JP2018162632A - Ground freezing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for applying a ground freezing method even if the groundwater flow velocity is fast and the ground has little or no groundwater, in which an original recovery process makes it possible to demonstrate excellent environmental restoration after completion of construction work.SOLUTION: The present invention comprises: <1> a step of injecting super absorbent polymer-containing liquid 4 into the ground around an injection pipe 3 through the injection pipe 3 in a borehole 2 of the ground 1 to prepare a super absorbent polymer mixed soil part 5; <2> a step of piping a freezing pipe 6 to the borehole 2 and sending cooling liquid 7 into the freezing pipe 6 to prepare a frozen soil part 5a of the super absorbent polymer mixed soil part 5; and <3> simultaneously or sequentially performing the steps of <1> and <2> at a plurality of predetermined positions and a step of connecting the frozen soil part 5a to construct a frozen wall 5b.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a ground freezing method that is one of ground improvement methods for constructing a structure on the ground.

  The ground freezing method is one of the ground improvement methods for constructing structures on the ground. In the ground freezing method, a freezing pipe is piped into a drilling hole provided by excavating the target ground, and a cooling liquid fed into the freezing pipe is circulated to create frozen soil. The soil in the ground gradually freezes together with the water contained in the ground to become frozen soil, gradually grows in a ring shape from the ground around the freezing pipe, and eventually becomes a cylindrical frozen soil pillar. The frozen soil pillars created in this way are interconnected with other frozen soil pillars formed around adjacent freezing pipes to construct a plate-shaped frozen soil wall.

  The frozen earth wall constructed in this way blocks groundwater. Moreover, such a frozen soil wall can exhibit the same strength and pressure resistance as concrete. Therefore, it is possible to construct a structure in an area surrounded by frozen soil walls and where the flow of groundwater is blocked.

  However, when constructing a frozen soil wall over a wide area, such as the impermeable wall of the Fukushima Daiichi nuclear power plant, the effect of the groundwater flow is a problem. For example, in ground where the groundwater flow velocity (limit flow velocity) exceeds 2.0 m / day (see, for example, “Japan Construction Mechanization Association, Ground Freezing Method-From Planning / Design to Construction”). However, due to the heat brought in by the groundwater, it was difficult to connect the frozen soil columns to each other, and there was a problem that unfrozen portions were generated on the frozen soil wall. Furthermore, when a frozen soil wall is being built around an unfrozen part, there is also a problem that the groundwater flows around the unfrozen part and the groundwater flow is concentrated at one point, thereby increasing the flow velocity and preventing freezing.

  Therefore, various improvements have been studied when constructing frozen soil walls in the ground where the flow rate of groundwater is high. For example, a soil improvement method that reduces the hydraulic conductivity of soil by, for example, injecting chemicals into the ground between multiple freezing pipes, pumping wells upstream and downstream of the construction site, pumping up groundwater, and relative groundwater flow rates A method of reducing the temperature of the cooling liquid, a method of increasing the number of rows of freezing tubes to reduce the pitch between the plurality of freezing tubes, a method of reducing the temperature of the cooling liquid, and the like have been proposed.

  However, in the method using chemical injection, the chemical solution may be washed away by the groundwater flow, and not only the effectiveness is not exhibited, but also environmental pollution due to diffusion of the flowed chemical solution may become a problem. In the ground freezing method, since the restoration of the original state after construction is generally required, such diffusion of chemicals and environmental pollution are not preferable.

  On the other hand, in the method of installing a pumping well, it was necessary to continuously pump up groundwater from the pumping well using a pump or the like during the construction period using the ground freezing method. In addition, in the method of increasing the number of rows of freezing tubes and the method of reducing the temperature of the cooling liquid, power consumption is reduced in order to send the cooling liquid to the additional freezing tubes, to lower the temperature of the cooling liquid, and to maintain the low temperature. There is a problem that the construction cost increases.

  In view of such problems, the ground freezing method disclosed in Patent Document 1 proposes a method of forming a heat transfer section around a cooling pipe piped in a borehole using a freezing material having a special composition. Yes.

  In the proposal of Patent Document 1, a liquid in which a thermal conductive material having a higher thermal conductivity than that of a base material is used as a base material and a dispersion material that increases fluidity are kneaded. Thus, a freezing material is formed. It has also been proposed to mix metal or carbon as a thermally conductive material in this freezing material. According to such a proposal, the freezing material has high fluidity and thermal conductivity, and the freezing latent heat of water released at the time of freezing is reduced, so that the ground to be frozen can be frozen in a short time. It is supposed to be possible.

JP 2008-69246 A

  However, even in the ground freezing method of Patent Document 1, no consideration has been given to freezing the ground where the flow rate of groundwater is fast, and there is a problem that the applicable ground is limited. In addition, since the metal contained in the freezing material remains in the soil constituting the ground, there is an aspect that is not necessarily a construction method with little environmental pollution.

  Furthermore, since the groundwater level is closely related to precipitation, it is known to change seasonally. In general, the water level is high in summer and the water level decreases in winter. Therefore, in winter, there may be no groundwater in the ground at the depth constructed by the frozen ground method. Or, a ground such as a sand layer or a gravel ground that has good drainage and little groundwater is known. In such ground where there is little or no groundwater, it was impossible to apply the ground freezing method.

  The present invention has been made in view of the circumstances as described above, and enables the application of the ground freezing method even in the ground where the flow velocity of the groundwater flow is fast, the groundwater is small or does not exist, and the original state is restored. The task is to realize a method that exhibits excellent environmental remediation after the construction work is completed.

  In the ground freezing method of the present invention, in order to solve the problems as described above, a freezing pipe is piped in a drilling hole provided by excavating the ground, and a frozen soil part is created with the coolant fed into the freezing pipe, A ground freezing method for constructing a frozen ground wall by connecting the frozen soil portions, comprising at least the following steps <1>, <2>, and <3>.

<1> A step of injecting a superabsorbent polymer-containing liquid into the ground around the injection pipe through an injection pipe in a ground excavation hole to create a superabsorbent polymer mixed soil part;
<2> A step of piping a freezing pipe into the excavation hole, sending a coolant into the freezing pipe, and forming a frozen soil portion of the superabsorbent polymer mixed soil portion;
<3> The step of <1> and <2> are performed simultaneously or sequentially at a plurality of predetermined positions, and the frozen soil part is connected to construct a frozen soil wall.

  In the ground freezing method of the present invention, in the step <1>, the ground soil and the superabsorbent polymer are mixed and agitated by a stirring blade of an injection pipe when forming the excavation hole to mix the superabsorbent polymer. It is preferred to create the soil.

  Further, in the ground freezing method of the present invention, the ground is at least one selected from the group consisting of sandy ground, gravel ground, ground where groundwater level is lowered due to seasonal variation, and ground above the groundwater surface. Are preferably considered.

  In the ground freezing method of the present invention, the superabsorbent polymer-containing liquid preferably contains superabsorbent polymer particles that are at least one selected from the group consisting of starch-based, cellulose-based and synthetic polymer-based. Be considered.

  Furthermore, in the ground freezing method of the present invention, it is preferably considered that the superabsorbent polymer-containing liquid contains a swelling degree adjusting agent.

  In the ground freezing method of the present invention, it is preferably considered that the swelling adjusting agent contains an electrolyte, and the electrolyte is a monovalent metal salt.

  Furthermore, the ground freezing method of the present invention includes the following original state restoration step <4> <5> following the frozen ground wall construction step of step <3> in the above ground freezing method.

<4> After melting the frozen earth wall and the frozen earth part, the injection pipe is piped into the excavation hole, and a separating agent is injected into the ground around the injection pipe through the injection pipe, and the high water absorption Releasing moisture taken up by the polymer particles;
<5> A step of removing the injection pipe from the excavation hole.

  Further, in the ground freezing method of the present invention, it is preferably considered that the separating agent contains an electrolyte, and the electrolyte is a divalent or higher polyvalent metal salt.

  According to the present invention, it is possible to apply the ground freezing method even in the ground where the flow velocity of the groundwater flow is fast or in the ground where there is little or no groundwater. Can demonstrate its sexuality.

(A) and (b) are the schematic process sectional drawing and the top view which showed the ground freezing method of this invention typically. (A) and (b) are the schematic process sectional drawing and the top view which showed typically the original state restoration process of the ground freezing method of this invention. It is the graph which showed the relationship between the electrical conductivity of a highly water-absorbing polymer containing liquid, and the water absorption magnification of a highly water-absorbing polymer particle. It is the graph which showed the relationship between the density | concentration (mass%) of NaCl which is a swelling degree adjusting agent, and the electrical conductivity of a super absorbent polymer containing liquid. It is the graph which showed the relationship of the free water rate and funnel viscosity (fluidity) in a super absorbent polymer containing liquid. It is the schematic which showed the apparatus which measures a hardly water-permeable layer formation. It is a graph which shows the result of the poorly water permeable layer formation test done using the apparatus of FIG.

  Below, the ground freezing construction method of this invention is demonstrated in detail along drawing.

  FIGS. 1A and 1B are a schematic cross-sectional view and a plan view schematically showing the ground freezing method of the present invention.

  In the ground freezing method of the present invention, a freezing pipe is formed in a drilling hole provided by excavating the ground, and a frozen soil part is created by using the coolant fed into the freezing pipe, and the frozen ground part is connected to construct a frozen soil wall. This is a ground freezing method that includes at least the following steps <1>, <2>, and <3>.

  That is, <1> a step of injecting the superabsorbent polymer-containing liquid 4 into the ground around the injection pipe 3 through the injection pipe 3 in the excavation hole 2 of the ground 1 to form the superabsorbent polymer mixed soil portion 5; <2> A step of constructing the frozen soil portion 5a of the superabsorbent polymer mixed soil portion 5 by piping the frozen tube 6 in the excavation hole 2 and feeding the coolant 7 into the frozen tube 6; <3> <1> The step <2> is performed simultaneously or sequentially at a plurality of predetermined positions, and the frozen soil wall 5b is constructed by connecting the frozen soil portions 5a.

  As shown in FIG. 1 (a), in the step <1> of the ground freezing method, after freezing the ground 1 to be frozen, after excavating the excavation hole 2, an injection pipe 3 is piped into the excavation hole 2. Then, the superabsorbent polymer-containing liquid 4 is injected into the injection tube 3.

  In the ground freezing method of the present invention, the target ground 1 may be at least one selected from the group consisting of sandy ground, gravel ground, ground in which groundwater level is lowered due to seasonal variation, and ground above the groundwater surface. Considered preferably.

  The hole diameter of the excavation hole 2 is, for example, about φ600 mm, but can be appropriately changed according to the diameter of the shaft used for excavation and the diameters of the injection pipe 3 and the freezing pipe 6.

  As the injection tube 3, for example, a stainless steel tube having corrosion resistance is used.

  The injection pipe 3 has a plurality of through holes 31 on the side wall surface thereof, and the highly water-absorbing polymer-containing liquid 4 fed into the injection pipe 2 is soil that forms the hole wall of the excavation hole 2 through the through holes. It can be injected into the voids between the particles and their soil particles. The through-hole 31 may be always open or may be covered with an openable / closable lid or the like.

  In addition, as the injection tube 3, a later-described freezing tube 6 can be used. In this case, since it is necessary to send and circulate the coolant 7 to the freezing tube 6, it is preferable to use a stainless steel tube having a through hole covered with an openable / closable lid as described above. Is done.

  In the ground freezing method of the present invention, in the step <1>, the soil of the ground and the superabsorbent polymer are mixed and stirred by the stirring blade of the injection pipe at the time of forming the excavation hole, and the superabsorbent polymer mixed soil portion is formed. The creation is preferably considered.

  In this case, the injection tube 3 may be, for example, a rotatable tube having a through-hole 31 at its tip or the like and a stirring blade or the like on the side surface. When such an injection pipe 3 is used, mixing and stirring of the superabsorbent polymer-containing liquid 4 from the through hole 31 and the soil particles can be simultaneously performed in the ground 1, and uniform polymer mixed soil 5 can be efficiently produced. Can be well built.

  The superabsorbent polymer-containing liquid 4 contains at least water and superabsorbent polymer particles swollen by absorbing water. The present inventors have previously proposed a stable composition of a swollen superabsorbent polymer for ground excavation and a construction method using the same, and apply the superabsorbent polymer particles used in this proposal. (See JP 2013-57061 A).

  Specifically, the highly water-absorbing polymer particle is a hydrophilic polymer having a cross-linked structure, has a water absorption of about 10 to 500 times its own weight, and hardly releases moisture even under pressure. It has the feature. Further, in the present specification, the term “superabsorbent polymer particles” means that individual particles maintain a granular form even after water absorption and swelling. For this reason, the term superabsorbent polymer particles does not include those in which a plurality of superabsorbent polymer molecules cannot maintain an independent particle form and form a pasty polymer, and are excluded. The amount of water absorption of the highly water-absorbing polymer particles is defined in JIS K 7223, and the method of measuring the amount of water absorption is performed based on the description in JIS K 7223.

  The type of the superabsorbent polymer particles used in the frozen ground method of the present invention can be used without particular limitation as long as the above conditions are satisfied. For example, starch-based, cellulose-based, synthetic polymer It is preferably considered that it is at least one selected from the group consisting of systems.

  Among the superabsorbent polymer particles described above, the synthetic polymer-based polysodium polyacrylate superabsorbent polymer particles are excellent in both performance and cost, and can be particularly preferably used.

  Polyacrylic acid sodium superabsorbent polymer particles are prepared by adding a cross-linking agent to sodium acrylate (CH2 = CH-COONa) to form a cross-linked acrylic acid polymer partial sodium salt having a three-dimensional network structure. It is a gel. A conventionally well-known thing can be used as a crosslinking agent.

  It is known that the sodium polyacrylate water-absorbing polymer particles absorb water and the carboxyl groups dissociate sodium ions in the gel, and if pure water produces a degree of swelling that is 100 to 1000 times its own weight. Yes.

  Examples of such sodium polyacrylate superabsorbent polymer particles include Geosap (manufactured by Sanyo Chemical Industries). In the case of Geosap, the water absorption ratio is about 450 times its own weight, the fluidity of the superabsorbent polymer-containing liquid 4 is good, and the voids in the gravel layer can be reliably plugged.

Moreover, the sodium polyacrylate high water-absorbing polymer particles contain a large amount of a crosslinking agent with respect to sodium acrylate, whereby the resulting gel becomes hard and its water absorption is reduced. In addition, if the amount of the crosslinking agent is reduced, the resulting gel becomes soft and the water absorption increases. Further, as the special superabsorbent polymer particles of sodium polyacrylate, the superabsorbent polymer particles polymerized with the crosslinker are used. The use of sodium polyacrylate superabsorbent polymer particles having a shell-core dual structure with the surface further crosslinked is exemplified.

  In the sodium polyacrylate highly water-absorbing polymer particles having a dual structure of shell and core, the thicker the shell as the outer shell, the harder the gel, and the amount of water absorption decreases. On the other hand, when the thickness of the shell is reduced, the gel becomes soft and the amount of water absorption increases.

  The above shell and core are usually crosslinked by an ester bond, but the cell and core are crosslinked by an ether bond excellent in alkali resistance and electrolyte resistance. There are also conductive polymer particles. In the present invention, it is more preferable to use sodium polyacrylate superabsorbent polymer particles in which the shell and the core are crosslinked by an ether bond.

  In addition to the above properties, dissociation of sodium ions in the sodium polyacrylate superabsorbent polymer particles also depends on conditions such as pH and electrolyte concentration where the gel is placed, so other superabsorbent polymers depending on the use conditions Particles can be appropriately selected and used.

  The superabsorbent polymer-containing liquid 4 is also assumed to be used at a great depth such as underground tunnel construction, and is supplied under pressure to the construction site. In this case, since the superabsorbent polymer particles in the superabsorbent polymer-containing liquid 4 are exposed to a high pressure, there is little decrease in water retention due to pressurization, and the polymer particles themselves have a crosslinked structure that is difficult to deform. It is necessary to select highly water-absorbing polymer particles. Moreover, in order to plug the gaps in the coarse gravel layer where the flow rate of groundwater is high, it is desirable that the particle size after swelling is 3 mm or less and the particle size distribution is better.

  In addition, when the ground 1 is a gravel layer (sand gravel ground), it is illustrated that the gap | interval amount between the soil particles in the ground 1 is 45 to 50%.

  In the present invention, as the superabsorbent polymer particles, when sodium polyacrylate superabsorbent polymer particles are used, they can be used without particular limitation as long as the above conditions are satisfied. What adjusted the sodium polyacrylate superabsorbent polymer particle which has a double structure of a shell and a core to said conditions can be used suitably.

  In the present invention, it is preferably considered that the superabsorbent polymer-containing liquid 4 contains a swelling degree adjusting agent. Furthermore, it is preferably considered that the swelling degree adjusting agent contains an electrolyte, and that the electrolyte is a monovalent metal salt. The degree of swelling of the superabsorbent polymer particles is maximized in pure water, but the degree of swelling is reduced in an aqueous solution containing a swelling degree adjusting agent. Based on such characteristics of the superabsorbent polymer particles, by adding a swelling degree adjusting agent to the superabsorbent polymer-containing liquid 4 in advance, the swelling of the superabsorbent polymer particles is suppressed and the particle size is controlled to be small. In addition, the content of the superabsorbent polymer particles contained per unit volume of the superabsorbent polymer-containing liquid 4 can be increased. Thereby, it becomes possible to send the highly water-absorbing polymer-containing liquid having a high content of the highly water-absorbing polymer particles to the ground 1 as the construction site without clogging the injection tube and the liquid feeding system. Then, in the ground 1, the superabsorbent polymer-containing liquid 4 and the groundwater come into contact with each other to dilute the electrolyte concentration in the superabsorbent polymer-containing liquid 4, so that the superabsorbent polymer particles have the desired particle diameter. It swells and can exhibit the effect of clogging voids between soil particles.

  As the swelling degree adjusting agent that can be added to the superabsorbent polymer-containing liquid 4, it is preferable to use an electrolyte that has a low environmental load and is unlikely to deteriorate the superabsorbent polymer particles. For example, sodium chloride, potassium chloride, Monovalent metal salts such as potassium hydroxide are preferably considered. In view of environmental load and economy, it is particularly preferable that the electrolyte is sodium chloride. By using such a swelling degree adjusting agent, it becomes possible to appropriately adjust the swelling degree of the superabsorbent polymer particles.

  As the superabsorbent polymer-containing liquid 4, for example, in a state where the superabsorbent polymer particles are swollen with pure water, for example, the specific gravity is adjusted to 1.20 or less, preferably the specific gravity is in the range of 0.95 to 1.20. This is illustrated.

  Moreover, the viscosity of the highly water-absorbing polymer-containing liquid 4 can be adjusted based on the free water rate (%) in the highly water-absorbing polymer-containing liquid 4.

  Further, an auxiliary agent can be added to the superabsorbent polymer-containing liquid 4 for the purpose of improving the hole wall stability of the excavation hole 2 and adjusting the viscosity of the superabsorbent polymer-containing liquid 4.

  Examples of the auxiliary agent include bentonite, sawdust, pulp, rock wool, fiber and the like, or a water-soluble polymer. These can be used alone or in combination of two or more.

  Furthermore, when the superabsorbent polymer-containing liquid 4 is mixed with electrolytes such as acidic substances, basic substances and salts, it is possible to suppress or recover the deterioration of the properties and quality of the superabsorbent polymer-containing liquid 4. Stabilizers can be added.

  Examples of the stabilizer include dilute sulfuric acid, aluminum sulfate, aluminum hydroxide, magnesium hydroxide, polyacrylamide, polyvinyl alcohol, sodium polyacrylate, polyethylene oxide, and the like. Examples of the stabilizer include bicarbonates such as sodium bicarbonate, calcium bicarbonate, potassium bicarbonate, and ammonium bicarbonate. Further, examples of the stabilizer include carbonates such as sodium carbonate, calcium carbonate, potassium carbonate, and ammonium carbonate. These can be used alone or in combination of two or more.

  When the superabsorbent polymer-containing liquid 4 having such a composition is injected into the excavation hole 2, first, the injection pipe 3 is embedded in the ground 1. An excavation hole 2 is excavated in the ground 1, and an injection pipe 3 is piped in the excavation hole 2. Then, the superabsorbent polymer-containing liquid 4 is fed and supplied to the injection tube 3, and the superabsorbent polymer-containing liquid 4 in a fluid state is injected into the ground 1 from the through hole of the injection tube 3.

  When salt is added to the superabsorbent polymer-containing liquid 4 to suppress the degree of swelling of the superabsorbent polymer particles, the superabsorbent polymer-containing liquid 4 and the groundwater W are in contact with each other in the ground 1. While the electrolyte concentration in the containing liquid decreases, the superabsorbent polymer particles swell 100 times or more of their own weight. The particle diameter reaches about 0.5 mm to 3 mm.

  Thus, the water-absorbing polymer particles having a high degree of swelling injected into the ground 1 are filled in the voids between the soil particles in the ground 1. And as shown to the top view of the lower figure of Fig.1 (a), the space | gap between these soil particles forms the polymer mixed soil 5 by which the high water absorption polymer particle was clogged, and the water permeability of the ground 1 is reduced. By doing so, the flow velocity of the groundwater W in the ground 1 can be reduced.

  In the above step <1>, the place where the frozen wall 5b is constructed by injecting the superabsorbent polymer-containing liquid 4 into the ground 1 through the injection pipe 3 piped into the excavation hole 2 excavated in the ground 1. Since the polymer mixed soil portion 5 is improved directly, the improvement range can be reduced. Therefore, compared with the geological improvement by injection | pouring of the chemical | medical solution which is the auxiliary work of the conventional frozen ground construction method, an environmental load is small and it is possible to suppress construction cost.

  In addition, the frozen ground construction method of the present invention can freeze the water in the superabsorbent polymer-containing liquid 4 by injecting the superabsorbent polymer-containing liquid 4 into the ground 1. It is possible to apply the frozen ground method to the ground without groundwater W where it was difficult to apply For example, it is known that the groundwater level is closely related to the amount of precipitation, so that it fluctuates seasonally. In general, the water level is high in summer and the water level decreases in winter. Therefore, in winter, there may be no groundwater in the ground at the depth constructed by the frozen ground method. In the frozen ground construction method of the present invention, the ground can be frozen by freezing the water in the superabsorbent polymer-containing liquid 4 even when there is no groundwater in the ground as described above. Moreover, the ground freezing method of the present invention is superior to the conventional method in that it can be applied to ground having good drainage and less groundwater, such as sand layers and gravel ground.

  Subsequently, as step <2>, a freezing pipe is piped into the excavation hole 2 and a cooling liquid is fed into the freezing pipe to form a frozen soil part 5a of the superabsorbent polymer mixed soil part 5. When freezing the ground 1, first, as shown in FIG. 1B, the freezing pipe 6 is embedded in the ground 1. When the freezing pipe 6 is installed in the excavation hole 2, the cooling liquid 7 is supplied into the freezing pipe 6 to start freezing of the ground 1 and the polymer mixed soil portion 5. When the cooling liquid 7 is supplied into the freezing pipe 6, as shown in the plan view of the lower part of FIG. 1 (b), it begins to freeze from the polymer mixed soil part 5 around the freezing pipe 6, and the freezing earth part 5a has an annual ring shape. After completion, the frozen soil pillar 5c is formed. Here, since the superabsorbent polymer particles are diffusion-filled in the polymer mixed soil portion 5, compared with the ground where the superabsorbent polymer-containing liquid 4 located at a position away from the excavation hole 2 is not injected. Even if the water permeability is lowered and the flow rate of the groundwater W is high, the water can be frozen without being affected by it. Therefore, the ground 1 and the polymer mixed soil 5 parts can be efficiently frozen without increasing the number of the freezing pipes 6 or lowering the temperature of the cooling liquid 7 as compared with the prior art.

  In addition, as described above, the superabsorbent polymer filled in the ground 1 even in the ground where the groundwater level is lowered due to seasonal variation and the ground where the amount of groundwater is low, such as the ground above the groundwater surface. It is possible to freeze the ground 1 by freezing the water in the containing liquid 4.

  In the ground freezing method of the present invention, as the step <3>, the steps <1> and <2> are simultaneously or sequentially performed at a plurality of predetermined positions to freeze the frozen soil portion 5a and to construct the frozen soil wall 5b. including.

  That is, as described above, the frozen soil column 5c formed around the freezing pipe 6 is connected to another adjacent frozen soil column 5c to construct the frozen soil wall 5b. Such a frozen earth wall 5b exhibits the same strength and pressure resistance as concrete, and can block the flow W of groundwater, so that it is possible to construct a structure in the area surrounded by the frozen earth wall 5b. Become.

  Further, the ground freezing method of the present invention includes the following original state restoration step <4> <5> following the frozen soil wall construction step of step <3> in the ground freezing method as described above. To do.

<4> After melting the frozen earth wall 5b and the frozen earth part 5a, the injection pipe 3 is provided in the excavation hole 2, and the separating agent 8 is injected into the ground 1 around the injection pipe 3 through the injection pipe 3. , Releasing the water taken up by the superabsorbent polymer particles;
<5> A step of removing the injection tube 3 from the excavation hole 2.

  2 (a) and 2 (b) are a schematic process cross-sectional view and a plan view schematically showing the original restoration process of the ground freezing method of the present invention.

  In step <4>, first, the circulating supply of the coolant is stopped. Thereby, the frozen soil constituting the frozen soil wall 5b and the frozen soil portion 5a is gradually melted and returned to the polymer mixed soil portion 5 made of the polymer mixed soil in a non-frozen state.

  As shown in FIG. 2 (a), the separating agent 8 is injected into the polymer-mixed soil portion 5 to release the water of the superabsorbent polymer particles and create voids between the soil particles. The water permeability of the ground 1 can be recovered.

  It is preferably considered that the separating agent 8 contains an electrolyte.

  When an electrolyte is injected into the polymer mixed soil 5 as the separating agent 8, the superabsorbent polymer particles in the polymer mixed soil 5 release the water taken in depending on the type of electrolyte and the electrolyte concentration of each electrolyte, Reduce its volume.

  The electrolyte can be used without particular limitation as long as it is an electrolyte that can release water by being poured into the polymer mixed soil 5. Examples include basic substances such as calcium hydroxide, salts such as calcium chloride, acidic substances such as hydrochloric acid, sulfuric acid, nitric acid and citric acid, and salts of these acidic substances. These electrolytes can be used alone or in combination of two or more. Among them, it is preferably considered that the electrolyte is a divalent metal salt such as calcium hydroxide or calcium chloride. In view of environmental load and economy, it is more preferable that the electrolyte is calcium chloride.

  As a specific example of the original restoration process of the ground freezing method of the present invention, for example, the component composition of the superabsorbent polymer-containing liquid 4 is 0.2 mass% of the superabsorbent polymer particles, 4.8 mass% of the weighting material, water In the case of 95.0% by mass, for example, 0.2% by mass or more of the normal superabsorbent polymer-containing liquid 4, or 0.3 to 0.4% by mass in the case of the superabsorbent polymer-containing liquid 4 having a high specific gravity. By adding calcium chloride, the superabsorbent polymer-containing liquid 4 is separated into a solid material of 5% by mass containing superabsorbent polymer particles that have released the weighting material and moisture, and an aqueous calcium chloride solution that accounts for 95% by mass. can do. In addition, 95 mass% here does not mean the mass% concentration of the calcium chloride aqueous solution.

  By injecting the separating agent 8 into the polymer mixed soil 5 by the above mechanism, as shown in the plan view of the lower diagram of FIG. 2 (a), from the superabsorbent polymer particles in the polymer mixed soil 5 Moisture is released and the volume and swelling degree of the superabsorbent polymer particles are greatly reduced. Therefore, the effect of clogging the voids in the soil particles by the superabsorbent polymer particles is reduced, and the water permeability of the ground 1 is restored.

  The superabsorbent polymer-containing liquid 4 does not contain heavy metal elements or the like that have a large environmental load. Therefore, even if the superabsorbent polymer particles that have released moisture are left in the ground as they are, There is almost no risk of contamination. Further, the superabsorbent polymer particles contact with the separating agent 8 to deteriorate over time, and eventually decompose in the ground. As described above, according to the frozen ground construction method of the present invention in which the superabsorbent polymer-containing liquid 4 can be separated into a solid and a liquid and discarded, the disposal can be facilitated and the processing cost is very low. Can be suppressed.

  Then, as process <5>, as shown in FIG.2 (b), the injection pipe 3 is removed from the excavation hole 2 provided in the ground 1. FIG. After removal of the injection pipe 3, the original state of the ground 1, which is a construction target, is completed by backfilling it with the dirt and sand of the excavation hole 2.

  Thus, according to the original restoration process of the ground freezing method of the present invention, which can separate the superabsorbent polymer-containing liquid 4 into a solid and a liquid and recover the original state, the environmental repairability is excellent. Of course, the processing cost can be kept very low.

<1. Basic Experiment of Superabsorbent Polymer Containing Liquid>
With respect to the superabsorbent polymer-containing liquid used in the ground freezing method of the present invention, the following properties have been confirmed from basic experiments.

(1) Relationship between electric conductivity of water-absorbing polymer-containing liquid and water absorption ratio As shown in FIG. 3, between electric conductivity x of water-absorbing polymer-containing liquid and reciprocal y of water absorption capacity, The following relational expression holds.

y = 3 × 10 ⁻⁶ +0.0016 (1)
Where x: electrical conductivity of the superabsorbent polymer-containing liquid (μS / cm)
y: Reciprocal of water absorption magnification (1 / times)
From the above formula (1), it was confirmed that the water absorption rate of the superabsorbent polymer particles can be controlled by adjusting the electrical conductivity of the superabsorbent polymer-containing liquid.

(2) Relationship between electrolyte concentration and electrical conductivity of swelling degree adjusting agent When sodium chloride NaCl is used as the swelling degree adjusting agent, the relationship between NaCl concentration (mass% concentration) and electric conductivity is shown in FIG. As shown in FIG. The following relational expression is established between the NaCl concentration (mass% concentration) x and the electric conductivity y of the superabsorbent polymer-containing liquid.

y = 17496.76x + 43.8913 (2)
Where x: NaCl concentration (mass% concentration)
y: electric conductivity of liquid containing superabsorbent polymer (μS / cm)
From the above formula (2), it was confirmed that the electrical conductivity of the superabsorbent polymer particles can be controlled by adjusting the addition amount of NaCl.

(3) Relationship between Free Water Ratio and Funnel Viscosity (Fluidity) of Liquid Containing High Water Absorbing Polymer Here, funnel viscosity means that 500 ml of sample liquid placed in a 500 ml funnel type container is discharged. The viscosity was measured using a Marsh Funnel viscometer that measures the viscosity according to the outflow time (seconds) required for. If the funnel viscosity is adjusted to an appropriate range, it becomes possible for the liquid containing the superabsorbent polymer to diffuse into the voids between the soil particles, and the superabsorbent polymer particles enter the voids between the soil particles. The effect of clogging can be demonstrated.

  As shown in FIG. 5, the funnel viscosity (fluidity) of the highly water-absorbing polymer-containing liquid is adjusted based on the free water ratio of the highly water-absorbing polymer-containing liquid, regardless of the water absorption value of the highly water-absorbing polymer particles. can do. For example, as shown in FIG. 5, it was confirmed that the funnel viscosity (fluidity) shows an equivalent value when the free water ratio of the water-absorbing polymer-containing liquid is the same. Therefore, by adjusting the free water ratio of the superabsorbent polymer-containing liquid, the superabsorbent polymer-containing liquid can be pumped according to the pumping machine.

  From the results of (1) to (3) above, adjust the electrical conductivity of the water used or the water-absorbing polymer-containing liquid that the water-absorbing polymer particles absorb in advance, and adjust the free water rate of the water-absorbing polymer-containing liquid. By doing so, it becomes possible to feed the highly water-absorbing polymer particles having a predetermined water absorption capacity adjusted to a viscosity (fluidity) according to the pressure feeder machine.

<2. Hard-permeable layer formation test and permeability recovery test>
Regarding the formation of the poorly permeable layer, it was confirmed by the following experiment that the highly water-absorbing polymer-containing liquid formed a hardly permeable layer around the wall surface of the excavation hole.

  FIG. 6 is a schematic diagram of an apparatus for measuring the formation of a hardly water-permeable layer. In FIG. 6, the simulated ground 11 has a thickness of 200 mm, assuming a gravel ground from a sand layer, and used two types of quartz sand (silica sand No. 1 having a coarse grain size and medium quartz sand No. 4) as sample soil. Moreover, as superabsorbent polymer particles (sodium polyacrylate: trade name Geosap, manufactured by Sanyo Kasei Kogyo Co., Ltd.), two types having different particle sizes (coarse particles in dry state: particle size 150 to 710 μm, fine particles: 20 to 50 μm) It was used.

1) Permeability test of sample soil, 2) Sample soil mixed with sample soil and water-swollen coarse polymer, coarse and fine superabsorbent polymer or fine superabsorbent polymer in a volume ratio of 1: 1. A water permeability test was conducted as (polymer mixed soil). Subsequent to the water permeation test of 2) above, a water permeation test was conducted using a calcium chloride CaCl ₂ 1 mass% solution instead of water.

In this experiment, in order to confirm the formability of the poorly permeable layer in the ground, a restraining pressure of 300 kN / m ² was applied to the first cylinder A and the second cylinder B by the compressor 9 and further to the cylinder A. adding 20 kN / ^{m 2} as a pressure head, the total 320kN / ^{m 2} was added by a compressor 9. The permeated water 10 that has permeated into the simulated ground 11 moves to the inside of the second cylinder B through a communication path that connects the first cylinder A and the second cylinder B. Thus, it moved to the inside of the 2nd cylinder B, the mass of stored osmotic water 10 was weighed with electronic scale 12, and the permeability coefficient was computed based on the mass of stored osmotic water 10.

  FIG. 7 shows the water permeability coefficient of the simulated ground 11 filled with the superabsorbent polymer-containing liquid obtained from the above experiment.

As a result of the water permeability test, the water permeability coefficient of silica sand is 2.0 to 4.75 × 10 ⁻² cm / s, but when the superabsorbent polymer-containing liquid is mixed with silica sand, the water permeability coefficient is silica sand No. 1 and silica sand No. 4 Both were as small as 10 ⁻⁵ cm / s, and it was confirmed that a hardly water-permeable layer was formed. However, the hydraulic conductivity of the simulated ground 11 which is a mixed soil of silica sand No. 1 and a coarse superabsorbent polymer was 7.5 × 10 ⁻² cm / s, and the decrease of the hydraulic conductivity was small. This indicates that it is necessary to adjust the particle size of the superabsorbent polymer particles so as to match the particle size of the target soil.

Next, when the calcium chloride CaCl ₂ 1 mass% solution is infiltrated into the simulated ground 11 as the permeated water 10, the hydraulic conductivity of the simulated ground 11 becomes large, and is approximately close to that of sand. This indicates that the superabsorbent polymer particles release water by the action of calcium chloride CaCl ₂ , and the volume decreases, so that the gaps between the soil particles cannot be clogged and the water is allowed to pass. Yes.

<3. Confirmation at construction site>
When one embodiment of the ground freezing method of the present invention is applied and the groundwater W flow is about 2.0 m / day or more and construction is performed by the frozen ground construction method, the groundwater W flow is blocked and the frozen ground wall It was confirmed that 5b can be constructed. Further, after the construction of the structure in the area where the flow of the groundwater W is blocked by the frozen ground wall 5b, the circulation of the cooling liquid 7 in the freezing pipe 6 is stopped to melt the frozen ground wall 5b, and then the excavation hole 2 When a 0.2 mass% calcium chloride aqueous solution was injected into the ground 1 as the separating agent 8 from the injection pipe 3 piped in the ground, the water permeability of the ground 1 was restored and the flow of the groundwater W was about 2.0 m / day or more. It was confirmed that it recovered. When soil samples and groundwater W samples were collected from the excavation hole 2 and subjected to instrumental analysis, it was confirmed that there was no contamination with chemical substances or the like, and that almost no environmental load was observed.

DESCRIPTION OF SYMBOLS 1 Ground 2 Drilling hole 3 Injection pipe 4 Liquid containing superabsorbent polymer 5 Polymer mixed soil part 5a Frozen earth part 5b Frozen earth wall 5c Frozen earth column 6 Freezing pipe 7 Coolant 8 Separating agent 9 Compressor 10 Infiltration water 11 Simulated ground 12 Electronic scale 31 Through hole A, B Cylinder W Groundwater

Claims (10)

It is a ground freezing method that constructs a frozen soil wall by connecting a frozen soil part to a frozen pipe in a drilling hole provided by excavating the ground, forming a frozen soil part with the coolant fed into the frozen pipe, A ground freezing method comprising at least the following steps <1>, <2> and <3>.
<1> A step of injecting a superabsorbent polymer-containing liquid into the ground around the injection pipe through an injection pipe in a ground excavation hole to create a superabsorbent polymer mixed soil part;
<2> A step of piping a freezing pipe into the excavation hole, sending a coolant into the freezing pipe, and forming a frozen soil portion of the superabsorbent polymer mixed soil portion;
<3> The step of <1> and <2> are performed simultaneously or sequentially at a plurality of predetermined positions, and the frozen soil part is connected to construct a frozen soil wall.   In the step <1>, when the excavation hole is formed, the ground soil and the superabsorbent polymer are mixed and stirred by a stirring blade of an injection pipe to form the superabsorbent polymer mixed soil portion. The ground freezing method according to claim 1.   The ground freezing according to claim 1 or 2, wherein the ground is at least one selected from the group consisting of a gravel ground, a ground in which groundwater level is lowered due to seasonal variation, and a ground above the groundwater surface. Construction method.   The superabsorbent polymer-containing liquid contains superabsorbent polymer particles that are at least one selected from the group consisting of starch, cellulose and synthetic polymer. The ground freezing method described in one item.   The ground freezing method according to any one of claims 1 to 4, wherein the superabsorbent polymer-containing liquid contains a swelling degree adjusting agent.   The ground freezing method according to claim 5, wherein the swelling adjusting agent contains an electrolyte.   The ground freezing method according to claim 6, wherein the electrolyte is a monovalent metal salt. 3. A ground freezing method comprising the following ground restoration step <4><5> following the frozen soil wall construction step of step <3> in the ground freezing method according to claim 1 or 2.
<4> After melting the frozen earth wall and the frozen earth part, the injection pipe is piped into the excavation hole, and a separating agent is injected into the ground around the injection pipe through the injection pipe, and the high water absorption Releasing moisture taken up by the polymer particles;
<5> A step of removing the injection pipe from the excavation hole.   The ground freezing method according to claim 8, wherein the separating agent contains an electrolyte.   The ground freezing method according to claim 9, wherein the electrolyte is a divalent or higher polyvalent metal salt.