JP3387006B2 - Sediment movement detection sensor for river embankment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sediment movement detection sensor for a river embankment using strain distribution by an optical fiber, and more particularly to a sediment movement detection sensor for a river embankment that accurately detects sediment movement. .

[0002]

2. Description of the Related Art In river management, it is important to detect collapse and deformation of levees. The present applicant is studying a method of burying a distributed strain sensor using an optical fiber cable in a river bank and detecting the movement of sediment from the strain generated in the optical fiber cable.

Various patents have been applied for a distributed strain sensor using an optical fiber cable. Some typical ones will be described.

Distortion detection sensor (JP-A-3-175328)
Then, when the optical fiber strand is straightly stretched and housed in the groove, a predetermined elongation is given in advance to apply compressive strain to the optical fiber strand, thereby enabling detection of a low strain amount.

In the strain detecting sensor (Japanese Patent Laid-Open No. 4-212004), the optical fiber can be bent by accommodating the optical fiber in a metal tube which is continuously bent in a wave pattern at a constant period. In addition, the change in optical transmission loss can be detected stably.

Optical fiber cable for strain measurement (JP-A-7-
181021), the sheath is filled with a substance that changes chemically so that the looseness between the optical fiber core and the sheath is ensured during cable laying so that no strain is applied to the optical fiber core. After the laying, the strain of the measuring object is accurately measured by transmitting the strain of the measuring object as it is to the core wire so that the adhesiveness is exhibited.

In the strain detecting sensor (Japanese Utility Model Laid-Open No. 2-120008), a nylon-coated optical fiber core wire used as a normal optical communication fiber, which is not simply nylon-coated, is adopted, Since the optical fiber element wire is fixed by vertically attaching it to the central member under tension and covering it with a protective layer, it is not necessary to use a special optical fiber, and an optical fiber cable for strain measurement can be easily and inexpensively used. You can make it.

The above is a typical prior art for a typical distributed strain sensor using an optical fiber.

[0009]

There are the following problems in using the distributed strain sensor as a sediment movement detection sensor on a river embankment.

In the strain sensor cable described in the strain detecting sensor (Japanese Patent Laid-Open No. 3-175328), an optical fiber is laid in advance by giving the spacer about 0.1% elongation. This method complicates the manufacturing process and makes it difficult to manufacture a long-distance cable. Also, the spacer and the optical fiber are fixed with resin, and then the extension of the spacer is restored to give compressive strain to the optical fiber, but the compressive strain applied to the optical fiber at that time may be non-uniform over the length direction. This results in variations in strain detection performance in the length direction of the optical fiber.

The strain sensor cable described in the strain detection sensor (Japanese Patent Application Laid-Open No. 4-212004) detects strain due to an increase in transmission loss due to bending of an optical fiber. When the distortion is added,
The transmission loss increases at that point, making it impossible to measure the cable after that. For example, one embankment
When this sensor cable was laid and measured for the purpose of monitoring over 0 km, if the deformation of the embankment occurred at the first 1 km and the cable was distorted, the remaining 9 k
The part of m becomes unmeasurable at that time, resulting in a system lacking in practicality.

In the strain sensor cable described in the strain distribution measuring optical fiber cable (Japanese Patent Laid-Open No. 7-81021), since the optical fiber core wire and the sheath are in close contact, the strain of the measuring object is Although it is easily transmitted to the fiber core wire, the rigidity of the cable increases after the filled resin hardens, and it may be difficult to follow the movement of the surrounding sediment when it is laid on a dike.

The sensor cable described in the strain detecting sensor (Japanese Utility Model Laid-Open No. 2-120008) is a cable in which an optical fiber core wire not covered with nylon is vertically attached to a central member and covered with a protective layer. However, with this structure, there is a high possibility that the optical fiber core wire will be broken during cable laying work. In order to prevent disconnection, a special laying method and excessive nerves must be paid, which complicates the process and is not practical.

The prior art of the strain sensor cable is as follows: 1) A minute bend is applied to the cable to give a microbend loss to the optical fiber core wire by the strain applied to the cable, and the microbend loss is measured by an optical fiber loss measuring instrument ( OTDR) and 2) a cable internal structure is devised so that the cable protection layer and the existing optical fiber core are integrated, and the strain applied to the optical fiber core is distributed. It can be roughly divided into two methods: measuring with a measuring instrument.

The method of measuring with the optical fiber loss measuring device has a drawback that when a strain is applied to a certain point, the optical fiber loss increases at that point and the remaining laid portion cannot be measured. The conventional method of measuring with an optical fiber strain distribution measuring device, especially when the soil is laid in a relatively soft place such as a river embankment, even if the surrounding sediment is displaced or moved, the cable does not follow the movement, There is a drawback that only the surrounding sediment moves, the cable is not distorted, and the displacement or movement of the sediment cannot be detected.

[0016]

In order to achieve the above object, the present invention is to embed an optical fiber cable extending along a river embankment in the sediment of a river embankment and to the side of the optical fiber cable. By embedding a weight connected to the section, the distribution of elongation strain applied to the above optical fiber cable is brilled.
A measuring device for measuring the frequency shift amount of the unscattered light and the time delay is provided, and the sediment movement of the river embankment is detected based on this strain distribution.

[0017]

In order to achieve the above object, the present invention is to embed an optical fiber cable extending along a river embankment in the sediment of a river embankment and to the side of the optical fiber cable. The weight connected to the section is buried, and a measuring device for optically measuring the distribution of strain applied to the optical fiber cable is provided, and the movement of sediment on the river embankment is detected based on this strain distribution.

The weight may be embedded more on the slope end side of the river bank than the optical fiber cable.

The weight may be embedded separately from the optical fiber cable, and a connecting member for connecting the weight and the optical fiber cable may be provided.

The weight may be attached to the optical fiber cable and the optical fiber cable may be laid in the vicinity of the slope of the river bank.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

As shown in FIG. 1, in the sediment movement detection sensor of the present invention, an optical fiber cable 1 extending along a river embankment is embedded in the sediment of the river embankment and the optical fiber cable 1 is embedded. The weight 2 connected to the side of the is embedded, the strain distribution measuring device 3 for optically measuring the strain distribution applied to the optical fiber cable 1 is provided, and the sediment movement of the river embankment is detected and displayed based on this strain distribution. The strain distribution display section 4 is provided. The embedded optical fiber cable 1 and the strain distribution measuring device 3 may be connected by an optical fiber cable 5 for signal transmission.

The optical fiber core wire used in the present invention and the optical fiber cable constituted by the optical fiber core wire are arbitrary in type and shape. For example, an optical fiber cable containing one or more optical fiber core wires,
An optical fiber cable in which the periphery of the optical fiber core is covered with a protective layer for protecting the optical fiber core.

As the measuring device 3 for measuring the strain applied to the optical fiber core, an optical fiber strain distribution measuring device utilizing Brillouin scattered light of the optical fiber is used. This optical fiber strain distribution measuring device makes short pulse laser light incident on the optical fiber and detects Brillouin scattered light scattered in the optical fiber. Here, measuring the Brillouin scattered light at each frequency while varying the optical frequency of the incident pulsed light,
Measure the spectrum distribution of Brillouin scattered light. The frequency at which the spectrum has the maximum light-receiving level is the Brillouin frequency shift. Since the shift amount of the Brillouin frequency shift is proportional to the strain applied to the optical fiber, the strain distribution in the length direction of the optical fiber can be obtained. The conversion of the distance along the optical fiber is based on the commonly known OT.
Similar to the method used in DR (Optical Time Domain Reflectometer), the principle of laser distance measurement is used. When the pulsed light is incident, the scattered light returning from each point in the optical fiber returns to the incident end with a time delay by a length corresponding to the distance from each point. The distance to the point can be found by dividing the time of returning to the incident end by the speed of light. In this way, the strain distribution in the length direction of the optical fiber is obtained.

As shown by the broken line in FIG. 2, it is assumed that the optical fiber cable when buried is in a straight line. Weights are attached to the optical fiber cable at regular intervals. The weight 2 is for facilitating the optical fiber cable 1 to follow the movement of the earth and sand when the earth and sand (soil) around which the optical fiber cable is buried is displaced or collapsed. When only the optical fiber cable 1 is laid without mounting the weight 2, only the earth and sand move, and the optical fiber cable 1 is likely to be stretched and to retain the initial burying position as it is.

Since it is considered that the optical fiber cable 1 is distorted following the movement of the earth and sand every time the weight 2 is attached, the distance resolution of the earth and sand movement is determined by the attachment interval of the weight 2.

The extension strain of the optical fiber cable 1 which is generated when the weight 2 moves due to the movement of the earth and sand is shown by a solid line in FIG. It is assumed that the weight 2 is attached to the optical fiber cable 1 at an interval x ₀ . At this time, sediment collapse occurs near the center weight 2 (dotted line indicates the sediment collapse line), the weight 2 moves by the displacement amount d, and when the optical fiber cable 1 extends to x ₁ at the weight attachment interval, strain e occurring in the fiber cable 1 is given by _{e = (x 1 -x 0)} / x 0 = {(x 0 2 + d 2) 1/2 -x 0} / x 0. Here, the minimum strain resolution of the optical fiber strain distribution measuring instrument is 0.02%, and the weight mounting interval is 2 m.
Then, the minimum detectable displacement d _min of the weight is 4 cm from the above equation. It can be seen that when the minimum strain resolution of the measuring device is determined, the minimum displacement detection amount of the weight, that is, the minimum movement amount of earth and sand that can be detected is determined by the weight attachment interval.

As described above, since the weight mounting interval determines the distance resolution of earth and sand movement and the minimum detectable movement amount, it is necessary to set the optimum distance depending on the object to be measured.

In the sediment movement detecting sensor shown in FIG. 1, an optical fiber cable 1 having a weight 2 attached is buried in the vicinity of the slope end of a bank. In this configuration, the levee was rained and the water level in the river was raised to promote the collapse of the levee and the performance was evaluated. At this time, the weight 2 was attached every 2 m. As described above, the distance resolution in this case is 2 m, and the minimum sediment movement detection amount is 4 cm. An optical fiber strain distribution measuring device was used as the measuring device 3, and a data display section 4 for displaying arbitrary measurement data was provided.

From the start of the test, the total amount of rainfall was 100 mm / h on the entire surface of the bank for 11.5 hours. After this, the river water level was raised to 2.2 m over 1 hour, and thereafter this water level was maintained. Figure 3 shows the changes over time in the strain detected by the optical fiber cable in the central portion of the bank. The cable burying part began to collapse at the stage of the test time of 45 hours, but the detected strain gradually increased with it, and it is clear that the collapse of the embankment can be detected.
Further, from about 10 hours before the collapse of the buried part, the fluctuation and slight increase of the detected strain could be confirmed, and it was confirmed that the sediment movement detection sensor of the present invention was also effective for the prediction of the embankment collapse.

Needless to say, the present invention is not limited to the above-described modes, but various modes are possible. For example, weight 2
Has a shape that creates resistance to the movement of earth and sand, and the weight 2 is embedded in a position and attitude that creates resistance. With this configuration, the weight 2 moves and the optical fiber cable 1 is pulled as the earth and sand move. For example, in FIG.
, The weight 2 is formed in a rod shape or a plate shape,
The weight 2 is erected vertically (with respect to the horizontal plane) and embedded in the embankment. The optical fiber cable 1 is located at the lower end of the weight 2. As shown in FIG. 4 (b), when the sediment collapses, the weight 2 becomes a resistance to the movement of the sediment (force is received from the sediment), so that the optical fiber cable 1 largely moves from the initial embedding position y. , Stress is applied. By setting the weight shape, arrangement, and posture as shown in FIG. 4A, a relatively minute sediment displacement can be detected.

Alternatively, the weight 2 may be placed away from the optical fiber cable 1 on the slope end of the bank, and the weight and the optical fiber cable may be connected by a connecting member. In this case, even if earth and sand move below the buried position of the optical fiber cable 1, the weight 2 moves and stress is applied to the optical fiber cable 1. For example, as shown in FIG. 5A, the weight 2 is formed in a wedge shape, and the weight 2 is laid horizontally and embedded in the embankment. Aim the center of gravity of the weight 2 to the buttock side,
The pointed part is directed toward the optical fiber cable 1. This weight 2
And the optical fiber cable 1 are connected by a connecting member such as a metal fitting. As shown in FIG. 5B, the weight 2 moves with the movement of the earth and sand below the cable burying position, and the optical fiber cable 1 largely moves from the initial burying position y and stress is applied. By adopting the weight shape, arrangement, and posture as shown in FIG. 5A, it is possible to detect a relatively small sediment displacement below the embedded position of the optical fiber cable 1.

[0033]

The present invention exhibits the following excellent effects.

(1) Since the weight moves due to the sediment, it is possible to reliably detect the sediment displacement.

(2) Since it is possible to detect a minute sediment displacement when detecting the deformation of the embankment due to the rise of river water level or an earthquake, it can be used for predicting the embankment collapse.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a sediment movement detection sensor showing an embodiment of the present invention.

FIG. 2 is a detection principle diagram of the sediment movement detection sensor of the present invention.

FIG. 3 is a time change waveform diagram of distortion in an operation experiment of the present invention.

FIG. 4 is an embedded view of a weight showing another embodiment of the present invention.

FIG. 5 is an embedded view of a weight showing another embodiment of the present invention.

[Explanation of symbols]

1 optical fiber cable 2 weights 3 Strain distribution measuring instrument 4 Strain distribution display section

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Keisuke Fukuchi Inventor Keiichi Fukuchi 5-1-1 Hidaka-cho, Hitachi City, Ibaraki NITRO ELECTRIC CO., LTD. 5-1-1, Machi, Nititsu Electric Cable Co., Ltd., Optoro System Research Center (56) Reference JP-A-6-34401 (JP, A) JP-A-5-99761 (JP, A) Jitsukaihei 6-82532 (JP, U) (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01D 21/00 E02B 3/10 G01L 1/24

Claims (4)

(57) [Claims] 1. An optical fiber cable extending along the river embankment is embedded in the sediment of the river embankment, and a weight connected to the side portion of the optical fiber cable is embedded,
The distribution of elongation strain applied to the above optical fiber cable is blurred.
A sensor for detecting sediment movement on a river embankment, which is provided with a measuring device for measuring the frequency shift amount and time delay of the Luang scattered light, and detects sediment movement on the river embankment based on this strain distribution. 2. The sediment movement detection sensor for a river embankment according to claim 1, wherein the weight is buried more on the slope end side of the river embankment than the optical fiber cable. 3. The weight is embedded so as to be separated from the optical fiber cable, and a connecting member for connecting the weight and the optical fiber cable is provided.
Or the sediment movement detection sensor of the river embankment according to 2. 4. The sediment movement detection sensor for a river embankment according to claim 1, wherein the weight is attached to the optical fiber cable, and the optical fiber cable is laid near the slope of the river embankment.