Liquefaction-resistant piles and liquefaction-resistant construction methods using these piles

Abstract

To provide a countermeasure pile using a substitute material having high permeability and capable of suppressing degradation of draining performance caused by mixing of sand, in place of conventional ballast, to thereby develop suppressing effect of liquefaction occurrence over a long period, and a liquefaction countermeasure method using the countermeasure pile.SOLUTION: A countermeasure pile is used in a liquefaction countermeasure method of driving a countermeasure pile for a liquefaction countermeasure, composed of ballast in ground. The countermeasure pile is structured by using crushed shell in place of ballast.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to liquefaction-resistant piles and liquefaction-resistant construction methods using these piles. [Background technology]

[0002] Numerous instances of liquefaction damage have been confirmed and reported as a result of past large-scale or moderate-scale earthquakes. In this context, the gravel drain method is generally known as one of the liquefaction countermeasures that suppress damage caused by liquefaction. The gravel drain method is a construction method that suppresses liquefaction by shortening the drainage distance by driving countermeasure piles made of crushed stone into soft ground, thereby promoting the early dissipation of excess pore water pressure in liquefiable ground. According to the gravel drain method, by driving the countermeasure piles into loosely deposited sandy ground, the occurrence of liquefaction can be suppressed and prevented with simple construction and low cost.

[0003] In other words, the gravel drain method involves installing countermeasures piles made of crushed stone in the sandy ground to shorten the horizontal drainage distance, thereby suppressing the rise in pore water pressure that occurs during earthquakes and preventing liquefaction.

[0004] However, in recent years, earthquakes that cause liquefaction have resulted in surrounding sand mixing into the gaps between the crushed stone that makes up the countermeasure piles. This narrows the drainage range within the countermeasure piles made of crushed stone, reducing the permeability of the countermeasure piles, and creating a problem where the liquefaction suppression effect will decrease when another earthquake occurs.

[0005] In other words, when conventional gravel drain construction methods are used, if an earthquake that causes liquefaction occurs, the sand surrounding the installed countermeasure piles gets mixed into the gaps between the crushed stones that make up the countermeasure piles, which leads to a problem in that the drainage performance of the countermeasure piles is reduced.

[0006] Furthermore, it was pointed out that if the drainage performance of the reinforced piles deteriorates and earthquakes that cause liquefaction, for example, occur repeatedly over a short or long period, the liquefaction suppression effect of the reinforced piles will further decrease, raising concerns about a further exacerbation of liquefaction damage. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2015-101938 [Overview of the project] [Problems that the invention aims to solve]

[0008] Thus, the present invention was devised to solve the aforementioned conventional problems, and aims to provide a countermeasure pile and a liquefaction countermeasure method using the countermeasure pile, which can exert a liquefaction suppression effect over a long period of time by using a substitute material that has high water permeability and can suppress the decrease in drainage performance due to the mixing of sands as the pile material for the countermeasure pile instead of conventional crushed stone.

[0009] In the past, there have been cases where numerous earthquakes causing liquefaction were observed, and cases where multiple earthquakes causing liquefaction occurred in a short period of time, including foreshocks, main shocks, and aftershocks, within a few days. In such cases, even when seismic motions causing liquefaction occur consecutively, it is possible to provide a countermeasure pile and a liquefaction countermeasure method using that countermeasure pile that can suppress the mixing of sand from the liquefiable sand layer into the gaps of the substitute material constituting the countermeasure pile, thereby exhibiting a liquefaction suppression effect over a long period of time. [Means for solving the problem]

[0010] The present invention Liquefaction layer A pile used in a liquefaction countermeasure method, which involves driving a countermeasure pile into the ground to prevent liquefaction, The aforementioned countermeasure piles are In the borehole excavated in a nearly vertical direction through the aforementioned liquefiable layer, crushed seashells, broken into flat, plate-like shapes, were placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, and crushed seashells, also broken into flat, plate-like shapes, were laid flat and stacked inside the vertically positioned crushed seashells to construct the structure. characterized in that or the construction using the crushed shells is carried out such that the relative density (Dr) is 60% to 80%. characterized in that or the crushed shells are composed of particles with a particle size of 75 mm or less and have a flat plate-like shape. characterized in that or It is a liquefaction countermeasure construction method in which countermeasure piles for liquefaction prevention are constructed and installed in a ground composed of a liquefied layer. The liquefiable layer of ground is excavated in a nearly vertical direction to form a borehole, and in the formed borehole, Crushed seashells, broken into flat, plate-like shapes, are placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, with the crushed seashells arranged almost vertically. Inside the vertically arranged crushed seashells, more crushed seashells, also broken into flat, plate-like shapes, are stacked horizontally to construct a pile for liquefaction prevention, and liquefaction prevention is carried out by this constructed pile. characterized in that or It is a liquefaction countermeasure construction method in which countermeasure piles for liquefaction prevention are constructed and installed in a ground composed of a liquefied layer. The liquefiable layer of ground is excavated in a nearly vertical direction to form a borehole, and in the formed borehole, The crushed seashells, broken into flat, plate-like shapes, are placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, and inside the vertically placed crushed seashells, more crushed seashells, broken into flat, plate-like shapes, are stacked horizontally to construct a pile for liquefaction prevention. These countermeasure piles are distributed in multiple locations across the target area of ​​the ground in the liquefiable layer. characterized in that or It is a liquefaction countermeasure construction method in which countermeasure piles for liquefaction prevention are constructed and installed in a ground composed of a liquefied layer. The liquefiable layer of ground is excavated in a nearly vertical direction to form a borehole, and in the formed borehole, The crushed seashells, broken into flat, plate-like shapes, are placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, and inside the vertically placed crushed seashells, more crushed seashells, broken into flat, plate-like shapes, are stacked horizontally to construct a pile for liquefaction prevention. The countermeasure piles are distributed across the target area of ​​the ground in the liquefiable layer, with multiple piles positioned at the intersections of vertical and horizontal lines forming a grid pattern. characterized in that or At the exposed upper end opening of the countermeasure piles that are buried in a dispersed arrangement, a shell mat is formed, which is blocked by crushed shells stacked so as to connect to the upper end opening. characterized in that or The delivery of the crushed shells in a stacked state into the ground is carried out such that the crushed shells are compacted in a horizontally stacked state. characterized by the above.

Advantages of the Invention

[0011] According to the present invention, crushed seashells, which have high water permeability and can suppress the decrease in drainage performance due to the mixing of sand and other materials, are used in place of conventional crushed stone in the countermeasure piles, thereby providing countermeasure piles that can exert a long-term effect of suppressing the occurrence of liquefaction and a liquefaction countermeasure construction method using these countermeasure piles, which is an excellent effect. [Brief explanation of the drawing]

[0012] [Figure 1] This is a plan view of the vibrating device. [Figure 2] This is a front view of the vibration device. [Figure 3] This is a front view of the soil tank. [Figure 4] This is an external view of the vibration device shown in the photograph. [Figure 5] This is a plan view of the soil tank. [Figure 6] This is a photograph showing a pile containing crushed stone as a countermeasure. [Figure 7] This is a photograph showing a protective stake containing crushed seashells. [Figure 8] This photograph shows the countermeasures piles containing crushed stone and crushed seashells installed inside the soil tank. [Figure 9] This is a photo of the sheet being removed from the protective stake. [Figure 10] This is a table showing the cases that were tested. [Figure 11] This is a table showing the experimental results. [Figure 12] Figure (1) shows the waveform of acceleration. [Figure 13] Figure (1) shows the results from the pore water pressure gauge. [Figure 14] Figure (2) shows the waveform of acceleration. [Figure 15] Figure (2) shows the results from the pore water pressure gauge. [Figure 16] Figure (3) shows the waveform of acceleration. [Figure 17] Figure (3) shows the results from the pore water pressure gauge. [Figure 18] Figure (4) shows the waveform of acceleration. [Figure 19] Figure (4) shows the results from the pore water pressure gauge. [Figure 20] Figure (5) shows the waveform of acceleration. [Figure 21] Figure (5) shows the results from the pore water pressure gauge. [Figure 22] Figure (6) shows the waveform of acceleration. [Figure 23] Figure (6) shows the results from the pore water pressure gauge. [Figure 24] This figure shows the relationship between the period and acceleration when uniform vibration is applied for a certain period of time. [Figure 25] This is an explanatory diagram showing the general structure of liquefaction countermeasures. [Figure 26] This is an explanatory diagram illustrating the configuration of the shell mat according to the present invention and a conventional gravel mat. [Figure 27] This is an explanatory diagram showing the particle size distribution when seashells are crushed using heavy machinery. [Figure 28] This is an explanatory diagram illustrating the configuration of the pile construction device according to the present invention. [Modes for carrying out the invention]

[0013] The inventors of this case have first confirmed that if the crushed shells 7, such as scallops, have a predetermined particle size, then even when affected by vibrations from earthquakes or the like, sand from the upper sand layer is less likely to mix into the lower crushed shell layer, which is composed of crushed shells 7 containing a large amount of the aforementioned particle size.

[0014] Based on these findings, we have now invented a method in which crushed seashells 7, such as scallops, can be used as a substitute material for the pile material of the countermeasure pile 2. This method suppresses the incorporation of sand from the surrounding sand layer into the interior of the countermeasure pile during liquefaction, thereby preventing a decrease in drainage performance associated with sand incorporation.

[0015] In other words, the inventors of this case compared the sand mixing characteristics of conventional countermeasure piles 2, which are composed of crushed stone 6, and countermeasure piles using crushed seashells 7 as a substitute for crushed stone 6, under seismic motion. They evaluated the decrease in drainage performance associated with sand mixing and verified the mechanism of this decrease in drainage performance. As a result, they clarified the effectiveness of countermeasure piles 2, i.e., liquefaction prevention countermeasure piles 2 using crushed seashells 7, which can suppress the generation of excess pore water pressure even under repeated seismic motion, thereby suppressing the decrease in drainage performance. Furthermore, we have also invented a liquefaction countermeasure method that is effective in preventing liquefaction by using countermeasure piles 2 made of the aforementioned crushed seashells 7 as the pile material.

[0016] Therefore, the following will first describe the details of the experiment that clarified the effectiveness of the liquefaction prevention pile 2 using crushed seashells 7, and then explain the invention of a liquefaction prevention method that is effective in preventing liquefaction using the aforementioned prevention pile 2.

[0017] (Experiment to understand the three-dimensional sand contamination characteristics using a small vibration device 1) In this experiment, a vibration device 1 and a pore water pressure gauge 4 were used to qualitatively evaluate the reduction in drainage performance due to acceleration settings, the increase in pore water pressure due to liquefaction, and the three-dimensional sand incorporation into the countermeasure pile 2 due to large-scale vibrations. In addition, the changes in the drainage performance of the countermeasure pile 2 depending on the magnitude of the earthquake and the sand incorporation characteristics due to differences in the relative density (Dr) of the sand layer, which is the liquefiable layer, were confirmed.

[0018] (Overview of the vibration device experiment) Figure 1 shows a plan view of the vibrating device 1, Figure 2 shows a front view of the vibrating device 1, and Figure 3 shows a front view of the soil tank 3. The soil tank 3 is 406 mm high, 780 mm wide, and 280 mm deep. Figure 4 shows an external view of the vibrating device 1 as shown in the photograph. Figure 5 is a plan photograph of the soil tank 3. A 3 mm thick, 50 mm diameter acrylic plate and a coarse geotextile were rolled into a cylindrical shape and glued together, then filled with crushed stone 6 or crushed seashells 7 to form the countermeasure pile 2. The countermeasure pile 2 filled with crushed stone 2 is shown in Figure 6, and the countermeasure pile 2 filled with crushed seashells 2 is shown in Figure 7, both installed inside the soil tank 3 (see also Figures 3 and 8).

[0019] In addition, a sheet was wrapped around the countermeasure pile 2 so that dry sand would not mix into the gap of the countermeasure pile 2. The water injection port is made by cutting a pipe in half and using a pipe with a length that reaches the bottom of the soil tank 3. In order to prevent the backflow of sand from the water injection port, gauze was stuffed into the tip of the pipe. The pore water pressure gauges 4 were installed at positions 50 mm and 100 mm from the ground surface between the two countermeasure piles 2 filled with crushed stones 6 and the countermeasure piles 2 filled with crushed shells respectively (see Fig. 3).

[0020] (Physical properties of the samples) For the countermeasure piles 2 of the crushed stones 6, the samples were prepared by mixing crushed stones No. 5, No. 6, and No. 7 with particle sizes adjusted to 4.75 - 19 mm (see Fig. 6). For the countermeasure piles 2 of the crushed scallop shells 7, crushed scallop shells 7 from scallops produced in Aomori Prefecture crushed by heavy machinery were used (see Fig. 7). For the formation of the liquefied layer and the non-liquefied layer, Toyoura sand was used, and these three types of samples were employed.

[0021] (Experimental procedure) Attach the soil tank 3 to the vibration device 1 and install the pore water pressure gauges 4. Measure the mass of the sample to be used, and scoop the measured dry Toyoura sand into the soil tank 3. To form a non-liquefied layer with a relative density Dr = 69.8% (ρ d = 1.58 g / cm 3 ), use a sand compaction vibrator to compact until the height of the non-liquefied layer from the bottom of the soil tank 3 reaches 100 mm.

[0022] Next, install the countermeasure piles 2 filled with crushed stones 6 filled with crushed stones 6 at a relative density Dr = 60.2% (ρ d = 1.56 g / cm 3 ) and the countermeasure piles 2 filled with crushed scallop shells 7 filled with crushed scallop shells 7 of scallops at a relative density Dr = 59.9% (ρ d = 0.909 g / cm 3 ) as shown in the drawing of Fig. 3 and the photographic image of Fig. 8.

[0023] After that, use a funnel to uniformly drop dry sand into the soil tank 3 by the air-drop method to a relative density of Dr = 29.9% or Dr = 48.8% (ρ d = 1.47 g / cm 3 , ρ d = 1.52 g / cm3 It was then slowly compacted. When dropping the sand in using the aerial drop method, a bag was placed over the aforementioned support pile 2 to prevent dry sand from getting mixed in. Next, a hose connected to a water tank was inserted into the water inlet pipe, and water injection began (see photographic video in Figure 4).

[0024] To prevent boiling from occurring when vibrations are applied to the sand layer, which will become the liquefaction layer 5, while it is unsaturated, water was slowly poured in for about two hours until the water level reached the ground surface and saturated the layer. After pouring in the water, a hose connected to a water-only vacuum cleaner was inserted into the pipe to drain the water and lower the water level to the non-liquefaction layer (this was to prevent the surrounding area from liquefying due to the impact of pulling out the sheet when the water level was at the ground surface, or to prevent sand from getting into the countermeasure pile 2).

[0025] Afterward, the sheet used to prevent the inclusion of dry sand was slowly removed (see photographic video in Figure 9), and water was poured again until the water level reached the ground surface. Then, the vibration device 1 was set to a frequency of 5.1 Hz, 10 waves, and a vibration time of 2 seconds, and the acceleration for each case was set to apply sinusoidal horizontal vibration.

[0026] After vibration, the water in the soil tank 3 was drained using a water-only vacuum cleaner, and the surrounding sand was removed to prevent sand from contaminating the countermeasure pile 2. Then, the countermeasure pile 2 was removed. The sand, crushed stone 6, and crushed seashells 7 that had contaminated the countermeasure pile 2 were taken out into a container and dried in a drying oven (110±5 ℃) for 24 hours.

[0027] After drying in the aforementioned drying oven, the sand, crushed stone 6, and crushed seashells 7 were sieved apart, and the mass of the sand was measured. The average saturated hydraulic conductivity due to the sand mixing was then calculated, and the decrease in drainage performance of the reinforcement piles 2 containing crushed stone 6 and the reinforcement piles 2 containing crushed seashells 7 after sand mixing was evaluated.

[0028] (Experimental case) Figure 10 shows the experimental cases. In the experimental cases shown in Figure 10, a sinusoidal horizontal vibration was applied with 10 waves, a frequency of 5.2 Hz, a vibration duration of 2 seconds, and 1 vibration cycle. The acceleration was set differently for each experimental case. The amount of sand contamination was compared and confirmed based on the effect of earthquake magnitude on the magnitude of the maximum acceleration.

[0029] In actual construction work, the ground surrounding remedial pile 2 is compacted when remedial pile 2 is driven in, so in this experiment, a loosely compacted relative density Dr = 29.9%(ρ d = 1.47 g / cm 3 ) and the surrounding ground were compacted by the installation of two countermeasure piles, so the relative density Dr = 48.8% (ρ d = 1.52 g / cm³ 3 By conducting experiments in the case of ), we also compared the amount of sand mixed in due to differences in the relative density (Dr) of the liquefaction layer 5.

[0030] (Experimental results and discussion) The experimental results are shown in Figure 11. Before vibration was applied, the basic mixing amounts were 86.6g of crushed stone and 81g and 82g of crushed seashells at a relative density Dr=29.9%, and 93.40g of crushed stone and 96.73g of crushed seashells at a relative density Dr=48.8%.

[0031] In case A, the maximum acceleration was 101 gal (see Figure 12 for acceleration waveform and Figure 13 for pore water pressure gauge results). In case B, the maximum acceleration was 149 gal (see Figure 14 for acceleration waveform and Figure 15 for pore water pressure gauge results). In case C, the maximum acceleration was 188 gal (see Figure 16 for acceleration waveform and Figure 17 for pore water pressure gauge results). In case D, the maximum acceleration was 55 gal (see Figure 18 for acceleration waveform and Figure 19 for pore water pressure gauge results). In case E, the maximum acceleration was 95 gal (see Figure 20 for acceleration waveform and Figure 21 for pore water pressure gauge results). In case F, the maximum acceleration was 192 gal (see Figure 22 for acceleration waveform and Figure 23 for pore water pressure gauge results).

[0032] In cases B, C, and F, the increase in pore water pressure indicates that liquefaction is occurring. However, in the event of liquefaction, the pore water pressure gauge value at GL-10cm should normally be 10 g / cm³. 2 It can rise up to 5-8 g / cm³, but experimental results show it to be 5-8 g / cm³. 2 This result suggests that soil tank 3 was not completely saturated.

[0033] In cases A, D, and E, liquefaction did not occur because the pore water pressure did not rise. The amount of sand mixed in when liquefaction did not occur did not change much compared to the basic amount of sand mixed in, suggesting that sand does not mix in during small earthquakes that do not cause liquefaction.

[0034] Comparing the amount of sand mixed in by manually applied vibration (single vibration) (crushed stone: 128.05 g, crushed seashells: 137.12 g) with the experimental results of this experiment, both the 6-grained crushed stone pile 2 and the 7-grained crushed seashell pile 2 had more sand mixed in than the amount at a maximum acceleration of 149 gal in case B, but less than the amount at a maximum acceleration of 188 gal in case C. Therefore, it can be concluded that the manual experiment involved vibrations with a maximum acceleration of approximately 149 to 188 gal.

[0035] Although acceleration is not a value that represents seismic intensity, Figure 24 shows the relationship between the period and acceleration when uniform vibration is applied for a certain period of time. From this relationship, it was found that, since this experiment applied vibrations with a small vibration device 1 at a constant acceleration, period (T=0.196 seconds), and amplitude, the estimated seismic intensity for cases B, C, and F was approximately 5-minus.

[0036] Furthermore, in the case of an earthquake that causes liquefaction of magnitude 5 or lower, using crushed seashells 7 in the liquefaction countermeasure piles 2 can reduce the decrease in drainage performance due to sand mixing caused by vibrations during liquefaction compared to using crushed stone 6, as confirmed by the experimental results shown in Figure 11, such as the sand mixing rate. Furthermore, it was confirmed that using crushed seashells 7 in the liquefaction-resistant piles 2 could potentially withstand repeated earthquakes and provide long-term liquefaction suppression effects.

[0037] Since the maximum acceleration of 188 gal in case C and 192 gal in case F are of similar acceleration, a comparison of sand inclusion due to differences in the relative density (Dr) of the liquefaction layer 5 shows that in case C, which was loosely compacted with a relative density Dr = 29.9%, the amount of sand inclusion was 136.26 g of crushed stone and 139.67 g of crushed seashells. In contrast, in case F, which was compacted with a relative density Dr = 48.8%, the amount of sand inclusion was 124.00 g of crushed stone and 113.07 g of crushed seashells. This confirmed that the looser the sandy soil, the more sand is mixed in.

[0038] In this experiment, the amount of sand mixed in, the sand mixing rate, and the permeability coefficient of the crushed stone 6-filled pile 2 and the crushed seashell 7-filled pile 2 were compared and the results are shown. In almost all of the above items, the crushed seashell 7-filled pile 2 was superior, confirming that using the crushed seashell 7-filled pile 2 can provide a long-term liquefaction suppression effect. In particular, when comparing the sand content, it can be confirmed that in all cases, the sand content of the pile containing crushed seashells (pile 2) is considerably smaller than that of the pile containing crushed seashells (pile 2).

[0039] Here, the value of the permeability coefficient (m / sec) is determined by how much of the voids in the reinforcement piles 2 containing crushed seashells 7 and reinforcement piles 2 containing crushed stone 6 are filled with the incoming sand.

[0040] In other words, let's assume that sand is mixed into the aforementioned voids. In that case, even if the amount of sand mixed in is the same, the amount of voids in the crushed stone 6-filled pile 2 and the crushed seashell 7-filled pile 2 will differ significantly, with the crushed seashell 7-filled pile 2 having an overwhelmingly larger amount of voids. Therefore, even if the same amount of sand is mixed into the voids, the water permeability performance of the crushed seashell 7-filled pile 2 will not decrease.

[0041] As can be seen from the results shown in Figure 11, in all cases from Case A to Case F, the sand content of the countermeasure pile 2 containing crushed seashells 7 is overwhelmingly smaller, and its water permeability, or in other words, drainage performance, is not reduced.

[0042] (summary) The vibrations applied by the vibration device 1 were thought to be of a similar magnitude to earthquakes that cause liquefaction. By using crushed seashells 7 in the liquefaction countermeasure piles 2, the deterioration of the drainage performance of the countermeasure piles 2 due to sand mixing caused by vibrations during liquefaction can be suppressed, and the liquefaction suppression effect can be exerted over the long term.

[0043] Furthermore, it was confirmed that the lower the relative density (Dr) of the sandy ground in the liquefiable layer 5, the more likely sand mixing is to occur. In practical terms, it is thought that the surrounding ground will be compacted by the confinement pressure generated in the ground due to the driving of the countermeasure piles 2, embankment, and superimposed loads from building structures, which will further slow down the tendency for the drainage performance to deteriorate due to sand mixing into the countermeasure piles 2 caused by vibration.

[0044] Next, we will explain the invention of a liquefaction countermeasure method using the countermeasure pile 2 containing crushed seashells 7 according to the present invention. As shown in Figure 25, the ground where countermeasures against liquefaction should be taken by constructing the countermeasure piles 2 is the ground consisting of the liquefiable layer 5. The ground consisting of the liquefiable layer 5 is composed of water-containing sandy soil, and during an earthquake, the water saturated between the sand grains flows due to vibrations, breaking the intergranular bonds between the sand grains and causing it to behave like a liquid, thus losing its bearing capacity.

[0045] When the liquefiable layer 5 liquefies, the water pressure of the fluidized pore water rises sharply, resulting in the generation of excess pore water. In this invention, as shown in Figure 25, a liquefaction prevention pile 2 is constructed in the ground consisting of the liquefaction layer 5, and the liquefaction prevention method is implemented. First, it is best to construct the liquefaction prevention piles 2 by burying multiple piles scattered throughout the entire area of ​​the liquefaction layer 5.

[0046] Here, the liquefaction countermeasure pile 2 used in the gravel drain method, one of the liquefaction countermeasure construction methods, is configured as a columnar body embedded in the liquefaction layer 5 in a nearly vertical direction in order to drain excess pore water from the liquefaction layer 5 that occurs during an earthquake upwards.

[0047] In other words, multiple of these countermeasure piles 2 are embedded in the entire target area of ​​the ground in the liquefiable layer 5, at predetermined intervals (horizontal intervals), and are dispersed in a grid-like pattern, for example (see Figure 26). Furthermore, depending on the area to be buried, it is also acceptable to divide the area into multiple equilateral triangles and distribute them, burying the countermeasure piles 2 at the vertices of each triangle.

[0048] As mentioned above, conventional gravel drain construction methods used for liquefaction countermeasures consisted of piles 2 filled with crushed stone 6. However, the inventors of this invention focused on seashells, which are abundant in Japan, an island nation. When seashells are crushed, most of them become flat, almost plate-like shapes. They never form blocks. Furthermore, seashells inherently have more voids inside than the crushed stone 6. Focusing on these characteristics of seashells, the inventors conceived the idea of ​​using crushed seashells 7 as the filling material for the countermeasure piles 2.

[0049] For liquefaction prevention piles 2, where good drainage performance is required, it was considered that seashells with many voids and high drainage capacity are suitable. Furthermore, it was decided to use liquefaction prevention piles 2 composed of crushed seashells 7 that have been crushed by rolling them with heavy machinery to a predetermined particle size. By crushing with heavy machinery, crushed seashells 7 with a particle size distribution curve (particle size distribution) as shown in Figure 27 can be easily produced. This makes it easy to obtain crushed seashells 7 with a particle size that is effective as liquefaction prevention piles 2 containing crushed seashells 7.

[0050] After obtaining a large quantity of crushed seashells 7 with a predetermined particle size through the above process, the process proceeds to construct multiple countermeasure piles 2 containing the crushed seashells 7 in the area where liquefaction prevention measures should be taken, i.e., the area of ​​the liquefiable layer 5. For constructing the countermeasure pile 2 by burying it, it is advisable to use an excavation device 12, such as the one shown in Figure 28, which is a so-called sand compaction pile.

[0051] The drilling device 12 is equipped with a cylindrical drilling tool 10 as a cutting member. The drilling tool 10 has a crushed shell inlet 11 at its upper end, and is configured so that crushed shells 7 of an appropriate particle size can be introduced into the hollow part of the drilling tool 10 from the crushed shell inlet 11.

[0052] Furthermore, the lower end of the excavation tool 10 is provided with an opening 13 for sending out crushed seashells, and the crushed seashells 7 stored in the hollow section can be sent into the ground through this opening 13.

[0053] The process of constructing the countermeasure pile 2 containing crushed seashells 7 by sending the crushed seashells 7 into the ground is as shown in Figure 28. The excavating tool 10 is driven into the ground, and the crushed seashells 7 are sent into the ground through the opening 13. The movement trajectory of the lower end of the excavating tool 10 at that time is as shown in Figure 28. The crushed seashells 7 sent into the ground need to be compacted to a predetermined degree. Therefore, in order to achieve the predetermined degree of compaction, vibration or other means are applied to the crushed seashells 7 when they are sent into the ground to compact them. The degree of compaction of the crushed seashells 7 can be expressed by the relative density (Dr) of the crushed seashells 7. The required relative density (Dr) of the crushed seashells 7 here is a value of 60% to 80%.

[0054] Furthermore, the crushed seashells 7 that make up the countermeasure pile 2 have many voids inside and, in their crushed state, form a flat, plate-like shape with a predetermined particle size. Therefore, when vibrations are applied, the crushed seashells 7 on the outer surface of the countermeasure pile 2 will be positioned vertically, parallel to the vertical surface of the countermeasure pile 2. Thus, this arrangement of the crushed seashells 7 will further prevent the intrusion of sand from the liquefiable layer 5.

[0055] Furthermore, around the axial direction of the countermeasure pile 2, many flat, plate-shaped crushed seashells 7 are stacked horizontally in a layered state. Within the countermeasure pile 2 containing the crushed seashells 7 in this arrangement, the crushed seashells 7 arranged vertically on the outer surface of the countermeasure pile 2, in combination, can reliably prevent sand from entering from the outer liquefaction layer 6.

[0056] In conventional crushed stone piles containing crushed stone 6, not all of the crushed stone 6 is flat or slab-shaped. Most of the crushed stone is block-shaped.

[0057] However, when seashells are crushed, all of the crushed seashells 7 almost automatically become flat and plate-like in shape. Therefore, if these flat and plate-like crushed seashells 7 are used as the pile material for the countermeasure piles 2, the fact that the seashells have more voids inside than crushed stone, combined with this, will prevent sand from entering more effectively than the countermeasure piles 2 containing crushed stone, thereby suppressing the deterioration of the drainage performance of the countermeasure piles 2.

[0058] Alternatively, a method may be used in which a spiral-shaped drilling blade is attached to the outer surface of the drilling tool, and the tool is rotated while penetrating the ground to excavate a cylindrical shape. After excavating to a predetermined depth, the crushed seashells 7 are dropped into the excavated hole while the drilling tool 10 is withdrawn, thereby filling it with crushed seashells. By applying vibration during this dropping process, the crushed seashells 7 to be laid can be made to a predetermined degree of compaction, in other words, a relative density (Dr) of a predetermined value. Here, too, the optimal relative density (Dr) of the filled crushed seashells 7 is preferably 60% to 80%.

[0059] The aforementioned crushed seashell-filled countermeasure piles 2 are constructed by dispersing them, for example, on the intersections of vertical and horizontal lines forming a grid, in the ground of the liquefaction layer 5 where liquefaction prevention measures should be taken, thereby burying them in a scattered manner.

[0060] The upper end opening of the countermeasure pile 2 containing crushed seashells 7, which is embedded in the liquefaction layer 5, is exposed to the ground surface, but the exposed upper end opening is covered with crushed seashells 7 to seal it. This is the formation of a so-called shell mat 9. The formation of the shell mat 9, which is made of crushed seashells 7, allows excess pore water that has risen after passing through the countermeasure pile 2 containing crushed seashells 7 to be drained very smoothly in the horizontal direction. In other words, it can be drained onto the ground surface at a predetermined location. Here, the shell mat 9 of the present invention is made of crushed seashells 7 as described above, and is not formed in the form of a flat mat by crushed stone 6 as in the conventional method, but is configured in the form of a grid mat as shown in the figure. This configuration allows for faster and smoother horizontal drainage.

[0061] Furthermore, regarding the construction of the countermeasure pile 2 using crushed seashells 7, the crushed seashells 7 are vibrated and released so that they are positioned parallel to the boundary surface between the outer surface of the excavation hole and the ground of the liquefiable layer 5, and then compacted. Near the axis of the countermeasure pile 2, the crushed seashells 7 are released and compacted so that they are stacked perpendicular to the boundary surface.

[0062] With this method of filling, even in the event of an earthquake that causes liquefaction, the crushed seashells 7 positioned upright on the interface can more effectively prevent sand from entering from the liquefiable layer 5. Furthermore, in the vicinity of the axial direction where the crushed seashells 7 are stacked and compacted perpendicular to the interface, the layered crushed seashells 7 can more effectively prevent sand from entering from the liquefiable layer 5.

[0063] However, due to the multiple crushed seashell-filled countermeasure piles 2 constructed in the liquefaction layer 5, excess pore water flows into the countermeasure piles 2 during an earthquake that causes liquefaction and is discharged to the desired surface location via a shell mat 9 made of crushed seashells 7. As a result, the increase in the excess pore water pressure ratio is suppressed in the liquefaction layer 5, and liquefaction is prevented.

[0064] Furthermore, it has been pointed out that if excess pore water in the liquefiable layer 5 is drained into the countermeasure pile 2 during an earthquake, the liquefiable layer 5 may condense, potentially causing ground subsidence. However, if a paved road or ground improvement layer is formed above the liquefiable layer 5, it is believed that even if some ground subsidence occurs, significant ground subsidence can be prevented. [Industrial applicability]

[0065] This invention can be used not only as a drain method for liquefaction countermeasures, but also in other liquefaction countermeasures, such as compaction methods. [Explanation of Symbols]

[0066] 1 Vibration device 2. Countermeasure piles 3 Earthen tank 4. Pore pressure gauge 5. Liquefaction layer 6. Crushed stone 7. Crushed seashells 9 Shell Mat 10 Excavation Tools 11. Inlet for crushed shells 12 Drilling equipment 13 Aperture

Claims

1. A countermeasure pile used in a liquefaction countermeasure method in which a countermeasure pile for liquefaction countermeasures is driven into a liquefaction layer of ground, The aforementioned countermeasure pile was constructed by excavating a borehole through the liquefiable layer ground in a substantially vertical direction, and then placing crushed seashells, which had been crushed into a flat, plate-like shape, vertically on the circumferential surface of the borehole that was in contact with the liquefiable layer ground, and then laying flat, plate-like crushed seashells on the inside of the vertically placed crushed seashells and stacking them horizontally. Liquefaction-resistant pile characterized by the above.

2. The construction using the aforementioned crushed seashells was carried out so that the relative density (Dr) was between 60% and 80%. Liquefaction-resistant pile according to claim 1.

3. The crushed seashells are composed of particles with a diameter of 75 mm or less and are formed in a flat, plate-like shape. Liquefaction-resistant pile according to claim 1 or 2.

4. This is a liquefaction countermeasure construction method that involves constructing and implementing liquefaction prevention piles in ground consisting of a liquefaction layer. The liquefiable layer of ground is excavated in a nearly vertical direction to form a borehole, and in the formed borehole, Crushed seashells, broken into flat, plate-like shapes, are placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, with the crushed seashells arranged almost vertically. Inside the vertically arranged crushed seashells, more crushed seashells, also broken into flat, plate-like shapes, are stacked horizontally to construct a pile for liquefaction prevention, and liquefaction prevention is carried out by this constructed pile. A liquefaction prevention method using countermeasures piles for liquefaction prevention, characterized by the following features.

5. This is a liquefaction countermeasure construction method that involves constructing and implementing liquefaction prevention piles in ground consisting of a liquefaction layer. The liquefiable layer of ground is excavated in a nearly vertical direction to form a borehole, and in the formed borehole, The crushed seashells, broken into flat, plate-like shapes, are placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, and inside the vertically placed crushed seashells, more crushed seashells, broken into flat, plate-like shapes, are stacked horizontally to construct a pile for liquefaction prevention. These countermeasure piles are distributed in multiple locations across the target area of ​​the ground in the liquefiable layer. A liquefaction prevention method using countermeasures piles for liquefaction prevention, characterized by the following features.

6. This is a liquefaction countermeasure construction method that involves constructing and implementing liquefaction prevention piles in ground consisting of a liquefaction layer. The liquefiable layer of ground is excavated in a nearly vertical direction to form a borehole, and in the formed borehole, The crushed seashells, broken into flat, plate-like shapes, are placed vertically on the circumferential surface of the borehole in contact with the liquefiable layer, and inside the vertically placed crushed seashells, more crushed seashells, broken into flat, plate-like shapes, are stacked horizontally to construct a pile for liquefaction prevention. The countermeasure piles are distributed across the target area of ​​the ground in the liquefiable layer, with multiple piles positioned at the intersections of vertical and horizontal lines forming a grid pattern. A liquefaction prevention method using countermeasures piles for liquefaction prevention, characterized by the following features.

7. At the exposed upper end openings of the dispersed and buried countermeasure piles, a shell mat was formed, which was covered with crushed seashells stacked so that the upper end openings were connected. A liquefaction prevention method using countermeasure piles for liquefaction prevention according to claim 5 or 6.

8. The discharge of crushed seashells into the ground in the aforementioned ground in a stacked state is such that the crushed seashells are compacted in a horizontally stacked state. A liquefaction prevention method using countermeasure piles for liquefaction prevention according to claim 5 or 6.

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