Evaluation method for the effectiveness of ground improvement using cement-based ground improvement methods
By correlating uniaxial compressive strength with electrical resistivity and using a simultaneous testing apparatus, the method provides an efficient and accurate assessment of cement-based ground improvement effectiveness, overcoming the limitations of traditional evaluation methods.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- TODA CORP
- Filing Date
- 2024-12-13
- Publication Date
- 2026-06-25
Smart Images
Figure 2026104068000001_ABST
Abstract
Description
[Technical Field]
[0001] This invention relates to a method for evaluating the effectiveness of a cement-based ground improvement method based on the correlation between the electrical resistivity before the improved body solidification and the strength of the improved body. [Background technology]
[0002] Traditionally, ground improvement work has been carried out to strengthen soft ground in reclaimed land and other areas using ground improvement methods such as high-pressure injection mixing, deep mixing treatment, and soil-cement methods that utilize cement-based solidifying agents. In this ground improvement work, a construction confirmation survey is conducted to verify whether the improved body made of cement-based solidifying agent has been properly constructed.
[0003] The most common method for verifying the construction of cement-based ground improvement methods is to evaluate the uniaxial compressive strength (qu) of specimens taken by core sampling after hardening. However, this method has several drawbacks: it is time-consuming and costly to sample the material and conduct uniaxial compression tests in the laboratory; core sampling by boring can damage the core specimen due to drilling with a bit, making it difficult to evaluate continuity; and the uniaxial compressive strength test is based on 28-day strength, making it difficult to take corrective measures such as reconstruction even if problems are found in the improved structure due to the passage of time.
[0004] Furthermore, as an existing investigation method, there is a quality control method (mixing tester method) for columnar improved bodies using a slurry-type mechanical stirring deep mixing treatment method with electrical resistivity, as disclosed in Non-Patent Document 1 below. This method involves inserting a probe into an unconsolidated columnar improved body, measuring the electrical resistivity along its entire length, and quantifying and clarifying its homogeneity. The criteria for quality control based on resistivity are that (1) the number of data points exceeding twice the average resistivity of the entire improved layer is 10% or less, and (2) the coefficient of variation of resistivity in the design target layer is 35% or less. Improved bodies that satisfy both (1) and (2) are judged to be acceptable. [Prior art documents] [Non-patent literature]
[0005] [Non-Patent Document 1] Ministry of Land, Infrastructure, Transport and Tourism, "Technical Development for Establishing Resistivity Technology in Quality Surveys of Ground Improvement Bodies for Building Foundations," [online], November 11, 2015, [Retrieved October 25, 2024], Internet<https: / / www.mlit.go.jp / common / 001110403.pdf> [Overview of the Initiative] [Problems that the invention aims to solve]
[0006] However, after the applicant diligently conducted experiments to examine the validity of the evaluation method using the mixing tester, the following results were obtained.
[0007] The electrical resistivity of improved soil immediately after improvement (at 0 days old) tends to decrease significantly within the range of small amounts of solidifying agent added, but the change due to increasing amounts of solidifying agent becomes extremely small within the range of larger amounts added. As a result, the aforementioned mixing tester method, which uses the homogeneous electrical resistivity distribution immediately after improvement as an indicator, may incorrectly judge that there is an improvement effect even when the amount of solidifying agent added is small, making it difficult to apply to all improved soils.
[0008] Therefore, the main objective of the present invention is to provide a method for evaluating the improvement effect of a cement-based ground improvement method that makes it possible to estimate the uniaxial compressive strength of an improved body by measuring the electrical resistivity before the improved body solidifies, without relying on a uniaxial compressive strength test by sampling. [Means for solving the problem]
[0009] To solve the aforementioned problems, the present invention according to claim 1 is a method for evaluating the effect of ground improvement by a cement-based ground improvement method, The first step involves determining the relationship between the uniaxial compressive strength qu (28 days) and electrical resistivity R (0 days) of the improved soil, which was prepared by mixing in-situ soil and a solidifying agent, through laboratory testing. Based on the relationship obtained in the aforementioned laboratory test, the second step is to set a threshold for electrical resistivity with respect to the target improvement strength of the improved soil, A method for evaluating the effectiveness of a ground improvement method using a cement-based ground improvement method is provided, characterized by comprising: a third step of constructing an improved body at the field site, measuring the electrical resistivity R(0day) before the improved body solidifies, and determining whether the improved body secures the target improvement strength based on whether the condition that this is below the threshold value of the electrical resistivity is met.
[0010] In the invention described in claim 1 above, when evaluating the ground improvement effect (uniaxial compressive strength) of the cement-based ground improvement method, the relationship between the uniaxial compressive strength qu (28 days) and electrical resistivity R (0 days) of the improved soil prepared by mixing the in-situ soil with a solidifying agent is determined in advance by laboratory tests using in-situ soil (first step).
[0011] Next, based on the above relationship, a threshold value for electrical resistivity is set for the target improvement strength of the improved body (second step). That is, if the measured electrical resistivity is less than or equal to the threshold value, the target uniaxial compressive strength will be ensured.
[0012] Subsequently, after constructing the improved structure in the field, the electrical resistivity R(0day) of the improved structure before solidification is measured, and it is determined whether the improved structure achieves the target improved strength based on whether the condition that this is below the threshold for electrical resistivity is met (third step).
[0013] In this invention, the uniaxial compressive strength of the improved body can be estimated by measuring the electrical resistivity before the improved body solidifies.
[0014] As part of the present invention according to claim 2, a method for evaluating the ground improvement effect of the cement-based ground improvement method according to claim 1 is provided, wherein the in-situ soil is cohesive soil or sandy soil.
[0015] In the invention described in claim 2 above, cohesive soil or sandy soil is given as an example of the in-situ soil. [Effects of the Invention]
[0016] As described in detail above, according to the present invention, it becomes possible to estimate the uniaxial compressive strength of the improved body by measuring the electrical resistivity before the solidification of the improved body, without relying on the uniaxial compressive strength test by sampling.
Brief Description of Drawings
[0017] [Figure 1] It is a semi-logarithmic graph showing the relationship between the uniaxial compressive strength qu (28 days) of the improved body and the electrical resistivity R (0 day). [Figure 2] It is a double-logarithmic graph showing the relationship between the uniaxial compressive strength qu (28 days) of the improved body and the electrical resistivity R (0 day). [Figure 3] It is a cross-sectional view showing the test apparatus 1. [Figure 4] It shows the measurement probe 3, (A) is a front view, and (B) is a side view. [Figure 5] It is an exploded view of the measurement probe 3. [Figure 6] It shows the measurement probe main body 8, (A) is a front view, and (B) is a side view. [Figure 7] It is a soil columnar diagram, N value, and cross-sectional view of the improved body. [Figure 8] It is a diagram showing the measurement method by a conductivity meter. [Figure 9] It is a diagram showing the measurement method by an LCR meter. [Figure 10] It is a graph showing the relationship between the electrical resistivity R of the solidifying material slurry and the water / solidifying material ratio. [Figure 11] (A) to (D) are graphs showing the relationship between the uniaxial compressive strength qu of the improved body, the electrical resistivity R, and the addition amount of the solidifying material. [Figure 12] (A) to (D) are graphs showing the relationship between the uniaxial compressive strength qu of the improved body, the electrical resistivity R, and the age of the material. [Figure 13] It is a graph showing the relationship between the uniaxial compressive strength qu of the improved body and the electrical resistivity R, sorted by age for (A) and (C), and by addition amount of the solidifying material for (B) and (D). [Figure 14]This graph shows the relationship between the uniaxial compressive strength qu (28 days) and electrical resistivity R (0 days) of the improved material. [Figure 15] This is a plan view showing the improvement location and measurement location. [Figure 16] (A) is a depth distribution map of Nd values in unimproved ground obtained by a small-scale dynamic cone penetration test, and (B) is a depth distribution map obtained by a large-scale dynamic cone penetration test. [Figure 17] (A) is a depth distribution map of electrical resistivity R in unimproved ground obtained by a small dynamic cone penetration test, and (B) is a depth distribution map of electrical resistivity R obtained by a large dynamic cone penetration test. [Figure 18] This is a depth distribution diagram showing the changes in electrical resistivity R and Nd values as the material ages, as measured by a small-scale dynamic cone penetration test. [Figure 19] This is a depth distribution diagram showing the changes in electrical resistivity R and Nd values as the material ages, as measured by a large dynamic cone penetration test. [Figure 20] This is a flowchart illustrating the conventional quality control method for improved products. [Figure 21] Based on the conventional quality control method for improved materials, (A) is a histogram of electrical resistivity R obtained from a small dynamic cone penetration test, and (B) is a histogram of electrical resistivity R obtained from a large dynamic cone penetration test. [Modes for carrying out the invention]
[0018] Embodiments of the present invention will be described in detail below with reference to the drawings.
[0019] The present invention provides a method for evaluating the effectiveness of ground improvement methods using cement-based solidification materials such as high-pressure injection mixing, deep mixing treatment, and soil-cement methods, for strengthening soft ground such as reclaimed land. Specifically, the method follows the procedure described below.
[0020] The first step involves determining the relationship between the uniaxial compressive strength qu (28 days) and electrical resistivity R (0 days) of the improved soil, which was prepared by mixing in-situ soil and a solidifying agent, through laboratory testing. Based on the relationship obtained in the aforementioned laboratory test, the second step is to set a threshold for electrical resistivity with respect to the target improvement strength of the improved soil, The process consists of three steps: first, creating an improved body in the field; second, measuring the electrical resistivity R(0day) of the improved body before solidification; and third, determining whether the improved body achieves the target improved strength based on whether the condition that this value is below the threshold for electrical resistivity is met.
[0021] Further details will be provided below. (Step 1) Prior to ground improvement, this invention involves conducting mix design tests using local soil, specifically preparing multiple test specimens (ground improvement specimens) with varying amounts of cement-based solidifying agent added to local soil. For each specimen, electrical resistivity tests and unconfined compression tests are performed to obtain a correlation diagram between unconfined compressive strength qu(28 days) and electrical resistivity R(0 day), as shown in Figure 1. The experimental results revealed that unconfined compressive strength qu(28 days) decreased exponentially with increasing electrical resistivity R(0 day), indicating a clear correlation between the two.
[0022] As shown in Figure 1, when a correlation diagram is drawn on a semi-logarithmic graph with a linear scale on the horizontal axis and a logarithmic scale on the vertical axis, with the electrical resistivity R(0day) at age 0 on the horizontal axis and the uniaxial compressive strength qu(28days) at age 28 on the vertical axis, there is a correlation between the uniaxial compressive strength qu(28days) at age 28 and the electrical resistivity R(0day) at age 0, which can be approximated by the following equation (1).
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[0023] Furthermore, as shown in Figure 2, when a correlation diagram is drawn on a log-log graph with both the horizontal and vertical axes on logarithmic scales, with the horizontal axis representing the electrical resistivity R(0day) at age 0 and the vertical axis representing the uniaxial compressive strength qu(28days) at age 28, there is a correlation between the uniaxial compressive strength qu(28days) at age 28 and the electrical resistivity R(0day), which can be approximated by the following equation (2).
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[0024] Since the uniaxial compressive strength qu can be expressed using shear strength Su, and the electrical resistivity R can be expressed using conductivity ρ, it is also possible to use either shear strength Su or conductivity ρ for either the horizontal axis or the vertical axis, or both.
[0025] (Step 2) Based on the correlation diagram obtained in the first step described above, the threshold for electrical resistivity is determined from the target unconfined compressive strength of the improved soil. That is, if the electrical resistivity of the improved soil at age 0 is below the threshold, it means that the unconfined compressive strength of the improved soil at age 28 satisfies the target unconfined compressive strength. If the electrical resistivity exceeds the threshold, it means that the improved soil will not reach the target unconfined compressive strength. The target unconfined compressive strength can be arbitrarily set depending on the application, etc., and one example is that it can be set to 1000 kPa.
[0026] (Step 3) In the third step, after constructing the improved body at the ground improvement construction site, the electrical resistivity R(0day) at age 0 before the improved body solidifies is measured by electrical logging, and it is determined whether the improved body secures the target improvement strength based on whether the condition that this is below the threshold for electrical resistivity is met.
[0027] The electrical resistivity measurement by the aforementioned electrical logging is performed in accordance with the following procedure.
[0028] Electrical logging is performed by using a test apparatus 1, in which a measuring probe 3 is attached to the tip of a penetration rod 2, to strike an anvil 20 fixed to the penetration rod 2 in the axial direction with a hammer 21, thereby penetrating the measuring probe 3 into the ground.
[0029] As shown in Figure 4, the measurement probe 3 is provided with a tip cone 4 for use in dynamic cone penetration tests at its tip, and an electrode section 5 is provided above the tip cone 4, with an electrode 6 for use in electrical logging on its outer surface.
[0030] The outer diameter of the electrode portion 5 is smaller than the outer diameter of the tip cone 4. Furthermore, the axis of the tip cone 4 and the axis of the electrode portion 5 are eccentric, and the electrode 6 is positioned on the outer circumferential surface of the electrode portion 5 on the eccentric side.
[0031] As shown in Figure 3, this test apparatus 1 allows for the simultaneous performance of a dynamic cone penetration test and electrical logging during the penetration process of the measurement probe 3.
[0032] To perform a dynamic cone penetration test using this test apparatus 1, as shown in Figure 3, the anvil 20 fixed to the penetration rod 2 is struck axially with a hammer 21 with a predetermined force, driving the tip cone 4 into the ground, and the penetration depth and number of blows are measured. The penetration of the tip cone 4 is carried out to the required depth by sequentially adding sections of the penetration rod 2, which is divided into predetermined lengths.
[0033] The aforementioned electrical logging measures the electrical resistance (resistivity) of the surrounding ground using an electrode 6 provided on the electrode section 5 of the measuring probe 3, thereby confirming the effectiveness of ground improvement. In managing the progress of improved ground using the chemical grout injection method, when there is a large variation in strength due to reasons such as sampling disturbances and ground non-uniformity, and it is difficult to quantitatively grasp the change in characteristics before and after improvement, measuring electrical resistivity is effective in quantitatively grasping characteristic values that are more sensitive than strength.
[0034] The measurement probe 3 will now be described in more detail. The measurement probe 3 has a vertically elongated rod-like appearance with a substantially circular cross-section. A male threaded portion 8a is formed at its upper end for connection with the penetration rod 2, and it can be screwed into a female threaded portion provided at the lower end of the penetration rod 2. Below the male threaded portion 8a, there is an electrode portion 5 in which a plurality of electrodes 6 are arranged in a single row at predetermined positions in the circumferential direction with axial spacing between them. The tip cone 4 is positioned below this electrode portion 5 and at the lower end of the measurement probe 3.
[0035] As shown in Figure 5, the measurement probe 3 consists of a measurement probe body 8 connected to the penetration rod 2, an outer sleeve 9 fitted to the measurement probe body 8 at a position corresponding to the electrode portion 5, an eccentric spacer 10 fitted to the lower end of the measurement probe body 8, and a tip cone 4 fitted to the eccentric spacer 10.
[0036] As shown in Figure 6, the measuring probe body 8 is a vertically elongated rod-shaped member with a substantially circular cross-section, made of metal or the like that has sufficient strength to transmit the axial impact force input to the penetration rod 2 during a dynamic cone penetration test to the tip cone 4 at its lower end. A hollow section is formed in the axial direction from the upper end to at least the portion where the electrode 6 is located. A male threaded portion 8a is formed at the upper end, which is screwed onto the penetration rod 2.
[0037] Each electrode 6 is connected to the lower end of an electrical cable 7, which is routed from the upper end opening of the measuring probe body 8 through the hollow section. The upper end of the electrical cable 7 extends to the ground through the hollow section of the penetration rod 2, which is formed as a hollow cylinder along its entire length. Its tip is further connected to a computer 19 (see Figure 3) that records and analyzes measurement data, and a power supply (not shown).
[0038] Furthermore, one or more ring-shaped water-stopping members 11 are provided on the outer circumferential surface above and below the axial section in which the electrodes 6 of the measuring probe body 8 are positioned, respectively, to prevent groundwater and other substances from entering through the gap between them and the outer sleeve 9 (Figure 5).
[0039] Furthermore, a groove is provided on a part of the outer circumferential surface of the axial section of the measuring probe body 8 where the electrodes 6 are arranged, which runs axially and communicates with the hollow section from the outside. An insulating member 12 made of an electrically insulating material such as resin is fitted into this groove. The electrodes 6 are provided so as to penetrate the outer and inner surfaces of this insulating member 12.
[0040] The arrangement of the electrodes 6 may be a two-electrode or three-electrode method, but a four-electrode method is preferred. In the four-electrode method, as shown in Figures 4 to 6, four electrodes 6 are arranged in a row with predetermined intervals along the vertical direction (axial direction), with the two electrodes at the top and bottom being current electrodes 6A, and the two in the middle being potential electrodes 6V. The distance between the centers of adjacent electrodes is preferably 2.5 cm. Using the four-electrode method has advantages such as eliminating the need to install ground electrodes and providing more stable measurement values.
[0041] The electrode 6 is made of a conductive metal material and is provided to be electrically connected from the hollow interior of the measuring probe body 8 to the outer surface of the measuring probe 3. The lower ends of the electrical cables 7 are connected to each electrode 6 within the hollow interior of the measuring probe body 8.
[0042] The electrode section 5 is constructed by fitting an outer sleeve 9, which has multiple electrodes 6 exposed on its outer surface, onto a measuring probe body 8 connected to the penetration rod 2. The electrodes 6 on the outer sleeve 9 are provided penetrating the outer and inner surfaces of the outer sleeve 9 and are arranged in a single row with axial spacing, similar to the electrodes 6 on the measuring probe body 8. When the outer sleeve 9 is attached to the measuring probe body 8, each electrode 6 on the outer sleeve 9 is electrically connected to the corresponding electrode 6 on the measuring probe body 8.
[0043] The outer sleeve 9 is made of an electrically insulating material such as resin and is a substantially cylindrical member with both ends in the axial direction open. As shown in Figure 4, when the outer sleeve 9 is inserted into the measuring probe body 8, its outer diameter is formed to be larger than the outer diameter of the measuring probe 3 (measuring probe body 8) which is positioned above the electrode portion 5. By forming the outer diameter of the outer sleeve 9 to be larger than the outer diameter of the measuring probe 3 which is positioned above the electrode portion 5, when the measuring probe 3 is inserted into the ground, the electrode 6 exposed on the outer surface of the outer sleeve 9 is more likely to come into contact with the hole wall, thereby improving the measurement accuracy of electrical logging and suppressing damage to the measuring probe 3.
[0044] Furthermore, the outer diameter of the outer sleeve 9 (electrode portion 5) is smaller than the outer diameter of the tip cone 4. This allows the penetration resistance of the tip cone 4 to be measured primarily during dynamic cone penetration testing, while minimizing the influence of circumferential friction of the outer sleeve 9 (electrode portion 5). The difference between the outer diameter of the electrode portion 5 and the outer diameter of the tip cone 4 is preferably 2 to 10 mm.
[0045] The outer sleeve 9 is inserted from the outer side through a through hole provided in the outer sleeve 9 and fixed by a plurality of countersunk screws 14 that are screwed into screw holes provided in the measuring probe body 8, so as not to cause relative axial displacement or circumferential rotation with respect to the measuring probe body 8. With the outer sleeve 9 fixed by the plurality of countersunk screws 14, the electrode 6 provided on the measuring probe body 8 and the electrode 6 provided on the outer sleeve 9 can be aligned, and circumferential rotation between the measuring probe body 8 and the outer sleeve 9 can be prevented, so that no misalignment occurs between the electrode 6 provided on the measuring probe body 8 and the electrode 6 provided on the outer sleeve 9 when the measuring probe 3 is inserted or withdrawn.
[0046] As shown in Figure 4(B), the measuring probe 3 has an eccentric relationship between the axis of the tip cone 4 and the axis of the electrode portion 5, and the electrode 6 is positioned on the outer circumferential surface of the electrode portion 5 on the side where the axis of the electrode portion 5 is eccentric with respect to the axis of the tip cone 4. Here, the eccentricity between the axis of the tip cone 4 and the axis of the electrode portion 5 is such that the outer circumferential surface of the electrode portion 5 does not protrude diametrically outward from the outer circumferential surface of the tip cone 4. This makes it possible to reduce circumferential friction of the electrode portion 5 in dynamic cone penetration tests while ensuring that the electrode 6 of the electrode portion 5 makes reliable contact with the hole wall surface in electrical logging.
[0047] As shown in Figure 4(B), the measuring probe 3 has an eccentric relationship between the axis of the tip cone 4 and the axis of the electrode portion 5, and the electrode 6 is positioned on the outer circumferential surface of the electrode portion 5 on the side where the axis of the electrode portion 5 is eccentric with respect to the axis of the tip cone 4. Here, the eccentricity between the axis of the tip cone 4 and the axis of the electrode portion 5 is such that the outer circumferential surface of the electrode portion 5 does not protrude diametrically outward from the outer circumferential surface of the tip cone 4. This makes it possible to reduce circumferential friction of the electrode portion 5 in dynamic cone penetration tests while ensuring that the electrode 6 of the electrode portion 5 makes reliable contact with the hole wall surface in electrical logging.
[0048] As shown in Figure 4(B), the diametrical separation distance S between the outer surface of the tip cone 4 and the outer surface of the electrode portion 5 on the eccentric side of the electrode portion 5 is preferably 0 to 3 mm. This reduces circumferential friction during the dynamic cone penetration test, while ensuring that the electrodes 6 of the electrode portion 5 make reliable contact with the hole wall surface to perform electrical logging.
[0049] As shown in Figure 5, an eccentric spacer 10 is fitted onto the lower end of the measuring probe body 8, and the tip cone 4 is fitted onto this eccentric spacer 10.
[0050] The eccentric spacer 10 is a sleeve member made of resin, metal, or the like, formed in a substantially cylindrical shape with both axial ends open, and having a flange-like eccentric flange portion 16 formed at its upper end (base end). The eccentric spacer 10 is fixed to the measuring probe body 8 by a fixing means that prevents it from falling off during either the insertion or withdrawal of the measuring probe 3. Such fixing means include mechanical element connections such as screws, pins, and rivets, or welding, to the measuring probe body 8.
[0051] The eccentric spacer 10 is fixed in place so that relative rotation and axial displacement with respect to the measuring probe body 8 do not occur, by screwing embedded screws into screw holes provided on the circumferential surface of the eccentric spacer 10 from the outer side, and by fitting the tips of the embedded screws into recesses 17 for set screws (see Figure 6) provided on the measuring probe body 8.
[0052] The eccentric spacer 10 has an inner circumferential axis that coincides with the axis of the measuring probe body 8, while its outer circumferential axis is eccentric with respect to the axis of the measuring probe body 8. As a result, when the eccentric spacer 10 is fitted onto the measuring probe body 8, and the tip cone 4 is then fitted onto the eccentric spacer 10, the axis of the measuring probe body 8 and the axis of the tip cone 4 become eccentric.
[0053] The tip cone 4 is not fixed to the eccentric spacer 10, but is simply fitted onto it. Therefore, when the measuring probe 3 is pulled out after being driven into the ground, the tip cone 4 detaches from the measuring probe 3 and remains at the bottom of the hole.
[0054] The tip cone 4 is made to have a shape and dimensions that conform to the specifications of various dynamic cone penetration tests. The outer diameter of the tip cone 4 is made larger than the outer diameter of the electrode portion 5 so that the resistance during penetration in the dynamic cone penetration test acts on the tip cone 4. If the axis of the tip cone and the axis of the electrode portion were to be aligned after making the tip cone 4 larger in diameter than the electrode portion 5, the electrodes of the electrode portion would not contact the hole wall surface, making it impossible to perform accurate electrical logging. However, in the measurement probe 3 of the present invention, the axis of the tip cone 4 and the axis of the electrode portion 5 are eccentric, and the electrodes 6 are placed on the outer circumferential surface of the eccentric side, so that the electrodes 6 can come into contact with the hole wall surface during the penetration process of the measurement probe 3, enabling accurate electrical logging. At this time, since the contact portion of the electrode portion 5 with the hole wall surface is linear, circumferential friction can be reduced during the penetration process of the dynamic cone penetration test.
[0055] As shown in Figure 4, the part of the measuring probe body 8 that contacts the upper end of the outer sleeve 9 is provided with an inclined stepped portion 18, which is formed at an angle in the circumferential direction between the outer circumferential surface of the measuring probe body 8 extending upward and the outer circumferential surface of the outer sleeve 9. The inclined stepped portion 18 is formed such that the circumferential position where the electrode 6 is positioned is the highest in the vertical direction, and the circumferential position on the opposite side is the lowest. Furthermore, the stepped surface itself forms a radially inclined surface that is higher towards the radial center and gradually lower towards the outer circumference. By forming such an inclined stepped portion 18 at the boundary with the outer sleeve 9, when the measuring probe 3 is withdrawn, the soil and sand accumulated in this stepped portion can be efficiently removed to the outer circumference, making the work easier. Furthermore, when the measuring probe 3 is withdrawn, the inclined stepped portion 18 comes into contact with the borehole wall, scraping away the natural ground of the borehole wall and causing soil to accumulate on top of the inclined stepped portion 18. This acts to push the measuring probe 3 towards the electrode 6, allowing electrical logging to be performed even when the measuring probe 3 is withdrawn while the electrode 6 is securely in contact with the borehole wall.
[0056] The assembly of the measurement probe 3 is completed as shown in Figure 5 by attaching the electrode 6 and electrical cable 7 to the measurement probe body 8, attaching the watertight member 11, then inserting the outer sleeve 9 and eccentric spacer 10 in that order from the lower end side of the measurement probe body 8, fixing the eccentric spacer 10 to the measurement probe body 8, and finally fitting the tip cone 4 onto the eccentric spacer 10.
[0057] To conduct a test to confirm the effect of ground improvement using the test apparatus 1 configured as described above, as shown in Figure 3, a dynamic cone penetration test is performed in which the tip cone 4 is driven into the ground by striking it with a hammer 21 while sequentially adding the penetration rod 2 and the electrical cable 7. At the same time, during the same penetration process, electrical logging is performed at predetermined intervals in which current is passed through the current electrode 6A and the potential difference at that time is measured with the potential electrode 6V.
[0058] In this way, since the dynamic cone penetration test is performed and electrical logging is performed simultaneously during the penetration process of the measuring probe 3, it is possible to eliminate the need to change equipment between the dynamic cone penetration test and electrical logging, thereby reducing the labor involved in the measurement work.
[0059] The measurement interval for the electrical logging is preferably 0.5 to 5 Hz. That is, current is passed through the current electrode 6A at intervals of 0.2 to 2 seconds, and the potential difference at that time is measured with the potential electrode 6V. By performing electrical logging at this measurement interval, data errors due to poor contact caused by vibrations during dynamic penetration are avoided, and electrical logging can be performed stably.
[0060] Furthermore, it is preferable to evaluate the reliability of the electrical logging by comparing the resistivity values of the electrical logging during hammer striking and when the hammer stops in the dynamic cone penetration test. In other words, data errors are verified by confirming that there is no significant difference in the resistivity values during hammer striking and when the hammer stops. When the resistivity values during hammer striking and when the hammer stops are within a range of ±10%, preferably within a range of ±5%, it can be determined that there is no difference in the resistivity values. [Examples]
[0061] 1. Introduction In this example, field measurement tests were conducted with the aim of investigating a quality evaluation method for improved soil targeting cohesive soil. Specifically, after obtaining detailed electrical resistivity characteristics of improved soil from laboratory tests of improved soil using cohesive soil sampled from a test yard, field measurement tests were conducted on an improved body created using the high-pressure injection mixing method, using dynamic cone penetration tests and electrical logging with the above-mentioned test apparatus 1. The purpose was to investigate whether it was possible to capture the change in electrical resistivity of a cohesive soil improved body with small changes in electrical resistivity before and after improvement, and whether conventional methods for evaluating improved ground could be applied.
[0062] 2. Overview of the test site The test site is located in Fuseda-cho, Fukui City, Fukui Prefecture. Figure 7 shows the soil column, N-value, and improved cross-section, and Table 1 shows the physical properties of the improved layer. The improved layer is cohesive soil, with a natural water content w0 lower than the liquid limit wL, and the value w0 / wL, which is the natural water content w0 normalized by the liquid limit wL, is in the range of 0.7 to 0.8. According to the literature (Lorenzo, GA et al.: "New and Economical Mixing Method of Cement-admixed Clay for DMM Application," Geotechnical Testing Journal, Vol.29, No.1, 2006. and Takashi Tsuchida et al.: Relationship between initial water content and mixing quality in cement-solidified soil, Japanese Geotechnical Society Journal, Vol.17, No.1, 2021.), there are reports that when w0 / wL falls below 0.85-1.1, the strength of the solidified soil becomes difficult to achieve due to a decrease in mixing quality. Therefore, the soil targeted for improvement in this case is also a ground where a decrease in mixing quality is a concern. In Table 1, ρs is the density of soil particles, Fc is the fine-grained content, Pc is the clay content, wp is the plastic limit, Ip is the plasticity index, and Li is the loss on ignition. [Table 1]
[0063] 3. Indoor testing of improved soil (1) Exam Content In the laboratory tests, we used cohesive soil sampled from the improved soil construction depth GL-2m to -4m in the field measurement tests. Based on the results of electrical resistivity tests and uniaxial compression tests of the improved soil prepared in the laboratory, we investigated quality evaluation indicators for the cohesive soil-improved soil.
[0064] The test involved mixing a cohesive soil sample, a solidifying agent slurry, and an admixture in a soil mixer. As shown in Figure 8, the electrical resistivity immediately after mixing (age 0) was measured using a conductivity meter. Then, cylindrical specimens with a diameter of 5 cm and a height of 10 cm were prepared, and electrical resistivity tests and uniaxial compression tests were performed at ages 1, 3, 7, 14, and 28. The electrical resistivity of the cylindrical specimens was measured using an LCR meter (Hioki E.E. Corporation, sinusoidal AC voltage) with electrodes placed on the upper and lower end faces of the improved soil specimens via filter paper, as shown in Figure 9. Electrical resistivity measurement using an LCR meter is called the AC impedance method. It is a method that measures the impedance Z of the electrical resistance in an AC circuit and the phase difference θ between the current and potential, and then calculates the electrical resistivity R using the following equation (3) (see reference: Kentaro Hatada et al.: Experiment to examine the accuracy and effectiveness of resistivity measurement using the AC impedance method in geological samples, JAMSTEC Rep. Res. Dev., Vol.20, 2015). The measurement frequency was set to 1 kHz. Uniaxial compression tests were performed on the specimens after electrical resistivity measurement.
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[0065] Table 2 shows the conditions for preparing the test specimens. Two types of cohesive soil samples were used: GL-2.0m to -3.5m (improved soil 1) and GL-3.5m to -4.0m (improved soil 2). A cement-based solidifying agent with reduced hexavalent chromium leaching was used as the solidifying agent, and a polycarboxylic acid-based admixture was used as the admixture. The type and amount of admixture added were determined to ensure the fluidity of the improved soil. Figure 10 shows 200 kg / m 3 This shows the relationship between the electrical resistivity of the added solidifying agent slurry and the water / solidifying agent ratio. The electrical resistivity of the solidifying agent slurry is hardly affected by the water / solidifying agent ratio and is in the range of 1.2 to 1.8 Ωm. [Table 2]
[0066] (2) Test results Figure 11 shows the relationship between the unconfined compressive strength qu, electrical resistivity R of the improved soil, and the amount of solidifying agent added. The electrical resistivity R of the improved soil immediately after mixing with the solidifying agent (age 0) is lower compared to the unimproved soil, but even with small amounts of solidifying agent added (25 and 50 kg / m³), the resistance decreases. 3 Even with additives, the resistance decreased to below 10 Ωm, and the change due to increasing the amount of additive after the decrease in electrical resistivity was extremely small. Furthermore, with a solidifying agent additive amount of 100 kg / m³, the resistance was significantly reduced. 3 And 200, 300 kg / m 3 Comparing the uniaxial compressive strength qu and electrical resistivity R, the additive amount 100 kg / m³ 3 For a uniaxial compressive strength qu of approximately 250 kPa and electrical resistivity of approximately 4 Ωm, the additive amounts are 200 and 300 kg / m³. 3 The unconfined compressive strength (qu) was approximately 1800 and 5800 kPa, and the electrical resistivity (R) was approximately 2.5 and 1.5 Ωm. While the unconfined compressive strength (qu) varied significantly depending on the amount added, the electrical resistivity (R) remained almost constant at around a few Ωm. Therefore, it is considered difficult to evaluate the quality of cohesive soils using the decrease in electrical resistivity distribution immediately after improvement as an indicator.
[0067] Figure 12 shows the relationship between the uniaxial compressive strength qu, electrical resistivity R, and age of the improved soil. The electrical resistivity R of the improved soil increases with age. Furthermore, based on the literature (Ohno, Y. et al.: “Performance Evaluation of Jet Grouting by Dynamic Probing and Electrical Logging”, Proc. 34th International Ocean and Polar Engineering Conference, Rhodes, 2024.), the applicability range of the investigation method using the above test apparatus 1 to cement-improved soil is approximately 3 days of age. Therefore, Table 3 summarizes the increments of uniaxial compressive strength qu and electrical resistivity R from 0 to 3 days of age. [Table 3]
[0068] The increment in the age range of 0 to 3 days is such that at a solidifying agent addition amount of 25, 50 kg / m 3 Δqu = 10 - 50 kPa and ΔR = 0 - 3 Ωm, and at a solidifying agent addition amount of 100 kg / m 3 Δqu = 120 - 200 kP and ΔR = 3 Ωm, and at a solidifying agent addition amount of 200 - 300 kg / m 3 Δqu = 900 - 2200 kPa and ΔR = 4 - 6 Ωm. Therefore, even at a small solidifying agent addition amount of 50 kg / m 3 the electrical resistivity R increases with the passage of age, and it hardly changes compared to the same increment at a solidifying agent addition amount of 100 - 300 kg / m 3 Also, in the improved soil with a solidifying agent addition amount of 100 - 300 kg / m 3 while the uniaxial compression strength qu increases significantly depending on the solidifying agent addition amount, the electrical resistivity R hardly changes. Therefore, it is considered difficult to conduct quality evaluation using the change in electrical resistivity with the passage of age as an index.
[0069] Regarding the solidifying agent addition amount of 100 - 300 kg / m 3 where the strength development of the improved soil is recognized in Fig. 13, the relationships between the uniaxial compression strength qu and the electrical resistivity R are arranged for each age and addition amount. In the relationship between the uniaxial compression strength qu and the electrical resistivity R of the improved soil arranged for each age in the upper part of Fig. 13, no correlation is recognized, but in the same relationship arranged for each solidifying agent addition amount in the lower part of Fig. 13, a correlation is recognized. This is considered to be due to the fact that the uniaxial compression strength qu of the improved soil changes significantly depending on the solidifying agent addition amount, while the electrical resistivity R hardly changes, and the change in the electrical resistivity of the improved soil with the passage of age varies depending on the solidifying agent addition amount. Therefore, when estimating the improvement strength from the electrical resistivity R using this relationship, it is considered that it is limited to the condition where the solidifying agent addition amount of the in-situ improved soil is clear.
[0070] Figure 14 shows the relationship between the uniaxial compressive strength qu(28 days) of improved soil at 28 days and the electrical resistivity R(0 day) at 0 days. For comparison, the figure also shows the relationship for the test results of silica sand No. 7 conducted by Tamura et al. as an improved soil for sandy soil (Tamura Masahito et al.: Field experiment on quality evaluation of improved soil by electrical resistivity, Journal of Structural Engineering, Architectural Institute of Japan, No. 531, 2000), similarly organized. There is a correlation between the uniaxial compressive strength qu(28 days) at 28 days and the electrical resistivity R(0 day), and this relationship is approximated by equation (1) above. Comparing improved cohesive soil and improved sandy soil, it can be seen that the change in electrical resistivity with respect to the change in strength is small, about 5 Ωm, for improved cohesive soil, but changes are larger, about 30 Ωm, for improved sandy soil.
[0071] (3) Summary The laboratory tests were conducted to understand the electrical resistivity characteristics of improved cohesive soil and to examine quality evaluation indicators. The results of the tests are shown in 1) to 4) below.
[0072] 1) The electrical resistivity R(0day) of improved soil at age 0 decreases with the addition of a small amount of solidifying agent. Although the uniaxial compressive strength qu of improved soil varies greatly depending on the amount of solidifying agent added, the electrical resistivity R hardly changes, remaining at around a few ohms. Therefore, in the case of improving cohesive soil, it is difficult to evaluate quality using the decrease in the electrical resistivity distribution immediately after improvement as an indicator.
[0073] 2) The electrical resistivity R of improved soil increases with age, even with a small amount of solidifying agent added. 3 In the improved soil, the increase in unconfined compressive strength qu over 1 to 3 days increases significantly with the amount of solidifying agent added, whereas the increase in electrical resistivity R over the same period shows almost no change with the amount of solidifying agent added.
[0074] 3) No correlation was found between the uniaxial compressive strength qu and electrical resistivity R of improved soil at different ages, but a correlation was found between the same relationship and the amount of solidifying agent added. Therefore, when estimating the improved strength from electrical resistivity R, it is limited to conditions where the amount of solidifying agent added is clearly defined.
[0075] 4) There is a correlation between the uniaxial compressive strength qu (28 days) at 28 days and the electrical resistivity R (0 days) at 0 days, which is approximated by equation (1) above. From this relationship, the change in electrical resistivity with respect to the change in strength of the improved soil is small for cohesive soil improved soil, but is somewhat large for sandy soil improved soil.
[0076] 4. Field measurement test (1) Exam Overview In the field measurement test, the above-described test apparatus 1 was applied to field test improved ground bodies constructed using the high-pressure injection mixing method, and the electrical resistivity R and Nd values were measured immediately after construction and at a young age (1-3 days old). In addition, samples of the improved ground were taken near the electrical logging measurement location immediately after construction, and the collected samples were visually inspected and subjected to uniaxial compression tests. Based on the test results, the following were investigated: 1) whether the electrical logging measured in the field can capture changes in electrical resistivity before and after improvement and due to changes in age; 2) whether the conventional method (see Non-Patent Document 1 above) that uses the decrease in electrical resistivity distribution immediately after improvement as an indicator can be applied to the evaluation of improved ground; and 3) a method for evaluating the quality of cohesive soil improved ground bodies based on the results of laboratory tests and field measurements.
[0077] (2) Overview of Dynamic Cone Penetration Test and Electrologging In this embodiment, the Nd value was measured from a dynamic cone penetration test and the electrical resistivity R from an electrical logging test in the same hole using the test apparatus 1 described above. Two types of dynamic cone penetration tests were performed using a small dynamic cone penetration test and a large dynamic cone penetration test, each with a different cross-sectional area of the tip cone. The specifications of the dynamic cone penetration tests used in this field measurement test are shown in Table 4. [Table 4]
[0078] (3) Exam Content The soil profile and N-value of the test site are shown in Figure 7. The test improved body was constructed using high-pressure jet agitation to create a cylindrical body with a planned improvement diameter of 2.5 m and an improved layer thickness of 2.0 m (GL-2.0 m to -4.0 m). The construction specifications for the high-pressure jet agitation method were a solidifying agent slurry injection pressure of 35 MPa, injection volume of 190 L / min, and injection device withdrawal speed of 10 min / m. A cement-based solidifying agent with reduced hexavalent chromium leaching was used, with a water / solidifying agent ratio of W / B = 100%. Furthermore, based on the planned volume of the improved soil and the construction specifications for the high-pressure jet agitation method, the amount of solidifying agent added to the improved body was calculated to be 280 kg / m³. 3 It will be to that extent.
[0079] Based on the results of the laboratory test of the improved soil shown in Figure 11, assuming a ratio of 1 / 3 between the field strength and the laboratory mix strength, the expected field improved strength is approximately 1.3 to 1.5 MPa.
[0080] Measurements were taken approximately 30 minutes after the completion of the improved body construction by pressing an electrode probe into the improved body and measuring the electrical resistivity R immediately after construction (age 0). Subsequently, dynamic cone penetration tests (during penetration) and electrical logging (during withdrawal) were performed on the improved bodies at ages 1, 2, and 3, and the Nd value and electrical resistivity R were measured. In addition, improved body samples (all core, age 28) were taken near the location of the electrical logging performed immediately after improvement, and the solidification state was visually confirmed and a uniaxial compression test was performed. The improved body plan and measurement locations are shown in Figure 15.
[0081] (4) Test results 1) Confirmation of the continuity of the improved soil and results of the unconfined compression test Visual inspection of improved soil cores (all cores) of T-1 and T-2 (Figure 15), taken at positions 0.6m and 0.8m from the center of the improved body, revealed that unconsolidated areas were scattered throughout both T-1 and T-2.
[0082] Furthermore, areas that appeared relatively solidified were selected by visual inspection, and the results of unconfined compression tests (at 28 days of age) are shown in Table 5. The average unconfined compressive strength of the improved soil was 130 kPa at T-1 and 350 kPa at T-2, which is about 10-20% of the unconfined compressive strength of 1.3-1.5 MPa expected from the laboratory test results, with a solidifying agent addition amount of 100 kg / m³. 3The situation falls short of that. [Table 5]
[0083] 2) Results of dynamic cone penetration test and electrical logging First, to confirm the variability of the unimproved ground, Figures 16 and 17 show the depth distribution of Nd values and electrical resistivity for the entire test yard. From GL-2.0m to -3.5m, sand is irregularly mixed in, resulting in ground with large variations in Nd values and electrical resistivity R depending on the degree of sand mixing.
[0084] Figures 18 and 19 show the depth distribution of electrical resistivity R, Nd value, and uniaxial compressive strength qu at 28 days of age of the test improved material, measured by small dynamic cone penetration tests and large dynamic cone penetration tests, respectively, as the material ages (0-3 days). The electrical resistivity R immediately after the improvement material was created decreased to 4-10 Ωm and increased to 10-20 Ωm at 3 days of age. In contrast, the increase in electrical resistivity from 0-3 days of age was 6-10 Ωm in the laboratory test results shown in Figure 12, suggesting that the electrical resistivity R measured in the field generally accurately evaluates the improved material.
[0085] First, as an example of existing evaluations, the quality of the improved soil was evaluated based on the evaluation method described in Non-Patent Document 1, which uses the electrical resistivity distribution immediately after improvement, as shown in Figure 20, as an indicator. As shown in Figures 18 and 19, the electrical resistivity R immediately after the improvement was constructed decreased uniformly, and as shown in Figure 21, from the histogram of electrical resistivity R in the improved range GL-2m to -4m, there were 0% of the data exceeding twice the average value of electrical resistivity R, and the coefficient of variation was 25%, 29%, and 35% or less. According to the flow shown in Figure 20, the improvement quality would be evaluated as good, but as mentioned above, the sampled improved soil core was of poor quality, so it was found that the improvement quality evaluation using the uniform decrease in electrical resistivity R immediately after construction as an indicator cannot be applied to cohesive soil improved soil. This is thought to be due to the electrical resistivity characteristics of cohesive soil improved soil, which decrease even with the addition of a small amount of solidifying agent.
[0086] Next, we compare the changes in electrical resistivity R and Nd values of the improved soil sample as the age progresses (0-3 days). As mentioned above, the electrical resistivity R of the improved soil sample increased from 4-10 Ωm immediately after improvement to 10-20 Ωm at 3 days. This indicates the characteristic of the improved cohesive soil where electrical resistivity increases with age, even at low additive amounts, similar to the results of the laboratory test. On the other hand, no clear increase in Nd value was observed after 1-3 days. The laboratory test results were for a solidifying agent additive amount of 200 kg / m³. 3 Since the unconfined compressive strength qu of the improved soil reaches qu = 500-1000 kPa at 1-3 days of age, it is considered that there is almost no increase in strength with the passage of time.
[0087] For reference, using the correlation between the uniaxial compressive strength qu of the improved soil at 28 days old and the electrical resistivity R at 0 days old, obtained from the indoor test results shown in Figure 14, the threshold for electrical resistivity R for a target improved strength of 1000 kPa or more is found to be 3 Ωm or less. Comparing this threshold with the electrical resistivity distribution at 0 days old (immediately after improvement body construction) shown in Figures 18 and 19, it was not possible to confirm measurements of 3 Ωm or less in most improvement depth ranges.
[0088] (5) Summary The results of this field measurement test are shown in 1) to 5) below, and the electrical resistivity characteristics of the improved cohesive soil obtained from the laboratory test results were confirmed in the field measurement.
[0089] 1) In poorly improved cohesive soil bodies, the electrical resistivity immediately after construction decreases uniformly compared to unimproved ground. This represents a characteristic of cohesive soil where the electrical resistivity R decreases with the addition of a small amount of solidifying agent, and there is almost no change with increasing the amount of additive after the decrease.
[0090] 2) When the quality of the improved ground was evaluated on-site using an evaluation method that uses the electrical resistivity distribution immediately after improvement as an indicator (Non-Patent Document 1 mentioned above), the improved ground was judged to be in good condition despite being poorly improved. Therefore, it is difficult to evaluate the quality of cohesive soil improved ground using this method.
[0091] 3) It was confirmed that the electrical resistivity R measured by this method captures subtle changes associated with the aging of the improved material.
[0092] 4) In the poorly modified specimens, the electrical resistivity R of the locally modified specimens increased with the aging period of 0 to 3 days. On the other hand, the change in Nd value was not clear.
[0093] 5) When comparing the threshold for judging the improvement effect, which was set based on the correlation between the unconfined compressive strength qu of the improved soil at 28 days old and the electrical resistivity R at 0 days old, with the electrical resistivity distribution of the improved soil at 0 days old in the field, it was determined that the improvement was insufficient at most improvement depths, as measurements of 3 Ωm or less, the target value, could not be confirmed.
[0094] 5. Conclusion In this example, field measurement tests were conducted with the aim of investigating a quality evaluation method for improved soil targeting cohesive soil, and the following conclusions were obtained (1) to (5).
[0095] 1) The electrical resistivity R of improved cohesive soil immediately after improvement (at age 0) decreases even with the addition of a small amount of solidifying agent. Therefore, existing evaluation methods that use a uniform decrease in the electrical resistivity distribution as an indicator (Figure 20) may incorrectly judge poorly improved soil as effective, making it difficult to apply this indicator to improved cohesive soil.
[0096] 2) The electrical resistivity measurement using the above-described test apparatus 1 confirmed that it could capture even subtle changes associated with the age of the material, even at low electrical resistivity values.
[0097] 3) The electrical resistivity of improved soil increases with age, even with only a small amount of solidifying agent added. Therefore, it is difficult to use the change in resistivity with age as a quality evaluation indicator. On the other hand, since the Nd value is an indicator of the strength of improved soil, the change in Nd value with age may be a quality evaluation indicator.
[0098] 4) No correlation was found between the uniaxial compressive strength qu of the improved soil and its electrical resistivity R. However, a correlation was observed when the same relationship was analyzed for each amount of solidifying agent added. Therefore, when estimating the improved strength from electrical resistivity R, it is limited to conditions where the amount of solidifying agent added is clearly defined.
[0099] 5) There is a correlation between the uniaxial compressive strength qu (28 days) at 28 days of age and the electrical resistivity R (0 days) at 0 days of age, and this can be approximated by equation (1) or (2) above. [Explanation of Symbols]
[0100] 1…Testing apparatus, 2…Penetration rod, 3…Measuring probe, 4…Tip cone, 5…Electrode section, 6…Electrode, 7…Electrical cable, 8…Measuring probe body, 9…Outer sleeve, 10…Eccentric spacer, 11…Waterproofing member, 12…Insulating member, 14…Countersunk screw, 16…Eccentric flange, 17…Recess for set screw, 18…Inclined step section, 19…Computer, 20…Anvil, 21…Hammer
Claims
1. A method for evaluating the effectiveness of ground improvement using cement-based ground improvement methods, The first step involves determining the relationship between the uniaxial compressive strength qu (28 days) and electrical resistivity R (0 days) of the improved soil, which was prepared by mixing in-situ soil and a solidifying agent, through laboratory testing. Based on the relationship obtained in the aforementioned laboratory test, the second step is to set a threshold for electrical resistivity with respect to the target improvement strength of the improved soil, A method for evaluating the effectiveness of ground improvement using a cement-based ground improvement method, characterized by comprising: a third step of constructing an improved body at the field site, measuring the electrical resistivity R(0day) before the improved body solidifies, and determining whether the improved body secures the target improvement strength based on whether the condition that this is below the threshold value of the electrical resistivity is met.
2. A method for evaluating the ground improvement effect of a cement-based ground improvement method according to claim 1, wherein the in-situ soil is cohesive soil or sandy soil.