JP7696565B2 - Method for evaluating the effect of ground improvement using chemical grouting method - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act, Proceedings of the 30th Survey, Design and Construction Technology Report Session, June 2021, Chubu Branch of the Japan Geotechnical Society, "Confirmation of progress using electrical resistivity of chemical injection improvement body - Field demonstration experiment on coastal reclaimed land -", June 10, 2021 (distribution date)

The present invention relates to a method for evaluating the effectiveness of ground improvement using a chemical injection method, the main purpose of which is to prevent liquefaction.

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. Furthermore, in Patent Document 2, the improvement effect is evaluated by measuring the electrical resistivity before and after ground improvement, but the measured resistivity before improvement can have large errors from location to location, and it is sometimes impossible to accurately judge the improvement effect based on the difference in electrical resistivity before and after improvement.

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 after improvement.

In order to solve the above problems, the present invention relates to a method for evaluating a ground improvement effect by a chemical injection method,
A procedure of obtaining a first correlation diagram between silica concentration (SiO ₂ ) and uniaxial compressive strength (q u ) in advance using the test results of fracture strain ε _f <2%, and obtaining a second correlation diagram between silica concentration (SiO ₂ ) and electrical resistivity (R);
A second step of determining a target silica concentration (SiO ₂ ) from a target unconfined compressive strength (quck) based on the first correlation diagram, and then setting a target electrical resistivity (Rk) from the target silica concentration (SiO ₂ ) based on the second correlation diagram;
The present invention provides a method for evaluating the effect of ground improvement using a chemical injection method, characterized in that after the ground improvement, electrical resistivity (Rimp) is measured by electrical logging using a penetration hole formed vertically in the ground, and a third step is used to determine whether the target uniaxial compressive strength (quck) has been secured based on whether the condition that the electrical resistivity (Rimp) of the improved ground is equal to or lower than the target electrical resistivity (Rk) is satisfied.

In the invention described in claim 1 above, when evaluating the effect of ground improvement (unconfined compressive strength) using the chemical grouting method, a first correlation diagram between silica concentration ( _SiO2 ) and unconfined compressive strength (qu) is obtained in advance using test results for a fracture strain εf _< 2% by indoor experiments using in-situ soil, and a second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R) is obtained in advance (first procedure).

Next, the target silica concentration ( _SiO2 ) is calculated from the target unconfined compressive strength (quck) based on the first correlation diagram, and then the target electrical resistivity (Rk) is set from the target silica concentration ( _SiO2 ) based on the second correlation diagram (second procedure). That is, the electrical resistivity (Rk) for obtaining the target unconfined compressive strength (quck) is set via the silica concentration ( _SiO2 ). If the measured electrical resistivity (Rimp) is smaller than the electrical resistivity (Rk), the target unconfined compressive strength (quck) is ensured.

After carrying out ground improvement work, the electrical resistivity (Rimp) is measured by electrical logging using a penetration hole drilled vertically in the ground, and it is determined whether the target unconfined compressive strength (quck) has been achieved based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition that it is less than the target electrical resistivity (Rk) (third step).

In the present invention, it is possible to determine whether a specified unconfined compressive strength (quck) has been achieved based solely on the measured value of electrical resistivity after ground improvement. In other words, while in the past, a qualitative assessment was made as to whether or not a chemical solution had been filled by comparing the electrical resistivity before and after improvement, the present invention makes it possible to grasp the unconfined compressive strength after improvement from the electrical resistivity after improvement, and quantitatively evaluate the improvement effect. In addition, since the effect of ground improvement can be confirmed without using the electrical resistivity before ground improvement, which can cause errors, it is possible to reduce the amount of work (measurement) required and improve the accuracy of effect confirmation.

According to a second aspect of the present invention, there is provided a method for evaluating a ground improvement effect by a chemical injection method, comprising the steps of:
A procedure of obtaining a first correlation diagram between silica concentration ( _SiO2 ) and liquefaction resistance ratio (RL) or cohesion (c) in advance using the test results for fracture strain εf _< 2%, and obtaining a second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R);
A second step of determining a target silica concentration (SiO2) from a target liquefaction strength ratio (RL) or cohesion (c) based on the first correlation diagram, and then setting a target electrical resistivity (Rk) from _the target silica concentration ( _SiO2 ) based on the second correlation diagram;
The present invention provides a method for evaluating the effect of ground improvement using a chemical injection method, characterized in that after the ground improvement, electrical resistivity (Rimp) is measured by electrical logging using a penetration hole formed vertically in the ground, and a third step is used to determine whether the target uniaxial compressive strength (quck) has been secured based on whether the condition that the electrical resistivity (Rimp) of the improved ground is equal to or lower than the target electrical resistivity (Rk) is satisfied.

In the invention described in claim 2 above, in consideration of the fact that unconfined compressive strength (qu) and liquefaction strength ratio (RL) can be converted using a certain conversion formula, and that unconfined compressive strength (qu) and cohesion strength (c) can be converted using a certain conversion formula, the axis of unconfined compressive strength in the first correlation diagram is replaced with the axis of liquefaction strength ratio (RL) or cohesion strength (c), and the effect of ground improvement is evaluated based on this correlation diagram.

As a third aspect of the present invention, there is provided a method for evaluating a ground improvement effect by a chemical injection method, comprising the steps of:
A procedure of obtaining a first correlation diagram between silica concentration (SiO ₂ ) and uniaxial compressive strength (q u ) in advance using the test results of fracture strain ε _f <2%, and obtaining a second correlation diagram between silica concentration (SiO ₂ ) and electrical conductivity (σ);
A second step of determining a target silica concentration (SiO ₂ ) from a target unconfined compressive strength (q uck ) based on the first correlation diagram, and then setting a target electrical conductivity (σ c ) from the target silica concentration (SiO ₂ ) based on the second correlation diagram;
The present invention provides a method for evaluating the effect of ground improvement using a chemical injection method, which comprises a third step of measuring the electrical resistivity (Rimp) by electrical logging using a penetration hole formed vertically in the ground after the ground improvement, calculating the electrical conductivity (σimp) after the ground improvement from the measured electrical resistivity, and judging whether the target uniaxial compressive strength (quck) has been secured based on whether the electrical conductivity (σimp) of the improved ground satisfies the condition that it is equal to or greater than the target electrical conductivity (σc).

In the invention described in claim 3 above, in view of the fact that electrical resistivity (R) and electrical conductivity (σ) can be converted using a conversion formula (R = 1/σ), the axis of electrical resistivity (R) in the second correlation diagram is replaced with the axis of electrical conductivity (σ), and the effect of ground improvement is evaluated based on this correlation diagram.

As the present invention according to claim 4, a depth distribution map of the Nd value showing the relationship between depth and Nd value is obtained by a small dynamic cone penetration test before and after ground improvement, and a primary effect confirmation is performed to evaluate the effect of ground improvement from the increment of the Nd value before and after ground improvement,
When the ground improvement effect is not clear by the primary effect confirmation, a method for evaluating the ground improvement effect by a chemical injection method is provided, characterized in that, as a secondary effect confirmation, electrical resistivity (Rimp) is measured using the penetration hole of the small dynamic cone penetration test, and the ground improvement effect is evaluated by a method described in any one of claims 1 to 3.

The invention described in claim 4 above involves a step-by-step evaluation that involves a primary confirmation of effects using a small dynamic cone penetration test, and a secondary confirmation of effects using electrical resistivity. Rather than confirming effects using electrical resistivity in all cases, the primary confirmation of effects is performed, and then the secondary confirmation of effects is performed only in cases where the ground improvement effect cannot be clearly confirmed, thereby making it possible to reduce the labor required for the entire work process.

As explained above in detail, the present invention provides a method for evaluating the effect 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 after improvement.

FIG. 2 is a flow diagram of a method for evaluating a ground improvement effect according to the present invention. FIG. 2 is a vertical cross-sectional view showing an outline of a measuring device used for electrical logging. FIG. 2 is a front view of the measurement probe 2 with the outer sleeve 8 attached. 1A shows an exterior sleeve main body 12, in which (A) is a front view, (B) is a cross-sectional view taken along line B-B, (C) is a back view, and (D) is a cross-sectional view taken along line D-D. 1 shows the outer sleeve tip 13, in which (A) is a front view and (B) is a top view. FIG. 13 is an explanatory diagram of a method for confirming secondary effects. 1 shows a plan view, a cross-sectional view and an investigation location diagram of the improved body in the embodiment. FIG. 1 shows soil log and N values. FIG. 13 is a graph showing the particle size accumulation curve of in-situ soil. This is a graph showing the salinity and electrical resistivity of groundwater. FIG. 1 is a graph showing the relationship between electrical resistivity and the salinity of pore water. FIG. 1 shows the particle size accumulation curve of the improved product. FIG. 13 is a diagram showing the results of a repeated triaxial test. FIG. 1 is a correlation diagram showing the relationship between unconfined compressive strength (qu) and liquid silica concentration (SiO ₂ ). FIG. 1 is a correlation diagram showing the relationship between electrical resistivity (R) and chemical solution silica concentration (SiO ₂ ). This is a depth distribution diagram of the Nd value of improved body No. 3. This is a depth distribution map of electrical resistivity R of improved body No. 3. This is a depth distribution diagram of the Nd value of improved body No. 4. This is a depth distribution map of electrical resistivity R of improved body No. 4. 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.

As shown in Figure 1, a depth distribution map of the Nd value showing the relationship between depth and Nd value was obtained by a small dynamic cone penetration test before and after ground improvement, and a primary effect confirmation was performed to evaluate 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 clear from the above-mentioned primary effect confirmation, as a secondary effect confirmation, electrical logging is carried out using the penetration hole of the above-mentioned small dynamic cone penetration test to determine whether the target uniaxial compressive strength (quck) has been secured.

The secondary effect confirmation includes a procedure of obtaining a first correlation diagram between silica concentration ( _SiO2 ) and uniaxial compressive strength (qu) and a second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R) in advance;
A second step of determining a target silica concentration (SiO ₂ ) from a target unconfined compressive strength (quck) based on the first correlation diagram, and then setting a target electrical resistivity (Rk) from the target silica concentration (SiO ₂ ) based on the second correlation diagram;
After the ground improvement, the electrical resistivity (Rimp) is measured by electrical logging using a penetration hole drilled vertically in the ground, and a third step is performed to determine whether the target unconfined compressive strength (quck) has been achieved based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition that it is less than the target electrical resistivity (Rk).

The specific details are provided below.

<Primary effect confirmation>
The small dynamic cone penetration test is a penetration test performed using a small dynamic cone penetration tester manufactured by Tecnotest, Italy, called PENNY. The test method involves automatically free-falling a hammer with a mass of 294 N (30 kgf) from a height of 20 cm using a hydraulic motor, and continuously measuring the number of blows (Nd value) required to penetrate a tip cone with a cross-sectional area of 10 ^cm2 and a tip angle of 60° 10 cm. The rotational torque of the rod is measured every 1 m, and the effect of the frictional force acting on the rod is corrected, making it possible to convert the value into an Nd value equivalent to the N value of the standard penetration test. The advantages of the small dynamic cone penetration test compared to the standard penetration test are as follows.
(1) In the standard penetration test, measurement points are spaced at 1-m intervals, so if the layer thickness of the injected chemical solution is around 1 to 2 m, it is not possible to secure measurement points. In contrast, the small dynamic cone penetration test allows measurements to be taken every 10 cm.
(2) The uniaxial compressive strength qu of the improved soil is about 50 to 100 kPa, so the impact energy is too large for the standard penetration test to be accurate, whereas the impact energy of the small dynamic cone penetration test is small (just right for the target strength range), ensuring measurement accuracy. In the standard penetration test, the hammer mass is 63.5 kg and the free fall energy is 473 J with a drop height of 76 cm, whereas in the small dynamic cone penetration test, the hammer mass is 30 kg and the free fall energy is 58.8 J with a drop height of 20 cm, which is about 12% of the impact energy.
(3) Since the dropping process is fully automatic, there is little variation in impact energy.
(4) The testing machine is light and easy to handle.

In this way, by measuring the Nd value using a small dynamic cone penetration test, it requires a smaller installation space than a standard penetration test, is highly portable, and is fully automated, so there is less variation in impact energy. Therefore, the effects of ground improvement can be reliably confirmed even in uneven reclaimed land where variation is likely to occur.

The small dynamic cone penetration test produces a depth distribution map of Nd values showing the relationship between depth and Nd value (see Figures 16 and 18).

A primary effect check is performed to confirm the effect of ground improvement using the depth distribution map of the Nd value obtained by the above-mentioned small dynamic cone penetration test. The method of checking the effect of ground improvement in this primary effect check is to write the Nd value before ground improvement on the Nd value-depth graph as shown in Figures 16 and 18, and write the Nd value after ground improvement on top of it, and check the increase in the Nd value before and after ground improvement.
The primary effectiveness can be judged as shown in Figure 1, when an increase in the Nd value is confirmed and it is determined that liquefaction will not occur based on the Nd value after improvement, or when the soil layer is determined not to liquefy due to the physical characteristics of the improved ground, such as being a clay layer or having a large amount of clay.

In cases where the Nd value before ground improvement is close to the target improvement strength, the increase in the Nd value is not very large, and the ground improvement effect of this Nd value may not be recognized. In such cases, a secondary effect confirmation is performed using electrical logging as described below.

<Secondary effect confirmation>
In the secondary effect confirmation, first, after the small dynamic cone penetration test, electrical logging is performed using the penetration hole H. The electrical logging is a geophysical exploration method in which a measurement probe 2 equipped with one current electrode 3 and two potential electrodes 4, 4 is inserted into the penetration hole H of the small dynamic cone penetration test by an injection device 1 shown in Fig. 2, as shown in Fig. 2 and Fig. 3, and the potential when a current is passed through the current electrode 3 is detected by the potential electrode 4 while these electrodes 3, 4 are in contact with the hole wall, and the electrical resistivity R of the ground near the hole wall is continuously measured in the depth direction.

As shown in FIG. 2, the press-in device 1 is configured such that pistons 20, 20 that are extendable and retractable in the vertical direction are arranged on both sides of the penetration hole H on the ground surface directly above the penetration hole H, and a chuck 22 that holds a penetration rod 5 with a measurement probe 2 connected to its lower end is provided in the center of a stand 21 that straddles the upper ends of the pistons 20, 20, as well as a control unit 23 that controls the operation of the pistons 20, 20. A hydraulic unit 24 consisting of an engine and a hydraulic pump is connected to the control unit 23.

In the press-fitting device 1, the pistons 20 on both sides expand and contract in unison, and the base 21 moves vertically, causing the penetration rod 5 clamped by the chuck 22 to move vertically, and the measurement probe 2 is pushed into and pulled out of the penetration hole H.

As shown in Fig. 3, the measuring device used for electrical logging includes a measuring probe 2 equipped with the electrodes 3, 4, an electric cable 7 extending from the upper end of the measuring probe 2 and having its tip connected to the electrodes 3, 4 inside the measuring probe 2, and a hollow exterior sleeve 8 into which the measuring probe 2 is removably inserted. The electric cable 7 is extended to the ground through the hollow part of a hollow cylindrical penetration rod 5, where its tip is connected to the measuring device.

The measurement probe 2 has a rod-like appearance with a roughly circular cross section, and at its upper end is formed a male threaded section 6 for connecting a penetration rod 5, which is adapted to be screwed into a female threaded section provided at the lower end of the penetration rod 5. In addition, a main body section 10 is provided, which is connected to the lower end of the male threaded section 6 via an intermediate section, and in which a number of electrodes are arranged at predetermined intervals in the axial direction (vertical direction), and a tip section 11 having a smaller diameter than the main body section 10 is provided to continue to the lower end of the main body section 10.

The electrode arrangement for the electrical logging may be a four-pole or three-pole method, but a two-pole method is preferable. In the two-pole electrode arrangement, as shown in Figures 2 and 3, one current electrode 3 and two potential electrodes 4, 4 are arranged at a predetermined interval in the vertical direction, and the current is returned to a current remote electrode (not shown) installed near the ground surface. The potential is measured by the potential electrodes 4, 4 while a constant current is passed from the current electrode 3, based on the potential remote electrode (not shown) also installed near the ground surface. Compared to the four-pole and three-pole electrode arrangements, the effect of ground improvement can be more clearly understood.

As shown in FIG. 3, of the three electrodes provided on the main body 10 of the measurement probe 2, the electrode located at the top is the current electrode 3, and the two electrodes located below it are potential electrodes 4. It is preferable that the electrode spacing a between the current electrode 3 and the upper potential electrode 4 is 2.5 cm, and the electrode spacing b between the current electrode 3 and the lower potential electrode 4 is 5 cm. In this way, by arranging the two potential electrodes 4, 4 with different electrode spacings from the current electrode 3, two potential differences with different electrode spacings can be measured simultaneously, improving measurement accuracy and shortening measurement time.

The electrodes 3, 4, etc. are made of a conductive metal material and extend from the inside to the outer surface of the measurement probe 2, and the tips of the electrical cables 7 are connected to each of them inside the measurement probe 2.

Next, we will explain the outer sleeve 8 that is attached to the tip side of the measurement probe 2 when the measurement probe 2 is inserted into the insertion hole H. In order to facilitate manufacture, it is preferable to divide the outer sleeve 8 into an outer sleeve main body 12 that is fitted onto the main body 10 of the measurement probe 2, and an outer sleeve tip 13 that is fitted onto the tip 11 of the measurement probe 2, as shown in FIG. 4.

The outer sleeve body 12 is formed in a generally cylindrical shape with open ends in the axial direction as shown in FIG. 4, and is formed so that its outer diameter is larger than that of the measurement probe 2 when inserted into the measurement probe 2 as shown in FIG. 3. By making the outer diameter of the outer sleeve 8 larger than that of the measurement probe 2, the outer sleeve 8 is more likely to come into contact with the hole wall when it penetrates the penetration hole H, improving measurement accuracy and suppressing damage to the measurement probe 2. It is preferable that the outer diameter of the outer sleeve 8 is approximately equal to the outer diameter of the tip cone used in the small dynamic cone penetration test.

As shown in FIG. 5, the outer sleeve tip 13 has an upper portion formed in a bottomed cylindrical shape that opens upward, and a lower portion formed in a cone shape that is pointed downward. The outer diameter of the upper bottomed cylindrical portion is formed to be approximately equal to the outer diameter of the outer sleeve body 12. The cone tip angle is preferably about 45° to 90°, and more preferably 60°.

As shown in Figs. 4 and 5, the outer sleeve 8 has a hollow portion 14 into which the measurement probe 2 is inserted over a range including the attachment positions of the electrodes 3, 4, etc. from the insertion tip side into the penetration hole 1, and outer electrodes 15, 16, 16 that penetrate continuously from inside the hollow portion 14 to the outer surface at positions corresponding to the electrodes 3, 4, etc. of the measurement probe 2, and whose inner tips contact the electrodes 3, 4, etc. when the measurement probe 2 is inserted into the hollow portion 14. The electrodes 3, 4, etc. of the measurement probe 2 correspond to the outer electrodes 15, 16, etc. of the outer sleeve 8, and the outer electrode 15 arranged at the top is a current electrode, and the two outer electrodes 16, 16 arranged below it are potential electrodes.

After the measurement probe 2 with the outer sleeve 8 attached is inserted into the penetration hole H by the pressing device 1 to perform electrical logging, if it becomes difficult to pull out the measurement probe 2 from the penetration hole H, the measurement probe 2 can be removed from the outer sleeve 8 by the pulling resistance, and the measurement probe 2 can be recovered. In this way, the outer sleeve 8 is removed and left in the ground due to the pulling resistance when pulling out the measurement probe 2, and the measurement probe 2 that has been removed from the outer sleeve 8 can be reliably recovered, eliminating the risk of being unable to recover the expensive measurement probe 2 in electrical logging. As described above, the measurement probe 2 is formed with an outer diameter smaller than the outer sleeve 8, so it can be relatively smoothly pulled out from the penetration hole H into which it was pressed with the outer sleeve 8 attached.

When the outer sleeve 8 is inserted into the penetration hole H, the outer electrodes 15, 16... are preferably provided with a contact-promoting protrusion 17 that protrudes outward on the outer surface opposite the outer electrodes 15, 16... in order to bring the outer electrodes 15, 16... into contact with the hole wall. The contact-promoting protrusion 17 is a vertically elongated protrusion formed in a range including the entire length of the arrangement section of the outer electrodes 15, 16... in the axial direction of the outer sleeve 8. The height is preferably 1 to 8 mm, and more preferably 3 to 5 mm. By providing the contact-promoting protrusion 17, the outer electrodes 15, 16... provided on the outer sleeve 8 can be more reliably contacted with the hole wall of the penetration hole 1, and the measurement accuracy of electrical logging can be further improved.

As shown in FIG. 4, it is preferable that a circumferential fixing protrusion 18 is provided on the peripheral surface of the measurement probe 2, and the circumferential rotation of the outer sleeve 8 and the measurement probe 2 is fixed by fitting the circumferential fixing protrusion 18 into a fitting portion 19 provided on the outer sleeve 8. The circumferential fixing protrusion 18 is formed on the upper end of the main body 10 of the measurement probe 2, and the fitting portion 19 is formed on the upper end of the outer sleeve main body 12 of the outer sleeve 8. By fitting the circumferential fixing protrusion 18 into the fitting portion 19, the circumferential rotation of the measurement probe 2 and the outer sleeve main body 12 of the outer sleeve 8 is prevented, and when the measurement probe 2 is inserted into the insertion hole 1, the electrodes 3, 4... of the measurement probe 2 and the outer electrodes 15, 16... of the outer sleeve 8 are not misaligned.

The procedure for the electrical logging is as follows: after the small dynamic cone penetration test, the pressing device 1 shown in FIG. 2 is installed on the ground surface directly above the penetration hole H, the measurement probe 2 with the outer sleeve 8 attached is inserted into the penetration hole H, and the measurement probe 2 is gradually pressed in while continuously measuring the electrical resistivity R in the depth direction. The measurement interval for the electrical resistivity R is arbitrary, but should be 10 cm or less, preferably 5 cm or less, and more preferably 1 cm. When measurements have been completed up to the specified depth, the measurement probe 2 is withdrawn from the penetration hole H and recovered.

Prior to the electrical logging, in the present invention, a first correlation diagram between silica concentration ( _SiO2 ) and uniaxial compressive strength (qu) is obtained in advance, and a second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R) is obtained in advance (first procedure). Specifically, a blending test of improved sand using on-site soil and sand, i.e., a uniaxial compressive strength test is performed while changing the silica concentration of the chemical solution, to obtain the first correlation diagram between silica concentration ( _SiO2 ) and uniaxial compressive strength (qu) shown in Fig. 6 , and an electrical resistivity test is performed while changing the silica concentration of the chemical solution to obtain the second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R) shown in Fig. 6 .

Incidentally, the first and second correlation diagrams can be combined by adjusting the scale of the silica concentration ( _SiO2 ) on the horizontal axis, as shown in Fig. 6. This combined diagram makes it possible to plot the relationship between the uniaxial compressive strength (qu), electrical resistivity (R), and silica concentration ( _SiO2 ) so that it can be understood at a glance.

Then, a target silica concentration ( _SiO2 ) is calculated from a target unconfined compressive strength (quck) based on the first correlation diagram. Next, a target electrical resistivity (Rk) is set from the target silica concentration ( _SiO2 ) based on the second correlation diagram (second procedure).

In other words, the value of electrical resistivity (Rk) is set to satisfy the target uniaxial compressive strength (quck) from the first and second correlation diagrams. If the measured electrical resistivity (Rimp) is smaller than the electrical resistivity (Rk), then the uniaxial compressive strength (q) greater than the target uniaxial compressive strength (quck) is ensured, and if the measured electrical resistivity (R) is greater than the electrical resistivity (Rimp), then the target uniaxial compressive strength (quck) is not ensured.

Therefore, after ground improvement, the electrical resistivity (Rimp) is measured by electrical logging using a penetration hole H drilled vertically in the ground, and whether the target uniaxial compressive strength (quck) has been secured is determined based on whether the condition that the electrical resistivity (Rimp) of the improved ground is equal to or less than the target electrical resistivity (Rk) is satisfied (third step).

[Other examples]
(1) In the above example, a two-stage effect confirmation method was described in which a primary effect confirmation was performed using a small dynamic cone penetration test, and if the ground improvement effect was not clear through this secondary effect confirmation, the secondary effect confirmation was performed. However, the primary effect confirmation may be omitted and the ground improvement effect may be confirmed only through the secondary effect confirmation.

(2) In the above embodiment, the correlation diagram between silica concentration (SiO ₂ ) and uniaxial compressive strength (qu) was used as the first correlation diagram, but 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 cohesive 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 20 (Source: Penetration Solidification Treatment Method Technical Manual Revised Edition July 2020, Coastal Technology Research Center, General Incorporated Foundation), and the correlation formula between the uniaxial compressive strength (qu) and the cohesive strength (c) is shown in Figure 21 (Source: Penetration Solidification Treatment Method Technical Manual Revised Edition July 2020, Coastal Technology Research Center, General Incorporated Foundation). Therefore, the above-mentioned secondary effect confirmation may be performed using the correlation diagram between silica concentration (SiO ₂ ) and the liquefaction strength ratio (RL) or the cohesive strength (c) as the first correlation diagram.

(3) In the above embodiment, a correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R) is used as the second correlation diagram, but electrical resistivity and electrical conductivity can be converted using a conversion formula. Specifically, electrical resistivity (R) and electrical conductivity (σ) have a relationship of R (Ω·m) = 1/σ (S/m). Therefore, the secondary effect may be confirmed by using a correlation diagram between silica concentration ( _SiO2 ) and electrical conductivity (σ) as the second correlation diagram.

Below, we will explain how to evaluate the effectiveness of ground improvement using specific examples carried out on-site.

The field experiment was carried out at a certain reclaimed land. Four chemical improvement bodies with a diameter of 2.5m (improved soil volume: ^8m3 x 4 bodies = ^32m3 ) were constructed approximately 25m behind the quay, and the improvement effect of the improvement bodies was confirmed using this method. Figure 7 shows the plan and cross-sectional view of the improvement bodies and the survey location. The specifications of the improvement bodies were special silica liquid concentration 9wt%, injection rate 40.5%, and design standard strength qu=100kPa (average value).

1. Overview of the experimental site Figure 8 shows the soil columnar diagrams and N values for Bor. Advance-1 to 3, and Figure 9 shows the grain size accumulation curve for each depth for Bor. Advance-1. From the surface, the strata consist of gravel-mixed sand, sandy silt, gravel-mixed sand, and silty fine sand. The gravel-mixed sand in the layer targeted for chemical improvement is coarse sand with an average grain size _D50 = 0.89 mm, fine grain content Fc = 3.8%, and uniformity coefficient Uc = 5.14, and contains layers of clay and silt at depths of GL-3m and below.

Figure 10 shows the depth distribution of groundwater salinity and electrical resistivity. As shown in Figure 11, the salinity of the groundwater is in the range of 700 to 7,400 ppm, compared to the salinity of 24,500 ppm for seawater collected near the experimental yard.

2. Experimental method The experiment was carried out at the measurement positions shown in Figure 7, with small dynamic cone penetration tests and electrical logging carried out on the unimproved soil, immediately after injection (material age 0 days), and 14 days after injection (material age 14 days). After the test was completed, the improved soil was excavated to GL-2.0m to confirm the finished shape, and block samples were taken of the improved soil and uniaxial compression tests, cyclic triaxial tests, triaxial CUB tests, etc. were carried out to confirm the improvement strength.

The improvement effect of this method was confirmed according to the flow shown in Figure 1. Specifically, by performing laboratory tests (mixing test (unconfined compressive strength test), electrical resistivity test), a first correlation diagram between silica concentration ( _SiO2 ) and unconfined compressive strength (qu) was obtained in advance, as shown in Figure 6, and a second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R).

Then, the chemical solution silica concentration (SiO ₂ ) equivalent to the design standard strength (quck) is determined from the relationship between the uniaxial compressive strength (qu) of the improved body and the chemical solution silica concentration (SiO ₂ ) (first correlation diagram), and the electrical resistivity value (Rk) equivalent to the chemical solution silica concentration (SiO ₂ ) equivalent to the design standard strength (quck) is determined from the relationship between electrical resistivity and the chemical solution silica concentration (SiO ₂ ) (second correlation diagram).
In other words, the electrical resistivity (Rk) is equivalent to the electrical resistivity value that satisfies the design standard strength (quck). Therefore, an improved body is judged to satisfy the design standard strength (quck) if the electrical resistivity (Rimp) of the improved body measured on-site is equal to or less than the electrical resistivity (Rk). In this experiment, gravel-mixed sand collected from Bor. Advance-3 was used to prepare improved sand with chemical silica concentrations of 3, 5, 7, and 9 wt% under the density conditions of the on-site ground, and a mix test and electrical resistivity test were carried out.

3. Experimental Results
(1) Excavated and improved bodies Figure 12 shows the grain size accumulation curve of unimproved sand. The grain size of the improved bodies at a depth of GL-2m varies from place to place, with improved bodies No. 1 and 3 being mainly made up of gravel-mixed sand ( _D50 = 0.62-0.82mm, Fc = 3.8-4.2%), and improved bodies No. 2 and 4 being mainly made up of silty sand ( _D50 = 0.12mm, Fc = 38.4%).

(2) Results of uniaxial compression test and cyclic triaxial test of collected samples The uniaxial compressive strength (qu) of the improved bodies was qu = 50-128kPa [average: qu = 101kPa] for the improved bodies mainly composed of gravel-mixed sand (No. 1, 3), and qu = 82-85 [average: qu = 83kPa] for the improved body mainly composed of silty sand (No. 4). As shown in the results of the cyclic triaxial test of sample No. 3 in Figure 13, the liquefaction strength ratio R _L20(imp) of the improved body No. 3 was R _L20(imp) = 1.07, which was about six times that of the unimproved sand (gravel-mixed sand).

(3) Mixing test and electrical resistivity test results Figure 14 shows the relationship between the unconfined compressive strength (qu) of the improved sand obtained from the mixing test and the silica concentration in the chemical solution ( _SiO2 ), and Figure 15 shows the relationship between the electrical resistivity (R) of the improved sand obtained from the electrical resistivity test and the silica concentration in the chemical solution ( _SiO2 ).

The results of the blending test shown in Figure 14 were obtained by taking into account the disturbance of the sample during molding and using the test results for fracture strain ε _f < 2% to obtain the correlation between the uniaxial compressive strength (qu) and the silica concentration in the chemical solution (SiO _{2 )} . From Figure 14, the silica concentration in the chemical solution corresponding to the design standard strength (quck) = 100 kPa is approximately SiO 2 = 5 wt%. Also, from Figure 15, the electrical resistivity value (Rk) of the improved body corresponding to SiO ₂ = 5 wt% was Rk = 5 Ω·m. From these results, if the electrical resistivity value (average value) of the improved body measured on-site is Rk (= 5 Ω·m) or less, it is judged that the design standard strength (quck) of the improved body is satisfied.

(4) Small dynamic cone penetration test and electrical logging results The surveys were conducted at the near center, +60cm from the center (1/2 the radius of the improvement body), and +100cm from the center of the improvement bodies No.3 and No.4, as shown in Figure 7. The ages of the improvement bodies were 0 days and 14 days. Figures 16 and 17 show the depth distribution of the Nd value and the electrical resistivity (R) of improvement body No.3, and Figures 18 and 19 show the depth distribution of the Nd value and the electrical resistivity (R) of improvement body No.4.

(a) Improved body No.3
The Nd value at the planned improvement depth (GL-1.75m to -3.75m, layer thickness 2m) varies widely even at 14 days of material age, when the strength of the improved body stabilizes. At the same material age, the Nd value increment for every 50cm is generally in the range of 1 to 12, but at depths of GL-3.5m and deeper, it was about 1. This is thought to be due to the influence of the clayey soil layer distributed at depths of GL-3.25m and deeper, which was confirmed in advance by Bor-3 and other studies.

Electrical resistivity (Rimp) has less variation compared to Nd values. As mentioned above, with this method, resistance values are measured at two different electrode spacings, 25mm and 50mm, and the lack of a large difference between the two electrical resistivity values (Rimp) confirms that there is no disturbance around the hole wall and that the electrodes are pressed against the hole wall. At measurement point No. 4, where the material is aged 0 days and 14 days, there is a large difference in the electrical resistivity (Rimp) obtained from the two different electrode spacings, which suggests poor adhesion of the electrodes to the hole wall or disturbance around the measurement hole wall, so this point will be excluded from the evaluation of the effects of this improvement.

The results of measurements No. 3 (100cm from the center of the improved body) and No. 5 (near the center of the improved body) taken at 14 days of age show that at the planned improvement depth of GL-1.75m to -3.75m, electrical resistivity (Rimp) is significantly lower than unimproved. In addition, electrical resistivity (Rimp) from GL-1.75m to -3.5m was in the range of Rimp = 3.1 to 6.8 Ω·m, with an average value of 5 Ω·m.

Meanwhile, the electrical resistivity value (Rimp) at depths of GL-3.5m and deeper was around 10 Ω·m, influenced by the clay layer. When the unconfined compressive strength (qu) is estimated from the electrical resistivity value (Rk) that satisfies the design criteria strength mentioned above, the unconfined compressive strength (qu) is ≧100 kPa in the range of GL-1.75m to -3.5m, which can be evaluated as satisfying the target improvement strength. These results are generally appropriate when compared with the unconfined compressive strength (qu) of block sampling samples taken from the improved body.

(b) Improved body No.4
The Nd values at the planned improvement depth (GL-1.75m to -3.75m, layer thickness 2m) showed a large variation at 14 days of material age, similar to that of improvement body No. 3. Also, at the same material age, the Nd value increment per 50cm was generally in the range of 1 to 15, but at depths of GL-3.25m and deeper, it was about 1 to 2. This is thought to be due to the influence of the clayey soil layer and silty sand layer.

At measurement points 5, 6, and 7 where the material age was 0, and at measurement point 7 where the material age was 14 days, the electrical resistivity (Rimp) was about 5 Ω·m (average value) from the top of the improved area to GL-2.5m to GL-3.0m. There was also almost no difference in the electrical resistivity (Rimp) due to material age.
On the other hand, at measurement points 5, 6, and 7, where the material age is 0, no change was observed before and after improvement at depths of GL-3m or deeper. Additionally, at measurement point 7, where the material age is 14 days, the change before and after improvement was small at depths of GL-3.25m or deeper. Excavation photographs and block sample samples show that the improved body at this location contains layers of silty sand (Fc=about 40%), so these influences are thought to be the cause.

If the improvement effect cannot be clearly confirmed even after the secondary effect check, we will consider conducting electrical logging again in a separate hole or repeating triaxial tests, etc.

4. Summary In this experiment, we applied this method, which combines a small dynamic cone and electrical logging, to the chemically improved body of a coastal reclaimed land, to verify the scope of application of electrical logging and to evaluate the improvement effect of this method. The conclusions are as follows.
(1) The Nd values obtained from the small dynamic cone penetration test after improvement showed a large variation, but an increase in the Nd value was confirmed at the planned improvement depth. In addition, there was almost no difference between the material ages of 0 and 14 days, and it is considered that the influence of variation is large.

(2) The electrical resistivity (Rimp) of the improved ground obtained by the push-in type micro-electrical logging method (two types: point electrode and two-pole method, electrode spacing 25 and 50 mm) showed a clear change in resistivity before and after the injection of chemicals in the area where the groundwater salinity was in the range of 700 to 7,400 ppm.

(3) The improvement effect of this method can be evaluated from the electrical resistivity value (Rimp) obtained from this electrical logging based on the relationship between unconfined compressive strength and chemical solution silica concentration (first correlation diagram) and the relationship between electrical resistivity and chemical solution silica concentration (second correlation diagram) obtained from mix tests and electrical resistivity tests using local sand.

(4) This electrical logging method verifies the measurement data by using electrodes with different spacing. Even in the measurement data from this experiment, there were some areas where the measurement results for different electrode spacings showed discrepancies, but the reliability of most of the measurement data is considered to be high thanks to this checking function.

From the above, it was found that this method of confirming the improvement effect, which combines small dynamic cone testing and push-type micro logging, is effective in confirming the improvement effect of the chemical injection method because it provides a function to check the measurement data in the electrical logging, and the data is highly reliable.

1... Press-in device, 2... Measurement probe, 3... Current electrode, 4... Potential electrode, 5... Penetration rod, 6... Male threaded portion, 7... Electric cable, 8... Outer sleeve, 9... Middle portion, 10... Main body, 11... Tip portion, 12... Outer sleeve main body, 13... Outer sleeve tip, 14... Hollow portion, 15... Outer electrode (current electrode), 16... Outer electrode (potential electrode), 17... Contact promoting protrusion, 18... Circumferential fixing protrusion, 19... Fitting portion, 20... Piston, 21... Stand, 22... Chuck, 23... Control unit, 24... Hydraulic unit, H... Penetration hole

Claims (4)

A method for evaluating the effect of ground improvement using a chemical grouting method, comprising:
A procedure of obtaining a first correlation diagram between silica concentration (SiO ₂ ) and uniaxial compressive strength (q u ) in advance using the test results of fracture strain ε _f <2%, and obtaining a second correlation diagram between silica concentration (SiO ₂ ) and electrical resistivity (R);
A second step of determining a target silica concentration (SiO ₂ ) from a target unconfined compressive strength (quck) based on the first correlation diagram, and then setting a target electrical resistivity (Rk) from the target silica concentration (SiO ₂ ) based on the second correlation diagram;
A method for evaluating the effect of ground improvement using a chemical injection method, characterized in that it comprises a third step of measuring the electrical resistivity (Rimp) by electrical logging using a penetration hole formed vertically in the ground after the ground improvement, and determining whether the target uniaxial compressive strength (quck) has been secured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition that it is equal to or lower than the target electrical resistivity (Rk). A method for evaluating the effect of ground improvement using a chemical grouting method, comprising:
A procedure of obtaining a first correlation diagram between silica concentration ( _SiO2 ) and liquefaction resistance ratio (RL) or cohesion (c) in advance using the test results for fracture strain εf _< 2%, and obtaining a second correlation diagram between silica concentration ( _SiO2 ) and electrical resistivity (R);
A second step of determining a target silica concentration (SiO2) from a target liquefaction strength ratio (RL) or cohesion (c) based on the first correlation diagram, and then setting a target electrical resistivity (Rk) from _the target silica concentration ( _SiO2 ) based on the second correlation diagram;
A method for evaluating the effect of ground improvement using a chemical injection method, characterized in that it comprises a third step of measuring the electrical resistivity (Rimp) by electrical logging using a penetration hole formed vertically in the ground after the ground improvement, and determining whether the target uniaxial compressive strength (quck) has been secured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition that it is equal to or lower than the target electrical resistivity (Rk). A method for evaluating the effect of ground improvement using a chemical grouting method, comprising:
A procedure of obtaining a first correlation diagram between silica concentration (SiO ₂ ) and uniaxial compressive strength (q u ) in advance using the test results of fracture strain ε _f <2%, and obtaining a second correlation diagram between silica concentration (SiO ₂ ) and electrical conductivity (σ);
A second step of determining a target silica concentration (SiO ₂ ) from a target unconfined compressive strength (q uck ) based on the first correlation diagram, and then setting a target electrical conductivity (σ c ) from the target silica concentration (SiO ₂ ) based on the second correlation diagram;
This method for evaluating the effect of ground improvement using a chemical injection method is characterized by comprising a third step of measuring the electrical resistivity (Rimp) by electrical logging using a penetration hole formed vertically in the ground after the ground improvement, calculating the electrical conductivity (σimp) after the ground improvement from this, and judging whether the target uniaxial compressive strength (quck) has been secured based on whether the electrical conductivity (σimp) of the improved ground satisfies the condition that it is equal to or greater than the target electrical conductivity (σc). Before and after ground improvement, a small dynamic cone penetration test was used to obtain a depth distribution map of the Nd value showing the relationship between depth and the Nd value, and a primary effect confirmation was performed to evaluate the effect of the ground improvement from the increment in the Nd value before and after the ground improvement.
A method for evaluating the effect of ground improvement using a liquid injection method, characterized in that, when the effect of ground improvement is not clear by the primary effect confirmation, as a secondary effect confirmation, electrical resistivity (Rimp) is measured using the penetration hole of the small dynamic cone penetration test, and the effect of ground improvement is evaluated by a method according to any one of claims 1 to 3.

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