Method for Evaluating Ground Improvement Effect by Chemical Solution Injection Method

Abstract

To provide an evaluation method of a ground improvement effect which can evaluate uniaxial compression strength of an improvement body by measuring an electric specific resistance after improvement.SOLUTION: An evaluation method of a ground improvement effect by a chemical injection method comprises first to third steps. In the first step, a first correlation diagram of silica concentration (SiO2) and uniaxial compression strength (qu) is obtained and a second correlation diagram of silica concentration (SiO2) and electric specific resistance (R) is obtained in advance. In the second step, a target silica concentration (SiO2) is obtained from a target uniaxial compression strength (quck) based on the first correlation diagram and a target electric specific resistance (Rk) is set from the target silica concentration (SiO2) based on the second correlation diagram. In the third step, an electric specific resistance (Rimp) by an electric logging is measured using a penetration hole vertically formed in a ground after ground improvement, and it is determined whether the target uniaxial compression strength (quck) is secured according to whether a condition that the electric specific resistance (Rimp) of the improved ground is equal to or less than the target electric specific resistance (Rk) is satisfied.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for evaluating the ground improvement effect by a chemical injection method mainly aimed at countermeasures against liquefaction.

Background Art

[0002] Conventionally, for ground strengthening of soft ground such as landfill sites, ground improvement work has been carried out by a chemical injection method in which a chemical solution composed of water glass (sodium silicate) or the like is injected into the ground. In the ground improvement work by the chemical injection method, after implementation, a construction confirmation survey is conducted to check whether the chemical solution is evenly distributed in the target ground.

[0003] The most common method for the construction confirmation survey of the chemical injection method is a method of evaluating the improved soil by the uniaxial compression strength (qu). However, the uniaxial compression strength of the improved soil by chemical injection is as small as about qu = 50 to 100 kPa, and depending on the target ground, there may be variations in strength and it may not be properly evaluated. That is, in the evaluation by the uniaxial compression strength qu, in the case of a small ground with qu of about 50 to 100 kPa, disturbance leading to a decrease in strength is likely to occur during sample collection or specimen preparation in the post - investigation. Also, depending on the target ground, shells, wood chips, silt, organic soil, etc. may be mixed into the specimen, resulting in variations in strength and inability to properly evaluate.

[0004] As a method for directly evaluating the quality of improved ground by means other than such uniaxial compression strength tests, in the study committees etc. regarding ground improvement work by chemical solution injection method in reclaimed land etc. of the Ministry of Land, Infrastructure, Transport and Tourism, a dynamic cone penetration test capable of measuring pore water pressure such as piezo drive cone (PDC) has been proposed. The piezo drive cone penetrates the ground by hitting a cone with a built-in pressure sensor with a hammer, and measures the penetration amount per blow and the response value of the pore water pressure during penetration. From the penetration amount, the dynamic penetration resistance value (Nd value) of the ground corresponding to the N value of the standard penetration test is calculated for each blow. Also, from the pore water pressure in the ground generated by impact penetration, the fine particle content ratio Fc is estimated, and it has been proposed in the above-mentioned study committees etc. that the cumulative excess pore water pressure ratio obtained using this pore water pressure can be an index for evaluating the penetration of the chemical solution into the ground.

[0005] In addition, electric logging is cited as another method for the construction confirmation survey of the chemical solution injection method. Electric logging utilizes the fact that in the chemical solution injection method, the pore water in the ground is replaced by the chemical solution, the compression rate of the ground changes, and the strength of the ground increases due to the solidification of the chemical solution, so the characteristics of the electrical conductivity of the improved ground change. In this electric logging, a qualitative judgment of the improvement effect can be made based on the decrease in the electrical resistivity value before and after construction. The measurement procedure of the electric logging is to insert a measurement probe equipped with a plurality of electrodes at predetermined intervals in the vertical direction into a boring hole, then energize the current electrode, and obtain the resistivity from the potential difference between the electrodes.

[0006] As a method for confirming the quality of ground improvement work by such electric logging, in Patent Document 1 below, an electrode attachment body with an annular electrode attached to the outer surface is inserted into the improvement body, the improvement body formed around the electrode attachment body is energized, and a method for obtaining the resistivity using the current between the current electrodes and the potential difference between the potential electrodes measured in such a state is disclosed. Also, in Non-Patent Document 1, a method for obtaining the chemical solution filling rate from the change in electrical resistivity before and after chemical solution injection is disclosed.

[0007] Even the present applicants have proposed, in Patent Document 2 below, a method for confirming the ground improvement effect with little variation and directly confirming the quality of the improved ground. The method for confirming the ground improvement effect by the chemical solution injection method involves, after ground improvement, obtaining a depth distribution diagram of the Nd value showing the relationship between depth and Nd value by a small dynamic cone penetration test, performing a primary effect confirmation to confirm the ground improvement effect from the increment of the Nd value before and after ground improvement, and when the ground improvement effect is not recognized by the primary effect confirmation, performing an electrical logging to measure the specific resistance by inserting a measurement probe equipped with electrodes into the penetration hole of the small dynamic cone penetration test, obtaining a depth distribution diagram of the specific resistance showing the relationship between depth and specific resistance, and performing a secondary effect confirmation to confirm the ground improvement effect from the decrement of the specific resistance before and after ground improvement.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0009]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, the method according to Patent Document 1 calculates the production volume of the improved body, i.e., the cross-sectional area, size, diameter, etc., from the measured specific resistance. The method according to Non-Patent Document 1 calculates the chemical solution filling rate from the electrical specific resistance. The method according to Patent Document 2 enables confirmation of the presence of the ground improvement consolidation body from the reduction in specific resistance even when the ground improvement effect cannot be judged only from the increased amount of the Nd value.

[0011] As described above, although the primary evaluation method of the ground improvement effect by the chemical solution injection method is the method of evaluating by the uniaxial compression strength qu, none of the above-mentioned conventional techniques directly targets the strength of the consolidation body for evaluation. Further, in Patent Document 2, the improvement effect is evaluated by measuring the electrical specific resistance before and after the ground improvement. However, the measured value of the specific resistance before the improvement may have a large error for each location, and it may not be possible to accurately judge the improvement effect from the difference in the electrical specific resistance before and after the improvement.

[0012] Therefore, the main object of the present invention is to provide a method for evaluating the ground improvement effect by the chemical solution injection method using an electrical logging method capable of evaluating the uniaxial compression strength of the improved body by measuring the electrical specific resistance after the improvement.

Means for Solving the Problems

[0013] As the present invention according to claim 1 for solving the above problems, a method for evaluating the ground improvement effect by the chemical solution injection method, Prior to that, Fracture strain ε f Using test results of <2% A first procedure of obtaining a first correlation diagram between the silica concentration (SiO2) and the uniaxial compression strength (qu) and obtaining a second correlation diagram between the silica concentration (SiO2) and the electrical specific resistance (R); Based on the first correlation diagram, the target silica concentration (SiO2) is obtained from the target uniaxial compression strength (quck), and then based on the second correlation diagram, the target electrical specific resistance (Rk) is set from the target silica concentration (SiO2). A second procedure; After ground improvement, the electrical resistivity (Rimp) by electrical logging is measured using the penetration holes formed vertically in the ground, and it is determined whether the target uniaxial compression strength (quck) is ensured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being equal to or less than the target electrical resistivity (Rk). A method for evaluating the ground improvement effect by a chemical solution injection method is provided, which comprises a third step.

[0014] In the invention described in claim 1 above, when evaluating the ground improvement effect (uniaxial compression strength) by the chemical solution injection method, through indoor experiments using in-situ soil, Fracture strain ε f Using test results of <2% A first correlation diagram between the silica concentration (SiO2) and the uniaxial compression strength (qu) is obtained, and a second correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R) is obtained (first step).

[0015] Next, based on the first correlation diagram, the target silica concentration (SiO2) is obtained from the target uniaxial compression strength (quck), and then based on the second correlation diagram, the target electrical resistivity (Rk) is set from the target silica concentration (SiO2) (second step). That is, the electrical resistivity (Rk) for obtaining the target uniaxial compression strength (quck) is set through the silica concentration (SiO2). If the measured electrical resistivity (Rimp) is smaller than the electrical resistivity (Rk), it means that the target uniaxial compression strength (quck) is ensured.

[0016] After that, after ground improvement, the electrical resistivity (Rimp) by electrical logging is measured using the penetration holes formed vertically in the ground, and it is determined whether the target uniaxial compression strength (quck) is ensured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being equal to or less than the target electrical resistivity (Rk) (third step).

[0017] In the present invention, it is possible to determine whether a predetermined uniaxial compression strength (quck) is ensured only by the measured value of the electrical resistivity after ground improvement. That is, conventionally, it was a qualitative evaluation of whether the chemical solution was filled by comparing the electrical resistivity before and after improvement. According to the present invention, it becomes possible to grasp the uniaxial compression strength after improvement from the electrical resistivity after improvement, and it becomes possible to quantitatively evaluate the improvement effect. Further, since the confirmation of the ground improvement effect is performed without using the electrical resistivity of the ground before improvement, which is a cause of error, labor saving of the work (measurement) can be achieved accordingly, and the accuracy of the effect confirmation can be improved.

[0018] As the present invention according to claim 2, a method for evaluating the ground improvement effect by a chemical solution injection method, comprising: Prior to that, Fracture strain ε f Using test results of <2% a first step of obtaining a first correlation diagram between the silica concentration (SiO2) and the liquefaction strength ratio (RL) or the adhesion (c), and obtaining a second correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R); a second step of obtaining the target silica concentration (SiO2) from the target liquefaction strength ratio (RL) or the adhesion (c) based on the first correlation diagram, and then setting the target electrical resistivity (Rk) from the target silica concentration (SiO2) based on the second correlation diagram; a third step of measuring the electrical resistivity (Rimp) by electrical logging using the penetration hole formed in the vertical direction in the ground after ground improvement, and determining whether the target uniaxial compression strength (quck) is ensured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being equal to or less than the target electrical resistivity (Rk). A method for evaluating the ground improvement effect by a chemical solution injection method is provided.

[0019] In the invention described in claim 2 above, in view of the fact that the uniaxial compression strength (qu) and the liquefaction strength ratio (RL) can be converted by a fixed conversion formula, and that the uniaxial compression strength (qu) and the adhesion (c) can be converted by a fixed conversion formula, a correlation diagram is provided in which the axis of the uniaxial compression strength in the first correlation diagram is replaced by the liquefaction strength ratio (RL) or the adhesion (c), and based on this, the ground improvement effect is evaluated.

[0020] As the invention according to claim 3, a method for evaluating the ground improvement effect by a chemical solution injection method, comprising: Prior to this, Fracture strain ε f Using test results of <2% A first step of obtaining a first correlation diagram between the silica concentration (SiO2) and the uniaxial compression strength (qu), and obtaining a second correlation diagram between the silica concentration (SiO2) and the conductivity (σ); A second step of obtaining the target silica concentration (SiO2) from the target uniaxial compression strength (quck) based on the first correlation diagram, and then setting the target conductivity (σc) from the target silica concentration (SiO2) based on the second correlation diagram; After ground improvement, the electrical resistivity (Rimp) by electrical logging is measured using the penetration holes formed vertically in the ground, and the conductivity (σimp) after ground improvement is calculated from this. It is determined whether the target uniaxial compression strength (quck) is ensured based on whether the condition that the conductivity (σimp) of the improved ground is equal to or higher than the target conductivity (σc) is satisfied. A method for evaluating the ground improvement effect by a chemical solution injection method is provided, which is characterized by comprising the above three steps.

[0021] In the invention described in claim 3 above, in view of the fact that the electrical resistivity (R) and the conductivity (σ) can be converted by a conversion formula (R = 1 / σ), a correlation diagram is provided in which the axis of the electrical resistivity (R) in the second correlation diagram is replaced by the conductivity (σ), and based on this, the ground improvement effect is evaluated.

[0022] As the invention according to claim 4, before and after ground improvement, obtain a depth distribution diagram of the Nd value showing the relationship between the depth and the Nd value by a small dynamic cone penetration test, and perform a primary effect confirmation to evaluate the ground improvement effect from the increment of the Nd value before and after ground improvement. When the ground improvement effect is not clear by the above primary effect confirmation, as a secondary effect confirmation, measure the electrical resistivity (Rimp) using the penetration hole of the small dynamic cone penetration test, and evaluate the ground improvement effect by the method according to any one of claims 1 to 3. There is provided a method for evaluating the ground improvement effect by a chemical solution injection method.

[0023] In the invention described in claim 4 above, it is a step-by-step evaluation by a primary effect confirmation by a small dynamic cone penetration test and a secondary effect confirmation by electrical resistivity. In all cases, instead of performing an effect confirmation by electrical resistivity, after performing the primary effect confirmation, the secondary effect confirmation is performed only in cases where the ground improvement effect cannot be clearly confirmed, so that the entire work process can be labor-saving.

Effect of the Invention

[0024] As described in detail above, according to the present invention, it is possible to provide a method for evaluating the ground improvement effect by a chemical solution injection method using an electrical logging that enables evaluation of the uniaxial compressive strength of the improved body by measuring the electrical resistivity after improvement.

Brief Description of the Drawings

[0025]

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Mode for Carrying Out the Invention

[0026] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0027] The present invention is a method for evaluating the ground improvement effect by a chemical injection method in which a chemical solution composed of water glass (sodium silicate) or the like is injected into the ground for ground reinforcement of soft ground such as landfills, and specifically, it is carried out according to the following procedure.

[0028] As shown in FIG. 1, before and after ground improvement, a depth distribution diagram of the Nd value showing the relationship between the depth and the Nd value is obtained by a small dynamic cone penetration test, and a primary effect confirmation for evaluating the ground improvement effect is performed 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, as a secondary effect confirmation, electrical logging using the penetration hole of the small dynamic cone penetration test is performed to determine whether the target uniaxial compressive strength (quck) is ensured.

[0029] The secondary effect confirmation includes, in advance, one procedure of obtaining a first correlation diagram between the silica concentration (SiO2) and the uniaxial compressive strength (qu), and obtaining a second correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R), Based on the first correlation diagram, the target silica concentration (SiO2) is obtained from the target uniaxial compressive strength (quck), and then based on the second correlation diagram, the target electrical resistivity (Rk) is set from the target silica concentration (SiO2). The second procedure, After ground improvement, the electrical resistivity (Rimp) by electrical logging is measured using the penetration holes formed vertically in the ground, and it is determined whether the target uniaxial compressive strength (quck) is ensured by whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being equal to or less than the target electrical resistivity (Rk). It consists of a third procedure.

[0030] Specific details are described below.

[0031] <Primary effect confirmation> The small-scale dynamic cone penetration test is a penetration test conducted using a small-scale dynamic cone penetrometer manufactured by Tecnotest of Italy, which is called "PENNY" so to speak. The test method is to automatically drop a hammer with a mass of 294 N (30 kgf) from a height of 20 cm freely using a hydraulic motor, and continuously measure the number of blows (Nd value) required to penetrate a tip cone with a cross-sectional area of 10 cm 2 and a tip angle of 60° by 10 cm. The rotational torque of the rod is measured every 1 m, and by correcting the influence of the frictional force acting on the rod, it can be converted into an Nd value equivalent to the N value of the standard penetration test. The advantages of the small-scale dynamic cone penetration test compared with the standard penetration test are as follows. (1) In the standard penetration test, since the measurement points are at a 1 m pitch, when the layer thickness of chemical solution injection is about 1 to 2 m, measurement points cannot be secured. In contrast, the small-scale dynamic cone penetration test can be measured every 10 cm. (2) Since the uniaxial compressive strength qu of the improved soil is about 50 to 100 kPa, in the case of the standard penetration test, the impact energy is too large to obtain accuracy. In contrast, the small-scale dynamic cone penetration test has a small impact energy (just right for the target strength range), and the measurement accuracy can be ensured. In the case of the standard penetration test, the free fall energy with a hammer mass of 63.5 kg and a drop height of 76 cm is 473 J, while in the small-scale dynamic cone penetration test, the free fall energy with a hammer mass of 30 kg and a drop height of 20 cm is 58.8 J, which is about 12% of the impact energy. (3) Since the dropping operation is fully automatic, there is little variation in the impact energy. (4) The test machine is light and has good handling performance.

[0032] Thus, by measuring the Nd value using the small-scale dynamic cone penetration test, compared with the standard penetration test, in a narrow installation space, it has excellent portability, and because it is fully automatic, there is little variation in the impact energy. Therefore, the ground improvement effect can be surely confirmed even in uneven landfill ground etc. where variation is likely to occur.

[0033] By the small-scale dynamic cone penetration test, a depth distribution diagram of the Nd value showing the relationship between depth and Nd value can be obtained (see FIGS. 16 and 18).

[0034] Using the depth distribution diagram of the Nd value obtained by the small-scale dynamic cone penetration test, a primary effect confirmation for verifying the ground improvement effect is performed. As shown in FIGS. 16 and 18, the method for confirming the ground improvement effect in this primary effect confirmation is to write the Nd value before ground improvement on the Nd value-depth graph and overlay and write the Nd value after ground improvement, and to confirm the increment of the Nd value before and after ground improvement. As shown in FIG. 1, the judgment of the primary effect confirmation includes cases where an increment of the Nd value is confirmed and it can be judged that liquefaction does not occur from the Nd value after improvement, or cases where it can be judged that liquefaction does not occur in a soil layer due to reasons such as the physical properties of the improved ground having more clay layers or clay.

[0035] The increment of the Nd value may not be very large in a ground where the Nd value before ground improvement is close to the target improvement strength, and the ground improvement effect by this Nd value may not be recognized. In that case, a secondary effect confirmation by the following electric logging is performed.

[0036] <Secondary effect confirmation> In the secondary effect confirmation, first, after the small-scale dynamic cone penetration test, an electric logging is performed using the penetration hole H. The electric logging is a geophysical exploration method in which, as shown in FIGS. 2 and 3, a measurement probe 2 having one current electrode 3 and two potential electrodes 4, 4 is inserted into the penetration hole H of the small-scale dynamic cone penetration test by the press-fitting device 1 shown in FIG. 2, and while bringing these electrodes 3, 4 into contact with the hole wall, the potential when a current is passed through the current electrode 3 is detected by the potential electrode 4, and the electrical resistivity R of the ground near the hole wall is continuously measured in the depth direction.

[0037] As shown in Fig. 2, the press-fitting device 1 has pistons 20, 20 that are stretchable along the vertical direction on both sides of the penetration hole H on the ground surface directly above the penetration hole H. A chuck 22 that clamps a penetration rod 5 with a measurement probe 2 connected to its lower end is provided at the center of a gantry 21 straddling the upper ends of these pistons 20, 20. A control unit 23 that controls the operation of the pistons 20, 20 is also provided. Further, a hydraulic unit 24 consisting of an engine and a hydraulic pump is connected to the control unit 23.

[0038] In the press-fitting device 1, the pistons 20, 20 on both sides expand and contract in synchronization, and the gantry 21 moves in the vertical direction, so that the penetration rod 5 clamped by the chuck 22 moves in the vertical direction, and the measurement probe 2 is pushed into and pulled out of the penetration hole H.

[0039] As shown in Fig. 3, the measuring device used for electrical logging includes a measurement probe 2 provided with the electrodes 3, 4..., an electrical cable 7 extending from the upper end of the measurement probe 2 and having its tip connected to the electrodes 3, 4... inside the measurement probe 2, and a hollow outer sleeve 8 into which the measurement probe 2 is detachably inserted. The electrical cable 7 extends to the ground through the hollow part of a penetration rod 5 formed in a hollow cylindrical shape, and at the ground, its tip is connected to a measuring device.

[0040] The measurement probe 2 has a rod-like appearance with a substantially circular cross-section. At its upper end, a male thread portion 6 for connecting the penetration rod 5 is formed so that it can be screwed into a female thread portion provided at the lower end of the penetration rod 5. Further, a main body portion 10 with a plurality of electrodes arranged at predetermined intervals in the axial direction (vertical direction) is provided continuously at the lower end of the male thread portion 6 via an intermediate portion, and a tip portion 11 with a smaller diameter than the main body portion 10 is provided continuously at the lower end of the main body portion 10.

[0041] The electrode arrangement of the electric logging layer may be the four - electrode method or the three - electrode method, but preferably the two - electrode method. As shown in FIGS. 2 and 3, in the electrode arrangement of the two - electrode method, one current electrode 3 and two potential electrodes 4, 4 are arranged at a predetermined interval in the vertical direction, and a current return is taken from a current far - electrode (not shown) installed near the ground surface. With a potential far - electrode (not shown) also installed near the ground surface as a reference, while flowing a constant current from the current electrode 3, the potential is measured by the potential electrodes 4, 4. Compared with the electrode arrangements of the four - electrode method and the three - electrode method, the improvement effect of the ground can be grasped more clearly.

[0042] As shown in FIG. 3, among the three electrodes provided on the main body 10 of the measurement probe 2, the electrode arranged at the uppermost part is the current electrode 3, and the two electrodes arranged below it are the potential electrodes 4 respectively. The electrode interval a between the current electrode 3 and the upper potential electrode 4 is preferably 2.5 cm, and the electrode interval b between the current electrode 3 and the lower potential electrode 4 is preferably 5 cm. In this way, by arranging two potential electrodes 4, 4 with different electrode intervals from the current electrode 3, two potential differences with different electrode intervals can be measured simultaneously, so that the measurement accuracy is improved and the measurement time can be shortened.

[0043] The electrodes 3, 4... are made of a conductive metal material, penetrate from the inside to the outside of the measurement probe 2, and the tips of the electric cables 7 are respectively connected inside the measurement probe 2.

[0044] Next, when the measurement probe 2 is inserted into the penetration hole H, the outer sleeve 8 attached to the tip side of the measurement probe 2 will be described. For the ease of manufacturing, as shown in FIG. 4, the outer sleeve 8 is preferably composed of an outer sleeve body 12 fitted on the main body 10 of the measurement probe 2 and an outer sleeve tip 13 fitted on the tip 11 of the measurement probe 2 and is divided.

[0045] As shown in Fig. 4, the outer sleeve body 12 is formed in a substantially cylindrical shape with both ends open in the axial direction. As shown in Fig. 3, in a state of being inserted into the measurement probe 2, the outer diameter is formed to be larger than the outer diameter of the measurement probe 2. By making the outer diameter of the outer sleeve 8 larger than the outer diameter of the measurement probe 2, when the outer sleeve 8 penetrates into the penetration hole H, the outer sleeve 8 is likely to come into contact with the hole wall, improving the measurement accuracy and suppressing damage to the measurement probe 2. The outer diameter of the outer sleeve 8 is preferably approximately equal to the outer diameter of the tip cone used in the small-scale dynamic cone penetration test.

[0046] As shown in Fig. 5, the tip of the outer sleeve 13 is formed in a bottomed cylindrical shape with the upper part open upward, and the outer shape of the lower part is formed in a conical shape (cone shape) with the tip pointing downward. The outer diameter of the upper bottomed cylindrical part 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°.

[0047] 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 mounting positions of the electrodes 3, 4,... from the insertion tip side into the penetration hole 1, and at positions corresponding to the electrodes 3, 4,... of the measurement probe 2, it continuously penetrates from the inside of the hollow portion 14 to the outer surface, and in a state where the measurement probe 2 is inserted into the hollow portion 14, the inner tips are in contact with the electrodes 3, 4,... respectively, and outer electrodes 15, 16, 16 are provided. The electrodes 3, 4,... provided on the measurement probe 2 and the outer electrodes 15, 16,... provided on the outer sleeve 8 correspond to each other. The outermost upper outer electrode 15 is a current electrode, and the two outer electrodes 16, 16 arranged below it are potential electrodes.

[0048] After the measurement probe 2 with the outer sleeve 8 attached is penetrated into the penetration hole H by the press-fitting device 1 for electrical logging, if it becomes difficult to pull out the measurement probe 2 from the penetration hole H, the measurement probe 2 comes off the outer sleeve 8 due to the pulling resistance, and the measurement probe 2 can be recovered. In this way, due to the pulling resistance when pulling out the measurement probe 2, the outer sleeve 8 comes off and remains in the ground, and the measurement probe 2 that has come off the outer sleeve 8 can be surely recovered. Therefore, the risk of the expensive measurement probe 2 in electrical logging not being recoverable is eliminated. As described above, since the measurement probe 2 is formed with an outer diameter smaller than that of the outer sleeve 8, it can be pulled out relatively smoothly from the penetration hole H into which the outer sleeve 8 is press-fitted.

[0049] When the outer sleeve 8 is inserted into the penetration hole H, in order to bring the outer electrodes 15, 16... into contact with the hole wall, it is preferable to provide contact promoting protrusions 17 protruding outward on the outer surface on the opposite side of the outer electrodes 15, 16.... The contact promoting protrusions 17 are longitudinally long protrusions formed in a range including the entire length of the arrangement section of the outer electrodes 15, 16... with respect to 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 protrusions 17, the outer electrodes 15, 16... provided on the outer sleeve 8 can surely contact the hole wall of the penetration hole 1, and the measurement accuracy of electrical logging can be further improved.

[0050] As shown in FIG. 4, a circumferential fixing projection 18 is provided on the circumferential surface of the measurement probe 2, and by fitting this circumferential fixing projection 18 into a fitting portion 19 provided on the outer sleeve 8, it is preferable to fix the rotation in the circumferential direction between the outer sleeve 8 and the measurement probe 2. The circumferential fixing projection 18 is formed at the upper end of the main body portion 10 of the measurement probe 2, and the fitting portion 19 is formed at the upper end of the outer sleeve main body 12 of the outer sleeve 8. By fitting the circumferential fixing projection 18 into the fitting portion 19, the rotation in the circumferential direction between 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 penetration hole 1, etc., displacement between the electrodes 3, 4... of the measurement probe 2 and the outer electrodes 15, 16... of the outer sleeve 8 does not occur.

[0051] The procedure of the electrical logging is as follows: after performing the small-scale dynamic cone penetration test, install the press-fitting device 1 shown in FIG. 2 on the ground surface directly above the penetration hole H, insert the measurement probe 2 with the outer sleeve 8 attached into the penetration hole H, and continuously measure the electrical resistivity R in the depth direction while gradually press-fitting the measurement probe 2. The measurement interval of the electrical resistivity R is arbitrary, but it is preferably 10 cm or less, more preferably 5 cm or less, and still more preferably 1 cm. When the measurement is completed up to a predetermined depth, pull out the measurement probe 2 from the penetration hole H and recover it.

[0052] Prior to the electrical logging, in the present invention, first, a first correlation diagram between the silica concentration (SiO2) and the uniaxial compressive strength (qu) is obtained, and a second correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R) is obtained (first step). Specifically, a mixing test of improved sand using in-situ soil and sand, that is, a uniaxial compressive strength test is performed while changing the chemical liquid silica concentration, and to 6 a first correlation diagram between the silica concentration (SiO2) and the uniaxial compressive strength (qu) as shown in the figure is obtained, and an electrical resistivity test is performed while changing the chemical liquid silica concentration, and to 6 a second correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R) as shown in the figure is obtained.

[0053] Note that when the scales of the silica concentration (SiO2) on the horizontal axes of the first correlation diagram and the second correlation diagram are matched, the two correlation diagrams can be combined as shown in Fig. to 6 According to this combined diagram, the relationship between the uniaxial compressive strength (qu), the electrical resistivity (R), and the silica concentration (SiO2) can be graphically represented so that it can be understood at a glance.

[0054] Then, based on the first correlation diagram, the target silica concentration (SiO2) is obtained from the target uniaxial compressive strength (quck). Next, based on the second correlation diagram, the target electrical resistivity (Rk) is set from the target silica concentration (SiO2) (second step).

[0055] That is, from the first correlation diagram and the second correlation diagram, the numerical value of the electrical resistivity (Rk) for satisfying the target uniaxial compressive strength (quck) is set. If the measured electrical resistivity (Rimp) is smaller than the electrical resistivity (Rk), it means that a uniaxial compressive strength (q) greater than the target uniaxial compressive strength (quck) is ensured. If the measured electrical resistivity (R) is greater than the electrical resistivity (Rimp), it means that the target uniaxial compressive strength (quck) is not ensured.

[0056] Therefore, after ground improvement, the electrical resistivity (Rimp) by electrical logging is measured using the penetration hole H formed vertically in the ground, and it is determined whether the target uniaxial compressive strength (quck) is ensured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being less than or equal to the target electrical resistivity (Rk) (third step).

[0057] 〔Other Embodiment Examples〕 (1) In the above embodiment example, a two-step effect confirmation method is described in which the primary effect confirmation by the small dynamic cone penetration test is performed, and when the ground improvement effect is not clear by this primary effect confirmation, the secondary effect confirmation is performed. However, the primary effect confirmation may be omitted and only the secondary effect confirmation may be used to confirm the ground improvement effect.

[0058] (2) In the above-described embodiment, the correlation diagram between the silica concentration (SiO2) and the uniaxial compression strength (qu) was used as the first correlation diagram. However, the uniaxial compression strength (qu) and the liquefaction strength ratio (RL) can be converted by a fixed conversion formula. Also, the uniaxial compression strength (qu) and the adhesion force (c) can be converted by a fixed conversion formula. Specifically, the correlation formula between the uniaxial compression strength (qu) and the liquefaction strength ratio (RL) is shown in Fig. 20 (Source: Technical Manual for Permeation and Solidification Treatment Method, Revised Edition, July R2, Coastal Technology Research Center, Incorporated Foundation). The correlation formula between the uniaxial compression strength (qu) and the adhesion force (c) is shown in Fig. 21 (Source: Technical Manual for Permeation and Solidification Treatment Method, Revised Edition, July R2, Coastal Technology Research Center, Incorporated Foundation). Therefore, as the first correlation diagram, a correlation diagram between the silica concentration (SiO2) and the liquefaction strength ratio (RL) or the adhesion force (c) may be used to perform the secondary effect confirmation.

[0059] (3) In the above-described embodiment, the correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R) was used as the second correlation diagram. However, the electrical resistivity and the conductivity can be converted by a conversion formula. Specifically, the electrical resistivity (R) and the conductivity (σ) are in the relationship of R (Ω·m) = 1 / σ (S / m). Therefore, as the second correlation diagram, a correlation diagram between the silica concentration (SiO2) and the conductivity (σ) may be used to perform the secondary effect confirmation.

Example

[0060] Hereinafter, with respect to the method for evaluating the ground improvement effect, a specific example conducted on-site will be used for explanation.

[0061] The in-situ experiment was conducted at a certain landfill site. The experiment was carried out by constructing four chemical improvement bodies with a diameter of 2.5 m at a position about 25 m behind the seawall (Improved soil volume: 8 m 3 × 4 bodies = 32 m 3 ), and the improvement effect of the improvement bodies was confirmed by this method. Fig. 7 shows the plan view, cross-sectional view of the improvement bodies, and the investigation positions. The specifications of the improvement bodies are a special silica solution concentration of 9 wt%, an injection rate of 40.5%, and a design standard strength qu = 100 kPa (average value).

[0062] 1. Overview of the experimental site Figure 8 shows the soil columnar diagrams and N-values of Bor.Pre-1 to 3, and Figure 9 shows the particle size accumulation curves for each depth of Bor.Pre-1. The strata from the ground surface are gravelly sand, sandy silt, gravelly sand, and silty fine sand deposited. The gravelly sand in the layer to be improved by chemical solution has a mean particle size D 50 = 0.89 mm, a fine particle content Fc = 3.8%, and is coarse sand with a uniformity coefficient Uc = 5.14. Below GL - 3 m, there are clay and silt layers in layers.

[0063] Figure 10 shows the depth distributions of the salt concentration and electrical resistivity of the groundwater. As shown in Figure 11, the salt concentration of the groundwater is in the range of 700 - 7,400 ppm with respect to the salt concentration of the seawater collected near the experimental yard, which is 24,500 ppm.

[0064] 2. Experimental Method The experiments were carried out with small dynamic cone penetration tests and electrical logging at the measurement positions shown in Figure 7 for the unimproved, immediately after chemical solution injection (age 0 days), and 14 days after injection (age 14 days). Also, after the test, the improved body was excavated to GL - 2.0 m to confirm the formed shape, and the improved body was block sampled to conduct uniaxial compression tests, repeated triaxial tests, triaxial CUB tests, etc. to confirm the improvement strength.

[0065] The confirmation of the improvement effect by this method was carried out according to the flow shown in Figure 1. Specifically, through laboratory tests (mixing tests (uniaxial compression strength tests), electrical resistivity tests), as shown in Figure 6, a first correlation diagram between the silica concentration (SiO2) and the uniaxial compression strength (qu) was obtained in advance, and a second correlation diagram between the silica concentration (SiO2) and the electrical resistivity (R) was obtained.

[0066] Then, after obtaining the silica concentration (SiO2) corresponding to the design reference strength (quck) from the relationship between the uniaxial compression strength (qu) of the improved body and the chemical solution silica concentration (SiO2) (the first correlation diagram), the electrical resistivity value (Rk) corresponding to the silica concentration (SiO2) corresponding to the design reference strength (quck) was obtained from the relationship between the electrical resistivity and the chemical solution silica concentration (SiO2) (the second correlation diagram). That is, the electrical resistivity (Rk) corresponds to the electrical resistivity value that satisfies the design reference strength (quck). Therefore, it is determined that the improved body with the electrical resistivity (Rimp) of the improved body measured on-site being less than or equal to the electrical resistivity (Rk) is an improved body that satisfies the design reference strength (quck). In this experiment, using gravel-mixed sand collected from Bor. pre-3, improved sand with chemical solution silica concentrations of 3, 5, 7, and 9 wt% was prepared under the density conditions of the in-situ ground, and a mixing test and an electrical resistivity test were carried out.

[0067] 3. Experimental Results (1) Excavated improved body Figure 12 shows the particle size accumulation curve of the unimproved sand. The improved bodies at a depth of GL - 2m have different sand particle sizes depending on the location. Improved bodies No. 1 and 3 are improved bodies mainly composed of gravel-mixed sand (D 50 = 0.62 - 0.82 mm, Fc = 3.8 - 4.2%), and improved bodies No. 2 and 4 are improved bodies mainly composed of silty sand (D 50 = 0.12 mm, Fc = 38.4%).

[0068] (2) Uniaxial compression test and cyclic triaxial test results of the collected samples The uniaxial compression strength (qu) of the improved body was qu = 50 - 128 kPa [average value: qu = 101 kPa] for the improved bodies mainly composed of gravel-mixed sand (No. 1 and 3), and qu = 82 - 85 [average value: qu = 83 kPa] for the improved body mainly composed of silty sand (No. 4). Also, as shown in the cyclic triaxial test results of sample No. 3 in Figure 13, the liquefaction strength ratio R L20(imp) of improved body No. 3 was R L20(imp) = 1.07, which was about 6 times that of the unimproved sand (gravel-mixed sand).

[0069] (3) Mixing test and electrical resistivity test results Figure 14 shows the relationship between the uniaxial compression strength (qu) of the improved sand obtained from the mixing test and the chemical solution silica concentration (SiO2), and Figure 15 shows the relationship between the electrical resistivity (R) of the improved sand obtained from the electrical resistivity test and the chemical solution silica concentration (SiO2).

[0070] The mixing test results shown in Fig. 14 take into account the disturbance of the sample during molding, and the fracture strain ε f <2% test results were used to determine the correlation between the uniaxial compression strength (qu) and the chemical solution silica concentration (SiO2). From Fig. 14, the chemical solution silica concentration corresponding to the design reference strength (quck) = 100 kPa is about SiO2 = 5 wt%. Also, from Fig. 15, the electrical resistivity value (Rk) of the improved body corresponding to SiO2 = 5 wt% is Rk = 5 Ω·m. From these results, if the electrical resistivity value (average value) of the improved body measured on-site is below Rk (=5 Ω·m), it is judged that the design reference strength (quck) of the improved body is satisfied.

[0071] (4) Small dynamic cone penetration test · Electrical sounding results The investigation was carried out at positions near the center, center + 60 cm (1 / 2 of the improved body radius), and center + 100 cm from the center of the improved bodies No. 3 and No. 4 as shown in the investigation positions in Fig. 7. The ages of the improved bodies are 0 days and 14 days. Figs. 16 and 17 show the depth distribution of the Nd value and the depth distribution of the electrical resistivity (R) of the improved body No. 3, and Figs. 18 and 19 show the depth distribution of the Nd value and the depth distribution of the electrical resistivity (R) of the improved body No. 4.

[0072] (a) Improved body No. 3 The Nd values in the planned improvement depth (GL - 1.75 m to - 3.75 m, layer thickness 2 m) have large variations even at the age of 14 days when the strength development of the improved body is stable. At the same age, the Nd value increment per 50 cm is generally in the range of 1 to 12, but below GL - 3.5 m, it is about 1. This may be due to the influence of the viscous soil layer distributed below GL - 3.25 m confirmed in advance by Bor - 3 etc.

[0073] The electrical resistivity (Rimp) has less variation compared to the Nd value. As described above, in this method, the resistance values are measured at two types of electrode intervals of 25 mm and 50 mm, and since there is no significant difference between the two electrical resistivity values (Rimp), it is confirmed that there is no disturbance around the hole wall and the electrode is crimped to the hole wall. At the measurement point No. 4 with an age of 0 days and an age of 14 days, the difference in the electrical resistivity (Rimp) obtained from the two types of electrode intervals is large, and poor crimping of the electrode to the hole wall or disturbance around the measurement hole wall is considered, so it will be excluded from the evaluation of this improvement effect.

[0074] From the results of Measurement No. 3 (100 cm from the center of the improved body) and No. 5 (near the center of the improved body) at an age of 14 days, it can be seen that in the planned improvement depth of GL - 1.75 m to - 3.75 m, the electrical resistivity (Rimp) has decreased significantly compared to the unimproved case. Also, the electrical resistivity (Rimp) in the range of GL - 1.75 m to - 3.5 m was in the range of Rimp = 3.1 to 6.8 Ω·m, and the average value was 5 Ω·m.

[0075] On the other hand, the electrical resistivity value (Rimp) deeper than GL - 3.5 m was affected by the clay layer and was about 10 Ω·m. When estimating the uniaxial compressive strength (qu) from the electrical resistivity value (Rk) that satisfies the above - mentioned design standard strength, in the range of GL - 1.75 m to - 3.5 m, the uniaxial compressive strength (qu) ≥ 100 kPa, and it can be evaluated that the target improvement strength is satisfied. These results are generally reasonable compared to the uniaxial compressive strength (qu) of the block sampling samples taken from the improved body.

[0076] (b) Improved body No. 4 The Nd value in the planned improvement depth (GL - 1.75 m to - 3.75 m, layer thickness 2 m) has large variation at an age of 14 days, similar to the improved body No. 3. Also, the Nd value increment per 50 cm at the same age is generally in the range of 1 to 15, but deeper than GL - 3.25 m, it was about 1 to 2. This is considered to be affected by the viscous soil layer and the silty sand layer.

[0077] The electrical resistivity (Rimp) was about 5 Ω·m (average value) from GL - 2.5 m to about 3.0 m from the improved ground surface at measurement points No. 5, 6, 7 at age 0 and measurement point No. 7 at age 14 days. Also, there was almost no difference in the electrical resistivity value (Rimp) according to the age. On the other hand, at measurement points No. 5, 6, 7 at age 0, no change was observed before and after improvement at depths deeper than GL - 3 m. Also, at measurement point No. 7 at age 14 days, the change before and after improvement was small at depths deeper than GL - 3.25 m. The improved body at the same location contains a silty sand layer (Fc = about 40%) in layers from the excavation photos and the sampled samples by block sampling, so these effects are considered.

[0078] In addition, when the improvement effect cannot be clearly confirmed even in the secondary effect confirmation, separately consider conducting electrical sounding again in another hole, or conducting repeated triaxial tests, etc.

[0079] 4. Summary In this experiment, this method combining a small - scale dynamic cone and electrical sounding was applied to the chemical - improved body in the coastal landfill area, and the applicable range of electrical sounding was verified and the improvement effect by this method was evaluated. The conclusions are shown below. (1) Although the Nd value after improvement obtained from the small - scale dynamic cone penetration test has large variations, an increase in the Nd value was confirmed at the planned improvement depth. Also, almost no difference was seen between age 0 days and 14 days, and it is considered that the influence of variations is large.

[0080] (2) The electrical resistivity (Rimp) of the improved ground obtained from the push - in type micro - electrical sounding method (point electrode, two - electrode method, two types with electrode intervals of 25 and 50 mm) showed a clear change in resistivity before and after chemical solution injection in the area where the salt concentration of groundwater was in the range of 700 - 7,400 ppm.

[0081] (3) The evaluation of the improvement effect by this method can be carried out based on the uniaxial compressive strength - chemical solution silica concentration relationship (the first correlation diagram) and the electrical resistivity - chemical solution silica concentration relationship (the second correlation diagram) obtained from the mixing test using in - situ sand and the electrical resistivity test, and the improvement effect can be evaluated from the electrical resistivity value (Rimp) obtained from this electrical logging.

[0082] (4) This electrical logging method verifies the measurement data by using electrodes with different intervals. In the measurement data of this experiment, although there were also some parts where the measurement results of different electrode intervals showed deviations, most of the measured values are considered to have high data reliability due to this checking function.

[0083] From the above, it is found that this improvement effect confirmation method combining the small - scale dynamic cone penetration test and the push - in type micro - logging is effective for confirming the improvement effect of the chemical solution injection method because it has a checking function for measurement data in electrical logging, and the data reliability is high.

Explanation of Symbols

[0084] 1... Press - in device, 2... Measurement probe, 3... Current electrode, 4... Potential electrode, 5... Penetration rod, 6... Male screw part, 7... Electrical cable, 8... Outer sleeve, 9... Intermediate part, 10... Main body part, 11... Tip part, 12... Outer sleeve main body, 13... Outer sleeve tip, 14... Hollow part, 15... Outer electrode (current electrode), 16... Outer electrode (potential electrode), 17... Protrusion for promoting contact, 18... Protrusion for circumferential fixation, 19... Fitting part, 20... Piston, 21... Stand, 22... Chuck, 23... Control unit, 24... Hydraulic unit, H... Penetration hole

Claims

1. A method for evaluating the ground improvement effect by the chemical solution injection method, First, using test results with a failure strain εf < 2%, obtain a first correlation diagram between silica concentration (SiO 2 ) and uniaxial compressive strength (qu), and obtain a second correlation diagram between silica concentration (SiO 2 ) and electrical resistivity (R), which is one step; Based on the first correlation diagram, obtain the target silica concentration (SiO 2 ) from the target uniaxial compressive strength (quck), and then based on the second correlation diagram, set the target electrical resistivity (Rk) from the target silica concentration (SiO 2 ), which is the second step; After ground improvement, measure the electrical resistivity (Rimp) by electrical logging using the penetration holes formed vertically in the ground, and determine whether the target uniaxial compressive strength (quck) is ensured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being less than or equal to the target electrical resistivity (Rk). The method for evaluating the ground improvement effect by the chemical solution injection method is characterized by comprising the above three steps.

2. A method for evaluating the ground improvement effect by the chemical solution injection method, First, using test results with a failure strain εf < 2%, obtain a first correlation diagram between silica concentration (SiO 2 ) and liquefaction strength ratio (RL) or cohesion (c), and obtain a second correlation diagram between silica concentration (SiO 2 ) and electrical resistivity (R), which is one step; Based on the first correlation diagram, obtain the target silica concentration (SiO 2 ) from the target liquefaction strength ratio (RL) or cohesion (c), and then based on the second correlation diagram, set the target electrical resistivity (Rk) from the target silica concentration (SiO 2 ), which is the second step; After ground improvement, the electrical resistivity (Rimp) is measured by electrical logging using the penetration holes formed vertically in the ground, and it is determined whether the target uniaxial compressive strength (quck) is ensured based on whether the electrical resistivity (Rimp) of the improved ground satisfies the condition of being equal to or less than the target electrical resistivity (Rk). A method for evaluating the ground improvement effect by a chemical solution injection method, characterized by comprising a third step.

3. A method for evaluating the ground improvement effect by a chemical solution injection method, In advance, using the test results with a failure strain εf < 2%, a first correlation diagram between the silica concentration (SiO 2 ) and the uniaxial compressive strength (qu) is obtained, and at the same time, a second correlation diagram between the silica concentration (SiO 2 ) and the conductivity (σ) is obtained, which is a first step, Based on the first correlation diagram, the target silica concentration (SiO 2 ) is obtained from the target uniaxial compressive strength (quck), and then based on the second correlation diagram, the target conductivity (σc) is set from the target silica concentration (SiO 2 ), which is a second step, After ground improvement, the electrical resistivity (Rimp) is measured by electrical logging using the penetration holes formed vertically in the ground, and from this, the conductivity (σimp) after ground improvement is calculated. It is determined whether the target uniaxial compressive strength (quck) is ensured based on whether the conductivity (σimp) of the improved ground satisfies the condition of being equal to or greater than the target conductivity (σc). A method for evaluating the ground improvement effect by a chemical solution injection method, characterized by comprising a third step.

4. Before and after ground improvement, a depth distribution diagram of the Nd value showing the relationship between the depth and the Nd value is obtained by a small dynamic cone penetration test, and a primary effect confirmation for evaluating the ground improvement effect is performed 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, as a secondary effect confirmation, the electrical resistivity (Rimp) is measured using the penetration hole of the small dynamic cone penetration test, and the ground improvement effect is evaluated by the method according to any one of Claims 1 to 3. A method for evaluating the ground improvement effect by a chemical solution injection method, characterized by this.

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