International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 01, January 2019, pp. 2059–2065, Article ID: IJMET_10_01_201
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=1
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
Scopus Indexed
INFLUENCE OF EXCAVATION TO THE
TEMPERATURE CONDITION OF
PERMAFROST
E. V. Markov, S. A. Pulnikov, Yu. S. Sysoev, N.V.Kazakova
Transportation institute, Tyumen industrial university,
Tyumen - 625048, Tyumen Oblast, Russia
ABSTRACT
Permafrost is a very complex ground for foundation. Predicting the stability and
reliability of constructions in a rapidly changing climate is impossible without
predicting the temperature condition of permafrost soil. Stabilization of permafrost
soils on local object is provided with a considerable margin, due to the use of
ventilated cellar, thermopiles and monitoring systems. On linear objects such as
pipelines, laying above the ground or the use of thermopiles is a very expensive
engineering protection method. Therefore, the pipeline is laid underground with the
use of ring insulation. Thawing of permafrost soils leads to significant uneven vertical
deformations of the pipeline. Therefore, the accuracy of predicting the temperature of
the permafrost is a priority problem.
Underground pipeline laying is accompanied by excavation and backfilling of the
soil. In this case, the structure of the soil is destroyed, the density is decreased and the
water content is increased due to mixing with snow and penetrating of precipitation.
The article presents the results of a numerical experiment of influence of
excavation to the temperature condition of soil. Soil loosening and filling the empty
pore by water significantly increases the average soil temperature of plastic frozen
soil (the soil with annual average temperature in the interval of the most intense phase
transformations). Thus, the stability of constructions is determined not only by their
impact on the permafrost, but also by changing the structure of the soil during
excavation.
Key words: excavation of permafrost, temperature of permafrost.
Cite this Article: E. V. Markov, S. A. Pulnikov, Yu. S. Sysoev, N.V. Kazakova,
Influence of Excavation to the Temperature Condition of Permafrost, International
Journal of Mechanical Engineering and Technology 10(1), 2019, pp. 2059–2065.
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E. V. Markov, S. A. Pulnikov, Yu. S. Sysoev, N.V. Kazakova
1. INTRODUCTION
Permafrost soils cover about 65% of the territories of the Russian Federation. Volume of
construction on permafrost soil increases every year. These are mainly oil and gas fields in the
north of Western and Central Siberia.
Usually local objects are built using cellar and have a high safety margin and special
systems for monitoring the temperature condition of the permafrost. Linear objects, such as
pipelines, are laid underground in many cases. An overground laying or thermopiles is a very
expensive technical solution, so thermal insulation of the surface of the pipeline is used to
protect permafrost from the thermal influence of the pipeline. Despite the calculations
performed at the design stage, it is not always possible to ensure the normative level of
reliability of pipeline systems. This is obviously related to some unaccounted factors in
predicting the temperature of the permafrost. One of these factors is the destruction of the soil
[1-3].
The excavation of soil in the trench destroys the bonds between the frozen soil particles.
The soil is removed from the trench in a temporary dump using an excavator. After the
pipeline is lowered into the trench, the soil from the temporary dump is moved back to the
trench using a bulldozer. At the same time the mixing of soil and snow is happed. As the
result, the soil in the backfilling is no longer identical to the one that was investigated at the
geotechnical survey stage: the density and content of water differ. Therefore, any calculations
of the temperature condition of the permafrost are not quite correct, if they do not take into
account the destruction of the soil.
In this article, the authors solved the problem of estimating of changes in the temperature
condition of permafrost due to changes in the soil structure in the pipeline trench: an increase
in the content of water and decrease of density of dry soil.
2. MATERIAL AND METHODS
To calculate the temperature condition of the permafrost, the authors used a mathematical
model, which is described in earlier articles [4-6]. Authors used the classical non-stationary
heat equation [7-12]:
(
(
)
),
(1)
where
– isobaric heat capacity of soil, J/(kg∙K);
– soil density, kg/m3;
– latent
3
heat capacity of water, J/kg;
– content of liquid phase of water, kg/m ;
– thermal
conductivity of soil, W/(m∙K).
The approximation of isobaric heat capacity, content of liquid phase of water and thermal
conductivity is done with the using of transition function:
(
(
)
)
(
(
(
)
),
(2)
),
(
(3)
),
(4)
where
,
– isobaric heat capacity of frozen and thawed soil, J/(kg∙K);
,
– thermal
conductivity of thawed and frozen soil, W/(m∙K);
– the temperature of the beginning of
freezing, ºC;
– total water content of thawed soil, kg/m3;
– content of non3
freezing water in soil, kg/m ;
– temperature interval of freezing, ºC;
Transition function between temperature of the beginning of freezing
and temperature
of the end of freezing
calculated as follows:
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Influence of Excavation to the Temperature Condition of Permafrost
(
(
)
(
)
)
(
(
)
)
(5)
If the soil temperature is greater than the beginning of freezing, then the transition
function is 1. If the soil temperature is lower than the end of freezing, then the transition
function is 0.
The author explored two main types of soil: sand and clay. The soil temperature at a depth
of 20 m was chosen as
°С and
°С. The calculation scheme is shown
in Figure 1. On the left for the trench in sand, on the right for the trench in clay.
a)
b)
z
z
Inward heat flux
Inward heat flux
0.5 m
1.5 m
45
8.0 m
Sand 1
12.0 m
Clay loam 2 or
Clay loam 3 1.5 m
45
0.5 m
8.0 m
Clay loam 1
12.0 m
Zero inward heat flux
Sand 2 or
Sand 3
y
Zero inward heat flux
Zero inward heat flux
y
Clay loam 1
Sand 1
Zero inward heat flux
Zero inward heat flux
Figure 1 Calculation schemes for numerical investigation
Six calculations were performed, each of which consisted of two stages. At first, the
mathematical model was adapted to the temperature
at a depth of 20 m below the surface
of the ground. To adapt the models the coefficient of snow cover thickness reduction was
used [13-15]. Then, at stage 2, the soil in the trench was replaced with a less dense one. The
duration of the calculation at stage 2 was 30 years.
Table 1 shows a list of numerical studies, including the number of the design scheme,
number of soil at a depth of 8 m, soil in a trench and temperature
at the depth of 20 m.
Table 2 shows the thermal properties of soils, table 3 shows the physical properties of soils.
The thermal properties of soils were calculated in accordance with the methodology given in
the state regulatory documentation [16].
Sand 1 corresponds to sand 2 taking into account the decrease in the density of the soil
skeleton as a result of excavation. Sand 3 can be obtained from sand 2 if you fill all the empty
pores with water. Similarly for clay loams 1, 2, 3.
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E. V. Markov, S. A. Pulnikov, Yu. S. Sysoev, N.V. Kazakova
Table 1 Parameters of numerical studies
№
1
2
3
4
5
6
Calculation
scheme
a
a
b
b
b
b
Soil
Soil in trench
Sand 1
Sand 1
Clay loam 1
Clay loam 1
Clay loam 1
Clay loam 1
Sand 2
Sand 3
Clay loam 2
Clay loam 3
Clay loam 2
Clay loam 3
Temperature
, °С
Table 2 The thermal characteristics of the soils
Soil
Name
1
Sand 1
750
1489
1143
1,57
2
Sand 2
750
1544
1163
3
Sand 3
750
1638
4
Clay loam 1
950
5
Clay loam 2
6
Clay loam 3
(J/(kg∙K))
(W/(m∙K))
(°C)
(kg/m3)
1,86
-0,1
0,8
1,42
1,65
-0,1
0,8
1197
1,57
1,82
-0,1
0,8
1280
960
2,51
2,93
-0,2
0,2
950
1335
981
2,15
2,5
-0,2
0,2
950
1443
1024
2,5
2,92
-0,2
0,2
Table 3 The physical characteristics of the soils
Soil
Name
1
(kg/m3)
(kg/m3)
(kg/m3)
(kg/m3)
(kg/m3)
Sand 1
2157
1800
357
357
120
2
Sand 2
1957
1600
357
428
115
3
Sand 3
2028
1600
428
428
115
4
Clay loam 1
2125
1800
325
325
0
5
Clay loam 2
1925
1600
325
400
0
6
Clay loam 3
2000
1600
400
400
0
It the tables 2 and 3 was used the next conventions:
skeleton, J/(kg∙K);
– density of dry soil, kg/m3.
– isobaric heat capacity of soil
3. RESULT AND DISSCUSION
The results of calculations No. 1, 2, 5, 6 show insignificant fluctuations in the temperature of
the soil, approximately ∓0.03 ºC. This indicates that the excavation of a trench with a change
in density and water content does not significantly influence to the temperature of the
permafrost if average annual temperature below the temperature of the end of freezing . The
result of calculation No. 3 also shows an insignificant change in temperature, which means
that loosening without changing the water content does not influence to the temperature
conditions of the permafrost.
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Influence of Excavation to the Temperature Condition of Permafrost
The result of calculation No. 4 shows an increase in the temperature of the soil by 0.1 C
(Figure 2, 3). The isotherm of -0.45 C under the section with trenches goes deeper by 2.5 m
than on the section with soil of natural structure. Such an increase in temperature significantly
changes the strength properties of the soil and can be a significant danger, because soil is in a
plastic-frozen state. The risks of the development of thermokarst and uncontrolled subsidence
of the soil are increasing, which represents a danger to the any construction.
-0.40 T, C
-0.42
-0.44
-0.46
-0.48
-0.50
0
5
10
15
20
Year
25
30
Figure 2 Dependence of the soil temperature at point z=-3 m, y=0 m
0
T, C
0.05
1
-0.12
2
-0.28
3
-0.45
-0.62
4
-0.78
5
y, m
0
3
6
9
-0.95
12
Figure 3 The distribution of the temperature in the soil at 30 year.
Thus, for any construction changing of water content in permafrost after excavation
presents a significant danger if average annual temperature in the interval of the most intense
phase transformations of water (plastic frozen soil)
4. CONCLUSIONS
Numerical studies of the temperature condition of permafrost under trenches with loosened
soil showed a slight temperature change in the case when the average annual temperature of
the permafrost is lower the interval of intense phase transformations of water. In addition, the
temperature condition is slightly affected only by loosening the soil without changing the
content of water.
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E. V. Markov, S. A. Pulnikov, Yu. S. Sysoev, N.V. Kazakova
The loosening of permafrost and filling the empty pores by water (during precipitation)
and at the same time at the average annual permafrost temperature inside the interval of
intense phase transformations (plastic frozen soil), a significant increase in the average soil
temperature is observed. In the case considered in the article, the increase was +0.1 ºC. For
plastic-frozen soils, such an increase in temperature is a significant factor that accelerates the
degradation of permafrost and thermal erosion. Thus, the stability of structures on permafrost
is determined not only by the thermal effect by constructions, but also by the change in the
soil structure during excavation.
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