JP4823377B2 - Ground strain detection end - Google Patents

Description

  The present invention relates to a ground strain detection end using an optical fiber for detecting ground strain.

  Monitoring and prediction of ground deformation such as landslides and ground collapses in mountains and cliffs is extremely important for disaster prevention. Conventionally, as a method for monitoring ground deformation, a method of detecting ground strain by attaching a sensor using a strain gauge to an appropriate place to be monitored is known. However, with this method, even when detecting ground strain at one location, sensors using strain gauges are placed at multiple depth positions in order to measure at which depth ground strain occurs. Therefore, it is necessary to connect to a monitoring place by a signal line, which causes a problem that the cost is extremely high. Moreover, the sensor using the strain gauge has a problem in durability.

  Therefore, recently, a landslide sensor using an optical fiber has been proposed as an alternative method (see, for example, JP-A-2-52222). The landslide sensor described in this publication is in a predetermined position by a flexible casing tube inserted into a deep excavation hole formed in the ground to be measured for landslide and a plurality of spacers arranged in the depth direction. An optical fiber strain sensor (hereinafter referred to as an optical fiber sensor) that is disposed and inserted into the casing tube and a filler filled in the casing tube, and when the ground is distorted, Accordingly, the optical fiber sensor is deformed, and the intensity of the scattered light in the portion where the distortion is generated in the optical fiber sensor changes accordingly. Therefore, an optical pulse is incident from one end of the optical fiber sensor and the optical fiber sensor is incident from the incident side By measuring the intensity of backscattered light in relation to the position in the length direction of the sensor, the center of elongation (contraction) strain of the optical fiber sensor is measured. Measuring the distribution in the longitudinal direction, whereby it detects the position and size of the variation occurring in the soil.

  For the installation of this landslide sensor, a drilling hole is drilled to a position deeper than the expected depth of the slip surface in the ground, a flexible casing tube is inserted along the drilling hole, and then a plurality of spacers are installed. The optical fiber sensor placed at a predetermined position is inserted into the casing tube while being suspended so that the spacer is guided in the same direction, and then the filler is filled into the casing. It was.

JP-A-2-52222

  By the way, since an optical fiber sensor will interfere with the measurement when it is in a slack state, a certain tension of a certain magnitude is required when the optical fiber sensor is installed in order to accurately measure the ground strain with the optical fiber sensor. It is necessary to generate a strain in the length direction in the optical fiber sensor in accordance with the ground strain. However, as in the method described in the above publication, in the configuration in which the optical fiber sensor is simply inserted in the casing tube in a suspended state, a desired and constant tension is applied to the optical fiber sensor. There was a problem that it was difficult.

  Recently, the optical fiber body is covered with a corrosion-resistant alloy sheath to prevent damage and deterioration during installation of the optical fiber sensor and during subsequent use, and to extend the service life. An optical fiber sensor having a structure in which a fiber main body is fixed to a corrosion-resistant alloy sheath has been proposed, but when an optical fiber sensor having this structure is used, in order to apply a desired tension to the optical fiber main body during installation, The stretch-resistant corrosion-resistant alloy sheath body is also subjected to elongation strain. Therefore, it is necessary to fix the entire optical fiber sensor to the ground in a state where a large tension (for example, about 300 N) is applied. . However, the configuration in which the optical fiber sensor is simply suspended and inserted into the casing tube as in the method described in the above publication cannot be laid with a desired tension.

  The present invention has been made in view of such a problem, and an optical fiber sensor is placed in a hole formed in the ground with an appropriate tension applied, and the optical fiber sensor is appropriately spaced from the ground. It is an object of the present invention to provide a ground strain detection end that can be installed in a fixed state.

  The ground strain detection end of the present invention forms a displacement marker tube by nesting a plurality of short tubes and forms an optical fiber sensor along the displacement marker tube with tension applied thereto. And a plurality of positions of the optical fiber sensor are fixed to the displacement marker tube, and a short tube disposed at the lower end of the displacement marker tube is used as a tip cap, A lid member fixed to the lower end of the optical fiber sensor, and a slit for passing a U-turn portion at the lower end of the optical fiber sensor is formed to extend downward in the axial direction from the upper end of the main body. The other short tube body of the displacement marker tube body includes a cylindrical main body having a cylindrical surface portion having a diameter smaller than the outer diameter of the cylindrical main body of the tip cap, and the optical fiber sensor is , U-turn part at the bottom Through the slit formed in the main body of the tip cap, the part above the U-turn part is arranged along the outer surface of the cylindrical surface part of another short tubular body, and a plurality of places are fixed to the outer surface of the cylindrical surface part It is the composition of doing. In the ground strain detection end of this configuration, this is inserted into a hole formed in the ground, filled with a suitable filler and integrated with the ground, the displacement marker tube is deformed according to the strain of the ground, On the other hand, the optical fiber sensor is also deformed in accordance with the deformation of the displacement marker tube, and causes distortion in the length direction. At that time, since the optical fiber sensor is provided with a plurality of places fixed to the displacement marker tube in a tensioned state, a strain distribution in the length direction is generated in the optical fiber sensor, and the strain distribution is It corresponds to the strain distribution with high accuracy. Therefore, by detecting the strain distribution in the length direction generated in the optical fiber sensor by observing the scattered light intensity, the distribution of the strain generated in the ground can be accurately measured. Also, when installing this ground strain detection end, while connecting the short tube bodies and fixing the optical fiber sensor along the short tube body in a tensioned state, connect the connected part in the hole. Therefore, the ground strain detection end can be easily installed in a deep hole without requiring a large work space.

  As described above, the ground strain detection end of the present invention forms a displacement marker tube by joining a plurality of short tubes, and an optical fiber sensor is placed along the displacement marker tube with tension applied thereto. In addition, since the optical fiber sensor has a configuration in which a plurality of locations of the optical fiber sensor are fixed to the displacement marker tube body, if the optical fiber sensor is inserted into a hole formed in the ground and filled with an appropriate filler, it is integrated with the ground. When strain occurs in the ground, the optical fiber sensor has a lengthwise strain distribution corresponding to the ground strain distribution with high accuracy, and therefore, by observing the scattered light intensity distribution in the optical fiber sensor, This has the effect of detecting the strain distribution in the length direction and accurately measuring the strain distribution generated in the ground. Also, when installing this ground strain detection end, while connecting the short tube bodies and fixing the optical fiber sensor along the short tube body in a tensioned state, connect the connected part in the hole. Thus, the ground strain detection end can be easily installed in a deep hole without requiring a large work space.

1 is a schematic sectional view showing a ground strain detection system using a ground strain detection end according to an embodiment of the present invention. Schematic perspective view showing the vicinity of the lower end of the displacement marker tube at the ground strain detection end and the optical fiber sensor attached to it FIG. 2 is an exploded perspective view showing a part shown in FIG. (A) is schematic sectional drawing which decomposes | disassembles and shows the lower end part of a displacement label | marker pipe body, (b) is a schematic sectional drawing seen from the AA arrow of (a). (A) is a schematic perspective view which shows a stopper in an assembly state, (b) is a schematic perspective view which decomposes | disassembles and shows a stopper Schematic perspective view showing a state in which a short tube is connected to the tip cap The schematic perspective view which shows the state which inserts the front-end | tip cap connected to the lower end of a short tube body in a vertical hole, and connects the next short tube body on a short tube body

  The ground strain detection end of the present invention includes a displacement marker tube inserted into a hole formed in the ground to detect strain, and a plurality of short tubes connected in a nested manner, and the displacement marker tube It is arranged in a state where tension is applied along, and is provided with a strain detection optical fiber sensor fixed to the displacement marker tube at a plurality of locations, and the ground strain detection end of this configuration is used as the ground. If the ground is distorted by laying in the formed hole, filling the filler and integrating it with the ground, the displacement marker tube will be deformed according to the strain, and the optical fiber placed along it The sensor is also deformed, and a strain distribution is generated in the length direction of the optical fiber sensor. For this reason, the strain distribution in the length direction of the optical fiber sensor can be measured by observing the scattered light distribution in the optical fiber sensor, and the generation position, direction, size, etc. of the ground strain can be detected from the measurement information. .

  Here, since the short tube disposed at the lower end of the displacement marker tube constitutes a tip cap, it is provided with a cylindrical main body and a lid member fixed to the lower end thereof, and the cylindrical tube In the main body, a slit for passing a U-turn part at the lower end of the optical fiber sensor is formed so as to extend downward in the axial direction from the upper end of the main body. A cylindrical main body having a cylindrical surface portion having a diameter smaller than the outer diameter of the cylindrical main body of the tip cap is provided. In the optical fiber sensor, the lower U-turn portion is passed through a slit formed in the body of the tip cap, and the portion above the U-turn portion is along the outer surface of the cylindrical surface portion of another short tubular body. And a plurality of locations are fixed to the outer surface of the cylindrical surface portion. With this configuration, it becomes easy to connect a plurality of short tubes, and in the connected state, the outer shape can be made constant in the longitudinal direction in the region above the tip cap, and the optical fiber sensor is attached to the outer surface. By arranging the optical fiber sensors along the optical fiber sensor, the optical fiber sensor can be attached without being bent, and the strain measurement accuracy by the optical fiber sensor can be improved.

  The tubular main body of the other short tubular body excluding the distal end cap of the displacement marker tubular body is provided with a plurality of ribs formed so as to protrude outward from the cylindrical surface portion and extend in the axial direction. However, it is preferable because the strength can be increased.

  The optical fiber sensor includes means for fixing the optical fiber sensor to the displacement marker tube, a pressing plate for pressing the optical fiber sensor against the displacement marker tube, and a screw for attaching the pressing plate to the displacement marker tube. It is preferable that the screw also serves as a screw for fixing two short tubes connected in a nested manner to each other. With this configuration, the fixing of the optical fiber sensor and the fixing of the joined portions of the short tubular bodies can be performed simultaneously, and the installation work can be facilitated.

  Hereinafter, preferred embodiments of the present invention shown in the drawings will be described. FIG. 1 is a schematic cross-sectional view showing a ground strain detection system using a ground strain detection end according to an embodiment of the present invention. 1 is a ground, 2 is a vertical hole formed in the ground 1, and 3 is an embodiment of the present invention. It is a ground strain detection end according to an example. The ground strain detection end 3 is disposed in a state where a substantially cylindrical displacement marker tube 5 inserted into the vertical hole 2 and tension is applied to the outer peripheral surface of the displacement marker tube 5 along the axial direction. The optical fiber sensor 6 etc. which were fixed to the displacement marker tube 5 with the fixing jig 7 are provided at a plurality of locations. After being installed in the vertical hole 2, the filler 8 such as grout is placed in the vertical hole 2 and the displacement marker tube. By being filled in 5, it is integrated with the ground. In this embodiment, four optical fiber sensors 6 are attached at four locations in the circumferential direction of the displacement marker tube 5, and each optical fiber sensor 6 makes a U-turn at the lower end of the displacement marker tube 5. ing. Further, the upper ends of the four optical fiber sensors 6 are connected in series by a connecting tool 10 of a connecting portion 9 provided on the ground, and both ends thereof are measured by a connecting optical fiber 11 (not shown). It is connected to the. As the measuring device, a BOTDR (Brillouin Optical Time Domain Reflectometer) that measures the strain distribution in the longitudinal direction of the optical fiber by using the Brillouin scattered light in the optical fiber can be used, whereby four optical fiber sensors can be used. 6 can measure the strain distribution in the lengthwise direction generated in 6.

  Next, the ground strain detection end 3 will be described in detail. 2 is a schematic perspective view showing the vicinity of the lower end of the displacement marker tube 5 of the ground strain detection end 3 and the optical fiber sensor 6 attached thereto, and FIG. 3 is a schematic perspective view showing the part shown in FIG. 4 (a) is a schematic cross-sectional view showing the lower end portion of the displacement marker tube 5 in an exploded manner, and FIG. 4 (b) is a schematic cross-sectional view taken along line AA in FIG. The displacement marker tube 5 is formed by connecting a plurality of short tubes 15 having the same shape and a short tube (hereinafter referred to as a tip cap) 15A which is disposed at the lower end and forms a tip cap. The tip cap 15A is made of aluminum, and has a cylindrical main body 16, a cylindrical socket 17 fitted with a part protruding from the upper end thereof, and fixed by welding, and a lid fixed by welding at the lower end. A member 18 is provided. This socket 17 is for nesting and connecting to the adjacent short tube body 15 and is provided with a screw hole 19 for fixing. Further, the main body 16 is formed with four slits 20 for passing the U-turn portion at the lower end of the optical fiber sensor 6 at positions equally spaced in the circumferential direction so as to extend downward in the axial direction from the upper end of the main body 16. Has been.

  The short tube body 15 is also made of aluminum, and includes a cylindrical main body 21 and a cylindrical socket 22 that is fitted with a part protruding from the upper end and fixed by a screw 23. The main body 21 has a structure including a cylindrical surface portion 21a and four ribs 21b formed to protrude outwardly from the cylindrical surface portion 21a and to extend in the axial direction at four locations equally spaced in the circumferential direction. In addition, it has an inner diameter capable of fitting the socket 17 provided on the tip cap 15A. Thus, the short tubular body 15 can be telescopically connected to the tip cap 15A. Further, the socket 22 attached to the short tubular body 15 can be fitted to the main body 21 of the other short tubular body 15, and therefore the short tubular body 15 is socketed to the other short tubular body 15. 21 can be connected in a nested manner. The cylindrical surface portion 21a of the main body 21 of the short tubular body 15 is made smaller in diameter than the outer diameter of the cylindrical main body 16 of the tip cap 15A, and is an optical fiber disposed through the slit 20 of the main body 16 through the U-turn portion at the lower end. The sensor 6 can be arranged along the outer surface of the cylindrical surface portion 21a without bending. This socket 22 is provided with a screw hole 24 for fixing at a portion to be fitted to another short tube body 15. Further, the main body 21 is formed with a screw hole 25 at a position corresponding to the screw hole 19 or 24 provided in the socket 17 fitted to the main body 21 or the socket 22 of the other short tubular body 15. .

  The screw holes 19 and 24 and the fool hole 25 at a position overlapping with the screw holes 19 and 24 are formed at positions where the optical fiber sensor 6 attached along the short tubes 15A and 15 is sandwiched, and the fixed treatment for fixing the optical fiber sensor 6 is performed. It is also used for attaching the tool 7. That is, the fixing jig 7 has a holding plate 30 and a screw 31, the optical fiber sensor 6 is arranged along the short tubes 15 </ b> A and 15, the holding plate 30 is pressed thereon, and the screw 31 is attached. The optical fiber sensor 6 can be fixed to the short tube body 15 with the presser plate 30 by screwing into the screw holes 19 and 24 through the flaw hole 25, and the short tube bodies 15A and 15 or the short tube body with the screw 31. 15, 15 can be fixed to each other.

  If the length of the short tubular body 15 is too short, the number of used parts increases and costs increase, and the amount of connection work increases. If the length is too long, handling becomes difficult, and sufficient space for construction is provided above the vertical hole 2. If it cannot be secured, the connection work cannot be performed. In consideration of these, the length of the short tubular body 15 may be selected. However, in many cases, about 1 m is preferable, and in this embodiment, the length of the main body 21 of the short tubular body 15 is set to 1 m. . As can be seen from FIG. 2, in this embodiment, the optical fiber sensor 6 is fixed at one location for each short tube 15, and the interval between the fixed location and the fixed location of the optical fiber sensor 6 is the short tube 15. Is equal to the length of the main body 21 (that is, 1 m). Since this interval is a unit for detecting a strain position when detecting ground strain, it may be determined in accordance with the required accuracy of detecting the ground strain occurrence position. In this embodiment, the detection unit is 1 m. The structure shown in FIG. If it is desired to set the detection unit to a different length, the attachment position of the presser plate 30 may be changed appropriately.

  As can be seen from FIG. 3, the optical fiber sensor 6 has a lower end in a loop shape, and the other portions are aligned and bundled with tape or the like (not shown). The optical fiber sensor 6 may be composed only of an optical fiber, but in this embodiment, the optical fiber is made of a corrosion-resistant alloy sheath in order to prevent breakage, deterioration, etc. during construction and use. The one that is covered is used.

  Next, a method for installing the ground strain detection end 3 having the above configuration will be described. FIG. 5 shows the stopper 35 used when the ground strain detection end 3 is installed. The stopper 35 includes a pair of clamp members 36 that can be attached to the outer surface of the short tube body 15, and the clamp members 36 are formed at arc ends 36 a for clamping the outer periphery of the short tube body 15 and both ends thereof. A flange portion 36b and a handle 36c on the back surface are provided. And as shown to Fig.5 (a), a short pipe 15 or 15A is pinched | interposed with a pair of clamp members 36, and the flange part 36b is connected with the volt | bolt 37, and it can attach to the short pipe 15 firmly. . Here, as shown in FIG. 7, the flange portion 36 b and the handle 36 c of the stopper 35 are made to have a size larger than the diameter of the vertical hole 2 formed in the ground, and therefore, the short length inserted into the vertical hole 2. When the stopper 35 is attached to the pipe body 15 or 15A, the stopper 35 is supported by the ground around the vertical hole 2 or the support plate 38 placed on the ground, so that the short pipe body 15 can be prevented from falling into the vertical hole 2. . At least two sets of the stoppers 35 are prepared.

  First, in order to install the ground strain detection end 3, a vertical hole 2 (see FIG. 1) is excavated, and a protective casing is inserted into the inner surface of the vertical hole 2 as necessary. Next, as shown in FIG. 6, the lower end of the short tube body 15 is inserted and joined to the socket 17 of the tip cap 15A, the loop portion at the lower end of the optical fiber sensor 6 is passed through the slit 20, and the other portions are short. It is drawn upward along the tube body 15. Then, the presser plate 30 is pressed against the short tubular body 15 from above the optical fiber sensor 6 and fixed with screws 31. Thereby, the optical fiber sensor 6 is firmly fixed by the presser plate 30 and the socket 17 of the tip cap 15A and the short tube body 15 are fixed by the screw 31.

  Next, as shown in FIG. 7, the first stopper 35 is attached to the upper end portion of the tip cap 15A, the tip cap 15A is inserted into the vertical hole 2, and the stopper 35 is placed on the support plate 38 on the ground. Support. Next, the next short tube body 15 is connected onto the short tube body 15, the optical fiber sensor 6 is disposed along the short tube body 15, and tension applying means (not shown) is provided to the optical fiber sensor 6. ) Is applied, and in this state, the presser plate 30 is pressed against the short tubular body 15 from above the optical fiber sensor 6 and fixed with a screw 31 (see FIG. 3). Thereby, the optical fiber sensor 6 is firmly fixed by the presser plate 30 and the short tubular bodies 15 and 15 are fixed by the screw 31. In addition, a predetermined tension T is applied to the optical fiber sensor 6 in the region sandwiched between the press plates 30, 30 arranged above and below. Such a fixing operation of the optical fiber sensor 6 is sequentially performed on the four optical fiber sensors 6 arranged around the short tube body 15, and when this is finished, the upper end portion of the lower short tube body 15 is placed on the upper end portion. Install the second stopper 35.

  Thereafter, with the short tubular body 15 joined together being supported by the hand, the lower stopper 35 is removed, and the connected short tubular body 15 is inserted into the vertical hole 2 until the upper stopper 35 reaches the support plate 38 on the ground. Go down inside. Then, after the connected short tube body 15 is supported by the stopper 35, the next short tube body 15 is connected to the short tube body 15 in the same manner as the previous time, and the tension T is applied to the optical fiber sensor 6. In the applied state, the holding plate 30 is attached and the optical fiber sensor 6 is fixed. Thereafter, by repeating the same operation, the short tubes 15 are connected one after another, fixed to the optical fiber sensor 6 with a constant tension T, and the connected short tubes 15 are placed in the vertical holes 2. You can take it down. During this operation, the connected short tube 15 is always supported by the stopper 35, so that it can be safely operated without being accidentally dropped into the vertical hole 2. In addition, the operation of joining the short tubular bodies 15 and the operation of fixing the optical fiber sensor 6 with the presser plate 30 in a tensioned state are performed in a state where the connected short tubular bodies 15 are supported by the stoppers 35. Easy to implement. Furthermore, since the short tubular body 15 is connected and lowered into the vertical hole 2, a large work space is not required above the vertical hole 2. That is, if there is a working space for connecting the two short tubes 15, 15 above the vertical hole 2 and arranging a means for applying tension to the optical fiber sensor 6, the work is possible. You can work in a narrow space such as in a tunnel.

  After the tip cap 15A and the short tubular body 15 connected thereto are lowered to a desired depth in the vertical hole 2, as shown in FIG. 1, the four optical fiber sensors 6 are pulled out, and the inside of the vertical hole 2 and the short length The tube 15 is filled with a filler 8 such as grout, and the short tube 15 and the optical fiber sensor 6 are integrated with the ground 1. Thereafter, the four optical fiber sensors 6 drawn upward are connected in series with each other and connected to a measuring instrument (not shown). Thus, the laying operation of the ground strain detection end 3 is completed.

  The ground strain detection end 3 installed as described above is integrated with the ground 1. A measuring instrument (not shown) connected to the optical fiber 11 connected to the optical fiber sensor 6 measures the scattered light distribution in the optical fiber sensor 6 at an appropriate timing, and determines the length in the optical fiber sensor 6. The strain distribution in the direction is monitored. If the ground 1 forming the vertical hole 2 is distorted, the displacement marker tube 5 installed in the vertical hole 2 is distorted according to the distortion of the ground 1. For example, when a landslide-like distortion occurs at a substantially intermediate position of the vertical hole 2, the displacement marker tube 5 bends in accordance with the ground at that portion. Due to the bending of the displacement marker tube 5, the optical fiber sensor 6 is also bent, causing a distortion in the length direction. At this time, in the four optical fiber sensors 6 along the displacement marker tube 5, the magnitude of strain differs depending on the mounting position (depending on the direction of bending of the displacement marker tube 5), and the length direction The magnitude of distortion varies depending on the position. Accordingly, by measuring the strain distribution in the optical fiber sensor 6 with a measuring instrument (not shown) connected to the optical fiber 11, the magnitude, direction, etc. of the strain (bending) of the displacement marker tube 5 is detected. The magnitude and direction of ground strain can be detected. As described above, the distortion of the ground 1 in which the vertical holes 2 are formed can be monitored, and the ground collapse can be predicted.

  In the above description, when connecting the short tubes 5, the connected portion is lowered into the vertical hole 2 each time one short tube 5 is connected, but it is large above the vertical hole 2. If the work space can be secured, the connected parts may be lowered into the vertical hole 2 after the plurality of short tubes 5 are joined together. In the above embodiment, the case where the ground strain detection end 3 is installed in the vertical hole 2 has been described. However, the ground strain detection end 3 is not limited to the vertical hole 2 and may be a horizontal hole, an oblique hole, or the like. In addition, when attaching the ground distortion | strain detection end 3 to the horizontal hole extended horizontally, the stopper 35 does not need to be used.

DESCRIPTION OF SYMBOLS 1 Ground 2 Vertical hole 3 Ground strain detection end 5 Displacement marker tube 6 Optical fiber sensor 7 Fixing jig 8 Filler 15 Short tube 15A Short tube (tip cap)
16, 21 Body 17, 22 Socket 19, 24 Screw hole 25 Buff hole 30 Presser plate 31 Screw 35 Stopper 36 Clamp member 37 Bolt 38 Support plate

Claims (3)

  Displacement marker tube inserted into a hole formed in the ground where strain is to be detected, which is formed by nesting together a plurality of short tubes, and arranged in a tensioned state along the displacement marker tube And a strain detection optical fiber sensor fixed at a plurality of locations to the displacement marker tube, and a short tube disposed at the lower end of the displacement marker tube constitutes a tip cap and is cylindrical. The main body and a lid member fixed to the lower end thereof are provided, and the cylindrical main body is formed with a slit for passing a U-turn part at the lower end of the optical fiber sensor extending downward in the axial direction from the upper end of the main body. The other short tube body of the displacement marker tube body includes a cylindrical main body having a cylindrical surface portion having a diameter smaller than the outer diameter of the cylindrical main body of the tip cap, and the optical fiber sensor is The U-turn part at the lower end is A portion formed above the U-turn portion is disposed along the outer surface of the cylindrical surface portion of another short tubular body, and a plurality of locations are fixed to the outer surface of the cylindrical surface portion. Ground strain detection end, characterized by being provided.   The tubular main body of the other short tubular body excluding the distal end cap of the displacement marker tubular body has a plurality of ribs formed so as to protrude outward from the cylindrical surface portion and extend in the axial direction. The ground strain detection end according to claim 1, wherein:   The means for fixing the optical fiber sensor to the displacement marker tube includes a pressing plate for pressing the optical fiber sensor against the displacement marker tube and fixing the screw, and a screw for attaching the pressing plate to the displacement marker tube. The ground strain detection end according to claim 1 or 2, wherein the screw also serves as a screw for mutually fixing two short tubular bodies connected in a nested manner.

Images