RU1824562C - Method of laboratory investigation of frozen ground - Google Patents

Abstract

The invention relates to the field of materials research using electrical means and relates, in particular, to methods for laboratory studies of the thermal conductivity of frozen soil. The method provides a qualitative determination of the coefficient of thermal conductivity of frozen soil. The method consists in the following. that the thawed soil is placed in a mold (parallelepiped, ball, cylinder) with a given moisture and density, the soil is divided in height into several layers of a given thickness, on the boundary, the conductors are placed on the bottom of each of them, and then layer-by-layer freezing of the soil, fix the time of freezing of each layer by changing the electrical conductivity of the soil and determine the coefficient of thermal conductivity of frozen soil by the formula LmG 2) 1 1 A | mg / p W / m K, where Amg is the desired coefficient of thermal conductivity of frozen soil, Ayg is the coefficient of thermal conductivity NOSTA dimensional evil soil 1st layer, n - the number of layers of soil which are determined for the 1st value of thermal conductivity, 3-yl, 1 tab. Ј

Description

The invention relates to the field of research of materials using electric means and can be applied, in particular, to methods of laboratory research of thermal conductivity of frozen soil

Impaired soil samples are prepared from uniformly moistened to a predetermined humidity and compacted to the required density, after which the samples, with thermocouple junctions previously introduced into them, are frozen at a temperature below minus 10 ° C for a time providing complete freezing, and additionally maintained at this temperature for 6-8 hours. When testing the soil, the thermos of the device is filled with a cooling or cryohydrate mixture with a temperature not exceeding minus 20 ° С. Non-freezing liquid (alcohol, kerosene) is poured into the tank of the heat receiver. The appliance is closed with a lid, the mixer is turned on and the temperature in the heat receiver is adjusted to the temperature of the cooling mixture with an accuracy of 0.1 ° C. During the experiment, the temperature differences between the sample and the heat-transfer fluid in the heat receiver are monitored over time using a galvanometer connected to the terminals of the differential thermocouple.

with you ate

The thermal diffusivity is calculated by the formula:

0.006,. m2 / s.

where g is the cooling rate;

kf is the calorimeter shape factor. Then, the thermal conductivity of the soil is determined by the formula

I'm a-c-woo. W / (mK)

where c is the specific heat of the soil, J / (kg -K);

y0 is the density of the sample, kg / m.

The disadvantage of this method is that it requires the use of cooling or cryohydrate mixtures, non-freezing liquids related to harmful substances, as well as the determination of the specific heat of the soil at temperatures similar to the temperatures of the experiment. In addition, the process of determining the thermal conductivity coefficient is time-consuming and does not guarantee the exact determination of the desired soil characteristic.

The prototype is a laboratory method for studying the thermal conductivity of frozen soil by the stationary thermal regime method, in which the test sample is placed in a mold, frozen, and then thermoEMF of the soil is fixed.

Investigations are carried out using an instrument consisting of a flat electric heater and a low-inertia heat meter installed at a distance of 2 cm from the surface of the refrigerator, which is a bath with a flowing non-freezing liquid of constant negative temperature, controlled by a thermostat with an accuracy of 0.1 ° C. Thermocouples are laid on the surface of the sample. The device is placed in a metal casing filled with thermal insulation. The heater, heat meter and refrigerator have a circle shape with a diameter of 20-25 cm. A soil sample is prepared according to the dimensions of the device with a given humidity and density and is kept in a refrigerator until the sample is completely frozen at a temperature below minus 10 ° C. The prepared sample is placed in the device, paying attention to the fact that the bases of the frozen soil sample must be flat and strictly parallel to each other. The thermostats of the heater and thermostat of the refrigerator are set to the set values of the test temperature and include the current of the electric heater.

10

fifteen

The temperature difference should be at least 5 ° C. 3-4 hours after the heater is turned on, periodic readings of the thermoEMF of the heat meter are made using a potentiometer. After the steady-state regime is established, which corresponds to the readings for 30 minutes or more with an accuracy of 0.05 MV, the readings of thermocouples mounted on the surface of the sample are recorded. After completing the measurements, the soil sample is weighed to an accuracy of 0.01 kg and at least 3 samples are taken to determine soil moisture, and the density of the sample is determined.

The calculation of the thermal conductivity of frozen soil is carried out according to the formula

twenty

, - W / (mK)

(3)

where h is the thickness of the sample in mm;

Vi is the temperature established on the surface of the sample from the side of the heater, in ° C;

V2 - established with the surface temperature of the soil sample from the side of the refrigerator, in ° C;

q is the amount of heat passing through a unit surface of the sample per unit time at a steady heat flow, defined as

q VR W / m

(4)

5

0

5

0

5

where E is measured by the potentiometer EMF of the thermopile of the heat meter in MV;

V is the constant of the heat meter, in W / (m2 MW).

The disadvantage of this method is that the device is complicated in constructive implementation, the experience requires considerable time, accurate observance of the geometric parameters of the sample, the temperature of the test, the use of stabilizing substances and coolants, does not provide high accuracy in determining the coefficient of thermal conductivity of frozen soil, and difficult to technologically perform research.

The aim of the invention is to increase the accuracy of research, simplification of technology.

This goal is achieved by the fact that in the proposed method of laboratory research of thermal conductivity of frozen soil, in which the test sample is placed in the mold, frozen and recorded changes in the electrical conductivity of the soil, the soil is divided in height into several layers of a given thickness, on the border of each of which are placed conductors , then layer-by-layer soil freezing is carried out, the time of freezing of each layer is recorded, and the thermal conductivity coefficient of frozen soil is determined by the formula:

1

i 1

. mg

W / (mK)

(5)

where Amg is the desired coefficient of thermal conductivity of frozen soil, W / (m-K);

I | mg - thermal conductivity coefficient of frozen soil of the 1st layer, W / (mK);

n is the number of soil layers for which the 1st values of the thermal conductivity are determined.

We do not know such a method for laboratory studies of the thermal conductivity of frozen soil.

Existing methods for determining the thermal conductivity coefficients of frozen soil either do not allow to achieve high accuracy of the results, or are associated with the use of a complex technical measurement process and require the use of harmful stabilizing substances.

The drawing shows a diagram of the proposed method for the laboratory study of thermal conductivity of frozen soil:

In Fig. 1, a laboratory method for studying the thermal conductivity of frozen soil in a parallelepiped shape.

Figure 2 is the same in the form of a ball.

In Fig.Z - the same, in the form of tsilinDRE.

Installation for laboratory research of thermal conductivity of frozen soil includes (figure 1) a form of organic glass or PCB 1 for thawed soil, consisting of an inner wall 2, an external wall 3, thermal insulation 4, a strip of plexiglass or PCB 5 glued to the walls 2 and 3 of form 1 for sealing thermal insulation 4, current sensors 6, for recording the freezing time of the 1st soil layer, enclosed in current-insulating tubes 7, conductor 8. common to the electric circuit, current source 9, ammeter 10 for recording changes in force when freezing the next layer of the investigated soil 12. switch 11 current sensors 6, the studied soil 12.

The method is as follows. In the form 1. made in the form of a parallelepiped, with dimensions not less than

15x15 cm and a depth of not more than 5 cm, lay melt soil 12, prepared with a given moisture and density. In form 1, the current sensors b are placed at fixed depths; / 1. tr.; / p, enclosed in current-insulating tubes 7. Current sensors 6 are connected to a current source 9 and to an ammeter 10 in the form of an electric circuit, for which a common current conductor 8 is connected to an ammeter 10.

The studied soil 12 is placed in mold 1 with a temperature close to 0 ° C, after which it is held for some time at this temperature. Then, on the surface of the test soil 12, the temperature changes sharply to negative, the electric circuit is turned on by the switch 11 of the current sensors 6 to the upper current sensor 6, and at the same time, at the time t 0, the process of freezing the test soil 12 begins with a one-dimensional heat flow from the surface. As soon as the layer r of the test soil 12 freezes, which detects the change in the current strength in the electric circuit recorded by the ammeter 10, the time ti is recorded. At the same time, the switch 11 of the current sensors 6 include the next current sensor 6, located at a depth tp, and fix the freezing time t. the studied soil 12. Then, the next current sensor 6, located at a depth of tft, is connected with the switch 11 of the current sensors 6, the freezing time ta of the layers is fixed, etc. before freezing the nth layer. The number of layers must be at least 3.

The coefficient of thermal conductivity of frozen soil is determined by the formula (5), and the coefficient of thermal conductivity of frozen soil of the 1st layer by the formula

Umg-Wn -L- 2 Vc Ti

W / (mK)

(6)

where UMG is the density of frozen soil, in

w l - ice content of frozen soil in fractional units;

L is the specific heat of melting ice, in J / kg;

1 - thickness of the 1st soil layer, in mm;

Vc is the negative temperature at which the test soil was frozen (taken modulo), in ° C;

ti - freezing time of the 1st soil layer, in sec.

In the form of FIG. 2, the upper hemisphere 1 of brass or copper with a side 2 is used, the lower hemisphere 3 of the same material with

a collar 2. which serves to connect the hemispheres 1 and 3 into a mold 11 in the form of a ball with a diameter of 2R.

Current sensors 4, enclosed in current-insulating tubes 5 and placed in the lower hemisphere 3 on fixed

distance x / 1, tjirjn, central current conductor 6, located in the center of form 11, current source 7, ammeter 8, recording changes in current strength when the next layer rj of the investigated soil freezes through 10, switch 9 of current sensors 4, the studied soil 10.

The method is as follows.

Thawed soil 10 is laid with a given humidity and density in hemispheres 1 and 3. then they are connected, using the sides 2, into a mold 11 in the form of a ball.

The test soil 10 is kept in the mold 11 for some time at a temperature of 0 ° C, for the entire mass of the test soil Y to acquire it. Then, the temperature on the surface of the mold 11 is sharply changed to negative.

Further studies are carried out similarly to the method of example 1, the thermal conductivity of frozen soil is determined by the formula (5), and the thermal conductivity of the 1st soil layer is determined by the formula:

mg

Mf-Wn-L -R) 2VlJfR} W / (m.k.) (7)

where R is the hemisphere radius in m.

In the mold of FIG. 3, an upper half cylinder 1 made of brass or copper with a flange 2 is used, a lower half cylinder 3 of the same material with a flange 2 used to connect the half cylinders 1 and 3 to a mold 12 in the form of a cylinder, end walls 4, current sensors

5 enclosed in insulating tubes

6 placed in the lower half cylinder 3

at fixed distances x / i, 772Cg

from the outer wall of the semicylinder 3, a central current conductor 7 located in the center of the mold 12, a current source 8, an ammeter 9, which records changes in the current strength when the next layer of the studied soil freezes 11, the switch 10 of the current sensors 5, the studied soil 11.

The method is as follows.

Thawed soil 11 is laid with a predetermined humidity and density in half-cylinders 1 or 3, after which they are connected using the sides 2 to a mold 12 in the form of a cylinder and closed at the ends by walls 4.

The studied soil 11 is kept in the form 12 for some time at a temperature

0 ° C to acquire it with the whole mass of soil 11. Then, the temperatures on the surface of mold 12 are sharply changed to negative.

Further research goes the same way.

According to Example 1, the coefficient of thermal conductivity of frozen soil is determined by the formula (5). and the thermal conductivity of the 1st soil layer is determined by

the formula

1 Umg Uy1- mg | Vc ti -), W / (mK),

4 p5-2ch

(8)

where R is the radius of the half cylinder in m.

Tests were conducted to determine the thermal conductivity of frozen soil by

the proposed method for laboratory studies of the thermal conductivity of frozen soil.

The test results showed the following (see table),

Total time spent on one experiment: by the claimed method - 3.5 hours; by the method of stationary thermal conditions - 8.7 hours

The time of one experiment does not take into account the determination of ice moisture and density of the sample, because is the same for both methods.

Thus, a comparison of the results of a laboratory study of the thermal conductivity of frozen ash and slag material of Vorkuta CHPP-2 showed that the claimed method has practically no scatter of test results in various experiments and is 0.01 W / (m

SE at that time, as in the stationary thermal regime, this spread was 0.15 W / (m K), which is 0.95% and 13.6%, respectively, of certain values of the coefficients of thermal conductivity of frozen soil and leads to an increase in accuracy research.

The authors' proposal makes it possible to use a simple technological process for research, which consists in freezing a soil sample, contains a minimum of recorded parameters, which improves the accuracy of determining the thermal conductivity of frozen soil and eliminates the use of harmful stabilizing substances in research.

Claims (1)

SUMMARY OF THE INVENTION A method for a laboratory study of the thermal conductivity of frozen soil, in which the test sample with a given moisture and density is placed in a mold, frozen and recorded changes in the electrical conductivity of the soil, and characterized in that, in order to improve the accuracy of research and simplify the technology, the soil is laid in the form in the form of a parallelepiped, they are divided in height into several layers of a given thickness tfl, on the border of each of which are placed conductors with current sensors, then layered freezing is carried out vanie soil freezing fixed time ti of each layer is determined and the first layer 1 of soil thermal conductivity I Nr. and then calculate the thermal conductivity of the soil Amg according to the formula ,, hmg , W / (mK), where n and: 3 - the number of soil layers for which the first values of the thermal conductivity coefficients A ig are determined, with 25 in case of filling the parallelepiped shape A | „is determined as . Hmg-vvL- lmg 5 10 15 0 5 in the form of a cylinder like -№ | And-x-L. in the form of a ball like D Umg-wLfa - R) 2 (2 9 + R) Lmg, where umg is the density of frozen soil, kg / m3: L / L - iodine content of frozen soil in fractional units; L is the specific heat of melting ice, J / kg; Vc — negative temperature at which freezing was carried out, C; R is the hemisphere radius, M; g is the radius of the half cylinder, M. Thermal conductivity coefficients of frozen ash and slag material of Vorkutinsky CHP-2, section No. 3. Fu1 Qtet Yu / j