JPS62112811A - Improving work of soft cohesive ground with heat pipe - Google Patents

Abstract

PURPOSE:To dewater and consolidate soft cohesive ground by means of water absorption and expanding pressure of frost-heaving phenomenon by a method in which a heat pipe packed with a working fluid in its air-tight container is buried under the soft ground, and the ground is frozen by the heat pipe and then thawed. CONSTITUTION:A working fluid, e.g., flon, etc., is packed into an air-tight container 1 to form a heat pipe A. The pipe A is vertically buried in soft cohesive ground 10 by projecting its upper part on the ground's surface. Under the condition, when the upper part of the pipe A is cooled in a cold atmosphere, the cold heat is transmitted to the lower part by the working fluid, and frozen soil F is formed around the pipe A. The frozen soil F is thawed by the atmospheric temperature in the summer season to settle the ground 10. The ground is dewatered and consolidated by water absorption and expanding pressure of the frost-heaving phenomenon, and the ground can thus be improved at low cost.

Description

Detailed Description of the Invention "Industrial Application Field" The present invention freezes a soft and viscous ground using a heat pipe, and dehydrates and consolidates the soft and viscous ground using the water absorption force and freezing expansion pressure generated when the ground freezes. This article is about the ground improvement method.

"Conventional technology" In general, soft and cohesive soil contains a lot of cohesive soil and nurturing substances, and has a thick layer of soft soil (soft soil layer) with a lot of water content, so the amount of ground subsidence is small. It is large and extremely vulnerable to external forces such as earthquakes. therefore,
In order to create a housing site on a soft and viscous ground and build a structure there, it is necessary to take measures such as increasing the strength of the ground and increasing its resistance to external forces in advance, so the ground must be improved. There must be.

Conventionally, as a means for improving such soft and viscous soil, a combination of the Santo Train method and the preload method has been used. The Santo Train method is
Collar x length for the purpose of shortening consolidation time: fit;
; Two sand piles are placed in a ten-width area and used as a drainage channel. In the pre-load construction method, embankment of an appropriate thickness is placed on soft ground. The weight of this embankment compresses the soft soil layer and compacts 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 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. There were a few problems.

The present invention has been made in view of the above-mentioned circumstances, and it is possible to omit the process of placing sand piles, embankment, etc., and is a heat pipe that is effective in dewatering and compacting soft and viscous soil with a simple process. The purpose of this study is to provide an improvement method for soft and viscous soil.

``Means for Solving the Problems'' In order to achieve this objective, the construction method of the present invention consists of (a) vertically burying an airtight container constituting a heat pipe in a soft and viscous ground 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, 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 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 frost heaving phenomenon that occurs at that time and the pressure caused by expansion.

"Operation" In such a process of the present invention, when the soft and viscous ground freezes, a layer of water called an ice lens is precipitated 1ξ concentrically with the heat pipe. This is because a force (water absorption force) that tries to draw water to the frozen surface is generated. As a result, water freezes and expands there, generating freezing expansion pressure, and these pressures (water absorption force and freezing expansion pressure) act as behavioral stress that consolidates the ground. When the ice lens is thawed, water is dewatered onto the ground by the weight of the improved soil along the gaps between the thawed ice lens layers. As a result, the bearing capacity of the ground increases.

"Example" An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of a heat pipe used in the construction method of the present invention. This heat pipe A is made by enclosing a working fluid G in a cylindrical airtight container I that is sufficiently deaerated and has a length of f. ,
Since it is configured so that heat transfer occurs with the phase change of the fluid G, it is buried in a soft and viscous ground lO to freeze it. Then, on the upper outer periphery of the airtight container l, there is a receiving/receiving entry 111/7'17'noq, +, <
Table 1 shows typical constituent materials and operating temperatures of this heat pipe A.

Copper or aluminum, which has good thermal conductivity, is used for the airtight container I in Table 1, but it should be 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, a ground improvement method of the present invention for improving the soft and viscous ground 10 using the heat pipe A configured as described above will be described.

Figures 2 (a) to 2) are side sectional views showing the process of improving soft and cohesive soil by the improved construction method of this embodiment, and the reference IO in the figure indicates the progress of the improvement. It is a soft and viscous land surface. The procedure for improving the soft and viscous land 10 is carried out through the following steps.

(1) Placement of heat pipes As shown in FIG. 2(a), heat pipes A are placed vertically at a predetermined pitch at arbitrary positions on the dredged soft and slightly viscous ground 10. The installation position of this heat pipe A in the soft and viscous ground lO is not particularly limited, but the predetermined pressure is As it happens,
It is preferable that the heat pipes A be arranged at appropriate intervals.

(11) Lateral construction in 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 liquid. , and falls along the stomach wall of the airtight container 1 due to gravity. As a result, cold heat is carried to the lower part, and by this carried cold heat,
The soft and viscous ground IO surrounding the airtight container 1 is frozen, and as shown in Figure 2 (b), a layer of water S called an ice lens is deposited on the frozen surface in a concentric circle with the heat pipe A. , frozen soil F is constructed (formed) around the airtight container l. Also, at this time, the soft viscous land plate 10 on the ground surface
is frozen by cold air on the ground surface separately from the heat pipe A, an ice lens layer S is precipitated, and frozen soil F is formed.

When such frozen soil F is constructed, that is, when the soft and viscous ground 10 freezes, water absorption force and freezing expansion pressure occur, and these pressures are shown in Fig. 3 as the effective stress that consolidates the ground 10. As shown, the stress increases and consolidates the ground 10, and as the effective stress increases, the soft and viscous ground 10 is dehydrated, increasing its bearing capacity.

The effective stress that consolidates the ground 10 is generated through the following process. In other words, when the soft and viscous land plate 10 freezes, a layer of water S called an ice lens forms a second layer on the frozen surface.
As shown in Figures (B) and (C), it is deposited concentrically with the heat pipe A. This is because a force (water absorption force) is generated that tries to draw water to the frozen surface. As a result, the water freezes and expands there, creating freeze-expansion pressure. Due to such pressure, the soft and viscous ground 10 is dehydrated and consolidated, thereby improving the ground.

On the other hand, the working fluid G in the lower part of the airtight container I is heated there, and evaporation begins from the surface of the liquid, as shown by arrow A in FIG. The generated steam moves upward due to the pressure difference with the upper part. This process is complete.

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 construction of frozen soil F is achieved more effectively than when so-called forced cooling using electric power is used. be done.

Since heat pipe A transports heat in the form of latent heat, heat pipe A should be able to transfer heat quickly, and secondly, the surface temperature is uniform, and since there are no moving parts, it is maintenance-free. Because of these advantages, the construction of frozen soil F can be achieved at a relatively low cost.

In addition, in the present invention, by constructing such frozen soil F, the soft and viscous ground IO can be dehydrated and consolidated, so that the construction of the frozen soil F can be carried out without the need to cast sand piles or embankments. It is possible to effectively demonstrate the powerful improvement effect of.

In addition, an explanation regarding the heat transport amount of heat pipe A is given below.
In addition, this is the resistance between the cold source in the atmosphere and the ground, ψl:l-, fg111. -During that time, Q times, 1-
1, Q' 7= (This is ψ11.

Heat transport ff1Q per 11 heat pies is given by the following equation.

Q = (Ta-Tg)/(rr+r2+r3)--(
1) Here, Ta, atmospheric temperature Tg Ground temperature rl: Thermal resistance between the ground and the heat vibrator r2 ・Thermal resistance of the heat vibrator r, J ・Thermal resistance of the fins Of the thermal resistance, r is the heat of the ground conductivity, frozen soil properties of soil,
Depending on the shape of the heat pipe, r3 is the fin pitch,
Determined by diameter etc.

(iii) Thawing of frozen soil Depending on the summer temperature, frozen soil F thaws, and the surrounding soft cohesive soil sinks following the improved soil, as shown in Figure 2 (d). From the position shown by the two-dot chain line, the entire ground sinks by the amount of consolidation in the step (11). At this time, the water generated by the thawing of the frozen soil F (that is, the water produced by the thawing of the ice lens) is caused by the subsidence of the ground 10 (due to the weight of the improved ground 10), and the water generated by the thawing of the frozen soil F. , drain to the surface along the gaps in the thawed ice lens layer 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 IO. This is preferable in terms of preventing penetration.

Then, as shown in FIG. 2(d), 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'' (+) Selected heat pipes In order to select economical heat pipes that transport a large amount of heat, four types of heat pipes, which will be described later, were selected and buried at the 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 steel pipe containing antifreeze. This is because the surface area of a single tube is small.
The idea is to use antifreeze convection to increase heat transfer.

The loop type has a large surface area and requires less processing time to make 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 heat pipe without grooves.

In addition, the joint 1″A of each of the heat pipe tubes
≦ improves heat conduction with adhesive containing aluminum powder.

(2) Buried location As shown in Figures 4 and 5, the four types of heat pipes A are arranged in two rows at a certain distance from the observation room R.
1. A2, i~3. A4 was buried.

The growth status of frost heave is shown for each heat pipe A + and A 2.
.

Measurement tube B+ buried 50cm away from A 3 and 7\4.

nnnrl-++, Ah-W;I)+-1・t)'trayLrgunogorIn the illustrated example, the change in soil temperature over time is measured at multiple measurement points 0 of the heat pipe A,), and It was inferred by

(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) of the heat pipe and °double loop type at a depth of −2 m and iJ in the soil 50 c + n away from the heat pipe.
-2 m) temperature change over time.

In the double-loop heat pipe, frozen soil with a diameter of about 20 cm was thought to have formed at a depth of -2 m, but since the temperature in the loop was above and below the zero temperature line, it is assumed that almost no frozen soil was formed. Seem.

Next, a description will be given of how to design the effective stress etc. when the soft and viscous ground l freezes.

"Design method" When freezing above saturation, expansion coefficient ξ due to freezing, water absorption rate ξ
W is given by the following equation as a relationship between the advancing speed V of the frozen surface and the nail effective stress σ' on the frozen surface.

ξ−ξo + (1+, r'rf7)σ0/σ゛...
...(2) ξ-ξo +n, r'+(1+r')
Uw --(3) Here, ξo1 σ0. Vo
is the soil solidification constant, n is the porosity, and r is the freezing expansion rate of water.

Heat transfer within frozen soil and unfrozen soil is considered to be a cylindrical model, so each can be determined as follows.

The heat transfer aTf/at- in the frozen ground is expressed as (θ'Tr/θr”+ I/r B
Tf/θr)・・・(4) Heat transfer in unfrozen soil El T Ll/(9t= /C2ca ”T u/E
l r2+ 1 /rθT r/a r)...
(5) Moisture movement in unfrozen soil El U w/a t= Cv(a 2U w/θr2
+ 1 /r El Uw/θr) (6) Here, T f and Tu are the temperatures of frozen soil and unfrozen soil, and U
w is the pore water pressure, 2 is 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 two must hold true at the boundary between frozen and unfrozen soil.

λ, 9Tr/ar-λtEJ T u/a r=L s
7 sV +L w7 wV w...(7) Here, λ1 and λ2 are the thermal conductivity 2L of frozen soil and unfrozen soil
s and Lw are the latent heat of freezing of the ground and water, γS and γW are the densities of the ground and water, and ■ 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 based on d'Arnaud's law.

Vw=ni/γw aUw/ar −(8)
A relationship is established. Also, from equations (2) and (3), the water absorption rate is Vt=(V+r blind ηvo/((ey −Uuto−5
,)(1+1"))-4n, r"/(1+I"
) -- given by (9).

Therefore, the stress increment on the frozen surface due to the forced displacement obtained by calculating the water absorption force Sr from equations (8) and (9) and subtracting the resulting dehydration consolidation amount is the increment in the freezing expansion pressure.

Furthermore, in addition to the boundary conditions of equations (1) and (7), equation (4
) to (6) to determine the temperature distribution, pore water pressure distribution, and effective stress distribution.

FIGS. 9(a) and 9(b) show an example of the boundary force and freezing expansion pressure when the temperature inside the heat vibrator is constant at 115°C and 130°C.

When the cooling temperature is -15°C, the water absorption force is approximately 0.7 kgf/cm2 and the freezing expansion pressure is approximately 2.0 kgf/cm2 at a point 15 cm away from the heat pipe, so the effective stress is approximately 2°7 kgf/cm2. cm2 is generated. This shows that the improvement effect is as strong as that obtained by embanking 15 to 20 meters.

In addition, in conventional preload construction methods, for example,
The soft and viscous soil is consolidated by embankment of about 5 m, but as mentioned above, in the present invention, when the cooling temperature of the heat pipe is 115°C, it is possible to suppress the standing stress beyond that without embankment. , which can be exerted and obtain a powerful improvement effect.

``Effects of the Invention'' As explained above, the method for improving soft and viscous soil according to the present invention is as follows: (a) An airtight container is installed in the soft and viscous soil, with its upper part protruding to form a groove with a heat pipe. (b) The upper part of the airtight container is cooled with cold air to make it airtight, the working fluid in the container is condensed, the liquefied working fluid is allowed to fall by gravity, and the cold heat transported at this time creates an airtight seal. A process of freezing the soft and viscous soil around the container 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. The method is characterized by the fact that natural cold energy is stored underground using a heat pipe, and the ground is dehydrated and consolidated by the water absorption force and pressure caused by expansion due to the frost heaving phenomenon that occurs at that time. Since it is Shino, it has various soaked effects such as:

(1) It is possible to omit the process of driving sand piles such as sand drains and placing embankments on top of them to consolidate the ground, thereby consolidating the soft and viscous soil. Can you improve your sex by 6g?

(2) The effect of improving the ground can also increase stress regardless of the depth of the ground.

[Brief explanation of drawings]

Figure 1 is a side sectional view showing an example of a heat pipe used in the construction method of the present invention, and Figures 2 (a) and 2) are shown to explain the steps of an embodiment of the construction method of the present invention. Something,
Figure 3 is a schematic diagram of changes in water absorption force and freezing expansion pressure over time, and Figures 4 and 5 are shown to explain experimental examples, so Figure 4 is a plan view and Figure 3 is a side sectional view, respectively. Figure 5 is a side sectional view, Figures 6 (a) and (b) are graphs showing changes in freezing index and atmospheric temperature over time, and Figures 7 (a) and (
b) is a graph showing the change over time in the heat pipe and soil temperature at a depth of -2 m for the grooved heat pipe, Figure 8 (a)
) and (b) are the depth of the double-loop heat pipe -
9(a) and (b) are graphs showing the influence of cooling temperature on water absorption power and freezing expansion pressure. A: heat pipe, l: airtight container, 3: heat receiving and dissipating fin, 10: soft and viscous ground, G: working fluid, F... ... Frozen soil, S ... Ice lens layer.

Claims (1)

[Claims] Soft, slightly viscous ground is frozen using a heat pipe formed by filling a working fluid inside an airtight container, and the soft, slightly viscous ground is dehydrated and consolidated by the water absorption force and freezing expansion pressure generated when the ground is frozen. It is a construction method that
(b) cooling the upper part of the airtight container with cold air, condensing the working fluid in the airtight container, and letting the liquefied working fluid fall by gravity; and (c) thawing the frozen soil to cause the ground to sink. A method for improving soft and viscous soil using heat pipes.