JPH0721179B2 - Method for preventing liquefaction of sandy ground and bubble injecting device used therefor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for preventing liquefaction of sandy ground in a completely saturated state or a nearly saturated state, and a bubble injection device used for the method.

“Conventional technology and its problems” Generally, saturated sandy ground may liquefy during an earthquake. Therefore, when building a building on such sandy ground, various ground improvement methods are used. Need to be improved. However, since such ground improvement requires extremely high cost, it is desired to develop a method for preventing liquefaction at low cost. In addition, when the above-mentioned ground improvement is performed on the sandy ground where the building has already been constructed, it may adversely affect the surrounding buildings. It is considered impossible to prevent liquefaction in the ground by the above-mentioned ground improvement method.

The present invention has been made in view of the above circumstances, by performing a ground improvement by press-fitting minute bubbles together with a liquid into the sandy ground that is almost completely saturated, the ground can be improved at a low cost, and the structure has already been constructed. It is an object of the present invention to provide a method for preventing liquefaction of a sandy ground that can be applied to the sandy ground constructed by, and a bubble injection device used for the method.

"Means for solving the problem" Recent research on elastic wave exploration shows that even in the ground below the groundwater level, when minute bubbles are present in the pore water of the ground, the P wave velocity of the ground is 1500 m / sec (P of water
It was clarified that the wave velocity decreased below.

On the other hand, it has already been researched that a slight decrease in the degree of saturation leads to an increase in strength in the sandy soil that is almost completely saturated. If it can be artificially created, it is considered promising as a new liquefaction prevention method.

Therefore, in the invention according to the first aspect of the present invention, the injection pipe is penetrated into the sandy ground close to the completely saturated state, a liquid containing bubbles is produced by the bubble injection device, and the liquid is injected into the liquid. By press-fitting from the upper end of the pipe into the interior of the pipe, by configuring a method for preventing liquefaction of the sandy ground such as injecting minute bubbles into the sandy ground from the injection pipe, the above problems are not solved. There is.

In addition, the invention according to the second claim is a bubble container, which is used in the liquefaction preventing device for a sandy ground according to the first claim, in which a pressure container and air are press-fitted into the pressure container. A compressor that makes the pressure inside the press-fitting container equal to or higher than the atmospheric pressure, a supply unit that supplies a liquid into the pressure container, a stirring unit that stirs the inside of the pressure container, and a communication unit that communicates with the inside of the pressure container through a switching unit. It is characterized in that it is provided with a decompression container that is provided and is maintained at a low pressure from the inside of the pressure container, and an injection pipe that is connected to this decompression container and penetrates into the ground.

"Operation" In the method for preventing liquefaction of sandy ground of the present invention, when building a building on a sandy ground that is almost completely saturated or when the sandy ground on which the building has already been built is completely saturated. If it is close, the injection pipe is penetrated into the sandy ground and a liquid in which bubbles are mixed by the bubble mixing device is prepared in advance. Then, after doing this, by injecting a liquid mixed with bubbles into the inside from the upper end of the injection pipe, the water pressure inside the pipe is increased above the groundwater pressure of the surrounding sandy ground, and this pipe is From this, minute bubbles are injected into the sandy ground together with the liquid. By doing so, the degree of saturation of the sandy ground can be reduced, and the rise in pore water pressure during an earthquake will be suppressed.

[Embodiment] An embodiment of the present invention will be described below with reference to FIG.

This embodiment is a liquefaction prevention method that is carried out when a building 2 is constructed on the sandy ground 1 that is almost completely saturated. The sandy ground is obtained by vertically penetrating the injection pipe 3 having minute air holes, producing a liquid in which bubbles are mixed by the bubble mixing device 4, and press-fitting the liquid into the inside from the upper end of the injection pipe 3. Liquefaction of 1 is prevented.

The injection pipe 3 is formed by closing the tip of a pipe made of stainless steel or synthetic resin with a closing plate, and forming innumerable minute air holes (not shown) on the side wall and the closing plate. Is directed downward and penetrates into the sandy ground 1. However, the structure of the injection pipe is not limited to this, and it is needless to say that it may be an injection pipe in which only the lower end portion is opened and the side walls are not provided with air holes.

The bubble mixing device 4 is provided for each of the injection pipes 3, ... Or concentratedly in one place, and as shown in FIG. 2, a closed type pressure vessel 10 having a structure capable of withstanding a pressure of atmospheric pressure or more, and a pressure. A compressor (compressor) 11 that pressurizes air into the container 10 to bring the pressure inside the pressure container 10 to atmospheric pressure or higher.
And a supply means (not shown) for supplying water 12 into the pressure vessel 10, and a stirring device 13 for stirring the inside of the pressure vessel 10. The stirring device 13 includes a stirring shaft 14 projecting from the upper part of the pressure vessel 10, stirring blades 15 planted on the side surface of the stirring shaft 14, ..., And a motor 16 for rotating the stirring shaft 14. It is configured.

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

The decompression container 18 and the upper ends of the injection pipes 3, ... Are hose 21 ,.
The hose 21 has a pump 22 with a discharge pressure adjustable.

When preventing the liquefaction of the sandy ground 1 by such a liquefaction prevention method, first, a large number of injection pipes 3, 3, ... Are respectively provided in the sandy ground 1 where the building 2 is to be constructed. Penetrate vertically into place.

Next, after the bubble mixing device 4 is connected 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). Then, after storing a predetermined amount of water in the pressure container 10, by pressurizing air into the pressure container 10 by the compressor 11, the pressure inside the pressure container 10 is increased to a predetermined pressure equal to or higher than atmospheric pressure. By driving the motor 16 and rotating the stirring shaft 14 together with the stirring blades 15, the water 12 in the pressure vessel 10 is stirred. As a result, the water 12 in the pressure vessel 10 is melted with air at a saturation amount or more at atmospheric pressure. The pressure in the pressure vessel 10 may be appropriately determined depending on the amount of bubbles required and the atmospheric pressure at that time.

After continuing the stirring for a while in this state, the pressurization by the compressor 11 and the stirring by the stirring blades 15, ...
The water 12 in the pressure vessel 10 is decompressed by opening the
Introduce inside. Next, the switching valves 19, 19 are closed, and then the pressure reducing valve 20 of the pressure reducing container 18 is opened so that the inside thereof is communicated with the atmosphere. As a result, the water in the decompression container 18
Of the air dissolved in 12, the air having a saturated amount or more under atmospheric pressure is eluted to form fine bubbles, and water 12 containing the bubbles can be obtained.

Then, the water 12 is pumped by the pump 22 and is injected into the inside from the upper end of the injection pipes 3, ... By increasing the water pressure in the injection pipes 3 ,. The water 12 in the injection pipes 3, ... Is discharged from the innumerable air holes of the injection pipes 3, ..., Injecting minute bubbles together with the water 12 into the sandy ground 1.

Injecting innumerable minute air bubbles into the sandy ground 1 in this way reduces the saturation of the sandy ground 1, but in that case, if the sandy ground 1 contains Check the saturation (liquefaction strength) of the sandy ground 1 and its change over time by installing a sensor to detect the saturation or installing an appropriate measuring device on the sandy ground 1. I will leave it. The detection of the saturation may be performed by measuring the P wave velocity by elastic wave exploration or the like based on the relationship between the decrease in the P wave velocity of the ground and the saturation as described above.

Then, the liquefaction strength of the sandy ground 1 is increased to a desired strength by decreasing the saturation degree of the sandy ground 1 while monitoring the improvement range and the improvement effect of the sandy ground 1. By doing so, the increase in pore water pressure during the earthquake in the sandy ground 1 is reduced and suppressed to an appropriate level, and thus even in the sandy ground 1 in a state close to complete saturation, Liquefaction will be prevented.

In particular, in the liquefaction prevention method of this embodiment, when injecting fine air bubbles into the sandy ground 1, water 12 in which air bubbles are mixed by the air bubble mixing device 4 is prepared, and this is injected into the injection pipe 3
Since the bubbles are injected into the ground 1 by being pressed into the ground 1 via the, the injection of bubbles into the ground 1 by appropriately controlling the injection amount and the injection pressure of the water 12 into the sandy ground 1. The amount of bubbles can be easily controlled, and moreover, the bubbles to the sandy ground 1 can be compared with the case where the bubbles are injected from the air holes of the injection pipe 3 by simply sending the compressed air into the injection pipe 3. Reliable and easy injection.

That is, according to the conventional research, it has been reported that the air permeability coefficient of sand decreases with an increase in the degree of saturation and becomes blocked at a degree of saturation of about 50%. This means that when simply injecting only bubbles into the sand, the degree of saturation of the sand does not reach below 50%. Therefore, it has been considered difficult to send air into the saturated ground without disturbing the skeletal structure of the soil. However, as in the present embodiment, by mixing fine air bubbles in the water 12 and injecting the fine air bubbles into the ground together with the water 12, it is possible to send air into the sand without causing blockage. The soil can be made to have a desired saturation.

Further, if the bubble mixing device 4 including the pressure vessel 10 and the decompression vessel 18 is used, bubbles having a uniform particle size and a minute distribution in the water 12 can be easily produced due to the pressure difference between the vessels 10 and 18. Moreover, it can be manufactured quickly.

In the present invention, the liquefaction prevention method to be carried out when constructing a building on a sandy ground close to a completely saturated state has been described, but the liquefaction prevention method of the present invention has already been constructed. It can also be applied to existing 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 provided 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, the sandy ground 1 has not been improved to prevent liquefaction.
Even for the existing building 2 built above, each building 2
By reducing the saturation of the surrounding sandy ground 1,
Liquefaction during an earthquake can be effectively prevented.

"Example" This example is a study conducted on a liquefaction countermeasure for injecting micro bubbles into a saturated sand ground to reduce the degree of saturation of the ground and reduce the pore water pressure generated during an earthquake. Based on the measurement results of the elastic wave velocity of the specimen due to the saturation process, the saturation degree, that is, the relationship between the B value and the elastic wave velocity is considered, and the ground saturation state is estimated by the elastic wave velocity for the purpose of monitoring the countermeasure effect. This is a study of the possibility of law.

Specimen and test method The specimens used are two types: Toyoura sand and gravel. The physical properties of the sample are shown in Table 1.

The test is a large triaxial tester (specimen: diameter 300 mm, height 600)
mm), and the effective confining pressure is 49
It was carried out in the process of keeping the specimen at kPa and saturating the specimen. The degree of saturation was obtained from Boyle's law by measuring the volume change of pore water with back pressure loading using a pressure resistant buret (capacity 1000 cm ³ ).

Saturation degree, B value and elastic wave velocity When using the poroelastic theory and considering the bulk elastic constant of pore water containing bubbles, the relationship between saturation (Sr) and P wave velocity (Vp) is as follows. Given by the formula.

Where ρ: Density (wet density) ρd: Density of soil skeleton (dry density) Vpd: P wave velocity of soil skeleton Kw: Volume of water containing no bubbles Elastic constant (2.2 × 10 kPa) Ka: Volume of air Elastic constant (pore pressure expressed in absolute pressure) n: Porosity FIG. 3 shows the change in Vp depending on the degree of saturation estimated using equation (1). Ratio of air bubbles in pore water (1-
Sr) is 10 ^-5 to 10 ^-3 (in terms of saturation, 99.999 to 99.9%)
The change in Vp in the region that changes to 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 saturation calculated from the equation (2). It can be seen that the B value is also significantly affected by the degree of saturation, and the change is large particularly in the region where (1-Sr) changes from 10 ⁻³ to 10 ⁻¹ .

Measurement Results and Discussion FIG. 5 shows the relationship between the elastic velocity measured in the saturation process and the saturation. Vp is significantly affected by saturation,
While it shows changes that correspond well with the estimated values shown in the figure,
The change in shear wave velocity (Vs) is extremely small. According to the calculation, the shear elastic constant showed an almost constant value in the saturation process even when the increase of the density with the increase of the saturation was taken into consideration. Comparison of (1-Sr) cal. Obtained from equation (1) and (1-Sr) meas. Obtained from measurement of volume change of pore water, using actually measured Vpd (Vp measured in dried sample) and Vp Results are shown in FIG. In the small region of (1-Sr), there are variations that are considered to be due to the measurement accuracy, but it can be determined that there is a good correlation as a whole.

FIG. 7 shows the relationship between the bulk elastic constant (K) obtained 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 the poroelastic theory to the equation (2).

B = 1-Kd / K (3) It can be seen that the calculation curve closely approximates the measured data. Figure 8 shows Kd from the elastic wave velocity measured on the dried specimen.
Is a result of comparison between the B value (Bcal.) Calculated using Equation (3) and the actually measured B value (Bmeas.). There is a wide range of good correlation between the two.

CONCLUSION The change in elastic wave velocity during the saturation process of sand and gravel specimens was measured, and it was shown that the P wave velocity decreased remarkably with a slight decrease in saturation from the state of complete saturation. Further, this can be explained by the theory of a poroelastic body if the bulk elastic constant of interstitial water containing a lot of bubbles is taken into consideration, and the degree of saturation, that is, the B value can be estimated from the elastic wave velocity. Revealed.

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

[Effect of the Invention] As described in detail above, according to the present invention, the injection pipe is penetrated into the sandy soil close to the completely saturated state, and the liquid in which the bubbles are mixed by the bubble mixing device is produced. By injecting into the inside of the injection pipe from the upper end of the injection pipe, it is possible to inject minute bubbles into the sandy ground from the injection pipe, thereby suppressing an increase in pore water pressure during an earthquake. Therefore, liquefaction during an earthquake can be prevented even in sandy soil that is almost completely saturated. Moreover, in the liquefaction prevention method of the present invention, since the bubbles are injected into the ground by press-fitting the liquid in which the bubbles are mixed into the ground, the injection amount and the injection pressure of the liquid to the sandy ground are controlled. It is possible to easily control the amount of bubbles injected into the ground by controlling appropriately, and moreover, in comparison with the case of injecting bubbles from the air holes of the pipe by simply sending compressed air into the pipe, Reliable and easy injection into sandy ground.

Further, by using a bubble injecting device equipped with a pressure vessel and a decompression vessel, it is possible to easily and quickly produce bubbles having a uniform and minute particle size and a uniform distribution in the liquid due to the pressure difference between these vessels. it can.

[Brief description of drawings]

1 and 2 are views showing an embodiment of the present invention, wherein FIG. 1 is a view for explaining a method for preventing liquefaction of sandy ground, and FIG. 2 is an example of a bubble injection device. FIG. 3 is a graph showing the relationship between saturation and P wave velocity, FIG. 4 is a graph showing the relationship between saturation and B value, and FIG. 5 is a change in elastic wave velocity due to saturation. 6 is a graph showing the relationship between the actually measured value and the estimated value of the saturation, FIG. 7 is a graph showing the relationship between the bulk elastic constant and the B value, and FIG. 8 is the calculated value of the B value. It is a graph showing the relationship with the measured value. 1 …… Sandy ground, 2 …… Building, 3 …… Pipe, 4 ……
Bubble mixing device, 12 ... Water (liquid).

Claims (2)

[Claims] 1. An injection pipe is penetrated into a sandy soil which is almost completely saturated, a liquid containing bubbles is produced by an air bubble mixing device, and the liquid is press-fitted into the inside from the upper end of the injection pipe. A method for preventing liquefaction of sandy ground, which comprises injecting fine bubbles into the sandy ground from the injection pipe. 2. A pressure vessel, a compressor for pressurizing air into the pressure vessel to bring the pressure inside the pressure vessel to atmospheric pressure or higher, a supply means for supplying a liquid into the pressure vessel, and an inside of the pressure vessel. And a decompression container that is provided in communication with the inside of the pressure vessel through a switching means and that is maintained at a low pressure from the inside of the pressure vessel, and an injection pipe that is connected to this decompression vessel and penetrates into the ground. A bubble injecting device used in a method for preventing liquefaction of sandy ground, which comprises