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Environ Earth Sci (2017)76:711
DOI 10.1007/s12665-017-7047-1
ORIGINAL ARTICLE
Simulation analysis of the impacts of underground mining
on permafrost in an opencast coal mine in the northern Qinghai–
Tibet Plateau
Wei Cao1 • Yu Sheng1 • Jichun Wu1 • Jing Li1 • Yaling Chou2 • Jinping Li3
Received: 22 November 2016 / Accepted: 11 October 2017
Ó Springer-Verlag GmbH Germany 2017
Abstract Based on two-dimensional heat-conduction
equations with a phase-change component, this study
investigates the impact of underground mining on the
permafrost environment in an opencast coal mining pit.
The dynamics of the maximum thawed and freezing depths
at different depths around a borehole wall are determined.
The spatial distributions of these dynamics are also determined at different locations of the wall profile. The results
show that (1) the maximum freezing depth tends to
increase over 100 years; (2) the maximum thawed depth
increases along a borehole wall over 100 years. In particular, the maximum thawed depth increases faster near the
junctions of permafrost and seasonally frozen soil; (3) due
to the small cross section of mining laneways, coal mining
does not cause rapid increases in permafrost temperature
around borehole walls. Once disturbance to permafrost
around a borehole wall decreases, the once-insignificant
effect of temperature will become more obvious. Underground mining does have some impacts on permafrost
surrounding borehole walls, but it does not cause large
areas of deformation due to thermal disturbance.
& Yu Sheng
sheng@lzb.ac.cn
Wei Cao
caowei@lzb.ac.cn
1
State Key Laboratory of Frozen Soil Engineering, Northwest
Institute of Eco-Environment and Resources, CAS,
Lanzhou 730000, China
2
Key Laboratory of Disaster Prevention and Mitigation in
Civil Engineering of Gansu Province, Lanzhou University of
Technology, Lanzhou 730000, China
3
CCCC First Highway Consultants Co., LTD, Xi’an 710065,
China
Keywords Underground mining Permafrost Opencast
coal mining pit Qinghai–Tibet Plateau
Introduction
Permafrost is a type of geological body which is extremely
sensitive to changes in temperature (Zhou et al. 2000).
Permafrost has undergone remarkable degradation in
recent decades due to global warming and human activities
(Wang et al. 2000; Zhang 2012; Zhou et al. 2000). Largescale linear projects in cold regions have resulted in severe
permafrost degradation and environmental impacts (Wu
et al. 2001). Particularly, engineering construction and
resource development in the Qinghai–Tibet Plateau have
caused widespread environmental damage (Jin et al.
2006, 2009). Mining can cause extensive environmental
damage as it may involve large areas (Cao et al. 2011).
Theses mining activities, especially opencast mining, will
lead to the deterioration of permafrost environments (Cao
et al. 2016). Many scholars have made broad-scale studies
of the environmental impacts of opencast mining on permafrost (Burov and Gresov 2011; Cao et al. 2016; Kaimonov and Panishev 2016; Makarov 2004; Wang
1993b, 1996; Zheng and Ma 1999, 2000). However, these
works have mainly focused on the effects of open-pit
mining, and little attention has been given to the influences
of underground mining on the permafrost environment.
Compared with the open-pit mining, the influence of
underground mining on permafrost environments is relatively small. Accordingly, most future coal well holes in
cold regions will be planned to employ underground mining. However, permafrost stability is a constraining factor
on underground coal mining in permafrost regions (Liu
2014; Zheng and Yu 2014; Xu et al. 2014b). Due to climate
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711
Page 2 of 10
change and the thermal disturbance of engineering activities, coal mining will change the conditions of heat
exchange in the atmosphere, borehole walls and earth
strata. It also will break the original thermal equilibrium.
This thermal change will lead to energy redistribution
between the terrestrial and atmospheric systems (Chang
et al. 2005; Wang 1993a, 2001; Wu et al. 2001). This
feedback process not only changes the local natural environment in permafrost regions, but also changes the temperature of permafrost around borehole walls. This can
make permafrost around borehole walls absorb heat from
the earth-atmosphere exchange. Thus, it will eventually
increase the temperature of permafrost around borehole
walls. These temperature rises may make the ice within
frozen soil turn into water, resulting in melting of permafrost around borehole walls and decreased permafrost
stability (He et al. 2011; Li and Guo 1980; Zheng et al.
2007). Reduced permafrost stability can cause a range of
environmental and engineering problems. Therefore, it is
critical to maintain the original thermal stability of permafrost around borehole walls. This has aroused the
interest of many scholars, who seek to solve the geological
engineering problems of mining in permafrost areas
(Cheng 2002; Ding 1988; Du et al. 2014; Jin and Brewer
2005; Makarov 2004). To this end, this paper will use
numerical simulation to predict the influence of coal mining on permafrost environments at various initial temperatures and boundary conditions. The study will investigate
the changing characteristics of permafrost environments
due to underground mining. It will provide a scientific basis
and theoretical foundation for the environmental protection
and ecological reconstruction of permafrost regions.
Materials and methods
Study area
The Muli coalfields are located in Qinghai Province
(38°100 –38°020 N, 98°590 –99°350 E), Tianjun Prefecture,
Haixi State, as well as in Gangcha Country, Haibei State,
China. The total area of the coalfield is about 400 km2 and
is the largest coal mining area in Qinghai Province. The
coal reserves are abundant and estimated at 3.54 billion
tons. The planned annual extraction of coal will reach 8.1
million tons per year by 2020 (Cao et al. 2008). This
coalfield is located at the alpine permafrost subregion in the
Altun and Qilian Mountains. These permafrost subregions
are part of the permafrost regions of the Qinghai–Tibet
Plateau. Due to the midlatitude, high-altitude location, this
mining area is largely covered by continuous permafrost
(Zhou et al. 2000). According to the topography, permafrost ice content, unconsolidated-formation lithology
123
Environ Earth Sci (2017)76:711
and thickness, the permafrost of this district mainly comprises four types. They are: bedrock-mountainous low-icecontent permafrost, river-terrace low-ice-content permafrost, ice-water terrace high-ice-content permafrost, and
mountain-front gentle-slope high-ice-content permafrost.
The Muli mining area is mainly comprised of mountainfront gentle-slope high-ice-content permafrost (48% of all
permafrost). The permafrost thickness is approximately
50–90 m, and its temperature ranges from approximately
- 1.0 to - 2.0 °C (Cao et al. 2010, 2011).
Reshui coal mine is adjacent to Gangcha and Qilian
Counties in Qinghai Province. The total area of the coal
mine is about 1000 km2, and it was operated in the 1970s.
Haitaer mine area is located in the Reshui coal mine. This
mine area is mainly excavated by underground mining.
Because the geographical location of the Reshui coal mine
is similar to that of the Muli coalfield, this mining area is
also covered by continuous permafrost (Zhou et al. 2000).
Numerical model and parameter settings
Permafrost is a type of geological body which is very
sensitive to temperature. Any variation in temperature will
markedly change the state of permafrost. Once the global
temperature rises due to climate change and human activities, permafrost will be significantly degraded and underground ice will melt. The stability of mine tunnels will thus
be affected. Therefore, the temperature field is usually
considered as an especially important index for characterizing the state of permafrost. In this study, an impact
analysis will simulate the distribution of the permafrost
temperature field. Due to its temperature sensitivity, permafrost is generally considered as a heat conductor in
numerical models. For simplicity, these models only consider permafrost as a heat conductor of the soil body where
the phase change of ice and water occurs. Thermal convection and other factors are ignored. This study assumes
that the content of unfrozen water is only a function of
temperature. So, the temperature-field distribution in a
mine tunnel cross section can be described by a two-dimensional heat-conduction equation with a phase-change
component (Li et al. 1996; Lai et al. 2003; Nan et al. 2005;
Sheng et al. 2006; Zhang et al. 2006). The equation is
expressed as follows:
oT
o
oT
o
oT
qC
¼
k
k
þ
ot
ox
ox
oy
oy
where:
8
C
>
< u C C
L oWi
u
f
C ¼ Cf þ
ð T Tb Þ þ
>
1 þ W oT
Tp Tb
:
Cf
T [ Tp
Tb T Tp
T\Tb
Environ Earth Sci (2017)76:711
8
T [ Tp
k
>
>
< u k k
u
f
k ¼ kf þ
ð T Tb Þ T b T T p
>
Tp Tb
>
:
kf
T\Tb
where q is the soil bulk density in kg/m3; C denotes the
specific heat of the soil body in J/(kg K); Cu and Cf are the
specific heat of melted soil and frozen soil, respectively, in
kJ/m3 °C; ku and kf represent the thermal conductivity of
melted soil and frozen soil, respectively, in J/(m °C h); L is
the phase-change latent heat of water; W and Wi denote the
total water content and total ice content of frozen soil (%);
Tp and Tb are the upper and lower boundary temperature
values in the severe phase-change zone of frozen soil,
respectively, in °C; T denotes temperature (°C); t is a time
variable (h); and x and y denote spatial variables (m).
The geometric dimension of the computational model is
illustrated in Fig. 1. The figure is depicted according to the
characteristics of regional permafrost distribution and the
current extent of underground mining. For simplicity, only
a quarter of the mine laneway is illustrated. In this model,
the radius and depth of the well holes used in underground
mining are 2.5 and 600 m, respectively. The calculation
only considers the thermal influence of the well hole wall
on the surrounding permafrost in the horizontal direction.
We neglect the mutual thermal influence of the soil layers
in the vertical direction. Therefore, the calculated area is
simplified as a two-dimensional area of horizontal profiles
at different depths. The calculated area is geometrically
symmetric on the well hole axis and is reduced to a quarter
of the well hole area. The circumference of the well hole
extends to 40 m from the center of the circular laneway in
Page 3 of 10
711
this calculation. The calculation model and mesh generation are shown in Fig. 1.
The physical parameters of the soil horizon are shown in
Table 1. The soil horizon is covered by weathered bedrock.
The natural volume weight of soil (q) is 2200 kg/m3. The
thermal conductivity values of the soil horizon are only
considered in the frozen and melted states, ignoring the
impact of temperature. The thermal conductivities of
melted soil (kf) and frozen soil (ku) are 4147 J/(m °C h)
and 5184 J/(m °C h), respectively. In addition, the heat
capacity of the soil horizon is simply taken as the volumetric heat capacities in the frozen and melted states in the
calculation process. The specific heat capacities of melted
soil (Cf) and frozen soil (Cu) are 2564 and 2088 kJ/m3 °C,
respectively. The soil moisture content is 5%.
This study does not consider the effect of global
warming. We assume that the thermal disturbance caused
by excavating the mine is 2 °C, based on observational
data obtained during underground mining in the Tumegela
Coal Mine, Xizang (Li and Guo 1980), and the temperature
change is simplified as the following trigonometric
function:
2pt
p
Tðx; y; tÞ ¼ T þ DT þ A sin
þ
ðx; yÞ 2 AB
8760 2
where T is the annual mean temperature at different depths
below the natural ground surface. The annual average
temperatures of the natural surface are - 0.5, - 1.0, - 1.5
and - 2.0 °C, respectively. The geothermal gradient at
different depths below the natural ground is 0.02 °C/m. DT
is the thermal disturbance caused by excavating the mine
and set as 2 °C. A is the amplitude of variation of the
annual average temperature. The temperature of the natural
ground is taken as 10 °C, and the temperature below the
natural ground is taken as 5 °C; t is time; and p/2 is the
initial calculation phase. The four sides of the computational model (BC, CD, DE, and EA) are considered as the
adiabatic boundary conditions. The initial temperature
(T = T0) is given by the annual mean temperature at different depths below the natural ground surface before
excavating the mine. Specific values are calculated from
the temperature gradients.
Results and discussion
Simulation results and influence analysis
To facilitate statistical analysis, thermal stability was
described by the maximum thawed depth in the permafrost
layer and the maximum freeze depth in the seasonally
frozen soil layer.
Fig. 1 Computing model of underground mining
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Page 4 of 10
Environ Earth Sci (2017)76:711
Table 1 Thermodynamic parameters of soil horizon
Soil horizon type
q (kg/m3)
kf [J/(m °C h)]
ku [J/(m °C h)]
Cf (kJ/m3 °C)
Cu (kJ/m3 °C)
Soil moisture content (%)
Weathered bedrock
2200
5184
4147
2088
2564
5
Temporal variation in maximum thawed depth around well
hole walls according to the annual mean ground
temperature near natural ground surfaces
Figure 2 shows the variation in the maximum thawed depth
around the well hole wall with time when it is at different
annual mean ground temperatures near the natural ground
surfaces. When the natural ground is at a mean annual
temperature of - 0.5 °C, it shows the change of the frozenthawed curve over time at the maximum thaw depth over
the 100-year life of the coalfield. It can be seen from the
figure that the maximum melted depth extends from 5.60 m
in the first year to 25.25 m in the 100th year. When the
natural ground is at a mean annual temperature of
- 1.0 °C, the maximum thawed depth expands from
5.10 m in the first year to 6.50 m in the 100th year. The
change is not obvious after the third year. When the natural
ground is at an average annual temperature of - 1.5 °C,
the maximum thawed depth is 4.85 m in the first year and
6.0 m in the 100th year. There is no remarkable change
after the third year. When the natural ground is at an
average annual temperature of - 2.0 °C, the maximum
thawed depth is 4.6 m in the first year and 5.75 m in the
100th year, with no significant change after the third year.
Temporal variation in maximum thawed depth according
to annual mean ground temperature
Figure 3 shows the temporal variation in the maximum
thawed depth at different annual mean ground temperatures.
-0.5
-1.0
-1.5
-2.0
Maximum thawed depth m
30
25
20
15
10
5
0
0
20
40
60
80
100
Time a
Fig. 2 Temporal variation in maximum thawed depth around well
hole walls according to the annual mean ground temperature near
natural ground surfaces
123
Figure 3a illustrates the change in the maximum thawed depth
over time at depths of 0, 10 and 20 m when the annual mean
temperature is - 0.5 °C. The figure shows that the maximum
thawed depth increases with time. Due to the influence of the
geothermal gradient, the maximum thawed depth of 10 m
increases to over 40 m in the 90th year. Meanwhile, the
maximum thawed depth of 20 m increases to over 40 m in the
9th year. Figure 3b shows these changes but at depths of 25,
35 and 45 m and an annual mean temperature of - 1.0 °C.
Because of the influence of the geothermal gradient, the
maximum thawed depth of 45 m is over 40 m in the 9th year.
Figure 3c shows the relationship at depths of 25, 50, 60 and
70 m and an annual mean temperature of - 1.5 °C. The
maximum thawed depth of 70 m is over 40 m in the 9th year
and is influenced by the geothermal gradient. Figure 3d shows
the relationship at depths of 50, 75, 85 and 95 m and an annual
average temperature of - 2.0 °C. The maximum thawed
depth increases with the time. As a result of the influence of the
geothermal gradient, the maximum thawed depth of 95 m is
over 40 m in the 9th year.
Temporal variation of the maximum frozen depth at various
annual mean ground temperatures
Figure 4 shows the temporal variation in the maximum
frozen depth with different annual mean ground temperatures. Figure 4a illustrates this relationship for depths of
30, 50, 100, 150 and 170 m with an annual mean temperature of - 0.5 °C. The maximum frozen depth increases
with the time. As shown in Fig. 4a, the maximum frozen
depth decreases with increasing well hole depth. This is
mainly due to the increase in the ground temperature. The
maximum frozen depth decreases with time during the
initial operation but no longer varies beyond 3 years.
Figure 4b shows the relationship at depths of 55, 75, 125,
175 and 195 m and an annual mean temperature of
- 1.0 °C. The maximum frozen depth decreases with
increasing well hole depth, mainly because of the increase
in ground temperature. As before, the maximum frozen
depth decreases with time during the initial operation but
no longer varies beyond 3 years. Figure 4c shows the
relationship at depths of 80, 100, 150, 200 and 220 m and
an annual mean temperature of - 1.5 °C. The maximum
frozen depth increases with time and decreases with
increasing well hole depth due to increasing ground temperature. As before, the maximum frozen depth initially
decreases with time and is stable after 3 years. Figure 4d
Environ Earth Sci (2017)76:711
Page 5 of 10
40
30
20
10
0
0
20
40
60
80
100
0
20
40
60
(b)
60
80
80
100
40
10
40
10
(a)
20
20
20
0
120
0m
25 m
50 m
60 m
70 m
0
30
Time/a
30
0
0m
25 m
35 m
45 m
Time/a
40
Maximum thawed depth/m
Maximum thawed depth/m
0m
10 m
20 m
Maximum thawed depth/m
Maximum thawed depth/m
40
711
100
0m
50 m
75 m
85 m
95 m
30
20
10
0
120
120
0
20
40
Time/a
60
80
100
120
Time/a
(c)
(d)
Fig. 3 Temporal variation in maximum thawed depth according to annual mean ground temperature. a - 0.5 °C, b - 1.0 °C, c - 1.5 °C,
d - 2.0 °C
Maximum frozen depth/m
20
10
0
0
20
40
60
80
100
30
Maximum frozen depth/m
30 m
50 m
100 m
150 m
170 m
30
20
10
0
120
55 m
75 m
125 m
175 m
195 m
0
20
40
Time/a
(a)
10
20
40
60
Time/a
(c)
100
120
80
100
120
30
Maximum frozen depth/m
Maximum frozen depth/m
20
0
80
(b)
80 m
100 m
150 m
200 m
220 m
30
0
60
Time/a
105 m
125 m
175 m
225 m
245 m
20
10
0
0
20
40
60
80
100
120
Time/a
(d)
Fig. 4 Temporal variation of the maximum frozen depth at various annual mean ground temperatures. a - 0.5 °C, b - 1.0 °C, c - 1.5 °C,
d - 2.0 °C
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Page 6 of 10
Environ Earth Sci (2017)76:711
shows the relationship at depths of 105, 125, 175, 225 and
245 m and an annual mean temperature of - 2.0 °C. As
before, the maximum frozen depth increases with time and
decreases with well hole depth due to increasing ground
temperature. The maximum frozen depth initially decreases with time and is stable after 3 years.
Temporal variation in maximum thawed depth and frozen
depth with different depths of well hole wall and various
annual mean ground temperatures
One single diagram simultaneously displays the temporal
variations in maximum thawed depth and frozen depth at
different depths of well hole wall to show the spatial
continuity visually.
Figure 5a illustrates the temporal changes in maximum
thaw and frozen depths along a borehole wall with a mean
annual temperature of - 0.5 °C. The permafrost extends
from the surface to a depth of 25 m. The maximum thaw
depth increases with time. In the vicinity of 25 m depth, the
maximum thaw depth increases from tens of meters in the
first year to over 40 m in the 100th year. The seasonal
frozen soil ranges in depth from 25 to 175 m. It can be seen
from the figure that the maximum thaw depth decreases
slightly with time, from[ 20 m in first year to around 20 m
in the 100th year. Melted soil is located below 175 m.
2.5 m
10 m
20 m
30 a
10 a
1a
Figure 5b shows the situation at a mean annual temperature of - 1.0 °C. The permafrost extends from the
ground to a depth of 50 m. The maximum thaw depth
increases with time; at approximately 50 m, the maximum
thaw depth increases from tens of meters in the first year to
over 40 m in the 100th year. The seasonal frozen soil
ranges in depth from 50 to 195 m. The maximum thaw
depth decreases slightly with time, from [ 20 m in first
year to around 20 m in the 100th year. Melted soil is
located below 195 m.
Figure 5c shows the situation at a mean annual temperature of - 1.5 °C. The permafrost extends from 0 to
75 m. The maximum thaw depth increases with time; at
approximately 75 m, the maximum thaw depth increases
from tens of meters in the first year to over 40 m in the
100th year. The seasonal frozen soil ranges from depths of
75–220 m. The maximum thaw depth decreases slightly
with time, from [ 20 m in first year to around 20 m in the
100th year. Melted soil is located below 220 m.
Figure 5d shows the situation at a mean annual temperature of - 2.0 °C. Permafrost extends from 0 to 100 m.
The maximum thaw depth increases with time. At
approximately 100 m, the maximum thaw depth increases
from tens of meters in the first year to over 40 m in the
100th year. The seasonal frozen soil ranges from 100 to
225 m depth. The maximum thaw depth decreases slightly
2.5 m
30 m
40 m Variation of
maximum
100 a Permafrost
thawed depth
80 a
50 a
10 m
20 m
1 a 10 a 30 a
50 a
40 m Variation of
30 m
100 a
80 a
maximum
thawed depth
Permafrost
Permafrost table
Permafrost table
25 m
50 m
100 a
Well wall
Variation of
maximum
frozen depth
1a
100 a
Well wall
Variation of
maximum
frozen depth
1a
Seasonal frozen soil
Seasonal frozen soil
175 m
200 m
Thawed soil
Thawed soil
600 m
600 m
(a)
2.5 m
10 m
(b)
20 m
30 m
2.5 m
40 m Variation of
Permafrost
10 m
20 m
40 m
30 m
maximum
thawed depth
Permafrost
100 a
1a
30 a
10 a
50 a
80 a
Permafrost table
75 m
Well wall
100 a
Variation of
maximum
thawed depth
Variation of
maximum
frozen depth
1a
1a
10 a
30 a
50 a
80 a
100 a
Permafrost table
100 m
Well wall
100 a
Seasonal frozen soil
Variation of
maximum
frozen depth
1a
Seasonal frozen soil
225 m
Thawed soil
600 m
(c)
225 m
600 m
Thawed soil
(d)
Fig. 5 Temporal variation in maximum thawed depth and frozen depth with different depths of well hole wall and various annual mean ground
temperatures. a - 0.5 °C, b - 1.0 °C, c - 1.5 °C, d - 2.0 °C
123
Environ Earth Sci (2017)76:711
with time, from [ 20 m in first year to around 20 m in the
100th year. Melted soil is located below 225 m.
Results validation and comparative discussion
Quaternary unconsolidated sediments are excavated during
underground mining in coal mining areas, resulting in the
formation of coal goafs. This inevitably induces surface
subsidence due to the instability of the coal goaf. Ultimately, surface subsidence leads to the movement and
deformation of strata and the ground surface overlying the
coal goaf. External factors (including climate, topography,
quaternary geology and, especially, human activity) will
not cause obvious thermal disturbance to the overlying
permafrost during the process of coal mining, because they
have no direct effects on permafrost. Surface subsidence
only causes minor deformation or movement of the overlying permafrost. Therefore, the permafrost’s characteristic
indicators (indexes of lithology, ice content, annual mean
temperature, seasonal thawing depth and permafrost
thickness) will not change significantly. However, surface
subsidence caused by underground mining will upset the
balance between water and rock at the overlying ground
surface. This will result in changes to shallow groundwater
systems, which can have consequences for overlying
meadows, such as death (Lv et al. 2014; Xu et al. 2014a;
Wang et al. 2015). Changes in surface vegetation can
change the thermal radiation balance of the ground surface,
which may increase the seasonal thawing depth to some
extent. In such cases, the permafrost table will lower and
the ground ice will melt. The lowering of a shallow water
table can lead to the withering or even death of overlying
meadows.
Both Muli coalfield and Reshui coal mine are located on
the southern slopes of the Qilian Mountains. Muli coalfield
is largely covered by continuous permafrost due to its
altitude and latitude. This area is mainly dominated by
mountain-front, gentle-slope, high-ice-content permafrost.
This is because at depths of 1–7 m below the surface, there
is an ice layer up to 9 m thick in a local belt. In the lower,
there is also a high-ice-content layer with a thickness of
0.3–1.0 m around the bedrock weathering layer. The ice
content is 40–90% in the fine-grained soil layer, while it is
15–40% in the moraine layer. The seasonal thawing depth
is 1.0–1.3 m in this coalfield. The permafrost thickness and
temperature are 50–90 m and - 1.0 to - 2.0 °C,
respectively.
Reshui coal mine is located in the northern edge of the
Datong Mountains at an elevation of 3480–3780 m. The
topography is mainly dominated by mountain-front gentleslope and ravine valleys. The seasonal frozen layer is
distributed in an island or strip shape crossing the permafrost regions. In this area, the annual mean permafrost
Page 7 of 10
711
temperature is - 0.1 to - 1.0 °C and the maximum seasonal thaw depth is 0.9–4.0 m. Therefore, there is little
difference in seasonal melting processes between Muli
coalfield and Reshui coal mine. The only difference is that
the frozen soil of Muli coalfield begins to thaw in late April
to early May, which is later than that of Reshui coal mine.
Additionally, the maximum thaw depth of Muli coalfield
appears in late September to early October, which is earlier
than that of Reshui coal mine.
Therefore, the construction experience of Reshui coal
mine has great reference value for the exploitation of the
Muli coalfield, because they are both located at the same
latitude and have similar permafrost formation and
development processes. The Haitaer old coal pit is a
feature of the Reshui coal mine. It was exploited in the
laneway method, and the mining depth reaches 540 m.
(Figure 6a). It is located in a permafrost region (Fig. 6b)
and is currently closed. This coal pit was a suitable site
at which to investigate the impact of underground mining on the permafrost environment. Data obtained here
may be a useful reference for similar mines in the Muli
coalfield. Our investigation found that surface subsidence formed above the coal goaf and has expanded to
surrounding areas (Fig. 6c). This has created a circular
pit 50–60 m deep and 30–100 m in diameter (Fig. 6d).
There is plentiful pond water and frozen ice in the pit.
Landslip has occurred at the edge of the pit. In places
where this extends 10–12 m from the pit, a ground fissure will occur with a depth of 50–60 m. Where slip
only extends 7–8 m from the pit, ground fissure 30–40 m
occurs. The subsidence height of the ground fissure is
30–100 m, and the whole subsidence area spreads out for
several kilometers. Moreover, ground temperature
observations from the 1970s indicate that the ground
temperature increased gradually with increasing depth
during the early stages of mining in the Haitaer coal pit
(Fig. 7). This is mainly because underground mining
caused thermal disturbance to the surrounding permafrost. However, once the mining depth exceeds
100 m, the ground temperature will continue to decline
and become close to 0 °C. Generally, coal mining in
permafrost regions will bring about a range of geological
disasters of different degrees, such as land collapses and
ground fissures. According to the subsidence situation in
the Haitaer old coal pit, the properties and thicknesses of
the permafrost layers have not been changed by the
mechanical deformation of the ground surface rock.
Ground fissures caused by underground mining are the
most serious mechanical deformation. However, the
change in balance between water and rock has some
influence on overlying meadows in permafrost regions.
In the worst cases, this can lead to the death of surface
vegetation.
123
711
Page 8 of 10
Environ Earth Sci (2017)76:711
Fig. 6 Current mining situation and its surrounding environment in Haitaer coal pit
Temperature/°C
0
-8
-6
-4
-2
0
2
4
100
Depth/m
200
300
400
1970-11-30
1971-11-30
1972-11-30
1973-10-30
500
600
2.
Fig. 7 Change of ground temperature with depths at the early stage
of mining in Haitaer coal pit
Conclusion
1.
This study reveals dynamics of the maximum thawed
and frozen depths at different depths around a bore
hole wall. It also demonstrates the spatial distribution
123
of the maximum thawed and frozen depth dynamics at
different locations along the wall profile. The results
show that (a) the maximum frozen depth tends to
increase over time; (b) the maximum thawed depth
increases along the borehole wall over time. In particular, the maximum thawed depth increases faster
near the junctions of permafrost and seasonally frozen
soil; (c) due to the small cross section of mining
laneways, coal mining does not induce rapid increases
in permafrost temperature around borehole walls. Once
it reduces disturbance to the permafrost around the
borehole wall, the insignificant change of temperature
will become more obvious. It is a fact that underground
mining has some impacts on the permafrost surrounding borehole walls; however, it will not cause
large areas of thawing deformation.
Compared with opencast mining, underground mining
does not have dramatic impacts on permafrost environments. However, it does have some effects and,
accordingly, it is necessary to take some constructive
and effective measures to mitigate the impacts of
exploitation on permafrost. Improvements in construction methods during the operation period can also
mitigate mining impacts. Thermal disturbance to
Environ Earth Sci (2017)76:711
permafrost should be aimed to be reduced as much as
possible when designing mine laneways. If necessary,
artificial freezing technology can be adopted. During
the operation period, it is crucial to reduce the
destruction of vegetation as much as possible. Ground
fissures caused by opencast mining should be filled
with coal gangue. Coal pit surfaces should be covered
with soil and transplanted vegetation and irrigated with
reclaimed mine waste water. These measures will
facilitate the recovery of permafrost environments.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 41501079 and 91647103), the
self-determined Project Funded by State Key Laboratory of Frozen
Soil Engineering (No. SKLFSE-ZQ-43), the Chinese Academy of
Sciences (CAS) Key Research Program (No. KZZD-EW-13), the
Foundation for Excellent Youth Scholars of NIEER, CAS. The
authors are grateful for valuable comments and suggestions for
improvements from the anonymous referees.
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