JP2733540B2 - Ground improvement method - Google Patents

Description

The present invention relates to a method for improving a ground by injecting microbubbles into the ground.

"Conventional technology and its problems" Generally, saturated sandy ground may be liquefied during an earthquake, so when constructing a building on such sandy ground, various types of ground improvement methods are required. Need to be improved. However, since such a ground improvement requires an extremely high cost, development of a method for preventing liquefaction at low cost is desired. In addition, if the ground improvement as described above is performed on sandy ground on which a building has already been constructed, there is a risk of adversely affecting surrounding buildings. It is thought that it is impossible to prevent liquefaction in the quality ground by the above-mentioned ground improvement method.

Therefore, the present applicant has penetrated a pipe having a large number of fine air holes into a sandy ground near full saturation, and press-fits air from the upper end of the pipe into the inside of the pipe through an air press-fitting device. We proposed a method to prevent liquefaction by injecting minute air bubbles into the ground through the porosity, thereby suppressing the rise in pore water pressure during an earthquake (Japanese Patent Application No. 63-163).
-276999).

At this time, it is thought that the air bubbles injected into the ground gradually diffuse from the periphery of the pipe to the entire ground to be improved, but it is desirable to surely mix the air bubbles into the ground in a specific area.
Therefore, further improvement was awaited.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a ground improvement method capable of surely mixing air bubbles in an area of the ground to be improved.

"Means to solve the problem" Recent research on elastic wave exploration shows that even in the ground below the groundwater level, if there are microbubbles in the pore water of the ground, the P-wave velocity of the ground is 1500m / It was found that the temperature dropped below sec (P wave velocity of water).

On the other hand, it has been already studied that a slight decrease in the degree of saturation leads to an increase in strength in a sandy ground near a fully saturated state. If it can be made artificially, it is considered promising as a new liquefaction prevention method.

Therefore, the invention according to the first claim of the present invention,
In the ground where the structure was constructed, the injection means and the drainage means are arranged so as to surround the structure, and the liquid mixed with bubbles is pressed into the inside of the means from the upper end of the means,
The above problem is solved by a soil improvement method in which minute bubbles are injected into the ground from the injection means, and groundwater is drained by the drainage means to improve the ground.

According to a second aspect of the present invention, in the ground improvement method according to the first aspect, the injection means and the drainage means are arranged as a set so as to face each other across the structure. The above-mentioned problem is being solved by a simple ground improvement method.

[Operation] In the ground improvement method of the present invention, when the ground on which a structure is already constructed is to be improved, an injection means and a drainage means are arranged in the ground so as to surround the structure. . Next, by injecting the liquid mixed with air bubbles into the inside of the injection means from the upper end thereof, the water pressure in the means is raised above the groundwater pressure of the surrounding ground, and the liquid is injected into the ground together with the liquid from this means. Inject small bubbles. Correspondingly, the groundwater is drained by the drainage means, and the injection of bubble injection water by the injection means is facilitated. By doing so, the degree of saturation of the ground can be reduced, and the rise in pore water pressure during an earthquake is suppressed.

"Embodiment" Hereinafter, a first embodiment of the present invention will be described with reference to FIG. 1 to FIG.

This embodiment is an embodiment in which the ground improvement method according to the present invention is applied to a liquefaction prevention method that is performed when a building 2 is already constructed on a sandy ground 1 that is almost completely saturated. An injection pipe (injection means) 3 having a large number of fine air holes is vertically penetrated into the sandy ground 1, and a well point (drainage means) 5 is similarly penetrated into the sandy ground 1 and air bubbles are formed. The liquefaction of the sandy ground 1 is performed by producing a liquid in which air bubbles are mixed by the mixing device 4 and pressing the liquid into the inside from the upper end of the injection pipe 3 and draining groundwater from the well point 5. I try to prevent it.

The injection pipe 3 has a pipe made of stainless steel, synthetic resin, or the like closed at a tip thereof with a closing plate, and has a number of minute air holes (not shown) formed on its side wall and the closing plate. Is penetrated into the sandy ground 1 in a state where is directed downward. However, the configuration of the injecting means is not limited to this, and may be, for example, an injecting means in which only the lower end is opened and the side wall is not provided with a vent.

The air bubble mixing device 4 is provided corresponding to the injection pipe 3, and as shown in FIG. 3, a sealed pressure vessel 10 having a structure capable of withstanding a pressure higher than the atmospheric pressure, and pressurizing air into the pressure vessel 10. By doing so, a compressor (compressor) 11 that raises the pressure inside the pressure vessel
It is schematically composed of a supply means (not shown) for supplying the pressure vessel 12 and a stirring device 13 for stirring the inside of the pressure vessel 10. The stirring device 13 includes a stirring shaft 14 protruding from the upper portion of the pressure vessel 10,
A stirring blade 15 is mounted on a side surface of the stirring shaft 14, and a motor 16 for driving the stirring shaft 14 to rotate.

Further, the pressure vessel 10 is connected to a hermetic pressure reducing vessel 18 via a pressure-resistant hose 17.
Switching valves 19 and 19 are attached to the pressure-resistant hose 17 in the foreground, respectively. The pressure reducing vessel 18 is provided with a pressure reducing valve 20 communicating with the atmosphere.

The upper end of the decompression container 18 and each injection pipe 3,.
, And the hose 21 is provided with a pump 22 whose discharge pressure can be adjusted.

On the other hand, the well point 5 also penetrates vertically into the sandy ground 1 like the injection pipe 3, and is disposed in the sandy ground 1 so that at least its tip is below the groundwater level of the sandy ground 1. I have. The well point 5 has a well-known structure used for ordinary ground improvement. As shown in FIG. 4, a nozzle portion (not shown) for penetrating into the ground is formed at the tip of a cylindrical main body. Further, a large number of drain holes (not shown) for draining groundwater are formed on the side surface. A riser pipe 30 is connected to the upper end of the well point 5, and a drain pump is connected to the riser pipe 30.
With the connection of 31, the groundwater around the well point 5 is drained.

The groundwater drained at the well point 5 in this manner is temporarily stored in a sand settling tank 32, whereby sediment mixed into the groundwater is settled, and as shown in FIG. Stored in the pump as needed
It is appropriately returned into the pressure vessel 10 of the bubble mixing device 4 via 33. The water pressure provided by the pump 33 must be higher than the pressure provided by the compressor.

At this time, it is preferable that a partition plate 34 having a certain height is provided inside the sand setting tank 32 to serve as a measure of the amount of groundwater drainage. The sediment stored at the bottom of the sand settling tank 32 is
32 It may be discharged from sand discharge holes 35, 35 formed in the lower part of the side surface. Naturally, these sand discharge holes 35 are plugged in a normal state.
36, 36 are fitted.

Then, as shown in FIG. 2, the injection pipe 3 and the well point 5 are paired with each other so as to surround the building 2 in a plan view in a boundary of the area A of the ground to be improved. Many are provided along the line B. However, the arrangement positions of the injection pipe 3 and the well point 5 are not limited to this, and may be appropriately determined depending on the properties of the ground to be improved, the groundwater level, and the like.

When the liquefaction of the sandy ground 1 is prevented by such a ground improvement method, first, the injection pipe 3 and the well point 5 are paired in the sandy ground 1 on which the building 2 is constructed. It is arranged to penetrate the position vertically. In particular, the well point 5 is provided with a nozzle portion at the tip thereof as described above. By sending high-pressure water to the well point 5, water is jetted from the tip of the nozzle portion and penetrates into the ground. .

Next, after the aeration device 4 is connected to the upper end of the injection pipe 3 and the drainage pump 31 is connected to the well point 5 via the riser pipe 30, a supply unit (not shown) is provided in the pressure vessel 10 of the aeration device 4. Inject water 12 through. Then, after storing a predetermined amount of water in the pressure vessel 10, the compressor 11
By pressing air into the pressure vessel 10 to increase the pressure inside the pressure vessel 10 to a predetermined pressure equal to or higher than the atmospheric pressure, the motor 16 is driven to rotate the stirring shaft 14 together with the stirring blades 15,. Thus, the water 12 in the pressure vessel 10 is stirred. As a result, air equal to or greater than the saturation amount at atmospheric pressure is dissolved in the water 12 in the pressure vessel 10. Note that the pressure in the pressure vessel 10 may be appropriately determined depending on the required amount of bubbles and the atmospheric pressure at that time.

After stirring for a while in this state, the pressurization by the compressor 11 and the stirring by the stirring blades 15,... Are stopped, and the switching valve 19,
By opening 19, the water 12 in the pressure vessel 10 is
Introduce within 18. Next, after the switching valves 19 and 19 are closed, the decompression valve 20 of the decompression container 18 is opened to communicate the inside with the atmosphere. As a result, the water in the
Of the air that has dissolved in the air, air that is equal to or greater than the saturation amount under atmospheric pressure elutes and becomes fine bubbles, so that water 12 containing the bubbles can be obtained.

Then, the bubble-containing water 12 is pressure-fed by the pump 22, and is injected into the inside of the injection pipes 3,... From the upper end, so that the water pressure in the injection pipes 3,. , Whereby the injection pipe 3,
.. Are discharged from the innumerable air holes of the injection pipes 3,... And fine bubbles are injected into the sandy ground 1 together with the water 12.

On the other hand, the drainage pump 31 is operated to drain the groundwater around the well point 5 in response to the bubbled water 12 being pumped into the injection pipe 3. The timing of the operation of the drainage pump 31 is also arbitrary, and may be performed before the injection of the bubbled water 12 by the injection pipe 3 or may be slightly delayed after the injection of the bubbled water 12. In short, it is sufficient that excess groundwater can be drained using the well point 5 by pressurizing the bubble-mixed water 12 with the injection pipe 3 and may be appropriately determined in consideration of the construction process and the like. Furthermore, air mixed with air 12
In order to further secure the ground improvement effect by lowering the groundwater level of the ground 1 together with the injection, the groundwater drainage amount is reduced from the injection amount of the bubbled water 12 so that the injection range is expanded beyond the drainage range to expand the bubbled water. It is also possible to inject 12.

In this case, the injection of the bubbled water 12 by the injection pipe 3 may be performed simultaneously on all the pipes 3 penetrating into the sandy ground 1, but as described above, the injection pipe 3 and the well point 5 are paired. And the pair of injection pipes 3 and well points 5 are arranged so as to surround the building 2, so that the bubble-containing water 12 is injected by a more efficient method. It is also possible.

That is, as shown in FIG. 2, among the many injection pipes 3 arranged in the sandy ground, the injection pipe 3 located at the point a is selected, and the bubble-mixed water 12 is pressed into this. next,
With respect to the selected injection pipe 3, a well point 5 located at a point b which is opposite to the building 2 is selected, and groundwater is drained only from this well point 5. After a certain period of time, after the injection of the bubbled water 12 and the drainage of the groundwater are completed, the bubbled water 12 is pressed from the injection pipe 3 located at the point b, and from the well point 5 located at the point a. Drain groundwater.

That is, the injection pipes 3 facing each other across the building 2
In the area connecting the well points 5 (the area surrounded by the dotted line in FIG. 2), a flow of aerated water flowing from the point a to the point b is generated in the sandy ground 1, and fine bubbles are generated through the flow. Is injected into the sandy ground 1, and then a reverse flow of bubble-containing water flowing from the point b to the point a is generated in the ground 1, so that the injection of bubbles in the area surrounded by the dotted line is ensured. is there.

Then, the injection pipe 3 into which the aerated water 12 is injected and the well point 5 for draining groundwater are sequentially moved,
Bubbles may be injected over the entire area A of the ground to be improved.

Injecting innumerable and minute bubbles into the sandy ground 1 in this way reduces the degree of saturation (liquefaction strength) of the sandy ground 1, but in that case, A sensor for detecting the degree of saturation of the sandy ground 1 is provided, or an appropriate measuring device or the like is provided on the sandy ground 1 to grasp the saturation of the sandy ground 1 and its temporal change. To keep. The detection of the degree of 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 degree of saturation as described above.

The liquefaction strength of the sandy ground 1 is increased to a desired strength by lowering the saturation of the sandy ground 1 while monitoring the improvement range and the improvement effect of the sandy ground 1. In this way, the sandy ground 1
The rise in pore water pressure during an earthquake in the middle is reduced to an appropriate level, and thereby liquefaction during an earthquake is prevented even in the sandy ground 1 in a state of being almost fully saturated.

In particular, in the ground improvement method of this embodiment, the groundwater is drained using the well point 5 in correspondence with the injection of the bubbled water 12 by the injection pipe 3, so that the conventional groundwater is mixed with bubbles. It will be replaced with water 12,
The injection of the bubble-containing water 12 into the ground is performed quickly and easily at a low pressure as compared with the case where the groundwater drainage by the well point 5 is not performed, and the bubble-injection water 12 is also injected into the ground immediately below the building 2. Is performed reliably. In addition, since the bubble-containing water 12 can be injected at a low pressure, it is possible to prevent the occurrence of undesired phenomena such as swelling of the ground and lifting of the structure due to the water pressure accompanying the high-pressure injection.

Furthermore, since the injection pipe 3 and the well point 5 are paired and arranged in the sandy ground 1 so as to surround the building 2, the injection pipe 3 opposed to the building with the injection pipe 3 and the well point 5 interposed therebetween.
If the air containing water 12 is injected into the well point 5 and the groundwater is drained, a flow of water in a direction that penetrates the building 2 can be generated in the sandy ground 1, and the air bubbles can be injected into the ground immediately below the building 2. The injection of water 1 is more reliable.

Furthermore, by controlling the amount of air bubbles mixed in the groundwater drained in this way, it is possible to determine whether the amount of air bubbles mixed in the ground to be improved has reached a predetermined value. Is also easy and highly reliable construction is possible.

Further, in this embodiment, when the minute bubbles are injected into the sandy ground 1, the water mixed with the bubbles by the bubble mixing device 4 is used.
Air bubbles are injected into the ground 1 by press-fitting this into the ground 1 through the injection pipe 3, so that the injection amount and the injection pressure of the water 12 mixed with air into the sandy ground 1 are adjusted. In the case where the amount of air bubbles injected into the ground 1 can be easily controlled by appropriately controlling, and air bubbles are injected from the air holes of the injection pipe 3 simply by sending compressed air into the injection pipe 3. In comparison with the above, air bubbles can be reliably and easily injected into the sandy ground 1.

That is, according to the conventional research, it has been reported that the permeability coefficient of sand decreases with an increase in the degree of saturation and closes at about 50% of the degree of saturation. This means that if only air bubbles are simply injected into the sand, the saturation of the sand will not reach 50% or less. Therefore, it has been considered difficult to send air into the saturated ground without disturbing the skeletal structure of the soil. However, if micro bubbles are mixed into the water 12 as in the present embodiment and the micro bubbles are injected into the ground together with the water 12, air can be sent into the sand without clogging, and The quality ground can be brought to the desired degree of saturation.

In addition, if the bubble mixing device 4 including the pressure vessel 10 and the decompression vessel 18 is used, the pressure difference between the vessels 10 and 18 makes it easy to form bubbles having a uniform and fine particle diameter and a uniform distribution in the water 12. Moreover, it can be manufactured quickly.

Next, a second embodiment of the present invention will be described with reference to FIGS.

In this embodiment, a soil improvement method according to the present invention is applied to a liquefaction prevention method implemented when a building 2 has already been constructed on a sandy ground 1 which is almost completely saturated as in the first embodiment. This is an embodiment applied to the present invention. In the following description, the same components as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.

The difference between this embodiment and the first embodiment lies in the arrangement of the injection means and the drainage means. That is, as shown in FIGS. 5 and 6, on the sandy ground 1 on which the building 2 is constructed, deep wells 40, 40,. , And the injection pipe 3 and the submersible pump 41
Are appropriately inserted to form an injection means and a drainage means, respectively.

Therefore, according to this method, the same operation and effect as those of the first embodiment can be obtained. The procedure for arranging the injection pipe 3 and the submersible pump 41 is as follows. For example, one deep well 40 is selected, the injection pipe 3 is inserted into this, and the submersible pump 4 is inserted into the other deep wells 40.
Inserting 1, ... to inject the aerated water 12 and drain the groundwater, and sequentially move the deep well 40 where the infused pipe 3 is inserted, so that the aerated air enters the sandy ground 1 directly under the building 2. Procedures such as injecting water 12 may be mentioned.

On the other hand, as shown in FIGS.
May be excavated so as to surround the deep wells 40,.

The details of the ground improvement method of the present invention are not limited to the above-described embodiment, and various modifications are possible.

As an example, in the above-described embodiment, a well point is used as a means for draining groundwater. However, the present invention is not limited to this. For example, when the groundwater level is particularly low, a deep well (deep well) is used.
And a drainage means such as draining groundwater with a submersible pump or the like may be employed.

Furthermore, the method of mixing bubbles in water is also optional,
For example, nitrogen gas, carbon dioxide gas or the like may be blown into water to produce water mixed with air bubbles, or a solid foaming agent may be poured into water to produce water, and there is no particular limitation.

On the other hand, the soil improvement method of the present invention is not only applied to the prevention of liquefaction of sandy ground, but also, for example, when anaerobic bacteria abnormally grow in a landfill, air bubbles are injected into the landfill to remove bacteria. It can also be suitably applied as a ground improvement method that suppresses the propagation of the soil.

"Experimental example" This experimental example is one of the studies conducted on liquefaction countermeasures to inject microbubbles into saturated sand ground to lower the degree of soil saturation and reduce 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 degree of saturation, that is, the relationship between the B value and the elastic wave velocity, was considered, and the ground saturation state based on the elastic wave velocity was estimated for monitoring the effect of countermeasures. We examined the possibility of the law.

Samples and test methods The samples used were Toyoura sand and gravel. Table 1 shows the physical properties of the samples.

The test is a large triaxial test device (specimen: 300 mm in diameter, height 6
00mm), and the measurement of the elastic wave
The test was carried out while maintaining the pressure at 49 kPa to saturate the specimen. The degree of saturation was determined by measuring the change in volume of pore water due to back pressure loading using a pressure proof burette (capacity: 1000 cm ³ ) and using the boiling rule.

Saturation, B value and elastic wave velocity Using the theory of porous elastic body 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: bulk elastic constant of water without any bubbles (2.2 × 10 kPa) Ka: volume of air Elastic constant (pore water pressure expressed in absolute pressure) n: porosity FIG. 8 shows a change in Vp depending on the degree of saturation estimated using equation (1). Ratio of bubbles in pore water (1-
Sr) is 10 ^-5 to 10 ^-3 (99.999 to 99.9% in terms of saturation)
The change of Vp in the region changing to is extremely large.

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

FIG. 9 shows a change in the B value depending on the degree of saturation obtained from equation (2). It can be seen that the B value is also significantly affected by the degree of saturation, and the change is particularly large in a region where (1-Sr) changes from 10 ^-3 to 10 ^-1 .

Measurement Results and Discussion FIG. 10 shows the relationship between the elastic velocity measured in the saturation process and the degree of saturation. Vp is remarkably influenced by the degree of saturation, and shows a change corresponding well to the estimated value shown in FIG. 8, whereas the change in the shear wave velocity (Vs) is extremely small. According to the calculation, the shear elastic constant showed an almost constant value during the saturation process, even if the increase in density with the increase in saturation was taken into account. Comparison between (1-Sr) cal. Obtained from equation (1) and (1-Sr) meas. Obtained from pore water volume change measurement using actually measured Vpd (Vp measured on a dry specimen) and Vp. Result 11
Shown in the figure. In a small region of (1−Sr), there is a variation considered to be due to the measurement accuracy, but it can be determined that there is a good correlation as a whole.

FIG. 12 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. 12 represents the following equation obtained by applying the porous elastic body theory to equation (2).

B = 1−Kd / K (3) It can be seen that the calculated curve closely approximates the measured data. FIG. 13 shows Kd from the elastic wave velocity measured on the dried specimen.
Is a comparison result between the B value (Bcal.) Calculated using Equation (3) and the actually measured B value (Bmeas.). There is a good correlation between the two over a wide range.

Conclusion The change of elastic wave velocity during the saturation process of sand and gravel specimens was measured, and it was shown that the P-wave velocity was significantly reduced by only slightly decreasing the degree of saturation from the state of complete saturation.
This can be explained by the theory of a porous elastic body, considering the bulk elastic constant of pore water containing many bubbles, and the saturation degree, that is, the B value can be estimated from the elastic wave velocity. Revealed.

From the above results, the effectiveness of the method of monitoring the effect of the measurement by measuring the elastic wave velocity of the ground in the liquefaction countermeasures by injecting bubbles was suggested.

[Effects of the Invention] As described above in detail, according to the present invention, the injection means and the drainage means are arranged in the ground on which the structure has been constructed, and the liquid mixed with air bubbles is discharged from the upper end of the means. By injecting into this, minute bubbles are injected into the ground from the injection means, and groundwater is drained by the drainage means to improve the ground. Is replaced by the liquid mixed with the water, and the injection of air bubbles into the ground is performed quickly and easily as compared with the case where the groundwater is not drained by the drainage means. In addition, it is possible to suppress the occurrence of a phenomenon such as swelling of the ground due to an increase in groundwater.

In addition, in the present invention, since the injection means and the drainage means are arranged so as to surround the structure, if the liquid injection and the groundwater drainage are performed by the injection means and the drainage means, the water flowing in the direction of penetrating the structure can be obtained. The flow can be generated in the ground, and the injection of air bubbles into the ground immediately below the structure is ensured.

[Brief description of the drawings]

1 to 4 are views showing a ground improvement method according to a first embodiment of the present invention. FIG. 1 is a perspective view showing a state in which an injection means and a well point are penetrated into the ground. 2 (a) and 2 (b) are a plan view and a side view schematically showing an injection means and a well point, respectively. FIG. 3 is a schematic view showing an example of a bubble injection device, and FIG. 4 is an example of a well point. 5 and 6 are views showing a ground improvement method according to a second embodiment of the present invention. FIG. 5 shows a state in which an injection means and a well point are penetrated into the ground. FIG. 6 is a plan view of the same, FIG. 6 is a plan view of the same, and FIG. 7 is a view of a ground improvement method according to a third embodiment of the present invention. , FIG. 8 is a graph showing the relationship between the saturation and the P-wave velocity, and FIG. 9 is a graph showing the relationship between the saturation and the B value. Graph, FIG. 10 is a graph showing a change in elastic wave velocity depending on saturation, FIG. 11 is a graph showing a relationship between an actually measured value and an estimated value of the saturation, and FIG. 12 is a relationship between a bulk elastic constant and a B value. FIG. 13 is a graph showing the relationship between the calculated value of B value and the actually measured value. 1 ... sandy ground, 2 ... building (structure), 3 ... injection pipe (injection means), 4 ... air bubble mixing device, 5 ... well point (drainage means), 12 ... water (liquid) ).

Continuation of the front page (56) References JP-A-51-16708 (JP, A) JP-A-47-33412 (JP, A) JP-A-54-84303 (JP, A)

Claims (2)

(57) [Claims] 1. An injection means and a drainage means are arranged in a ground on which a structure is constructed so as to surround the structure, and a liquid mixed with air bubbles is pressed into the inside of the means from the upper end of the means. Thus, a ground improvement method in which minute bubbles are injected into the ground from the injection means, and groundwater is drained by the drainage means to improve the ground. 2. The soil improvement method according to claim 1, wherein said injection means and drainage means are arranged in pairs so as to face each other with said structure interposed therebetween.