Liquefaction countermeasure structure and liquefaction countermeasure method for structure

Abstract

The liquefaction reduction effect can be improved.SOLUTION: There are provided a lattice-shaped improvement body 20 which reaches a non-liquefaction layer G1 below a liquefaction layer G2 and is installed on a lower foundation of a new structure 2a, a gravel drain 30 which is installed on a surface layer G2 which does not reach the non-liquefaction layer G0 in a wall surrounded by the lattice-shaped improvement body 20 and has a water permeability coefficient larger than that of the lattice-shaped improvement body 20, and a crushed stone improvement board 40 which is installed over the entire surface of an installation portion 2a of the new structure 2a.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a liquefaction countermeasure structure and a liquefaction countermeasure construction method for a structure. [Background technology]

[0002] Conventionally, lattice improvement has been adopted as a construction method to prevent liquefaction in soft ground that may liquefy during an earthquake (see, for example, Patent Document 1). Lattice improvement bodies have the effect of preventing liquefaction by restraining the shear deformation of the ground that occurs during an earthquake. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-12977 Summary of the Invention [Problem to be solved by the invention]

[0004] However, with conventional liquefaction prevention measures using grid-like improvement, in extremely soft and susceptible ground to liquefaction, a very high improvement rate may be required to achieve liquefaction prevention effects, which increases construction costs and lengthens construction time.

[0005] Furthermore, when constructing liquefaction countermeasures using grid-like improvement, if the new structure is built on a spread foundation, the weight of the structure is transferred to the ground, generating confining pressure in the surface ground, so the liquefaction resistance does not become extremely low, and as a result, liquefaction is prevented by grid-like improvement. However, if the structure is very light (for example, a single-story steel-framed structure), if the load of the structure is borne by the grid-like improvement body and not transmitted to the ground, or if the pile foundation does not generate ground pressure at the base, the confining pressure in the surface ground may not increase sufficiently, and liquefaction may occur.

[0006] Furthermore, the ground inside the grid is watertight around the periphery due to the improvement body, which prevents horizontal pore water movement. Therefore, if liquefaction occurs, it may take a long time for the water pressure to dissipate. In particular, in the case of a large earthquake where a second earthquake occurs within a few tens of minutes to a few hours, there is a possibility that liquefaction will occur again with the second earthquake. As such, conventional liquefaction prevention measures using lattice-type improvement cannot reliably prevent liquefaction in surface ground using lattice-type improvement bodies alone, so there is a need to improve the liquefaction mitigation effect, and there is room for improvement in this regard.

[0007] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a liquefaction countermeasure structure and a liquefaction countermeasure construction method for a structure that can improve the liquefaction mitigation effect. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the liquefaction countermeasure structure of the present invention is a liquefaction countermeasure structure for reducing damage caused by liquefaction to structures constructed on liquefied ground, and is characterized by comprising a lattice-shaped improvement body that reaches the non-liquefied layer below the liquefied layer and is installed in the ground below the structure, and a gravel drain that is installed in the surface portion within the wall surrounded by the lattice-shaped improvement body and does not reach the non-liquefied layer, and has a higher hydraulic conductivity than the lattice-shaped improvement body.

[0009] In addition, the liquefaction countermeasure method for structures of the present invention is a liquefaction countermeasure method for structures for reducing damage caused by liquefaction to structures constructed on liquefied ground, and is characterized by having the steps of installing a lattice-shaped improvement body in the ground below the structure so that it reaches the non-liquefied layer below the liquefied layer, and installing a gravel drain with a higher permeability coefficient than the lattice-shaped improvement body in the surface portion within the wall surrounded by the lattice-shaped improvement body, which does not reach the non-liquefied layer.

[0010] In this invention, in addition to the installation of lattice-shaped improvement bodies, a drainage effect is added by gravel drains that are shorter than the lattice-shaped improvement bodies, limited to the surface portion of the liquefied layer. In this way, by installing short gravel drains in addition to the lattice-shaped improvement bodies in the surface portion of the liquefied layer where water pressure is likely to increase, it is possible to quickly dissipate the water pressure that increases during an earthquake and prevent liquefaction. Therefore, it is possible to effectively reduce liquefaction of the surface ground surrounded by the lattice-shaped improvement bodies while maintaining the conventional improvement rate, and it is also possible to effectively reduce re-liquefaction due to a second earthquake. Furthermore, since the present invention can achieve a similar effect to some extent in ground deeper than the bottom end of the gravel drain, the cost of the gravel drain can be reduced by shortening the length of the gravel drain so that it does not reach the bottom end of the liquefied layer.

[0011] Furthermore, in the liquefaction prevention structure of the present invention, the structure is a newly constructed structure, a crushed stone improvement plate is installed over the entire installation area of ​​the newly constructed structure, the lattice-shaped improvement body is connected to the crushed stone improvement plate and supports the newly constructed structure from below via the crushed stone improvement plate, and the gravel drain preferably extends vertically downward from the crushed stone improvement plate.

[0012] In this case, since the target is a new structure, liquefaction countermeasures can be constructed in advance of the new structure's construction. Therefore, a grid-like improvement body and gravel drain can be installed in the liquefied layer below the new structure's installation area, and a crushed stone improvement plate connected to the top of the installed grid-like improvement body and gravel drain is installed on the surface of the liquefied layer. In the event of liquefaction, excess pore water that rises through the gravel drain can be efficiently drained to the ground surface outside the new structure via the crushed stone improvement plate. Therefore, a grid-like improvement body with multiple compartments can be installed in the liquefied layer below the new structure, and gravel drains can be installed in each compartment. This allows for the rapid dissipation of water pressure that builds up during an earthquake, providing excellent liquefaction countermeasures.

[0013] In addition, in the liquefaction countermeasure structure according to the present invention, it is preferable that the crushed stone improvement plate protrudes outward from the outer peripheral portion of the lattice-shaped improvement body.

[0014] In this case, when liquefaction occurs, excess pore water rising through the gravel drain can be reliably drained to the ground surface outside the newly constructed structure via the crushed stone improvement plate.

[0015] Furthermore, in the liquefaction prevention structure of the present invention, it is preferable that the structure is an existing structure, the lattice-shaped improvement body is installed outside the existing structure, and the gravel drain forms an inclined drain that extends from the outside to the inside of the existing structure as it extends downward.

[0016] In this case, since the target is an existing structure, the liquefaction countermeasure structure can be constructed from the outside of the existing structure. In this invention, the inclined drain can be installed by constructing the gravel drain from the outside to the inside of the existing structure, resulting in a structure in which the gravel drain is installed in the liquefied layer surrounded by the lattice-shaped improvement body below the existing structure. Then, when liquefaction occurs, excess pore water that rises through the inclined drain can be efficiently drained to the ground surface outside the existing structure. Therefore, the water pressure that rises during an earthquake can be quickly dissipated, providing an excellent liquefaction countermeasure. [Effects of the Invention]

[0017] According to the liquefaction countermeasure structure and liquefaction countermeasure construction method for a structure of the present invention, the liquefaction mitigation effect can be improved. [Brief explanation of the drawings]

[0018] [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. 4 is a vertical cross-sectional view of a liquefaction countermeasure structure according to a second embodiment of the present invention. [Figure 3] FIG. 2 is a plan view of an experimental model according to an embodiment, viewed from above. [Figure 4]FIG. 4 is a cross-sectional view taken along line II shown in FIG. 3, showing a longitudinal cross-sectional view of the experimental model. [Figure 5] 10 shows a time history of input acceleration according to an embodiment. [Figure 6] FIG. 10 is a diagram showing the time history of the excess pore water pressure ratio during vibration (time history of the excess pore water pressure ratio) observed in an experiment according to the embodiment. [Figure 7] FIG. 10 is a diagram showing the time history of the excess pore water pressure ratio during water pressure dissipation after excitation (time history of excess pore water pressure ratio) observed in an experiment according to an embodiment. [Figure 8] FIG. 1 is a schematic diagram of preventing re-liquefaction using a liquefaction countermeasure method according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, a liquefaction countermeasure structure and a liquefaction countermeasure construction method for a structure according to an embodiment of the present invention will be described with reference to the drawings.

[0020] (First embodiment) As shown in Figure 1, the liquefaction countermeasure structure 1 and liquefaction countermeasure construction method for a structure 2 according to the first embodiment are employed to reduce damage caused by liquefaction to a structure 2 constructed on liquefiable ground. The structure 2 according to the first embodiment is intended for a newly constructed structure 2A.

[0021] The liquefaction prevention structure 1 comprises a lattice-shaped improvement body 20 that reaches the non-liquefaction layer G2 below the liquefaction layer G1 and is installed in the ground below the new structure 2A, a gravel drain 30 that is installed in the surface portion G0 within the wall surrounded by the lattice-shaped improvement body 20 and does not reach the non-liquefaction layer G2 and has a higher permeability coefficient than the lattice-shaped improvement body 20, and a crushed stone improvement board 40 that is installed over the entire surface of the installation portion 2a of the new structure 2A.

[0022] The upper end 20a of the lattice-shaped improvement body 20 is connected to the lower surface 40a of the crushed stone improvement board 40. The lattice-shaped improvement body 20 supports the new structure 2A from below via the crushed stone improvement board 40. The lattice-shaped improvement body 20 forms multiple compartment areas 21 with a rectangular (square) cross section using improvement walls arranged vertically and horizontally to form a rectangular tube in the liquefaction layer G1.

[0023] The grid-like improved body 20 is constructed by a known construction method, for example, by mixing and stirring cement or cement-based solidification material with the original ground to form a wall. The thickness of the improved wall, the number of compartment areas 21 formed by the lattice-shaped improved body 20, the size of the cross-sectional shape of one compartment area, etc. can be set appropriately depending on the ground conditions, structure conditions, etc.

[0024] The gravel drain 30 is connected to the crushed stone improvement board 40 and extends vertically downward from the crushed stone improvement board 40. The upper end 30a of the gravel drain 30 is connected to the lower surface 40a of the crushed stone improvement board 40. At least one gravel drain 30 is arranged in each compartment of the partitioned lattice-shaped improvement body 20. As mentioned above, the depth of the gravel drain 30 is shallower than that of the lattice-shaped improvement body 20. The gravel drain 30 is made of crushed stone that has the same permeability as the crushed stone improvement board 40.

[0025] By providing multiple gravel drains 30, excess pore water generated in the liquefied ground within the lattice-shaped improved body 20 partitioned by an earthquake can be collected, and the water can be pumped up from the gravel drains 30 to the ground surface through the crushed stone improvement plate 40, and then drained to the ground through the crushed stone improvement plate 40.

[0026] The crushed stone improvement board 40 protrudes outward from the outer periphery of the lattice-shaped improvement body 20. In other words, the crushed stone improvement board 40 is slightly larger than the new structure 2A in plan view and has a rectangular shape in plan view. The crushed stone improvement board 40 is formed by laying crushed stone in the installation area of ​​the new structure 2A. The crushed stone improvement board 40 is a crushed stone layer with drainage function and a higher permeability coefficient than the liquefied ground and the lattice-shaped improvement body 20.

[0027] The crushed stone improvement board 40 is constructed by backfilling, for example, with natural crushed stone, an area that has been excavated to a predetermined depth over a large area corresponding to the installation area of ​​the new structure 2A after the construction of the lattice-shaped improvement body 20 and the gravel drain 30. Note that various artificial permeable materials can also be used as crushed stone as long as they provide equivalent permeability. The crushed stone improvement board 40 is installed on the surface layer G0 of the liquefaction layer G1, and has the function of efficiently draining excess pore water to the surface when liquefaction occurs in the liquefied ground below the new structure 2A, thereby preventing sand boiling onto the surface.

[0028] A construction method for the above-mentioned liquefaction countermeasure structure 1 will be described with reference to FIG. In other words, as a liquefaction countermeasure method for reducing damage caused by liquefaction to a new structure 2A to be constructed on liquefied ground G, first, a lattice-shaped improvement body 20 is installed in the liquefied layer G1 at the construction location of the new structure 2A so that the improvement wall is arranged in a lattice pattern and reaches the non-liquefied layer G2 below the liquefied layer G1.

[0029] Next, in the surface layer G0 within the wall surrounded by the lattice-shaped improvement body 20, which does not reach the non-liquefaction layer G2, multiple gravel drains 30 with a higher hydraulic conductivity than the lattice-shaped improvement body 20 are installed. Furthermore, in the surface layer G0, a crushed stone improvement board 40 is constructed so as to connect to the upper ends of the lattice-shaped improvement body 20 and multiple gravel drains 30 installed in the liquefaction layer G1. In this way, the liquefaction countermeasure structure 1 can be constructed. In this embodiment, a new structure 2A is constructed on top of the liquefaction countermeasure structure 1 that has been constructed in advance.

[0030] Next, the operation of the liquefaction countermeasure structure 1 and the liquefaction countermeasure construction method for a structure described above will be described in detail with reference to FIG. The liquefaction countermeasure structure 1 according to this embodiment is a structure for reducing damage caused by liquefaction to a structure 2 constructed on liquefied ground. The liquefaction countermeasure structure 1 comprises a lattice-shaped improvement body 20 that reaches the non-liquefied layer G2 below the liquefied layer G1 and is installed in the ground below the new structure 2A, and a gravel drain 30 that is installed in the surface layer G0 within the wall surrounded by the lattice-shaped improvement body 20 but does not reach the non-liquefied layer G2 and has a higher hydraulic conductivity than the lattice-shaped improvement body 20.

[0031] In this embodiment, in addition to the installation of the lattice-shaped improvement bodies 20, the drainage effect of gravel drains 30, which are shorter than the lattice-shaped improvement bodies 20, is added only to the surface portion G0 of the liquefaction layer G1. In this way, by installing short gravel drains 30 in combination with the lattice-shaped improvement bodies 20 in the surface portion G0 of the liquefaction layer G1, where water pressure is likely to increase, it is possible to quickly dissipate the water pressure that increases during an earthquake and mitigate liquefaction. Therefore, it is possible to effectively prevent liquefaction of the surface ground surrounded by the lattice-shaped improvement bodies 20 while maintaining the conventional improvement rate, and it is also possible to effectively mitigate re-liquefaction due to a second earthquake. In addition, in this embodiment, since the same effect can be achieved to some extent in ground deeper than the lower end of the gravel drain 30, the cost of the gravel drain 30 can be reduced by shortening the length of the gravel drain 30 so that it does not reach the lower end of the liquefaction layer.

[0032] Furthermore, since this embodiment targets a new structure 2A, the liquefaction countermeasure structure 1 can be constructed in advance before the construction of the new structure 2A. Therefore, a lattice-shaped improvement body 20 and a gravel drain 30 can be installed in the liquefaction layer G1 below the installation portion of the new structure 2A. Furthermore, a crushed stone improvement plate 40 connected to the top of the installed lattice-shaped improvement body 20 and gravel drain 30 is installed in the surface portion G0 of the liquefaction layer G1. In the event of liquefaction, excess pore water rising through the gravel drain 30 can be efficiently drained to the ground surface outside the new structure via the crushed stone improvement plate 40. Therefore, the lattice-shaped improvement body 20, which forms multiple compartment areas 21 in the liquefaction layer below the new structure 2A, and gravel drains 30 can be placed in each compartment area 21. This allows for the rapid dissipation of water pressure that rises during an earthquake, providing excellent liquefaction countermeasures.

[0033] In addition, in this embodiment, the crushed stone improvement board 40 extends outward from the outer periphery of the lattice-shaped improvement body 20, so that when liquefaction occurs, excess pore water rising through the gravel drain 30 can be reliably drained to the ground surface outside the newly constructed structure 2A via the crushed stone improvement board 40.

[0034] As described above, the liquefaction countermeasure structure 1 and the liquefaction countermeasure construction method for the structure 2 according to this embodiment can improve the liquefaction mitigation effect.

[0035] (Second embodiment) Next, we will explain the liquefaction countermeasure structure 1A and liquefaction countermeasure construction method for structures according to the second embodiment, but we will use the same symbols for components and parts that are the same or similar to those in the first embodiment described above, omit their explanation, and only explain the configurations that differ from the first embodiment. As shown in Figure 2, the liquefaction countermeasure structure 1A according to the second embodiment is applied to an existing structure 2B as the target structure 2. Therefore, this is an example of application when liquefaction damage is reduced without requiring work on the original ground (liquefaction layer G1) directly below the existing structure 2B.

[0036] The lattice-shaped improvement body 20A is installed outside the existing structure 2B. The lattice-shaped improvement body 20A has a planar shape and an outer frame shape that is slightly larger than the existing structure 2B. In this case, only one partitioned area 21 is formed in the lattice-shaped improvement body 20A.

[0037] The gravel drain 30A forms an inclined drain that extends downward from the outside to the inside of the existing structure 2B. The installation angle θ of the gravel drain 30A is set in the range of 0 to 45° with respect to the vertical direction.

[0038] In this second embodiment, since the target is an existing structure 2B, the liquefaction countermeasure structure 1A can be constructed from the outside of the existing structure 2B. In this second embodiment, an inclined drain can be installed by constructing the gravel drain 30A from the outside toward the inside of the existing structure 2B, resulting in a structure in which the gravel drain 30A is installed in the liquefied layer surrounded by the lattice-shaped improvement body 20A below the existing structure 2B. In the event of liquefaction, excess pore water rising through the inclined drain (gravel drain 30A) can be efficiently drained to the ground surface outside the existing structure 2B. This allows the water pressure that rises during an earthquake to be quickly dissipated, providing an excellent liquefaction countermeasure.

[0039] Next, examples conducted to verify the effectiveness of the liquefaction countermeasure structure and liquefaction countermeasure construction method for structures according to the above-described embodiment will be described below.

[0040] (Example) In this example, a centrifugal shaking table experiment was carried out using a scaled-down model of a liquefaction countermeasure structure similar to that of the first embodiment described above, and the effects thereof were confirmed. Fig. 3 is a plan view of the experimental model 100 as seen from above. Fig. 4 is a cross-sectional view taken along line II in Fig. 3, which is a longitudinal cross-sectional view of the experimental model 100. The experimental model 100 is a scaled-down model that is 1 / 50 of the actual size, and is subjected to vibration in a centrifugal field of 50 G, thereby achieving the same effect as a full-size experiment.

[0041] The experimental conditions, such as the dimensions of the experimental model 100 (values ​​in parentheses are converted to full-size values), are as follows: The experimental model 100 consists of a non-liquefaction layer 101 with a thickness of 50 mm (2.5 m), a soft liquefaction layer 102 with a thickness of 300 mm (15 m), a grid-like improvement body 103 located within the liquefaction layer 102, and four gravel drains 104 with a length (depth) shorter than the grid-like improvement body 103. In this experiment, no structures were included in the experimental model 100, and conditions were set that made the soil prone to liquefaction. The non-liquefaction layer 101 was made of No. 3 silica sand with a relative density of 80%, and the liquefaction layer 102 was made of No. 7 silica sand with a relative density of 50% mixed with 5% kaolin clay by volume. The dominant frequency of the ground was approximately 2 Hz. Other dimensions of the experimental model 100 are as shown in Figure 3.

[0042] The lattice-shaped improvement body 103 was made to reach the lower end of the liquefaction layer 102. The wall spacing of the lattice-shaped improvement body 103 was a square lattice of 200 mm (10 m) in both directions. The improvement rate of the lattice-shaped improvement body 103 was equivalent to 24.3%. There were two lattices. One of the two lattices was ground treated with only the lattice-shaped improvement body, and the other was ground to which the construction method of the first embodiment described above was applied, using a short gravel drain 104 in combination. In addition, the lattice was installed slightly offset from the center of the soil tank, so that the ground on one side outside the lattice was left untreated.

[0043] The gravel drains 104 were made of silica sand No. 3, which had the same relative density of 80% as the non-liquefaction layer 101, and were 150 mm (7.5 m) long (depth), reaching half the depth of the liquefaction layer 102. The cross section of the gravel drains 104 was a square measuring 22 mm x 22 mm (1.1 m x 1.1 m). The four gravel drains 104 were spaced 100 mm (5 m) apart, in a square arrangement of two drains vertically and two drains horizontally.

[0044] Numerous accelerometers 201, 202 and water pressure gauges 203 were installed within the experimental model 100. Acceleration and water pressure measurements were taken for the ground without any measures, the ground with only grid improvement, and the ground to which this construction method was applied. The accelerometers used were a horizontal accelerometer 201 and a vertical accelerometer 202. Reference numeral 204 denotes a laser displacement meter, and reference numeral 205 denotes a bender element. The positions of the water pressure gauges 203 in the experimental model 100 are indicated by P1 to P6, P11 to P16, P23 to P26, and P34 to P36 in Figures 3 and 4.

[0045] Figure 5 shows the time history of input acceleration. The horizontal axis of Figure 5 is time (s) and the vertical axis is acceleration (m / s 2 ) As shown in Figure 5, the amplitude is 1.2 m / s in full-scale equivalent. 2 A 1.2 Hz sine wave was input to the bottom of the experimental model 100. Then, a 5-second gradual increase and decrease section was added before and after the input wave, and the vibration was applied for a total of 20 seconds, after which the model was left to stand until the excess pore water pressure in each part dissipated.

[0046] Figure 6 shows the time history of the excess pore water pressure ratio during vibration observed in the experiment, i.e., the excess pore water pressure ratio time history (during vibration). Figure 6 shows the results measured by the water pressure gauge 204 placed at each ground level GL. In Figure 6, the solid line indicates the ground without any countermeasures, the dashed line indicates the ground with only the lattice-shaped improvement body 103, and the dotted line indicates the ground to which the liquefaction countermeasure method according to the first embodiment described above was applied. In Figure 6, (a) and (b) show the time history at GL-2.5m, (c) shows the time history at GL-5m, (d) shows the time history at GL-10m, (e) shows the time history at GL-15m, and (f) shows the time history at GL-16.25m.

[0047] When only the lattice-shaped improved body 103 was applied, no clear liquefaction mitigation effect was obtained under the experimental conditions of this example. As shown in (a) and (b) in Figure 6, although the water pressure ratio increased more slowly near the ground surface compared to the ground without measures, it eventually reached a water pressure ratio equal to or greater than that of the ground without measures, leading to liquefaction. This is presumably due to the fact that the liquefaction strength of the ground near the ground surface is low, making it difficult for the lattice-shaped improved body to exert its effect, and also because the vibration of the lattice-shaped improved body 103 itself acted as an excitation input source for the ground within the lattice, promoting liquefaction.

[0048] When the liquefaction countermeasure method according to the first embodiment, which uses both the lattice-shaped improvement body 103 and the gravel drain 104, is applied, a momentary high water pressure ratio occurs at the ground surface due to the drainage of the gravel drain 104, as shown in (a) of Figure 6. However, as shown in (a) to (c) of Figure 6, within the length range of the gravel drain 104, the water pressure ratio is generally kept lower than that of the conventional method in which only the lattice-shaped improvement body 103 is applied, and a relatively high liquefaction mitigation effect is obtained.

[0049] Figure 7 shows the time history of the excess pore water pressure ratio during water pressure dissipation after excitation, i.e., the time history of the excess pore water pressure ratio (after excitation, during water pressure dissipation) observed in the experiment. Similar to Figure 6, Figure 7 shows the results measured by the water pressure gauges 204 installed at each ground level GL. In Figure 7, the solid line indicates the ground without countermeasures, the dashed line indicates the ground with only the lattice-shaped improvement body 103, and the dotted line indicates the ground to which the liquefaction countermeasure method according to the first embodiment described above was applied. Because the water pressure dissipation time is long, the horizontal axis (time (seconds)) in Figure 7 is displayed as a logarithmic scale. In Figure 7, (a) and (b) show the time history at GL-2.5m, (c) shows the time history at GL-5m, (d) shows the time history at GL-10m, (e) shows the time history at GL-15m, and (f) shows the time history at GL-16.25m.

[0050] When only the lattice-shaped improvement body 103 is applied, as shown in (a) and (b) in Figure 7, the lattice-shaped improvement body 103 stops water around it, hindering the horizontal movement of interstitial water, and the start of dissipation of water pressure (1000 to 2000 seconds = 17 to 34 minutes after the end of vibration), especially near the surface, tends to be slower than in ground without any measures.

[0051] When the liquefaction countermeasure method according to the first embodiment, which uses both the lattice-shaped improvement body 103 and the gravel drain 104, is applied, as shown in Figures 7(a) to (d), it can be seen that the start of dissipation of water pressure (200 seconds = 3 minutes after the end of vibration) tends to be significantly faster not only within the length range of the gravel drain 104 but also at depths deeper than the bottom end of the gravel drain 104, compared to untreated ground or when only the lattice-shaped improvement body 103 is applied. This indicates that the effectiveness of this liquefaction countermeasure method can be demonstrated to some extent even in ground deeper than the bottom end of the short gravel drain 104. In addition, it is suggested that this liquefaction countermeasure method is effective in preventing re-liquefaction due to aftershocks.

[0052] Figure 8 is a schematic diagram of the prevention of re-liquefaction using the liquefaction countermeasure method. In Figure 8, the horizontal axis is time and the vertical axis is excess pore water pressure ratio. The solid line Q1 in Figure 8 shows the results of the conventional method in which only the lattice-shaped improvement body 103 was applied, and the dashed line Q2 shows the results of the liquefaction countermeasure method according to this embodiment in which the lattice-shaped improvement body 103 and gravel drain 104 were used in combination.

[0053] As shown in Figure 8, in the case of the conventional construction method in which only grid-like improvement is applied (graph Q1), it takes a long time for the water pressure that was built up during the first earthquake to completely dissipate, so the water pressure remains high during the second earthquake, and even a small earthquake could lead to re-liquefaction. In the case of the liquefaction countermeasure method according to this embodiment (Graph Q2), water pressure drops relatively quickly after the first earthquake, so that when a second earthquake occurs, water pressure does not rise again, preventing re-liquefaction. This effect of preventing re-liquefaction is thought to be particularly effective in cases where a second earthquake occurs within a few tens of minutes to a few hours, such as the 2011 Tohoku Pacific Coast Earthquake and the 2023 Turkey-Syria Earthquake.

[0054] From the experimental results of these examples, it was confirmed that the preferred interval of gravel drains is 5m, which is twice the radius of influence based on the unsteady well theory. It was also confirmed that the preferred drain length of gravel drains is long enough to reach the center depth of the liquefied layer.

[0055] The above describes embodiments of the liquefaction countermeasure structure and liquefaction countermeasure construction method for structures according to the present invention, but the present invention is not limited to the above embodiments and can be modified as appropriate within the scope of its intent.

[0056] For example, in this embodiment, the liquefaction prevention structure 1 for the newly constructed structure 2A is configured so that a crushed stone improvement board 40 is installed over the entire installation area of ​​the newly constructed structure 2A, but it is not limited to installing a crushed stone improvement board 40, and it is also possible to omit the crushed stone improvement board 40. Furthermore, the crushed stone improvement board 40 is not limited to a configuration in which it extends outward from the outer periphery of the lattice-shaped improvement body 20.

[0057] In addition, the configurations of the lattice-shaped improvement bodies 20, 20A in the above-mentioned embodiments, such as the wall thickness, the number of partitioned areas 21, and the depth-wise length of the gravel drains 30, 30A, are set appropriately taking into account the size, weight, ground conditions, etc. of the structure 2.

[0058] In addition, it is possible to replace the components in the above-described embodiments with well-known components as appropriate, without departing from the spirit of the present invention. [Explanation of symbols]

[0059] 1, 1A Liquefaction prevention structure 2 structures 2A New structure 2B Existing structures 20, 20A lattice-shaped improved body 21 Compartment Area 30, 30A Gravel Drain 40 Crushed Stone Improvement Plate G0 Surface layer G1 liquefied layer G2 Non-liquefiable layer

Claims

1. A liquefaction countermeasure structure for reducing damage caused by liquefaction to structures built on liquefied ground, A lattice-shaped improvement body that reaches a non-liquefied layer below the liquefied layer and is installed on the ground below the structure; A liquefaction prevention structure comprising a gravel drain installed in a surface layer within the wall surrounded by the lattice-shaped improvement body that does not reach the non-liquefaction layer, and having a permeability coefficient higher than that of the lattice-shaped improvement body.

2. The structure is a new structure, A crushed stone improvement board is installed over the entire installation area of ​​the new structure, The grid-like improvement body is connected to the crushed stone improvement board and supports the new structure from below via the crushed stone improvement board; The liquefaction countermeasure structure according to claim 1 , wherein the gravel drain extends vertically downward from the crushed stone improvement plate.

3. 3. The liquefaction prevention structure according to claim 2, wherein the crushed stone improvement plate extends outward from the outer peripheral portion of the lattice-shaped improvement body.

4. The structure is an existing structure, The grid-shaped improvement body is installed outside the existing structure, 2. The liquefaction countermeasure structure according to claim 1, wherein the gravel drain forms an inclined drain that extends downward from the outside to the inside of the existing structure.

5. A liquefaction countermeasure method for a structure to reduce damage caused by liquefaction to a structure built on liquefiable ground, a step of installing a grid-shaped improvement body in the ground below the structure so as to reach the non-liquefaction layer below the liquefaction layer; a step of installing a gravel drain having a permeability coefficient greater than that of the grid-shaped improvement body in a surface layer portion within the wall surrounded by the grid-shaped improvement body that does not reach the non-liquefaction layer; A liquefaction prevention method for structures.

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