JPH0577807B2 - - Google Patents

Description

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

(Prior art) Due to repeated shearing forces caused by earthquakes, water-saturated, low-density sandy ground loses its effective stress due to the sudden increase in the pressure of the interspace water between soil particles, causing it to become as if it were a liquid. It behaves like this. This phenomenon is called liquefaction phenomenon. Due to this liquefaction phenomenon,
Ground failure and structures toppling, destruction, and uneven settling have been reported in the Niigata Earthquake, the Chubu Japan Sea Earthquake, and other events.

One of the liquefaction prevention methods is the gravel drain method. This construction method uses a diameter of 40 mm in the ground.
~60cm gravel piles are placed at 1 to 2m intervals, with the aim of quickly dissipating excess water pressure that occurs during earthquakes.

(Problem to be solved by the invention) However, the effectiveness of this construction method varies depending on the permeability of the crushed stone used for the gravel drain, and if crushed stone with a high permeability coefficient is used, the sand particles in the soil There is a problem that the liquid flows into the water, causing a so-called clogging phenomenon. Furthermore, when crushed stone with a low hydraulic conductivity is used, there is a problem in that the drainage capacity is limited and liquefaction cannot be sufficiently prevented. Normally, it is considered appropriate for the hydraulic conductivity of crushed stone used for gravel drains to be 200 to 400 times that of the target ground, but this level of hydraulic conductivity causes a time delay in drainage, so the well resistance of the drain is must be taken into account. Recently, designs have been made that take this well resistance into consideration, but there is a problem in that the intervals between drains are smaller than in the past.

The purpose of this invention is to further improve the drainage capacity of gravel drains, and to liquefy sandy ground, which can effectively drain excess water that occurs during earthquakes even if the drainage interval is increased. The purpose is to provide a prevention method.

(Means for Solving the Problems) To explain the means of the present invention for achieving the above object, the present invention has a cross section that is approximately rectangular, and the opposing wide surfaces are the water flow plate surfaces. comprising a water-permeable material and a nonwoven fabric covering the surface of the water-permeable material,
A plurality of water passages continuous in the longitudinal direction are arranged in the water passage material in parallel in the width direction, partitioned by a plurality of ribs connecting the two water passage plate surfaces at intervals in the width direction, and A belt-like structure having a structure in which a large number of grooves are formed along the width direction of both water passage plates at intervals in the longitudinal direction, and each groove has a large number of water passage holes that communicate with the water passage. Using a drain, the sandy ground is excavated to a predetermined depth using a casing auger with a built-in mandrel for the belt-shaped drain, and when the auger is pulled up, the belt-shaped drain is installed in the sand, and then the belt-shaped drain is thrown from above. The invention is characterized in that a composite drain of the belt-shaped drain and the gravel drain is formed by surrounding the belt-shaped drain with crushed stone to create a gravel drain.

(Function) When a special strip-shaped drain is installed in a gravel drain to create a composite structure, the filter action of the non-woven fabric on the surface of the strip-shaped drain and the multiple ribs between the two water-passing plate surfaces of the water-passing material and the The structure prevents the water passageway from being crushed by the confining pressure, which secures the water passageway, increases the drainage capacity along with the drainage capacity of the gravel drain, and efficiently drains excess water at intervals during an earthquake. In addition,
Liquefaction can be effectively prevented. Furthermore, since the drainage capacity is increased, there is no need to consider well resistance, and effective drainage can be performed even when the drain interval is large.

(Example) Examples of the present invention will be described in detail below with reference to the drawings.

In the present invention, an improved belt-shaped drain conventionally used to promote consolidation of soft ground is used, so first, see FIGS. 3 to 6 for specific examples of the structure of the novel belt-shaped drain material 1 used in the present invention. and explain.

This belt-shaped drain 1 is composed of a water-permeable material 2 whose cross section is approximately rectangular and whose opposing wide surfaces are water-permeable plate surfaces 2A, and a nonwoven fabric 3 that covers the surface of the water-permeable material 2. has been done. Water-permeable material 2 has a width W of 100~
It has a rectangular cross-sectional shape of 150 mm and a thickness T of about 10 to 15 mm. In the water passage material 2, there are a plurality of water passages 2B that are continuous in the longitudinal direction and partitioned by a plurality of ribs 2C that connect both water passage plate surfaces 2A at intervals in the width direction. They are installed in parallel. There are 5 grooves 2D along the width direction on both water passage plate surfaces 2A.
A large number of holes are formed with a width of about mm and spaced apart from each other by about 5 mm in the longitudinal direction. The grooves 2D are formed so as to exist in the longitudinal direction of both water passage plate surfaces 2A. A large number of rectangular water passage holes 2E are formed in each groove 2D along the longitudinal direction of the groove 2D. The water-permeable material 2 is made of a resin material such as vinyl chloride or polyethylene whose hardness and softness can be controlled by adjusting the composition.

Such a belt-shaped drain 1 is formed by partitioning the water passage 2E between both water passage plate surfaces 2A and ribs 2C, and is designed not to be crushed by the restraining pressure in the ground, so it is restrained in the ground. It has the advantage that the water permeability does not decrease even when pressure is applied, and the decrease in water permeability can be suppressed even when the length is increased. Furthermore, due to the material of the water passing material 2 and the presence of the grooves 2D on the surfaces of both water passing plate surfaces 2A, it is possible to follow the deformation of the ground, and it is also easy to wind up into a roll shape, making it easy to transport.

Figure 7 shows conventional strip-shaped drain materials A, B, and C.
The water flow test results of the new strip-shaped drain material D used in this invention (width 100 mm and thickness 10 mm at the time of the test), where the lateral pressure during the test was increased stepwise in consideration of the confining pressure in the soil. The amount of water flowing through each area is shown.
As a result, it was found that the novel strip-shaped drain material D of the present invention exhibited a water flow rate approximately three times larger than that of the conventional strip-shaped drain materials A, B, and C, and did not affect the lateral pressure.

Figures 8A and B and 9A and B show the liquefaction state of sandy ground during an earthquake when no belt-shaped drain material is used and when the new belt-shaped drain material D of the present invention is used. It is a figure showing a comparison.

In Fig. 8A, a saturated sand 5 with a loose density is placed in a soil tank 4, a water pressure gauge 6 is set at a point P in the saturated sand 5, and a strip drain 1 is inserted into the saturated sand 5. An example of no comparative experimental equipment is shown.

FIG. 8B is a diagram showing temporal changes in the excess spacing water pressure ratio at point P when a vibration experiment was conducted using the equipment shown in FIG. 8A. Here, Δu is excess pore water pressure,
σv′ is the effective overburden pressure and Δu/ _σv ′ is the excess spacing water pressure ratio. In this case, Δu/σ _v ' in the sand is approximately 1, indicating that the ground has completely liquefied and behaves like water.

In FIG. 9A, a soil tank 4 is filled with saturated sand 5 of a loose density, a new strip-shaped drain material 1 (for example, width 100 mm, thickness 10 mm) used in the present invention is placed in the saturated sand 4, and a strip-shaped drain material 1 (for example, width 100 mm, thickness 10 mm) P1 in drain material 1 and saturated sand 5
An example of experimental equipment is shown in which interval water pressure gauges 6 are set at points P2, P2, and P3.

FIG. 9B is a diagram showing temporal changes in the excess interval water pressure ratio at points P1, P2, and P3 when a vibration experiment was conducted using the equipment shown in FIG. 9A.

As is clear from the figure, when the novel strip-shaped drain material 1 used in the present invention is placed, the excess spacing water pressure does not increase much and is quickly dissipated. From this, it was found that the novel strip-shaped drain material 1 used in the present invention has a liquefaction prevention effect.

Next, the construction method of the present invention will be explained with reference to FIGS. 1A to 1E and FIG. 2.

The driving device used in the present invention uses a casing auger 9 having an auger tip cap 8 at its tip, and a mandrel 10 arranged concentrically within the casing auger 9.

As shown in FIG. 1A, a novel strip-shaped drain 1 used in the present invention is passed through a mandrel 10, and an anchor 11 is attached to the lower end of the strip-shaped drain 1 to prepare for casting.

As shown in FIG. 1B, the casing auger 9
While rotating the saturated sandy ground 12 to a predetermined depth.
to excavate. At this time, the mandrel 10 is not rotated by the rotation control device 13 and reaches a predetermined depth.

As shown in FIG. 1C, when the excavation is completed to a predetermined depth, the rotation of the casing auger 9 is stopped, and
The mandrel 10 is press-fitted so that it protrudes from the tip of the casing auger 9 by about 50 to 100 cm.

As shown in FIG. 1D, the casing auger 9
The crushed stone loading lid 14 at the top of the container is opened, and an appropriate amount of crushed stone 15 is loaded. Thereafter, while reversing the casing auger 9, the casing auger 9 is raised to a depth corresponding to the amount of crushed stone input, and at the same time the mandrel 10 is pulled up. During rotation of the casing auger 9, the strip drain 1 is protected by a mandrel 10. This operation is repeated to form the gravel drain 16 and pull out the casing auger 9 and mandrel 10.

As shown in FIG.
is formed, and then crushed stone mats 18 are laid on the saturated sandy ground 12 to complete the series of construction methods.

When the composite drain 17 is formed in this way, as shown in FIG. The water in the gravel drain 16 flows into the strip drain 1 as shown by the arrow 21. The water that has flowed into the strip drain 1 is discharged upward as shown by an arrow 22 due to the drainage effect of the strip drain 1. Note that some of the water is discharged to the upper part by the gravel drain 16 as shown by an arrow 23. Due to the drainage effect of these two drains 1 and 16, the excessive spacing water pressure generated in the sandy ground 12 can be quickly dissipated, making it possible to prevent liquefaction and solving the problem of the gravel drain 16.

(Effects of the Invention) As described above, in the present invention, a specially structured belt-shaped drain is installed in a gravel drain to form a composite drain, so that the drainage capacity of the belt-shaped drain is weighted in accordance with the drainage capacity of the gravel drain. . In particular, this belt-shaped drain has a filter effect due to the nonwoven fabric on the surface, and a structure that prevents the water passageway from being crushed by confining pressure due to both water passage plate surfaces of the internal water passage material and a plurality of ribs between them. This ensures that drainage can be carried out efficiently. Therefore, according to the composite drain of the present invention, there is no need to consider the well resistance of gravel drains, the drain interval can be increased, the number of composite drains installed can be reduced, and construction costs can be reduced. I can do it.

[Brief explanation of drawings]

1A to 1E are longitudinal sectional views showing steps of an example of the construction method of the present invention, FIG. 2 is a longitudinal sectional view showing an example of a composite drain formed by the construction method of the invention, and FIG. A plan view showing an example of the belt-shaped drain to be used with a portion of the nonwoven fabric removed, FIG. 4 is a side view of the same,
Figure 5 is a vertical sectional side view of the same, Figure 6 is - in Figure 5.
Line sectional view, Figure 7 is a comparison diagram of the change in water flow rate with respect to lateral pressure between the belt-shaped drain used in the present invention and the conventional belt-shaped drain, and Figures 8A and 9A are the cases where the belt-shaped drain is not used and when it is used. Figures 8B and 9B are longitudinal cross-sectional views of the experimental equipment shown in Figures 8A and 9A. It is a diagram. 1... Band-shaped drain, 2... Water-permeable material, 2A...
Water passage plate surface, 2B... Water passage, 2C... Rib, 2D
...Groove, 2E...Water hole, 3...Nonwoven fabric, 8...
Auger tip lid, 9...Casing auger, 1
0...Mandrel, 11...Anchor, 12...
Sandy ground, 14...crushed stone, 16...gravel drain, 17...composite drain, 18...crushed stone pine.

Claims (1)

[Scope of Claims] 1. A water-permeable material comprising: a water-permeable material having a substantially rectangular cross section and opposite wide surfaces serving as water-permeable plate surfaces, and a nonwoven fabric covering the surface of the water-permeable material; Inside the water material, a plurality of water passages that are continuous in the longitudinal direction are partitioned by a plurality of ribs that connect the two water passage plate surfaces at intervals in the width direction, and are arranged in parallel in the width direction. A large number of grooves are formed along the width direction of the water plate at intervals in the longitudinal direction, and each groove has a belt-shaped drain having a structure in which a large number of water holes communicating with the water passage are formed respectively. A casing auger with a built-in mandrel for the belt-shaped drain is used to excavate sandy ground to a predetermined depth, and when the auger is pulled up, the belt-shaped drain is placed in the sand, and crushed stone thrown from above is used to excavate the sandy ground to a predetermined depth. A method for preventing liquefaction of sandy ground, characterized by forming a composite drain of the belt-shaped drain and the gravel drain by creating a gravel drain surrounding the belt-shaped drain.