degrees with respect to the Z axis, intersects with the spherical shell 12. YZ45 The second load cell 20 is located on the spherical surface of the spherical shell 12. Y and the third load cell 20 Z That is, the +YZ45 line extending on the YZ plane at an angle of +45 degrees with respect to the Y axis intersects with the spherical shell 12.

[0040] In this way, out of the total six load cells 20, three load cells 20 X ,20 Y ,20 Z are arranged in three mutually perpendicular axes, and the remaining three load cells 20 XY45 ,20 ZX45 ,20 YZ45By placing the axes X, Y, and Z at 45 degrees, the stress values ​​σ acting in six directions inside the ground or rock mass can be calculated. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 can be detected effectively.

[0041] Each load cell 20 measures a stress value σ acting in the vertical direction on the pressure-receiving surface of the load button 22. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 can be detected as a load acting on the pressure-receiving surface, it is possible to effectively reduce the influence of local heterogeneity such as minute cracks in the ground or rock. In other words, even if minute cracks exist in the ground or rock around the hemispherical hole where the sensor unit 11A is installed, it is possible to calculate the three-directional principal stresses σ1, σ2, and σ3 without being affected by these.

[0042] Here, each load cell 20 may be attached to the spherical shell 12 by, for example, drilling a through hole 12B in the spherical shell 12 and providing a circular groove 24 extending in the circumferential direction on the side of the case 21 of the load sensor 20, as shown in Figure 3, and engaging the periphery of the through hole 12B with the groove 24. Alternatively, each load cell 20 may be fixed to the spherical shell 12 with bolts and nuts or the like.

[0043] Furthermore, as shown in Fig. 4, when the load button 22 of each load cell 20 is fixed with adhesive or the like to the surface of the hemispherical hole h2 drilled at the bottom of the insertion hole h, a contact jig 25 may be interposed between the load button 22 and the surface of the hemispherical hole h2. The jig 25 is preferably made of a high-strength material so that stress from the ground or bedrock is transmitted to the load button 22 without attenuation. Note that the reference numeral 27 in Fig. 4 denotes a spacer for obtaining a reaction force.

[0044] The hole in which the sensor unit 11A is installed is not limited to the hemispherical hole h2, but may be a small diameter hole h3 with a substantially flat bottom as shown in FIG. 5. In this case, three load cells 20 X ,20 Y The load cell 20 easily contacts the side wall surface of the small diameter hole h3. Z The load cell 20 easily contacts the bottom surface of the small diameter hole h3. ZX45 ,20 YZ45 The jig 26 is placed in a position facing the side wall surface of the small diameter hole h3 with a gap between them. To fill these gaps, a jig 26 with a substantially trapezoidal cross section can be used instead of the jig 25. The jig 26 is placed so that the load cell 20 is positioned at the pair of interior angles θ1 and θ2 on the lower base side. ZX45 ,20 YZ45 The inner angle θ1 on the side of the small diameter hole h3 should be 90 degrees, and the inner angle θ2 on the side of the small diameter hole h3 should be 45 degrees.

[0045] Although not shown in detail, each load cell 20 can be arranged in the opposite direction except for the -Z axis direction by using a roughly spherical shell depending on the purpose of measurement, etc. X Alternatively, the second load cell 20 may be disposed in the −X-axis direction. Y The fourth load cell may be placed in the -Y-axis direction. XY45 The fifth load cell may be arranged in a direction extending in the XY plane at an angle of -135 degrees relative to the X axis. ZX45 The sixth load cell may be arranged in a direction extending such that the ZX plane is at an angle of -135 degrees relative to the Z axis. YZ45 may be oriented such that the YZ plane extends at an angle of −135 degrees relative to the Y axis.

[0046] (Measurement method) Next, a method for measuring an initial stress using the measurement device 10A of the first embodiment will be described.

[0047] FIG. 6 is a schematic diagram showing a site where initial stress is measured. FIG. 6(A) shows an example of measuring the initial stress of the surrounding ground during mountain tunnel construction. In mountain tunnel construction, a drilling device such as a jumbo drill (not shown) may be used to drill multiple insertion holes h forward from the tunnel face 70, or multiple insertion holes h may be drilled radially outward from the tunnel side wall 71. The hemispherical bit 17 or cylindrical bit 18 (both see FIG. 1) may be attached to the drilling rod (not shown) of the drilling device.

[0048] Figure 6(B) shows an example of measuring the initial stress of the ground by drilling a vertical insertion hole h in the ground. A boring machine 80 is installed on the ground surface as a drilling device. The boring machine 80 is equipped with a drilling rod 82 with a bit 81 attached to its tip, a drive unit 83 including a cylinder for raising and lowering the drilling rod 82 and a power source for rotating the drilling rod 82, and a swivel 84 attached to the upper end of the drilling rod 82. A hose 85 is connected to the swivel 84, and a pump P for pumping drilling water and a tank T for storing the drilling water are connected to the hose 85. A hemispherical bit 17 or a cylindrical bit 18 (both see Figure 1) can be attached to the drilling rod 82 of the boring machine 80.

[0049] Fig. 7 is a flow diagram illustrating a method for measuring initial stress using the measurement device 10A of the first embodiment, and Fig. 8 is a schematic diagram illustrating some of the steps in Fig. 7. The measurement method described below has the same procedure for either the site shown in Fig. 6(A) or Fig. 6(B).

[0050] In step S100, an insertion hole h is drilled in the ground or bedrock using a drilling device (see FIG. 8(A)). After drilling the insertion hole h, in step S110, a hemispherical bit 17 is attached to the drilling rod of the drilling device, and a hemispherical hole h2 is drilled at the bottom of the insertion hole h (see FIG. 8(B)). After drilling the hemispherical hole h2, the surface of the hemispherical hole h2 is preferably polished or cleaned. Note that surface polishing may be omitted depending on the surface condition of the hemispherical hole h2. Furthermore, in this embodiment, in which a load cell 20 is used for the sensor unit 11A, the method is less susceptible to the influence of local inhomogeneities such as minute cracks in the ground or bedrock, and therefore the bottom-hole observation required in the conical bottom-hole strain method can be omitted.

[0051] In step S120, the sensor unit 11A is attached to the insertion rod 16 and inserted into the insertion hole h, and the sensor unit 11A is installed in the hemispherical hole h2 (see FIG. 8(C)). At this time, the load button 22 of each load cell 20 is preferably fixed to the surface of the hemispherical hole h2 with an adhesive or the like using a jig 25. The above steps S100 to S120 constitute the preparation process.

[0052] Once the adhesive has hardened, the process proceeds to the measurement step. In step S130, a cylindrical bit 18 is attached to the drilling rod of the drilling device, and overcoring is performed to drill a hole in the rock around the sensor unit 11A (see FIG. 8(D)).

[0053] After overcoring, in step S140, the stress values ​​σ in six directions detected by each load cell 20 during overcoring are calculated. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 In this embodiment, the principal stresses σ1, σ2, and σ3 are calculated based on the stress values ​​σ in six directions detected by the six load cells 20, and the measurement is completed. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45Since the calculation is based on the above, there is no need to calculate Poisson's ratio or Young's modulus. This means that laboratory testing using core recovery, which is required for the cone-hole strain method, can be omitted.

[0054] In the above-described measurement method, an example has been described in which a hemispherical hole h2 is drilled in the bottom of the insertion hole h in step S110, but in step S110, a small-diameter hole h3 having a diameter smaller than the insertion hole h may be drilled from the bottom of the insertion hole h, and the sensor unit 11A may be installed in the small-diameter hole h3, as shown in Fig. 5. In this case, overcoring can be performed efficiently by performing the step portion S between the insertion hole h and the small-diameter hole h3, as shown by the dashed line in Fig. 5.

[0055] 9(A), a small-diameter hole h3 having a smaller diameter than the insertion hole h may be drilled at the bottom of the insertion hole h, and then a hemispherical hole h2 for installing the sensor unit 11A may be drilled at the bottom of the small-diameter hole h3 as shown in FIG. 9(B). In this case, too, overcoring may be performed at the stepped portion S between the insertion hole h and the small-diameter hole h3 as shown by the dashed line in FIG. 9(B).

[0056] According to the first embodiment described above in detail, the sensor unit 11A includes a hemispherical shell 12, and the shell 12 includes six load cells 20 for measuring stress values ​​σ in six directions. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 By installing such a sensor unit 11A in the hemispherical hole h2 and over-coring the periphery of the sensor unit 11A, the stress values ​​σ in six directions directly detected by each load cell 20 can be detected. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 The initial stresses σ1, σ2, and σ3 can be calculated easily and with high accuracy based on the above.

[0057] In other words, compared to the cone-hole bottom strain method, which assumes the ground or rock mass is an isotropic, homogeneous, linear elastic body and calculates initial stresses from inverse analysis of measured values, this method can reliably ensure the accuracy of calculating initial stresses σ1, σ2, and σ3. It also reduces the labor required for indoor testing by core recovery, making it possible to significantly reduce the effort and time required to determine initial stresses σ1, σ2, and σ3.

[0058] Further, each load cell 20 of the sensor unit 11A detects a stress value σ acting in the vertical direction on the pressure receiving surface of the load button 22. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 can be detected as a load acting on the pressure-receiving surface, the initial stresses σ1, σ2, and σ3 can be calculated without being significantly affected by local inhomogeneities such as minute cracks in the ground or rock mass.

[0059] In other words, it is possible to omit the bottom-hole observation using a borehole camera, which is required in the conical bottom-hole strain method, and it is also possible to significantly reduce the time and cost required to determine the initial stresses σ1, σ2, and σ3.

[0060] In addition, the stress value σ in six directions X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 By detecting this, it is possible to evaluate not only the stress component perpendicular to the hole axis, but also the stress component in the hole axis direction. In other words, there is no need to drill multiple boreholes at different angles as in hydraulic fracturing, which significantly reduces the cost and labor required for measurement compared to hydraulic fracturing. In addition, since there is no need for packers or high-pressure pumps, it is possible to simplify the entire equipment compared to hydraulic fracturing.

[0061] [Second embodiment] Next, a measurement device 10B according to a second embodiment and a method for measuring an initial stress using the measurement device 10B will be described with reference to FIGS.

[0062] As shown in Figure 10, the measuring device 10B of the second embodiment includes a sensor unit 11B, an insertion rod 16 for inserting the sensor unit 11B into the insertion hole, a cylindrical bit 18 for overcoring the ground or bedrock around the sensor unit 11B, a small-diameter bit 19 for drilling a small-diameter hole, and a processing device 30 that calculates the principal stresses in three directions (σ1, σ2, σ3) based on the electrical signals input from the sensor unit 11B.

[0063] The same components as those in the first embodiment are denoted by the same reference numerals, and their functions are also the same. Therefore, detailed descriptions of the insertion rod 16, the cylindrical bit 18, and the arithmetic processing unit 30 will be omitted.

[0064] The small diameter bit 19 is formed in a cylindrical shape with a diameter smaller than the diameter of the insertion hole to be drilled by the drilling bit of the drilling device and slightly larger than the outer diameter of the sensor part 11 B. This small diameter bit 19 is used when drilling a small diameter hole smaller than the insertion hole from the bottom of the insertion hole in the preparation step described below.

[0065] (sensor part) FIG. 11 is a schematic perspective view showing a sensor unit 11B of a measurement device according to the second embodiment.

[0066] As shown in FIG. 11, a sensor unit 11B of the second embodiment includes a main rod 13 attached to the tip of an insertion rod 16, and a plurality of support rods 14A to 14F extending radially from the main rod 13.

[0067] The main rod 13 is attached to the tip of the insertion rod 16 and extends in the axial direction of the insertion rod 16. In this embodiment, among the multiple support rods 14A to 14F, the third support rod 14C extends from the tip O of the main rod 13 in the axial direction (Z-axis direction) of the main rod 13, and has a third load cell 20 attached to its tip. ZAlthough the main rod 13 and the third support rod 14C are formed as separate bodies, they may be formed as a single rod.

[0068] The first support rod 14A, the second support rod 14B, and the third support rod 14C are perpendicular to one another with the tip O of the main rod 13 as the intersection point. The first support rod 14A extends in the X-axis direction from the tip O of the main rod 13, and the second support rod 14B extends in the Y-axis direction from the tip O of the main rod 13. A first load cell 20 is attached to the tip of the first support rod 14A. X is attached to the tip of the second support rod 14B, and a second load cell 20 Y is attached.

[0069] The fourth support rod 14D extends in the +XY 45 degree direction from the tip O of the main rod 13 on the XY plane at an angle of +45 degrees with respect to the X axis. XY45 The fourth support rod 14D may also be provided in a direction extending in the XY plane at an angle of −135 degrees with respect to the X axis.

[0070] The fifth support rod 14E extends in the -ZX45 degree direction from the tip O of the main rod 13 on the ZX plane at an angle of -135 degrees with respect to the Z axis. A fifth load cell is mounted on the tip of the fifth support rod 14E. ZX45 The fifth support rod 14E may also be provided so that the ZX plane extends at an angle of +45 degrees with respect to the Z axis.

[0071] The sixth support rod 14F extends in the -ZY 45 degree direction from the tip O of the main rod 13 on the YZ plane at an angle of -135 degrees with respect to the Y axis. YZ45 The sixth support rod 14F may also be provided so that the YZ plane extends at an angle of +45 degrees with respect to the Y axis.

[0072] In this way, out of the total six load sensors 20, three load cells 20 X ,20 Y ,20 Z are arranged in three mutually perpendicular axes, and the remaining three load cells 20 XY45 ,20 ZX45 ,20 YZ45 By placing the axes on the XYZ plane at +45 / -135 degrees to each axis X, Y, Z, the stress σ acting in six directions inside the ground or rock mass can be calculated. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 As in the first embodiment, each load cell 20 detects a stress value σ acting in the vertical direction on the pressure-receiving surface of the load button 22. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 This allows the load acting on the pressure-receiving surface to be detected, which makes it possible to effectively reduce the effects of local heterogeneity such as minute cracks in the ground or rock mass.

[0073] (Measurement method) FIG. 12 is a flow diagram illustrating a method for measuring an initial stress using the measurement device 10B of the second embodiment, and FIG. 13 is a schematic diagram illustrating some of the steps in FIG.

[0074] In step S200, an insertion hole h is drilled in the ground or bedrock using a drilling device (see FIG. 13A). After drilling the insertion hole h, in step S110, a small-diameter bit 19 is attached to the drilling rod of the drilling device, and a small-diameter hole h3, smaller in diameter than the insertion hole h, is drilled from the bottom of the insertion hole h (see FIG. 13B). After drilling the small-diameter hole h3, the side and bottom walls of the small-diameter hole h3 are preferably polished or cleaned. Note that polishing the walls may be omitted depending on the condition of the walls of the small-diameter hole h3. Furthermore, in this embodiment, in which a load cell 20 is used for the sensor unit 11B, the bottom of the hole, which is required for the conical bottom-hole strain method, can be omitted because it is less susceptible to local inhomogeneities such as minute cracks in the ground or bedrock.

[0075] In step S220, the sensor unit 11B is attached to the insertion rod 16 and inserted into the insertion hole h, and the sensor unit 11B is installed in the small diameter hole h3 (see FIG. 13(C)). At this time, the load buttons 22 of each load cell 20 are preferably fixed to the wall surface of the small diameter hole h3 with adhesive or the like using jigs 25, 26. The above steps S200 to S220 constitute the preparation process.

[0076] Once the adhesive has hardened, the process moves to the measurement step. In step S230, a cylindrical bit 18 is attached to the drilling rod of the drilling device, and overcoring is performed by drilling the rock around the sensor unit 11B from the stepped portion between the insertion hole h and the small-diameter hole h3 (see FIG. 13(D)).

[0077] After overcoring, in step S240, the stress values ​​σ in six directions detected by each load cell 20 during overcoring are calculated. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 In this embodiment, as in the first embodiment, the principal stresses σ1, σ2, and σ3 are calculated based on the stress values ​​σ in six directions detected by the six load cells 20, and the measurement is completed. X ,σ Y ,σ Z,σ XY45 ,σ ZX45 ,σ YZ45 Since the calculation is based on the above, there is no need to calculate Poisson's ratio or Young's modulus. This means that laboratory testing using core recovery, which is required for the cone-hole strain method, can be omitted.

[0078] According to the second embodiment described above in detail, the sensor unit 11B includes three support rods 14A to 14C extending in the three-axis X, Y, and Z directions from the tip O of the main rod 13, and three support rods 14D to 14F extending in the +45 / -135 degree direction with respect to each of the X, Y, and Z axes on the XYZ plane from the tip O. Six load cells 20 attached to the tips of these support rods 14A to 14F measure stress values ​​σ in six directions. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 By installing such a sensor unit 11B in the small diameter hole h3 and over-coring the periphery of the sensor unit 11B, the stress values ​​σ in six directions directly detected by each load cell 20 can be detected. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 The initial stresses σ1, σ2, and σ3 can be calculated easily and with high accuracy based on the above.

[0079] In other words, compared to the conical hole bottom strain method, which assumes the ground or bedrock to be an isotropic, homogeneous, linear elastic body and calculates the initial stress from an inverse analysis of the measured values, it is possible to reliably ensure the accuracy of calculating the initial stresses σ1, σ2, and σ3.

[0080] Furthermore, each load cell 20 of the sensor unit 11B detects a stress value σ acting in the vertical direction on the pressure receiving surface of the load button 22. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45can be detected as a load acting on the pressure-receiving surface, the initial stresses σ1, σ2, and σ3 can be calculated without being significantly affected by local inhomogeneities such as minute cracks in the ground or rock mass.

[0081] In other words, it is possible to omit the bottom-hole observation using a borehole camera, which is required in the conical bottom-hole strain method, and it is also possible to significantly reduce the effort, time, and cost required to determine the initial stresses σ1, σ2, and σ3.

[0082] In addition, the stress value σ in six directions X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45 By detecting this, it is possible to evaluate not only the stress component perpendicular to the hole axis, but also the stress component in the hole axis direction. In other words, there is no need to drill multiple boreholes at different angles as in hydraulic fracturing, which significantly reduces the cost and labor required for measurement compared to hydraulic fracturing. In addition, since there is no need for packers or high-pressure pumps, it is possible to simplify the entire equipment compared to hydraulic fracturing.

[0083] Furthermore, by adopting a structure in which six load cells 20 are supported by six support rods 14A to 14F, it is possible to increase the degree of freedom in the arrangement direction of each load cell 20 compared to when each load cell 20 is arranged in a hemispherical spherical shell.

[0084] [others] The present disclosure is not limited to the above-described embodiments, and can be appropriately modified and implemented within the scope of the present disclosure.

[0085] For example, in the first and second embodiments, the calculation processing unit 30 calculates the stress values ​​σ in six directions when overcoring is performed. X ,σ Y ,σ Z ,σ XY45 ,σ ZX45 ,σ YZ45However, the sensor units 11A and 11B may be left in place to monitor long-term fluctuations in stress. In this case, the sensor units 11A and 11B may be buried and fixed by injecting grout in step S120 shown in Fig. 7 and step S220 shown in Fig. 12.

[0086] In the first and second embodiments, the sensor units 11A and 11B use the electric load cell 20, but they can also be configured to detect stress using an optical fiber strain gauge. Using an optical fiber strain gauge makes it possible to more appropriately monitor long-term fluctuations in stress.

[0087] In the first and second embodiments, the third load cell 20 Z In the above description, the Z-axis direction in which the insertion rod 16 is disposed is set to coincide with the axial direction of the insertion rod 16, but the Z-axis may also be configured to be inclined at a predetermined angle relative to the axial direction of the insertion rod 16.

[0088] In the first and second embodiments, the load cells 20 can be arranged in any of six different directions. A specific calculation procedure for the case where the load cells 20 are arranged in any of six directions will be described below.

[0089] The triaxial stress state of the ground or rock mass to be measured is expressed as normal stress σ X ,σ Y ,σ Z and shear stress τ XY ,τ ZX ,τ YZ and the direction cosine (l i ,m i ,n i :i=1~6) any plane π i The axis perpendicular to the plane π is the ξ axis. i The normal stress to σ iξ (However, l i 2 +m i 2 +ni 2 = 1, and the normal stress σ iξ is positive in the tensile direction and negative in the compressive direction).

[0090] Any six planes π i corresponds to the pressure receiving surface (load button 22) of each of the six load cells 20, the measured compressive and tensile stress is iξ As shown in Figure 15, the plane π i Resultant stress p acting on i When each stress component is resolved in the Cartesian coordinate direction, p ix ,p iy ,p iz Let the plane π i However, when the area of ​​the triangle ABC formed by the intersection with a small rectangular parallelepiped part with a vertex O in the target ground or rock mass is Δ, and the balance of the forces acting on the small triangular pyramid OABC is considered, the direction of the X axis is p ix Δ=σ X △OBC+τ XY △OAC+τ ZX △OAB (where △OBC represents the area of ​​triangle OBC. The same applies to others), i.e., p ix Δ=σ X (l i Δ)+τ XY (m i Δ)+τ ZX (n i Δ) The same applies to the Y-axis and Z-axis directions, and rearranging these gives p ix ,p iy ,p iz can be expressed by the following formulas (1.1 to 1.3).

[0091] p ix =l i σ X +m i τ XY +n i τ ZX (1.1) p iy =l i τ XY +m i σ Y +n i τ YZ(1.2) p iz =l i τ ZX +m i τ YZ +n i σ Z (1.3)

[0092] plane π i The stress vector p on i and the direction cosine (l i ,m i ,n i ) is the dot product of the stress vector p i plane π i Since the component is in the normal direction (ξ axis direction) to the stress, the relationship between the component in the ξ axis direction and the stress components in the X, Y, and Z axes can be expressed by the following formula (2).

[0093] σ iξ =l i p ix +m i p iy +n i p iz ···(2)

[0094] By substituting the formulas (1.1 to 1.3) into the formula (2), the following formula (3) corresponding to the coordinate transformation formula for stress can be obtained.

[0095] σ iξ =l i 2 σ X +m i 2 σ Y +n i 2 σ Z +2(l i m i τ XY +m i n i τ YZ +n i l i τ ZX ) ···(3)

[0096] The direction of the pressure receiving surface of the six load cells 30 (ξ axis direction) is known as a set value, and the compressive and tensile stress measurements detected by these load cells 20 are known, that is, the direction of the pressure receiving surface of the six load cells 30 is known as a set value. i The direction cosine of (l i ,m i ,n i ) and normal stress σ iξ Since both are known, equation (3) can be solved with six unknowns (σ X ,σ Y ,σ Z ,τ XY ,τ ZX ,τ YZ ) can be regarded as simultaneous linear equations. By finding the solution to this equation (3), the three-dimensional stress state (σ1, σ2, σ3) of the ground or rock mass being measured can be obtained. [Explanation of symbols]

[0097] 10A, 10B... measuring device, 11A, 11B... sensor unit, 12... spherical shell, 13... main rod, 14A to 14F... support rod, 16... insertion rod, 17... hemispherical bit, 18... cylindrical bit, 19... small diameter bit, 20... load cell, 21... case, 22... load button, 25, 26... jig, 30... calculation processing device, 31... six-direction stress value acquisition unit, 32... principal stress calculation unit

Claims

1. a sensor unit including a plurality of load cells that are inserted into a hole drilled in the ground or bedrock and fixed to the surface of the hole; an insertion rod for inserting the sensor unit into the hole and fixing the plurality of load cells to the surface of the hole; a stress calculation unit that calculates a principal stress inside the ground or the rock mass based on stress values ​​detected by the plurality of load cells; a hemispherical bit for drilling a hemispherical hole at the bottom of the hole, the sensor unit is attached to a tip of the insertion rod and has a hemispherical or spherical shell having a diameter smaller than that of the hemispherical bit, the plurality of load cells each have a case and a load button, and the case is attached to the spherical shell so as to detect stress values ​​in six arbitrary directions different from one another; the sensor unit further includes a contact jig that is fixed to the load button and interposed between the load button and the surface of the hemispherical hole, and a spacer that is disposed between the surface of the spherical shell and the surface of the hemispherical hole to obtain a reaction force, and the plurality of load cells are configured to be fixed to the surface of the hemispherical hole via the jig by inserting the spherical shell into the hemispherical hole with the insertion rod, The stress calculation unit calculates the principal stress based on the stress values ​​in the six directions detected by the plurality of load cells. A measuring device characterized by:

2. The plurality of load cells Assuming that the three mutually orthogonal axes are the X-axis, Y-axis, and Z-axis, and the positive directions along the X-axis, Y-axis, and Z-axis are respectively the +X-axis direction, +Y-axis direction, and +Z-axis direction, and the negative directions along the X-axis, Y-axis, and Z-axis are respectively the -X-axis direction, -Y-axis direction, and -Z-axis direction, a first load cell for detecting a stress value in a +X-axis direction or a −X-axis direction perpendicular to the YZ plane; a second load cell for detecting a stress value in the +Y-axis direction or the −Y-axis direction perpendicular to the ZX plane; a third load cell for detecting a stress value in a +Z-axis direction or a −Z-axis direction perpendicular to the XY plane; a fourth load cell for detecting a stress value in a direction extending from the intersection of the three axes on the XY plane at an angle of +45 degrees or −135 degrees with respect to the X axis; a fifth load cell for detecting a stress value in a direction extending from the intersection of the three axes on the ZX plane at an angle of +45 degrees or −135 degrees with respect to the Z axis; and a sixth load cell that detects a stress value in a direction extending from the intersection of the three axes on the YZ plane at an angle of +45 degrees or −135 degrees with respect to the Y axis, The sensor unit is configured by arranging the plurality of load cells on the surface of the spherical shell so that the center of the spherical shell coincides with the intersection of the three axes. The measurement device according to claim 1 .

3. A sensor unit including a plurality of load cells that are inserted into a hole drilled in the ground or bedrock and fixed to the surface of the hole; an insertion rod for inserting the sensor unit into the hole and fixing the plurality of load cells to the surface of the hole; a stress calculation unit that calculates a principal stress inside the ground or the rock mass based on stress values ​​detected by the plurality of load cells, the plurality of load cells are arranged to detect stress values ​​in six arbitrary directions different from one another, Assuming that the three mutually orthogonal axes are the X-axis, Y-axis, and Z-axis, and the positive directions along the X-axis, Y-axis, and Z-axis are respectively the +X-axis direction, +Y-axis direction, and +Z-axis direction, and the negative directions along the X-axis, Y-axis, and Z-axis are respectively the -X-axis direction, -Y-axis direction, and -Z-axis direction, a first load cell for detecting a stress value in a +X-axis direction or a −X-axis direction perpendicular to the YZ plane; a second load cell for detecting a stress value in the +Y-axis direction or the −Y-axis direction perpendicular to the ZX plane; a third load cell for detecting a stress value in a +Z-axis direction or a −Z-axis direction perpendicular to the XY plane; a fourth load cell for detecting a stress value in a direction extending from the intersection of the three axes on the XY plane at an angle of +45 degrees or −135 degrees with respect to the X axis; a fifth load cell for detecting a stress value in a direction extending from the intersection of the three axes on the ZX plane at an angle of +45 degrees or −135 degrees with respect to the Z axis; and a sixth load cell that detects a stress value in a direction extending from the intersection of the three axes on the YZ plane at an angle of +45 degrees or −135 degrees with respect to the Y axis, The sensor unit a main rod attached to the tip of the insertion rod; a first support rod having a tip end of the main rod as an intersection of the three axes, extending linearly from the intersection in the +X-axis direction or the −X-axis direction, and having the first load cell attached to its tip end; a second support rod that extends linearly from the intersection in the +Y-axis direction or the −Y-axis direction and has the second load cell attached to its tip; a third support rod that extends linearly from the intersection in the +Z-axis direction or the −Z-axis direction and has the third load cell attached to its tip; a fourth support rod that extends linearly from the intersection point on the XY plane at an angle of +45 degrees or −135 degrees with respect to the X axis, and has the fourth load cell attached to its tip; a fifth support rod that extends linearly on the ZX plane from the intersection at an angle of +45 degrees or −135 degrees with respect to the Z axis, and has the fifth load cell attached to its tip; a sixth support rod that extends linearly from the intersection point on the YZ plane at an angle of +45 degrees or −135 degrees with respect to the Y axis, and has the sixth load cell attached to its tip end; The stress calculation unit calculates the principal stress based on the stress values ​​in the six directions detected by the plurality of load cells. A measuring device characterized by:

4. The plurality of load cells are compression-tension type load cells. The measuring device according to any one of claims 1 to 3.

5. Further provided is a cylindrical bit for over-coring the ground or rock around the sensor unit, The stress calculation unit calculates an initial stress generated in the ground or the rock mass based on stress values ​​detected by the plurality of load cells when overcoring is performed by the cylindrical bit. The measuring device according to any one of claims 1 to 4.

6. A method for measuring an initial stress using the measurement device according to claim 5, Drilling the hole in the ground or rock using a drilling device; inserting the sensor unit into the hole using the insertion rod to fix the plurality of load cells to a surface of the hole; a step of overcoring the ground or rock around the hole in which the sensor unit is installed using the cylindrical bit; and calculating an initial stress generated in the ground or the rock mass based on stress values ​​detected by the plurality of load cells when the overcoring is performed. A measuring method characterized by:

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