JPH01127717A - Preventing work for liquidizing of sandy ground - Google Patents

Abstract

PURPOSE:To raise the draining function of water by a method in which a specific structure of belt-like drain is set in a gravel drain to form a composite drain. CONSTITUTION:A belt-like drain 1 is made up of water-permeable material 2 having mutually facing water-permeable wide plates 2A of a nearly rectangular cross section and a nonwoven fabric material 3 covering the surface of the material 2. The drain 1 is set in a gravel drain 16 to form a composite drain 17. The draining function of water can thus be enhanced.

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 pore water existing between soil particles, causing it to become as if it were a liquid. behave. This phenomenon is called liquefaction phenomenon. It has been reported that this liquefaction phenomenon causes the ground to fail, causing structures to overturn, be destroyed, and to sink unevenly, such as in the Niigata Earthquake 1 and the Chubu Japan Sea Earthquake.

One of the liquefaction prevention methods is the gravel drain method. This construction method involves placing gravel piles with a diameter of 40 to 60 cm in the ground at intervals of 1 to 2 m, with the aim of quickly dissipating excess pore water pressure that occurs during an earthquake.

(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 gravel drains, and if crushed stone with a high permeability coefficient is used, sand particles in the soil will flow into the gaps. However, there is a problem that a so-called clogging phenomenon occurs. 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. usually,
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.
Drain well resistance must be considered. 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 improve the drainage capacity of gravel drains (even if the interval between drains is wide), it is possible to effectively drain excess pore water that occurs during earthquakes. The purpose is to provide a liquefaction prevention method.

(Means for Solving the Problems) To explain in detail 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. and a non-woven fabric covering the surface of the water-permeable material, and the water-permeable material includes a water passageway that is continuous in the longitudinal direction and spaced apart in the width direction between the surfaces of the water plates. A plurality of grooves are arranged in parallel in the width direction, partitioned by a plurality of ribs connected to each other, and a large number of grooves are formed along the width direction on the surfaces of both water passage plates at intervals in the longitudinal direction. For this purpose, a belt-shaped drain having a structure in which a large number of water holes leading to the water passageway are formed respectively is used, and a casing auger with a built-in mandrel for the belt-shaped drain is used to excavate the sandy ground to a predetermined depth. When the auger is pulled up, the band-shaped drain is installed in the sand, and the periphery of the band-shaped drain is created by surrounding it with crushed stones thrown from above, thereby creating a composite drain of the band-shaped drain and the gravel drain. It is characterized by forming.

(Function) When a special strip-shaped drain is arranged in a gravel drain to create a composite structure, the non-woven fabric on the surface of the strip-shaped drain has a filtering effect, and both water-passing plate surfaces of the water-passing material and the plurality of ribs between them can be used. The structure prevents the water passage from being crushed by the confining pressure, ensuring the water passage, increasing the drainage capacity in conjunction with the drainage capacity of the gravel drain, and efficiently draining excess pore water during earthquakes. In addition, liquefaction can be effectively prevented. In addition, since the drainage capacity is increased, there is no need to consider well resistance (and even if the drain spacing is widened, effective drainage will still occur).

(Example) Embodiments 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 20. ing. The water-permeable material 2 has a width W of 100 to 150 mm and a thickness T of 10 mm.
It has a rectangular cross-sectional shape of about 151 m. 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 28 at intervals in the width direction. They are arranged side by side.

There are 5 grooves 2D along the width direction on both water passage plate surfaces 2A.
They are about the width of a crane and are formed in large numbers at intervals of about five fins in the longitudinal direction. The grooves 2D are formed so as to exist in the longitudinal direction of both water passage plate surfaces 2A. Zendao 2D
A large number of rectangular water holes 2E are formed along the longitudinal direction of the groove 2D. The water-permeable material 2 is made of vinyl chloride whose hardness and softness can be controlled by adjusting its composition.

It is made of resin material such as polyethylene.

Such a belt-shaped drain 1 is formed by partitioning the water passage 2E between both water flow 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 surface of both water passing plate surfaces 2, it is possible to follow the deformation of the ground, and it is also easy to wind up into a roll shape and transport it.

Figure 7 shows conventional strip-shaped drain materials A, B, and C, and the new strip-shaped drain material D used in this invention (100 m wide at the time of testing).
The thickness is 10++n. ) shows the water flow rate when the lateral pressure during the test was increased step by step, taking into account the confining pressure in the soil. As a result, the novel strip-shaped drain material of this invention is different from the conventional strip-shaped drain materials A, B, and C.
It was found that the water flow rate was approximately three times larger than that of the previous model, and that it did not affect the lateral pressure.

Figures 8A and B and Figures 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 of the present invention is used. It is a figure showing a comparison.

In FIG. 8A, saturated sand 5 with a loose density is placed in a soil tank 4, a pore water pressure gauge 6 is set at point P in the saturated sand 5, and the saturated sand 5 is
An example of comparative experimental equipment in which the belt-shaped drain 1 is not inserted is shown.

FIG. 8B is a diagram showing the temporal change in the excess pore water pressure ratio at point P when a vibration experiment was conducted using the equipment shown in FIG. 8A.

Here, Δμ is excess pore water pressure, σV′ is effective overburden pressure, Δ
U/σV' is the excess pore water pressure ratio. In this case, ΔU/σV' in the sand is approximately 1, indicating that the ground has completely liquefied and behaves like water.

In FIG. 9A, saturated sand 5 with a loose density is placed in a soil tank 4, and a novel strip-shaped drain material 1 (for example, width 100 fins, thickness 10 mm) used in the present invention is placed in the saturated sand 4, and An example of experimental equipment is shown in which pore water pressure gauges 6 are set at points 21, P2, and P3 in a strip of drain material 1 and saturated sand 5.

FIG. 9B is a diagram showing temporal changes in the excess pore water pressure ratio at the pt point, P2 point, and P3 point 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 pore 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. 1(A) to (IE) 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. 1(A), a novel strip-shaped drain 1 used in the present invention is passed through a mandrel 10, and an anchor 1) is attached to the lower end of the strip-shaped drain 1 to prepare for pouring.

As shown in FIG. 1(B), the saturated sandy ground 12 is excavated to a predetermined depth while rotating the casing auger 9. At this time, the mandrel 10 does not rotate due to the rotation control value ff13 and reaches a predetermined depth.

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

As shown in FIG. 1(0), the crushed stone loading lid 14 at the top of the casing auger 9 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. 1(E), a composite drain 17 consisting of the belt-shaped drain 1 and the gravel drain 16 is formed, and then a crushed stone mantle 18 is laid on the saturated sandy ground 12. end.

When the composite drain 17 is formed in this way, as shown in FIG.
Gravel drain 1 flows into the gravel drain 1 as shown by arrow 20.
The water in the drain 6 flows into the strip drain 1 as shown by an 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 drained upward by the gravel drain 16 as indicated by an arrow 23.

Due to the drainage effect of these two drains 1.16, excess pore water pressure generated in the sandy ground 12 can be quickly dissipated, making it possible to prevent liquefaction.
6 problems can be solved.

(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 filtering effect due to the non-woven fabric on the surface, and a structure that prevents the water passageway from being crushed by confining pressure due to the water plate surfaces of the internal water-permeable material and the multiple 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 spacing can be increased, the number of composite drains installed can be reduced, and construction costs can be reduced. can.

[Brief explanation of the drawing]

Figures 1 (A) to (E) are longitudinal sectional views showing steps of an example of the construction method of the present invention, Figure 2 is a longitudinal sectional view showing an example of a composite drain formed by the construction method of the invention, and Figure 3. is a plan view showing an example of the belt-shaped drain used in the present invention with a portion of the non-woven fabric removed, FIG. 4 is a side view of the same, FIG. 5 is a longitudinal cross-sectional side view of the same, and FIG. ■ Line cross-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 case where the belt-shaped drain is not used and the case where the belt-shaped drain is used. Figures 8B and 9B are vertical cross-sectional views of the experimental equipment for the case, and the excess pore water pressure ratio shows the change in the excess pore water pressure ratio during shaking in the experimental equipment shown in Figures 8A and 9A. It is a characteristic diagram. ■... Band-shaped drain, 2... Water passing material, 2A... Water passing plate surface, 2B... Water passage, 2C... Rib, 2D...
・Groove, 2E...Water hole, 3...Nonwoven fabric, 8...Auger tip cover, 9...Casing auger, 10...
・Mandrel, 1)...Anchor, 12...Sandy ground, 14...Crushed stone, 16...Gravel drain, 1
7... Composite drain, 18... Crushed stone mat.

Claims (1)

[Claims] (1) 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; has a plurality of water passages that are continuous in the longitudinal direction and 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; uses a band-shaped drain having a structure in which a large number of grooves are formed along the width direction at intervals in the longitudinal direction, and each groove has a large number of water holes communicating with the water passage, A casing auger with a built-in mandrel for the drain is used to excavate sandy ground to a predetermined depth, and when the auger is pulled up, the belt-shaped drain is installed in the sand, and crushed stones thrown from above are used to dig the belt-shaped drain. 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 surrounding area.