JPH035514A - Liquefaction prevention of sandy ground and bubble injection device - Google Patents

Abstract

PURPOSE:To improve a ground at a low cot when a building is constructed on a saturated sandy ground, or when a building is already constructed thereupon, by press fitting a fine bubble by an injection pipe along with a liquid, into the ground. CONSTITUTION:When a building 2 is constructed on a sandy ground 1 which is almost completely saturated, an injection pipe 3 having a number of fine permeable holes is inserted vertically in the sandy ground 1, and a liquid in which a bubble is incorporated by a bubble incorporation device 4 is formed, which is press fitted into the inside of the injection pipe 3 from its upper end. Degree of saturation of the ground is lowered by the fine bubble, whereby liquefaction of the ground 1 is prevented. A ground can thus be improved at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a method for preventing liquefaction of sandy ground that is fully saturated or nearly so, and a bubble injection device used therein.

"Conventional technology and its issues" In general, saturated sandy ground may liquefy during an earthquake, so when building on such sandy ground, various ground improvement methods are used to improve the ground. needs to be improved. However, since such ground improvement requires extremely high costs, there is a desire to develop a low-cost method of preventing liquefaction. In addition, if the above-mentioned ground improvement is carried out on sandy ground on which a building has already been constructed, it may have a negative impact on surrounding buildings, so if a building has already been constructed, In sandy ground, it is considered impossible to prevent liquefaction using the ground improvement methods described above.

This invention was made in view of the above circumstances, and by injecting liquid and minute air bubbles into sandy ground that is nearly completely saturated to improve the ground, it is possible to improve the ground at a low cost, and it is The object of the present invention is to provide a method for preventing liquefaction of sandy ground and an air injection device that can be applied to sandy ground on which objects are built.

``Means to solve the problem'' Recent research on elastic wave exploration has shown that even in the ground below the groundwater level, if there are minute air bubbles in the pore water of the ground, the P wave velocity in the ground is 1500i+/ se
c (P wave velocity of water) or less.

On the other hand, it has already been studied that in sandy ground that is close to a completely saturated state, a slight decrease in the degree of saturation causes an increase in strength. If it can be artificially created, it is considered to be a promising new method for preventing liquefaction.

Therefore, the invention according to the first claim of the present invention involves penetrating an injection pipe into sandy ground that is close to a completely saturated state, producing a liquid with air bubbles mixed therein using a bubble mixing device, and injecting the liquid into the sandy ground. We have attempted to solve the above problem by configuring a method for preventing liquefaction of sandy ground in which minute air bubbles are injected into the sandy ground from this injection pipe by press-fitting it into the pipe from the upper end. There is.

In addition, the invention according to the second claim includes the air bubble injection device used in the liquefaction prevention device for sandy ground according to the first claim, which includes a pressure vessel and a pressure vessel in which air is injected into the pressure vessel. A compressor for levering the pressure inside the pressure container to above atmospheric pressure, a supply means for supplying liquid into the pressure container, an agitation means for stirring the inside of the pressure container, and communication with the inside of the pressure container via a switching means. The system is characterized in that it consists of a reduced pressure vessel that is installed at a pressure vessel and maintained at a lower pressure than the inside of the pressure vessel, and an injection pipe that is connected to this reduced pressure vessel and penetrates into the ground.

"Operation" In the method for preventing liquefaction of sandy ground according to the present invention, when a building is constructed on sandy ground that is close to a completely saturated state, or when the sandy ground on which a building has already been constructed is completely saturated. If the soil is close to the sandy ground, an injection pipe is penetrated into the sandy ground, and a liquid containing air bubbles is prepared in advance using a bubble mixing device.

After doing this, by injecting the liquid mixed with air bubbles into the injection pipe from the upper end of the pipe, the water pressure inside the pipe is made higher than the groundwater pressure of the surrounding sandy ground, and this pipe Microscopic air bubbles are injected into the sandy ground along with the liquid. In this way, the degree of saturation of the sandy ground can be reduced, and an increase in pore water pressure during an earthquake can be suppressed.

"Example" Hereinafter, an example of the present invention will be described with reference to FIG.

This example is a liquefaction prevention method that is carried out when constructing a building 2 on sandy ground 1 that is nearly completely saturated. The sandy ground is penetrated vertically through an injection pipe 3 having minute air permeable holes, and a bubble-mixing device 4 prepares a liquid with air bubbles mixed therein, and this is press-fitted into the inside of the injection pipe 3 from the upper end of the injection pipe 3. This is to prevent liquefaction of 1.

The injection pipe 3 is a tube made of stainless steel or synthetic resin whose tip is closed with a closing plate, and countless minute air permeable holes (as shown in the figure) are formed in the side wall and the closing plate. The sandy ground 1 is penetrated into the sandy ground 1 with the shaft facing downward. However, the configuration of the injection pipe is not limited to this (for example, it is of course possible to use an injection pipe in which only the lower end is open and no air permeable holes are provided in the side wall).

The bubble mixing device 4 is provided for each injection pipe 3, . . . or concentrated at one location, as shown in FIG.
Closed pressure vessel with a structure that can withstand pressures above atmospheric pressure
and a compressor (which makes the pressure inside the pressure vessel 10 equal to or higher than atmospheric pressure by pressurizing air into the pressure vessel 10).
The system is generally composed of a compressor) 11, a supply means (not shown) that supplies water 12 into the pressure vessel 10, and a stirring device 13 that stirs the inside of the pressure vessel 10. The stirring device 13 includes a stirring shaft 14 protruding from the top of the pressure vessel 10, stirring blades 15 installed on the side surface of the stirring shaft 14, and a motor 16 that rotationally drives the stirring shaft 14. It is composed of.

Further, a closed pressure reducing vessel 18 is connected to the pressure vessel 10 via a pressure hose 17, and a switching valve 19 is connected to the pressure vessel 10 and the pressure hose 17 in front of the pressure reducing vessel 18, respectively.
．． 19 is attached. This pressure reducing container 18 is provided with a pressure reducing valve 20 communicating with the atmosphere.

The upper ends of the reduced pressure container 18 and each injection pipe 3, . . . are connected by a hose 2L, and a pump 22 whose discharge pressure can be adjusted is interposed in the hose 21.

In order to prevent liquefaction of the sandy ground 1 using such a liquefaction prevention method, first, a large number of injection nozzles (e.g. Penetrate vertically into each predetermined position.

Next, after connecting the bubble mixing device 4 to the upper end of the injection pipe 3, water 12 is injected into the pressure vessel 10 of the bubble mixing device 4 via a supply means (not shown).

After storing a predetermined amount of water in the pressure vessel 10, the compressor 11 pressurizes air into the pressure vessel 10, thereby increasing the pressure inside the pressure vessel IO to a predetermined pressure higher than atmospheric pressure. , drives the motor 16 to drive the stirring blades 15,...
- By rotating the stirring shaft 14, the water 12 in the pressure vessel IO is stirred.

As a result, the water 12 in the pressure vessel 10 contains more air than the saturation amount at atmospheric pressure. In addition, the pressure vessel 10
The internal pressure may be appropriately determined depending on the amount of bubbles required and the atmospheric pressure at that time.

After continuing stirring for a while in this state, the pressurization by the compressor 11 and the stirring by the stirring blades 15, etc. are stopped, and the switching valves 19 and 19 are opened, so that the water in the pressure vessel 10 is 12 is introduced into the pressure reducing container 18. Next, the switching valve 19.
19 is closed, and then the pressure reducing valve 20 of the pressure reducing container 18 is opened, thereby communicating the inside of the container with the atmosphere. As a result, of the air dissolved in the water 12 in the reduced pressure container 18, more than the saturation amount under atmospheric pressure is eluted and becomes minute bubbles, and water 12 mixed with bubbles can be obtained. .

Then, this water 12 is pumped by the pump 22 and press-fitted into the injection pipe 3 from the upper end.
The water pressure in these injection pipes 3, . . . is made higher than the groundwater pressure in the surrounding sandy ground 1, thereby increasing the
The water 12 in the sandy ground 1 is discharged from the numerous air permeable holes of the injection pipe 3, and minute air bubbles are injected into the sandy ground 1 together with the water 12.

Injecting countless minute air bubbles into the sandy ground 1 in this way will reduce the degree of saturation of the sandy ground 1; The saturation level (liquefaction intensity) of the sandy ground 1 and its temporal changes can be ascertained by installing a sensor to detect the saturation level or by installing an appropriate measuring device on the sandy ground 1. Leave it there. This degree of saturation may be detected by measuring the P-wave velocity through elastic wave exploration or the like, based on the relationship between the decrease in the P-wave velocity of the ground and the saturation degree, as described above.

Then, by lowering the degree of saturation of the sandy ground 1 while monitoring the improvement range and improvement effect of the sandy ground 1, the liquefaction strength of the sandy ground 1 is increased to a desired strength. In this way, the increase in pore water pressure during an earthquake in the sandy ground 1 can be reduced and suppressed to an appropriate level, and as a result, even in the sandy ground 1 that is nearly completely saturated, the increase in pore water pressure during an earthquake can be reduced. Liquefaction will be prevented.

In particular, in the liquefaction prevention method of this embodiment, when injecting minute air bubbles into the sandy ground 1, water 12 mixed with air bubbles is prepared by the air bubble mixing device 4, and this is poured through the injection pipe 3. Since the air bubbles are injected into the ground 1 by injecting them into the ground 1, the amount of air bubbles injected into the ground 1 can be controlled by appropriately controlling the amount of water 12 injected into the sandy ground l and the injection pressure. It can be easily controlled, and compared to the case where air bubbles are injected from the air hole of the injection pipe 3 by simply sending compressed air into the injection pipe 3, it is possible to inject air bubbles into the sandy ground 1. Can be done reliably and easily.

That is, according to conventional research, it has been reported that the air permeability of sand decreases as the degree of saturation increases, and that sand becomes clogged at a degree of saturation of about 50%. This means that if only air bubbles are injected into the sand, the saturation of the sand will not reach 50% or less. Therefore, it has been considered difficult to pump air into saturated ground without disturbing the soil's skeletal structure. but,
By mixing minute air bubbles into the water 12 as in this embodiment and injecting the minute air bubbles into the ground together with the water 12, it is possible to send air into the sand without causing blockage, and it is possible to improve the sandy ground. can be set to a desired saturation level.

Furthermore, if the bubble mixing device 4 equipped with the pressure vessel 10 and the vacuum vessel 18 is used, the pressure difference between these vessels 10118 makes it possible to easily and quickly form bubbles with uniform and minute particle diameters and uniform distribution in the water 12. It can be made into

In addition, in this invention, the liquefaction prevention method that is implemented when constructing a building on sandy ground that is close to a completely saturated state has been explained, but the liquefaction prevention method of this invention is applicable even if a building has already been constructed. It can also be applied to sandy ground. In that case, as shown in FIG. 1, the injection pipe 3 is driven around each building 2 on the sandy ground 1, and an air bubble mixing device is placed at the upper end of the injection pipe 3, for example, at the four corners of each building 2. 4 etc. should be installed. As a result, sandy ground that has not been improved to prevent liquefaction1
For the existing building 2 built on top of each building 2,
By reducing the saturation level of the surrounding sandy ground 1,
Liquefaction during earthquakes can be effectively prevented.

``Experiment example'' This experiment example is based on research conducted on liquefaction countermeasures that involve injecting microbubbles into saturated sand ground to lower the saturation level of the ground and reduce pore water pressure that occurs during earthquakes. Based on the measurement results of the elastic wave velocity of the specimen during the saturation process, we considered the relationship between the degree of saturation, that is, the B value, and the elastic wave velocity, and estimated the state of ground saturation using elastic wave velocity for the purpose of monitoring the effectiveness of countermeasures. It examines the possibilities of law.

■ Samples and test methods Two types of samples were used: Toyoura sand and gravel.

The physical properties of the samples are shown in Table 1.

Table 1 ■ Using poroelastic body theory for saturation, B value, and elastic wave velocity, and considering the bulk elastic constant of pore water containing bubbles, the relationship between saturation (Sr) and P wave velocity (Vp) is calculated. The relationship is given by the following equation.

*: Measurement results using a mold with a diameter of 150xx and a height of 150ix.

The test was conducted using a large triaxial testing device (specimen, diameter 300 cm, height 600*z), and the elastic wave measurements were carried out while the effective confining pressure was maintained at 49 kPa and the specimen was saturated. The degree of saturation is the change in volume of pore water caused by back pressure loading.
) and calculated from Boyle's law.

Here, ρ: Density (wet density) ρd: Density of soil skeleton (dry density) Vpd: P-wave velocity of soil skeleton Kw: Bulk elastic constant of water that does not contain any air bubbles (2,2X 1okPa) Ka: of air Bulk elastic constant (pore water pressure expressed in absolute pressure) n: Porosity Figure 3 shows Vp due to the degree of saturation estimated using equation (1).
It shows the change in Proportion of air bubbles in pore water (1
-Sr) is 10-5 to 1O-3 (99 in terms of saturation)
．． 999% to 99.9%) is extremely large.

On the other hand, if the compressibility of soil particles is ignored, the B value is expressed by the following equation.

FIG. 4 shows the change in the B value depending on the degree of saturation determined from equation (2). It can be seen that the B value is also significantly affected by the degree of saturation, particularly in the region where (1-Sr) changes from 10-' to 10-'.

■ Measurement results and discussion Figure 5 shows the relationship between the elastic velocity measured during the saturation process and the degree of saturation. Vp is significantly affected by the degree of saturation and exhibits a change that corresponds well to the estimated value shown in FIG. 3, whereas the change in the precipitation velocity (Vs) is extremely small. According to calculations, the shear elastic constant remained almost constant during the saturation process, even when the increase in density with increasing saturation was taken into account. Actually measured Vpd (Vp measured on dry specimen) and Vp
(1-Sr)cal obtained from formula (1) using
(1-5r) m
Fig. 6 shows the comparison results of eas. Although there are variations in the small region of (isr) that are thought to be due to measurement accuracy, it can be determined that there is a good correlation overall.

FIG. 7 shows the relationship between the bulk elastic constant (K) determined from the elastic wave velocity and the actually measured B value at each saturation stage.

The curve in FIG. 7 represents the following equation obtained by applying poroelastic body theory to equation (2).

B=l−Kd/K (3) It can be seen that the calculated curve closely approximates the measured data. Figure 8 shows Kd from the elastic wave velocity measured on the dry specimen.
The B value (Bcal',
) and the actually measured B value (Bmeas, ). A good correlation is observed over a wide range between the two.

■ Conclusion We measured changes in elastic wave velocity during the saturation process of sand and gravel specimens, and showed that the P-wave velocity significantly decreases when the degree of saturation decreases even slightly from a completely saturated state. In addition, this can be explained by the theory of poroelastic bodies by considering the bulk elastic constant of pore water containing many bubbles, and the degree of saturation, that is, the B value, can be estimated from the elastic wave velocity. revealed.

From the above results, in liquefaction countermeasures by bubble injection,
The effectiveness of the method of measuring the elastic wave velocity in the ground and monitoring the effectiveness of countermeasures was suggested.

[Effects of the Invention] As explained in detail above, according to the present invention, an injection pipe is penetrated into sandy ground that is nearly completely saturated, and a liquid containing air bubbles is produced by an air bubble mixing device. By press-fitting into the inside of the injection pipe from the upper end, minute air bubbles can be injected from the injection pipe into the sandy ground, thereby suppressing an increase in pore water pressure during an earthquake. For this reason, it is possible to prevent liquefaction during an earthquake even in sandy ground that is nearly fully saturated.

Wear. Moreover, in the liquefaction prevention method of this invention,
Since air bubbles are injected into the ground by injecting a liquid containing air bubbles into the ground, the amount of air bubbles injected into the ground can be controlled by appropriately controlling the amount of liquid injected into sandy ground and the injection pressure. In addition, injection into sandy ground can be performed more reliably and easily than when air bubbles are injected through the pipe's permeable holes by simply sending compressed air into the pipe. .

Furthermore, if a bubble injection device equipped with a pressure vessel and a vacuum vessel is used, the pressure difference between these vessels makes it possible to easily and quickly create bubbles with uniform and minute particle diameters and uniform distribution within the liquid. can.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing one embodiment of the present invention, in which Figure 1 is a diagram for explaining a method for preventing liquefaction of sandy ground, and Figure 2 is an example of a bubble injection device. Figure 3 is a graph showing the relationship between saturation and P wave velocity, Figure 4 is a graph showing the relationship between saturation and B value, and Figure 5 is a graph showing the change in elastic wave velocity due to saturation. Figure 6 is a graph representing the relationship between the measured value and estimated value of saturation, Figure 7 is a graph representing the relationship between the bulk elastic constant and B value, and Figure 8 is a graph representing the calculated value of B value. It is a graph showing the relationship with actual measured values. 1...Sandy ground, 2...Buildings, 3.
...Pipe, 4...Bubble mixing device, 12
...Water (liquid).

Claims (2)

[Claims] (1) By penetrating an injection pipe into sandy ground that is close to a completely saturated state, and creating a liquid with air bubbles mixed in using a bubble mixing device, this is injected into the inside of the injection pipe from the upper end of the injection pipe. A method for preventing liquefaction of sandy ground, comprising injecting minute air bubbles into the sandy ground from an injection pipe. (2) A pressure vessel, a compressor that pressurizes air into the pressure vessel to make the pressure inside the pressure vessel equal to or higher than atmospheric pressure, a supply means for supplying liquid into the pressure vessel, and stirring the inside of the pressure vessel. a stirring means for stirring, a reduced pressure container provided in communication with the inside of the pressure vessel via a switching means and maintained at a lower pressure than the inside of the pressure vessel, and an injection pipe connected to this reduced pressure container and penetrated into the ground. Air bubble injection device.