Monitoring method used in high-pressure injection agitation method

Abstract

To propose a monitoring method capable of easily confirming an effective diameter of a soil improvement body.SOLUTION: A monitoring method used when creating a soil improvement body S in a high-pressure injection agitation method includes steps of: preparing two electrodes 4A, 4B installed on a surface of a soil G or a surface of a covering plate P laid on the surface of the soil, and a potentiometer 5 for measuring a potential difference between the electrodes 4A, 4B; installing at least one electrode 4A on the radially inside of a planned effective diameter R of the soil improvement body S on the surface of the soil G or the surface of the covering plate P, and installing the other electrode 4B on the surface of the soil G or the surface of the covering plate P; and rotating an injection pipe 3 while injecting a hardening material from a nozzle 3a, and measuring a potential difference between the two electrodes 4A, 4B by the potentiometer 5.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a monitoring method used when constructing a ground improvement body using a high-pressure jet mixing method. [Background technology]

[0002] The high-pressure injection mixing method is known as a construction method for improving weak ground that does not have the desired strength to a high strength. In the high-pressure injection mixing method, a hardening material is injected at high pressure from the nozzle of an injection pipe inserted into the ground while the injection pipe is rotated, thereby cutting and collapsing the surrounding ground with the high-pressure hardening material, and mixing and mixing the hardening material with the ground, thereby creating a cylindrical or fan-shaped ground improvement body in the ground.

[0003] In the high-pressure jet mixing method, the effective diameter of the ground improvement body can vary depending on the shear strength and hardness of the soil layer to be improved, as well as the unevenness of the target soil layer. For this reason, it is necessary to confirm that the ground improvement body has been constructed according to the planned effective diameter (hereinafter referred to as the planned effective diameter). Conventionally, after the ground improvement body has been constructed, follow-up survey borings have been conducted and the results confirmed using the extracted cores. However, with this method, the constructed ground improvement body must be allowed to cure for several days before the cores can be extracted, which means that it takes time to confirm the effective diameter of the ground improvement body.

[0004] As an alternative to the verification method of collecting cores, a method of verifying the extent of ground improvement work is known, as disclosed in Patent Document 1. In the method disclosed in Patent Document 1, a built-in pipe is provided around the injection pipe, and a sound collector is placed inside the built-in pipe, and this sound collector monitors the sound of the hardening material sprayed from the nozzle of the injection pipe hitting the built-in pipe. Then, based on the volume level of the monitored sound, it is determined whether the hardening material has reached the built-in pipe. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-62626 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the method disclosed in Patent Document 1 requires the installation of a built-in pipe in the ground, which requires time and effort to confirm the range of ground improvement.

[0007] In view of these points, the present invention aims to propose a monitoring method that can easily confirm the effective diameter of a ground improvement body. [Means for solving the problem]

[0008] The monitoring method of the present invention is a monitoring method used when creating a ground improvement body in a high-pressure injection mixing method in which a hardening material is sprayed from the nozzle of an injection pipe inserted into the ground while the injection pipe is rotated to create the ground improvement body, and includes the steps of preparing two electrodes to be installed on the surface of the ground or on the surface of a covering plate laid on the surface of the ground, and a potentiometer for measuring the potential difference between the electrodes, installing at least one of the electrodes radially inside the planned effective diameter of the ground improvement body on the surface of the ground or the surface of the covering plate, and installing the other electrode on the surface of the ground or the surface of the covering plate, and rotating the injection pipe while spraying the hardening material from the nozzle and measuring the potential difference between the two electrodes with the potentiometer. [Effects of the Invention]

[0009] According to the monitoring method of the present invention, two electrodes and a potentiometer installed on the surface of the ground or the surface of the lining plate can be used to measure the change in spontaneous potential associated with the flow of the excavated and collapsed ground and the hardening material, which are stirred and mixed when the hardening material is sprayed at high pressure from the nozzle of the injection pipe. At least one of the electrodes is installed on the surface of the ground or the surface of the lining plate, radially inward of the planned effective diameter of the ground improvement body. Therefore, if a change in spontaneous potential is measured when the hardening material is sprayed from the nozzle, it is understood that the stirred and mixed ground and hardening material have flowed at least up to the point where this electrode is installed, and it is possible to estimate that the ground improvement body has been constructed so that at least the effective diameter from the injection pipe to this electrode is secured. Furthermore, the monitoring method of the present invention does not require the installation of a built-in pipe in the ground as in the past, so the effective diameter of the ground improvement body can be easily confirmed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram schematically illustrating an embodiment of an apparatus used in a monitoring method according to the present invention. [Figure 2] 2 is a diagram showing a schematic view from above of the positions where the two electrodes shown in FIG. 1 are installed. FIG. [Figure 3] FIG. 1 is a diagram showing an example of the relationship between the potential difference between two electrodes and time in the monitoring method according to the present invention. [Figure 4] FIG. 1 is a diagram showing an example of the relationship between the potential difference between two electrodes and time in the monitoring method according to the present invention. [Figure 5] FIG. 10 is a diagram schematically illustrating another embodiment of an apparatus used in the monitoring method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of a monitoring method according to the present invention will be described with reference to the drawings.

[0012] 1 is a schematic diagram of a monitoring device 1 used in one embodiment of the monitoring method according to the present invention. The monitoring device 1 includes a construction machine 2, an injection pipe 3 supported vertically in a vertical hole H1 drilled in the ground G by the construction machine 2, two electrodes (the electrode located closer to the injection pipe 3 is referred to as electrode 4A, and the electrode located farther away is referred to as electrode 4B), and a potentiometer 5. In this embodiment, a lining plate P is laid on the top surface of the ground G, and the construction machine 2 and electrodes 4A and 4B are installed on the top surface of the lining plate P. In this embodiment, it is assumed that the top surface of the ground G extends horizontally and the vertical hole H1 extends vertically.

[0013] The construction machine 2 has the function of moving the injection pipe 3 in the vertical direction, rotating the injection pipe 3 around its central axis O, or swinging it around its central axis O within a predetermined rotation angle.

[0014] The injection pipe 3 has a single-pipe structure or a multiple-pipe structure, and a flow path for flowing a hardening material (for example, a cement-based hardening material) is formed inside the injection pipe 3. A nozzle 3a that communicates with this flow path is provided on the side of the lower part of the injection pipe 3. A hose (not shown) is attached to the upper part of the injection pipe 3, and this hose is connected to a plant (not shown) that supplies the hardening material at a predetermined pressure.

[0015] By operating the construction machine 2 and plant described above, the injection pipe 3 is raised while rotating, and the hardening material fed from the plant via a hose into the injection pipe 3 flows inside the injection pipe 3 and is sprayed at high pressure from the nozzle 3a. That is, the ground G is cut and collapsed by the hardening material sprayed at high pressure, and is further stirred and mixed with the hardening material, so that a cylindrical or fan-shaped ground improvement body S can be created in the ground G.

[0016] The electrodes 4A, 4B and the potentiometer 5 are configured to measure the natural potential of the ground G. The electrodes 4A, 4B are non-polarized electrodes, and for example, copper sulfate electrodes, silver chloride electrodes, hydrogen electrodes, calomel electrodes, etc. can be used. The potentiometer 5 is a voltmeter with a high input resistance, and for example, a multimeter can be used.

[0017] Next, a monitoring method using such a monitoring device 1 when constructing a ground improvement body S will be described.

[0018] First, as shown in Figure 1, an injection pipe 3 is inserted into a vertical hole H1 provided at a point that will be the center of the ground improvement body S to be constructed. In this embodiment, a construction machine 2 is installed at a point that will be the center of the ground improvement body S, and the construction machine 2 rotates and lowers the injection pipe 3, thereby drilling the vertical hole H1 and inserting the injection pipe 3 into the vertical hole H1. Alternatively, the vertical hole H1 may be drilled in advance using a boring machine or the like, and then the construction machine 2 is installed and the injection pipe 3 is inserted by the construction machine 2.

[0019] Furthermore, electrodes 4A and 4B are installed on the upper surface of the lining plate P. Note that electrodes 4A and 4B may also be installed on the upper surface of the ground G. In the monitoring method according to this embodiment, since the potential difference based on the natural potential between the locations where electrode 4A is installed and electrode 4B is installed is measured, it is preferable to install electrodes 4A and 4B at a certain distance as shown in the figure. As an example, the distance between electrodes 4A and 4B is 200 mm to 500 mm. In this embodiment, with respect to the planned effective diameter R of the ground improvement body S to be constructed, electrode 4A is installed near the planned effective diameter R on the inside of this planned effective diameter R, and electrode 4B is installed outside the planned effective diameter R. Note that electrode 4B may also be installed inside the planned effective diameter R. Furthermore, as shown in FIG. 2, in this embodiment, electrodes 4A and 4B are installed on a straight line L extending radially outward from the central axis O of the injection pipe 3 when facing the surface of the lining plate P (as viewed from above). Note that, by arranging the electrodes 4A and 4B on the same straight line L, the advantages described below can be obtained; however, the positions at which the electrodes 4A and 4B are arranged do not have to be on the same straight line L, and they may be arranged, for example, at the same position in the circumferential direction around the central axis O.

[0020] Then, by operating the construction machine 2 and plant, the hardening material is sprayed at high pressure from the nozzle 3a while the injection pipe 3 is rotated and raised. At this time, the ground G is excavated and collapsed by the hardening material sprayed at high pressure. The excavated and collapsed ground G is mixed with the hardening material and flows through the ground G, causing a change in the natural potential where the excavated and collapsed ground G and hardening material flow. In other words, when the flow of the excavated and collapsed ground G and hardening material reaches the location where the electrode 4A is installed, the change in potential can be measured using the potentiometer 5. Therefore, it can be estimated that the ground improvement body S is developed at least within the radius from the central axis O to the location where the electrode 4A is installed. Furthermore, because the hardening material is sprayed radially outward from the nozzle 3a, it is assumed that the excavated and collapsed ground G and hardening material also flow radially outward. Therefore, by placing the electrodes 4A and 4B on the same line L, the change in natural potential accompanying this flow can be more accurately measured.

[0021] Here, the confirmation results obtained by one embodiment of the monitoring method according to the present invention will be described with reference to FIGS.

[0022] Figure 3 is a graph showing the relationship between time and potential difference obtained when constructing ground improvement body S with a planned effective diameter R of 1750 mm. Electrodes 4A and 4B are both copper sulfate electrodes, and a multimeter is used as potentiometer 5. Electrode 4A is connected to the negative side of the multimeter and is installed 1.2 m from the central axis O of the top surface of ground G. Electrode 4B is connected to the positive side of the multimeter and is installed 1.7 m from the central axis O of the top surface of ground G. The geology of ground G varies depending on the depth; the soil is sandy where nozzle 3a of injection pipe 3 inserted into vertical hole H1 is initially located, and clayey soil is deposited on top of the sandy soil.

[0023] The confirmation results shown in Figure 3 show that the injection pipe 3 is rotated and raised by the construction machine 2, while the hardening material is sprayed from the nozzle 3a at time t0, the rotation and raising of the injection pipe 3 and the spraying of the hardening material from the nozzle 3a are temporarily stopped at time t1 and then resumed shortly thereafter, the rotation and raising of the injection pipe 3 and the spraying of the hardening material from the nozzle 3a continue at times t2 and t3, and the rotation and raising of the injection pipe 3 and the spraying of the hardening material from the nozzle 3a are stopped at time t4. Time t2 is the estimated time when the construction of the ground improvement body S with sandy soil is completed, calculated from the amount the injection pipe 3 is raised, and time t3 is the estimated time when the construction of the ground improvement body S with clayey soil is completed, calculated from the amount the injection pipe 3 is raised.

[0024] Figure 4 is a graph showing the relationship between time and potential difference obtained when constructing ground improvement body S with a planned effective diameter R of 1750 mm. Both electrodes 4A and 4B are copper sulfate electrodes, and a multimeter is used as potentiometer 5. Electrode 4A is connected to the negative side of the multimeter and is initially installed 1.0 m from the central axis O of the top surface of lining plate P. However, it is moved during construction and is now installed 1.2 m from the central axis O of the top surface of lining plate P. Electrode 4B is connected to the positive side of the multimeter and is installed 1.4 m from the central axis O of the top surface of lining plate P. The ground G examined in Figure 4 is also sandy soil where nozzle 3a of injection pipe 3 inserted into vertical hole H1 was initially located, with clayey soil deposited on top of the sandy soil.

[0025] The confirmation results shown in Figure 4 were obtained by rotating and pulling up the injection pipe 3 with the construction machine 2, spraying the hardening material from the nozzle 3a at time t10, and then at time t11, without changing the conditions for spraying the hardening material, moving the position of the electrode 4A from 1.0 m to 1.2 m from the central axis O. The confirmation was then continued in this state. Time t12 is the estimated time when the construction of the ground improvement body S with sandy soil, calculated from the amount the injection pipe 3 was pulled up, is completed, and time t13 is the estimated time when the construction of the ground improvement body S with clayey soil, calculated from the amount the injection pipe 3 was pulled up, is completed.

[0026] As shown in Figure 3, the potential difference fluctuates slightly between time t0 and time t4, when the hardening material is being sprayed from the nozzle 3a. However, the fluctuations in the potential difference become smaller after time t4, when the hardening material is no longer being sprayed from the nozzle 3a. As shown in Figure 4, when the electrode 4A is placed closer to the central axis O, a small fluctuation in the potential difference can be observed. However, when the electrode 4A is placed further away from the central axis O, the fluctuations in the potential difference become smaller, even though the spraying condition of the hardening material from the nozzle 3a remains unchanged. In other words, when a small fluctuation in the potential difference can be observed, it can be inferred that the excavated and collapsed ground G and hardening material have flowed at least up to the point where the electrode 4A is placed. Therefore, according to the monitoring method of this embodiment, the effective diameter of the ground improvement body S can be confirmed based on the state of the potential difference and the distance from the central axis O to the electrode 4A closest to the central axis O. Furthermore, because the difference in fluctuations in potential difference can be seen whether electrodes 4A and 4B are installed on the top surface of ground G or on the top surface of covering plate P, it can be seen that electrodes 4A and 4B may be installed on either the top surface of ground G or the top surface of covering plate P. As shown in Figure 3, the finely fluctuating potential difference can be confirmed both between time t1 and time t2, when the ground improvement body S is estimated to be created in sandy soil, and between time t2 and time t3, when the ground improvement body S is estimated to be created in clayey soil, so the effective diameter of the ground improvement body S can be confirmed regardless of differences in the geology of the ground G.

[0027] Note that the relationship between time and potential difference shown in Figures 3 and 4 is only an example, and therefore the fluctuations in potential difference may differ from those shown depending on the condition of the ground G, etc. However, even in this case, the effective diameter of the ground improvement body S can be confirmed in the same manner as explained in Figures 3 and 4 by comparing the potential difference when the hardening material is sprayed and when it is stopped, for example.

[0028] The monitoring method according to the present invention can also be realized by a monitoring device 1A shown in FIG. 5. The monitoring device 1A includes three or more electrodes (six electrodes 6A, 6B, 6C, 6D, 6E, and 6F in this embodiment) instead of the two electrodes 4A and 4B described above. The electrodes 6A to 6F and the potentiometer 5 are connected so that the potential difference between each of the electrodes 6A to 6E relative to the electrode 6F is measured (so that the potential difference between the electrodes 6A and 6F, the potential difference between the electrodes 6B and 6F, and the potential difference between the electrodes 6E and 6F are measured). While the electrodes 6A to 6F are installed on the upper surface of the lining plate P in FIG. 5, they may also be installed on the upper surface of the ground G as described for the electrodes 4A and 4B. The electrodes 6A to 6F are installed at different distances from the central axis O of the injection pipe 3. In this embodiment, the electrode 6A is installed closest to the central axis O, and the electrode 6F is installed farthest from the central axis O. Electrodes 6A to 6E are installed inside the planned effective diameter R of the ground improvement body S, and electrode 6F is installed outside the planned effective diameter R. Although not shown in the figure, all of electrodes 6A to 6F are installed on a straight line L that extends radially outward with the central axis O as the center when facing the surface of the covering plate P (as viewed from above). The distances between adjacent electrodes 6A to 6E (the distance between electrode 6A and electrode 6B, the distance between electrode 6B and electrode 6C, ... the distance between electrode 6E and electrode 6F) are all the same.

[0029] With this type of monitoring device 1A, for example, it is possible to confirm small fluctuations in the potential difference between electrode 6A and electrode 6F, the potential difference between electrode 6B and electrode 6F, and the potential difference between electrode 6D and electrode 6F. However, if small fluctuations cannot be confirmed in the potential difference between electrode 6E and electrode 6F, it is estimated that the excavated and collapsed ground G and hardened material have flowed at least up to where electrode 6D is installed, but have not flowed up to where electrode 6E is installed. In other words, by using this type of monitoring device 1A, it is possible to estimate that the effective diameter of the ground improvement body S to be constructed is between the distance from the central axis O to electrode 6D and the distance from the central axis O to electrode 6E. In this way, a monitoring method using the monitoring device 1A makes it possible to grasp the effective diameter of the ground improvement body S in more detail.

[0030] The potentiometer 5 described above was connected so that the potential difference between each of electrodes 6A to 6E relative to electrode 6F was measured. However, it may also be connected so that the potential difference between adjacent electrodes 6A to 6E is measured (so that the potential difference between electrode 6A and electrode 6B, the potential difference between electrode 6B and electrode 6C, and so on, and the potential difference between electrode 6E and electrode 6F are measured). When measuring the potential difference due to the natural potential, it changes not only depending on whether or not the ground G and the hardening material are flowing, but also on the speed of this flow and the distance between the electrodes. In other words, when using a potentiometer 5 connected in this manner, the influence of the distance between the electrodes is eliminated when comparing the potential difference between electrode 6A and electrode 6B, the potential difference between electrode 6B and electrode 6C, and the potential difference between electrode 6E and electrode 6F, and therefore, the change in the potential difference can be evaluated more accurately. In addition, when the distance between adjacent electrodes is made the same and the potential difference between adjacent electrodes is measured, it is preferable to install at least two of these electrodes at the same distance as the other electrodes and outside the planned effective diameter R (for example, in the monitoring device 1A shown in FIG. 5, a new electrode is installed radially outside electrode 6F so that the distance from electrode 6F is the same as the distance between electrodes 6A and 6B). When the collapsed ground G and the flow of hardened material reach the planned effective diameter R as expected, the fluctuations in the potential difference obtained from the two electrodes installed outside the planned effective diameter R are basically such that the collapsed ground G and the flow of hardened material are not measured. Therefore, by comparing the fluctuations in the potential difference obtained from the other electrodes installed radially inside these two electrodes, it can be determined that the collapsed ground G and the flow of hardened material have reached the planned effective diameter R as expected.

[0031] (Addendum) In one aspect, the present specification discloses the following technology.

[0032] (Technology 1) In a high-pressure injection mixing method in which a hardening material is injected from a nozzle (3a) of an injection pipe (3) inserted into the ground (G) while rotating the injection pipe (3) to create a ground improvement body (S), a monitoring method used when creating the ground improvement body (S): a step of preparing two electrodes (4A, 4B) to be installed on the surface of the ground (G) or on the surface of a covering plate (P) laid on the surface of the ground (G), and a potentiometer (5) to measure the potential difference between the electrodes (4A, 4B); A step of installing at least one of the electrodes (4A) radially inside the planned effective diameter (R) of the ground improvement body (S) on the surface of the ground (G) or the surface of the covering plate (P), and installing the other electrode (4B) on the surface of the ground (G) or the surface of the covering plate (P); and rotating the injection tube (3) while injecting the hardening material from the nozzle (3a) and measuring the potential difference between the two electrodes (4A, 4B) with the potentiometer (5).

[0033] This technology allows for the measurement of changes in spontaneous potential associated with the flow of the hardening material and the excavated and collapsed ground that is stirred and mixed when the hardening material is sprayed at high pressure from the nozzle of the injection pipe. At least one electrode is installed on the surface of the ground or on the surface of the covering plate, radially inward of the planned effective diameter of the ground improvement body. Therefore, if a change in spontaneous potential is measured when the hardening material is sprayed from the nozzle, it is understood that the stirred and mixed ground and hardening material have flowed at least up to the point where this electrode is installed, and it can be assumed that the ground improvement body has been constructed so that at least the effective diameter from the injection pipe to this electrode is secured. Furthermore, this technology does not require the installation of a built-in pipe into the ground as in the past, making it easy to confirm the effective diameter of the ground improvement body.

[0034] (Technology 2) The monitoring method described in Technology 1, wherein one electrode (4A) and the other electrode (4B) are installed on a straight line (L) extending radially outward from the injection pipe (3) as a center while facing the surface of the ground (G) or the surface of the covering plate (P).

[0035] Since the hardening material is sprayed radially outward from the nozzle, it is thought that the collapsed ground and hardening material will also flow radially outward. Therefore, this technology can more accurately measure changes in self-potential.

[0036] (Technology 3) The number of the electrodes (6A to 6F) is three or more, When facing the surface of the ground (G) or the surface of the covering plate (P), the distances from the injection pipe (3) to the electrodes (6A to 6F) are different, One (6F) of the electrodes (6A to 6F) is installed radially outside the planned effective diameter (R) on the surface of the ground (G) or the surface of the covering plate (P), and the other electrodes (6A to 6E) are installed radially inside the electrode (6F) installed outside the planned effective diameter (R), The monitoring method described in Technology 1, wherein the potentiometer (5) is capable of measuring the potential difference between the electrode (6F) installed radially outside the planned effective diameter (R) and each of the other electrodes (6A to 6E).

[0037] This technology allows for more accurate estimation of the effective diameter of the ground improvement body S.

[0038] (Technology 4) The number of the electrodes (6A to 6F) is three or more, The distances between adjacent electrodes (6A to 6F) when facing the surface of the ground (G) or the surface of the covering plate (P) are all the same, The monitoring method according to technique 1, wherein the potentiometer (5) is capable of measuring the potential difference between the adjacent electrodes (6A to 6F).

[0039] This technique eliminates the effect of electrode distance on fluctuations in potential difference, allowing for more accurate evaluation of changes in potential difference.

[0040] (Technology 5) The monitoring method described in Technology 3 or 4, wherein all of the electrodes (6A to 6F) are installed on a straight line (L) that extends radially outward from the injection pipe (3) as a center when facing the surface of the ground (G) or the surface of the covering plate (P).

[0041] This technique allows for more accurate measurement of changes in spontaneous potential.

[0042] Although one embodiment of the present invention has been described above, the present invention is not limited to this specific embodiment, and unless otherwise limited in the above description, various modifications and variations are possible within the spirit and scope of the present invention as defined in the claims. For example, the configurations of the above-described embodiment may be added or deleted as appropriate, and the configurations of one embodiment may be incorporated into other embodiments. Furthermore, the effects of the above-described embodiment are merely examples of the effects that can be obtained from the present invention. In other words, the effects of the present invention are not limited to the above-described effects, and additional effects may be obtained in addition to the above-described effects. [Explanation of symbols]

[0043] 3: Injection tube 3a: Nozzle 4A, 4B: Electrode 5: Potentiometer 6A~6F: Electrode G: Ground L: A straight line extending radially outward from the injection tube P: Lining board R: Planned effective diameter S: Ground improvement body

Claims

1. In a high-pressure injection mixing method in which a hardening material is injected from a nozzle of an injection pipe inserted into the ground while rotating the injection pipe to create a ground improvement body, a monitoring method used when creating the ground improvement body, A step of preparing two electrodes to be installed on the surface of the ground or on the surface of a covering plate laid on the surface of the ground, and a potentiometer to measure the potential difference between the electrodes; At least one of the electrodes is installed radially inside the planned effective diameter of the ground improvement body on the surface of the ground or the surface of the covering plate, and the other electrode is installed on the surface of the ground or the surface of the covering plate; rotating the injection tube while injecting the hardening material from the nozzle and measuring the potential difference between the two electrodes with the potentiometer.

2. 2. The monitoring method according to claim 1, wherein one electrode and the other electrode are installed on a straight line extending radially outward from the injection pipe as a center when facing the surface of the ground or the surface of the covering plate.

3. The number of electrodes is three or more, The distances from the injection pipe to the electrodes are different when the electrodes are facing the surface of the ground or the surface of the covering plate, One of the electrodes is installed radially outside the planned effective diameter on the surface of the ground or the surface of the covering plate, and the other electrode is installed radially inside the electrode installed outside the planned effective diameter, The monitoring method according to claim 1 , wherein the potentiometer is capable of measuring a potential difference between the electrode disposed radially outside the planned effective diameter and each of the other electrodes.

4. The number of electrodes is three or more, The distances between adjacent electrodes when facing the surface of the ground or the surface of the covering plate are all the same, The monitoring method according to claim 1 , wherein the potentiometer is capable of measuring a potential difference between adjacent electrodes.

5. 5. The monitoring method according to claim 3, wherein all of the electrodes are installed on a straight line extending radially outward from the injection pipe when facing the surface of the ground or the surface of the covering plate.

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