Condition measuring device and method for burying the condition measuring device

Abstract

Measurements will be taken simultaneously along the buried fiber optic cables. [Solution] The condition measuring device 100 comprises an optical fiber cable 50 that is folded and embedded in a borehole 2 formed vertically downward in the ground 1, a cover member 60 that covers the folded portion 50a of the optical fiber cable 50, a weight 262 attached to the cover member 60, and a measuring device 70 installed outside the borehole 2 to which one end of the optical fiber cable 50 is connected. The measuring device 70 has a light source of pulsed light that is incident on the optical fiber cable 50.

Description

Technical Field

[0001] The present invention relates to a state measurement device and a method for embedding the state measurement device.

Background Art

[0002] Patent Document 1 discloses a state measurement device that measures the state of the ground using an optical fiber cable buried in the ground.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the state measurement device disclosed in Patent Document 1, as an optical fiber cable buried in the ground, an optical fiber cable for strain measurement is provided, and a temperature compensation optical fiber cable is provided to correct the measured value measured in the strain measurement optical fiber cable. Since these optical fiber cables are separately connected to the scattered light measurement device, measurements are performed at different timings.

[0005] An object of the present invention is to simultaneously perform measurements with the buried optical fiber cable.

Means for Solving the Problems

[0006] The present invention relates to a condition measuring device for measuring the condition of an object to be measured using an optical fiber cable, comprising: an optical fiber cable folded and buried in a hole formed vertically downward in the ground; a cover member covering the folded portion of the optical fiber cable; a weight attached to the cover member; and a measuring device installed outside the hole to which one end of the optical fiber cable is connected, wherein the measuring device has a light source of pulsed light incident on the optical fiber cable.

[0007] Furthermore, the present invention relates to a method for burying a condition measuring device, which measures the condition of an object to be measured using an optical fiber cable, in a hole formed vertically downward in the ground, wherein the condition measuring device comprises an optical fiber cable that is folded back and buried so as to reciprocate along the axial direction in the hole, a cover member that covers the folded portion of the optical fiber cable, and a weight attached to the cover member, wherein the cover member is inserted into the hole together with the weight, or the weight is inserted into the hole before the cover member. [Effects of the Invention]

[0008] According to the present invention, measurements can be taken simultaneously in buried optical fiber cables. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows a state measuring device according to an embodiment of the present invention. [Figure 2] This is a cross-sectional view illustrating the configuration of the first optical fiber cable. [Figure 3] This is a cross-sectional view illustrating the configuration of the second optical fiber cable. [Figure 4] This figure shows a cover member of a condition measuring device according to an embodiment of the present invention. [Figure 5] This figure shows a modified example of the cover member of a condition measuring device according to an embodiment of the present invention. [Figure 6] This figure shows a modified example of a state measuring device according to an embodiment of the present invention. [Modes for carrying out the invention]

[0010] A state measuring device according to an embodiment of the present invention will be described below with reference to the drawings.

[0011] First, the configuration of the state measuring device 100 according to an embodiment of the present invention will be described with reference to Figures 1 to 4. The state measuring device 100 is a device that measures the state of the ground 1, which is the object to be measured, by using an optical fiber cable 50 embedded in the ground 1. The object to be measured is not limited to the ground 1, but can be anything in which an optical fiber cable 50 can be embedded, such as a concrete structure such as a dam. The embodiment shown in Figure 1 is a reference example that is not included in the technical scope of the invention claimed in this application, and is an embodiment disclosed in the application before division.

[0012] In the construction of civil engineering structures, it is important to understand the ground conditions, such as landslides, and to understand the progression of loosening areas in the ground that occur during the excavation of underground cavities such as tunnels, in order to proceed with excavation in a stable manner. Therefore, in order to detect underground strain and loosening areas, for example, as shown in Figure 1, an optical fiber cable 50 is embedded in a borehole 2 (insertion hole) that is excavated vertically upward in the ground 1. The borehole 2 is a bottomed hole in which one end is closed inside the ground or structure being measured, and the other end is open to the ground or structure being measured.

[0013] Since the optical fiber cable 50 embedded in the borehole 2 undergoes deformation in response to underground deformation and loosening of the ground 1, the underground deformation and loosening of the ground 1 can be measured by measuring the deformation of the optical fiber cable 50, as described later.

[0014] The state measurement device 100 shown in FIG. 1 mainly includes an optical fiber cable 50 that is folded and embedded so as to reciprocate along the axial direction of the boring hole 2, a pipe material 10 to which the optical fiber cable 50 is attached, a cover member 60 that is attached to the tip side of the pipe material 10 and covers the folded portion 50a of the optical fiber cable 50, an expanding body 20 that is attached to the cover member 60 and can expand in the radially outward direction of the boring hole 2, and a filling hose 30 provided along the pipe material 10.

[0015] The pipe material 10 is a hollow long member formed of a resin such as polyvinyl chloride and has an outer diameter sufficiently smaller than the inner diameter of the boring hole 2. It has an internal passage 11 formed to penetrate in the axial direction and a communication hole 12 that has one end opening on the inner peripheral surface of the internal passage 11 and communicates the inside and outside of the pipe material 10.

[0016] The communication hole 12 is formed on the tip side of the pipe material 10 when the state measurement device 100 is inserted into the boring hole 2, and functions as a discharge port for discharging the air in the boring hole 2 to the outside of the boring hole 2 when filling the grout 40 (filling material) into the boring hole 2 through the filling hose 30.

[0017] [[ID=IS]] The expanding body 20 has an expanding portion 22 that expands radially outward in response to the pressure of the supplied fluid, a support portion 21 that supports the expanding portion 22 on the radially inner side, and a supply pipe 24 that supplies the pressurized fluid. FIG. 1 shows a state in which the expanding portion 22 expands toward the radially outer side of the boring hole 2 and the expanding portion 22 presses against the inner wall surface of the boring hole 2.

[0018] The expanding portion 22 is a member made of rubber or metal whose outer shape is formed in a barrel shape or a cylindrical shape, and is supported by the support portion 21 so as to expand radially outward in response to the pressure of the fluid supplied to a space (not shown) formed between the support portion 21 and the expanding portion 22.

[0019] The support portion 21 is fixed to the outer surface of the cover member 60 via a fixture (not shown).

[0020] The supply pipe 24 is attached along the axial direction to the pipe material 10, and one end 24a thereof opens into the space formed between the support portion 21 and the expansion portion 22. The other end 24b of the supply pipe 24 is connected to a fluid supply source such as a pump (not shown) outside the boring hole 2 before the pressurized fluid is supplied to the expansion portion 22 through the supply pipe 24, and is housed in the boring hole 2 as shown in FIG. 1 when the supply of the fluid to the expansion portion 22 is completed. In addition, in order to maintain the expanded state of the once-expanded expansion portion 22, a check valve for preventing the backflow of the supplied fluid may be provided on the supply pipe 24.

[0021] The fluid supplied to the diameter-expanding body 20 through the supply pipe 24 is, for example, pressurized water, and the expansion portion 22 presses the inner wall surface of the boring hole 2 with a load corresponding to the water pressure. Note that the fluid supplied to the diameter-expanding body 20 is not limited to water, and may be compressed air or pressurized hydraulic oil.

[0022] The filling hose 30 is a hose through which the grout 40 filled in the boring hole 2 flows, and a discharge port 30a for discharging the grout 40 is attached along the pipe material 10 so as to open in the boring hole 2. The position of the discharge port 30a is set vertically below the communication hole 12 formed in the pipe material 10 as shown in FIG. 1. Note that the discharge port 30a only needs to be arranged vertically below the communication hole 12, and may be arranged, for example, near the opening end 2b of the boring hole 2. The other end of the filling hose 30 is connected to a grout delivery pump (not shown) outside the boring hole 2.

[0023] As shown in FIG. 1, the optical fiber cable 50 is folded at a folding portion 50a near the bottom surface 2a of the boring hole 2 and is provided in the ground 1 so as to reciprocate in the boring hole 2.

[0024] The tip 50b of the optical fiber cable 50 is positioned near the open end 2b of the borehole 2 and is sealed with a sealing material such as oil or silicone to prevent reflection on the tip surface. On the other hand, the base end 50c of the optical fiber cable 50 is connected to the measuring device 70 outside the borehole 2.

[0025] The optical fiber cable 50 is mainly composed of two different optical fiber cables: a first optical fiber cable 51 buried in the forward path from the open end 2b of the borehole 2 towards the bottom surface 2a, and a second optical fiber cable 52 buried in the return path from the bottom surface 2a towards the open end 2b. The terms "forward path" and "return path" are used to distinguish the paths in which the optical fiber cables are laid; the cable buried in the forward path may be called the second optical fiber cable, and the cable buried in the return path may be called the first optical fiber cable.

[0026] The first optical fiber cable 51 is an optical fiber sensor used to measure the strain occurring in the ground 1, and as shown in Figure 2, it has a first optical fiber strand 51a, a tension member 51e which is a steel wire, and a resin covering 51d which covers them. The first optical fiber strand 51a is composed of a core 51b, a cladding 51c which surrounds the outer circumference of the core 51b, and an ultraviolet-curing resin (not shown) which covers the cladding 51c. Figure 2 is a cross-sectional view of the first optical fiber cable 51.

[0027] The surface of the coating 51d is embossed to improve adhesion with the grout 40 that is filled into the borehole 2. This makes it easier for strain generated in the ground 1 to be transmitted to the first optical fiber strand 51a via the grout 40.

[0028] The second optical fiber cable 52 is an optical fiber sensor used to measure the temperature inside the ground 1, and as shown in Figure 3, it has a second optical fiber strand 52a and a metal tube 52d (covering) that is provided to cover the second optical fiber strand 52a.

[0029] The second optical fiber strand 52a, like the first optical fiber strand 51a, is composed of a core 52b, a cladding 52c surrounding the outer circumference of the core 52b, and an ultraviolet-curing resin (not shown) covering the cladding 52c.

[0030] The metal tube 52d is a steel tube made of stainless steel or nickel alloy, which has high corrosion resistance, and has an inner diameter larger than the outer diameter of the second optical fiber strand 52a. In other words, a small gap is formed between the second optical fiber strand 52a and the metal tube 52d, allowing the second optical fiber strand 52a to expand and contract freely without being constrained by the metal tube 52d. Figure 3 is a cross-sectional view of the second optical fiber cable 52.

[0031] Comparing the conformability of the cladding 51c (first optical fiber strand 51a) to the sheath 51d of the first optical fiber cable 51 used for strain measurement with the conformability of the cladding 52c (second optical fiber strand 52a) to the metal tube 52d (sheath) of the second optical fiber cable 52 used for temperature measurement, the former is higher and the latter is lower. In other words, comparing the coefficient of friction between the sheath 51d and cladding 51c of the first optical fiber cable 51 with the coefficient of friction between the sheath (metal tube 52d) and cladding 52c of the second optical fiber cable 52, the former is larger than the latter. That is, comparing the conformability of the first optical fiber strand 51a (cladding 51c) to the grout 40 (filling material) in which each sheath is embedded with the conformability of the second optical fiber strand 52a (cladding 52c), the former is higher and the latter is lower.

[0032] In particular, the surface of the sheath 51d of the first optical fiber cable 51 is embossed as described above, making it easier for the sheath 51d to adhere to the grout 40, thus improving the conformability of the first optical fiber strand 51a (clad 51c) to the grout 40. On the other hand, as described above, a small gap is provided between the second optical fiber strand 52a of the second optical fiber cable 52 and the metal tube 52d (sheath), making it difficult for the second optical fiber strand 52a to conform to the metal tube 52d (sheath), thus reducing the conformability of the second optical fiber strand 52a (clad 52c) to the grout 40.

[0033] In other words, when strain occurs in the ground 1, the strain in the first optical fiber cable 51 is more easily transmitted to the first optical fiber strand 51a via the sheath 51d, while in the second optical fiber cable 52, the strain is less easily transmitted to the second optical fiber strand 52a via the metal tube 52d.

[0034] Furthermore, in the second optical fiber cable 52 used for temperature measurement, a metal tube 52d with high thermal conductivity is used as a covering to enclose the second optical fiber strand 52a, making it easier for the temperature of the ground 1 to be transmitted to the second optical fiber strand 52a.

[0035] The first optical fiber cable 51 and the second optical fiber cable 52 are connected by fusing the first optical fiber strand 51a and the second optical fiber strand 52a within a cover member 60 that covers the folded portion 50a of the optical fiber cable 50. Specifically, the first optical fiber strand 51a and the second optical fiber strand 52a are connected by fusing the cores 51b and 52b together, and the cladding 51c and 52c together. The sheathing 51d of the first optical fiber cable 51 and the metal tube 52d (sheathing) of the second optical fiber cable 52, which would interfere with the fusing of the first optical fiber strand 51a and the second optical fiber strand 52a, are removed in advance over a predetermined length from the end. The portion where the first optical fiber strand 51a and the second optical fiber strand 52a are fused together is protected by a protective sleeve 54, which will be described later. The method of connecting the first optical fiber cable 51 and the second optical fiber cable 52 is not limited to fusion splicing; mechanical splicing or connector connections may also be used. However, fusion splicing is preferable in order to reduce connection loss at the connection point.

[0036] Furthermore, the first optical fiber cable 51 and the second optical fiber cable 52, which are provided along the axial direction on the outer surface of the pipe material 10, are laid on the pipe material 10 together with the supply pipe 24 and the filling hose 30 and fixed together with the pipe material 10 by fixing tape 14. Note that the pipe material 10 and the first optical fiber cable and the second optical fiber cable only need to be integrated when the pipe material 10 and the first optical fiber cable and the second optical fiber cable are inserted into the borehole 2. Multiple fixing tapes 14 are provided at predetermined intervals in the axial direction.

[0037] The cover member 60 is a resin housing formed by injection molding, and as shown in Figure 4, it has a housing portion 61 formed in a shape that can accommodate the folded portion 50a of the optical fiber cable 50, and a lid member (not shown) attached to the housing portion 61 so that the optical fiber cable 50 routed inside the housing portion 61 is not exposed to the outside. Figure 4 shows the internal shape of the housing portion 61 when the lid member has been removed from the housing portion 61.

[0038] The housing section 61 has a plate-shaped bottom wall 61a and a side wall 61b formed along the outer edge of the bottom wall 61a. Inside the housing section 61, there is a curvature holding section 63 capable of holding the folded portion 50a of the optical fiber cable 50 at a predetermined curvature, as well as a first holding section 64 capable of holding the first optical fiber cable 51, a second holding section 65 capable of holding the second optical fiber cable 52, a sleeve holding section 66 capable of holding a protective sleeve 54 that protects the fusion splice portion between the first optical fiber strand 51a and the second optical fiber strand 52a, and an excess length holding section 67 capable of holding the excess length.

[0039] The curvature-holding portion 63 has a bulging portion 63a that bulges out in a circular shape from the bottom wall 61a, and a groove portion 63b formed between the bulging portion 63a and the side wall 61b. The outer circumferential surface of the bulging portion 63a is an arcuate surface 63c with a predetermined curvature, and the optical fiber cable 50 inserted into the groove portion 63b is held along the arcuate surface 63c. By being held in this way by the curvature-holding portion 63, the direction of extension of the optical fiber cable 50 is changed. In other words, the portion held by the curvature-holding portion 63 becomes the folded portion 50a of the optical fiber cable 50.

[0040] The curvature of the arcuate surface 63c is set according to the minimum bending radius of the optical fiber cable 50 in order to avoid an increase in the bending loss of the optical fiber cable 50. For example, as shown in Figure 4, when the second optical fiber strand 52a is provided in the folded portion 50a, the curvature of the arcuate surface 63c is set according to the minimum bending radius of the second optical fiber strand 52a so as not to increase the bending loss.

[0041] To reduce bending loss at the folded portion 50a, it is preferable to reduce the curvature of the arcuate surface 63c. However, reducing the curvature of the arcuate surface 63c increases the width of the cover member 60, which necessitates increasing the inner diameter of the borehole 2 into which the cover member 60 is inserted. On the other hand, the cost and time required for drilling the borehole 2 increase as the inner diameter of the borehole 2 increases, so it is preferable to make the inner diameter of the borehole 2 as small as possible.

[0042] Furthermore, generally speaking, when comparing cables such as the first optical fiber cable 51 and the second optical fiber cable 52 with individual optical fiber strands such as the first optical fiber strand 51a and the second optical fiber strand 52a, the optical fiber strands are more flexible and have a smaller minimum bending radius.

[0043] Therefore, in this embodiment, by arranging strands such as the second optical fiber strand 52a in the folded portion 50a, the curvature of the arcuate surface 63c is increased, and the width of the cover member 60 is made as small as possible. This makes it possible to insert the cover member 60 even when the inner diameter of the boring hole 2 is small.

[0044] The shape of the bulging portion 63a on which the arcuate surface 63c is provided is not limited to a circular shape, and can be any shape as long as it has an arcuate surface 63c that is formed in a convex shape toward the tip side of the cover member 60 that abuts against the bottom surface 2a of the borehole 2 when inserted into the borehole 2, for example, it may be a semicircular shape or a crescent shape.

[0045] The first holding portion 64 is a groove 64a formed in a shape that allows the first optical fiber cable 51, with its sheath 51d intact, to be held by being fitted into it, and the second holding portion 65 is a groove 65a formed in a shape that allows the second optical fiber cable 52, with its metal tube 52d intact, to be held by being fitted into it. Each groove 64a, 65a is formed by cutting out a part of the side wall 61b so that the inside and outside of the housing portion 61 are in communication.

[0046] The sleeve holding portion 66 is a groove 66b formed between a linearly extending side wall 61b and an inner wall 66a extending parallel thereto, and the width of the groove 66b is set to be slightly smaller than the outer diameter of the protective sleeve 54. When the protective sleeve 54 is fitted into the groove 66b, the fusion splice portion of the first optical fiber strand 51a and the second optical fiber strand 52a is fixed to the cover member 60 via the protective sleeve 54.

[0047] The protective sleeve 54 is a tubular member provided to protect the fusion splice portion between the first optical fiber strand 51a and the second optical fiber strand 52a, which is a vulnerable part. Inside, a rod-shaped rigid member is provided that spans both the first optical fiber strand 51a and the second optical fiber strand 52a.

[0048] The excess length holding section 67 is the part that, when connecting the first optical fiber cable 51 and the second optical fiber cable 52, handles the excess length of the first optical fiber strand 51a, from which the sheath 51d has been removed with ample margin, and the excess length of the second optical fiber strand 52a, from which the metal tube 52d has been removed with ample margin. It has a bulging section 67a that bulges out in a semicircular shape from the bottom wall 61a. A part of the outer surface of the bulging section 67a is an arcuate surface 67b with a predetermined curvature, and the optical fiber cable 50 is held along the arcuate surface 67b.

[0049] The shape of the bulge 67a on which the arcuate surface 67b is provided is not limited to a semicircular shape, but can be any shape as long as it has an arcuate surface 67b, for example it may be circular, similar to the bulge 63a of the curvature holding part 63. If the excess length is short, the optical fiber cable 50 may be held along the groove 67c formed between the bulge 67a and the side wall 61b, as shown by the dashed line. If the excess length is long, the optical fiber cable 50 may be held by wrapping around the bulge 63a of the curvature holding part 63.

[0050] Furthermore, the excess length holding portion 67 is not limited to holding the excess length portion of the second optical fiber strand 52a, as shown in Figure 4, but may be formed in a position capable of holding the excess length portion of the first optical fiber strand 51a, either instead of or in addition to the above.

[0051] The cover member 60, in which the optical fiber cable 50 is routed internally, is attached to the tip side of the pipe material 10 such that the side with the curvature-holding portion 63 that holds the folded portion 50a faces the bottom surface 2a of the borehole 2. In other words, when inserting the pipe material 10 into the borehole 2, the cover member 60 is first inserted into the borehole 2, and is inserted until the tip of the cover member 60 abuts against the bottom surface 2a of the borehole 2.

[0052] Furthermore, the condition measuring device 100 has a flange 16 at the open end 2b of the borehole 2 for fixing the base end of the pipe material 10 to the borehole 2.

[0053] The flange 16 is a plate-shaped member provided to support the base end of the pipe material 10 that protrudes to the outside of the borehole 2. It has an insertion hole 16a through which the pipe material 10, the optical fiber cable 50 and filling hose 30 provided around the pipe material 10 can be inserted, and a plurality of bolt holes 16b through which anchor bolts 4 embedded in the ground 1 so as to surround the opening of the borehole 2 are inserted. The flange 16 is fixed to the borehole 2 by tightening nuts 5 that are screwed onto the anchor bolts 4.

[0054] Furthermore, the flange 16 has a sealant injection hole (not shown) at one end which opens inside the borehole 2. By injecting a sealant 18 such as urethane material near the flange 16 inside the borehole 2 through this sealant injection hole, the open end 2b of the borehole 2 is closed, and the space inside the borehole 2 is sealed off from the outside.

[0055] When the sealing material 18 is injected into the open end 2b, the space inside the borehole 2 is sealed, and grout 40 (filling material) such as cement bentonite is filled into the space through the filling hose 30. As a result, the optical fiber cable 50 is embedded in the borehole 2 together with the pipe material 10 and becomes almost integrated with the ground 1. When the grout 40 is filled, the air inside the borehole 2 is discharged to the outside of the borehole 2 through the communication hole 12 and internal passage 11 of the pipe material 10.

[0056] Next, a method for measuring the condition of the ground 1, which is the object to be measured, using the condition measuring device 100 with the above configuration will be described.

[0057] Generally, optical fiber cables have the property of slightly scattering incident pulsed light backward. By utilizing this property, it is possible to measure strain at multiple locations within the optical fiber cable. Since the frequency of scattered light depends on the strain of the optical fiber cable, the strain of the optical fiber cable can be measured by incidenting pulsed light into the optical fiber cable and measuring the frequency of the scattered light. Furthermore, by measuring the time it takes for scattered light generated within the optical fiber cable to return to the incident point after pulsed light is incident into the optical fiber cable, the location where the scattered light was generated, i.e., the location where strain occurred in the optical fiber cable, can be measured.

[0058] On the other hand, since the frequency of scattered light changes not only due to distortion but also due to changes in ambient temperature, if distortion is simply calculated from the frequency of scattered light, the calculated distortion will include the frequency change due to temperature changes. In other words, in order to accurately measure distortion, it is necessary to compensate for the frequency change corresponding to temperature changes.

[0059] To compensate for frequency changes due to temperature, it is conceivable to provide a separate optical fiber cable for temperature compensation. However, if the timing of measurements differs between the optical fiber cable used for strain measurement and the optical fiber cable used for temperature compensation, for example, if the temperature changes between the time the measurement by the optical fiber cable used for strain measurement is performed and the time the measurement by the optical fiber cable used for temperature compensation is completed, the amount of strain will be corrected at a temperature different from the temperature at which the measurement by the optical fiber cable was performed.

[0060] Therefore, simply installing a fiber optic cable for temperature compensation may not reliably eliminate the effects of temperature changes, potentially leading to a decrease in the accuracy of strain measurement.

[0061] Therefore, in this embodiment, as described above, a single optical fiber cable 50 is formed by connecting a first optical fiber cable 51 used to measure the strain occurring in the ground 1 and a second optical fiber cable 52 used to measure the temperature inside the ground 1, and this cable is folded back and buried in the borehole 2. This makes it possible to measure the strain of the ground 1 and the temperature of the ground 1 simultaneously, and reliably eliminates the influence of temperature changes on the measurement of the amount of strain.

[0062] Specifically, first, a predetermined pulse of light is incident on the optical fiber cable 50 from a light source (not shown) inside the measuring device 70 (light incidence step).

[0063] Next, the strain of the ground 1 is measured along the borehole 2 based on the scattered light (reflected light) returning from the first optical fiber cable 51 of the optical fiber cable 50 (strain measurement process). The strain is measured, for example, by a known BOTDR (Brillouin Optical Time Domain Reflectometry) method that utilizes Brillouin scattering.

[0064] Almost simultaneously, the temperature of the ground 1 is measured along the borehole 2 based on the scattered light (reflected light) returning from the second optical fiber cable 52 of the optical fiber cable 50 (temperature measurement process). The temperature measurement is performed using the known BOTDR method, similar to the strain measurement.

[0065] When the axial strain distribution and temperature distribution within the borehole 2 are measured simultaneously in this manner, the strain measured in the first optical fiber cable 51 is corrected using the temperature measured in the second optical fiber cable 52 (correction step).

[0066] Here, the frequency difference (frequency shift) obtained in the first optical fiber cable 51 to calculate the strain includes the difference corresponding to temperature, as described above. On the other hand, the frequency difference (frequency shift) obtained in the second optical fiber cable 52 is only the difference corresponding to temperature and does not include the difference corresponding to strain.

[0067] Therefore, by subtracting the frequency difference obtained in the second optical fiber cable 52 from the frequency difference obtained in the first optical fiber cable 51, the frequency difference caused solely by distortion can be determined. Based on the frequency difference caused solely by distortion determined in this way, the distortion generated at a predetermined position in the borehole 2 can be measured with high accuracy.

[0068] The measurement of strain and temperature is not limited to the above method, and may be performed by other methods as long as they can be measured based on the backscattered light of the optical fiber cable 50. For example, it may be performed using the known OFDR (Optical Frequency Domain Reflectometry) method, or it may be performed in combination with the above method and the OFDR method.

[0069] According to the above embodiments, the following effects are achieved.

[0070] According to the condition measuring device 100 of this embodiment, the optical fiber cable 50 embedded in the borehole 2 is integrated by connecting the first optical fiber cable 51 embedded in the forward path and the second optical fiber cable 52 embedded in the return path.

[0071] Therefore, measurements taken with the first optical fiber cable 51 and measurements taken with the second optical fiber cable 52 are performed at the same time. This makes it possible to correct the measurement value taken with the first optical fiber cable 51 using the measurement value taken with the second optical fiber cable 52 at the same time, and as a result, the measurement accuracy of the strain of the ground 1 (the condition of the object being measured) can be improved. In addition, since measurements taken with the first optical fiber cable 51 and measurements taken with the second optical fiber cable 52 are performed simultaneously, the time required to measure the strain of the ground 1 can be shortened.

[0072] Furthermore, at the folded portion 50a, the optical fiber cable 50 is held at a predetermined curvature by the cover member 60. As a result, bending loss at the folded portion 50a is suppressed, and consequently, measurement accuracy can be maintained throughout the entire optical fiber cable 50 even when the folded portion 50a is present.

[0073] Furthermore, the connection point between the first optical fiber cable 51 and the second optical fiber cable 52 is located inside the cover member 60. This allows the vulnerable connection point to be protected from external elements.

[0074] Furthermore, the following modifications are also within the scope of the present invention, and it is possible to combine the configurations shown in the modifications with the configurations described in the embodiments described above, or to combine the configurations described in the following different modifications.

[0075] In the above embodiment, a second optical fiber strand 52a is arranged in the folded portion 50a. Alternatively, as shown in the modified example in Figure 5, a third optical fiber strand 53 (third optical fiber cable), separate from the first optical fiber strand 51a and the second optical fiber strand 52a, may be arranged in the folded portion 50a, and this third optical fiber strand 53 may be held by the curvature holding portion 63. Note that Figure 5 corresponds to Figure 4.

[0076] In the modified example shown in Figure 5, the first optical fiber cable 51 and the second optical fiber cable 52 are connected via a third optical fiber strand 53. Specifically, one end 53a of the third optical fiber strand 53 is fused to the first optical fiber strand 51a, and the other end 53b of the third optical fiber strand 53 is fused to the second optical fiber strand 52a, with each fusion splice being protected by a first protective sleeve 154 and a second protective sleeve 155, respectively.

[0077] The third optical fiber strand 53 is an optical fiber specially designed for bending resistance, such as a Holy fiber, and is used that has a smaller minimum bending radius than the first optical fiber strand 51a and the second optical fiber strand 52a.

[0078] In this modified example, as shown in Figure 5, the housing portion 161 of the cover member 160 is provided with a curvature holding portion 63 similar to that in the above embodiment, as well as a first sleeve holding portion 166 capable of holding the first protective sleeve 154 and a second sleeve holding portion 167 capable of holding the second protective sleeve 155.

[0079] The first sleeve holding portion 166 is a groove 166b formed between a linearly extending side wall 61b and an inner wall 166a extending parallel thereto, and the width of the groove 166b is set to be slightly smaller than the outer diameter of the first protective sleeve 154. By fitting the first protective sleeve 154 into the groove 166b, the fusion splice portion of the first optical fiber strand 51a and the third optical fiber strand 53 is fixed to the cover member 160 via the first protective sleeve 154.

[0080] The second sleeve holding portion 167, like the first sleeve holding portion 166, is a groove 167b formed between a linearly extending side wall 61b and an inner wall 167a extending parallel thereto, and the width of the groove 167b is set to be slightly smaller than the outer diameter of the second protective sleeve 155. When the second protective sleeve 155 is fitted into the groove 167b, the fusion splice portion of the second optical fiber strand 52a and the third optical fiber strand 53 is fixed to the cover member 160 via the second protective sleeve 155.

[0081] Since the third optical fiber strand 53, which is positioned in the folded portion 50a, has a smaller minimum bending radius than the first optical fiber strand 51a and the second optical fiber strand 52a, this modified example makes it possible to increase the curvature of the arcuate surface 63c of the curvature holding portion 63 compared to the above embodiment, and to further reduce the width of the cover member 160. This makes it possible to embed the optical fiber cable 50 in a boring hole 2 with an even smaller inner diameter.

[0082] Furthermore, the portion of the third optical fiber strand 53 held by the curvature holding portion 63 may be in the form of an optical fiber cable covered with a resin coating, similar to the first optical fiber cable 51. In addition, the housing portion 161 of the cover member 160 may be provided with an excess length holding portion 67 capable of holding the excess length portion, similar to the embodiment described above.

[0083] Furthermore, in the above embodiment, the optical fiber cable 50 is embedded in a borehole 2 excavated vertically upward in the ground 1. Alternatively, the optical fiber cable 50 may be embedded in a borehole 2 excavated horizontally, or, as shown in the modified example in Figure 6, it may be embedded in a borehole 2 excavated vertically downward. Note that the embodiment shown in Figure 6, disclosed as a modified example in the original application, the combination of the configuration shown in this embodiment and the configuration described in the above-described embodiment, and the combination of the configuration shown in this embodiment and the configuration described in the modified example shown in Figure 5 are all within the technical scope of the invention claimed in this application.

[0084] In the modified example shown in Figure 6, the optical fiber cable 50 is inserted into the borehole 2 together with a steel rope 264, rather than with a pipe 10.

[0085] Specifically, the condition measuring device 200 shown in Figure 6 includes an optical fiber cable 50 that is folded back and embedded along the axial direction of the borehole 2, a cover member 260 that covers the folded portion 50a of the optical fiber cable 50, a weight 262 attached to the cover member 260, and a rope 264 with one end connected to the cover member 260.

[0086] The cover member 260, like the cover member 60 in the above embodiment, has a configuration that allows it to hold the folded portion 50a of the optical fiber cable 50 with a predetermined curvature, and also has a configuration that allows it to hold the connection portion between the first optical fiber cable 51 and the second optical fiber cable 52.

[0087] The weight 262 is a metal member attached to the side of the cover member 260 and functions as a weight to stabilize the position of the cover member 260 when it is inserted into the borehole 2. The weight 262 may also be a cylindrical metal member with a space formed inside that can accommodate the cover member 260. Alternatively, the weight 262 may be connected to the tip of the cover member 260 via a wire or the like and inserted into the borehole 2 before the cover member 260.

[0088] The rope 264 is a steel wire rope that suspends and supports the cover member 260 and the weight 262 when they are inserted into the borehole 2, and the other end is attached to a winding device such as a winch (not shown). Note that the rope 264 is not limited to a steel wire rope, but may be a resin rope made of polyethylene or the like. Furthermore, it is preferable that the rope 264 is a non-rotating rope. In addition, one end of the rope 264 may be connected to the weight 262 instead of the cover member 260.

[0089] The optical fiber cable 50, which is folded back and installed inside the borehole 2, is fixed to the rope 264 by a plurality of fixing tapes 14 that are provided at predetermined intervals in the axial direction, and is inserted into the borehole 2 in this state.

[0090] Then, as shown in Figure 6, with the tip of the cover member 260 in contact with the bottom surface 2a of the borehole 2 and the tip 50b of the optical fiber cable 50 positioned near the open end 2b of the borehole 2, the borehole 2 is filled with grout 40.

[0091] Even when the optical fiber cable 50 is embedded in a borehole 2 excavated vertically downward, the measurement accuracy of the ground deformation (condition of the object being measured) can be improved, similar to the embodiment described above.

[0092] Furthermore, in the above embodiment, a second optical fiber strand 52a is arranged in the folded portion 50a. Alternatively, a first optical fiber cable 51 with its sheath 51d intact or a second optical fiber cable 52 with its metal tube 52d intact may be arranged in the folded portion 50a. However, as described above, in order to increase the curvature of the arcuate surface 63c of the curvature holding portion 63 that holds the folded portion 50a, it is preferable to arrange the optical fiber strands 51a and 52a in the folded portion 50a.

[0093] Furthermore, in the above embodiment, the first optical fiber cable 51 is buried in the forward path from the open end 2b of the borehole 2 toward the bottom surface 2a, and the second optical fiber cable 52 is buried in the return path from the bottom surface 2a toward the open end 2b. Alternatively, the second optical fiber cable 52 may be buried in the forward path and the first optical fiber cable 51 may be buried in the return path.

[0094] Although embodiments of the present invention have been described above, these embodiments only represent a part of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments.

[0095] The problem that the invention of the present application aims to solve is as follows. In the condition measuring device disclosed in Patent Document 1, a fiber optic cable for strain measurement is provided as a fiber optic cable buried in the ground, and a fiber optic cable for temperature compensation is provided to correct the measurement value measured by the fiber optic cable for strain measurement. Since these fiber optic cables are connected separately to the scattered light measuring device, measurements are taken at different timings. If the timing of measurements is different in this way, for example, if the temperature changes between the time the measurement by the fiber optic cable for strain measurement is taken and the time the measurement by the fiber optic cable for temperature compensation is taken, the amount of strain will be corrected at a temperature different from the temperature at which the measurement by the fiber optic cable for strain measurement was taken. As a result, the accuracy of the measurement of the amount of strain may decrease.

[0096] Furthermore, the invention described in the original application of this application aims to improve the measurement accuracy of measuring the state of an object to be measured using an optical fiber cable.

[0097] Furthermore, the original claims of the present application (Japanese Patent Application No. 2022-145245) are as follows: [Claim 1] A condition measuring device that measures the condition of an object to be measured using an optical fiber cable, The optical fiber cable is embedded within the object to be measured, folded back so as to move back and forth within the object to be measured, The optical fiber cable comprises a cover member that covers the folded portion, The optical fiber cable comprises a first optical fiber cable buried in the forward path and a second optical fiber cable buried in the return path and connected to the first optical fiber cable within the cover member. Condition measuring device. [Claim 2] The first optical fiber cable and the second optical fiber cable each have a core, a cladding surrounding the outer periphery of the core, and a covering surrounding the outer periphery of the cladding, The conformability of the cladding to the sheath of the first optical fiber cable is higher than that of the cladding to the sheath of the second optical fiber cable. The condition measuring device according to claim 1. [Claim 3] The cover member has a curvature-holding portion that can hold the folded portion of the optical fiber cable at a predetermined curvature. The condition measuring device according to claim 1. [Claim 4] The first optical fiber cable and the second optical fiber cable each have a core, a cladding surrounding the outer periphery of the core, and a covering surrounding the outer periphery of the cladding, The curvature-holding portion holds either the first optical fiber cable or the second optical fiber cable, whose sheathing has been removed, in order to connect the first optical fiber cable and the second optical fiber cable. The condition measuring device according to claim 3. [Claim 5] The first optical fiber cable and the second optical fiber cable are connected via a third optical fiber cable having a smaller allowable bending radius than the first optical fiber cable and the second optical fiber cable. The curvature holding portion holds the third optical fiber cable. The condition measuring device according to claim 3. [Claim 6] The optical fiber cable is inserted into an insertion hole provided in the object to be measured. The gap between the optical fiber cable and the insertion hole is filled with a filler material. A condition measuring device according to any one of claims 1 to 5. [Claim 7] The first optical fiber cable and the second optical fiber cable each have a core, a cladding surrounding the outer periphery of the core, and a covering surrounding the outer periphery of the cladding, The conformability of the cladding of the first optical fiber cable to the filler is higher than that of the cladding of the second optical fiber cable to the filler. The condition measuring device according to claim 6. [Claim 8] A method for measuring the state of an object to be measured using a state measuring device according to any one of claims 1 to 5, The process of injecting light into the optical fiber cable, The process of measuring the distortion of the object to be measured based on the scattered light returning from the first optical fiber cable, The process involves measuring the temperature of the object to be measured based on the scattered light returning from the second optical fiber cable among the optical fiber cables. The process includes correcting the measured strain using the measured temperature, Method for measuring the state. [Explanation of Symbols]

[0098] 100,200... Condition measuring device 1. Ground (object being measured) 2. Boring hole (insertion hole) 40. Grout (filling material) 50... Fiber optic cable 50a...Folding section 51...First Fiber Optic Cable 51a...First optical fiber strand 51b...core 51c...Clad 51d...covering 52...Second Fiber Optic Cable 52a...Second optical fiber strand 52b...core 52c...Clad 52d... Metal pipe (coated) 53. Third optical fiber strand (third optical fiber cable) 60, 160, 260... Cover parts 63...curvature holding part

Claims

1. A condition measuring device that measures the condition of an object to be measured using an optical fiber cable, The optical fiber cable is folded and buried in a hole formed vertically downward in the ground, A cover member that covers the folded portion of the optical fiber cable, The weight attached to the cover member, The device comprises a measuring device installed outside the hole and to which one end of the optical fiber cable is connected, The measuring device has a light source of pulsed light that is incident on the optical fiber cable. Condition measuring device.

2. The system further comprises a rope with one end connected to the cover member or the weight, The rope is capable of suspending and supporting the cover member and the weight when the cover member is inserted into the hole together with the weight. The condition measuring device according to claim 1.

3. The optical fiber cable is folded back and embedded in the grout filled in the hole. The condition measuring device according to claim 1.

4. The cover member has a housing portion formed in a shape capable of accommodating the folded portion of the optical fiber cable. A condition measuring device according to any one of claims 1 to 3.

5. A method for burying a condition measuring device, which measures the condition of an object to be measured using an optical fiber cable, in a hole formed vertically downward in the ground, The aforementioned condition measuring device is The optical fiber cable is buried in the hole, folded back so as to reciprocate along the axial direction, A cover member that covers the folded portion of the optical fiber cable, The cover member is equipped with a weight attached to it, The cover member is inserted into the hole together with the weight, or the weight is inserted into the hole before the cover member. Method for burying a condition measurement device.

6. With the tip of the cover member in contact with the bottom of the hole, grout is filled into the hole. The method for burying a condition measuring device according to claim 5.

7. The condition measuring device further comprises a rope with one end connected to the cover member or the weight, When inserting the cover member together with the weight into the hole, the rope is used to suspend and support the cover member and the weight. A method for burying a condition measuring device according to claim 5 or claim 6.

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