JP5161815B2 - Ground improvement method - Google Patents

Description

  The present invention relates to a ground improvement method, and preferably relates to a ground improvement method that requires countermeasures against liquefaction.

  The liquefaction of the ground is accompanied by a sudden rise in the water pressure of pore water that exists in a saturated state between the soil particles as the ground with high moisture content undergoes deformation due to impact and vibration due to an earthquake. This is a phenomenon in which the friction resistance between soil particles disappears and the ground behaves like a liquid and loses its proof strength.

  In order to prevent liquefaction, methods such as increasing the strength of the ground or injecting fine particles into the ground are known, but recently, liquefaction prevention methods by pumping groundwater have proven effective. doing. However, in order to keep the groundwater level low at all times, the groundwater must be continuously and permanently pumped, resulting in a problem that the operation cost and maintenance cost required for it are increased.

  Therefore, the applicant does not keep pumping the groundwater and keep the groundwater level low, but gives the ground ground an overconsolidation history (preloading) by temporarily lowering the groundwater level. A liquefaction prevention method was proposed (Patent Document 1) in which (the volume of groundwater in the ground / the gap volume of the ground) is reduced, thereby increasing the ground strength against liquefaction (Patent Document 1).

  On the other hand, Patent Document 2 discloses a method of injecting compressed gas into the ground for preventing liquefaction, but simply mixing compressed gas has a limit in reducing the degree of saturation, and the ground There was a risk of raising.

  In order to solve the above-mentioned problem, the applicant of the present invention uniformly mixes in a wide range by mixing mixed water having a large number of ultrafine bubbles having a diameter of less than 100 micrometers into a target region for improving ground strength. A method for reducing the saturation of the ground was presented (Patent Document 3).

Japanese Patent No. 3678828 Japanese Patent No. 3757216 JP 2008-2170 A

However, according to the method disclosed in Patent Document 3, after the mixed water having a large number of ultrafine bubbles is adjusted in the container in advance and then mixed into the target region, a large amount of bubbles are bonded together. As a result, there is a problem that the efficiency of mixing into the ground is reduced. Such a phenomenon tends to occur particularly when mixing at a low flow rate.
Therefore, the main problem to be solved by the present invention is to improve the ground by providing water having a large number of ultrafine bubbles to the ground more efficiently than before in reducing the saturation of a wide range of target areas of the ground. It is to provide a method that can be performed.

The present invention that has solved this problem is as follows.
[Invention of Claim 1]
An improvement method of the ground in which water having a large number of ultrafine bubbles is mixed in the ground of the target area,
In the ground of the target area, provide a mixing tube with a bubble generating nozzle near the tip,
Supplying premixed mixed water containing a large amount of air to the mixed pipe;
Immediately before mixing the mixed water into the ground of the target area, the bubbles are mixed into the ground while generating a large number of ultrafine bubbles with an average diameter of 10 to 60 micrometers in the mixed water by the bubble generating nozzle.
A ground improvement method characterized by this.

(Function and effect)
In the present invention, after water containing a large number of air is generated in advance in the container, it is mixed to the vicinity of the target region where the ground strength is to be improved, and ultrafine bubbles are generated immediately before mixing. According to the present invention, water containing a large number of bubbles having an average diameter of 10 to 60 micrometers, preferably 10 to 45 micrometers, and more preferably 20 to 30 micrometers can be mixed in the ground.

  Due to the mixing, the mixed water flows in the ground, for example, the gap in the liquefied layer, and the ultrafine bubbles are adsorbed on the sand particles and remain. This phenomenon is considered to be the following factor. In other words, the ultrafine bubbles are reduced by the surface tension of water under high pressure, but the curvature of the bubbles is created by the ultrafine bubbles released from pressure adhering to the sand particle surface in the process of drifting in the groundwater. It is considered that bubbles are precipitated when the radius is reduced, the bubble internal pressure is reduced, and the bubbles are bonded to each other.

  Even if the water in which bubbles are generated in advance is mixed, there is a problem that the bubbles are combined before the mixed water reaches the ground, and the bubbles cannot efficiently enter the gap in the ground. However, since the present invention generates bubbles in the mixed water immediately before mixing into the ground, it is possible to minimize the influence of bonding between the bubbles.

[Invention of Claim 2]
The ground improvement method according to claim 1, wherein an orifice is disposed at a discharge port of the bubble generating nozzle.

(Function and effect)
By providing an orifice at the discharge port of the bubble generating nozzle, it is possible to stably generate fine bubbles even at a low flow rate.

[Invention of Claim 3]
The ground improvement method according to claim 1 or 2, wherein a sleeve check valve is provided on an outer wall of the mixing pipe.

(Function and effect)
A sleeve check valve is arranged on the outer wall of the mixing tube, preferably on the outer wall near the tip. A plurality of check valves can be installed. In addition to the function of the sleeve check valve itself to generate ultrafine bubbles in the mixed water, it is possible to prevent the mixing of fine particles from the ground into the nozzle.

[Invention of Claim 4]
The pre-generated mixed water containing a large amount of air has air bubbles from the bubble generating nozzle in the water in a state where the bubble generating nozzle faces the water in the pressure resistant container and the container is maintained in a pressurized state. The ground improvement construction method of any one of Claims 1-3 obtained by generating.

(Function and effect)
If only the bubble generation nozzle is used immediately before mixing the mixed water into the ground, the mixed water does not contain sufficient air, so bubbles cannot be generated efficiently. Therefore, it is necessary to include as much air as possible in the mixed water in advance.

  It has been found that when air bubbles are generated from the air bubble generating nozzle while the inside of the container is maintained in a pressurized state, more air can be contained in the water. The amount of dissolved oxygen DO in water varies depending on the water temperature. If the pressure is about 20 mL / L at atmospheric pressure, the solubility up to about 80 mL / L, that is, the amount of dissolved air can be increased under a pressure of 400 kPa.

  It is desirable that the pressure in the container is significantly higher than the pressure in the target area of the ground. For example, when desaturation is performed up to a depth of 10 m in the target region of the ground, by generating bubbles in the water at a pressure higher than the pressure at the depth of the ground of 10 m (depending on the ground density, but about 100 kPa), It becomes possible to generate a large amount of dissolved air as fine bubbles.

[Invention of Claim 5]
The ground improvement construction method according to any one of claims 1 to 3, wherein the pre-generated mixed water containing a large amount of air is generated using a vortex turbine pump.

(Function and effect)
A large amount of air-containing water can also be generated using a vortex turbine pump. In this case, water and air are mixed using a vortex turbine pump to generate supersaturated air-dissolved water. After the amount of dissolved air is sufficiently increased in the surplus air separation tank, it is mixed into the mixing tube.

[Invention of Claim 6]
The ground improvement method according to any one of claims 1 to 5, wherein the mixed water is mixed after the groundwater is pumped by a pumping well provided on the target ground to lower the groundwater level and cause overconsolidation.

(Function and effect)
Applicants do not keep pumping groundwater and keep the groundwater level low, but rather lower the groundwater level temporarily to reduce the saturation of the ground, thereby increasing the ground strength against liquefaction. A prevention method has been proposed (Patent Document 1). This method has an advantage of eliminating the problem of increased operation costs and maintenance costs due to the fact that the groundwater must be continuously and permanently pumped in order to keep the groundwater level always low.

  According to the invention of claim 3, when groundwater is pumped by a pumping well provided in the target ground, the groundwater level is lowered to cause overconsolidation, and then the groundwater level is naturally restored to the original groundwater level state, or When the mixed water of the present invention is mixed in the recovery process, the ultrafine bubbles can be uniformly dispersed in the target region. As a result, the effect of increasing the ground strength against liquefaction due to overconsolidation history and the effect of lowering the degree of saturation accompanying the dispersion of ultrafine bubbles can be combined to obtain a high ground strength against liquefaction.

[Invention of Claim 7]
The ground improvement construction method according to any one of claims 1 to 6, wherein the bubble generating nozzle is configured to eject pressurized air along the center of the contracted flow of the swirling water flow around the nozzle central axis.

(Function and effect)
If the configuration is such that the pressure air is ejected along the center of the contraction of the swirling water flow around the central axis of the nozzle, the vortex will collapse at the contracted portion of the high-speed swirling water flow, and the bubbles accompanying the ejection of the pressure air Is mixed, the bubbles are destroyed and become small bubbles. In this case, if the inside of the container is kept in a pressurized state, the bubbles become finer.

  According to the present invention, in order to reduce the saturation of the target region of the ground, when mixing the ultrafine bubble water into the ground, the ultrafine bubbles are stably included in the mixed water even in a low flow rate state. It becomes possible to.

It is an outline explanatory view of construction concerning the present invention. It is a general | schematic perspective view of the example of a bubble generator. It is a longitudinal cross-sectional view of a guide body. It is a side view of a guide body. It is a longitudinal cross-sectional view of the other bubble generator example. It is a schematic explanatory drawing near the front-end | tip of a mixing rod. (A) It is a schematic explanatory drawing of a reducer rod, and (b) It is BB sectional drawing. It is (a) rubber sleeve type check valve appearance, (b) internal structure, and (c) AA sectional view. It is a construction outline figure of other examples. It is an actual ground model experiment outline figure. It is a graph of an experimental result.

Next, an embodiment of the present invention will be described by taking improvement of the liquefaction countermeasure ground as an example.
<Outline of construction method>
FIG. 1 shows an example of an ultrafine bubble water ground mixing system. The ultrafine bubble water (hereinafter referred to as microbubble water) generating device includes a pressure-resistant tank 1, a bubble generating nozzle 2, a circulating high-lift pump 3, and a compressor 4, and is capable of generating microbubble water under pressure. Microbubble water, for example, draws groundwater from the pumping hole 5, fills the pressure tank, pressurizes the tank and circulates the water in the tank for a predetermined time, thereby generating microbubbles and high concentration ultrafine water. Make a bubbly water. After the generation is completed, microbubble water is mixed from the mixing tube 6 (for example, the mixing rod 10). At that time, by adding the bubble generating nozzle 7 near the tip of the mixing boring hole or mixing rod 10, micro bubbles are further generated, and the ground is desaturated.

  This system mixes microbubble water from the mixing pipe and promotes lowering of the groundwater level and penetration of microbubble water in the improved ground. At the same time, the pumping pump 9 draws groundwater from the water hole 5 and circulates the groundwater. It is a mechanism that desaturates the target ground. However, in the present invention, microbubble water can be mixed after pumping water and causing overconsolidation in the liquefaction countermeasure ground. Even if overconsolidation is not performed, the effect of the present invention is sufficiently exerted.

<Example of bubble generation nozzle>
2 to 4 exemplify the bubble generating nozzles 2 and 7, and a guide body 22 with wings is provided in the cylinder 21. The cylindrical body 21 includes a cylindrical main body 21A, a conical constricted flow portion 21B that is tapered, and a small cylindrical ejection portion 21C. A guide body 22 is housed inside the cylindrical main body 21A.

  The guide body 22 has a cylindrical main body portion 22A and a rear end (left side in FIG. 5) having a substantially hemispherical inlet portion 22B, and wing bodies 22C and 22C are provided on the outer periphery from the inlet portion 22B to the front end of the main body portion 22A. ... are integrated. Here, the tip portions of the wing bodies 22C, 22C,... In the groove space surrounded by the wing bodies 22C and 22C and the cylinder body 21, water W0 from the high head pump 3 or the pressure tank 1 is introduced into the cylinder body 21 from the rear. Further, an introduction hole 22D for pressurized air A that enters the inside from the outer peripheral surface of the main body 22A and exits from the center of the tip (right side in FIG. 5) is formed.

  In the bubble generation nozzles 2 and 7, the water W0 introduced from the high head pump 3 or the pressure tank 1 from the rear into the cylinder 21 enters the groove spaces from the inlet 22B. The water that has entered the groove space flows to the tip side of the groove space. At the tip portion, the wing bodies 22C, 22C,... It flows out into the main body 21A. As a result, a swirl flow F1 is generated in the main body 21A forward from the front end of the main body 22A. The swirling flow F1 gradually becomes a small vortex F2 in the contracted flow portion 21B and goes to the tip, and particularly in the tip portion of the contracted flow portion 21B, the vortex flow F2 collapses and bubbles are generated.

  On the other hand, the pressurized air A is ejected from the tip of the introduction hole 22D, and the energy is given to the vortex F2 or its collapsed portion. As a result, the generated bubbles are refined and ejected from the ejection part 21C.

  Thus, micronized microbubbles are formed in the pressure tank 1 or in the water near the tip of the mixing rod 10. This water is mixed as mixed water W into the target area where the ground strength is to be improved.

  FIG. 5 shows another example of the bubble generating nozzle 2a, in which an enlarged diameter portion is formed on the tip side of the ejection portion 21Ca.

  As the bubble generation method, an ultrasonic method, an ultra-high-speed swirling method, a gas-liquid two-layer convection mixing / shearing method, and the like can be used. However, according to the study by the present inventors, it has been found that the above examples are optimal in the generation rate and stability of microbubbles.

<Example of mixed rod>
FIG. 6 shows a distal end portion of the mixing rod 10 as an example of a mixing tube used for mixing water into the ground. For the purpose of simple construction at the site, a 33.5 mm diameter boring rod that is relatively small in diameter and can be constructed even with a simple sounding machine is used.

A portion 31 surrounded by a broken line of the 33.5 mm diameter mixing rod 10 is a reducer rod (FIG. 7), and the bubble generating nozzle shown in FIGS. 2 to 5 is attached to the gap 38 of this portion. The reducer rod 31 has an orifice-like lid 33 having a hole 37 as shown in FIG. Microbubbles are restored when water containing high-concentration air passes through a bubble generating nozzle installed on the reducer rod. At this time, since the reducer rod has the orifice, the microbubbles can be stably restored even at a low flow rate. A plurality of rubber sleeve check valves are attached to the tip side of the reducer rod for mixing microbubble water into the ground (FIG. 8). The microbubble water is structured such that when the pressure is applied to the rubber sleeve check valve from the inside, the rubber sleeve valve 35 closing the hole portion 34 is opened, and the microbubble water can be mixed into the ground. The check valve is closed when microbubble water is not mixed, such as when the rod penetrates the ground, so that sand and clay in the ground do not enter the rod.
The tip of the rod has a conical cone shape for easy penetration.

<Construction conditions, etc.>
In the present invention, mixed water having many microbubbles having a diameter of less than 100 micrometers is used. The diameter of a preferable microbubble is 10 to 60 micrometers on average, preferably 10 to 45 micrometers, and more preferably 20 to 30 micrometers. The diameter of the bubbles defined in the present invention is 70% or more, particularly 80% or more as long as it has the diameter defined above, and all the bubbles need not have the diameter defined above.

  The pressure in the bubble generating container (for example, the pressure tank 1) is preferably about 0.2 MPa to 0.7 MPa. The pressure at the time of mixing can be about 0.05 MPa to 0.6 MPa.

  The target region for improving the ground strength is not limited as long as the ground needs improvement. It is desirable that the mixing amount of the mixing water is a mixing amount that can be replaced with microbubbles. Usually, the degree of saturation is set to 90% or less, and whether or not the target degree of saturation has been reached can be determined by, for example, measuring the specific resistance of the ground.

  Since the ground replaced with microbubbles remains in the particles constituting the ground, for example, the gap between the ground as described above, the replacement state can be maintained for a long time. If intense groundwater flow is expected, saturation can be measured at time intervals of about 2 to 3 years, and mixed water can be mixed again if necessary.

<Other embodiments>
FIG. 9 shows another embodiment. The form shown in FIG. 9 is the same as the form shown in FIG. 1 until the groundwater is pumped up, but the vortex turbine pump 43 is used to generate supersaturated air-dissolved water, and the excess air is separated in the excess air separation tank 41. Except for the above, water sufficiently high in the dissolved rate of air is injected into the mixing tube. Immediately before mixing water into the ground, micro bubbles are generated using the bubble generation nozzle 7 of the mixing rod 10, and water is poured into the ground from the check valve 11.

The effect of lowering the degree of saturation of the ground in the present invention was verified by the microbubble water mixing experiment on the actual ground model. The test apparatus is shown in FIG. The experiment was performed using an acrylic cylindrical container (column) 52 having an inner diameter of 75 mm and a height of 300 mm, and using a ground 51 having a wet density ρt = 1.6 g / cm ³ similar to the actual ground and saturated. Then, a decrease in the saturation Sr due to the mixing of microbubble water was investigated. Furthermore, in order to make it the same ground condition as the actual ground experiment, the back pressure of the specimen was set to 50 kPa, and mixed microbubble water was generated using a device that was used on site. Sr was calculated from the following formula [Equation 1] by measuring the weight change of the column.

Here, Sr: ground saturation (%), Vw (m ³ ): volume of pore water, Vv (m ³ ): volume of gap, ΔW (g / min): weight change (g), ρw: water Density (g / cm ³ ).

  The average diameter of the microbubbles in the actual ground 51 in this test apparatus was calculated from the rising speed of the microbubbles using Stokes' law. The microbubble rising speed was 400 μm / s, and the average diameter was calculated to be 30 μm.

  Table 1 shows the test conditions of Examples and Comparative Examples. A case in which microbubble water was directly mixed was used as a comparative example, and a case in which a microbubble generating nozzle was used again immediately before mixing was used as an example.

  FIG. 11 shows the results of an actual ground model experiment in this experimental apparatus. In an example in which microbubbles were generated immediately before mixing, it was found that the saturation of the ground can be reduced more efficiently.

  The present invention is an invention for the purpose of ground improvement, particularly prevention of liquefaction, and can be used mainly in the field of architecture.

  DESCRIPTION OF SYMBOLS 1 ... Pressure-resistant tank, 2 ... Bubble generating nozzle, 3 ... High lift pump, 4 ... Compressor, 5 ... Water hole, 6 ... Mixing pipe, 7 ... Bubble generating nozzle, 8 ... Notch tank, 9 ... Lifting pump, 10 ... Mixing Rod, 11 ... Check valve, 21 ... Tube, 22 ... Guide body with turbine blades, W0 ... Introduced water, W ... Mixed water, 31 ... Nozzle reducer, 32 ... Boring rod screw, 33 ... Orifice lid, 34 ... Hole, 35 ... Packer rubber sleeve check valve, 36 ... Screw, 37 ... Hole, 38 ... Bubble generation nozzle mounting position, 41 ... Excess air separation tank, 43 ... Eddy current turbine pump, 51 ... Practical ground, 52 ... Column, 53 ... load cell, 54 ... pore water pressure gauge, 55 ... back pressure compressor.

Claims (7)

An improvement method of the ground in which water having a large number of ultrafine bubbles is mixed in the ground of the target area,
In the ground of the target area, provide a mixing tube with a bubble generating nozzle near the tip,
Supplying premixed mixed water containing a large amount of air to the mixed pipe;
Immediately before mixing the mixed water into the ground of the target area, the bubbles are mixed into the ground while generating a large number of ultrafine bubbles with an average diameter of 10 to 60 micrometers in the mixed water by the bubble generating nozzle.
A ground improvement method characterized by this.   The ground improvement method according to claim 1, wherein an orifice is disposed at a discharge port of the bubble generating nozzle. The ground improvement method according to claim 1 or 2, wherein a sleeve check valve is provided on an outer wall of the mixing pipe.   The pre-generated mixed water containing a large amount of air has air bubbles from the bubble generating nozzle in the water in a state where the bubble generating nozzle faces the water in the pressure resistant container and the container is maintained in a pressurized state. The ground improvement construction method of any one of Claims 1-3 obtained by generating.   The ground improvement construction method according to any one of claims 1 to 3, wherein the pre-generated mixed water containing a large amount of air is generated using a vortex turbine pump.   The ground improvement method according to any one of claims 1 to 5, wherein the mixed water is mixed after the groundwater is pumped by a pumping well provided on the target ground to lower the groundwater level and cause overconsolidation.   The ground improvement construction method according to any one of claims 1 to 6, wherein the bubble generating nozzle is configured to eject pressurized air along the center of the contracted flow of the swirling water flow around the nozzle central axis.

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