Pore ​​water pressure gauge unit

Abstract

To solve the problem of conventional technique that pore water pressure measurement for multiple aquifers at a landslide location or the like requires an expensive device and drilling of a large-diameter borehole, which incurs costs of drilling and installing pore water pressure gauges.SOLUTION: Pore water pressures of multiple aquifers at different depths in a borehole are measured by providing a pipe through the inside of pore pressure gauges and feeding a composite cable (consisting of power lines and signal lines) for the pore water pressure gauges installed below through the pipe, or measuring pore water pressures inside the pipe and feeding the composite cable for the pore water pressure gauges installed below along the side of the pipe.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a pore water pressure gauge unit for measuring the pore water pressure in aquifers in landslide areas and the like.

Background Art

[0002] In May 2014, the National Institute of Technology and Evaluation, a national research and development agency, created "Guidelines (Draft) for Pore Water Pressure Observation Using Partial Strainer Holes" (Non-Patent Document 1). According to the field surveys conducted by the National Institute of Technology and Evaluation in landslide areas, in many sites, observation using full-section strainer holes is adopted, and it is said that the pore water pressure on the slip surface cannot be correctly observed. In the case of observation using full-section strainer holes, the pore water pressure is observed based on the water pressure or water level in the boring hole. The same document states that there are three types of methods for pore water pressure observation in landslide areas: 1) full-section strainer holes, 2) partial strainer holes, and 3) embedded pore water pressure gauges. 1) does not necessarily correctly observe the pore water pressure in the target aquifer. 2) can observe the pore water pressure in the aquifer near the slip surface at the target depth. 3) requires technology for installation and maintenance, etc. on the slip surface at a deep depth. As such, it has become clear, and the spread of method 2) is expected. In addition to this, 3) is also a factor hindering the spread because the pore water pressure gauge is expensive. In the case of Patent Document 1, although the pore water pressure in the target aquifer can be measured, it is necessary to drill a boring hole with a large diameter (φ80 mm or more), and a high cost is required for the drilling work. On the other hand, there is also a pore water pressure gauge that is pressed into the ground without drilling a boring hole, as in Patent Document 2, but according to Non-Patent Document 1, it is said that technology is required for installation and maintenance, etc. on the slip surface at a deep depth.

[0003] Recently, the Japan Meteorological Agency has frequently issued warnings about "unprecedented heavy rainfall," leading to a surge in landslides and levee breaches. Levee breaches cause immense damage, making levee reinforcement a top national priority. The Public Works Research Institute, aiming to promote the qualitative improvement of river levees, created the River Levee Monitoring Technology Guidelines (draft) (September 30, 2020), which pointed out the indispensability of observing the water level (infiltration line) within the levee and developed and released a driven-in water level observation well method (Non-Patent Literature 2, Patent Literature 3). From the photographs in the publicly released materials from Non-Patent Literature 2, it appears that the water level gauge has a diameter of about 30 mm, making it difficult to drive into deep parts of the levee. Furthermore, a drawback is that if the depth at which the aquifer to be observed is unknown, there is no guarantee that the well can be driven to a depth where the water level related to the aquifer can be observed. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 3353714 [Patent Document 2] Japanese Patent Publication No. 11-6777 [Patent Document 3] Patent No. 5044852 [Non-patent literature]

[0005] [Non-Patent Document 1] Public Works Research Institute Material No. 4283 (May 2014) [Non-Patent Document 2] Public Works Research Institute New Technology Showcase 2020 in Tokyo (September 2020) [Overview of the project] [Problems that the invention aims to solve]

[0006] Landslide areas have numerous aquifers, and if the slip surface is not clearly defined, it is not possible to pinpoint a specific aquifer, making it impossible to perform the "observation using partial strainer holes" recommended by the Public Works Research Institute. While it is desirable to measure the pore water pressure of multiple aquifers as described in Patent Document 1, this technique requires drilling large boreholes, and the drilled boreholes are used exclusively for measuring pore water pressure, making them unsuitable for other purposes. Even in river embankments, there are multiple aquifers, and it is impossible to know the exact depth of the aquifer within the embankment without drilling boreholes. Moreover, even with the same pore water pressure gauge, the diameter of the borehole to be drilled differs between landslide areas where the boreholes are deep and embankments where the depth is limited to about 15m. Therefore, it is necessary to select a pore water pressure gauge unit with an outer diameter suitable for the diameter of each borehole and an installation method suitable for the installation depth.

[0007] In landslide boreholes, ground deformation is measured using pipe-type strain gauges and inclinometers, and it is preferable that these observations and the measurement of pore water pressure in the aquifer near the slip surface can be performed in the same borehole. Furthermore, it is preferable that the pore water pressure of multiple aquifers at different depths can be measured in the same borehole. The same applies to dam bodies, where a multi-stage pore water pressure gauge capable of measuring the pore water pressure of multiple aquifers is preferable. Considering these circumstances, a unitized pore water pressure gauge (integrating a strainer and pore water pressure gauge) capable of measuring the pore water pressure of the target aquifer is preferable. Hereafter, the unitized pore water pressure gauge will be referred to as a pore water pressure gauge unit or simply a unit. It is necessary to connect the pore water pressure gauge unit to the ground-level equipment (data processing equipment, communication equipment, power supply equipment, etc.) with signal lines and power lines (hereafter, signal lines, etc. will be referred to as composite cables). However, if the borehole is deep, the composite cable becomes long, making installation difficult. In such cases, it is preferable to have a configuration in which composite cables for pore water pressure gauges, pipe-type strain gauges, inclinometers, etc. installed below can pass through the unit, such as by providing a dedicated pipe for the composite cable inside the pore water pressure gauge unit. With such a configuration, even when installing a large number of observation devices in the same borehole, the composite cable can be housed inside the unit, simplifying the installation work. Furthermore, the unit allows for the observation of pore water pressure in the aquifer, while simultaneously observing ground deformation with a pipe-type strain gauge and ground inclination with an in-hole inclinometer, enabling multi-purpose comprehensive observation in the borehole. [Means for solving the problem]

[0008] To measure the pore water pressure of an aquifer, a through-hole is made in the lower wall of a fixed-length pipe that is closed at both ends, and this fixed-length pore water pressure gauge is installed in the aquifer of the borehole. Then, the space between the pore water pressure gauge and the borehole wall (the pipe wall) is sealed at both ends of the aquifer. When this is done, water from the aquifer permeates into the pipe through the through-hole in the lower pipe wall, and the air inside the pipe is compressed by the permeated groundwater, causing the water level inside the pipe to rise. When the pressure of the compressed air and the permeated groundwater balances out and the pipe comes to a standstill, the internal pressure is measured. Since the internal pressure and water level correspond 1:1, the water level can also be measured and converted to pressure. With a fixed-length pore water pressure gauge unit of this type, the pore water pressure of the aquifer can be measured by installing this unit at the target depth of the aquifer in the borehole and sealing the space between the unit and the borehole wall at both ends of the unit. Furthermore, by passing composite cables, such as those for pore water pressure gauges installed below, through this unit, multiple pore water pressure gauges can be installed in multiple aquifers at different depths within the same borehole, and pipe-type strain gauges and in-hole inclinometers can also be installed. When conducting geological surveys during house construction, there is a method called the Swedish Sounding Test (SWS) which involves drilling small boreholes. This method allows for inexpensive borehole drilling, sample acquisition as needed, and determination of the aquifer depth. Therefore, when observing pore water pressure, after conducting an SWS to determine the aquifer depth, a small-diameter pore water pressure gauge unit can be installed in the drilled borehole to measure the pore water pressure in the aquifer at the known depth. This method is suitable for observing pore water pressure in dam bodies where shallow boreholes are acceptable. To utilize boreholes drilled using the SWS method, it is necessary to manufacture a pore water pressure gauge with an outer diameter of approximately 15 mm.

[0009] Boring holes are often drilled vertically, and drilling costs can be reduced by using a smaller diameter bore. Therefore, a pore water pressure gauge unit is preferably a long, vertically elongated pipe-shaped unit with a small outer diameter. [Effects of the Invention]

[0010] By inserting the above-mentioned pore water pressure gauge unit into a borehole and sealing it above and below a known aquifer, groundwater from the aquifer will seep into the unit through through-holes in the pipe wall, balancing with the internal pressure and becoming stationary. By observing the pressure inside the installed unit, the pore water pressure of the known aquifer can be measured. When installing a unit, it is necessary to connect it to the ground-level equipment (data processing unit, communication equipment, power supply unit, etc.) with a composite cable. If the composite cable is long, it can hinder the installation of the observation equipment, making the process difficult. When installing multiple units in the same borehole, the installation process becomes even more difficult. This is because the composite cable of the lower unit passes along the side of the upper unit, making it difficult to prevent water from entering the aquifer. However, if the composite cable of the lower unit passes through the inside of the unit, the installation of the composite cable and the unit becomes simpler. By installing the above unit in the aquifer near the slip surface at the bottom of the borehole and sealing the area above it, pore water pressure observation using a partial strainer hole, as recommended by the Public Works Research Institute in Non-Patent Document 1 for the purpose of widespread adoption, can be performed. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1A is an overall view of the first embodiment, Figure 1B is an axial cross-sectional view of the first embodiment (with the direction of the borehole as the axial direction), and Figure 1C is a cross-sectional view of the first embodiment perpendicular to the axial direction. [Figure 2] Figure 2A is an overall view of the second embodiment, and Figure 2B is an axial cross-sectional view of the second embodiment (with the direction of the borehole defined as the axial direction). [Figure 3] Figure 3 is an axial cross-sectional view of the third embodiment. [Figure 4] Figure 4A is an axial cross-sectional view of the fourth embodiment, and Figure 4B is a cross-sectional view perpendicular to the axial direction of the fifth embodiment. [Figure 5] Figure 5A is an axial cross-sectional view of the fifth embodiment, and Figure 5B is a cross-sectional view of the fifth embodiment perpendicular to the axial direction. [Figure 6]Fig. 6A is an axial cross-sectional view of the sixth embodiment, and Fig. 6B is a cross-sectional view perpendicular to the axial direction of the sixth embodiment. [Figure 7A] Fig. 7A is an axial cross-sectional view of the seventh embodiment. [Figure 7B] Fig. 7B is a cross-sectional view perpendicular to the axial direction of the seventh embodiment. [Figure 8] Fig. 8 is a conceptual diagram of the state where the pore pressure gauge is installed in the boring hole.

Embodiments for Carrying Out the Invention

[0012] This invention has been made to solve the above-described problems. This invention is defined as follows. That is, a pore pressure gauge unit for measuring the pore pressure of an aquifer, A measuring pipe having an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, and a through hole provided at the lower part of the pipe wall, A water pressure specifying part for specifying the pore pressure based on the water level of the water in the aquifer that has entered the measuring pipe through the through hole, A pore pressure gauge unit comprising the above.

[0013] When the pore pressure gauge unit defined as above is present in the aquifer, the water in the aquifer enters the first pipe through the through hole provided in the pipe wall. The air inside the observation pipe is compressed by the infiltrated groundwater. Then, the pressure of the compressed air rises until it balances with the pore pressure of the aquifer. In the state where the pressures are balanced, the water level of the water that has entered the observation pipe stabilizes. Based on this water level, the pore pressure of the aquifer can be measured. For example, since the pressure in the space defined by the water surface defined by this water level, the upper wall of the observation pipe, and the pipe wall balances with the pore pressure, the pore pressure can be specified by measuring the pressure in this space. In the above, the through hole only needs to have a function of allowing the groundwater in the aquifer to pass through. Therefore, it may be formed in a slit shape from the lower edge of the observation pipe. A water-permeable filter may be provided to prevent foreign matter from entering the aquifer.

[0014] Furthermore, by measuring the change in distance between the water level, defined by the water level, and the upper wall of the observation pipe, the volume ratio between the volume of air in the space corresponding to the water level height balanced with the pore water pressure and the volume of air in the initial state at the time of installation can be determined. Once the volume ratio is determined, Boyle's Law (at a constant temperature, the volume of a given amount of gas is inversely proportional to its pressure) can be applied to determine the ratio to the pressure of the air in the initial state. From this ratio, the pressure of the air inside the observation pipe can be determined, and the pore water pressure in the aquifer that is balanced by this pressure can be identified.

[0015] Since the water that enters the observation pipe is essentially a dielectric, by arranging a pair of electrodes on either side of it, the change in water level and the capacitance of the pair of electrodes become proportional. As previously described, the pore water pressure can be determined from the change in water level (change in air volume), so if the relationship between the capacitance and the pore water pressure is predetermined, the pore water pressure can be determined from the measurement of the capacitance. Focusing on the fact that the water entering the observation pipe also has some conductivity, this water is used as one of the electrodes. The change in capacitance between this electrode and an electrode placed inside the observation pipe and covered with an insulator corresponds to the change in water level. Based on this change, the pore water pressure can be determined in the same way as described above.

[0016] If the water pressure identification section provided in the observation pipe consists of, for example, a pair of electrodes, it is easy to create space inside it. As a result, a composite cable can be easily passed through the observation pipe in the axial direction. A second pipe through which this composite cable is inserted can also be passed through the observation pipe in the axial direction. By passing a composite cable through, multiple observation pipes can be connected in serial order. The water pressure detection unit receives power and control signals from this composite cable, and also sends the electrical signals of the measurement results to a processing unit (not shown) via this composite cable.

[0017] Observation pipes, which can be connected serially by passing a composite cable through them, can be positioned in multiple aquifers within a single borehole via spacers. The spacers are adjustable in length and capable of passing through the composite cable. The boreholes used can be those drilled using the Swedish sounding test. The unit of this invention can be designed to have any diameter, and as a result, it can be easily inserted into boreholes drilled using the general-purpose test described above.

[0018] This invention can also be defined as follows (first aspect): A pore water pressure gauge unit comprising a first pipe, which is closed at the top and bottom and has a through-hole in the lower pipe wall, and a second pipe through which a composite cable of a pressure measuring means and an observation device installed below passes, wherein the pressure inside the first pipe is measured by the pressure measuring means.

[0019] If the pore water pressure gauge unit is as defined in the first phase, after it is inserted into the borehole, if the space between the outer wall of the unit and the borehole wall is sealed above and below the unit (by a sealing method not described in detail here), groundwater in the aquifer will permeate into the first pipe through through holes provided in the pipe wall of the first pipe, and the air inside the first pipe will be compressed by the permeated groundwater. The pressure of the compressed air will then rise until it balances with the pore water pressure of the aquifer. When the pressures are balanced, the pore water pressure of the aquifer can be determined by measuring the air pressure or water pressure inside the first pipe with a pressure measuring device.

[0020] The second aspect of this invention is defined as follows: A pore water pressure gauge unit comprising a first pipe, which is closed at both ends and has a through-hole in the lower pipe wall, a third pipe, which is closed at the top and allows groundwater that has permeated into the first pipe to flow into it, and a pressure measuring means, wherein a composite cable of an observation device installed below passes between the first pipe and the third pipe, and the pressure inside the third pipe is measured by the pressure measuring means.

[0021] In the pore water pressure gauge unit defined in the second phase, if groundwater that has seeped in through the through hole below the first pipe is allowed to seep into the third pipe, the air inside the third pipe will be compressed, similar to the first phase, and the pressure of the air inside the third pipe will rise until it balances with the pore water pressure of the aquifer. Once the pressures are balanced, the pore water pressure of the aquifer can be determined by measuring the pressure inside the third pipe using a pressure measuring device.

[0022] When the pressure of compressed air in the first and third pipes balances the pore water pressure of the aquifer, the water level of groundwater that has infiltrated into the first and third pipes also rises. From the height of the rise in water level, the volume ratio of the volume of air inside the pipe to the volume of air in the initial state at the time of installation can be determined. Once the volume ratio is determined, Boyle's Law (at a constant temperature, the volume of a given amount of gas is inversely proportional to its pressure) can be applied to find the ratio to the pressure of the air in the initial state. From this ratio, the pressure of the air inside the pipe can be determined, and the pore water pressure in the aquifer that balances this pressure can be determined. By passing the composite cable of observation devices such as pore water pressure gauges installed below through the second pipe, or between the first and third pipes, multiple pore water pressure gauges can be installed in the same borehole.

[0023] The third aspect of this invention is defined as follows: A pore water pressure gauge unit that calculates the pressure in the first and second phases from the water level of groundwater that has permeated into the pipe, detected by a water level measuring device.

[0024] In the pore water pressure gauge unit defined in the third phase, if the shape of the cross-section perpendicular to the axial direction of the borehole is constant regardless of height, the volume of compressed air can be determined from the water level due to groundwater infiltration into the first or third pipe. The pressure of the compressed air can be determined by applying Boyle's Law to the compression ratio between the volume of compressed air and the volume of air in its initial state. Then, the pore water pressure of the aquifer that balances the pressure of the compressed air can be determined. In other words, the pore water pressure of the aquifer can be calculated from the water level detected by the water level detection means.

[0025] The fourth aspect of this invention is defined as follows: A pore water pressure meter unit comprising a first pipe, which is closed at the top and bottom and has a through hole in the lower pipe wall, a first electrode covered with an insulating material on its inner surface, a second pipe through which a composite cable for an observation device installed below passes, and a second electrode covered with an insulating material on its outer surface, which detects changes in electrical capacitance due to groundwater seeping between the first electrode and the second electrode, detects the groundwater level from the change in electrical capacitance, and calculates the pore water pressure of the aquifer from the detected water level.

[0026] In the pore water pressure gauge unit defined in the fourth phase, the first electrode provided on the inner surface of the first pipe and the second electrode provided on the outer surface of the second pipe are electrically insulated from the first and second pipes, and also from the groundwater seeping into the first pipe. As a result, capacitance is generated between the first and second electrodes, and the capacitance changes in accordance with the height of the groundwater seeping between the electrodes. If this changing capacitance is incorporated into an oscillation circuit and oscillated, the frequency output from the circuit will change in accordance with the change in capacitance. If the relationship between the change in output frequency and the change in water level in the first pipe is determined in advance, the water level in the first pipe can be detected from the change in frequency using that relationship equation. From the detected change in water level, the compression ratio of the air inside the first pipe can be determined, and the pressure of the compressed air, in other words, the pore water pressure of the aquifer, can be calculated using Boyle's Law. In this case, water with a high dielectric constant (close to 80 at room temperature) is interposed between the electrodes installed inside the first pipe, and the change in capacitance accompanying the change in water level is large, making it easy to detect the change in water level. Even if a composite cable for other observation devices, such as a pore water pressure gauge installed below, passes through inside the second pipe, the composite cable is inside the second electrode, so it does not affect the change in capacitance, and the change in water level can be detected.

[0027] To determine the pore water pressure of the aquifer using the method in the fourth phase, it is sufficient to detect the change in capacitance corresponding to the change in the water level inside the first pipe using the first and second electrodes installed inside the first pipe. Therefore, even if the inner surface of the first pipe is divided axially (for example, into three 120mm square sections) and the first and second electrodes are placed at opposing positions, and the change in capacitance formed between these electrodes is detected, the water level inside the first pipe can be determined, and the pore water pressure of the aquifer can be calculated from the detected water level.

[0028] The fifth aspect of this invention is defined as follows: A first pipe, which is closed at the top and bottom and has a through-hole in the lower pipe wall, contains a second pipe through which a composite cable of an observation device installed below it passes, and a first electrode with a viewing angle of 180 degrees or less, covered with insulating material and provided opposite the inner surface of the first pipe, and a second electrode, A pore water pressure meter unit that detects changes in electrical capacitance associated with changes in the water level of groundwater that has seeped between the first electrode and the second electrode, detects the water level of groundwater from the change in electrical capacitance, and calculates the pore water pressure of the aquifer from the detected water level.

[0029] In the pore water pressure gauge unit defined in the fifth phase, the first and second electrodes are insulated, and capacitance is generated between the first and second electrodes. As the amount of dielectric changes in accordance with the height of the infiltrated groundwater, the capacitance changes. If this capacitance is incorporated into an oscillation circuit and oscillated, the frequency output from the circuit changes in accordance with the change in capacitance. Then, using the previously determined relationship between water level and frequency change, the water level change can be determined from the frequency change, and the pore water pressure of the aquifer can be calculated from the detected water level.

[0030] In the fourth and fifth phases, the second pipe exists inside the first pipe, and a composite cable for the observation device installed below passes through the second pipe. However, the relative permittivity of the composite cable material is nearly an order of magnitude smaller than that of water. Therefore, in the capacitance formed between the first and second electrodes, the proportion of capacitance accounted for by water, which has a high relative permittivity, is large, making it easier to detect changes in water level.

[0031] In the fourth and fifth phases, the pore water pressure is measured by using the increase or decrease in capacitance accompanying changes in water level, with groundwater sandwiched between the first and second electrodes as the dielectric. Since groundwater is a conductor, it can be used as the second electrode. In this case, the insulating material covering the first electrode becomes the dielectric, and capacitance is formed between the first electrode and the groundwater acting as the second electrode. The size of the second electrode changes in response to changes in water level, and the capacitance changes in response to this change.

[0032] The sixth aspect of this invention is defined as follows: A first pipe, which is closed at the top and bottom and has a through-hole in the lower pipe wall, contains a second pipe through which a composite cable of an observation device installed below it passes, and a first electrode covered with an insulating material is provided on the inner surface of the first pipe. A pore water pressure meter unit that uses groundwater that has permeated into the first pipe as the second electrode, detects the change in electrical capacitance associated with the change in the water level of the permeating groundwater formed by an insulating material sandwiched between the first and second electrodes, detects the water level of the groundwater from the change in electrical capacitance, and calculates the pore water pressure of the aquifer from the detected water level.

[0033] In the case of a pore water pressure gauge unit as defined in the sixth phase, the area of ​​the second electrode in contact with the insulating material covering the first electrode changes in accordance with the height of groundwater that has infiltrated the unit, and the capacitance between the first electrode and the groundwater (the second electrode) changes. In this case, if the thickness of the insulating material covering the first electrode is constant and the shape of the horizontal cross-section within the unit is the same regardless of height, the capacitance changes in accordance with the height of the infiltrated groundwater. In other words, the water level can be detected from the change in capacitance, and the pore water pressure of the aquifer can be calculated from the detected water level.

[0034] With the pore water pressure gauge unit from the first to the sixth phase, even in deep boreholes, pore water pressure can be observed using a partial strainer hole, as recommended by the Public Works Research Institute in Non-Patent Document 1 for the purpose of widespread adoption. [Examples]

[0035] The first embodiment will be explained with reference to Figures 1A, 1B, and 1C. Figure 1A is an overall view of the first embodiment, Figure 1B is a cross-sectional view in the axial direction, and Figure 1C is a cross-sectional view in a direction perpendicular to the axis. Although not shown here, the space between the unit and the borehole wall is sealed above and below, and groundwater from the aquifer located in the sealed intermediate area flows in through the through hole 3 at the bottom of the first pipe (observation pipe) 1, creating a state where the air pressure and the pressure of the infiltrated groundwater (pore water pressure of the aquifer) are balanced within the first pipe 1. Figure 1B shows the position of the water level L. A sealed space S is formed between the water surface L, the pipe wall 5 and the upper wall 7 of the first pipe 1. The water surface L rises and falls in response to changes in pore water pressure, and the pressure in the sealed space S changes depending on the position of the water surface L. This pressure is measured by the space pressure measuring unit 10. The electrical signal of the measurement result is sent to a composite cable (not shown) via a cable (not shown). This composite cable is inserted into the second pipe 11, which penetrates the upper wall 7 and lower wall 9 of the first pipe 1. In this first embodiment, the air pressure measuring unit 10 is located in the upper air portion. However, if the air pressure and the submerged groundwater pressure (pore water pressure of the aquifer) are in equilibrium, the pore water pressure of the aquifer that is in equilibrium with that pressure can be determined from the measured pressure, even in the lower groundwater or in the area between the groundwater and the air.

[0036] The second embodiment will be explained in Figures 2A, 2B, and 2C. Elements identical to those described in Figure 1 will be given the same reference numerals, and their descriptions will be omitted. Figure 2A is an overall view of the second embodiment, Figure 2B is a cross-sectional view in the axial direction, and Figure 2C is a cross-sectional view in a direction perpendicular to the axis. In this embodiment, a third pipe 20, through which groundwater infiltrates, is located inside the first pipe 1, with its upper end closed. A spatial pressure measuring unit 10 is located above the third pipe 20. A composite cable 15 passes between the first pipe 1 and the third pipe 20. The third pipe 20 is fixed to the first pipe 1 in a manner not shown, such that when groundwater flows into the first pipe 1, the groundwater also flows into the third pipe 20. The air inside the third pipe 20 is compressed until it balances with the pore water pressure of the aquifer. After compression to balance, the pore water pressure of the aquifer can be determined by measuring the pressure of the air inside the third pipe 20 with the spatial pressure measuring unit 10. In the third pipe 20, a sealed space S3 is formed by the water surface L3, the upper wall 27, and the pipe wall 25. In this embodiment, the water level in the first pipe 1 also rises, but there is also a method of installing a partition plate to prevent groundwater from seeping into the upper part of the first pipe 1. Even when a partition plate is installed, if groundwater seeps into the third pipe 20 and balances with the pressure of the air inside, the pore water pressure of the aquifer can be measured.

[0037] Figure 3 is an axial cross-sectional view of the third embodiment. The same reference numerals are used for elements identical to those in Figure 1, and their descriptions are partially omitted. In this third embodiment, an ultrasonic transceiver 17 is located at the top. Ultrasound is used to measure the distance to the water surface, detect the water surface L, and determine the pore water pressure from the detected water level (water surface height). If the water level of the infiltrated groundwater can be detected, the volume of compressed air inside the pipe can be determined, and the volume ratio between that volume and the volume of air in the initial state (volume of air at 1 atmosphere at the time of installation) can be determined. Then, using Boyle's law, the pressure of the compressed air can be determined from that volume ratio, and that pressure becomes the pore water pressure in the aquifer. Although not shown here, even if the ultrasonic transceiver is installed inside the third pipe 20 as in the second embodiment, the pore water pressure in the aquifer can be observed by detecting the water level inside the third pipe 20 and calculating the pressure from that water level.

[0038] The fourth embodiment will be described with reference to Figures 4A and 4B. Elements identical to those in Figure 1 are given the same reference numerals and their descriptions are omitted. Figure 4A is an axial cross-sectional view of the fourth embodiment, and Figure 4B is a cross-sectional view of the fourth embodiment in a direction perpendicular to the axis. In this fourth embodiment, a first electrode 31 covered with an insulating material on the entire inner surface of the first pipe 1 and a second electrode 33 covered with an insulating material on the entire outer surface of the second pipe 11 located inside the unit are used to detect volume changes due to groundwater seeping between these electrodes. The water level is detected from the volume change, and the pore water pressure of the aquifer is determined from the detected water level change. The method for detecting the water level from the volume change will be described below. If the water level can be detected, the compression ratio of the air can be determined as described in the second embodiment, and the pore water pressure of the aquifer can be determined from that compression ratio. Preferably, the electrodes placed inside the first pipe 1 extend across the entire axial area of ​​the first pipe 1. This ensures that even if the water level changes significantly, the area above the water surface and the area below the water surface are always separated, thereby allowing the change in volume to follow the change in water level.

[0039] Between the first electrode and the second electrode, which are composed of parallel plates, a capacitance C is generated as shown in equation (1), which is proportional to the relative permittivity ε of the material sandwiched between the electrodes, the area S of the electrodes, and inversely proportional to the distance d between the electrodes. C = ε * S / d (1) In the fourth embodiment, the first electrode 31 and the second electrode 33 are not parallel plates, but capacitance is formed by the material sandwiched between the first electrode 31 and the second electrode 33. In the case of the fourth embodiment, there is groundwater in the lower part, and the water level rises and falls in response to the increase and decrease in pore water pressure, and the capacitance increases or decreases according to the change in water level. Because the relative permittivity of water is large, even small changes in water level result in large changes in capacitance, making it easy to detect changes in water level.

[0040] Although not shown here, if the change in capacitance formed between the first electrode 31 and the second electrode 33 is incorporated into a part of the oscillation circuit not shown, the change in capacitance due to the change in water level can be detected as a change in the output frequency of the oscillation circuit. If the relationship between the change in water level and the change in the oscillation circuit frequency is determined in advance, the water level can be detected from the frequency change using that relationship, and the pore water pressure of the aquifer can be calculated from the detected water level. In this fourth embodiment, the second pipe 11 is located in the center, but the pipe does not necessarily have to be in the center; it is sufficient if it is positioned such that the change in water level and the change in electrical capacitance correspond in a 1:1 ratio. Furthermore, it is preferable to use the sensor output device proposed by the present inventors for detecting changes in capacitance (see WO2021-145428).

[0041] The fifth embodiment will be described with reference to Figures 5A and 5B. Elements identical to those in Figure 1 are denoted by the same reference numerals, and their descriptions are omitted. Figure 5A is an axial cross-sectional view of the fifth embodiment, and Figure 5B is a cross-sectional view of the fifth embodiment perpendicular to the axis. A second pipe 11 is located in the center. In this fifth embodiment, a first electrode 41 covered with a vertically elongated insulating material with a depth angle of 180 degrees or less is provided on the inner surface of the first pipe 1, and a second electrode 43 covered with an insulating material with a depth angle of 180 degrees or less is provided on the inner surface 180 degrees away from the first electrode. Changes in electrical capacitance due to changes in the water level of groundwater that has permeated between these electrodes are detected as frequency changes, the water level is detected from the frequency changes, the compression ratio of the air inside the unit is determined from the detected water level changes, and the pore water pressure of the aquifer is determined from the compression ratio.

[0042] In the fifth embodiment, both the first electrode 41 and the second electrode 43 have a viewing angle of approximately 180 degrees, but there is no limit to the size of the viewing angle. The first electrode 41 and the second electrode 43 do not contact each other but face each other, and the capacitance between the first electrode 41 and the second electrode 43 changes in response to the change in water level. In this case as well, if capacitance is incorporated into a part of the oscillation circuit (not shown), the change in capacitance can be detected as a change in the output frequency of the oscillation circuit. Then, the change in water level can be determined from the change in output frequency, and the pore water pressure of the aquifer can be calculated from that water level.

[0043] In the fifth embodiment, a first electrode 41 and a second electrode 43 are provided on the inner surface of the first pipe 1. However, as shown in Figure 2, a first electrode and a second electrode covered with an insulating material may be provided on the inner surface of the third pipe 20, and groundwater that has seeped in from the aquifer may be flowed into the third pipe 20 to detect its water level. In this case, a composite cable will pass between the first pipe 1 and the third pipe 20.

[0044] When electrodes covered with insulating material are installed inside the third pipe 20, a larger outer diameter of the third pipe 20 makes the installation of the electrodes easier. Even if the outer diameter of the third pipe 20 is increased to the point where it touches the inner diameter of the first pipe 1, it is sufficient that a composite cable for the observation device installed below can pass through the gap. In such a case, the composite cable will be arranged along the inner surface of the first pipe 1.

[0045] In the fourth and fifth embodiments, a second pipe 11 penetrates the inside of the first pipe 1, and a composite cable for an observation device installed below passes through the inside of the second pipe 11. The relative permittivity of the composite cable material is nearly an order of magnitude smaller than that of water. Therefore, in the capacitance formed between the first electrode 41 and the second electrode 43, the proportion of capacitance accounted for by water, which has a large relative permittivity, is large, making it easy to detect changes in water level.

[0046] The sixth embodiment will be explained with reference to Figures 6A and 6B. Elements identical to those in Figure 1 are given the same reference numerals and their explanations are omitted. Figure 6A is an axial cross-sectional view of the sixth embodiment, and Figure 6B is a cross-sectional view of the sixth embodiment in a direction perpendicular to the axial direction. A second pipe 11 is located in the center. In this sixth embodiment, the first electrode 51, covered with an insulating material provided on the inner surface of the first pipe 1, and the groundwater 53 that has permeated into the unit are used as the second electrode. Groundwater contains ions and is conductive, so it can be used as an electrode. In this embodiment, the insulating material covering the first electrode 51 acts as a dielectric, and when groundwater permeates, the area of ​​the second electrode changes, and the capacitance between the first electrode and the second electrode changes. According to equation (1) explained in the fourth embodiment, the capacitance increases when the distance between electrodes is short. Therefore, if the first electrode 51 is covered with a thin insulating material, the capacitance will change significantly with changes in water level. The change in electrical capacitance associated with the change in groundwater level is detected as a change in the frequency of an oscillation circuit (not shown), the water level is detected from the change in capacitance, and the pore water pressure of the aquifer is calculated from the detected water level.

[0047] In the sixth embodiment, the second pipe 11 is arranged concentrically with the first pipe 1. However, since groundwater is the second electrode, the second pipe 11 does not need to be arranged concentrically and may be placed closer to the wall of the first pipe 1. Although not shown in the figures, placing it closer to the wall of the first pipe 1 creates a large air space. This enlarged air space may be used to detect changes in electrical capacitance. In other words, the first electrode 51 and the second electrode (groundwater), covered with an insulating material, may be arranged parallel to each other in the axial direction, and the groundwater level may be detected from the change in electrical capacitance between the two electrodes.

[0048] As in the sixth embodiment, in a pore water pressure gauge unit where the first electrode 51 is covered with an insulating material on the wall surface and the groundwater 53 that has permeated into the first pipe 1 is used as the second electrode, the water level can be detected even without providing the second pipe 11. If the second pipe 11 is omitted, the outer diameter of the first pipe 1 can be reduced, and it can be installed in a borehole with a narrow diameter.

[0049] The seventh embodiment will be described with reference to Figures 7A and 7B. Figure 7A is an axial cross-sectional view of the seventh embodiment, and Figure 7B is a cross-sectional view of the seventh embodiment in a direction perpendicular to the axial direction. In Figure 7, elements having the same function as those in Figure 1 are denoted by the same reference numerals, and their descriptions are omitted. In the seventh embodiment, groundwater from an aquifer is allowed to permeate the interior of the central first pipe 1. The first electrode 61 and the second electrode 63, both covered with insulating material and located on the outer wall of the first pipe 1, are used to convert the change in capacitance caused by the groundwater that has permeated into the first pipe 1 into a frequency change. This change is then incorporated into an oscillation circuit (not shown here), from which the water level is determined, and the pore water pressure is calculated from the change in water level. Between the first electrode 61 and the second electrode 63, capacitance is generated due to the insulating material sandwiched between the electrodes, the material of the pipe wall 5 of the first pipe 1, and the groundwater that has permeated into the first pipe 1. Since the relative permittivity of groundwater is more than an order of magnitude larger than that of the other materials, the change in capacitance generally reflects the change in the groundwater level. Therefore, the change in water level can be determined from the change in capacitance. The electrodes positioned outside the first pipe 1 are preferably positioned to extend across the entire axial area of ​​the first pipe 1. This ensures that even if the water level changes significantly, the area above the water level and the area below the water level are always separated, thereby allowing the change in volume to follow the change in water level.

[0050] The seventh embodiment is an example that uses a borehole drilled using the Swedish sounding test (SWS), a well-known geological survey method for ground. An acrylic pipe with a diameter of 20 mm or less is used as a protective pipe 71, and a first pipe 1 with a diameter of 15 mm or less is placed inside it to measure the water level inside the first pipe 1. In SWS, the maximum diameter of the screw point at the tip of the borehole drilling instrument is specified as 33 mm, but the borehole diameter after the test will be smaller than this size due to the protrusion of the borehole wall. In order to install a pore water pressure gauge unit in the aquifer after drilling a borehole in a dam body or the like to determine the depth of the aquifer, it is preferable that the outer diameter of the unit be small. If the pore water pressure gauge unit can be installed at the bottom of the borehole, observations corresponding to pore water pressure observation using a partial strainer hole, which the Public Works Research Institute recommends in Non-Patent Literature 1 for the purpose of popularization, can be performed.

[0051] In the seventh embodiment, in order to reduce the diameter of the pore water pressure gauge, the two electrodes provided on the outer wall of the first pipe 1 fill the gap between the first pipe 1 and the protective pipe 71, and the composite cable 73 is placed in the gap between the two electrodes. In principle, the outer diameter of the pore water pressure gauge can be made to about 10 mm, but there is a limit to how thin it can be made because if it is made too thin, there will be no space to put the oscillation circuit in or for the composite cable to pass through.

[0052] Figure 8 shows how the pore water pressure gauge unit is used. In Figure 8, reference numeral 100 denotes the pore water pressure gauge unit of this invention, and reference numeral 101 denotes a spacer. Reference numeral 110 denotes a borehole, and 111 denotes an aquifer. The pore water pressure gauge unit 100 and the spacer 101 are cylindrical with the same diameter, and for example, the first pipe (without a through hole) of the pore water pressure gauge unit can be used as a cylindrical member constituting the spacer 101. The length of the spacer 101 can be adjusted as needed, thereby positioning the pore water pressure gauge unit 100 in the aquifer 111. A second pipe 113 passes through the pore water pressure gauge unit 100 and the spacer 101, and a composite cable is inserted through them. This makes it possible to set up a pore water pressure gauge 100 in each of the multiple aquifers present in the geological formation by drilling only one borehole 110. [Explanation of symbols]

[0053] 1. First pipe (observation pipe) 3 Through holes 5 Pipe wall 7 Upper wall 9 Lower wall 10 Spatial pressure measurement section 11. Second pipe 20 Third pipe 31, 33, 41, 43, 51, 53, 61, 63 electrode 71 Protective pipe 100 pore water pressure gauge unit 101 Spacer 110 boreholes 111 Aquifer

Claims

1. A pore water pressure gauge unit for measuring the pore water pressure of an aquifer, A measuring pipe comprising an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, wherein a sealed space is formed at the water level of the aquifer water that has entered through a through hole provided in the lower part of the pipe wall, and at the portion of the upper wall and the pipe wall where the through hole is not present, It comprises a water pressure identification unit that identifies the pore water pressure, The water pressure identification unit is a pore water pressure gauge unit that includes a space pressure measuring unit for measuring the pressure in the sealed space.

2. A pore water pressure gauge unit for measuring the pore water pressure of an aquifer, A measuring pipe comprising an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, wherein a sealed space is formed at the water level of the aquifer water that has entered through a through hole provided in the lower part of the pipe wall, and at the portion of the upper wall and the pipe wall where the through hole is not present, It comprises a water pressure identification unit that identifies the pore water pressure, The water pressure determination unit is a pore water pressure gauge unit that includes a length measuring unit for measuring the distance between the upper wall and the water surface.

3. A pore water pressure gauge unit for measuring the pore water pressure of an aquifer, A measuring pipe comprising an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, wherein a sealed space is formed at the water level of the aquifer water that has entered through a through hole provided in the lower part of the pipe wall, and at the portion of the upper wall and the pipe wall where the through hole is not present, It comprises a water pressure identification unit that identifies the pore water pressure, The water pressure determination unit comprises a capacity change measuring unit that measures the change in capacity between a pair of electrodes arranged inside the measuring pipe, and each of the pair of electrodes comprises a region exposed above the water surface and a region submerged in the water of the invading aquifer, in a pore water pressure gauge unit.

4. The unit according to claim 3, wherein the pair of electrodes are arranged on the inner surface of the measuring pipe so as to face each other.

5. A pore water pressure gauge unit for measuring the pore water pressure of an aquifer, A measuring pipe comprising an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, wherein a sealed space is formed at the water level of the aquifer water that has entered through a through hole provided in the lower part of the pipe wall, and at the portion of the upper wall and the pipe wall where the through hole is not present, It comprises a water pressure identification unit that identifies the pore water pressure, The water pressure determination unit is a cylindrical electrode positioned inside the measuring pipe, and is a cylindrical electrode covered with an insulating layer, A pore water pressure gauge unit further comprises a capacity change measuring unit that measures the change in capacity between the cylindrical electrode and the water of the aquifer that has entered the pore, using the aquifer water as the other electrode.

6. A pore water pressure gauge unit for measuring the pore water pressure of an aquifer, A measuring pipe comprising an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, wherein a sealed space is formed at the water level of the aquifer water that has entered through a through hole provided in the lower part of the pipe wall, and at the portion of the upper wall and the pipe wall where the through hole is not present, It comprises a water pressure identification unit that identifies the pore water pressure, The pore water pressure gauge unit comprises a water pressure determination unit which includes a capacity change measuring unit which measures the change in capacity between a pair of electrodes located on the outside of the measuring pipe, the pair of electrodes each having a region located above the water surface and a region located below the water surface, and the measuring pipe is made of an insulating material.

7. A protective pipe is further provided that is enclosed around the aforementioned measuring pipe. The pair of electrodes are positioned between the measuring pipe and the protective pipe. The unit according to claim 6, wherein a composite cable is inserted into the space formed by the measuring pipe, the protective pipe, and the pair of electrodes.

8. The unit according to any one of claims 1 to 5, wherein the composite cable is passed through the measuring pipe in the axial direction.

9. The unit according to claim 8, further comprising a second pipe that passes through the measuring pipe in the axial direction and into which the composite cable can be inserted.

10. The unit according to claim 4, further comprising a second pipe that penetrates the measuring pipe axially and into which a composite cable can be inserted, wherein one of the pair of electrodes is attached to the inner surface of the measuring pipe and the other electrode is attached to the outer surface of the second pipe.

11. A pore water pressure gauge unit for measuring the pore water pressure of an aquifer, A measuring pipe having an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, with a through hole provided in the lower part of the pipe wall, An auxiliary measuring pipe inserted into the aforementioned measuring pipe, comprising a lower-open type auxiliary measuring pipe having a second upper wall and a second pipe wall, It comprises a water pressure identification unit that identifies the pore water pressure, The pore water pressure gauge unit includes a space pressure measuring unit which is located inside the auxiliary measuring pipe and measures the water level of the aquifer water that has entered through the through-hole in the pipe wall, the pressure in the sealed space formed by the second upper wall and the second pipe wall.

12. A pore water pressure measuring device comprising the unit according to any one of claims 1 to 7 or claim 11, wherein a cylindrical unit and a cylindrical spacer having the same diameter as the unit are connected in the axial direction.

13. A pore water pressure measuring device according to claim 8, comprising a cylindrical first unit and a second unit, and a cylindrical spacer having the same diameter as these units, connected in the axial direction, wherein at least one of the spacers is positioned between the first and second units, and the composite cable passing through the first unit passes through the spacer and reaches the second unit.

14. The apparatus according to claim 12, wherein the outer diameters of the cylindrical unit and the spacer are such that they can be inserted into a borehole drilled in a Swedish sounding test.

15. The apparatus according to claim 13, wherein the outer diameters of the cylindrical unit and the spacer are such that they can be inserted into a borehole drilled in a Swedish sounding test.

16. A method for measuring pore water pressure in an aquifer, A measuring pipe is installed in the aquifer, comprising an upper wall, a lower wall, and a pipe wall capable of forming a sealed space, wherein the water surface of the aquifer enters through a through-hole provided in the lower part of the pipe wall, and a sealed space is formed in the upper wall and the pipe wall where there is no through-hole. Based on the water level of the aquifer water that has entered the measuring pipe through the aforementioned through hole, the pore water pressure is determined. Method for measuring pore water pressure.

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