JPH0573849B2 - - Google Patents

Description

[Detailed description of the invention]

"Industrial Application Field" The present invention relates to a ground improvement method in which soft and viscous soil is frozen using a heat pipe, and the soft and viscous soil is dehydrated and consolidated by the water absorption force and freezing expansion pressure generated when the ground is frozen. It is something. "Conventional technology" In general, soft and cohesive soil has a thick layer of soft soil (soft soil layer) that contains a lot of cohesive soil and organic matter and has a high moisture content, so the amount of ground subsidence is large. It is also extremely vulnerable to external forces such as earthquakes. Therefore, when creating a housing site on a soft and sticky ground and building a structure there, it is necessary to take measures such as increasing the strength of the ground and increasing its resistance to external forces. Improvements must be made. Conventionally, as a means for improving such soft and viscous soil, a combination of a sand drain method and a preload method has been applied. The sand drain method involves driving a large number of sand piles into soft, cohesive soil to shorten the consolidation time, and using these as drainage channels. Embankment of an appropriate thickness is placed on the ground, and the weight of the embankment compresses the soft soil layer to compact it, increasing the density of the soil and increasing its strength, that is, consolidation. ``Problems to be solved by the invention'' However, in the above-mentioned method of improving soft and viscous soil, it tends to be difficult to obtain high-quality sand for use in sand piles, and the effect of improving the soil depends on the depth. The deeper the embankment, the less the stress increase due to the embankment.Moreover, a large amount of embankment on a soft and viscous soil base must be left in place for a long period of time, and it must be removed after ground improvement, resulting in poor economic efficiency and work efficiency. A number of problems arose. The present invention was made in view of the above circumstances, and
It is possible to omit the process of placing sand piles and embanking, and dewaters soft and sticky soil with a simple process.
The purpose of this study is to provide a method for improving soft and viscous soil using heat pipes, which have the effect of compaction. ``Means for Solving the Problems'' In order to achieve the above object, the construction method of the present invention includes: (a) burying an airtight container constituting a heat pipe vertically in soft and viscous soil with its upper part protruding; (b) The upper part of the airtight container is cooled with cold air, the working fluid in the airtight container is condensed, and the liquefied working fluid is allowed to fall by gravity, and the cold heat carried at this time causes the surrounding area of the airtight container to cool. This method involves the steps of freezing soft and sticky soil to form frozen soil around an airtight container, and (c) thawing this frozen soil to cause the ground to sink.It uses heat pipes to store natural cold energy underground. It is characterized by dewatering and consolidating the ground using the water absorption force caused by the frozen ground phenomenon that occurs at that time and the pressure caused by expansion. "Operation" In the process of the present invention, when the soft and viscous ground freezes, a layer of ice called an ice lens is deposited concentrically with the heat pipe. This is because a force (water absorption force) is generated that tries to draw water to the frozen surface. As a result, water freezes and expands there, generating freezing expansion pressure, and these pressures (water absorption force and freezing expansion pressure) act as effective stress to consolidate the ground. When the ice lens is thawed, water is dehydrated onto the ground by the weight of the improved ground along the thawed ice lens layer. As a result, the bearing capacity of the ground increases. “Embodiment” An embodiment of the present invention will be described below with reference to the drawings. Figure 1 shows an example of a heat pipe used in the construction method of the present invention.
In this system, a working liquid G is sealed in a long cylindrical airtight container 1 that is sufficiently deaerated, and heat transfer occurs as the phase of the fluid G changes. It is designed to be buried and frozen. Fins 3 for receiving and dissipating heat are protruded from the upper outer periphery of the airtight container 1. Typical constituent materials and operating temperatures of this heat pipe A are shown in Table 1.

[Table] Copper or aluminum, which has good thermal conductivity, is used for the airtight container 1, but it can be arbitrarily selected depending on the usage conditions of the heat pipe (temperature, surrounding corrosion conditions, etc.). The working fluid G is selected depending on the vapor pressure determined by the operating temperature of the heat pipe A and the compatibility of the airtight container 1. Next, heat pipe A configured as described above
The soil improvement method of the present invention for improving the soft and viscous ground 10 using the following will be described. Figures 2A to 2D are side sectional views showing the process of improving soft and sticky soil by the improved construction method of this embodiment, and reference numeral 10 in the figure indicates the soft and sticky land on which the improvement is being carried out. It is a board. The procedure for improving the soft and viscous land 10 is carried out through the following steps. (i) Installation of heat pipes As shown in Fig. 2A, heat pipes A are installed vertically at predetermined pitches at arbitrary positions on the damp, soft and viscous ground 10. The installation position of the heat pipe A in the soft and viscous ground 10 is not particularly limited, but may be based on the temperature of the area where the heat pipe A is installed, the freezing expansion pressure of the ground,
It is preferable that the heat pipes A be arranged at appropriate intervals so that a predetermined pressure is generated, taking water absorption power into consideration. (ii) Construction of frozen soil When the heat pipe A is buried in this way and the upper part of the airtight container 1 is cooled, for example by cold air in winter, the working fluid G in the airtight container 1 condenses and becomes a liquid. , it falls along the pipe wall of the airtight container 1 due to gravity. As a result, cold heat is carried to the lower part, and this carried cold heat causes
When the soft and viscous ground 10 surrounding the airtight container 1 is frozen, an ice layer S called an ice lens is deposited on the frozen surface concentrically with the heat pipe A, as shown in FIG. Frozen soil F is constructed (formed) around the container 1. Also, at this time, the soft and viscous land plate 10 facing the ground surface is
Separately from the heat pipe A, it is frozen by cold air on the ground surface, an ice lens layer S is precipitated, and frozen soil F is constructed. And when such frozen soil F is constructed,
That is, when the soft and slightly viscous ground 10 freezes, water absorption force and freezing expansion pressure are generated, and these pressures increase as effective stress for consolidating the ground 10 as shown in FIG. 3, consolidating the ground 10,
Furthermore, as the effective stress increases, the soft and viscous soil 10 is dehydrated, increasing its supporting capacity. The effective stress that consolidates the ground 10 is generated through the following process. That is, when the soft and viscous ground 10 freezes, an ice layer S called an ice lens is deposited on the frozen surface concentrically with the heat pipe A, as shown in FIGS. 2B and 2C. This is because a force (water absorption force) is generated that tries to draw water to the frozen surface. the result,
There, the water freezes and expands, creating freeze-expansion pressure. Then, due to such pressure, the soft and viscous ground 10 is dehydrated and consolidated, and the ground is improved. On the other hand, the working fluid G in the lower part of the airtight container 1 is heated there, as shown by arrow A in FIG. 1, and evaporation begins from the surface of the liquid. The generated steam moves upward due to the pressure difference with the upper part.
By repeating this process, heat transfer occurs automatically. Therefore, in the present invention, the construction of frozen soil F is carried out by burying the heat pipe A in the soft and viscous ground 1, so that construction of frozen soil F can be achieved more effectively than with so-called forced cooling using electric power. Ru. In other words, heat pipe A transports heat in the form of latent heat, so it can quickly transfer heat, has a uniform surface temperature, and has no moving parts, so it has advantages such as being maintenance-free. , construction of frozen soil F can be achieved at relatively low cost. In addition, in the present invention, the soft and viscous ground 10 is dehydrated and consolidated by constructing such frozen soil F, so that it is possible to dewater and consolidate the soft and viscous ground 10 without placing sand piles or embanking. It is possible to effectively exhibit the powerful improvement effect of. It should be noted that the heat transfer amount of the heat pipe A is inversely proportional to the thermal resistance between the cold source in the atmosphere and the ground, and is proportional to the temperature difference therebetween. The heat transport amount Q per heat pipe is given by the following equation. Q = (Ta - Tg) / (r ₁ + r ₂ + r ₃ ) ... (1) where, Ta: Atmospheric temperature Tg: Ground temperature r ₁ : Thermal resistance between the ground and heat pipe r ₂ : Heat pipe Thermal resistance _r3 : Thermal resistance of the fins Of the thermal resistance, _r1 is determined by the thermal conductivity of the ground, frozen soil characteristics of the soil, and the shape of the heat pipe, and _r3 is determined by the pitch and diameter of the fins. . (iii) Thawing of frozen soil Due to summer temperatures, frozen soil F thaws, following the sinking improved soil, the surrounding soft and cohesive soil sinks, and as shown in Figure 2 D, the frozen soil F thaws. From the position shown by the two-dot chain line, the entire ground sinks by the amount of consolidation in step (ii) above. At this time, the water generated by the thawing of the frozen soil F (that is, the water generated by the thawing of the ice lens) is transferred to the ground 1.
Due to the subsidence of 0 (due to the weight of the improved ground 10), as the frozen soil F thaws, water is drained to the ground surface along the gap between the thawed ice lens layers S. In addition, if the water drained by the thawing of the frozen soil F is drained by using an appropriate drainage means (for example, laying a drainage ditch on the ground), it is possible to drain the water to the improved ground 10. This is preferable in terms of preventing penetration. Then, as shown in FIG. 2D, after the ground 10 has subsided, the heat pipe A is removed, and the improvement of the ground 10 is completed. Hereinafter, the present invention will be specifically explained with reference to experimental examples. ``Experiment example'' (1) Selected heat pipes In order to select economical heat pipes that transport a large amount of heat, we selected four types of heat pipes, which will be described later, and buried them on site. The container is made of copper, and the working fluid is Freon-22. The single-tube type has a heat pipe placed inside a copper tube filled with antifreeze. This is because the surface area of a single tube is small, so it is hoped that heat transfer by antifreeze convection will increase the amount of heat transport. The loop type has a large surface area and requires less processing time for the heat pipe. There were two types of this type: one with grooves on the inside of the heat pipe and one without grooves. The double loop type is a double loop type heat pipe without grooves. In addition, the joint portion of each of the heat pipe tubes was made with adhesive containing aluminum powder to improve heat conduction. (2) Buried location As shown in Figures 4 and 5, observation room R
The four types of heat pipes A ₁ , A ₂ , A ₃ , and A ₄ were buried in two rows at positions spaced apart from each other by a certain distance. The growth status of frozen soil is determined by each heat pipe A ₁ , A ₂ ,
Measurement tube buried 50cm away from A ₃ and A ₄
Using thermocouples in B ₁ , B ₂ , B ₃ , and B ₄ , changes in soil temperature over time were measured at multiple measurement points O of each heat pipe (heat pipe A ₁ in the illustrated example), and based on this, inferences were made. . (3) Experimental results Figures 6 a and b show the freezing index and atmospheric temperature changes over time since December 1, 1984, and Figure 7 a
and b, and 8th a and b show the temperature change over time at a point at a depth of −2 m for the grooved heat pipe and the double loop type, and in the soil (depth −2 m) 50 cm away from the heat pipe. In double-loop heat pipes, the depth -
It is thought that frozen soil with a diameter of about 20 cm was formed at the 2 m point, but since the temperature in the loop is above and below the zero degree line, it is thought that almost no frozen soil was formed. Next, a description will be given of how to design the effective stress and the like when the soft and viscous ground 1 freezes. "Design method" When saturated soil freezes, the expansion rate due to freezing ξ,
The water absorption rate ξw is given by the following equation as a relationship between the advancing speed V of the frozen surface and the effective stress σ' on the frozen surface. ξ=ξo+(1+√)σo/σ'...(2) ξ=ξo+n _f Γ+(1+Γ)ξw...(3) Here, ξo, σo, and Vo are constants unique to the soil, and n _f is the interpolation rate. , Γ is the freezing expansion coefficient of water. Heat transfer within frozen soil and unfrozen soil is considered to be a cylindrical model, so each can be determined as follows. Heat transfer in frozen soil ∂Tf/∂t=κ ₁ (∂ ² Tf/∂r ² +1/r∂Tf/∂r) ...(4) Heat transfer in unfrozen soil ∂Tu/∂t=κ ₂ ( ∂ ² Tu/∂r ² +1/r∂Tf/∂r) ...(5) Moisture movement in unfrozen soil ∂TU/∂t=Cv(∂ ² Uw/∂r ² +1/r∂Uw/∂r ) ...(6) Here, Tf and Tu are the temperatures of frozen soil and unfrozen soil,
Uw is the water pressure, κ ₁ and κ ₂ are the temperature conductivity of frozen soil and unfrozen soil, Cv is the consolidation coefficient, t is time, and r is the coordinate representing the position. Furthermore, the following equation must hold true at the boundary between frozen soil and unfrozen soil. λ ₁ ∂Tf／∂r−λ ₂ ∂Tu／∂r = LsγsV＋LwγwVw ...(7) Here, λ ₁ and λ ₂ are the thermal conductivities of frozen soil and unfrozen soil, and Ls and Lw are the latent heat of freezing of the ground and water. , γs, γw
is the ground and water density, V and Vw are the freezing rate and water absorption rate. The water absorption rate Vw on the frozen surface is given by the following equation according to Darcy's law. Vw=k/γw∂Uw/∂r...(8) The following relationship holds true. Also, from equations (2) and (3), the water absorption rate is Vw = (V+√)σ ₀ /{(σ−Uwo
−S _f )(1+Γ)}−Vn _f Γ/(1+Γ)……(9). Therefore, the stress increment on the frozen surface due to forced displacement obtained by calculating the water absorption force Sf from equations (8) and (9) and subtracting the resulting dehydration consolidation amount is the increment in the freezing expansion pressure. Furthermore, based on the boundary conditions of equations (1) and (7), equations (4) to (6) are solved to obtain the temperature distribution, water pressure distribution, and effective stress distribution. Figures 9a and b show that the temperature inside the heat pipe is -
An example of boundary force and freezing expansion pressure at constant temperatures of 15°C and -30°C is shown. At a point 15cm away from the heat pipe when the cooling temperature is -15℃, the water absorption power is approximately 0.7Kgf/cm ² and the freezing expansion pressure is approximately 2.0Kgf/cm ² , so the effective stress is approximately 2.7Kgf/cm ² has occurred. This is 15~
This shows that the improvement effect is as strong as that of a 20m embankment. In addition, normally, with conventional preload construction methods,
For example, a soft and viscous ground is consolidated by embankment of about 5 m, but in the present invention, as mentioned above, when the cooling temperature of the heat pipe is -15°C, an effective stress of more than that is applied to the embankment. without any
It is something that can be demonstrated and a great improvement effect can be obtained. "Effects of the Invention" As explained above, the method for improving soft and viscous soil of the present invention is as follows: (a) An airtight container constituting a heat pipe is buried vertically in a soft and viscous soil with its upper part protruding. (b) The upper part of the airtight container is cooled with cold air, the working fluid in the airtight container is condensed, and the liquefied working fluid is caused to fall by gravity, and the cold heat carried at this time causes the soft and weak viscosity around the airtight container to soften. A process of freezing the soil to form frozen soil around the airtight container, and (c) thawing this frozen soil and draining the water from the ice lens along the heat pipe.
It is characterized by the process of sinking the ground, storing natural cold energy underground using heat pipes, and dewatering and consolidating the ground using the water absorption power caused by the frost heaving phenomenon that occurs at that time and the pressure caused by expansion. As a result, various excellent effects are achieved, including the following. (1) It is possible to omit the process of driving sand piles such as sand drains and placing embankment on top of them to consolidate the ground, making it possible to consolidate soft and viscous soil, and improving work efficiency. can be done. (2) The effect of soil improvement can also increase stress regardless of the depth of the soil.

[Brief explanation of the drawing]

Fig. 1 is a side sectional view showing an example of a heat pipe used in the construction method of the present invention, and Fig. 2 A to D are shown to explain the steps of an embodiment of the construction method of the present invention. 3 is a schematic diagram of changes in water absorption force and freezing expansion pressure over time. 4 and 5 are shown to explain experimental examples. 4 is a plan view, and 5 is a side sectional view. The figure is a side sectional view, No. 6
Figures a and b are graphs showing changes in freezing index and atmospheric temperature over time; Figures 7 a and b are graphs showing changes in heat pipe and soil temperature over time at a depth of -2 m for the grooved heat pipe; Figure 8 a and b
9 is a graph showing the change over time in the heat pipe and soil temperature at a depth of -2 m in a double-loop heat pipe, and FIGS. 9a and 9b are graphs showing the influence of cooling temperature on water absorption and freezing expansion pressure. A... Heat pipe, 1... Airtight container, 3...
Heat receiving and dissipating fins, 10...Soft viscous land plate, G...
...Working fluid, F...Frozen soil, S...Ice lens layer.

Claims (1)

[Claims] 1. Freezing a soft, slightly viscous ground using a heat pipe formed by filling a working fluid inside an airtight container, and dewatering the soft, slightly viscous ground using the water absorption force and freezing expansion pressure generated when the ground freezes. It is a consolidation method that includes (a) burying the airtight container vertically in soft and cohesive soil with the upper part protruding, and (b) cooling the upper part of the airtight container with cold air to form an airtight container. A process of condensing the working fluid in the airtight container, causing the liquefied working fluid to fall by gravity, and freezing the soft and slightly viscous ground around the airtight container by the cold heat carried at this time to form frozen soil around the airtight container; (c) A method for improving soft and viscous soil using heat pipes, which includes the process of thawing this frozen soil and causing the ground to sink.