JPH0230806A - Liquefaction preventive construction for sandy ground at time of earthquake - Google Patents

Abstract

PURPOSE:To effectively prevent sandy ground from being liquefied, without pollution by arranging earth anchors between a pressure-resisting board set under the foundation of a structure and strong ground under the sandy ground, and by introducing prestress. CONSTITUTION:At the lower section and on the periphery of a structure 2 constructed on relaxed sandy ground forecast to be liquefied at the time of earthquake, a pressure-resisting board 3 made of reinforced concrete is set. Then, earth anchors 4 are driven into the pressure-resisting board 3, and the lower ends of the anchors 4 are set on the strong ground 5 of generating no liquefaction, and the earth anchors 4 are stretched and are set on the pressure- resisting board 3, and into the sandy ground 1, prestress is introduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a construction method for preventing liquefaction of sandy ground during an earthquake.

(Conventional technology) During a major earthquake, sand can be blown out along with groundwater at landfill sites, and the ground can become fluid, causing structures built on top to sink or topple over. This phenomenon is generally referred to as liquefaction of sandy ground, and has become a major problem since the Niigata earthquake. It is said that this liquefaction phenomenon tends to occur in saturated, loose sandy ground, and in the zero soil layer at least 20 meters from the ground surface.

Liquefaction generally refers to excessive pore water pressure that occurs in the ground due to repeated shear stress acting within the ground (the pressure of water between soil particles when the pore water pressure in the ground abnormally increases due to an earthquake, etc.)
becomes equal to the initial effective stress that restrains the soil particles (the stress that effectively acts on the compression of the skeleton of the soil particles and the friction between the soil particles, before it changes), and as a result, the effective stress becomes It is defined as the phenomenon of becoming O. Therefore, when this state is reached, the soil particles are no longer restrained, the shear resistance of the sandy ground is almost completely eliminated, and the sand becomes liquid. ((1) below)
(See formula) When the sandy ground liquefies during an earthquake, structures with direct foundations lose their supporting capacity and sink or fall.

Furthermore, in the case of a structure based on a pile foundation, the horizontal resistance of the piles is lost and the piles break, which may result in the structure overturning. Therefore, in places where loose sandy ground continues deep, such as in reclaimed land, or in sandy ground with a high groundwater level with an N value of about 25 or less, in the case of clean sand with a uniform particle size and not containing much clay, It is necessary to consider measures to prevent liquefaction in the design of structures.

Before considering ways to prevent sandy ground from liquefying, let's consider what kind of ground is prone to liquefaction.

Published by the Architectural Institute of Japan [Architectural Foundation Structure Design Guidelines Section 14.5 (p.
-163), the following conditions are listed as conditions for ground that is susceptible to liquefaction.

1) The lower the content of fine-grained soil (soil particles with a grain size of 0,074 m or less) in saturated ground, 2) The smaller the N value of saturated ground, 3) The closer the groundwater table is to the ground surface, the more likely it will be an earthquake. In order to prevent liquefaction as the input increases, the above conditions should be reversed, so basically the following countermeasures can be considered.

i) Increasing the shear strength of the ground ii) Increasing the density of the ground if) Decreasing the pore water pressure i) includes the injection method and the deep mixing method. Furthermore, if the liquefaction layer is shallow, a shallow layer mixing and stirring method can be adopted in which the liquefied layer is dug up and then filled back in by mixing and stirring the consolidation material. Both methods aim to increase shear strength by consolidating the ground.

11) Compaction methods include sand compaction and vibroflotation.

These construction methods increase the density of the ground (which increases the N value), resulting in an increase in shear strength.

Typical examples include gravel pillars created by filling excavated holes in the ground with gravel, and gravel drains that use gravel walls to extract pore water in the ground and reduce pore water pressure. There is a construction method.

(Problem to be solved by the invention) The typical construction methods currently used as measures to prevent liquefaction are as described above, but for example, the ground consolidation method increases construction costs and The construction method has problems with pollution such as noise and vibration, and furthermore, with the Geberbeld and Lane construction methods, it is difficult to extract pore water uniformly over the entire ground.

The present invention has been proposed in view of the problems of the prior art, and its purpose is to effectively prevent liquefaction of sandy ground during earthquakes without causing pollution problems. The point is to provide a liquefaction prevention construction method.

(Means for Solving the Problem) In order to achieve the above-mentioned object, the method for preventing liquefaction of sandy ground according to the present invention provides a method for preventing liquefaction of sandy ground in the lower part of the foundation of a structure on sandy ground that is expected to liquefy during an earthquake, and in the surrounding area. An earth anchor whose lower end is anchored to the same ground is placed between the horizontally arranged pressure plate and the firm ground below, and the anchor is tensioned to fix the upper end to the pressure plate. The structure is designed to introduce prestress into the ground below the pressure plate to prevent liquefaction of the sandy ground during an earthquake.

(Function) According to the present invention, there is a gap between a pressure plate installed horizontally under and around the foundation of a structure on sandy ground that is expected to liquefy in the event of an earthquake, and the underlying solid ground that will not liquefy. By tensioning the earth anchor, which is disposed across the ground and whose lower end is fixed to the firm ground, and whose upper end is fixed to the pressure plate, the ground anchor is placed between the pressure plate and the firm ground. Prestress is introduced into the sandy ground that is expected to liquefy, and the pressure plate suppresses the ground, resulting in an increase in the effective stress within the same ground, and a concomitant increase in the shear strength of the ground. Liquefaction during earthquakes is prevented.

(Example) The present invention will be described below with reference to the illustrated embodiments.

Figure 1 shows the case where the present invention is applied to the construction of a structure with a direct foundation, where (1) is a loose sandy ground that is expected to liquefy in the event of an earthquake, and (2) is on the same sandy ground (1). In the structure to be constructed, a pressure plate (3) made of reinforced concrete will be installed at the bottom and around the foundation, and an earth anchor (4) will be driven into the pressure plate (3) to ensure that the lower end is solid and will not cause liquefaction. It takes root on solid ground (5).

In the figure, (4a) shows the fixing part of the earth anchor (4).

Then, tighten the earth anchor (4) and attach it to the pressure plate (3).
When it settles on the sandy ground (1), prestress is introduced into the sandy ground (1), and the sandy ground (1) is suppressed by the pressure plate (3). (1) The effective stress within increases.

Letting this effective stress be σ, there is the following relationship between it and the shear strength τ of the sandy ground.

τ=σ tan φ +C...(1)σ=σ −・
...(2) However, φ: Angle of internal friction of the soil C: Adhesive force of the soil σ': Total stress acting on the soil U: Pore water pressure in the soil -II The increase in ground density due to compaction means that the internal frictional force As a result, the shear strength of the ground increases. In the case of this method, the shear strength of the sandy ground (1) is increased by increasing the effective stress of the sandy ground (1).

FIG. 2 shows a case in which the present invention is applied to construction of a structure having a pile foundation, in which (6) indicates a pile, and other parts equivalent to those in the above embodiment are given the same reference numerals.

Figures 3 to 8 show the process in which the present invention is applied to a pile foundation, in which a resistor (6) is constructed on solid ground (5) that does not cause liquefaction (see Figure 3). ) Next, install the earth anchor (4) (see Figure 4).
4) The lower end is anchored to the firm ground (5) described above, and the foundation and underground portion are excavated. (See Fig. 5) Next, install a horizontal pressure plate (3) on the excavated portion (7), and (see Fig. 6) earth anchor (4) (7) PC
A tensioning device (with steel material (4b) set on a pressure plate (3)
8), I was nervous, and the fixing tool (9) was attached to the same pressure plate (3).
), and the sandy ground (1) below the pressure plate (3)
The shear strength of the sandy ground (1) is increased by introducing prestress into it (Fig. 7), and then the base of the structure! (10) and a frame (not shown).

(Effects of the Invention) According to the present invention, as described above, there is a gap between the lower part of the foundation of a structure on sandy ground where liquefaction is expected, the pressure plate horizontally arranged around the periphery, and the solid ground below. An earth anchor with its lower end fixed to the same ground is placed over the entire area, and the earth anchor is tensioned and its upper end is fixed to the pressure plate, thereby uniformly prestressing the sandy ground through the pressure plate. The present invention can be used to increase the effective stress of the sandy ground, thereby increasing the shear strength of the sandy ground and preventing liquefaction.According to the present invention, construction is easy and noise and vibration are reduced. It is possible to effectively prevent liquefaction of sandy ground without causing pollution.

[Brief explanation of the drawing]

Figures 1 and 2 are longitudinal cross-sectional views showing the implementation status of each embodiment of the method of preventing liquefaction of sandy ground during earthquakes according to the present invention, and Figures 3 to 8 are steps of the method of the present invention. FIG. (1)-m-Sandy ground expected to liquefy, (2)--1 structure, (3)-11 pressure plate, (4)--earth anchor, (5)--1 solid Ground, (8) - one tension device, (9L fixed equipment, (10) - m = foundation of the structure S no W slope

Claims (1)

[Claims] Grounding with the lower end anchored to the same ground between the lower part of the foundation of a structure on sandy ground that is expected to liquefy during an earthquake, the surrounding pressure plate placed horizontally, and the firm ground below. A method for preventing liquefaction of sandy ground during an earthquake, characterized by installing an anchor, tightening the anchor, fixing the upper end to the pressure plate, and introducing prestress into the ground below the pressure plate.