Rock bolts and installation methods thereof

Abstract

To efficiently remove a tubular member buried in the ground.SOLUTION: A rock bolt 100 installed in a borehole 2 provided in the natural ground 1 comprises a rod-shaped main body 10 arranged along the axial direction in the borehole 2, and an optical fiber cable 20 attached to the rod-shaped main body 10 along the axial direction.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a rock bolt and a method for installing the rock bolt. [Background technology]

[0002] Rock bolts are widely used as reinforcing members for slopes and as supporting members for tunnels. Patent Document 1 discloses a configuration that makes it possible to measure the axial force of a rock bolt in order to confirm the reinforcing effect of the rock bolt. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-5180 Summary of the Invention [Problem to be solved by the invention]

[0004] In the invention described in Patent Document 1, a strain gauge is used as a detector for detecting axial force. The strain gauge detects the axial force at the location where it is attached, but cannot measure the distribution of axial force in the axial direction of the rock bolt, making it difficult to accurately grasp the behavior of the ground in which the rock bolt is installed.

[0005] It is possible to measure the distribution of axial force by installing multiple strain gauges on the rock bolt, but this would require an increase in wiring depending on the number of strain gauges installed, and multiple signal processing devices, which could complicate the wiring work and increase the costs of measurement.

[0006] The present invention aims to easily grasp with high accuracy the behavior of the ground in which a rock bolt is installed. [Means for solving the problem]

[0007] The present invention relates to a rock bolt to be installed in a borehole provided in a natural ground, the rock bolt comprising: a rod-shaped main body portion disposed in the borehole along an axial direction; a folded portion that is folded back at a tip end of the rod-shaped main body portion, a first attachment portion that extends from a base end of the rod-shaped main body portion toward the folded portion, and a second attachment portion that extends from the folded portion toward the base end of the rod-shaped main body portion, an optical fiber cable attached to the rod-shaped main body along an axial direction; a holding portion provided at the tip of the rod-shaped main body portion and holding the folded-back portion of the optical fiber cable; Equipped with The holding portion is a bottomed tubular member having an insertion hole into which the tip of the rod-shaped main body portion is inserted. .

[0008] The present invention also provides a method for installing a rock bolt, the method comprising the steps of forming a plurality of boreholes in a natural ground, a rod-shaped main body formed into a rod shape, a folded portion that is folded back at a tip end of the rod-shaped main body portion, a first attachment portion that extends from a base end of the rod-shaped main body portion toward the folded portion, and a second attachment portion that extends from the folded portion toward the base end of the rod-shaped main body portion, an optical fiber cable attached to the rod-shaped main body along an axial direction; a holding portion provided at the tip of the rod-shaped main body portion and holding the folded-back portion of the optical fiber cable; The present invention includes a step of inserting a rock bolt having the optical fiber cable into each of a plurality of boreholes, and a step of connecting an end of the optical fiber cable of the rock bolt inserted into the borehole to an end of the optical fiber cable of another rock bolt inserted into the borehole. The holding portion is a bottomed tubular member having an insertion hole into which the tip of the rod-shaped main body portion is inserted. . Effect of the Invention

[0009] According to the present invention, the behavior of the ground in which a rock bolt is installed can be grasped accurately and easily. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing a configuration of a lock bolt according to an embodiment of the present invention. [Diagram 2] 2 is a cross-sectional view showing a cross section taken along line AA in FIG. 1. [Diagram 3] 2 is a cross-sectional view showing a cross section taken along line BB in FIG. 1. [Figure 4] FIG. 2 is a diagram for explaining a method of installing a rock bolt according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, a rock bolt and a method for installing the rock bolt according to an embodiment of the present invention will be described with reference to the drawings.

[0012] First, the configuration of a rock bolt 100 according to an embodiment of the present invention will be described with reference to Figs. 1 to 3. Here, a case will be described in which the rock bolt 100 is inserted as a reinforcing member into a borehole 2 provided in a slope 1a in order to stabilize the natural ground 1. Fig. 1 is a diagram showing the rock bolt 100 inserted into the borehole 2, and shows the main configuration of the rock bolt 100. Fig. 2 is a cross-sectional view showing a cross section along line AA in Fig. 1, and Fig. 3 is a cross-sectional view showing a cross section along line BB in Fig. 1.

[0013] The lock bolt 100 comprises a rod-shaped main body 10 formed in a rod shape, an optical fiber cable 20 attached to the rod-shaped main body 10 along the axial direction, a holding portion 30 that holds the optical fiber cable 20 at the tip of the rod-shaped main body 10, a spacer 40 that is provided to position the rod-shaped main body 10 approximately in the center of the drilling hole 2, a support plate 15 having an insertion hole 15a through which the rod-shaped main body 10 is inserted, and a nut 16 that is screwed into the rod-shaped main body 10 to press the support plate 15 against the slope 1a.

[0014] The rod-shaped main body 10 is a so-called threaded rebar with the nodes on the surface formed in a spiral shape, and its outer circumferential surface is provided with a pair of threaded portions 11 with threads formed on either side of the central axis, and flat portions 12 with no threads formed are provided parallel to each other on either side of the central axis. When the drilled hole 2 is long, the rod-shaped main body 10 is formed by connecting multiple threaded rebars with couplers (not shown).

[0015] The optical fiber cable 20 is a linear member having an approximately flat cross-sectional shape, in which an optical fiber and steel wire (not shown) are covered with a resin material, and has a folded-back portion 21 that is folded back at the tip side of the rod-shaped main body 10 that is inserted into the borehole 2, a first attachment portion 22 that extends from the base end side of the rod-shaped main body 10, which is the opening end side of the borehole 2, toward the folded-back portion 21, and a second attachment portion 23 that extends from the folded-back portion 21 toward the base end side of the rod-shaped main body 10.

[0016] The first attachment portion 22 and the second attachment portion 23 are respectively attached along the axial direction to a pair of flat portions 12 that face each other across the central axis of the rod-shaped main body 10. In other words, the first attachment portion 22 and the second attachment portion 23 are disposed facing each other across the central axis of the rod-shaped main body 10. The first attachment portion 22 and the second attachment portion 23 are attached to the rod-shaped main body 10 via an adhesive such as epoxy resin.

[0017] As shown in Figure 1, the end 22a of the first attachment portion 22 and the end 23a of the second attachment portion 23 are pulled out to the outside of the borehole 2 through the through hole 15b formed in the support plate 15, connected to the optical fiber cable 20 of another rock bolt 100, and then connected to the measuring device 50 described below.

[0018] The optical fiber of the optical fiber cable 20 attached to the rod-shaped main body 10 in this manner has the property of slightly scattering the incident pulsed light backward, and by utilizing this property, it is possible to measure strain at multiple positions in the optical fiber.

[0019] Specifically, since the frequency of scattered light depends on the distortion of the optical fiber, the distortion of the optical fiber can be measured by measuring the frequency of scattered light generated when pulsed light is input to the optical fiber. Also, by measuring the time from when pulsed light is input to the optical fiber until the scattered light generated in the optical fiber returns to the input position, the position where the scattered light is generated, i.e., the position where the distortion occurs in the optical fiber, can be measured.

[0020] Therefore, by attaching the optical fiber cable 20 to the rod-shaped main body 10 along the axial direction, it becomes possible to measure the distribution of axial force in the axial direction of the rod-shaped main body 10. As a result, behavior such as loosening and displacement of the ground 1 around the borehole 2 into which the rod-shaped main body 10 is inserted can be understood by measuring the strain generated in the optical fiber cable 20.

[0021] Furthermore, by arranging the first attachment portion 22 and the second attachment portion 23 of the optical fiber cable 20 opposite each other across the central axis of the rod-shaped main body portion 10 as described above, it is possible to determine whether the rod-shaped main body portion 10 is in an axially deformed state or in a bending deformed state by comparing the strain detected at the first attachment portion 22 with the strain detected at the second attachment portion 23, that is, to accurately determine the behavior of the ground 1 around the rod-shaped main body portion 10, such as the displacement direction and displacement position of the ground 1.

[0022] As shown in Figures 1 and 2, the holding portion 30 is a member provided for holding the folded portion 21 of the optical fiber cable 20, and has a cap portion 31 covering the tip of the rod-shaped main body portion 10, and a pair of slope portions 35 extending from the cap portion 31 along the flat portion 12 of the rod-shaped main body portion 10.

[0023] The cap portion 31 is a bottomed cylindrical member having an insertion hole 33 formed therein into which the tip of the rod-shaped main body portion 10 is inserted, and is formed in an obtuse cone shape (a cone shape having a substantially hemispherical tip) whose outer diameter radially tapers toward the tip. The insertion hole 33 is a female threaded hole, and the tip of the rod-shaped main body portion 10 is covered by the cap portion 31 by screwing the tip of the rod-shaped main body portion 10 into the cap portion 31. The insertion hole 33 does not have to be a female threaded hole, and may be a hole into which the tip of the rod-shaped main body portion 10 is inserted or fitted, for example, a non-through hole formed to have a two-face width in accordance with the cross-sectional shape of the rod-shaped main body portion 10.

[0024] Further, an accommodation groove 32 for accommodating the folded-back portion 21 of the optical fiber cable 20 is provided along the axial direction on the outer peripheral surface of the cap portion 31. Specifically, as shown in Fig. 2, the accommodation groove 32 is a pair of grooves formed on either side of the central axis from an end surface 34 where the insertion hole 33 opens toward the tip, and the pair of grooves are connected to form one groove at the tip of the cap portion 31. The accommodation groove 32 functions as a guide portion that guides the optical fiber cable 20 so that the optical fiber cable 20 does not deviate from a predetermined attachment position when the optical fiber cable 20 is folded back and attached to the cap portion 31, so that the attachment operation can be performed smoothly.

[0025] The slope portion 35 is a member provided to eliminate the step that occurs between the bottom surface of the accommodating groove 32 of the cap portion 31 and the flat portion 12 of the rod-shaped main body portion 10, and has an inclined groove 36 formed to connect the accommodating groove 32 and the flat portion 12.

[0026] As shown in FIG. 2, the inclined groove 36 is formed so that its radial height on one end side is approximately the same height as the bottom surface of the accommodating groove 32, i.e., the surface of the cap portion 31 to which the optical fiber cable 20 is attached, and its radial height on the other end side, i.e., in the direction away from the cap portion 31, gradually decreases.

[0027] By providing the slope portion 35 having such an inclined groove 36 on the end face 34 of the cap portion 31, it is possible to prevent the optical fiber cable 20 from being bent excessively in the portion spanning between the cap portion 31 and the rod-shaped main body portion 10. In other words, the pair of slope portions 35 extending from the cap portion 31 along the rod-shaped main body portion 10 function as a step elimination portion that eliminates the step (height difference) between the surface of the cap portion 31 to which the optical fiber cable 20 is attached and the surface of the rod-shaped main body portion 10 to which the optical fiber cable 20 is attached.

[0028] A protrusion 37a for assembling the slope portion 35 to the cap portion 31 is formed protruding from the end face 37 of each slope portion 35 facing the end face 34 of the cap portion 31. After the tip of the rod-shaped main body 10 is screwed to the cap portion 31, the pair of slope portions 35 are assembled to the cap portion 31 by inserting the protrusions 37a into a pair of recesses 34a formed on the end face 34 of the cap portion 31. The cap portion 31 is fixed to the rod-shaped main body 10 such that the storage groove 32 is positioned on an extension line of the flat portion 12 of the rod-shaped main body 10, as shown in FIG.

[0029] By attaching the folded portion 21 of the optical fiber cable 20 along the accommodation groove 32 and the inclined groove 36 formed in the holding portion 30 in this manner, the optical fiber cable 20 is held with a relatively gentle curvature at the tip of the rod-shaped main body 10 without being bent extremely. Also, the cap portion 31 holds the folded portion 21 at the tip of the rod-shaped main body 10 by the accommodation groove 32 so that the folded portion 21 of the optical fiber cable 20 does not come off the tip of the rod-shaped main body 10.

[0030] Therefore, even if the optical fiber cable 20 is attached back and forth to the relatively thin rod-shaped main body 10, it is possible to avoid unnecessary distortion being detected or light being blocked at the turn-back portion 21, and it is also possible to prevent the optical fiber cable 20 from coming off the tip of the rod-shaped main body 10 when the rod-shaped main body 10 is inserted into the borehole 2. Therefore, by measuring the distortion generated in the optical fiber cable 20 attached along the rod-shaped main body 10, it is possible to accurately grasp the behavior of the ground 1 around the borehole 2 into which the rod-shaped main body 10 is inserted.

[0031] In addition, in order to prevent the folded portion 21 of the optical fiber cable 20 from hitting the inner wall surface of the borehole 2 and being damaged when the rod-shaped main body portion 10 is inserted into the borehole 2, the accommodating groove 32 and the inclined groove 36 in which the folded portion 21 is accommodated are filled with a resin material such as epoxy resin.

[0032] The spacer 40 is a member provided for positioning the rod-shaped main body 10 approximately in the center of the borehole 2, and has an attachment part 41 attached to the outer circumferential surface of the rod-shaped main body 10, and an extension part 42 extending from the attachment part 41 toward the inner wall surface of the borehole 2. Although not shown in the figure, two or more spacers 40 are provided.

[0033] The extensions 42 are strip-shaped metal plates provided between the pair of mounting portions 41, and a plurality of extensions, for example, four extensions, are arranged approximately evenly in the circumferential direction. The extensions 42 are formed so as to generate a restoring force when a load acting radially inward is applied to them, and function as positioning members that position the rod-shaped main body 10 in the borehole 2 so that the central axis of the rod-shaped main body 10 does not significantly deviate from the central axis of the borehole 2 when the rod-shaped main body 10 is inserted into the borehole 2.

[0034] As shown in FIG. 3, the mounting portion 41 is a band-shaped metal plate having a cross section formed in an approximately C-shape, and is attached to the outer peripheral surface of the rod-shaped main body portion 10 so as to sandwich the flat portion 12 formed on the rod-shaped main body portion 10.

[0035] In this manner, in the portion where the mounting portion 41 of the spacer 40 is attached to the rod-shaped main body 10, the force with which the mounting portion 41 grips the rod-shaped main body 10 acts, resulting in almost no gap between the flat portion 12 and the mounting portion 41. For this reason, if the optical fiber cable 20 is attached to the flat portion 12 in this portion as well, the optical fiber cable 20 will be pressed by the mounting portion 41 of the spacer 40, which may result in unnecessary distortion being detected or light being blocked.

[0036] Therefore, in this portion, the optical fiber cable 20 is not attached to the flat portion 12, but is attached to the outer surface of the attachment portion 41 in the radial direction, as shown in Fig. 3. In other words, the attachment portion 41 of the spacer 40 is disposed between the flat portion 12 and the optical fiber cable 20.

[0037] Next, a method for installing the rock bolt 100 having the above-mentioned configuration will be described with reference to Fig. 1 and Fig. 4. Fig. 4 is a view of a slope 1a on which a plurality of rock bolts 100 are installed, viewed from a direction perpendicular to the slope 1a. The cross-sectional view shown in Fig. 1 corresponds to a cross-section along line CC in Fig. 4.

[0038] First, a plurality of drilling holes 2 for inserting the rock bolts 100 are drilled into the slope 1a (drilling step). The drilling of the holes 2 may be performed after forming a crib work by spraying concrete onto the slope 1a.

[0039] Next, after filling the borehole 2 with a filler 3 such as cement milk or grout, the rod-shaped main body 10 to which the spacer 40 and the holding part 30 are attached in advance and to which the optical fiber cable 20 is attached as described above is inserted into the borehole 2 (insertion step). Note that the filler 3 may be filled after the rod-shaped main body 10 is inserted into the borehole 2.

[0040] In the insertion process, the rod-shaped main body 10 is inserted into the borehole 2 so that the first attachment portion 22 and the second attachment portion 23 of the optical fiber cable 20 are arranged in a predetermined direction, for example, with the first attachment portion 22 facing upward and the second attachment portion 23 facing downward.

[0041] By determining the orientations of the first attachment portion 22 and the second attachment portion 23 in this manner in advance, if a difference occurs between the strain detected by the first attachment portion 22 and the strain detected by the second attachment portion 23, the deformation direction and deformation position of the rod-shaped main body portion 10 can be identified, making it possible to easily grasp the displacement direction and displacement position of the ground 1 around the rod-shaped main body portion 10.

[0042] Next, the support plate 15 is installed by inserting the rod-shaped main body portion 10 protruding from the slope 1a into the insertion hole 15a of the support plate 15, and the support plate 15 is temporarily fixed to the slope 1a by screwing the nut 16 into the rod-shaped main body portion 10.

[0043] After the filling material 3 has sufficiently solidified, the nut 16 is tightened with a prescribed torque, whereby the bearing plate 15 is pressed against the slope 1a, and the installation of the rock bolt 100 is completed.

[0044] After the installation of the lock bolt 100 is completed, the optical fiber cable 20 is then connected (connecting step).

[0045] The installation of the rock bolts 100 on the slope 1a is generally performed in sequence from the top to the bottom. Therefore, first, the optical fiber cables 20 of the rock bolts 100 installed in a row at the top are connected.

[0046] Specifically, the end 22a of the first attachment portion 22 and the end 23a of the second attachment portion 23, which are pulled out from the adjacently installed rock bolts 100, are connected via the connector 25. Note that the connection between the end 22a of the first attachment portion 22 and the end 23a of the second attachment portion 23 may be performed by fusion.

[0047] When installation of the lower lock bolt 100 is completed, the end 23a of the second attachment portion 23 pulled out from the upper lock bolt 100 and the end 22a of the first attachment portion 22 pulled out from the lower lock bolt 100 are connected via the connector 25.

[0048] By connecting the optical fiber cables 20 in the horizontal and vertical directions in this manner, the optical fiber cables 20 of all the rock bolts 100 installed on the slope 1a are integrally formed as a single cable. The step of connecting the optical fiber cables 20 may be performed before the filling material 3 solidifies.

[0049] An end 22a of the first attachment portion 22, which is one end of the integrated optical fiber cable 20, is connected to a measuring device 50 via a first connection cable 27, and an end 23a of the second attachment portion 23, which is the other end, is connected to the measuring device 50 via a second connection cable 28.

[0050] The measuring device 50 is a device with a built-in light source, a photodetector, and a calculation unit, and is capable of injecting pulsed light into an optical fiber, and is capable of measuring the distortion caused in the optical fiber and the position where the distortion occurred by measuring the frequency of scattered light generated when the pulsed light is injected into the optical fiber and the time it takes for the scattered light to return to the injection position. A rock bolt system 200 capable of monitoring the behavior of the natural ground 1 is constructed by such a measuring device 50 and multiple rock bolts 100.

[0051] In addition, it is sufficient that either one end of the integrated optical fiber cable 20 is connected to the measuring device 50. In this case, the other end is sealed with a sealing material such as oil, silicone, etc. to prevent reflection from occurring at the end face.

[0052] By using this construction method, a plurality of rock bolts 100 equipped with optical fiber cables 20 are placed on the slope 1a, so that loosening and displacement of the entire slope 1a can be grasped.

[0053] Furthermore, by integrating the optical fiber cables 20 of multiple rock bolts 100, the work of installing the cables becomes easier and there is no need to prepare multiple measuring devices 50, which simplifies the measurement system and reduces the costs required for measurement.

[0054] According to the above embodiment, the following effects are achieved.

[0055] The rock bolt 100 according to this embodiment comprises a rod-shaped main body 10 arranged axially within a borehole 2, and an optical fiber cable 20 affixed to the rod-shaped main body 10 axially.

[0056] In this way, by attaching the optical fiber cable 20, which is capable of measuring the distribution of strain, along the rod-shaped main body 10, it becomes possible to measure the distribution of axial force in the axial direction of the rock bolt 100, and as a result, the behavior of the ground 1 in which the rock bolt 100 is installed can be easily grasped with high accuracy.

[0057] Furthermore, in the installation method of the rock bolt 100 according to this embodiment, the optical fiber cable 20 of a rock bolt 100 inserted into a borehole 2 is connected to the optical fiber cable 20 of a rock bolt 100 inserted into another borehole 2, and the optical fiber cables 20 of multiple rock bolts 100 are integrated.

[0058] By integrating the optical fiber cables 20 of multiple rock bolts 100 in this manner, the cable installation work becomes easier and there is no need to prepare multiple measuring devices 50 to which the optical fiber cables 20 are connected, thereby simplifying the measurement system and reducing the costs required for measurement.

[0059] In addition, the following modified examples are also within the scope of the present invention, and it is possible to combine the configuration shown in the modified example with the configuration described in the above embodiment, or to combine the configurations described in the different modified examples below.

[0060] In the above embodiment, the rod-shaped main body 10 is a threaded reinforcing bar with a helical node on the surface. Alternatively, the rod-shaped main body 10 may be a general deformed reinforcing bar having axially extending ribs and nodes intersecting the ribs. In this case, a threaded portion for attaching the cap portion 31 is formed at the tip, and a threaded portion for attaching the nut 16 is formed at the base end. The rod-shaped main body 10 may be configured to be fitted into the cap portion 31, i.e., the insertion hole 33 of the cap portion 31 may be a hole having an inner diameter slightly smaller than the outer diameter of the rod-shaped main body 10.

[0061] In the above embodiment, the rock bolt 100 is inserted into the slope 1a as a reinforcing member. Alternatively, the rock bolt 100 may be inserted into the outer wall of a tunnel constructed by, for example, the NATM construction method as a support member. In this case, too, by arranging a plurality of rock bolts 100 equipped with the optical fiber cable 20 along the outer wall of the tunnel, it is possible to accurately grasp the loosening and displacement of the ground on which the tunnel is constructed.

[0062] In the above embodiment, the optical fiber cables 20 of the multiple rock bolts 100 are integrated and connected to one measuring device 50. Alternatively, the optical fiber cables 20 of the rock bolts 100 may not be connected to each other, and one measuring device 50 may be provided for each rock bolt 100. In addition, when the slope 1a is relatively large, the rock bolts 100 may be divided into multiple groups, the optical fiber cables 20 may be integrated for each group, and a measuring device 50 may be provided for each group.

[0063] Furthermore, in the above embodiment, all of the lock bolts 100 are equipped with optical fiber cables 20. Alternatively, every other or every third lock bolt 100 may be equipped with an optical fiber cable 20, and the other lock bolts may not be provided with an optical fiber cable 20.

[0064] In the above embodiment, the optical fiber cable 20 is folded back at the tip of the rod-shaped main body 10 and arranged to face each other across the central axis of the rod-shaped main body 10. Alternatively, the optical fiber cable 20 may be arranged at the tip of the rod-shaped main body 10 without being folded back at the tip of the rod-shaped main body 10. In this case, the tip of the optical fiber cable 20 is held by the holding part. In this case, since the folded back part 21 is not provided, the holding part does not need to be the holding part 30 having the above-mentioned shape, and may have any configuration as long as it can hold the tip of the optical fiber cable 20 at the tip of the rod-shaped main body 10. For example, the holding part may be an epoxy resin applied to the tip of the rod-shaped main body 10.

[0065] In the above embodiment, the first and second attachment parts 22 and 23 of the optical fiber cable 20 are arranged opposite to each other across the central axis of the rod-shaped main body 10. In addition, an optical fiber cable may be arranged along the axial direction, for example, in the middle between the first and second attachment parts 22 and 23 in the circumferential direction so as not to overlap with the first and second attachment parts 22 and 23. By arranging a plurality of optical fiber cables along the axial direction at a predetermined interval in the circumferential direction of the rod-shaped main body 10 in this manner, it is possible to identify whether the rod-shaped main body 10 is in an axially deformed state or a bendingly deformed state by comparing the strains detected in the optical fiber cables, that is, it is possible to accurately identify the behavior of the natural ground 1, such as the displacement direction and displacement position of the natural ground 1 around the rod-shaped main body 10.

[0066] Although the embodiments of the present invention have been described above, the above-mentioned embodiments merely show some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above-mentioned embodiments. [Explanation of symbols]

[0067] 100···Rock bolt 1. Ground 2. Borehole 10... Rod-shaped main body 20...Fiber optic cable 30...Holding part 33 Insertion hole 40 Spacer 41 Mounting part 42...Extension part

Claims

1. A rock bolt installed in a borehole provided in the ground, A rod-shaped main body portion disposed axially in the borehole; an optical fiber cable having a folded portion folded back at a tip end of the rod-shaped main body portion, a first attachment portion extending from a base end of the rod-shaped main body portion toward the folded portion, and a second attachment portion extending from the folded portion toward the base end of the rod-shaped main body portion, the optical fiber cable being attached to the rod-shaped main body portion along the axial direction; a holding portion provided at the tip of the rod-shaped main body portion and holding the folded-back portion of the optical fiber cable, The holding portion is a bottomed tubular member having an insertion hole into which the tip of the rod-shaped main body portion is inserted. Rock bolt.

2. The outer peripheral surface of the holding portion is provided with an axially extending accommodation groove in which the folded-back portion of the optical fiber cable is accommodated. The rock bolt according to claim 1.

3. A spacer is further provided to position the rod-shaped main body at the center of the drilled hole. The spacer has an attachment portion attached to an outer circumferential surface of the rod-shaped main body and an extension portion extending from the attachment portion toward an inner wall surface of the borehole, The attachment portion is disposed between the rod-shaped main body portion and the optical fiber cable.

3. A rock bolt according to claim 1 or 2.

4. forming a plurality of boreholes in a natural ground; a lock bolt including a rod-shaped main body portion formed in a rod shape, a folded portion folded back at a tip of the rod-shaped main body portion, a first attachment portion extending from a base end of the rod-shaped main body portion toward the folded portion, and a second attachment portion extending from the folded portion toward the base end of the rod-shaped main body portion, the optical fiber cable being affixed to the rod-shaped main body portion along the axial direction, and a holding portion provided at the tip of the rod-shaped main body portion and holding the folded portion of the optical fiber cable, into each of the plurality of boreholes; connecting an end of the optical fiber cable of the rock bolt inserted in the borehole with an end of the optical fiber cable of another rock bolt inserted in the borehole; The holding portion is a bottomed tubular member having an insertion hole into which the tip of the rod-shaped main body portion is inserted. How to install rock bolts.

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