JPH051327B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is directed to the liquefaction of sandy ground, which prevents structures constructed on or near sandy ground from liquefaction of sandy ground caused by earthquakes, etc. This is related to the anti-oxidation method.

[Conventional technology and its problems]

When loose saturated sand ground undergoes repeated shear deformation in an almost undrained state during an earthquake, the water pressure increases and the contact force between sand particles, that is, the effective stress of the sand, decreases. = It appears as a so-called liquefaction phenomenon in which it floats in water and behaves as if it were a liquid with a high specific gravity. When liquefaction occurs in this sandy ground, the supporting capacity of the ground decreases, and structures built on or very close to the sandy ground may collapse or be damaged. This has been proven by the overturning of reinforced concrete apartment buildings, the collapse of bridges, and the surfacing of underground objects during the Niigata Earthquake.

In order to prevent the sandy ground from losing its strength and becoming liquefied due to repeated loads such as earthquakes, a vibration compaction method has been proposed in which the sandy ground is compacted in advance to increase its resistance to liquefaction. However, even if the sandy ground can be improved by this method, vibration noise is generated due to the method, which may cause deformation of the surrounding ground.In addition to the generation of vibration noise pollution, it also has a considerable impact on existing structures. Become something.

In order to prevent this, crushed stone drain construction methods have been proposed in recent years. This crushed stone drain construction method involves drilling a hole to a predetermined depth in sandy ground or other ground where there is a risk of liquefaction, and then inserting crushed stone into the hole to build a crushed stone pile in the ground. This is to stabilize the ground by suppressing and dissipating excess water. However, when this crushed stone drain (crushed stone pile) effectively exhibits the water permeability as designed, it has the effect of suppressing and dissipating excess interstitial water.
When sand particles with a smaller diameter than the crushed stone enter the crushed stone and cause clogging, the water permeability effect naturally decreases. In order to prevent clogging in this conventional crushed stone drain construction method, the standard set by the road earthwork guidelines, etc., is 15% particle size of crushed stone (filter material) / 85% of sand ground (material protected by filter) Although a grain size of <5 is adopted, various experiments have revealed that it is difficult to prevent ground liquefaction with this standard value.

Therefore, in order for a crushed stone drain to exhibit its liquefaction prevention function, it is necessary to select crushed stone as the drain material, which does not cause clogging during liquefaction and at the same time has sufficient drainage performance compared to the water permeability of the ground. be.
However, in the past, the particle size of crushed stone was selected based solely on workability, or crushed stone was selected with consideration only to prevention of clogging. In addition, when considering workability, the particle size becomes excessively large and causes clogging; on the other hand, even if consideration is given to clogging, safety standards for clogging prevention (road earthwork guidelines, etc.)
It was not possible to ensure sufficient water permeability to adopt a safety factor of approximately 2).

[Means for solving problems]

Based on various experimental results, the applicant of the present application has noticed that by using crushed stone in a crushed stone drain whose particle size corresponds to the particle size of surrounding sand particles, clogging can be prevented and good water permeability can be maintained. In the present invention, the crushed stone particle size of a crushed stone drain to prevent liquefaction constructed in sandy ground is determined as follows: 15% particle size of crushed stone/85% particle size of sandy ground<9 15% particle size of crushed stone/15% of sandy ground It is characterized by constructing crushed stone piles of the required diameter in sandy ground by setting the grain size to a range of >20.

〔Example〕

Pile diameter in sandy ground with average grain size of 0.25 mm and groundwater level of 1.0 m.
Construct a crushed stone drain D with a length of 40 cm and a pile length of 15 m, and install a vibrating rod (vibrator) L 1 m from this crushed stone drain.
An experiment was conducted in which the ground was penetrated at a remote location and artificially excited to forcefully increase the excess water pressure in the ground, causing the ground to liquefy. The experimental layout is shown in Figure 1, and the results of the experiment are shown in Figure 2.

In Figure 2, the water pressure increases rapidly due to the vibration, and reaches its maximum value after about 30 seconds, but after that, a rapid drop in pressure is observed and the amount of water discharged gradually increases. It is therefore clear that a dissipation of water pressure occurs.

Furthermore, Fig. 3 shows the results of comparing the dissipation process of the water pressure with reference to the peak time of the water pressure. In FIG. 3, the time axis indicates the peak time of the water pressure as 0 seconds.

By the way, the conventional standards for clogging are filter standards for drainage materials (for example, road construction guidelines): 15% particle size of crushed stone (filter material) / 85% particle size of sandy ground (material protected by filter) <5 However, according to the results of clogging experiments, the filter standards for conventional drainage materials are set on the very safe side, and crushed stone in the range of 15% particle size of crushed stone / 85% particle size of sandy ground <9 By using this method, it is possible to sufficiently prevent a decrease in water permeability due to clogging and the eruption of liquefied sand.
It was found to be more effective in maintaining water permeability.

In addition, the hydraulic permeability of crushed stone drains changes as the flow shifts from laminar to turbulent flow depending on the magnitude of the hydraulic gradient, but the hydraulic gradient obtained in field vibration experiments is approximately 0.3. There is a possibility that the inside of the crushed stone drain is in a turbulent state. Therefore, a model ground in which crushed stone and sand ground were in contact was created inside the test machine.
As a result of conducting a large-scale model permeability test that can reproduce turbulent flow conditions, as shown in Figure 4, the excess water pressure generated during the earthquake did not remain in the crushed stone drain, and was approximately the same level as when a hollow well was constructed. In order to have a sufficient liquefaction prevention effect, that is, a water pressure dissipation effect, it is necessary for the hydraulic conductivity of crushed stone to be greater than 400. became clear.

Furthermore, the experiments of Sherard et al. (ASCE, 110
[GT6] (1984) (USA) “Basic Properties of
According to "Sand and Gravel Filters" p.684-700), the hydraulic conductivity of crushed stone and sand is proportional to the square of the 15% grain size (mm), and the hydraulic conductivity (cm/sec) = 0.35 x 15% grain size ( It is known that it is the square of mm).

Therefore, when this relationship is substituted for the hydraulic conductivity ratio required to prevent liquefaction of the ground, it becomes 15% particle size of crushed stone / 15% particle size of sandy ground > 20, ensuring the permeability necessary to prevent liquefaction of the ground. In order to do so, it was found that it was necessary to use crushed stone with a 15% particle size that was 20 times larger than the 15% particle size of the sandy ground.

〔Effect of the invention〕

According to the present invention, the crushed stone particle size of the crushed stone drain is set to a particle size corresponding to the particle size of sand particles in the ground, that is, 15% particle size of crushed stone/85% particle size of sandy ground <9 15% particle size of crushed stone/sand By setting the grain size within the range expressed by the relational expression of 15% grain size of the ground > 20, the permeability of the crushed stone drain is improved and clogging is prevented, thereby preventing liquefaction during an earthquake even in loose saturated ground. It has the advantage of being able to prevent problems.

[Brief explanation of the drawing]

Figure 1 is a layout diagram of the vibration experiment, Figure 2 is a graph showing the results of the actual ground vibration experiment, Figure 3 is a graph showing the dissipation of excess water pressure, and Figure 4 is between the central part of the crushed stone drain. It is a graph diagram showing the water pressure ratio. D is the crushed stone drain, L is the vibrating rod.

Claims (1)

[Claims] 1. In a construction method in which crushed stone drains are constructed at appropriate intervals in loose saturated sandy ground, the crushed stone particle size of the crushed stone drains is set to 15% of the crushed stone particle size/sand, corresponding to the sand grain size of the surrounding ground. A liquefaction prevention construction method for sandy ground characterized by constructing crushed stone piles within the following range: 85% particle size of ground <9 15% particle size of crushed stone/15% particle size of sandy ground>20.