Ejecta suppression and ground subsidence suppression system of improvement ground

Abstract

To provide an ejecta suppression and ground subsidence suppression system of liquefaction ground which confirms that a crushed shell drain composed of a crushed shell suppresses reduction in drainage performance accompanied by mixing of sand compared to a gravel drain, and is effective as a liquefaction countermeasure drain, and in which since the crushed shell is small in dry density and is extremely lightweight compared to a crushed stone, the dead weight in a ground improvement area is small, and the system has an effect for further reducing a ground subsidence amount after liquefaction.SOLUTION: An ejecta suppression and ground subsidence suppression system, ejecta being from an improvement ground surface by earthquake time liquefaction in an improved ground where a crushed shell drain is laid, includes: selecting a crushed shell having such a coefficient of permeability that a seepage pressure by an excess pore water pressure difference of the crushed shell drain laid on the ground is not higher than an effective placement pressure of the crushed shell drain, after liquefaction; laying the crushed shell drain formed of the selected crushed shell; and improving the ground.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a system for suppressing ejecta and ground subsidence in improved ground. [Background technology]

[0002] Crushed shells have the material properties of being lightweight and highly permeable. In other words, when crushed shells are used instead of crushed stone in pore pressure dissipation methods, including CSP, the vertical load on the drain itself can be reduced in absolute terms compared to the current material, crushed stone.

[0003] Therefore, the inventors have discovered that when the drain penetrates incompletely into non-liquefiable ground (bearing layer), the method not only produces the expected effects, but also reduces the amount of ground subsidence when crushed shells are used, more so than when crushed stone is used.

[0004] Here, the pore water pressure dissipation method refers to a method in which, for example, vertical drainage column drains are installed in the ground, such as sandy soil, which is at risk of liquefaction during an earthquake, to increase the permeability of the ground and quickly dissipate excess pore water pressure generated during an earthquake, thereby preventing liquefaction.

[0005] One of the liquefaction countermeasures is the gravel drain method, which uses highly permeable crushed stone to suppress the increase in excess pore water pressure in the ground during an earthquake and to speed up the rate at which it dissipates.

[0006] Gravel drains have a high drainage performance due to the large porosity of the crushed stone, but in the event of liquefaction, fine sand (sand from the liquefied layer) is likely to get mixed into the gaps in the crushed stone layer, which can reduce drainage performance. Furthermore, for homes on ground with a thick liquefied layer, there are concerns that the home may tilt and the difference in the amount of settlement between the home and the surrounding ground (hereinafter referred to as "indentation settlement") may increase. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2022-24434 Summary of the Invention [Problem to be solved by the invention]

[0008] Thus, the present invention was devised to address the problems of the past, and it has been confirmed that crushed shell drains constructed from crushed shells have less reduction in drainage performance due to the inclusion of sand than gravel drains, and are effective as liquefaction countermeasure drains.Furthermore, crushed shells have a lower dry density than crushed stone and are extremely lightweight, so their own weight in the ground improvement area is smaller, and the purpose of this invention is to provide a system for suppressing ejecta and subsidence in liquefied ground, which has the effect of significantly reducing the amount of ground subsidence after liquefaction.

[0009] In other words, based on the results of a 1G field vibration table model experiment, the inventors have confirmed that crushed shell drains constructed from crushed shells have a smaller decrease in drainage performance due to the inclusion of sand than gravel drains, and are therefore effective as liquefaction prevention drains.Furthermore, because crushed shells have a lower dry density and are lighter than crushed stone, the weight of the ground improvement area (area where crushed shell drains are installed) is smaller, and the amount of ground subsidence after liquefaction can be reduced.This experimental result led to the invention of a system that can be used to suppress ejecta and ground subsidence in liquefied ground.

[0010] This invention assumes that crushed shell drains, which are considered to be an effective countermeasure for preventing liquefaction, are applied to a house on ground with a thick liquefaction layer, and confirms the amount of settlement after vibration through a 1G field vibration model experiment.Based on the results of the experiment, the amount of settlement of gravel drains and crushed shell drains is compared, and the reduction effect is clarified.

[0011] In recent years, there has been a demand for a shift to a recycling-oriented society, and the effective use of waste resources has become an important issue. One such waste resource is shells, such as scallop shells, which are a by-product of the fishing industry, and proposals have been made to use these as construction materials. However, although there have been attempts to use some of the lightweight crushed shells, such as scallop shells, as a sand substitute, this is limited and it is difficult to say that they are being used sufficiently. On the other hand, by adjusting the particle size, crushed shells can be used as a material that can more reliably and accurately ensure high permeability, and therefore are expected to be applied to pore water pressure dissipation construction methods.

[0012] In this invention, in a pore water pressure dissipation method using lightweight crushed shells, the suppression of the ejection of shell particles and other materials onto the ground surface was confirmed through indoor shaking table experiments based on the relationship between infiltration water pressure and effective overburden pressure during an earthquake.As a result, it was revealed that even lightweight crushed shells can suppress the ejection of ejecta onto the ground surface against the infiltration water pressure during an earthquake.

[0013] As mentioned above, crushed shells can be precisely adjusted to ensure high permeability by adjusting the particle size, etc., and therefore are highly promising for application in pore water pressure dissipation methods during earthquakes.

[0014] However, when using it in actual improved ground, there are issues as to whether it will actually have the effect of dissipating excess pore water pressure during an earthquake, and there are also issues arising from the fact that crushed shells are flat in shape, are difficult to compact when they are of a single particle size, and have a low dry density (whether ejecta can be reliably suppressed).

[0015] Furthermore, there is also the issue of whether crushed shell drains will have reduced drainage performance due to clogging, which is common in ordinary gravel drains. If these issues can be resolved and methods for setting and evaluating the physical properties of crushed shells can be established in the design of pore water pressure dissipation methods, it will be possible to contribute to the mass utilization of crushed shells and their recycling in a recycling-oriented society.

[0016] In order to address the above issues, in addition to various physical tests on crushed shells, we also conducted shaking table experiments to confirm the effect of dissipating excess pore water pressure. In these experiments, we also measured the pore water pressure inside drains, which had not been measured before. These results demonstrated the stability of lightweight crushed shells against seepage water pressure during earthquakes.

[0017] Furthermore, by adjusting the particle size and particle size distribution of the crushed shells, the crushed shells have a low dry density and are lightweight, so their weight in the ground improvement area can be reduced, thereby reliably and accurately reducing the amount of ground subsidence after liquefaction, and it is possible to provide a system for suppressing ejecta and ground subsidence in improved ground that can also suppress the ejecta ejecta in liquefied ground. [Means for solving the problem]

[0018] The present invention provides This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. After liquefaction, crushed shells having a hydraulic conductivity such that the seepage water pressure due to the excess pore water pressure difference of the crushed shell drain laid in the ground does not become higher than the effective overburden pressure of the crushed shell drain were selected, and the crushed shell drain formed from the selected crushed shells was laid to carry out ground improvement. It is characterized by the fact that or This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. After liquefaction, crushed shells with a low dry density and a hydraulic conductivity that ensures that the seepage water pressure due to the excess pore water pressure difference of the crushed shell drain laid in the ground will not become higher than the effective overburden pressure of the crushed shell drain were selected, and the crushed shell drain formed from the selected crushed shells was laid to carry out ground improvement. It is characterized by the fact that or This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. The ground that has been improved by laying the crushed shell drain is reproduced, and the reproduced ground is vibrated to cause liquefaction. After liquefaction, the effective overburden pressure of the crushed shell drain and the seepage water pressure due to the excess pore water pressure difference are determined. The crushed shells having a permeability coefficient that does not cause the seepage water pressure due to the excess pore water pressure difference to be higher than the effective overburden pressure are selected, and the crushed shell drain formed from the selected crushed shells is laid to carry out actual ground improvement. It is characterized by the fact that or This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. The ground improved using the crushed shells is reproduced, the reproduced ground is vibrated to cause liquefaction, the effective overburden pressure of the crushed shells and the seepage water pressure due to the difference in excess pore water pressure are determined after liquefaction, crushed shells having a low dry density and a permeability coefficient such that the seepage water pressure due to the difference in excess pore water pressure does not become higher than the effective overburden pressure are selected, and a crushed shell drain formed from the selected crushed shells is laid to carry out actual ground improvement. It is characterized by the fact that or The seepage water pressure due to the difference between the effective overburden pressure and the excess pore water pressure of the crushed shells is calculated, and the crushed shells having a hydraulic conductivity such that the seepage water pressure due to the excess pore water pressure difference is not higher than the effective overburden pressure are selected by calculating the safety factor using the formula below for calculating the safety factor related to ejecta suppression, and selecting crushed shells having a hydraulic conductivity with a high safety factor: It is characterized by the following. (Formula for calculating safety factor Fs) JPEG2025172418000002.jpg3677 where, i cr : Critical hydraulic gradient of crushed shell drain i: hydraulic gradient of the crushed shell drain γ': Unit weight of crushed shell in water gamma w : unit weight of water L: Distance traveled by infiltrated water h: head difference between two points [Effects of the Invention]

[0019] According to the present invention, crushed shell drains constructed from crushed shells have less of a decline in drainage performance due to the inclusion of sand than gravel drains, making them effective as liquefaction prevention drains.Since crushed shells have a lower dry density and are lighter than crushed stone, the weight of the ground improvement area can be reduced, the amount of ground subsidence after liquefaction can be reduced, and a system for suppressing ejecta and ground subsidence in improved ground can be provided that can suppress the ejecta ejecta that occurs from liquefied ground toward the ground, thereby achieving the excellent effect of providing a system for suppressing ejecta and ground subsidence in improved ground. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is an explanatory diagram (1) illustrating the contents of the first experiment. [Figure 2] FIG. 10 is an explanatory diagram (2) illustrating the contents of the first experiment. [Figure 3] FIG. 10 is an explanatory diagram (3) for explaining the contents of the first experiment. [Figure 4] FIG. 4 is an explanatory diagram (4) for explaining the contents of the first experiment. [Figure 5] FIG. 5 is an explanatory diagram (5) for explaining the contents of the first experiment. [Figure 6] FIG. 6 is an explanatory diagram (6) for explaining the contents of the first experiment. [Figure 7] FIG. 7 is an explanatory diagram (7) for explaining the contents of the first experiment. [Figure 8] FIG. 8 is an explanatory diagram (8) for explaining the contents of the first experiment. [Figure 9] FIG. 9 is an explanatory diagram (9) for explaining the contents of the first experiment. [Figure 10] FIG. 10 is an explanatory diagram for explaining the contents of the first experiment. [Figure 11] FIG. 11 is an explanatory diagram for explaining the contents of the first experiment. [Figure 12] FIG. 1 is an explanatory diagram (1) illustrating the contents of the second experiment. [Figure 13] FIG. 10 is an explanatory diagram (2) illustrating the contents of the second experiment. [Figure 14] FIG. 10 is an explanatory diagram (3) for explaining the contents of the second experiment. [Figure 15] FIG. 4 is an explanatory diagram (4) for explaining the contents of the second experiment. [Figure 16] FIG. 5 is an explanatory diagram (5) for explaining the contents of the second experiment. [Figure 17] FIG. 6 is an explanatory diagram (6) for explaining the contents of the second experiment. [Figure 18] FIG. 7 is an explanatory diagram (7) for explaining the contents of the second experiment. [Figure 19] FIG. 8 is an explanatory diagram (8) for explaining the contents of the second experiment. [Figure 20] FIG. 9 is an explanatory diagram (9) for explaining the contents of the second experiment. [Figure 21] FIG. 10 is an explanatory diagram for explaining the contents of the second experiment. [Figure 22] FIG. 1 is an explanatory diagram (1) illustrating the contents of the third experiment. [Figure 23] FIG. 10 is an explanatory diagram (2) illustrating the contents of the third experiment. [Figure 24] FIG. 10 is an explanatory diagram (3) for explaining the contents of the third experiment. [Figure 25] FIG. 10 is an explanatory diagram (4) illustrating the content of the third experiment. [Figure 26] FIG. 5 is an explanatory diagram (5) for explaining the contents of the third experiment. [Figure 27] FIG. 6 is an explanatory diagram (6) for explaining the contents of the third experiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] In order to explain and verify the configuration and effects of the present invention, various experiments were conducted, and the details of these experiments will be described below.

[0022] "First Experiment" (Outline of the experimental equipment and experimental conditions) As shown in Figure 1, in the first experiment, the experimental equipment consisted of a soil tank 2 (1000mm wide x 480mm high x 200mm deep) and a water tank 3 placed on a shaking table 1.

[0023] Toyoura sand was used as the sample for the liquefied and non-liquefied layers. Three types of drainage materials were used: crushed shells and crushed stone with particle sizes adjusted to 2.0 to 4.75 mm, and crushed shells with particle sizes adjusted to 4.75 to 19.0 mm (Figures 2 and 3).

[0024] A total of four experimental cases were conducted, including three types of drainage materials and one case where no countermeasures were used (Figure 4).

[0025] The model ground has a thick liquefaction layer, and the liquefaction layer (relative density D r = 30%) to 340 mm, and the non-liquefiable layer (D r =70%) was set to 50 mm.

[0026] In addition, the replacement rate of the drain was set to 100% to make it easier to examine the differences in the amount of ground subsidence and indentation settlement caused by the difference in the weight (dry density) of the drain material between the crushed shell drain and the gravel drain (see Figure 5).

[0027] (Experimental Procedure) To simulate the pouring of drains next to each other in succession, the ground improvement area was created by first excavating the existing ground and then backfilling it with drains.

[0028] After creating the model ground, a model structure simulating the weight of a two-story wooden detached house was placed on the ground improvement area, and the groundwater level was slowly raised (1.7 mm / min) to the ground surface. Horizontal vibrations were then applied with the acceleration waveform shown in Figure 6, a frequency of 5 Hz, and a vibration duration of 6 seconds. Figure 7 shows the state of ground subsidence after the vibration. It can be clearly seen that ground subsidence occurred in Case 3, where crushed stone was used as a drain.

[0029] (Ground subsidence and indentation subsidence after shaking) Before and after shaking, the distance between the model structure and the ground surface was measured using the top surface of soil tank 2 as the reference, and various amounts of settlement before and after shaking were calculated. Figures 8 to 10 compare the amount of ground settlement, amount of embedment settlement, and amount of change in colored sand line for each case.

[0030] Furthermore, Figure 11 shows the excess pore water pressure ratio for each case at pore water pressure gauge W1 in a representative ground improvement area.

[0031] (Conclusion) It was shown that the amount of ground subsidence, the amount of sinking, and the amount of change in the ground (subsidence amount) after liquefaction can be reduced in cases where ground improvement measures are implemented using crushed shell drains or crushed stone drains (Case 1 to Case 3) compared to no measures (Case 4).

[0032] In Case 3, it was clearly shown that crushed shell drains can reduce the amount of ground subsidence, etc., compared to gravel drains.

[0033] In other words, it was shown that Case 2, in which the weight (dry density) of the crushed shell drain is small, can further reduce the amount of ground subsidence, etc.

[0034] These results clearly show that if crushed shells, especially those with low dry density, are used to lay crushed shell drains for ground improvement, the amount of ground subsidence can be significantly reduced in the event of liquefaction.

[0035] Next, the second experiment will be explained. "Second Experiment" When crushed shells are used in place of crushed stone to apply the pore water pressure dissipation method, the following issues arise: a) whether the crushed shell drain's pore water pressure dissipation effect can be confirmed; and b) whether the shell particles will be ejected to the ground surface in response to the upward seepage water pressure due to the low dry density of the crushed shells used as the drain material.

[0036] Question a is whether crushed shells have the same pore water pressure dissipation effect as crushed stone, and question b is a problem specific to shells: because shell particles are flat, compaction is difficult when they are of a single grain size, and their dry density is low. In other words, the question is whether the upward seepage water pressure caused by excess pore water pressure generated during an earthquake will cause shell particles to erupt onto the ground surface (boiling phenomenon).

[0037] In response to the issues a and b above, a shaking table experiment simulating a level 1 earthquake motion was conducted to confirm the effect of dissipating excess pore water pressure, and based on the results, the stability of the crushed shell drain against the eruption of shell particles was analyzed.

[0038] (Shaking table experiment) (1) Sample The samples used were scallop shells from Aomori Prefecture, which were available in relatively large quantities at the time of construction. The shells were thoroughly washed with water, dried in a drying oven at 80°C for 24 hours, and then crushed after allowing the shells to cool to room temperature.

[0039] Two types of crushed shell samples with different particle size distributions were prepared for the drainage material and physical tests were conducted. Crushed shell A was a sample containing fine particles with a particle size of 4.75 mm or less (uniformity coefficient U c =4.79).

[0040] Crushed shell B is a sample with a particle size of 19 mm or less and 4.75 mm or more (U c = 2.21). These materials are difficult to compact when they are of a single grain size because the particles are flat, have many voids, and do not contain powder.

[0041] In the construction of the gravel drain method, the material is only compacted by vibration, and is not compacted with high pressure. For this reason, the relative density in the production of the crushed shell drain model was set to Dr = 60%. Toyoura sand was used for the liquefaction layer, with a maximum particle size of 0.25 mm (U c= 1.94) and Dr = 30%. Figure 12 shows the physical properties of the sample, and Figure 13 shows the particle size accumulation curve.

[0042] The results of a known test were used to determine the permeability coefficient of crushed shell B. This known test is based on an indoor permeability test method other than the Geotechnical Society's standards and criteria, and is suitable for coarse-grained soils mainly consisting of gravel, and highly permeable materials such as crushed shells.

[0043] According to Figure 12, the permeability coefficient k = 1.43 × 10 -1 m / s, and the required hydraulic conductivity (k>5×10 -2 ~1.5×10 -1 m / s) is satisfied.

[0044] On the other hand, the hydraulic conductivity of crushed shell A is k = 4.97 × 10 -3 m / s, which does not meet the recommended value in the PWB manual, but compared to Toyoura sand, which simulates a liquefied layer, it has a permeability about 35 times greater.

[0045] (2) Overview of the experimental soil tank Figures 14 and 15 show the shaking table model testing equipment for the second experiment. Soil tank 2 is fixed to shaking table 1, and the two work together to create a mechanism that can provide a predetermined hydraulic head difference using water injection tank 3. The measurement items were response acceleration and excess pore water pressure, with measuring instruments installed in the positions shown in the figure. The dimensions of the experimental soil tank, shaking table, etc. are shown below. Soil tank: Width 200mm x Length 1000mm x Height 480mm Shaking table: Medium-scale earthquake motion (Level 1) Acceleration (cm / s 2 ) Frequency 5Hz, vibration time 6 seconds Measuring instruments: Accelerometer A1, pore water pressure gauge W1~W6

[0046] The model crushed shell drain (Dr = 60%) was fabricated as a semicircular column using a formwork so that the excess pore water pressure inside the drain could be measured, and pore water pressure gauges W1 and W3 were installed on the side of the experimental soil tank, aligned with the center line of the drain.

[0047] The depth positions of the pore water pressure gauges were set at 4 cm from the ground surface and 14 cm from the ground surface near the bottom of the liquefied layer so that the excess pore water pressure of the model ground could be measured stably.

[0048] The Toyoura sand used in the model ground was adjusted to Dr = 30% in the liquefied layer and Dr = 70% in the non-liquefied layer. Figure 16 shows the acceleration waveform of the test soil tank in this experimental device.

[0049] (3) Experimental conditions Figure 17 shows the experimental cases for this experiment. Three cases were tested: one with a model crushed shell drain made from crushed shell samples A and B, and one with no countermeasures. This allowed us to verify the stability of shell particles erupting to the ground surface against the seepage water pressure during an earthquake, taking into account the differences in permeability and weight of drain materials made from crushed shells. The water level in soil tank 2 was set at the soil layer surface in soil tank 2 in all cases to eliminate the effects of capillary rise. The diameter of the model crushed shell drain was set to 90 mm, which is approximately five times the maximum particle size of crushed shells, 19 mm.

[0050] This experiment was carried out in the following procedure when a crushed shell drain was available. 1) The model ground was created by compacting a non-liquefiable layer (Toyoura sand Dr = 70%) with a thickness of 200 mm using a vibrator at 5 cm per layer. 2) A formwork for making model drains was fixed to the side of the two soil tanks, and a liquefied layer (Toyoura sand Dr = 30%) was made to a thickness of 190 mm using the air-drop method. 3) To prevent boiling, water was poured in over a sufficient period of time to allow the water level to rise to the soil surface, and then slowly drained. 4) While removing the formwork for making the drain, crushed shells were added in layers of 3 cm each, and compacted to a target Dr = 60% to create a columnar drain. 5) Water was refilled and the water level was raised to the soil surface. 6) Install the measuring instruments at the positions shown in Figure 15 and subject them to an acceleration of 200 cm / s 2 The specimen was vibrated at a frequency of 5 Hz for 6 seconds.

[0051] (4) Evaluation formula for seepage pressure and shell particle ejection The phenomenon that occurs in actual ground during an earthquake will be explained using soil tank 2 in the shaking table experiment. When soil tank 2 used in the experiment is shaken, excess pore water pressure is generated, and an upward hydraulic gradient i occurs, and the pore water starts to flow toward the surface of soil tank 2. At this time, when the excess pore water pressure generated in the crushed shell drain becomes equal to the initial effective overburden pressure of the drain, the hydraulic gradient i also reaches the critical hydraulic gradient i. cr The problem is that the temperature reaches a certain level, causing shell particles to erupt onto the surface (boiling phenomenon).

[0052] This phenomenon is known as sand boiling during liquefaction, and the reason why sand boiling can be seen even after the earthquake motion has ceased is that in actual ground, there is pavement or other material on the surface, and it takes time for the excess pore water pressure in the ground to dissipate.

[0053] In order to quantitatively grasp the danger of this boiling phenomenon (eruption of shell particles), the relationship between the infiltration water pressure and the effective overburden pressure can be expressed using the safety factor Fs. If we use the pore water pressure gauges W1 and W3 at the model crushed shell drain locations as the target, we get Equation 1.

[0054]

number

[0055] In addition, a schematic diagram of the effective overburden pressure of the crushed shell drain in Equation 1 and the infiltration water pressure due to the excess pore water pressure difference is shown in Figure 18.

[0056] (5) Experimental results (dissipation effect of excess pore water pressure) First, we will use the experimental results to confirm whether crushed shells are suitable as drain materials for the pore water pressure dissipation method. This suitability requires a quantitative evaluation of the excess pore water pressure dissipation effect. Here, we will evaluate this using the excess pore water pressure ratio in Equation 2. (Formula 2) Excess pore water pressure ratio = Excess pore water pressure (kPa) / Initial effective overburden pressure (kPa)

[0057] In Equation 2, when the excess pore water pressure ratio at the pore water pressure gauge installation position is 1, the effective stress σ' is z = 0, and it can be determined that liquefaction has occurred.

[0058] Figure 19 shows the excess pore water pressure ratios for Cases 1 and 2, where crushed shell drains were installed, and Case 3, where no countermeasures were taken.

[0059] According to Figure 19, in Case 3 (no countermeasures), the Toyoura sand, which simulates a liquefied layer, recorded an excess pore water pressure ratio of >0.95, except for the W3 pore water pressure gauge, and the occurrence of liquefaction can be confirmed.

[0060] On the other hand, in Cases 1 and 2, where crushed shell drains made from crushed shells, which have better permeability than Toyoura sand, were installed, the excess pore water pressure ratio was maintained at less than 1 in all pore water pressure gauges W1 to W4. This means that no liquefaction phenomenon occurred, and although it only accounts for 0.16% of the area of ​​test soil tank 2, the crushed shell drains were effective in dissipating excess pore water pressure.

[0061] In this experiment, Case 1, which has a lower hydraulic conductivity than Case 2, shows a tendency for the excess pore water pressure ratio to be smaller. This is thought to be due to the fact that the difference in hydraulic conductivity between the crushed shells and Toyoura sand in Case 1 was originally large at about 35 times, and the Toyoura sand is homogeneous, resulting in a clear drainage effect, and the submerged unit volume weight of the crushed shells is about 1.5 times larger than that of Case 2.

[0062] (6) Experimental results (stability against seepage water pressure) Here, we examine the stability of shell particle eruptions against upward seepage water pressure caused by excess pore water pressure.

[0063] First, Figure 20 shows the difference in excess pore water pressure u between W3 and W1. According to Figure 20, the pore water pressure difference between Case 1 and Case 2, where the crushed shell drains of the model were installed, is smaller than Case 3, where no countermeasures were taken, and the effect of pore water pressure dissipation due to drainage can be seen. Notably, there is no significant difference in the pore water pressure difference between the two cases for about 6 seconds after the start of vibration, but from 6 seconds after the end of vibration, there is a tendency for the pore water pressure difference in Case 2 to be clearly smaller. This is thought to be due to the permeability of the crushed shells that make up the crushed shell drains.

[0064] That is, the hydraulic conductivity of the crushed shells in Case 2 (k = 1.43 × 10 -1 m / s) is that of Case 1 (k=4.97×10 -3 m / s), the pore water pressure dissipates quickly and the pore water pressure difference tends to be small.

[0065] Next, the difference in pore water pressure was used to calculate the head difference, which was then substituted into the aforementioned equation 1. Figure 21 shows the result of expressing the stability of the shell particles against their eruption to the surface as a safety factor based on the relationship between the infiltration water pressure and the effective overburden pressure.

[0066] In Figure 21, the safety factor Fs against seepage water pressure for Cases 1 and 2 remains at a high value of generally Fs > 2 at all times from the start to the end of excitation. In other words, the safety factor Fs against seepage water pressure shows a high value.

[0067] This confirmed that the infiltration water pressure did not exceed the effective overburden pressure of the crushed shells, and no eruption of shell particles occurred.

[0068] The safety factors of Case 1 and Case 2 reverse at the 6-second mark when the excitation ended. This is because there is not much difference in the head difference between Case 1 and Case 2 up until about 6 seconds into the excitation, so the safety factor of Case 1 is higher due to the larger submerged unit volume. After the excitation ended, the pore water pressure dissipated quickly, resulting in a smaller head difference, and so the safety factor of Case 2 was higher.

[0069] summary In order to address the issues involved in applying crushed shells to pore water pressure dissipation works, we conducted a shaking table experiment simulating a level 1 earthquake to verify the effectiveness of crushed shell drains in dissipating pore water pressure and the stability of shell particles ejected to the ground surface against seepage water pressure. The main conclusions are as follows:

[0070] 1) In the shaking table experiment, when crushed shell drains were installed (Case 1 and Case 2), the excess pore water pressure ratio was less than 1, and liquefaction did not occur. This confirmed that crushed shells could be used as a material that can effectively dissipate pore water pressure.

[0071] 2) In the shaking table experiment, no visible ejection of shell particles was observed in the test soil tank. It was also confirmed that the safety factor Fs against seepage water pressure remained at a high value of Fs > 2 throughout the entire process, including the dissipation of pore water pressure from the start of shaking to the end of shaking. This confirmed that the seepage water pressure never exceeded the effective overburden pressure of the crushed shells, and no ejection of shell particles occurred.

[0072] 3) In the shaking table experiment, judging from Figure 21, which shows the safety factor for the stability of shell particles erupting onto the ground surface against infiltration water pressure, it was confirmed that the safety factor is affected not only by the hydraulic conductivity of the crushed shells, but also by the unit volume weight of the crushed shells, in other words, the dry density of the crushed shells.

[0073] 4) Shaking table experiments confirmed that crushed shell drains can potentially be used as a material that can dissipate pore water pressure in the same way as conventional crushed stone, provided that the density of the crushed shell particles used is similar to that of crushed stone and the material is highly permeable and has a drainage path.

[0074] Next, the third experiment will be explained. "Third Experiment" The third experiment was conducted to confirm the stability of crushed shells during the pore water pressure dissipation process, and verified what hydraulic conductivity and dry density (weight) of crushed shells would have the high stability (high safety factor) described above.

[0075] (Experiment overview) The effect of dissipating excess pore water pressure in the crushed shells was confirmed, and then the stability against seepage water pressure was confirmed. In this experiment, crushed shells of 4.75 mm or less and 4.75-19 mm were used.

[0076] First, a non-liquefied layer was created as a model ground, and then a formwork for a semicircular cross-section drain was fixed to the side of soil tank 2. Then, a liquefied layer was created at the same height as the drain length.

[0077] While the formwork was being removed, crushed shells were added and compacted to create a crushed shell drain. The experimental setup for the third experiment is shown in Figure 22.

[0078] (Experimental Procedure) As mentioned above, crushed shells were prepared in the shape of a semicircular column on the side of the soil tank 2, and in order to measure the pore water pressure inside the outer crushed shells, two pore water pressure gauges 4 were set up on the center line of the crushed shells. After that, the groundwater level was raised to the ground surface, and then vibrations were applied with a medium-scale earthquake motion.

[0079] The experimental cases used in this experiment are shown in Figure 23. Case 1 uses small crushed shell particle sizes, Case 2 uses large crushed shell particle sizes, and Case 3 uses no crushed shells and no countermeasures. We then compared the stability associated with seepage water pressure, i.e., the safety factor, due to differences in the dry density (weight) of the crushed shells and their hydraulic conductivity.

[0080] (Experimental results) First, the excess pore water pressure ratio (hereinafter referred to as the water pressure ratio) was confirmed from the experimental results. When the water pressure ratio is 1, the effective stress of the ground is 0.

[0081] Figure 24 shows a time-gravel comparison of the water pressure ratio for each case in W1 and W3. From this, in the case of Case 3 (no countermeasures), the water pressure ratio was recorded as approximately 1 in W1 and W3, confirming that liquefaction had occurred.

[0082] On the other hand, in Cases 1 and 2, where crushed shells were cast, the water pressure ratio was significantly below 1, so it is believed that liquefaction did not occur. In other words, the casting of crushed shells promoted drainage, and the liquefaction-suppressing effect of the rapid dissipation of excess pore water pressure was confirmed.

[0083] However, when the sum of the seepage water pressure and buoyancy force that tries to pull up the crushed shell particles during the process of excess pore water pressure dissipation reaches a critical value equal to the weight of the crushed shells, the effective stress becomes zero, and there is a concern that boiling may occur.

[0084] The degree to which the crushed shells are stable against this possibility can be expressed as the safety factor Fs, which can be determined using the formula for calculating the safety factor Fs mentioned above, i.e., Equation 1 mentioned above. JPEG2025172418000004.jpg3677

[0085] where: i cr : Critical hydraulic gradient of crushed shell drain i: hydraulic gradient of the crushed shell drain γ': Unit weight of crushed shell in water gamma w : unit weight of water L: Distance traveled by infiltrated water (L = 10 cm) h: water head difference

[0086] This is the critical hydraulic gradient i due to the submerged unit weight of the crushed shells in the drain material. cr This is the ratio of the hydraulic gradient i caused by the upward seepage water pressure generated within the drain to the safety factor Fs = 1, which indicates that when the hydraulic gradient generated within the crushed shells reaches the critical hydraulic gradient, boiling (eruption) of the crushed shell drain may occur.

[0087] As can be seen from Figure 22, the flow of calculating the safety factor is as follows: first, the difference in excess pore water pressure u generated at W3 and W1 is calculated using the pore water pressure gauge 4 installed inside the crushed shell. Then, the resulting head difference is found, and other material parameters are substituted to find the safety factor for each elapsed time.

[0088] As a result, as shown in Figure 25, it can be seen that the difference in excess pore water pressure is greatest in Case 3, while it is small in Cases 1 and 2.

[0089] Furthermore, Figure 26 shows that the safety factor for Case 3 is around 1, a low value. Therefore, it can be determined that in the case of no countermeasures, as shown in Case 3, there is a high possibility of boiling occurring. On the other hand, since the safety factors for Cases 2 and 3 are greater than 1, it can be determined that upward seepage water pressure is occurring within the crushed shells in an extremely small range that does not reach the critical hydraulic gradient of each drain material. And, as mentioned above, crushed shells, which have a high safety factor, are lighter in weight than crushed stone, so the feared boiling will not occur, and it can be confirmed that they do not lose stability against seepage water pressure.

[0090] Here, the relationship between the dry density of the crushed shells and the hydraulic conductivity of the crushed shells, and the relationship between the hydraulic conductivity of the crushed shells and the safety factor will be explained with reference to FIG.

[0091] As shown in Figure 27, if the dry density of the crushed shell is high, the hydraulic conductivity of the crushed shell is low. For example, if the dry density of the crushed shell is 1.5 (Mg / m 3 ), the hydraulic conductivity of the crushed shells is approximately 1.0 × 10 -4 m / s. If the dry density of the crushed shell is small, the hydraulic conductivity of the crushed shell will be large. For example, if the dry density of the crushed shell is about 0.9 (Mg / m 3 ), the hydraulic conductivity of the crushed shells is approximately 1.5×10 -4 It is m / s.

[0092] Furthermore, the high dry density of crushed shells can be said to indicate that the particle size of the crushed shells is small, and the low hydraulic conductivity of crushed shells can be said to indicate that the particle size is small.

[0093] The relationship between the hydraulic conductivity and the safety factor will be explained with reference to Figure 27. The hydraulic conductivity of the crushed shells is 7.0 × 10 -3 As shown by the arrow in Figure 27, it can be seen that the safety factor drops sharply towards the left at m / s.

[0094] In addition, the hydraulic conductivity of the crushed shells is approximately 7.0 × 10 -3It can be seen that the safety factor is almost the highest when measured in m / s.

[0095] Therefore, the crushed shells used as drainage materials have a minimum hydraulic conductivity k min =7.0×10 -3 It can be concluded that it is extremely desirable to define the permeability coefficient k as m / s and use crushed shells with a permeability coefficient k equal to or greater than this.

[0096] As mentioned above, crushed shells have a smaller rate of decline in drainage performance than crushed stone, so they can still be effective as a drain even if their hydraulic conductivity is lower than the minimum hydraulic conductivity indicated for crushed stone drain material.

[0097] (Conclusion) As described above, the first to third experiments have made it possible to easily select crushed shells with a low dry density and a hydraulic conductivity that prevents the infiltration pressure due to excess pore water pressure difference from becoming higher than the effective overburden pressure. Therefore, by laying crushed shell drains using the crushed shells selected in the first to third experiments and carrying out actual ground improvement, it has become possible to reliably suppress ejecta and ground subsidence in the improved ground. [Explanation of symbols]

[0098] 1. Shaking table 2 Earthen tank 3 Water tank 4. Pore pressure gauge

Claims

1. This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. After liquefaction, crushed shells having a hydraulic conductivity such that the seepage water pressure due to the excess pore water pressure difference of the crushed shell drain laid in the ground does not become higher than the effective overburden pressure of the crushed shell drain were selected, and the crushed shell drain formed from the selected crushed shells was laid to carry out ground improvement. A system for suppressing ejecta and subsidence in improved ground.

2. This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. After liquefaction, crushed shells with a low dry density and a hydraulic conductivity that ensures that the seepage water pressure due to the excess pore water pressure difference of the crushed shell drain laid in the ground will not become higher than the effective overburden pressure of the crushed shell drain were selected, and the crushed shell drain formed from the selected crushed shells was laid to carry out ground improvement. A system for suppressing ejecta and subsidence in improved ground.

3. This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. The ground that has been improved by laying the crushed shell drain is reproduced, and the reproduced ground is vibrated to cause liquefaction. After liquefaction, the effective overburden pressure of the crushed shell drain and the seepage water pressure due to the excess pore water pressure difference are determined. The crushed shells having a permeability coefficient that the seepage water pressure due to the excess pore water pressure difference does not become higher than the effective overburden pressure are selected, and the crushed shell drain formed from the selected crushed shells is laid to carry out actual ground improvement. A system for suppressing ejecta and subsidence in improved ground.

4. This is a system to prevent ejecta from the improved ground surface and prevent ground subsidence due to liquefaction during an earthquake in ground that has been improved by laying crushed shell drains. The ground improved using the crushed shells is reproduced, the reproduced ground is vibrated to cause liquefaction, the effective overburden pressure of the crushed shells and the seepage water pressure due to the difference in excess pore water pressure are determined after liquefaction, crushed shells having a hydraulic conductivity such that the seepage water pressure due to the difference in excess pore water pressure does not become higher than the effective overburden pressure and having a low dry density are selected, and a crushed shell drain formed from the selected crushed shells is laid to carry out actual ground improvement. A system for suppressing ejecta and subsidence in improved ground.

5. The seepage water pressure due to the difference between the effective overburden pressure and the excess pore water pressure of the crushed shells is calculated, and the crushed shells having a hydraulic conductivity such that the seepage water pressure due to the excess pore water pressure difference is not higher than the effective overburden pressure are selected by calculating the safety factor using the formula below for calculating the safety factor related to ejecta suppression, and selecting crushed shells having a hydraulic conductivity with a high safety factor:

4. The improved ground ejecta suppression and ground subsidence suppression system according to claim 3. (Formula for calculating safety factor Fs) where: i cr : Critical hydraulic gradient of crushed shell drain i: hydraulic gradient of the crushed shell drain γ': Unit weight of crushed shells in water gamma w : unit volume weight of water L: Distance traveled by infiltrating water (L = 10 cm) h: water head difference

Images