JP7696563B2 - Method for evaluating the effect of ground improvement using chemical grouting method - Google Patents

Description

Patent Law Article 30, Paragraph 2 Application 14th Ground Improvement Symposium Proceedings (USB), "Confirmation of Completion of Chemical Injection Method Using Electrical Logging", November 5, 2020 (shipping date)

The present invention relates to a method for evaluating the effectiveness of ground improvement using a chemical injection method that uses electrical logging.

Traditionally, to strengthen soft ground such as reclaimed land, ground improvement work has been carried out using a chemical injection method in which a chemical solution made of water glass (sodium silicate) or the like is injected into the ground. After construction using the chemical injection method, a construction inspection is carried out to check whether the chemical solution has been evenly distributed throughout the target ground.

The most common method for construction confirmation surveys of the chemical grouting method is to evaluate the improved soil using its unconfined compressive strength (qu). However, the unconfined compressive strength of soil improved by chemical grouting is small, at around qu = 50 to 100 kPa, and depending on the target ground, there are cases where the strength varies and the soil is not properly evaluated. In other words, when evaluating using the unconfined compressive strength qu, in cases where the ground has a small qu of around 50 to 100 kPa, disturbances that lead to a decrease in strength are likely to occur when taking samples or preparing test specimens in follow-up surveys. Also, depending on the target ground, there are cases where the test specimens contain shells, wood chips, silt, organic soil, etc., which causes strength variations and makes it impossible to properly evaluate the soil.

As a method to directly evaluate the quality of improved ground other than the uniaxial compressive strength test, the Ministry of Land, Infrastructure, Transport and Tourism's review committee on ground improvement work using chemical injection methods on reclaimed land, etc. has proposed a dynamic cone penetration test that can measure pore water pressure, such as a piezo-driven cone (PDC). The piezo-driven cone penetrates a cone with a built-in pressure sensor into the ground by striking it with a hammer, and measures the penetration amount and the pore water pressure response value at the time of penetration for each strike. From the penetration amount, the dynamic penetration resistance value (Nd value) of the ground, which corresponds to the N value of the standard penetration test, is calculated for each strike. In addition, the above review committee has proposed that the fine grain content Fc can be estimated from the pore water pressure in the ground generated by the strike penetration, and that the cumulative excess pore water pressure ratio obtained using this pore water pressure can be an index for evaluating the penetration of chemicals into the ground.

Another method for confirming the construction of the liquid injection method is electrical logging. Electrical logging takes advantage of the fact that in the liquid injection method, the pore water in the ground is replaced with a liquid chemical, changing the compressibility of the ground, and the strength of the ground increases as the liquid solidifies, resulting in a change in the electrical conductivity characteristics of the improved ground. With electrical logging, a qualitative assessment of the improvement effect can be made by checking the decrease in electrical resistivity before and after construction. The measurement procedure for electrical logging involves inserting a measurement probe equipped with multiple electrodes at a specified interval in the vertical direction into the borehole, then passing electricity through the current electrodes and determining the resistivity from the potential difference between the electrodes.

As a method for checking the quality of ground improvement work using such electrical logging, the following Patent Document 1 discloses a method in which an electrode attachment body with a ring-shaped electrode attached to its outer surface is inserted into the improvement body, electricity is passed through the improvement body constructed around the electrode attachment body, and resistivity is calculated using the current between the current electrodes and the potential difference between the potential electrodes measured in this state. In addition, Non-Patent Document 1 discloses a method for calculating the chemical solution filling rate from the change in electrical resistivity before and after chemical solution injection.

The present applicants have also proposed in Patent Document 2 below a method for confirming the effect of ground improvement using a chemical injection method, as a method for confirming the effect of ground improvement with little variation and allowing direct confirmation of the quality of the improved ground, in which after ground improvement, a small dynamic cone penetration test is used to obtain a depth distribution map of Nd values showing the relationship between depth and Nd value, and a primary effect confirmation is performed to confirm the effect of ground improvement from the increment in the Nd value before and after ground improvement. If the effect of ground improvement is not confirmed by the primary effect confirmation, an electrical logging is performed to measure resistivity by inserting a measuring probe equipped with an electrode into the penetration hole of the small dynamic cone penetration test, a depth distribution map of resistivity showing the relationship between depth and resistivity is obtained, and a secondary effect confirmation is performed to confirm the effect of ground improvement from the decrement in resistivity before and after ground improvement.

JP 2000-46510 A JP 2021-4473 A

Hideo Komine, "Evaluation method of filling rate of improved part by grouting using electrical resistivity", Journal of Japan Society of Civil Engineers, No. 463/III-22, pp. 153-162, March 1993 Takahiro Kanno et al., "Strength evaluation method for in-situ investigation of ground injected with chemicals as a liquefaction countermeasure", Port and Airport Research Institute Materials, No. 1366, pp. 2020

However, the method of Patent Document 1 calculates the volume of the improved body, i.e., cross-sectional area, size, diameter, etc., from the measured resistivity, while the method of Non-Patent Document 1 calculates the chemical solution filling rate from the electrical resistivity, and the method of Patent Document 2 makes it possible to confirm the presence of a ground improvement body from the decrease in resistivity even when the effect of ground improvement cannot be determined from the increase in the Nd value alone.

As mentioned above, the primary method for evaluating the effect of ground improvement using chemical injection methods is to evaluate the uniaxial compressive strength qu, but none of the above-mentioned conventional techniques directly evaluate the strength of the solidified body.

It has also been pointed out that when evaluating the effectiveness of ground improvement by measuring electrical resistivity, in cases where the salinity of the pore water is high and the electrical resistivity is low, such as in reclaimed land in coastal areas, it is unlikely that this method can be applied to determining the infiltration of chemical solutions (Non-Patent Document 2).

The main objective of the present invention is to provide a method for evaluating the effectiveness of ground improvement using a chemical injection method with electrical logging that makes it possible to evaluate the uniaxial compressive strength of the improved body by measuring the electrical resistivity before and after improvement.

The second objective is to clarify the scope of application (the range of salt concentration of pore water) of the method for evaluating the effect of ground improvement using the chemical injection method of the present invention.

In order to solve the above problem, as the present invention according to claim 1, a correlation diagram is obtained in advance, in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and a threshold value is set for the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) under conditions that can ensure a target uniaxial compressive strength (quck),
A method for evaluating the effect of ground improvement using a liquid grouting method, comprising measuring electrical resistivity by electrical logging using a penetration hole formed vertically in the ground before and after ground improvement, determining the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and judging whether the target uniaxial compressive strength (quck) is achieved based on whether the electrical resistivity ratio (Rimp/Runimp) is greater than or less than the threshold value ,
The correlation diagram is obtained by: a first step of diluting a silica solution with water and measuring the electrical resistivity for each silica concentration in order to grasp the electrical resistivity characteristics of the silica solution used for ground improvement, and obtaining a correlation diagram between silica concentration (SiO ₂ ) and electrical resistivity (Rc);
The second step is to obtain a correlation diagram between the electrical resistivity of unimproved and improved ground (Rs and Rimp) and the electrical resistivity of pore water and chemical solutions (Rpw and Rc).
a third step of obtaining a correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rimp) of the improved ground based on the correlation diagram between silica concentration (SiO2) and electrical resistivity (Rc) obtained in the first step and the correlation diagram between the electrical resistivity (Rs·Rimp) of the unimproved and improved ground and the electrical resistivity (Rpw _· Rc) of the pore water and chemical solution obtained in the second step;
A fourth step is to obtain a correlation diagram between silica concentration (SiO ₂ ) and unconfined compressive strength (q u ) using a test specimen prepared indoors in order to set the liquid silica concentration corresponding to the target unconfined compressive strength (q u );
a fifth step in which the correlation diagram between the silica concentration ( _SiO2 ) obtained in the third step and the electrical resistivity (Rimp) of the improved ground is arranged to obtain a correlation diagram in which one axis represents the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co) and the other axis represents the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp);
a sixth step of arranging the correlation diagram between the silica concentration ( _SiO2 ) and the unconfined compressive strength (qu) obtained in the fourth step to obtain a correlation diagram in which one axis represents the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co) and the other axis represents the unconfined compressive strength (qu);
Based on the correlation diagram obtained in the fifth step and the correlation diagram obtained in the sixth step, a correlation diagram is obtained by a seventh step in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp).

In the invention described in claim 1, when evaluating the effect of ground improvement (uniaxial compressive strength) by the chemical grouting method, a correlation diagram is obtained in advance, with one axis being the uniaxial compressive strength (qu) and the other axis being the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and a threshold value is set for the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) under conditions that can ensure the target uniaxial compressive strength (quck). The specific method of obtaining the correlation diagram is as follows: Steps 1 to 7 below .

Then, at the site, electrical resistivity is measured by electrical logging using a penetration hole drilled vertically in the ground before and after ground improvement, and the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) is calculated. Whether or not the target uniaxial compressive strength (quck) has been achieved is determined based on whether this electrical resistivity ratio (Rimp/Runimp) is greater or smaller than the threshold value.

Therefore, by measuring the electrical resistivity before and after improvement, it becomes possible to evaluate the uniaxial compressive strength of the improved body. In other words, in the past, a qualitative assessment of whether or not the chemical solution was filled was made by comparing the electrical resistivity before and after improvement, but according to the present invention, it is possible to easily grasp the numerical value of the uniaxial compressive strength after improvement from the electrical resistivity before and after improvement, making it possible to quantitatively evaluate the improvement effect.

The correlation diagram is obtained by: a first step of diluting a silica solution with water and measuring the electrical resistivity for each silica concentration in order to grasp the electrical resistivity characteristics of the silica solution used for ground improvement, and obtaining a correlation diagram between silica concentration (SiO ₂ ) and electrical resistivity (Rc);
The second step is to obtain a correlation diagram between the electrical resistivity of unimproved and improved ground (Rs and Rimp) and the electrical resistivity of pore water and chemical solutions (Rpw and Rc).
a third step of obtaining a correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rimp) of the improved ground based on the correlation diagram between silica concentration (SiO2) and electrical resistivity (Rc) obtained in the first step and the correlation diagram between the electrical resistivity (Rs·Rimp) of the unimproved and improved ground and the electrical resistivity (Rpw _· Rc) of the pore water and chemical solution obtained in the second step;
A fourth step is to obtain a correlation diagram between silica concentration (SiO ₂ ) and unconfined compressive strength (q u ) using a test specimen prepared indoors in order to set the liquid silica concentration corresponding to the target unconfined compressive strength (q u );
a fifth step in which the correlation diagram between the silica concentration ( _SiO2 ) obtained in the third step and the electrical resistivity (Rimp) of the improved ground is arranged to obtain a correlation diagram in which one axis represents the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co) and the other axis represents the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp);
a sixth step of arranging the correlation diagram between the silica concentration ( _SiO2 ) and the unconfined compressive strength (qu) obtained in the fourth step to obtain a correlation diagram in which one axis represents the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co) and the other axis represents the unconfined compressive strength (qu);
Based on the correlation diagram obtained in the fifth step and the correlation diagram obtained in the sixth step, a correlation diagram is obtained in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) .

The present invention according to claim 2 provides a method for evaluating the effect of ground improvement by the liquid grouting method according to claim 1 , for improving ground having a salinity concentration of pore water of 5,000 to 10,000 ppm.

The invention described in claim 2 clarifies the application range (pore water salinity range) of the method for evaluating the effect of ground improvement using the chemical grouting method according to the present invention. By clarifying the application range, reliable evaluation is possible even in coastal areas where evaluation using the electrical resistivity method has been considered unsuitable.

As the present invention of claim 3 , there is provided a method for evaluating the effect of ground improvement using the chemical injection method described in either claim 1 or 2, in which, instead of the correlation diagram in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), a correlation diagram in which one axis is the liquefaction resistance ratio (RL) or adhesion (c) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground, modified based on the conversion formula between the uniaxial compressive strength (qu) and the liquefaction resistance ratio (RL) or the conversion formula between the uniaxial compressive strength (qu) and the adhesion (c), is used.

The invention described in claim 3 above takes into consideration that the uniaxial compressive strength (qu) and the liquefaction resistance ratio (RL) can be converted using a certain conversion formula, and that the uniaxial compressive strength (qu) and the cohesion (c) can be converted using a certain conversion formula, and instead of a correlation diagram in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), the effect of ground improvement is judged based on a correlation diagram in which the uniaxial compressive strength (qu) on one axis is the liquefaction resistance ratio (RL) or the cohesion (c).

As the present invention of claim 4 , there is provided a method for evaluating the effect of ground improvement using the chemical injection method described in either claim 1 or 2, which uses a correlation diagram in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), modified based on the conversion formula for electrical resistivity (R) and electrical conductivity (σ) (R(Ω·m)＝1/σ(S/m)). The correlation diagram has one axis being the uniaxial compressive strength (qu) and the other axis being the ratio (σunimp/σimp) of the electrical conductivity of the unimproved ground (σunimp) to the electrical conductivity of the improved ground (σimp).

The invention described in claim 4 above takes into consideration that electrical resistivity (R) and electrical conductivity (σ) can be converted using a conversion formula (R = 1/σ), and instead of a correlation diagram in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of improved ground (Rimp) to the electrical resistivity of unimproved ground (Runimp), the effect of ground improvement is judged based on a correlation diagram in which the ratio (Rimp/Runimp) of the electrical resistivity of improved ground (Rimp) to the electrical resistivity of unimproved ground (Runimp) on the other axis is the ratio (σunimp) of the electrical conductivity of unimproved ground (σunimp) to the electrical conductivity of improved ground (σimp) (σunimp/σimp).

As explained above in detail, the present invention makes it possible to provide a method for evaluating the effect of ground improvement using a chemical injection method with electrical logging that can evaluate the uniaxial compressive strength of the improved body by measuring the electrical resistivity before and after improvement.

In addition, the scope of application of the method for evaluating the effect of ground improvement using the chemical injection method of the present invention (range of salt concentration of pore water) will be clarified.

This is a vertical cross-sectional view of a small cylindrical soil tank used in small soil layer experiments. This is the particle size accumulation curve of the sand used. FIG. 2 is a plan view showing the positions at which electrical resistivity measurements are performed. 1 is a graph showing the relationship between silica concentration (SiO ₂ ) and electrical resistivity (Rc). This is a graph showing the relationship between electrical resistivity and pore water salinity of unimproved and improved ground using silica sand No. 7 and Enshu silica sand. This is a graph showing the relationship between the electrical resistivity of unimproved and improved ground and the electrical resistivity of interstitial water and chemical solutions. 1 is a graph showing the relationship between the electrical resistivity of improved ground and the silica concentration in the chemical solution. 1 is a graph showing the relationship between the unconfined compressive strength and silica concentration of silica sand No. 7 (produced in Seto). FIG. 13 is a side view of a device for pressing a probe into a penetration hole used in a field demonstration experiment. This is a soil column diagram of the in-situ soil. This is the particle size accumulation curve of the in-situ soil. FIG. 2 is a plan view of the chemical solution improver. FIG. 2 is a cross-sectional view of a chemical solution improver. The values are qu (uniaxial compressive strength), _Nd value ( _Nd value equivalent to the N value obtained from a small dynamic cone penetration test), R (electrical resistivity obtained from a push-type micro-electrical logging (electrode spacing 2.5 cm)), and Rimp/Runimp (the ratio of electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp)) of the improved body A. The values are qu (uniaxial compressive strength), _Nd value ( _Nd value equivalent to the N value obtained from a small dynamic cone penetration test), R (electrical resistivity obtained from a push-type micro-electrical logging (electrode spacing 2.5 cm)), and Rimp/Runimp (the ratio of electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp)) of the improved body B. This is a graph showing the relationship between the electrical resistivity of unimproved and improved ground and the interstitial water and chemical solution. 1 is a graph showing the relationship between the electrical resistivity of improved ground and the silica concentration in the chemical solution. 1 is a graph showing the relationship between unconfined compressive strength and silica concentration obtained by a blending test. This is a graph showing the relationship between the ratio (c/co) of the silica concentration (c) after dilution of improved body A to the intended silica concentration (co) and the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity (Runimp) of the unimproved ground. This is a graph showing the relationship between the ratio (c/co) of the silica concentration (c) after dilution of improved body B to the intended silica concentration (co) and the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity (Runimp) of the unimproved ground. 1 is a graph showing the relationship between the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) for improved bodies A and B, and the unconfined compressive strength (qu). This is a graph showing the relationship between the unconfined compressive strength qu and the ratio (Rimp/Runimp) of the electrical resistivity of improved ground (Rimp) to the electrical resistivity of unimproved ground (Runimp). FIG. 1 shows the correlation between unconfined compressive strength (qu) and liquefaction strength ratio (RL). FIG. 1 shows the correlation between unconfined compressive strength (qu) and adhesive strength (c).

The following describes in detail the embodiments of the present invention with reference to the drawings.

The present invention is a method for evaluating the effect of ground improvement using a chemical injection method in which a chemical solution made of water glass (sodium silicate) or the like is injected into the ground to strengthen soft ground such as reclaimed land, and specifically, the method involves the following steps.

A correlation diagram is prepared in advance, with the vertical axis representing the uniaxial compressive strength (qu) and the horizontal axis representing the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and a threshold value is set for the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) under conditions that can ensure the target uniaxial compressive strength (quck) based on the correlation diagram.
Before and after ground improvement, electrical resistivity is measured by electrical logging using a penetration hole formed vertically in the ground, and the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) is determined. Depending on whether this electrical resistivity ratio (Rimp/Runimp) is greater or smaller than the threshold value, it is determined whether the target uniaxial compressive strength (quck) has been secured.

In other words, in this invention, prior to ground improvement using the chemical injection method, a correlation diagram (hereinafter also referred to as a uniaxial compressive strength correlation diagram) is obtained by performing an indoor experiment using in-situ soil from the improved ground, with the vertical axis representing the uniaxial compressive strength (qu) and the horizontal axis representing the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp). At this time, based on the uniaxial compressive strength correlation diagram, a threshold value is set for the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) under conditions that can ensure the target uniaxial compressive strength (quck).

An electrical logging probe is then inserted into the penetration hole formed in the ground before ground improvement to measure electrical resistivity (Rimp) at each specified depth. Also, after ground improvement, an electrical logging probe is inserted into the penetration hole formed in the ground to measure electrical resistivity (Rimp) at each specified depth.

Next, it is possible to determine whether the target uniaxial compressive strength (quck) has been secured by determining the ratio of electrical resistivity (Rimp/Runimp) before and after ground improvement and whether this ratio of electrical resistivity (Rimp/Runimp) is greater than or less than the threshold value. In addition to determining whether the target uniaxial compressive strength (quck) has been secured, it is also possible to actually fit the ratio of electrical resistivity (Rimp/Runimp) before and after ground improvement onto the horizontal axis of the uniaxial compressive strength correlation diagram and obtain the specific value of the uniaxial compressive strength qu of the improved ground from the intersection with the correlation line.

Next, we will explain in detail the procedure for obtaining the uniaxial compressive strength correlation diagram.

(First Step)
First, in order to understand the electrical resistivity characteristics of the silica solution used for ground improvement, the silica solution is diluted with water and the electrical resistivity is measured for each silica concentration, and a correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rc) is obtained. In other words, the amount of water glass (silica solution) injected into the ground for ground improvement is quantitatively understood to what extent the electrical resistivity decreases. An example of this correlation diagram is shown in Figure 4 below.

(Second Procedure)
A correlation diagram is obtained between the electrical resistivity (Rs/Rimp) of unimproved and improved ground and the electrical resistivity (Rpw/Rc) of pore water and chemical solutions. In other words, the electrical resistivity (Rs/Rimp) of unimproved and improved ground is governed by the electrical resistivity (Rpw/Rc) of pore water and chemical solutions, and the relationship between the electrical resistivity (Rs/Rimp) of unimproved and improved ground and the electrical resistivity (Rpw/Rc) of pore water and chemical solutions is extremely high. An example of this correlation diagram is shown in Figure 6 below.

(Third Procedure)
A correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rc) obtained in the first step and a correlation diagram between the electrical resistivity (Rs/Rimp) of unimproved and improved ground and the electrical resistivity (Rpw/Rc) of pore water and chemical solution obtained in the second step is used to obtain a correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rimp) of improved ground. That is, the silica concentration (SiO ₂ ) in the correlation diagram between silica concentration (SiO _{2 ) and electrical resistivity (Rc) obtained in the first step is left as it is, and the electrical resistivity (Rc) is converted using the correlation diagram between silica concentration (SiO 2 )} and electrical resistivity (Rimp) of the improved ground based on the correlation diagram between the electrical resistivity (Rs・Rimp) of the unimproved and improved ground and the electrical resistivity (Rpw・Rc) of the pore water and chemical solution obtained in the second step, to obtain a correlation diagram between silica concentration (SiO ₂ ) and electrical resistivity (Rimp) of the improved ground. This correlation diagram becomes one side of the correlation diagram data (electrical resistivity data) required to obtain the desired correlation diagram of uniaxial compressive strength. An example of this correlation diagram is shown in Figure 7 described later.

(Fourth Procedure)
Next, in order to set the silica concentration of the chemical solution corresponding to the target unconfined compressive strength (quck), a correlation diagram between the silica concentration ( _SiO2 ) and the unconfined compressive strength (qu) is obtained using a specimen prepared indoors. That is, a correlation diagram between the silica concentration ( _SiO2 ) and the unconfined compressive strength (qu) is obtained in correspondence with the correlation diagram between the silica concentration ( _SiO2 ) and the electrical resistivity (Rc) obtained in the first step. An example of this correlation diagram is shown in Figure 8, which will be described later.

(Fifth Step)
The correlation diagram between the silica concentration ( _SiO2 ) obtained in the third step and the electrical resistivity (Rimp) of the improved ground is rearranged to obtain a correlation diagram with the horizontal axis representing the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) and the vertical axis representing the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp).

That is, in order to obtain a correlation diagram with the uniaxial compressive strength (qu) as the vertical axis and the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) as the horizontal axis, the correlation diagram between the silica concentration (SiO ₂ ) and the electrical resistivity of the improved ground obtained in the third step is converted. Since the silica concentration (SiO ₂ ) is an absolute concentration, it is generalized as a relative value. In other words, the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co). This makes it possible to generalize the silica concentration parameter on the horizontal axis regardless of the desired concentration value. In addition, the electrical resistivity (Rimp) of the improved ground on the vertical axis is set to the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground, thereby generalizing the parameter. Examples of this converted correlation diagram are shown in Figures 19 and 20 described later.

(Sixth Step)
The correlation diagram between silica concentration ( _SiO2 ) and unconfined compressive strength (qu) obtained in the fourth step above is rearranged to obtain a correlation diagram with the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) on the horizontal axis and the unconfined compressive strength (qu) on the vertical axis.

That is, the correlation diagram between silica concentration ( _SiO2 ) and unconfined compressive strength (qu) obtained in the fourth step is converted. Since silica concentration ( _SiO2 ) is an absolute value, it is generalized as a relative value. In other words, the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co). This makes it possible to generalize the silica concentration parameter on the horizontal axis regardless of the intended concentration value. Examples of this converted correlation diagram are shown in Figures 21(A) and 21(B).

(Seventh Step)
The uniaxial compressive strength correlation diagram is obtained based on the correlation diagram obtained in the fifth step and the correlation diagram obtained in the sixth step.

In other words, in the fifth step, a correlation diagram was obtained between the electrical resistivity (Rimp) of the improved ground when the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) and the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and in the sixth step, a correlation diagram was obtained between the unconfined compressive strength (qu) when the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co).

These correlation diagrams share the common horizontal axis, which is the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co), so it is possible to draw a correlation diagram using the relationship between the vertical axes. This graph is the aforementioned uniaxial compressive strength correlation diagram that we ultimately want to obtain. An example of this correlation diagram is shown in Figure 22.

In the uniaxial compressive strength correlation diagram, if a threshold value for the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) that satisfies the conditions for securing the target uniaxial compressive strength (quck) is set in advance, the electrical resistivity can be measured before and after improvement to determine the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and whether the target uniaxial compressive strength (quck) has been secured can be determined based on whether this electrical resistivity ratio (Rimp/Runimp) is greater or smaller than the threshold value.

The method of the present invention requires that the electrical resistivity of the improved ground (Rimp) is clearly different from the electrical resistivity of the unimproved ground (Runimp) before and after improvement. Therefore, as will be shown in the experiment described below, ground that meets this condition is ground with a salinity of the pore water of 5,000 to 10,000 ppm. Therefore, the method of the present invention should be applied to ground with a salinity of the pore water of 5,000 to 10,000 ppm.

1. Small-scale soil tank experiment First, a small-scale soil tank experiment was conducted to clarify the electrical resistivity characteristics of the improved sand and the range of application of this method.

(1) Experimental overview This experiment was conducted to confirm the effect of the salinity (electrical resistivity) of pore water on the resistivity of the ground, and to verify whether it is possible to distinguish improved areas from the change in resistivity before and after improvement. The experiment was conducted using a small cylindrical soil tank as shown in Figure 1. Unimproved ground was prepared using pore water with different salinity concentrations, and the same ground was improved by injecting a special silica liquid into the same ground to prepare chemically improved ground, and the electrical resistivity of each ground was measured.

(2) Experimental method Unimproved ground was prepared by filling a soil tank with dry sand using the air-drop method to a specified density, and then injecting carbon dioxide gas from the bottom of the soil tank to replace the air in the gaps with carbon dioxide gas. After that, deaerated water with a specified salt concentration was allowed to infiltrate from the bottom of the soil tank at a head difference below the critical hydraulic gradient. Improved ground was prepared by injecting a special silica liquid with a specified silica concentration into the bottom of the soil tank after the unimproved ground was prepared. The sands used in the experiment were silica sand No. 7 (from Seto) and Enshu silica sand. The physical properties and particle size accumulation curves of the sands used are shown in Table 1 and Figure 2 below, and the conditions for preparing the soil tank ground are shown in Table 2 below.

An electrical logging probe was used to measure the electrical resistivity. The probe has a point electrode (four-pole method) arrangement with an electrode spacing of 25 mm. Furthermore, with previous circularly arranged electrodes, the ratio of the measurement hole diameter to the electrode diameter has a significant effect on the measured electrical resistivity, so a point electrode was used that does not have this effect.

Measurements were carried out at the plan position shown in Figure 3, with the probe bottom end at depths of 14cm, 15cm, 16cm and 17cm from the ground surface. Therefore, the number of measurement points per soil tank was 20 for unimproved ground and 8 for improved ground. After resistivity measurements were taken for the improved ground, a thin wall liner with an inner diameter of 75mm and height of 150mm was pressed into the improved ground to collect samples, which were then used for uniaxial compression tests and silica content tests.

(3) Electrical resistivity of special silica liquid In order to understand the electrical resistivity characteristics of the special silica liquid during the experiment, the liquid was diluted with water and the electrical resistivity was measured for each silica concentration in the liquid. The relationship between the electrical resistivity of the special silica liquid used in the experiment and the silica concentration in the liquid is shown in Figure 4. The electrical resistivity increases sharply when the silica concentration is lower than 1.5wt%, but when it exceeds 2.0wt%, the electrical resistivity decreases due to the increase in silica concentration, but remains in the range of 0.4 to 1.3Ω・m.

(4) Experimental results Figure 5 shows the relationship between electrical resistivity and pore water salinity for unimproved and improved ground using silica sand No. 7 (from Seto) and Enshu silica sand. Figure 6 shows the relationship between the electrical resistivity Runimp and Rimp of the unimproved and improved ground and the electrical resistivity Rpw and Rc of the pore water and chemical solution. The electrical resistivity is the average value of each measurement point shown in Figure 3, measured with an electrode spacing of 25 mm. The coefficient of variation of the measured electrical resistivity was small, with silica sand No. 7 (from Seto) unimproved: approximately 0.04, improved: approximately 0.02, and Enshu silica sand unimproved: approximately 0.06, improved: approximately 0.02.

The electrical resistivity of unimproved and improved ground is governed by the electrical resistivity of the pore water and chemical solutions, and there is an extremely high correlation between the electrical resistivity of unimproved and improved ground and the electrical resistivity of the pore water and chemical solutions. The figure also shows that in unimproved ground, the resistivity decreases as the salinity of the pore water increases.

On the other hand, in improved ground, the electrical resistivity is almost constant regardless of the salinity of the pore water. The electrical resistivity of unimproved ground and improved ground is similar when the salinity of the pore water is around 10,000 ppm, and once the salinity exceeds this concentration, the electrical resistivity of the pore water becomes smaller than the resistivity of the chemical solution.

From the above, the difference in electrical resistivity between unimproved and improved soils depends on the type of soil, but it is believed possible to distinguish between soils with pore water salinity concentrations of up to about 5,000 to 10,000 ppm.

Figure 7 shows the relationship between the electrical resistivity of the improved ground and the silica concentration of the chemical solution, calculated from the relationship between the electrical resistivity of the unimproved and improved ground, pore water, and chemical solution shown in Figure 6, and the relationship between the electrical resistivity of the chemical solution and silica concentration shown in Figure 4. The figure also shows the electrical resistivity of the chemical solution. When the chemical solution silica concentration is 2 wt% or more, the electrical resistivity of the chemical solution is in the range of 0.4 to 1.3 Ω·m, while the range of the electrical resistivity of the improved ground is wider, at 1.0 to 3.3 Ω·m.

The results of this experiment, which used electrical logging, showed that there was almost no variation in the electrical resistivity values, and the electrical resistivity ratio Rimp/Runimp of improved and unimproved ground was proportional to the electrical resistivity ratio Rc/Rpw of the chemical solution and pore water.

From the above experimental results,
(a) There is an extremely high correlation between the electrical resistivity of unimproved ground and improved ground and the electrical resistivity of pore water and chemical solutions.
(b) The difference in electrical resistivity between unimproved and improved soil depends on the type of soil, but it is possible to distinguish up to a salinity concentration of pore water of approximately 5,000 to 10,000 ppm.
(c) The electrical resistivity of the special silica liquid increases sharply when the silica concentration is lower than 1.5 wt%. On the other hand, when the silica concentration exceeds 2.0 wt%, the electrical resistivity of the liquid does not change much and is in the range of 0.4 to 1.3 Ω m, but in improved ground, the range becomes wider, from 1.0 to 3.3 Ω m.
(d) Electrical resistivity measurements using this electrical logging method produce values with little variation regardless of the value of Rc/Rpw.

Next, the improvement effect is evaluated using electrical resistivity, and Rimp for each silica concentration can be obtained from the relationship between the electrical resistivity Rimp and silica concentration of the improved ground shown in Figure 7. In addition, in actual construction, uniaxial compression tests for each silica concentration are carried out using specimens prepared indoors, with the aim of setting the chemical silica concentration that corresponds to the design standard strength quck. Figure 8 shows the relationship between the uniaxial compressive strength qu and silica concentration ( _SiO2 ) of silica sand No. 7 (from Seto).

(5) Summary In this experiment, we verified the effect of the salinity of pore water (electrical resistivity) on the resistivity of the ground and whether it is possible to distinguish the improved area from the change in resistivity before and after improvement. The results of the experiment, as shown in (1) to (3) below, were obtained.
(a) The difference in electrical resistivity between unimproved and improved ground depends on the type of ground, but it is possible to distinguish between salinity concentrations of pore water of up to about 5,000 to 10,000 ppm. In addition, the applicability of this method can be determined by conducting experiments in advance using on-site soil and sand.
(b) Electrical resistivity measurements using point electrodes based on electrical logging provide values with little variation regardless of the value of Rc/Rpw. Therefore, minute changes in resistivity can be detected.
(c) The improvement effect using electrical resistivity can be evaluated using the relationship between the electrical resistivity of the ground and the silica concentration, and the relationship between the improvement strength and silica concentration.

2. On-site demonstration experiment
(1) Experimental overview In this experiment, small dynamic cone penetration tests were conducted on the ground before and after improvement using the chemically improved ground obtained through the improvement test construction, and the electrical resistivity was measured by electrical logging using the same penetration hole to evaluate the improvement effect of the improved area. In addition, the Nd values obtained from the small dynamic cone penetration tests were compared with the results of uniaxial compression tests and cyclic triaxial tests conducted using undisturbed samples to verify the effectiveness of this method. Note that the indentation-type micro-electrical logging method was used to measure the electrical resistivity.

(2) Overview of the Push-in Micro-Electrical Logging Method Unlike general electrical logging, which uses electrodes inserted into a borehole for measurement, the push-in micro-electrical logging method uses the borehole for measurement after a dynamic cone penetration test or the like is performed. The probe for the push-in micro-electrical logging method is a point electrode (two-pole method) with an electrode spacing of 2.5 cm and 5.0 cm. The diameter of the probe is 32 mm, assuming that it will be pushed into the penetration hole of a small dynamic cone penetration test. In addition, a 3 mm protrusion is provided on the side of the sleeve to press the probe electrode against the wall of the penetration hole. Figure 9 shows the device 1 for pushing the probe into the penetration hole.

The pressing device 1 is configured such that pistons 10, 10 that are extendable and retractable in the vertical direction are arranged on both sides of the penetration hole 3 on the ground surface directly above the penetration hole 3, and a chuck 12 that holds a penetration rod 4 with a measurement probe 2 connected to its lower end is provided in the center of a stand 11 that straddles the upper ends of the pistons 10, 10, and a control unit 13 that controls the operation of the pistons 10, 10. A hydraulic unit 14 consisting of an engine and a hydraulic pump is connected to the control unit 13.

In the press-fitting device 1, the pistons 10 on both sides expand and contract in unison, and the base 11 moves vertically, causing the penetration rod 4 clamped by the chuck 12 to move vertically, and the measurement probe 2 is pushed into and pulled out of the penetration hole 3.

(3) Overview of the experimental site Figures 10 and 11 show the soil column diagram, N value, and grain size accumulation curve of the experimental site, and Table 3 below shows the physical properties. The strata consist of embankment, sand-mixed silt, and silt-mixed sand below that, from the surface. The silt-mixed sand, which is the target layer for chemical improvement, contains organic soil from GL-2m to -4m below ground level, and contains a layer of silt below GL-5m. The groundwater level is GL-1.1m, the electrical resistivity of the groundwater is 4-12 Ω・m, and the salinity is in the range of 400-1,300ppm.

Figure 12 shows a plan view of the chemical improvement body, Figure 13 shows its cross-sectional view, and the improvement specifications are shown in Table 4 below. Both Improvement A and Improvement B satisfied the planned improvement diameter of 2.5 m, and no unconsolidated areas were observed on the surface of the improvement body.

(4) Experimental method After backfilling the improved body, a small dynamic cone penetration test (Penny) was conducted, and a push-type micro-electrical logging was carried out using the same penetration hole. Measurements were carried out at one unimproved location, two improved locations A, and two improved locations B, as shown in Figure 12. The measurement depths were Unimproved: GL-10m, Improved A: GL-8m, and Improved B: GL-5m. The age of the improved body at the time of measurement was approximately 23 months. In addition, small model ground similar to that described above was created using local soil and sand, and the electrical resistivity of the unimproved and improved ground was measured.

(5) Experimental results Figures 14 and 15 show the depth distribution of the uniaxial compressive strength q, the _Nd value equivalent to N obtained from a small dynamic cone penetration test, the electrical resistivity R obtained from a push-type micro-electrical logging (electrode spacing 2.5 cm), and the ratio of electrical resistivity between the improved ground and the unimproved ground (Rimp/Runimp) for Improvement A and Improvement B, respectively.

(a) Uniaxial compressive strength q
The unconfined compressive strength test was carried out on improved sand samples (28 days old) taken with block sampling and a rotary triple tube sampler during the improvement test construction. The strength of the improved body was evaluated using samples taken at 1/2 the radius of the improved body, so Figures 14 and 15 show the test results of samples taken from the same position. Furthermore, when organic soil, humus, etc. was present within the sampled specimens, cracks occurred along the same points, resulting in significantly lower values, so they were classified according to the presence or absence of organic soil or humus.

The uniaxial compressive strength qu varies particularly widely in improvement A. This is mainly due to cracks occurring due to the presence of organic soil and humus in the collected specimens, but similar specimens were often seen in samples collected with a triple tube sampler. It was difficult to quantitatively grasp the change in properties before and after improvement in uniaxial compressive strength in natural ground.

(b) _Nd value With improvement A, the _Nd value generally increased after improvement. The average increase in _Nd value due to improvement was approximately 16 at +10cm from the improvement center, and approximately 6 at +60cm from the improvement center in the upper improved body from GL -2m to -4m, and approximately 6 at +10cm from the improvement center, and approximately 6 at +60cm from the improvement center in the lower improved body from GL -4m to -6m. On the other hand, with improvement B, which had a target improvement strength that was half that of improvement A, an increase in the _Nd value was observed in the upper half of the improved body, but no increase was observed in the lower half.

(c) Electrical resistivity R
In unimproved ground, electrical resistivity R shows high values of around 90 to 130 Ω·m at around GL-3.0m to -4.0m, but is generally 30 to 60 Ω·m at other depths. On the other hand, in improved ground, improvement A shows low values of R = 1 to 3 Ω·m at all improvement depths from GL-2.0m to GL-6.0m, and improvement B shows 2 to 10 Ω·m at GL-2.0m to -3.5m, and 20 to 70 Ω·m at GL-3.5m to 4.0m. It can be seen that electrical resistivity is greatly reduced by the improvement work.

(d) Evaluation of improvement effect In order to evaluate the improvement effect based on the electrical resistivity measured on-site, a model ground was prepared using on-site soil in the same manner as in the small soil tank experiment described above, and the electrical resistivity of the unimproved and improved ground was measured. The soil tank preparation conditions were: soil density: D _r =50% (e=0.911), salinity of pore water: 400, 1,300, 20,000 ppm, chemical solution used: special silica liquid 8 wt%. The improved ground was prepared by injecting the special silica liquid into the same ground.

Therefore, the electrical resistivity of the improved ground measured in this experiment is considered to be under conditions where the chemical filling rate is close to 100%.

Figure 16 shows the relationship between the electrical resistivity of unimproved and improved ground and the pore water and chemical solution, and Figure 17 shows the relationship between the electrical resistivity Rimp of the improved ground and the chemical solution silica concentration ( _SiO2 ), calculated from the same figure and the relationship between the electrical resistivity and silica concentration of the chemical solution shown in Figure 4. Figure 18 shows the relationship between the unconfined compressive strength qu and the silica concentration ( _SiO2 ) obtained from the mix test, and Figure 22 shows the relationship between the unconfined compressive strength qu and the ratio of electrical resistivity Rimp/Runimp of the improved ground to the unimproved ground, compiled from Figures 18 and 17.

Specifically, the correlation diagram in Figure 22 (relationship between the uniaxial compressive strength qu and the ratio of electrical resistivity Rimp/Runimp of improved ground to unimproved ground) is obtained by the following procedure.

The correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rimp) of the improved ground shown in Figure 17 is rearranged (converted) to obtain a correlation diagram as shown in Figures 19 and 20, in which the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) and the vertical axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp).

Figure 19 is a graph converted for improvement A. In this case, it is assumed that Runimp = 15 Ω・m (salt concentration 1,300 ppm groundwater salinity). The intended silica concentration is co = 8 wt%. In the figure, the left side of the figure is a graph with the horizontal axis representing the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co), and the right side of the figure is a graph with the vertical axis representing the electrical resistivity (Rimp) of the improved ground as the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp).

Figure 20 is a graph converted for improvement B. In this case, Runimp = 15 Ω・m (salt concentration 1,300 ppm groundwater salinity). The intended silica concentration is co = 5 wt%. In the figure, the left side of the figure is a graph with the horizontal axis representing the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co), and the right side of the figure is a graph with the vertical axis representing the electrical resistivity (Rimp) of the improved ground as the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp).

Next, the correlation diagram between silica concentration ( _SiO2 ) and unconfined compressive strength (qu) shown in Figure 18 is rearranged to obtain a correlation diagram in which the horizontal axis represents the ratio (c/co) of the silica concentration after dilution (c) to the initial silica concentration (co) and the vertical axis represents the unconfined compressive strength (qu), as shown in Figure 21. Note that Figure 21(A) is Improvement A and Figure 21(B) is Improvement B.

The correlation diagrams in Figures 19 and 20 and the correlation diagram in Figure 21 have a common horizontal axis of the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co), so by sorting the two by the vertical axis relationship, we can obtain the correlation diagram shown in Figure 22, between the unconfined compressive strength qu and the ratio of electrical resistivity Rimp/Runimp of improved ground to unimproved ground.

In FIG. 22, when an approximation line is drawn using the least squares method, a threshold value is set to judge the improvement effect of improvement A (target improvement strength quck≧100 kPa) and improvement B (target improvement strength quck≧50 kPa), and for improvement A, Rimp/Runimp≦0.13, and for improvement B, Rimp/Runimp≦0.16.
(e) Observations Looking at the depth distribution of Rimp/Runimp shown in Figures 14 and 15, in Improved A, Rimp/Runimp was generally 0.1 or less at depths of GL-2.0m to -6.0m. On the other hand, in Improved B, Rimp/Runimp was 0.1 or less at depths of GL-2.0m to -3.5m, and was 0.25 to 0.8 at depths of GL-3.5m to -4.0m.

The thresholds for judging the improvement effect, set from Figure 22, were Improvement A: Rimp/Runimp≦0.13, Improvement B: Rimp/Runimp≦0.16. Therefore, in Improvement A, filling of the chemical solution was observed at GL -2.0m to -6.0m, and in Improvement B, filling of the chemical solution was observed at GL -2.0m to -3.5m, but sufficient filling was not observed at GL -3.5m to -4.0m.

(6) Summary In this experiment, the electromagnetic resistivity of the improved ground was measured by electrical logging (push-type micro logging) using point electrodes, and the improvement effect was evaluated. The results of the experiment are shown in (a) and (b) below.
(a) The resistivity measured by the newly developed indentation micro-logging method can sensitively express the difference before and after chemical improvement.
(b) The evaluation of the improvement effect using the electrical resistivity measured by the same method is valid when compared with the increment in _Nd value, unconfined compressive strength and liquefaction strength ratio in the improved area.

3. Conclusions In this experiment, we focused on the change in electrical resistivity before and after improvement to confirm the progress of the chemical grouting method, and conducted a small soil tank experiment and a field demonstration experiment using electrical logging with point electrodes. The following conclusions (1) to (4) were obtained from this experiment.
(1) The electrical resistivity of the special silica liquid increases sharply when the silica concentration falls below 1.5 wt%, and when it exceeds 2.0 wt%, there is no significant change in the electrical resistivity of the liquid, remaining in a narrow range of 0.4 to 1.3 Ω·m. However, in improved ground, the range becomes wider, from 1.0 to 3.3 Ω·m, when the silica concentration exceeds 2.0 wt%.
(2) The difference in electrical resistivity between unimproved and improved ground depends on the type of ground, but it is possible to distinguish salinity concentrations of pore water up to about 5,000 to 10,000 ppm. In addition, the applicability of this method can be determined by conducting experiments in advance using on-site soil and sand.
(3) Electrical resistivity measurements using point electrodes can obtain values with less variation regardless of the value of Rc/Rpw compared to measurements using resistivity tomography. In other words, highly accurate values can be obtained even in ground with low electrical resistivity of pore water.
(4) The effectiveness of improvement using electrical resistivity can be evaluated using the relationship between the electrical resistivity of the ground and the silica concentration, and the relationship between the improvement strength and the silica concentration. However, in order to set the threshold value of electrical resistivity for judging the improvement effectiveness, it is necessary to clarify the effects of dilution of the silica concentration, etc.

[Other examples]
(1) In the above example, the effect of ground improvement was judged using a correlation diagram (uniaxial compressive strength correlation diagram) in which the vertical axis is the uniaxial compressive strength (qu) and the horizontal axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to that of the unimproved ground (Runimp). The uniaxial compressive strength (qu) and the liquefaction strength ratio (RL) can be converted using a certain conversion formula. The uniaxial compressive strength (qu) and the cohesion strength (c) can also be converted using a certain conversion formula. Specifically, the correlation formula between the uniaxial compressive strength (qu) and the liquefaction strength ratio (RL) is shown in Figure 23 (Source: Seepage Solidification Treatment Method Technical Manual Revised Edition July 2019, Coastal Technology Research Center), and the correlation formula between the uniaxial compressive strength (qu) and the cohesion strength (c) is shown in Figure 24 (Source: Seepage Solidification Treatment Method Technical Manual Revised Edition July 2019, Coastal Technology Research Center).
Therefore, in a correlation diagram (uniaxial compressive strength correlation diagram) in which the vertical axis is the uniaxial compressive strength (qu) and the horizontal axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), it is also possible to replace the uniaxial compressive strength (qu) on the vertical axis with the liquefaction strength ratio (RL) or adhesion strength (c) and use this to judge the effectiveness of ground improvement.

(2) In the above example, the effect of ground improvement was judged using a correlation diagram (unconfined compressive strength correlation diagram) with the vertical axis being unconfined compressive strength (qu) and the horizontal axis being the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to that of the unimproved ground (Runimp), but electrical resistivity and electrical conductivity can be converted using a conversion formula. Specifically, electrical resistivity (R) and electrical conductivity (σ) have the relationship R (Ω·m) = 1/σ (S/m).
Therefore, by applying the conversion formula, the ratio (Rimp/Runimp) of the electrical resistivity of improved ground (Rimp) to the electrical resistivity of unimproved ground (Runimp) can be converted into the ratio (σunimp/σimp) of the electrical conductivity of unimproved ground (σunimp) to the electrical conductivity of improved ground (σimp).
Therefore, in a correlation diagram (uniaxial compressive strength correlation diagram) in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), it is also possible to create a correlation diagram in which the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) on the other axis is replaced with the ratio (σunimp) of the electrical conductivity of the unimproved ground (σimp) to the electrical conductivity (σimp) of the improved ground (σunimp/σimp), and use this to judge the effectiveness of ground improvement.

(3) In the above example, in the uniaxial compressive strength correlation diagram, the vertical axis is the uniaxial compressive strength (qu) and the horizontal axis is the ratio of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) (Rimp/Runimp), but it is also possible to use the vertical axis as the ratio of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) (Rimp/Runimp) and the horizontal axis as the uniaxial compressive strength (qu).
Similarly, it is also possible to interchange the horizontal and vertical axes in the correlation diagram in the fifth step in which the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) and the vertical axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity (Runimp) of the unimproved ground, and in the correlation diagram in the sixth step in which the horizontal axis is the ratio (c/co) of the silica concentration after dilution (c) to the intended silica concentration (co) and the vertical axis is the uniaxial compressive strength (qu).

1... Press-fitting device, 2... Measurement probe, 3... Penetration hole, 4... Penetration rod, 10... Piston, 11... Stand, 12... Chuck, 13... Control unit, 14... Hydraulic unit

Claims (4)

A correlation diagram is prepared in advance, in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and a threshold value is set for the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp) under conditions that can ensure the target uniaxial compressive strength (quck) based on the correlation diagram.
A method for evaluating the effect of ground improvement using a liquid grouting method, comprising measuring electrical resistivity by electrical logging using a penetration hole formed vertically in the ground before and after ground improvement, determining the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), and judging whether the target uniaxial compressive strength (quck) is achieved based on whether the electrical resistivity ratio (Rimp/Runimp) is greater than or less than the threshold value ,
The correlation diagram is obtained by: a first step of diluting a silica solution with water and measuring the electrical resistivity for each silica concentration in order to grasp the electrical resistivity characteristics of the silica solution used for ground improvement, and obtaining a correlation diagram between silica concentration (SiO ₂ ) and electrical resistivity (Rc);
The second step is to obtain a correlation diagram between the electrical resistivity of unimproved and improved ground (Rs and Rimp) and the electrical resistivity of pore water and chemical solutions (Rpw and Rc).
a third step of obtaining a correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (Rimp) of the improved ground based on the correlation diagram between silica concentration (SiO2) and electrical resistivity (Rc) obtained in the first step and the correlation diagram between the electrical resistivity (Rs·Rimp) of the unimproved and improved ground and the electrical resistivity (Rpw _· Rc) of the pore water and chemical solution obtained in the second step;
A fourth step is to obtain a correlation diagram between silica concentration (SiO ₂ ) and unconfined compressive strength (q u ) using a test specimen prepared indoors in order to set the liquid silica concentration corresponding to the target unconfined compressive strength (q u );
a fifth step in which the correlation diagram between the silica concentration ( _SiO2 ) obtained in the third step and the electrical resistivity (Rimp) of the improved ground is arranged to obtain a correlation diagram in which one axis represents the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co) and the other axis represents the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp);
a sixth step of arranging the correlation diagram between the silica concentration ( _SiO2 ) and the unconfined compressive strength (qu) obtained in the fourth step to obtain a correlation diagram in which one axis represents the ratio (c/co) of the silica concentration after dilution (c) to the desired silica concentration (co) and the other axis represents the unconfined compressive strength (qu);
A method for evaluating the effect of ground improvement using a chemical injection method, characterized in that a correlation diagram is obtained by a seventh step, based on the correlation diagram obtained in the fifth step and the correlation diagram obtained in the sixth step, in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp). 2. A method for evaluating the effect of ground improvement using the liquid grouting method according to claim 1 , in which the target of ground improvement is ground having a salinity of pore water of 5,000 to 10,000 ppm. 3. A method for evaluating the effect of ground improvement using a chemical injection method as described in claim 1 or 2, in place of the correlation diagram in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), a correlation diagram in which one axis is the liquefaction resistance ratio (RL) or adhesion (c) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground, modified based on the conversion formula between the uniaxial compressive strength (qu) and the liquefaction resistance ratio (RL) or the conversion formula between the uniaxial compressive strength (qu) and the adhesion (c), is used . A method for evaluating the effect of ground improvement using the chemical injection method described in either claim 1 or 2, in place of the correlation diagram in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (Rimp/Runimp) of the electrical resistivity of the improved ground (Rimp) to the electrical resistivity of the unimproved ground (Runimp), a correlation diagram is used in which one axis is the uniaxial compressive strength (qu) and the other axis is the ratio (σunimp/σimp) of the electrical conductivity of the unimproved ground (σunimp) to the electrical conductivity of the improved ground (σimp), corrected based on the conversion formula (R(Ω·m)＝1/σ(S/m)) between electrical resistivity (R) and electrical conductivity (σ) .

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