JP7842984B2 - Pore water pressure gauge unit - Google Patents

Description

This invention relates to a pore water pressure meter unit for measuring pore water pressure in aquifers in landslide areas and the like.

In May 2014, the Public Works Research Institute, a national research and development agency, created a draft of "Guidelines for Pore Water Pressure Observation Using Partial Strainer Holes" (Non-Patent Document 1). According to a survey of landslide areas conducted by the Public Works Research Institute, observation using strainer holes for the entire section is employed in many sites, and it is said that the pore water pressure on the sliding surface cannot be accurately observed. In the case of observation using strainer holes for the entire section, the pore water pressure is observed based on the water pressure or water level inside the borehole. The same document states that there are three types of methods for observing pore water pressure in landslide areas: 1) strainer holes for the entire section, 2) partial strainer holes, and 3) buried pore water pressure gauges.
1) The pore water pressure of the target aquifer is not always accurately observed.
2) This allows observation of the pore water pressure in the aquifer near the slip surface at the target depth.
3) Installation and maintenance on deep sliding surfaces require specialized skills.
These points have been clarified, and the widespread adoption of method 2) is expected. In addition, the high cost of pore water pressure gauges is a factor hindering the widespread adoption of method 3).
In the case of Patent Document 1, the pore water pressure of the target aquifer can be measured, but it is necessary to excavate a large borehole (φ80 mm or larger), which requires high excavation costs. On the other hand, as in Patent Document 2, there are pore water pressure gauges that are pressed into the ground without excavating a borehole, but Non-Patent Document 1 states that installation and maintenance at deep slip surfaces require technical expertise.

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 a draft of the River Levee Monitoring Technology Guidelines (September 30, 2020). Within this guideline, it 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 system (Non-Patent Document 2, Patent Document 3). From the photographs in the publicly released materials (Non-Patent Document 2), it appears that the water level gauge, with a diameter of approximately 30 mm, is not easily driven into deep parts of the levee. Furthermore, a drawback is that if the depth of 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.

Patent No. 3353714 Japanese Patent Publication No. 11-6777 Patent No. 5044852

Public Works Research Institute Material No. 4283 (May 2014) Public Works Research Institute New Technology Showcase 2020 in Tokyo (September 2020)

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 and cannot be used 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.

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. Hereinafter, 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 (hereinafter, 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 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.

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 by SWS, it is necessary to manufacture a pore water pressure gauge with an outer diameter of approximately 15 mm.

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 with a small outer diameter and a long, vertical pipe-like shape is preferable.

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 becomes an obstacle to the installation work when installing observation equipment, making it difficult. When installing multiple units in the same borehole, the installation work 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 work 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.

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 defined as the axial direction), and Figure 1C is a cross-sectional view of the first embodiment perpendicular to the axial direction. 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 is an axial cross-sectional view of the third embodiment. 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 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 6A is an axial cross-sectional view of the sixth embodiment, and Figure 6B is a cross-sectional view of the sixth embodiment perpendicular to the axial direction. Figure 7A is an axial cross-sectional view of the seventh embodiment. Figure 7B is a cross-sectional view of the seventh embodiment perpendicular to the axial direction. Figure 8 is a conceptual diagram showing a pore water pressure gauge installed in a borehole.

This invention was made to solve the problems described above. This invention is defined as follows: 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,
A water pressure determination unit that determines the pore water pressure based on the water level of the aquifer that has entered the measuring pipe through the through hole,
A pore water pressure gauge unit equipped with the following.

When a pore water pressure gauge unit defined in this way is placed in an aquifer, water from the aquifer enters the first pipe through a through-hole provided in the pipe wall. The air inside the observation pipe is compressed by the infiltrating groundwater. The pressure of the compressed air then rises until it balances with the pore water pressure of the aquifer. When the pressures are balanced, the water level inside the observation pipe stabilizes. Based on this water level, the pore water pressure of the aquifer can be measured.
For example, the water level defined by this water level, the pressure in the space defined by the upper wall of the observation pipe and the pipe wall are in equilibrium with the pore water pressure, so the pore water pressure can be determined by measuring the pressure in this space.
In the above, the through-hole only needs to have the function of allowing groundwater from the aquifer to pass through. Therefore, it may be formed in the shape of a slit from the lower edge of the observation pipe. To prevent foreign matter from entering from the aquifer, a permeable filter may be provided.

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 initial volume of air 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 initial air. From this ratio, the pressure of the air inside the observation pipe can be determined, and the pore water pressure in the aquifer balanced by this pressure can be identified.

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.
Noting 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.

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.

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.

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.

If the pore water pressure gauge unit is as defined in the first phase, after insertion 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 the through-holes in the pipe wall, 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. Once 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.

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.

In the pore water pressure gauge unit defined in the second phase, if groundwater that has infiltrated through the through-hole below the first pipe is allowed to permeate into the third pipe, the air inside the third pipe will be compressed, similar to the first phase, and the air pressure 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.

When the pressure of compressed air within the first and third pipes balances the pore water pressure of the aquifer, the water level of groundwater that has infiltrated the first and third pipes will also rise. From the height of this rise, the volume ratio of the air inside the pipe to the initial volume of air at the time of installation can be determined. Once this 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 initial air pressure. 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 then 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.

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.

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 remains 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 compression ratio between this compressed air volume and the initial air volume allows for the calculation of the compressed air pressure by applying Boyle's Law. Then, the pore water pressure of the aquifer that balances this compressed air pressure 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.

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.

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.

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, by dividing the inner surface of the first pipe axially (for example, dividing it into three 120mm square sections) and placing the first and second electrodes at opposing positions, and detecting the change in capacitance formed between these electrodes, 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.

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.

In the pore water pressure gauge unit defined in the fifth phase, the first and second electrodes are insulated, generating capacitance between them. This capacitance changes as the amount of dielectric material changes in response to the height of the infiltrated groundwater. By incorporating this capacitance into an oscillation circuit, the frequency output from the circuit changes in response to the change in capacitance. Then, using a pre-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.

In the fourth and fifth phases, a 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.

In the fourth and fifth phases, the pore water pressure is measured by utilizing the increase or decrease in capacitance due to changes in water level, using groundwater sandwiched between the first and second electrodes as a dielectric. Since groundwater is a conductor, it can be used as the second electrode. In this case, the insulating material covering the first electrode acts as the dielectric, forming capacitance between the first electrode and the groundwater (the second electrode). The size of the second electrode changes in response to the water level change, and the capacitance changes accordingly.

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.

In the case of a pore water pressure gauge unit 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 infiltrating 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 remains the same regardless of height, the capacitance changes in accordance with the height of the infiltrating 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.

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.

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 from 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. The position of the water level L is shown in Figure 1B.
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 the 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.

The second embodiment will be explained with reference to 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 space 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 space 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 providing a partition plate to prevent groundwater from seeping into the upper part of the first pipe 1. Even when a partition plate is provided, 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.

Figure 3 is an axial cross-sectional view of the third embodiment. Elements identical to those in Figure 1 are denoted by the same reference numerals, 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 this volume and the volume of air in the initial state (volume of air at 1 atmosphere at installation) can be determined. Then, using Boyle's law, the pressure of the compressed air can be determined from this volume ratio, and this 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.

The fourth embodiment will be described with reference to Figures 4A and 4B. Elements identical to those in Figure 1 are denoted by 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.

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 and 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.

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 frequency of the oscillation circuit 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).

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 perpendicular to the axis of the fifth embodiment. 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. A second electrode 43, also 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 seeping between these electrodes are detected as frequency changes. The water level is detected from these frequency changes, the compression ratio of the air within the unit is determined from the detected water level changes, and the pore water pressure of the aquifer is determined from this compression ratio.

In the fifth embodiment, both the first electrode 41 and the second electrode 43 have a viewing angle of approximately 180 degrees. However, 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 water level change. In this case as well, by incorporating capacitance 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 water level change can be determined from the change in output frequency, and the pore water pressure of the aquifer can be calculated from that water level.

In the fifth embodiment, the first electrode 41 and the second electrode 43 were provided on the inner surface of the first pipe 1. However, as shown in Figure 2, the first and second electrodes, covered with an insulating material, may be provided on the inner surface of the third pipe 20, and groundwater infiltrating from the aquifer may be introduced into the third pipe 20 to detect its water level. In this case, a composite cable would pass between the first pipe 1 and the third pipe 20.

When installing electrodes covered with insulating material 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 the 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.

In the fourth and fifth embodiments, a second pipe 11 penetrates the inside of the first pipe 1, and a composite cable for the 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 high relative permittivity, is large, making it easier to detect changes in water level.

The sixth embodiment will be described with reference to Figures 6A and 6B. Elements identical to those in Figure 1 are given the same reference numerals and their descriptions 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 frequency change in an oscillation circuit (not shown). The water level is then detected from the change in capacitance, and the pore water pressure of the aquifer is calculated from the detected water level.

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.

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, water level detection is possible even without the second pipe 11. Without the second pipe 11, the outer diameter of the first pipe 1 can be reduced, allowing it to be installed in a borehole with a narrow diameter.

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) to determine the water level, 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 by 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 length of the first pipe 1. This ensures that even if the water level changes significantly, the region above the water level and the region below the water level are always separated, thereby allowing the change in volume to follow the change in water level.

The seventh embodiment uses a borehole drilled using the Swedish Sounding Test (SWS), a well-known geological survey method for ground conditions. 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. While the maximum diameter of the screw point at the tip of the drilling instrument for the SWS is specified as 33 mm, the borehole diameter after the test will be smaller due to the protrusion of the borehole wall. To install a pore water pressure gauge unit in the aquifer after drilling a borehole to determine the depth of the aquifer, a small outer diameter of the unit is preferable. 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, as recommended by the Public Works Research Institute in Non-Patent Document 1 for the purpose of popularization, can be performed.

In the seventh embodiment, 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 are limits because making it too narrow would eliminate the air space for the oscillation circuit and the gap for passing the composite cable.

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.

1. First pipe (observation pipe)
3 Through hole 5 Pipe wall 7 Upper wall 9 Lower wall 10 Space pressure measuring section 11 Second pipe 20 Third pipes 31, 33, 41, 43, 51, 53, 61, 63 Electrode 71 Protective pipe 100 Pore water pressure gauge unit 101 Spacer 110 Borehole 111 Aquifer

Claims (16)

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 in the upper wall and the pipe wall where there is no through hole,
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. 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 in the upper wall and the pipe wall where there is no through hole,
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. 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 in the upper wall and the pipe wall where there is no through hole,
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. The unit according to claim 3, wherein the pair of electrodes are arranged opposite each other on the inner surface of the measuring pipe. 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 in the upper wall and the pipe wall where there is no through hole,
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. 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 in the upper wall and the pipe wall where there is no through hole,
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. 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. 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. The unit according to claim 8, further comprising a second pipe that penetrates the measuring pipe axially and into which the composite cable can be inserted. 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 . 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 meter 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. 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 their respective axial directions. 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 axially, 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. 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 using a Swedish sounding test. 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 using a Swedish sounding test. 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.