Estimation method for penetration length of clayey soil at tip of vertical drain

Abstract

To provide an estimation method for a penetration length of clayey soil at a tip of a vertical drain.SOLUTION: An estimation method of the present disclosure is an estimation method for a penetration length L of clayey soil at a tip of a vertical drain 1. The estimation method includes a penetration length calculation step for determining the penetration length L at which a suction force F1 that draws the clayey soil into the vertical drain 1 is balanced with a resistance force F2 that resists the suction force F1 and varies depending on the penetration length L. The resistance force F2 is expressed as a product of the penetration length L, an adhesion resistance f2 of the clayey soil in the vertical drain 1, and a perimeter Ld of a drainage groove Dr of the vertical drain 1.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for estimating the penetration length of clayey soil at the tip of a vertical drain. [Background technology]

[0002] One of the methods for improving the ground of soft clayey soil is the consolidation promotion method using vertical drains.

[0003] This method aims to promote consolidation of clay by pouring highly permeable drain material vertically into the clay layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-190237 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-022561 Summary of the Invention [Problem to be solved by the invention]

[0005] However, depending on the condition of the clayey soil into which the vertical drain is cast, the clayey soil may enter the drainage ditch of the vertical drain from the tip of the vertical drain and clog the drainage ditch.

[0006] Furthermore, since drainage ditches clogged with clayey soil have a significantly reduced drainage function, it is considered important to know the length over which clayey soil penetrates from the tip of the vertical drain in order to carry out appropriate ground improvement work.

[0007] The present disclosure has been made in consideration of the above circumstances, and aims to provide a method for estimating the penetration length of clay soil at the tip of a vertical drain. [Means for solving the problem]

[0008] The estimation method disclosed herein is a method for estimating the penetration length of clay soil at the tip of a vertical drain, and includes a penetration length calculation step for determining the penetration length at which the suction force that draws the clay soil into the vertical drain and the resistance force that resists the suction force, which changes depending on the penetration length, are balanced. [Effects of the Invention]

[0009] According to the present disclosure, a method for estimating the penetration length of clay soil at the tip of a vertical drain can be provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a cross-sectional view of the tip of the vertical drain of the first embodiment according to the present disclosure. [Figure 2] FIG. 2 is a diagram for explaining the penetration length of clay soil from the cross section of the tip of the vertical drain of the first embodiment according to the present disclosure. [Figure 3] 1 is a flowchart illustrating the steps of a method for estimating the penetration length of clay soil at the tip of a vertical drain according to a first embodiment of the present disclosure. [Figure 4] 1 is a graph showing the relationship between relative water content and undrained shear strength. [Figure 5] FIG. 2 is a diagram for explaining the contents of an experiment according to the first embodiment of the present disclosure. [Figure 6] 1 is a graph in which the penetration length determined by an experiment is plotted on a graph of the penetration length of clay soil estimated by the estimation method of the first embodiment according to the present disclosure. [Figure 7] 10 is a flowchart illustrating the steps of a method for estimating the penetration length of clay soil at the tip of a vertical drain according to a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, modes for carrying out the invention (hereinafter referred to as "embodiments") will be described in detail with reference to the accompanying drawings. It should be noted that the same elements are denoted by the same numbers or symbols throughout the description of the embodiments.

[0012] <<First Embodiment>> A method for estimating the penetration length L of the clay soil at the tip of the vertical drain 1 according to the first embodiment of the present disclosure will be described with reference to FIGS. 1 to 6. FIG.

[0013] FIG. 1 is a diagram showing a cross section CS of a tip end of a vertical drain 1 of a first embodiment according to the present disclosure. In the following description, the wider direction in cross section will be referred to as the width direction (the left-right direction in FIG. 1), and the narrower direction will be referred to as the height direction (the up-down direction in FIG. 1).

[0014] FIG. 2 is a diagram for explaining the penetration length L of clay soil from the cross section CS at the tip of the vertical drain 1 of the first embodiment according to the present disclosure. 2 is a diagram that schematically shows only one drain groove Dr along the length direction of the vertical drain 1, and the length direction is the direction in which the vertical drain 1 extends. The length direction is also the direction in which the drain grooves Dr extend.

[0015] As shown in FIG. 1, the vertical drain 1 includes a core body 11 that forms the framework of the drain groove Dr, and a pair of filters 12 provided on both sides of the core body 11 in the height direction.

[0016] In this embodiment, the core 11 is made of plastic and the filter 12 is made of nonwoven fabric, but there is no need to be limited to this.

[0017] For example, the core 11 may be formed of a material having the strength to function as a skeleton, and the filter 12 may be formed of a material that is permeable and has the strength to not break when pouring.

[0018] The core body 11 includes rod-shaped portions 11A that are provided at equal intervals in the width direction and extend in the height direction, and connecting portions 11B that connect adjacent rod-shaped portions 11A. However, it is not necessary that the rod-shaped portions 11A extending in the height direction are provided at equal intervals in the width direction.

[0019] In this embodiment, the vertical positions of adjacent connection parts 11B are alternately changed, so that the vertical drain 1 has drainage grooves Dr, which are a wide groove Dr1 with a large rectangular cross section formed by the core body 11 and the filter 12, and a narrow groove Dr2 with a small rectangular cross section formed by the core body 11 and the filter 12.

[0020] All of the connection portions 11B may be provided at the center position in the vertical direction of the rod-shaped portion 11A, and all of the drain grooves Dr of the vertical drain 1 may have the same rectangular cross-sectional area.

[0021] When such a vertical drain 1 is placed in the ground for ground improvement, water permeates from the filter 12 into the drainage ditch Dr, promoting the consolidation of the clayey soil.

[0022] However, at the cross section CS at the tip of the vertical drain 1, as shown in Figure 1, the drainage ditch Dr is open, so depending on the condition of the clayey soil being poured, the clayey soil may enter the drainage ditch Dr.

[0023] Then, noticing that the penetration length L of this clayey soil changes depending on the state of the clayey soil, and referring to Figure 2, we came to the conclusion that the penetration length L of the clayey soil from the cross section CS at the tip of the vertical drain 1 is determined by the mechanism that will be explained below, and based on that mechanism, we discovered a method to estimate the penetration length L of the clayey soil from the cross section CS at the tip of the vertical drain 1.

[0024] In order for the clayey soil to penetrate through the cross section CS at the tip of the vertical drain 1, a suction force F1 is required to draw the clayey soil into the vertical drain 1 (more precisely, into the drainage ditch Dr), as shown in Figure 2.

[0025] This suction force F1 (kN) is calculated based on the cross-sectional area Ad (m 2 ) and the suction pressure P (kN / m 2 ) (see Equation 1). F1 = Ad × P (1)

[0026] For example, when the drainage ditch Dr is vacuumed to promote drainage in the vertical drain 1, the suction pressure P (kN / m 2 ) is thought to be dominated by the force caused by the vacuum, and when vacuuming or the like is not performed, it is thought that the consolidation pressure is dominant.

[0027] On the other hand, there is a resistance force F2 (kN) that resists the suction force F1 (kN). The resistance force F2 (kN) is determined by the penetration length L (m) and the adhesion resistance f2 (kN / m 2 ) and the perimeter Ld (m) of the drainage ditch Dr of the vertical drain 1 (see Equation 2). F2 = Ld × L × f2 (2)

[0028] In equation (2), the perimeter Ld (m) and adhesion resistance f2 (kN / m 2 ) is determined by the shape of the vertical drain 1 and the condition of the clayey soil of the land to be improved.

[0029] Therefore, when vertical drain 1 is cast into the land to be improved, it is thought that clayey soil will penetrate into the drainage ditch Dr of vertical drain 1 until the penetration length L (m) reaches a point where the resistance force F2 (kN), which changes depending on the penetration length L (m), and the suction force F1 (kN) are balanced.

[0030] And adhesion resistance f2 (kN / m 2) is the undrained shear strength USS (kN / m 2 ) and the coefficient α, but in the experimental example shown later, α is set to 1.0, and the undrained shear strength USS (kN / m 2 ) adhesion resistance f2 (kN / m 2 )

[0031] The coefficient α is set in consideration of the case where a correction is required to convert the undrained shear strength USS of the clay soil into the bond resistance f2. In such cases, the coefficient α can be determined experimentally, as will be explained later.

[0032] The penetration length L (m) of the clay soil from the cross section CS at the tip of the vertical drain 1 can be calculated based on the condition that the suction force F1 (kN) and the resistance force F2 (kN) that resists the suction force F1 (kN) are balanced, as shown in Figure 2 (see Equation 3). L = [Ad × P] / [Ld × f2] (3)

[0033] Next, a method for estimating the penetration length L of the clay soil at the tip of the vertical drain 1 of this embodiment will be described with reference to FIG.

[0034] FIG. 3 is a flowchart for explaining the procedure of a method for estimating the penetration length L of the clay soil at the tip of the vertical drain 1 according to the first embodiment of the present disclosure.

[0035] However, the procedure is not necessarily limited to that shown in FIG. 3, and the order of steps that can be reversed without causing any problems may be reversed.

[0036] In addition, since this is assumed to be done to determine the length of the clay soil that will penetrate from the tip of Vertical Drain 1 when proceeding with ground improvement, in the explanation of the flowchart below, it will simply be referred to as clay soil, but this clay soil is the clay soil of the land where ground improvement is planned (where Vertical Drain 1 is planned to be poured).

[0037] Specifically, the clayey soil was sampled from land where ground improvement work was planned (where vertical drain 1 was planned to be installed).

[0038] However, if the particle size and composition are similar, it is thought that they will exhibit similar behavior with respect to the penetration length L, etc.

[0039] Therefore, the clay soil to be used in the procedure described below is not limited to samples taken from the land where ground improvement is planned (where vertical drain 1 is planned to be installed), and any clay soil having a particle size and composition similar to that of the clay soil in the land where ground improvement is planned (where vertical drain 1 is planned to be installed) may be used.

[0040] Furthermore, each step appearing in the following description of the procedure with reference to FIG. 3 is a step included in the method for estimating the penetration length L of clay soil at the tip of the vertical drain 1 of the first embodiment according to the present disclosure.

[0041] (S1) First, in S1, a moisture content determination step is carried out to determine the moisture content of the clayey soil using a test method that complies with JIS A 1203:2020 "Testing method for moisture content of soil," for example, to determine the moisture content of the clayey soil.

[0042] (S2) Next, in S2, a liquid-plastic limit determination step is carried out to determine the liquid limit and plastic limit of the clayey soil using a test method that complies with JIS A 1205:2020 "Test method for liquid limit and plastic limit of soil," for example, and the liquid limit and plastic limit of the clayey soil are determined.

[0043] (S3) Then, in S3, a relative water content determination step is carried out to determine the relative water content of the clayey soil based on the water content, liquid limit, and plastic limit determined in S1 and S2.

[0044] Specifically, the relative water content is expressed as in Equation 4 using the water content, liquid limit, and plastic limit, so the relative water content of clayey soil can be determined by substituting the water content, liquid limit, and plastic limit into Equation 4. Relative water content = [water content - plastic limit] / [liquid limit - plastic limit] (4)

[0045] (S4) Then, in S4, once the relative water content is determined, an undrained shear strength determination step is carried out to determine the undrained shear strength USS based on the relationship between the relative water content and the undrained shear strength USS, and the undrained shear strength USS of the clayey soil is determined.

[0046] Figure 4 is a graph showing the relationship between relative water content and undrained shear strength USS. Once the relative water content is determined, the undrained shear strength USS of the clayey soil can be determined from the relationship between relative water content and undrained shear strength USS shown in Figure 4.

[0047] The graph showing the relationship between the relative water content and the undrained shear strength USS as shown in FIG. 4 may be obtained experimentally in advance, or data from literature may be used.

[0048] (S5) Then, in S5, an adhesion resistance determination step is carried out to determine the adhesion resistance f2 of the clayey soil in the vertical drain 1, and the adhesion resistance f2 is determined. Specifically, the bond resistance f2 is determined based on the undrained shear strength USS of the clayey soil.

[0049] As already explained, the bond resistance f2 is expressed as the product of the coefficient α and the undrained shear strength USS of the clayey soil, but in many cases, the undrained shear strength USS and the bond resistance f2 are considered to be roughly the same.

[0050] For this reason, in this embodiment, the adhesion resistance f2 is calculated with the coefficient α set to 1.0, and the adhesion resistance determination step is a step in which the adhesion resistance f2 is determined to be the undrained shear strength USS itself.

[0051] (S6) Next, in S6, a suction force setting step is carried out to set a suction force F1 for drawing the clayey soil into the vertical drain 1, and the suction force F1 to be used in the calculation for obtaining the penetration length L, which will be performed later, is set.

[0052] As explained above, for example, if drainage is to be promoted by vacuuming the inside of the vertical drain 1, the suction pressure P due to the vacuuming is multiplied by the cross-sectional area Ad of the drainage groove Dr to obtain the suction force F1 (see Equation 1 shown above), and the obtained suction force F1 is set as the suction force F1 to be used in the calculation to obtain the penetration length L later.

[0053] (S7) Finally, in S7, an intrusion length calculation step is carried out to determine the intrusion length L at which the suction force F1 that draws the clayey soil into the vertical drain 1 and the resistance force F2 that resists the suction force F1 and changes depending on the intrusion length L are balanced, and the intrusion length L of the clayey soil from the tip of the vertical drain 1 is determined.

[0054] Specifically, the penetration length calculation step is a step in which the suction force F1 is divided by the product of the adhesion resistance f2 (in this example, the undrained shear strength USS) and the perimeter Ld of the drainage groove Dr to obtain the penetration length L (see equation 3 above).

[0055] By following the above procedure, when a vertical drain 1 is cast into land to be improved, it is possible to estimate the penetration length L, that is, the length of penetration of clayey soil from the tip of the vertical drain 1.

[0056] <Experimental Example> Next, we will show the penetration length L (calculated result) estimated as described above and the results of an experiment (experimental result) to determine the actual length over which clayey soil penetrates from the tip of the vertical drain 1.

[0057] As described with reference to FIG. 1, in this embodiment, the vertical drain 1 has, as drainage grooves Dr, a wide groove Dr1 with a large area and a rectangular cross section, and a narrow groove Dr2 with a small area and a rectangular cross section.

[0058] Specifically, as shown in FIG. 1, the wide groove Dr1 has a rectangular cross section with a height H and a width W, and the narrow groove Dr2 has a rectangular cross section with a height h and a width W. The height H is 1.4 mm, the height h is 0.4 mm, and the width W is 1.9 mm.

[0059] Therefore, the circumferential length Ld and cross-sectional area Ad of the inner peripheral surface of the wide groove Dr1 are 6.6 mm and 2.66 mm, respectively. 2 is.

[0060] The circumferential length Ld and cross-sectional area Ad of the inner peripheral surface of the narrow groove Dr2 are 4.6 mm and 0.76 mm, respectively. 2 is.

[0061] In the above, the units of the circumferential length Ld and the cross-sectional area Ad are shown in mm, but the units in the calculation formulas explained above are m, and actual calculations are performed in m, and the same applies hereinafter.

[0062] FIG. 5 is a diagram for explaining the contents of an experiment according to the first embodiment of the present disclosure. As shown in FIG. 5, the experiment used a container 2 containing clayey soil and a vertical drain 1 with a cap 3 attached to the top for vacuuming.

[0063] The cap 3 is connected to a vacuum pump 4 via a pipe, and the vacuum pump 4 is capable of evacuating the interior of the vertical drain 1 to a predetermined vacuum pressure.

[0064] Then, as shown in Figure 5, after the vertical drain 1 was inserted into the clayey soil and the inside of the vertical drain 1 was kept at a predetermined vacuum pressure for a sufficient time, the vacuum was stopped, the vertical drain 1 was removed, and the penetration length L of the clayey soil from the cross section CS at the tip of the vertical drain 1 was measured.

[0065] Specifically, four clay samples with different relative water content were prepared, and the above-mentioned experiment was carried out for each sample to experimentally determine the relationship between the relative water content and the penetration length L. In all experiments, the vacuum pressure was set at -50 kPa (-50 kN / m 2 ) and the suction pressure P is 50 kN / m 2 It is set to.

[0066] FIG. 6 is a graph in which the penetration length L of clay estimated by the estimation method of the first embodiment according to the present disclosure is plotted against the penetration length L determined by an experiment. That is, it is a graph showing the relationship between the calculated penetration length L and the actually measured penetration length L.

[0067] In Figure 6, the horizontal axis represents the relative water content and the vertical axis represents the penetration length L (cm). The solid line represents the calculated penetration length L of the wide groove Dr1, and the dotted line represents the calculated penetration length L of the narrow groove Dr2.

[0068] In other words, the undrained shear strength USS at each sample point of the relative water content is determined from the relationship between the relative water content and the undrained shear strength USS shown in Figure 4, as explained above, and this line shows the relationship between the penetration length L calculated using the undrained shear strength USS as the adhesion resistance f2 and the relative water content.

[0069] The circular points represent the penetration length L of the thick groove Dr1, which was measured using four clay samples with different relative water content ratios, and the triangular points represent the penetration length L of the thin groove Dr2, which was measured and plotted on the graph.

[0070] As can be seen from the graph in Figure 6, there is a slight deviation in areas with low relative water content, i.e., areas where it is difficult for clayey soil to penetrate into the vertical drain 1, but in areas with high relative water content where the clayey soil that is actually causing problems can easily penetrate into the vertical drain 1, it can be seen that the penetration length L can be estimated with good accuracy.

[0071] If the difference between the calculated value and the measured value becomes large, the coefficient α explained above can be adjusted to reduce the difference, and the coefficient α can be determined by collecting experimental data for this purpose. In other words, the coefficient α can be adjusted with a small amount of experimental data so that the calculated value approaches the actually measured value in the graph of FIG.

[0072] <<Second embodiment>> Next, a method for estimating the penetration length L of the clay soil at the tip of the vertical drain 1 according to the second embodiment of the present disclosure will be described with reference to FIG.

[0073] Note that the second embodiment also has some similarities to the first embodiment, and the following mainly describes the differences from the first embodiment, and may omit a description of the similarities to the first embodiment.

[0074] FIG. 7 is a flowchart illustrating the procedure of a method for estimating the penetration length L of the clay soil at the tip of the vertical drain 1 according to the second embodiment of the present disclosure, and corresponds to FIG.

[0075] In FIG. 7, the same step numbers are used for steps that are the same as those explained in FIG. Therefore, in FIG. 7, explanations of steps having the same step numbers as those in FIG. 3 may be omitted.

[0076] Steps S1 to S4 described with reference to FIG. 3 are steps performed to determine the undrained shear strength USS.

[0077] However, undrained shear strength USS can be determined using a test method that conforms to JIS A 1216:2020 "Unconfined compression test method for soil."

[0078] In addition, undrained shear strength USS can also be determined using a test method that conforms to JGS 0521-0524:2020 "Triaxial compression test method for soil."

[0079] (S4') For this reason, in the estimation method of the second embodiment, as shown in Figure 7, instead of S1 to S4 (see Figure 3) in the first embodiment, the undrained shear strength USS of the clayey soil is determined in S4'.

[0080] Specifically, in S4', an undrained shear strength determination step is carried out to determine the undrained shear strength USS using a test method that complies with JIS A 1216:2020 "Uniaxial compression test method for soil" or JGS 0521-0524:2020 "Triaxial compression test method for soil", and the undrained shear strength USS of the clayey soil is determined.

[0081] In this way, the estimation method of the second embodiment omits the water content determining step, the liquid plastic limit determining step, and the relative water content determining step of the estimation method of the first embodiment.

[0082] Furthermore, in the second embodiment, instead of the "undrained shear strength determination step in which the undrained shear strength USS is determined based on the relationship between the relative water content and the undrained shear strength USS" of the first embodiment, an "undrained shear strength determination step in which the undrained shear strength USS is determined using a test method that conforms to JIS A 1216:2020 "Uniaxial compression test method for soil" or JGS 0521-0524:2020 "Triaxial compression test method for soil" is provided.

[0083] As shown in FIG. 7, the estimation method of the second embodiment includes an adhesion resistance determining step, a suction force setting step, and an intrusion length calculating step, similar to the estimation method of the first embodiment.

[0084] Even with the estimation method of the second embodiment as described above, it is possible to obtain the penetration length L similar to that of the estimation method of the first embodiment.

[0085] Although the present disclosure has been described above based on specific embodiments, it should be understood that the present disclosure is not limited to the above embodiments.

[0086] The present disclosure also encompasses modifications and improvements to the embodiments within its technical scope, which will be apparent to those skilled in the art from the claims. [Explanation of symbols]

[0087] 1···Vertical drain, 11···Core body, 11A···Rod-shaped part, 11B···Connection part, 12···Filter, 2···Container, 3···Cap, 4···Vacuum pump, CS···Cross section, Dr···Drainage groove, Dr1···Thick groove, Dr2···Narrow groove, F1···Suction force, F2···Resistance force, f2···Adhesion resistance, H, h···Height, L···Penetration length, Ld···Circumference, USS···Undrained shear strength, W···Width, α···Coefficient

Claims

1. A method for estimating the penetration length of clay soil at the tip of a vertical drain, comprising: The estimation method includes: The estimation method includes a penetration length calculation step of determining the penetration length at which the suction force that draws the clayey soil into the vertical drain and the resistance force that resists the suction force, which changes depending on the penetration length, are balanced.

2. The resistance force is the penetration length; Adhesion resistance of the clay soil in the vertical drain; and the perimeter of the drainage groove of the vertical drain, The estimation method includes: An adhesion resistance determination step of determining the adhesion resistance based on the undrained shear strength of the clayey soil, The estimation method according to claim 1 , wherein the penetration length calculation step is a step of obtaining the penetration length by dividing the suction force by the product of the adhesion resistance and the perimeter of the drain groove.

3. The estimation method according to claim 2 , further comprising an undrained shear strength determination step of determining the undrained shear strength based on a relationship between the relative water content and the undrained shear strength.

4. The estimation method includes: A moisture content determination step of determining the moisture content of the clayey soil by a test method in accordance with JIS A 1203:2020 "Test method for moisture content of soil"; A liquid plastic limit determination step for determining the liquid limit and plastic limit of the clayey soil by a test method conforming to JIS A 1205:2020 "Soil Liquid Limit and Plastic Limit Test Method"; The estimation method according to claim 3 , further comprising: a relative water content determining step of determining the relative water content based on the water content, the liquid limit, and the plastic limit.

5. The estimation method comprises a test method conforming to JIS A 1216:2020 “Uniaxial compression test method for soil” or JGS 0521-0524:2020 “Triaxial compression test method for soil”. The estimation method according to claim 2, further comprising an undrained shear strength determination step for determining the undrained shear strength.

Images