Liquefaction countermeasure structure

Abstract

To provide a liquefaction countermeasure structure which can reduce damage caused by liquefaction, and which can be adopted even for an existing bank without greatly modifying the existing bank.SOLUTION: A permeable banking structure includes an impervious wall 3 buried in a ground 11 around a toe 22 of a slope of an existing embankment 2 (embankment) in parallel with the existing embankment 2 from a non-liquefaction layer 12 to a liquefaction layer 13 in a plan view from a vertical direction, a first permeable drain 4 buried in the ground 11 between the existing embankment 2 and the impervious wall 3 from the non-liquefaction layer 12 to the liquefaction layer 13, and a permeable banking 5 integrally provided on the ground 11 in which the first permeable drain 4 is buried and on the toe 22 of the slope of the existing embankment 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a liquefaction prevention structure. [Background technology]

[0002] If the ground beneath a riverbank levee liquefies during an earthquake, the levee may move significantly toward the toe of the slope, causing large cracks and subsidence, significantly reducing the levee's performance. If the river water level rises due to heavy rain or other factors while the levee is damaged, it could break and cause significant damage to the surrounding area. However, the length and area of ​​the levee are very large, and implementing measures to prevent liquefaction would require enormous costs and time. Furthermore, improving the ground directly below an existing levee could damage the levee to some extent, and it would be difficult to place heavy objects such as large heavy machinery on top of the levee.

[0003] It is known that after liquefaction, a water film forms on the top of the liquefied layer as the water pressure dissipates. When the ground liquefies, the water pressure of the water film is equal to the overburden pressure of the soil at that point, so below a levee with a slope, the water pressure is high in the center of the width direction where the height is high and the overburden pressure is high, and low at the foot of the slope where the overburden pressure is low. Because a difference in water pressure occurs across the plane, the water in the water film flows from the center of the width direction toward the foot of the slope, increasing the water pressure at the foot of the slope. The increased water pressure can cause sand boils at the foot of the slope or cause the ground, including the levee, to flow sideways, which could cause major damage to the levee.

[0004] For example, Patent Documents 1 and 2 disclose liquefaction countermeasure structures that cause minimal damage to existing levees. The liquefaction countermeasure structure disclosed in Patent Document 1 installs a crushed stone mat and permeable liquefaction-preventing sheet piles at the foot of the embankment (bankment). The liquefaction countermeasure structure disclosed in Patent Document 2 uses curved boring to improve the liquefaction layer from a position away from the embankment (bankment). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 2803409 [Patent Document 2] Japanese Patent Application Publication No. 2019-112836 Summary of the Invention [Problem to be solved by the invention]

[0006] The liquefaction prevention structure disclosed in Patent Document 1 requires the liquefaction prevention sheet piles to penetrate the liquefaction layer by a distance equal to the thickness of the liquefaction layer plus a specified length, which is costly. Furthermore, because the liquefaction prevention sheet piles are permeable horizontally, if the water pressure in the water film cannot be fully controlled, there is a risk that the water pressure will propagate from the foot of the slope to the adjacent ground. Furthermore, when applied to an existing levee, excavation using the jacking method is required to install tie rods to connect the liquefaction prevention sheet piles on both sides of the levee, which may damage the levee.

[0007] The liquefaction prevention structure disclosed in Patent Document 2 has the problem that curved boring is performed, which makes the cost very high and requires a large working space on the side of the embankment.

[0008] Therefore, an object of the present invention is to provide a liquefaction prevention structure that can reduce damage caused by liquefaction and can be adopted for existing levees without significantly modifying the existing levees. [Means for solving the problem]

[0009] In order to achieve the above-mentioned objective, the liquefaction prevention structure of the present invention comprises a watertight wall buried in the ground around the toe of the embankment, parallel to the embankment in a plan view from above and below, extending from the non-liquefaction layer to the liquefaction layer, a first permeable drain buried in the ground between the embankment and the watertight wall, extending from the non-liquefaction layer to the liquefaction layer, and a permeable embankment laid over the ground in which the first permeable drain is buried and over the toe of the embankment.

[0010] In this invention, when an earthquake occurs and water rises in the liquefied layer, the water in the water film formed between the liquefied layer and the non-liquefied layer is discharged to the ground at the side of the levee through the first permeable drain and the permeable embankment. Because the impermeable wall is buried from the non-liquefied layer to the liquefied layer, the water in the water film is prevented from flowing between one side of the impermeable wall and the other. This allows the water below the levee to be quickly discharged to the ground. As a result, the increase in water pressure outside the impermeable wall is almost eliminated, preventing lateral ground movement and reducing damage caused by liquefaction. When applied to an existing levee, the levee itself is not altered, so liquefaction damage during an earthquake can be significantly reduced while maintaining its functionality. Since the impermeable wall only needs to restrain the flow of the water film that forms on the top surface of the liquefaction layer, it does not need to be installed all the way to the bottom of the liquefaction layer, and it is sufficient to build it from the ground surface to about 2 to 3 meters below the top of the liquefaction layer. Therefore, the liquefaction countermeasure structure of this invention can reduce costs compared to liquefaction countermeasures that are implemented all the way to the bottom of the liquefaction layer.

[0011] The liquefaction countermeasure structure of the present invention may have a second permeable drain buried in the ground on the side opposite to the side where the first permeable drain of the watertight wall is buried, extending from the non-liquefaction layer to the liquefaction layer, and the permeable embankment may be integrally provided over the ground where the second permeable drain is buried, over the ground where the first permeable drain is buried, and over the toe of the embankment.

[0012] This configuration allows the water in the water film that forms on the top surface of the liquefaction layer to be drained not only inside the impermeable wall but also outside. Because there is almost no water flow in the water film outside the impermeable wall, water rising from deep within the wall may remain on the underside of the non-liquefaction layer, forming a thick water film. A thick water film reduces friction on the underside of the non-liquefaction layer, allowing the non-liquefaction layer above the water film to easily move. The impermeable wall is subjected to water pressure from the water flowing inside the impermeable wall. If the water film on the outside of the impermeable wall becomes thick and the ground reaction becomes weak, this water pressure may cause the impermeable wall to move outward. By providing permeable drains on the outside of the impermeable wall and draining the water, the formation of a water film on the outside of the impermeable wall can be prevented, ensuring friction on the underside of the non-liquefaction layer outside the impermeable wall, and further suppressing the movement of the impermeable wall. This reduces the amount of movement of the impermeable wall and further reduces damage. [Effects of the Invention]

[0013] According to the present invention, damage caused by liquefaction can be reduced and the present invention can be applied to existing levees without making major modifications to the existing levees. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a vertical cross-sectional view of a liquefaction countermeasure structure according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a vertical cross-sectional view of the liquefaction countermeasure structure of the first embodiment when liquefaction occurs. [Figure 3] FIG. 4 is a vertical cross-sectional view of a liquefaction countermeasure structure according to a second embodiment of the present invention. [Figure 4] FIG. 10 is a vertical cross-sectional view of the liquefaction countermeasure structure of the second embodiment when liquefaction occurs. DETAILED DESCRIPTION OF THE INVENTION

[0015] (First embodiment) Hereinafter, a liquefaction countermeasure structure according to an embodiment of the present invention will be described with reference to FIGS. As shown in Figure 1, the liquefaction countermeasure structure 1 according to the first embodiment is used for an existing levee 2 built along a riverbank. The existing levee 2 is built at a predetermined distance from the river. The existing levee 2 is constructed using embankments, concrete structures, etc. The ground 11 directly below the existing levee 2 is a non-liquefaction layer 12. Below the non-liquefaction layer 12 is a liquefaction layer 13. The horizontal direction in which the existing levee 2 extends, i.e., the direction in which the river flows in a plan view, is referred to as the length direction. The horizontal direction perpendicular to the length direction is referred to as the width direction. The vertical cross-sectional shape of the existing levee 2 perpendicular to the length direction is approximately trapezoidal. The existing levee 2 has slopes 21 formed on both sides in the width direction. The portions of the existing levee 2 where the slopes 21 are formed at both ends in the width direction are referred to as the toes of the slopes 22, 22. The middle part of the existing levee 2 in the width direction, between the toe parts 22, 22, is referred to as the middle part 23.

[0016] The liquefaction countermeasure structure 1 has a water impermeable wall 3, a first permeable drain 4, and permeable embankment 5. The water impermeable wall 3 is buried in the ground 11 around the toe 22 of the existing levee 2, parallel to the existing levee 2 in a plan view viewed from the top and bottom, and extending from the non-liquefaction layer 12 to the liquefaction layer 13. The first permeable drain 4 is buried in the ground 11 between the existing levee 2 and the water impermeable wall 3, extending from the non-liquefaction layer 12 to the liquefaction layer 13. The permeable embankment 5 is provided on top of the ground 11 in which the first permeable drain 4 is buried, and over the toe 22 of the existing levee 2.

[0017] The impermeable wall 3 is located several meters away from the toe 22 of the existing levee 2. The impermeable wall 3 extends longitudinally along the existing levee 2. The impermeable wall 3 is a cement-based ground improvement body or a steel sheet pile, for example. The impermeable wall 3 has water-impermeable properties. The lower end 31 of the impermeable wall 3 is embedded about 2 to 3 m into the liquefaction layer 13. In other words, the lower end 31 of the impermeable wall 3 is located about 2 to 3 m below the upper end 131 of the liquefaction layer 13. The upper end 131 of the liquefaction layer 13 corresponds to the groundwater level or the lower end of a clayey soil layer near the ground surface, for example. The upper end 32 of the impermeable wall 3 is located at approximately the same height as the ground surface 111.

[0018] The first permeable drain 4 is a drain formed by filling a columnar hole in the ground 11 with a material such as crushed stone. The material used for the first permeable drain 4 is a material with a particle size that can form gaps that do not clog during liquefaction, such as No. 7 crushed stone. The first permeable drain 4 is permeable. The lower end 41 of the first permeable drain 4 is embedded approximately 1 m into the liquefaction layer 13. The impermeable wall 3 is embedded deeper into the liquefaction layer 13 than the first permeable drain 4. The upper end 42 of the first permeable drain 4 is located below the upper end 32 of the impermeable wall 3. A crushed stone mat 43 is located above the first permeable drain 4, spanning the entire area between the existing levee 2 and the impermeable wall. The crushed stone mat 43 is constructed by paving crushed stone. The crushed stone mat 43 is permeable. The upper surface of the crushed stone mat 43 is located at approximately the same height as the ground surface 111.

[0019] The permeable embankment 5 is an embankment formed by laying, for example, crushed stone on top of the crushed stone mat 43 and the toe 22 of the existing levee 2. The permeable embankment 5 is permeable. The material used for the permeable embankment is shaped and configured to withstand water pressure during liquefaction. Permeable embankments may be made of a box-shaped wire mesh basket filled with crushed stone. A slope 51 is formed on the outer side of the width of the permeable embankment 5, i.e., on the top surface away from the existing levee 2. The slope 51 is located above the first permeable drain 4 and does not overlap the existing levee 2 in the vertical direction. The outer width portion of the permeable embankment 5 where the slope 51 is formed is referred to as the toe 52.

[0020] The first permeable drain 4 and the crushed stone mat 43 are provided continuously. The crushed stone mat 43 and the permeable embankment 5 are provided continuously. Water can pass between the first permeable drain 4 and the crushed stone mat 43, and between the crushed stone mat 43 and the permeable embankment 5.

[0021] The operation and effect of the liquefaction countermeasure structure 1 according to the first embodiment will be described. As shown in Figure 2, when an earthquake occurs and the water in the liquefied layer 13 rises, a water film 14 is formed above the upper end 131 of the liquefied layer 13, i.e., between the liquefied layer 13 and the non-liquefied layer 12. The water film 14 is located at the midpoint of the first permeable drain 4 in the height direction. The water in the water film 14 flows into the first permeable drain 4. The water that flows into the first permeable drain 4 is discharged to the ground through the first permeable drain 4, the crushed stone mat 43, and the permeable embankment 5. In Figure 2, the flow of water is indicated by arrows. The water film 14 is located at the midpoint in the height direction of the impermeable wall 3. This blocks the flow of water between one side of the impermeable wall 3 and the other side of the water film 14. This prevents water on the outside of the water film 14, i.e., the side of the water film 14 that is away from the existing levee 2 relative to the impermeable wall 3, from entering the inside of the water film 14, i.e., the side of the water film 14 where the existing levee 2 is located relative to the impermeable wall 3. This allows water below the existing levee 2 to be quickly drained to the ground. As a result, the increase in water pressure outside the impermeable wall 3 can be almost eliminated, preventing lateral movement of the ground 11 and reducing damage due to liquefaction. The liquefaction countermeasure structure 1 according to the first embodiment can be adopted for an existing levee 2 without modifying the existing levee 2. Therefore, the liquefaction countermeasure structure 1 according to the first embodiment can reduce damage caused by liquefaction during an earthquake while maintaining the function of the existing levee 2 as a levee.

[0022] The water impermeable wall 3 only needs to restrain the flow of the water film that forms on the upper surface of the liquefaction layer 13, so it does not need to be installed all the way to the bottom of the liquefaction layer 13, and it is sufficient to build it from the ground surface 111 to about 2 to 3 meters below the top end 131 of the liquefaction layer 13. Therefore, the liquefaction resistant structure 1 according to the first embodiment can reduce costs compared to when liquefaction countermeasures are implemented all the way to the bottom end of the liquefaction layer 13.

[0023] (Second embodiment) Next, a second embodiment will be described. The same or similar members and parts as those in the first embodiment will be denoted by the same reference numerals, and the description will be omitted. Only the configurations different from the first embodiment will be described. As shown in Figure 3, the liquefaction countermeasure structure 1B according to the second embodiment is the liquefaction countermeasure structure 1 according to the first embodiment, with a second permeable drain 6 provided. The second permeable drain 6 is buried in the ground outside the impermeable wall 3, extending from the non-liquefaction layer to the liquefaction layer. Permeable embankment 5B is provided over the ground in which the second permeable drain 6 is buried, over the ground 11 in which the first permeable drain 4 is buried, and over the toe 22 of the existing levee 2.

[0024] The second permeable drain 6 has the same configuration as the first permeable drain 4, but is installed in a different position than the first permeable drain 4. A crushed stone mat 63 of the first permeable drain 4 is provided on top of the second permeable drain 6. The crushed stone mat 63 has the same configuration as the crushed stone mat 43. The crushed stone mat 63 is provided outside the impermeable wall 3, i.e., in a predetermined range on the side of the impermeable wall 3 away from the existing levee 2.

[0025] The permeable embankment 5B is a permeable embankment in which, for example, crushed stone is laid on top of the crushed stone mats 43, 63, the impermeable wall 3, and the toe 22 of the existing levee 2. The same materials as those used for the permeable embankment 5 of the first embodiment are used for the permeable embankment 5B. A slope 51B is formed on the outer upper surface of the permeable embankment 5B in the width direction. The slope 51B is located above the first permeable drain 4, the second permeable drain 6, and the impermeable wall 3, and is located in a position that does not overlap with the existing levee 2 in the vertical direction. The portion of the permeable embankment 5B on the outer side in the width direction where the slope 51B is formed is referred to as the toe 52B.

[0026] The first permeable drain 4 and the crushed stone mat 43 are provided continuously. The crushed stone mat 43 and the permeable embankment 5B are provided continuously. The second permeable drain 6 and the crushed stone mat 63 are provided continuously. The crushed stone mat 63 and the permeable embankment 5B are provided continuously. Water can pass between the first permeable drain 4 and the crushed stone mat 43, and between the crushed stone mat 43 and the permeable embankment 5B. Water can pass between the second permeable drain 6 and the crushed stone mat 63, and between the crushed stone mat 63 and the permeable embankment 5B.

[0027] As shown in Figure 4, in the liquefaction countermeasure structure 1B according to the second embodiment, similarly to the first embodiment, when an earthquake occurs and water rises in the liquefaction layer 13, the water in the water film 14 formed on the upper end 131 of the liquefaction layer 13 below the existing levee 2 and inside the impermeable wall 3 is discharged to the ground through the first permeable drain 4, the crushed stone mat 43, and the permeable embankment 5B. Therefore, the liquefaction countermeasure structure 1B according to the second embodiment achieves the same effects as the first embodiment. In the liquefaction prevention structure 1B according to the second embodiment, just outside the water barrier 3, the water in the water film 15 outside the water barrier 3 can be discharged to the ground through the second permeable drain 6, the crushed stone mat 63 and the permeable embankment 5B.

[0028] In the liquefaction countermeasure structure 1 according to the first embodiment, water pressure accumulates at the toe 52 of the permeable embankment 5. Because the overburden pressure is low, the water pressure increases due to the inflow of water, potentially damaging the permeable embankment 5. Furthermore, if the water film 15 on the outside of the impermeable wall 3 becomes thicker, its effectiveness in preventing the movement of the impermeable wall 3 decreases. Increased water pressure inside the impermeable wall 3 may cause the impermeable wall 3 to move outward. In the liquefaction countermeasure structure 1B according to the second embodiment, permeable drains 4, 6 and crushed stone mats 43, 63 are provided on both sides of the impermeable wall 3, and the permeable embankment 5B is placed on top of them. This reduces damage to the toe 52B of the permeable embankment 5B. Furthermore, because water is also drained outside the impermeable wall 3, a ground reaction force can be expected on the outside of the impermeable wall 3, minimizing the movement of the impermeable wall 3. In Figure 4, arrow A indicates the direction in which the ground reaction force acts on the outside of the impermeable wall 3. In the liquefaction countermeasure structure 1 according to the first embodiment, the permeable embankment 5 is not the main body of the embankment, so even if it is damaged, the embankment function is maintained.

[0029] Although the embodiments of the liquefaction prevention structure according to the present invention have been described above, the present invention is not limited to the above-described embodiments and can be modified as appropriate within the scope of the invention. For example, in the above-described embodiment, the liquefaction countermeasure structures 1, 1B are applied to the existing embankment 2, but they may also be applied to a newly constructed embankment. The embankment of the liquefaction countermeasure structures 1, 1B may be an embankment built on a coast. In the above embodiment, crushed stone mats 43, 63 are provided on the permeable drains 4, 6, and permeable embankments 5, 5B are provided on top of that, but the permeable embankments 5, 5B may be provided on the permeable drains 4, 6 without providing the crushed stone mats 43, 63.

[0030] The Sustainable Development Goals (SDGs) are 17 international goals adopted at the United Nations Summit in September 2015. The liquefaction prevention structure of this embodiment can contribute to achieving one of the 17 SDGs, such as goal 9, "Create indispensable infrastructure for industry, innovation and sustainable development." [Explanation of symbols]

[0031] 1,1B Liquefaction prevention structure 2 Existing levee (levee) 3. Water-impermeable walls 4. First permeable drain 5,5B Permeable embankment 6. Second permeable drain 11 Ground 12 Non-liquefiable layer 13 Liquefied layer 14 Water film 21 Slope 22 Butt part 51,51B Slope 52,52B Butt part

Claims

1. a watertight wall embedded in the ground around the toe of the embankment, parallel to the embankment in a plan view from above and below, and extending from the non-liquefaction layer to the liquefaction layer; a first permeable drain buried in the ground between the embankment and the impermeable wall, spanning from the non-liquefaction layer to the liquefaction layer; A liquefaction prevention structure comprising: a permeable embankment integrally provided over the ground in which the first permeable drain is buried and over the toe of the embankment.

2. a second permeable drain buried in the ground on the side of the impermeable wall opposite to the side where the first permeable drain is buried, spanning from the non-liquefaction layer to the liquefaction layer; 2. The liquefaction prevention structure described in claim 1, wherein the permeable embankment is integrally provided over the ground in which the second permeable drain is buried, over the ground in which the first permeable drain is buried, and over the toe of the embankment.

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