close

Вход

Забыли?

вход по аккаунту

?

j.ijheatmasstransfer.2018.07.146

код для вставкиСкачать
International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
Thermal performance of a novel crushed-rock embankment structure
for expressway in permafrost regions
Minghao Liu a, Wei Ma a, Fujun Niu a,b,⇑, Jing Luo a, Guoan Yin a
a
b
State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China
South China Institute of Geotechnical Engineering, School of Civil Engineering and Transportation, South China University of Technology, Guangzhou, Guangdong 510641, China
a r t i c l e
i n f o
Article history:
Received 20 March 2018
Received in revised form 21 June 2018
Accepted 30 July 2018
Keywords:
Thermal performance
Convection heat transfer
Crushed-rock interlayer
Expressway
Permafrost
Porous media
a b s t r a c t
The current crushed-rock interlayer structure, which was successfully adopted in construction of the
Qinghai-Tibet Railway, cannot maintain the foundation stability of expressways in permafrost regions
because of the strong heat absorption of the wide and dark-colored asphalt pavement surface. To satisfy
the higher cooling requirement of expressways, a novel crushed-rock interlayer structure, which especially focuses on enhancing the cooling performance on the embankment core, is presented. A heat transfer model, which includes air convection in the crushed-rock interlayer and the heat conduction with a
phase change in the soil layers, was developed to simulate the temperature evolution of a full-scale testing expressway embankment section built in Huashixia, the Qinghai-Tibet Plateau. The numerical results
indicated that the new structure has a significant cooling performance and especially plays an effective
role in lowering the permafrost temperature beneath the centerline of the expressway. Moreover, the
new structure has the benefit of maintaining symmetry of the embankment temperature distribution.
Therefore, it can be concluded that the new structure is an effective method to prevent permafrost degradation under expressways and can ensure the long-term thermal stability of embankments under the
climate warming. The study provides reference and guidance for expressway design and construction
in permafrost regions, such as the planned Qinghai-Tibet Expressway.
Ó 2018 Published by Elsevier Ltd.
1. Introduction
The Qinghai-Tibet Plateau (QTP), which has an average elevation of more than 4000 m a. s. l., contains the largest lowlatitude permafrost area on the earth [1] (Fig. 1). With the rapid
economic development in China, many pivotal engineering projects have traversed the special regions of the QTP, e.g., the
Qinghai-Tibet Railway (QTR), the Qinghai-Tibet Highway, and the
Qinghai-Tibet Power Transmission Line. However, engineering
problems frequently occur, including frost heave and thaw settlement of foundation soils, leading to damage, malfunction or infrastructure failures [2,3], and the problems will be exacerbated under
the climate warming [4,5]. Under combination of the climate
warming and thermal disturbances from engineering constructions, infrastructures on the degrading permafrost in the QTP are
at a clear risk [6]. The QTR is the first transportation infrastructure
designed in the plateau permafrost region of China to take climate
warming into consideration [6]. This roadway project has resulted
⇑ Corresponding author at: State Key Laboratory of Frozen Soil Engineering,
Northwest Institute of Eco-Environment and Resources, Chinese Academy of
Sciences, Lanzhou, Gansu 730000, China.
E-mail address: niufujun@lzb.ac.cn (F. Niu).
https://doi.org/10.1016/j.ijheatmasstransfer.2018.07.146
0017-9310/Ó 2018 Published by Elsevier Ltd.
in the introduction of a roadbed cooling approach and the development of various special embankment structures with cooling features, including crushed rocks, ventilation ducts, thermosyphons,
and sun-shadings, to protect the permafrost from warming and
thawing beneath the railway embankments [7,8].
A crushed-rock embankment, which is a cost-effective mitigation technique to reduce the effects of permafrost degradation,
has been widely applied in the QTR construction in permafrost
regions, and its ability of protecting the underlying permafrost
has been validated by numerous field experiments and numerical
simulations [9–12]. Experimental embankments with crushedrock materials on the Alaska Highway in the USA and at the Beaver
Creek test site in Canada have also demonstrated the excellent
cooling performances [13,14]. A crushed-rock embankment is usually constructed using coarse, poorly-graded rocks with a high
porosity, which allows natural/forced air convection to occur
within them and can accelerate heat extraction from the embankment and underlying soil during winter. In summer, the heat transfer within the porous layer primarily occurs through conduction
because the crushed-rock layer has a lower thermal conductivity
than the typical embankment material [13], and thus less heat is
conducted to the underlying soil. In summary, its cooling mecha-
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
1179
Nomenclature
m
m
q
P
k
c
B
k
T
b
C
L
velocity (ms1)
dynamic viscosity (Pas)
density (kgm3)
pressure (Pa)
thermal conductivity (Wm1°C-1)
specific heat capacity (Jkg1°C1)
inertial resistance factor (m1)
permeability of the medium (ms1)
temperature (°C)
thermal expansion coefficient of air
volumetric heat capacity (Jm3°C1)
latent heat (Jm3)
nism results from the increased winter cooling rates and decreased
summer warming rates [15].
Although a crushed-rock embankment has been used as a mainstream technique to maintain the road foundation stability of railways and ordinary highways on the plateau [9–11], the current
structure used for the QTR cannot satisfy the higher cooling
requirement of an expressway, which has a wider and darkcolored asphalt pavement. This is mainly because of the adoption
of an asphalt pavement and the much wider (more than 20 m
wide) road surface [16,17], both of which significantly increase
the heat absorption and decrease the heat dissipation from the
top surface compared with railway and ordinary highway embankment, causing more severe permafrost thaw settlement [18]. Based
on the data from several monitoring units installed along the
newly built Gonghe-Yushu Expressway in the east part of the
QTP, obvious permafrost warming trends were observed under
some crushed-rock embankments, especially under the centerlines
[19], indicating that the crushed-rock configuration used for the
QTR cannot be directly applied to expressway construction in
t
q
time (s)
heat flux (Wm2)
Subscripts/superscripts
x
x-direction
y
y-direction
a
air
⁄
equivalent
f
frozen
u
unfrozen
H
height
permafrost regions. According to the proposed development
project by China’s Ministry of Transport, the plan is for the
Qinghai-Tibet Expressway (QTE) to run across the largest plateau
permafrost region in China, and it is expected to play an important
role in promoting the economic development of Tibet [18]. The
QTE will face more severe permafrost degradation problems than
the QTR, especially under impacts of the climate warming.
Some composite embankments that combine crushed-rock and
other cooling measures such as a forced-air ventilation duct,
L-shaped thermosyphons, and insulating material for better cooling performances have been developed, and their effects in
increasing the thermal stability of expressway embankments have
been verified by laboratory tests or numerical experiments [20,21].
However, the combining of two or more cooling measures
multiplies the construction costs. Nanofluid can be selected as an
ideal working fluid to enhance the convection heat transfer
efficiency in porous media [22–24], but such a material at present
cannot be directly applied in the construction of crushed-rock
embankment in permafrost regions.
Fig. 1. Permafrost distribution and embankment section with new design at Huashixia in QTP. The data in this permafrost map was from the China Cold and Arid Scientific
Data Center (http://westdc.westgis.ac.cn/).
1180
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
This paper presents a novel crushed-rock interlayer embankment (CRIE) for expressways in permafrost regions. This new
embankment structure focuses on enhancing the convection cooling performance of a large-width expressway, especially the
embankment center. To evaluate the cooling effect of the new CRIE
on the permafrost, a full-scale embankment section for an expressway using the new design located in a warm permafrost region
with an elevation of 4500 m at Huashixa in the QTP was selected
for study. A numerical heat transfer model, which included the
air convection heat transfer in the crushed-rock layer and heat conduction with a phase change in the soil layers, was developed to
simulate the temperature evolution of the embankment section
with the new design, followed by model verification. This model
was used to forecast the long-term cooling performance of the
new structure based on the assumption that the air temperature
in the QTP will warm up by 2.6 °C in the next 50 years [25]. It is
hoped that this work would provide guidance for expressway
design and constructions in permafrost regions.
below which there is ice-rich permafrost. The mean annual ground
temperature ranges from 1.0 °C to 0.5 °C. This warm and icerich permafrost condition is typical in the QTP and is troublesome
for engineering infrastructures.
3. Numerical model
A coupled heat transfer model was developed to investigate the
thermal performance of the new designed embankment for an
expressway. In the model, the embankment model is divided into
three zones based on the different heat transfer characteristics of
the different media, i.e., the air zone outside the crushed-rock
layer, crushed-rock zone, and soil layer zone. The crushed-rock
zone was taken as porous media because of its highly permeability.
Based on the heat and mass transfer theories, the air convection
heat transfer in the crushed-rock layer and heat conduction with
a phase change in the soil layers were considered in the model.
3.1. Physical model
2. New embankment design and site description
Fig. 2 presents photos and structural diagram of the new CRIE.
Compared with the traditional structure, a layer of crushed rock
was added to the central portion of the current crushed-rock interlayer to form a connective reverse T-shaped crushed-rock frame.
The open boundary of this layer was intended to improve the ventilation capacity of a wider crushed-rock layer, especially in the
center. Thus, it was expected to enhance the air convection heat
transfer in winter time within the layer and results in greater heat
dissipation from the embankment. The added crushed-rock layer is
located in the central isolation of the expressway. Therefore, it will
not influence driving safety.
The studied embankment with the new design, which was completed in August 2014, is located at Huashixia, with an elevation of
approximately 4500 m a. s. l. on the QTP (Fig. 1). The site is an open
area and is close to the Qing-Kang Highway. The vegetation cover
in the site is less than 50%. The soil types mainly consist of silty
clay, sand with gravel in the upper layers, and a highly weathered
mudstone layer. The permafrost table is nearly 2.0 m in depth,
The physical model of the new embankment was established
based on the full-scale embankment located at Huashixia, which
has an elevation of approximately 4500 m a. s. l. on the QTP, as
shown in Fig. 3. A traditional CRIE was also studied for comparison.
In practical permafrost engineering problems, the rock layer can be
considered infinitely long in the longitudinal direction. Thus, the
simulation model was established in a 2-D cylindrical coordinate
system.
In the model, the new CRIE has side slopes of 1:1.5, a width of
24.0 m at the crest of the driving surface, and a height of 2.5 m,
including a 1.2 m thick crushed rock layer at its bottom and a
1.2 m wide crushed-rock layer in the center. There was no central
crushed-rock layer in the traditional embankment model. As
shown in Fig. 3, the numerical model has 30 m wide horizontal
extensions from the right and left slope toes, which were designed
to eliminate the boundary effect. In the vertical direction, the
model has a 30 m deep extension, which includes a 1.0 m thick
silty clay layer, 6.0 m thick sandy soil layer with gravel, and 23 m
thick strongly weathered mudstone layer.
Fig. 2. The full-scale testing expressway embankment section with the new CRIE design in Huashixia. (a) Crushed-rock in the basement; (b) crushed-rock in the center; and
(c) structural diagram.
1181
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
J
I
Air flow
K
A
L
N
M
O
P
1.2m
.5
1:1
Q
R
B
H
G
C
F
23m
6m 1m
30m
24m
1.2m
Inlet
1.3m
30m
Outlet
D
E
Part I- embankment fill; Part II- crushed-rock layer; Part III- silty clay; Part IV- gravel soil; Part Vweathered mudstone; Part VI- air
Fig. 3. Physical model of new CRIE for expressway.
3.2. Governing equations
3.2.1. Air zone outside crushed-rock layer
The airflow/wind is considered to be a turbulent flow outside
the crushed-rock layer [26]. Because the air is assumed to be an
incompressible fluid with constant physical properties, the influence of the air temperature on the airflow velocity is negligible.
Consequently, the physical problem can be described by the Reynolds averaged equations, and we have the following governing
equations for the airflow’s turbulent heat transfer process [27].
Continuity:
@v x @v y
þ
¼0
@x
@y
ð1Þ
where v x and v y are the x and y components of the air velocity,
respectively.
Momentum:
!
@ mx
@ðmx mx Þ @ðmy mx Þ
@p
@ 2 mx @ 2 mx
¼ þl
þ
q
þ 2
þq
@x
@y
@x
@t
@x2
@y
@m
@ðmx my Þ @ðmy my Þ
@p
@ 2 my @ 2 my
¼ þl
þ
q yþq
þ 2
@x
@y
@y
@t
@x2
@y
ð2Þ
qa g
@T
@
@T
@
@T
@ðmx TÞ @ðmy TÞ
¼
ka
þ
ka
qc a
þ
@t @x
@x
@y
@y
@x
@y
ð3Þ
ð4Þ
where ka and ca are the thermal conductivity and specific heat
capacity of air at a constant pressure, respectively.
3.2.2. Crushed-rock layer zone
Air convection inside the crushed-rock layer can occur when
unstable air pressure gradients exist. Thus, crushed-rock layer in
the embankment model can be considered as porous media [28].
In the model, only the motion of the interstitial air is considered.
Therefore, the governing equations can be written as follows [29]:
Continuity:
@v x @v y
þ
¼0
@x
@y
@p
l
¼ mx qBjmjmx
@x
k
ð6Þ
@p
l
¼ my qBjmjmy qa g
@y
k
ð7Þ
where jmj ¼
qa ¼ q0 ½1 bðT T 0 Þ
ð8Þ
where T0 is the corresponding temperature of q0, and b is the thermal expansion coefficient of air.
Energy:
@x
@
@T
@
@T
@ @ðmx TÞ @ðmy TÞ
¼
k
þ
k
qca
þ
@t @x
@x
@y
@y
@x
@x
@y
ð9Þ
where C⁄ and k⁄ are the effective volumetric heat capacity and
effective thermal conductivity, respectively.
3.2.3. Soil layers and embankment fill zones
Based on the assumption that the heat conduction is far larger
than the convective heat transfer in the soil layers [30], the convective heat transfer in these layers can be ignored. Thus, only the heat
conduction and phase change in the soil layers are considered. The
heat transfer process in the soil layers can be described as follows
[27,30]:
C
@x
@
@T
@
@T
¼
k
þ
k
@t @x
@x
@y
@y
ð10Þ
We assume that the phase change of the media occurs in a
range of temperatures ðT m DTÞ. Based on the sensible heat capacity method, C⁄ and k⁄can be expressed as follows:
ð5Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
m2x þ m2y , B is the Beta factor of the non-Darcy flow, k is
the permeability of the porous medium, m is the dynamic viscosity
of air, p is the air pressure, and qBjmjmx is the inertia-turbulent term.
Because the air is incompressible, its density qa is a function of
the temperature and obeys the Boussinesq approximation:
C
!
where m is the dynamic viscosity of air, q is the air density, and p is
the air pressure.
Energy:
qca
where v x and v y are the x and y components of the air velocity in
the porous medium, respectively.
Momentum:
C ¼
8
>
<
L
> 2D T
:
Cf
C f þC u
2
þ
Cu
T < ðT m DTÞ
ðT m DTÞ 6 T 6 ðT m þ DTÞ
T > ðT m þ DTÞ
ð11aÞ
1182
k ¼
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
8
>
<
>
:
T < ðT m DTÞ
kf
kf þ
ku kf
2DT
Table 2
Temperature parameters of air, natural ground surfaces, and embankment surfaces.
½T ðT m DTÞ ðT m DTÞ 6 T 6 ðT m þ DTÞ
T > ðT m þ DTÞ
ku
ð11bÞ
where the subscripts f and u represent the frozen and unfrozen
states, respectively; C and k are the volumetric heat capacity and
thermal conductivity of the media, respectively; and L is the latent
heat per unit volume.
Based on the measured data and previous laboratory test results
[31], the related thermal parameters are given in Table 1. The mean
particle size of the crushed rock is approximately 20.0 cm (diameter range from 10 cm to 30 cm), which accords with the construction of the studied embankment section. The permeability and
inertial resistance factor are k = 1.39 105 m2 and B = 211.20
m1, respectively [29,32]. The specific heat of air at an elevation
of more than 4500 m is ca = 1.004 103 Jkg1°C1, the thermal
conductivity is ka = 2.0 102 Wm1°C1, the air density is qa =
0.641 kgm3, and the dynamic viscosity is l = 1.75 105
kgm1s1.
3.4. Boundary and initial conditions
3.4.1. Boundary conditions
In the computational model, temperatures are used as the thermal boundary conditions, including the air and the natural ground
surface and embankment surface. Based on the observed data and
adherent layer theory [33], the thermal boundary conditions can
be expressed as follows:
2p
p
t þ þ a0
8760
2
þ
DT
t
8760
ð12Þ
where T0 and A are the mean annual temperature and annual amplitude of the temperature, respectively; t is the time in hours; and a0
is the phase angle, as determined by the completion time for the
embankment. Based on a widely accepted climate warming scenario for the plateau [25], an increase rate of DT = 0.052 °Ca1 is
taken into account for the mean annual air temperature. The mean
annual air temperature is 4.5 °C in this region, and based on the
adherent layer theory [33] and field observations, the values of T0
and A for different surfaces in Eq. (12) are listed in Table 2.
Based on the in-situ observed data for the wind at Huashixia on
the QTP, the ambient wind velocity outside the embankment can
be expressed as follows:
m10 ¼ 4:6 þ 1:52 sin
T0 (°C)
A
Air (IH)
Natural ground surface (AK and RH)
Asphalt pavement (MN and OP)
Side slope surfaces (LM and PQ)
Median strip surface (NO)
4.5
0.52
2.0
0.2
0.2
11.5
12
15
13
13
mH ¼ m10
3.3. Physical parameters
T ¼ T 0 þ A sin
Variables
2p
3p
tþ
þ a0
8760
2
a
H
10
ð14Þ
where a is the power law exponent, and a value of a = 0.16 was
obtained in this study.
A constant heat flux of q = 0.06 Wm2 is applied to the bottom
surface of the computational model. The lateral boundaries in Fig. 3
are assumed to be adiabatic.
3.4.2. Initial conditions
The construction of the studied embankment took place in summer, thus it is assumed that the embankment was constructed on
July 15 when the warmest time of the year occurs. The initial temperature fields of underneath the embankment (Parts III, IV and V
in Fig. 3) on July 15 were obtained through a long-term transient
solution with the upper boundary condition of natural ground surface (Eq. (12)) without considering climate warming. According to
numerous numerical simulation tests, the stable thermal regime
can be obtained after 100 years of computations. The initial temperature fields of embankment (Parts I and II in Fig. 3) were determined by the air temperature at the construction time. The initial
wind temperature is taken as air temperature on that date. The initial velocity field of the airflow on July 15 is calculated by using a
wind boundary condition (Eqs. (13) and (14)) without considering
the effect of thermal boundary conditions.
3.5. Solution method
Because the governing equations in the model are highly nonlinear, a numerical solution must be employed. The spatial and
temporal discretization of the above governing equations is carried
out by using the Control Volume Integration Method [27]. The discrete coupled equations are solved in an iterative manner using a
Successive Under-Relaxation Method for every time interval Dt
[27], and the iteration sequence is continued until the maximum
normalized changes of all variables are less than 103, Using these
methods, we can obtain the variable fields of the three zones. Additional details concerning this numerical method used in this model
can be found in Refs. [26,27].
ð13Þ
where m10 is the wind velocity at a height of 10 m. On the basis of
the ‘‘power law for the wind profile” of the atmospheric surface
layer [34], the wind velocity at a height of H from the natural
ground surface at boundary IH can be simplified as follows:
3.6. Model validation
To validate the aforementioned numerical model, the ground
temperatures at a natural borehole and a central borehole of the
new embankment were monitored, and a comparison between
Table 1
Thermal parameters of different materials in computational model (k: thermal conductivity; C: heat capacity; L: latent heat of freezing; subscript f: frozen; subscript u: unfrozen).
Physical variables
kf
Wm1°C1
Cf
Jm3°C1
ku
Wm1°C1
Cu
Jm3°C1
L
Jm3
Embankment fill (I)
Crushed-rock layer (II)
Silty clay (III)
Gravel soil (IV)
Weathered mudstone (V)
Air (VI)
2.053
0.442
1.351
2.610
1.824
0.020
1.625 106
1.016 106
1.879 106
1.863 106
2.122 106
0.644 103
1.794
0.442
1.125
1.910
1.474
0.020
1.982 106
1.016 106
2.357 106
2.401 106
2.413 106
0.644 103
2.04 107
0
6.03 107
2.32 107
3.81 107
0
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
1183
Fig. 4. Comparison of measured and simulated ground temperatures: (a–b) natural ground boreholes and (c–d) embankment center boreholes.
the computed and measured temperatures is given in Fig. 4. The
results indicated that the computed temperatures agreed well with
the measured data for the permafrost layer. A large difference
between the measure data and simulated data was found for the
active layer because a perfect condition was considered for the
temperature boundary, whereas it is actually variable and complex
on the QTP. In general, the numerical model is reasonable for simulating the thermal regime of a crushed-rock embankment in a
permafrost region.
4. Results and analysis
4.1. Thermal regime evolution in warm seasons
4.1.1. Traditional CRIE
Fig. 5a–d shows the evolutions of the geothermal regimes of the
traditional CRIE on October 15 in the 2nd, 5th, 20th, and 30th years
after construction, respectively. The 0 °C isotherm is defined as the
permafrost table because the maximum seasonal thaw depth usually occurs in October on the QTP. From Fig. 5a, the temperature
field of the traditional CRIE is basically symmetrical in the 2nd year
after construction and when the permafrost table under the
embankment elevates to near the original ground surface. However, the permafrost table beneath the embankment center is relatively deeper than that under the embankment shoulders,
indicating a higher heat absorption in the center. The permafrost
temperature beneath the embankment increases, as shown by
the descending lines for 0.5 and 0.6 °C. This is mainly caused
by the thermal disturbance from embankment construction in
the warm season.
As shown in Fig. 5b, the traditional CRIE begins to show a weak
cooling effect on the underlying permafrost after five years of operation. However, a marked asymmetry for the temperature distribution is observed at this time, as shown by the asymmetrical
distribution of the 0.8 °C isotherms under the embankment. In
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
1
-0.8
-0.6
-15
-25
-20
-10
-5
10
15
20
25
0
-10
-5
5
0
10
15
Y/m
20
25
5
0
1
-0.5
-5
0
4
0 1
-0.8
-10
-15
-25
-20
-15
-10
-5
5
0
2
10
15
20
25
0
-5
4
-20
2
1
0
0
-0.8
-15
-10
4
-0.5
1
.8
-20
-15
-10
-5
0
5
10
20
25
X/m
Fig. 5. Thermal regimes of traditional CRIE on October 15 in 2nd (a), 5th (b), 20th
(c), and 30th (d) years after construction (unit: °C).
the 20th year after construction, a 1.0 °C isotherm occurs under
the embankment as a result of the cooling effect of the CRIE, but
the asymmetry of the temperature distribution intensifies. This
implies that the cooling performance of the traditional CRIE is
obviously weakened from the windward side to the leeward side
because of the much wider embankment for the expressway.
In the 30th year after construction, the 1.0 °C isotherm disappears beneath the embankment, which indicates that the permafrost temperature obviously increases as a result of climate
warming. The warm permafrost may cause instability in the
embankment because the deformation of the permafrost is promoted by the increase in temperature. Therefore, the traditional
CRIE structure cannot satisfy the higher cooling requirement of
an expressway with a wide and dark-colored asphalt pavement,
and the serious asymmetry of its ground temperature distribution
will be intensified with an increase in the width of the embankment and may cause uneven thaw settlement.
4.1.2. New CRIE
Fig. 6 shows the geotemperature distributions of the new CRIE
on October 15 in the 2nd, 5th, 20th, and 30th years after construc-
0 -0.5
-1
-5
5
0
10
15
20
25
5
-1
0
-0.5
-1.5
-20
-15
-10
-5
5
0
1
0
10
15
20
25
X/m
(d)
5
2
-5
0
5
6
-15
-25
4
0
-1
-0.5
1
2
-0.5
-1.5
-10
15
1
-1.5
-1.5
1
-0
25
(c)
0
2
-0.5
-0.5
-10
-15
-25
1
20
X/m
-15
-25
5
15
1
-1
-5
5
6
4
-1
X/m
(d)
10
4
3
1
-10
-1
-1
5
0
(b)
0
-0.5
-5
-10
-5
5
(c)
1
-0.6
-0.8
X/m
-15
-25
X/m
-0.5 0
-0.5
-10
-0.8
-15
-15
-0.5 0
-0.8
-20
-20
-0.5
-0.8
-0.5
3
1
-0.6
0
Y/m
5
1
4
4
1
0
-0.8
-15
-25
1
-0.5 0
-15
-25
-5
5
4
-10
Y/m
5
5
-5
Y/m
0
(b)
1
3
1
-10
X/m
0
Y/m
-15
(a)
0
1
-0.5
-0.6
-0.8
0
-10
5
1
0
-0.5
-5
5
3
Y/m
4
0
Y/m
5
(a)
Y/m
5
-1
1184
-1
-20
-15
-10
-5
0
5
10
15
20
25
X/m
Fig. 6. Thermal regimes of new CRIE on October 15 in 2nd (a), 5th (b), 20th (c), and
30th (d) years after construction (unit: °C).
tion. Significant differences in the ground temperature field evolution can be observed for the two different embankment structures.
As shown in Fig. 6a, similar to the traditional structure, a weak
warming of the underlying permafrost caused by the heat disturbance associated with embankment construction occurs in the initial two years after construction. However, the permafrost table
under the embankment moves upward near the original ground
surface, especially under the center line, with a magnitude that is
nearly 1.0 m higher than the same position in the traditional CRIE.
The ground is significantly cooled by the new embankment structure in the 5th year after construction, as indicated by the existence of 1.0 °C and 1.5 °C isotherms under the embankment
center, as shown in Fig. 6b. In addition, the new CRIE improves
the symmetry of the ground temperature field. In the 20th year
after construction, the cold-temperature zone of 1.0 °C expands
and covers most of the area under the embankment (Fig. 6c). This
indicates that the warm permafrost experiences a significant cooling process as a result of the cooling performance of the new
embankment structure.
After 30 years of operation, the permafrost beneath the
embankment warms slightly under the impact of climate warming.
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
1185
Fig. 7. Variations in permafrost table beneath centerlines of new structure and
traditional CRIE.
However, the foundation soil is still cooler than that under the natural ground surface. As shown in Fig. 6d, the 1.0 °C and 1.5 °C
isotherms are still found under the embankment, and the permafrost table is also kept near the bottom of the embankment in
the 30th year after construction. As a result, the ground temperature is clearly lower under the new embankment than under the
traditional one, with a difference of nearly 0.7 °C. The differences
demonstrate that the new CRIE has a better cooling effect than
the traditional one, and its thermal regime is beneficial to maintain
the long-term thermal stability of an expressway in a permafrost
region.
To clearly show the variations in the permafrost tables beneath
the embankments, Fig. 7 summarizes the depths of the permafrost
tables in the centerlines of the two embankments over a period of
30 years after their construction. As shown, considerable upward
movements of the permafrost tables occur under the two embankments in the 2nd year after construction. In particular, the permafrost table beneath the new embankment structure shows a
sharper uprising of 2.8 m and then enters the interior of the
embankment in only 2 years after construction, implying that the
seasonal freeze-thaw process will only occur in the embankment
body. Although a slight downward movement of the permafrost
table occurs as a result of climate warming, it is also kept above
the original ground surface in the 30th year after construction. This
result clearly indicates the effectiveness of the new embankment
structure design in cooling the permafrost stratum under the
expressway, especially under the embankment center.
Fig. 8. (a) Variations of outside air temperature and air temperature in crushedrock layer; (b) variations of instantaneous heat fluxes at bottom of new structure
and traditional CRIE in 5th year after construction.
During the warm seasons from April to October, the positive heat
fluxes at the bottom of the embankments are similar for the new
structure and traditional one, and the maximum heat flux values
are both approximately 3.0 Wm2. However, during the cold seasons, the negative heat fluxes are different. The heat flux of the
new structure increases with the air temperature and reaches a
maximum value of approximately 8.0 Wm2 in January when
the lowest air temperature occurs. In contrast, for the traditional
structure, the maximum heat flux in winter is less than 7.0 Wm2.
The negative heat flux is clearly larger in the new embankment
than in the traditional one in winter, with a mean annual difference of nearly 0.6 Wm2, demonstrating the superior convection
heat transfer of the new embankment in winter. Therefore, compared with the traditional CRIE, the new structure has a stronger
cooling capacity and can effectively stabilize the permafrost stratum under an expressway in a permafrost region.
4.2. Heat fluxes
Fig. 8 illustrates the air temperatures outside and inside the
rock layer, along with the instantaneous heat fluxes at the bottom
of the new structure and traditional CRIE in the 5th year after construction when the embankment is in a relatively stable state. It
can be found that the air temperature in the rock layer decreases
with a decrease in the air temperature outside in cold seasons,
with a minimum temperature of approximately 7.5 °C. The long
and cold winter on the QTP increases the convection cooling rate
of the crushed-rock layer.
The variations of the instantaneous heat fluxes at the bottom of
the new CRIE and traditional CRIE in the 5th year after construction
are shown in Fig. 8b, where positive values stand for heat absorption and negative values stand for heat release. Data of the instantaneous heat fluxes in this figure were computed and collected
during the numerical computation. It can be seen that the change
processes for the heat fluxes are consistent with the air temperature variation, and both the new and traditional CRIE experience
strong heat release processes in the 5th year after construction.
4.3. Cooling performances of crushed-rock interlayers with new and
traditional structures in cold seasons
The active cooling effect of a crushed-rock layer comes from a
heat insulation effect during summer and a convection cooling
effect during winter caused by the air convection heat transfer
through its pore structure [35]. Fig. 9 shows the temperature distribution of the traditional CRIE on January 15 in the 2nd, 5th, 20th,
and 30th years after construction. As shown, the cold energy enters
the crushed-rock interlayer from the air inlet in the right toe and
first cools the foundation soil under this region, as indicated by
the descending line for 0.8 °C (Fig. 9b). The cooling trend for
the permafrost continues in the 20th year after construction
because of the performance of the crushed-rock interlayer, which
can be seen in the existence of the 1.0 °C isotherm (Fig. 9c). However, the low-temperature zone of 1.0 °C only exists under the
right hand portions (windward side) of the embankment, implying
that the traditional structure cannot effectively cool the foundation
soil under the left hand portion of the embankment because of the
1186
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
5
-8
-4
-4 -8 .5 -1
-0
-0.5
-0.5
-0
-15
-25
5
-0
-15
-5
-10
5
0
10
20
25
(b)
-10
-4
-8
-1
-8
-15
-25
-0.8
-10
-8
-1
-5
0
-4
-0.
8
-0
-15
-25
-20
-15
-5
-10
0
5
10
15
20
25
-1
-8
-4
-0.5
0
-8
-1
-4
-0.5
5
10
15
20
25
-0
-1
-0.5
-0.7
.5
-0
-5
-0
.5
-4
-0
.7
-15
-5
-10
5
0
10
15
20
25
(c)
-10
-8
-1
-10
-8
-4
-8
-0.7
-4
-0.5
-1.5
-0
-1.5
-20
-15
-10
-5
-8
-1
-1.5
-0.5
0
5
.7
10
15
20
25
(d)
-5
-1
-8
-8 -4
-0.5
-8
-4
-1.5
-10
-4
-8
-1
-0.5
-1
-0.7
-0
.7
-15
-25
-8
-1.5
X/m
-10
-10
-1.
5
-1
0
-1 -4 -8 -0.7
-0.5
-10
-8
-4
.7
-20
-5
5
-10
-4
25
X/m
Y/m
-8 -4
-1
20
-1
-15
-25
X/m
0
15
-1
0
(d)
-8
Y/m
-5
-10
10
-0.
7
-15
5
0
-1
-20
-5
-1
-10
-0.7
-15
-25
5
-1
-0.7
-10
-10
-10
.7
-5
-0.5
-0.7
-10
-0.7
-15
-0
Y/m
0
-4
-0.7
-4 -8 -10
-1
-15
-25
Y/m
-8
-10
-0.7
-0.5
.7
(b)
5
(c)
-4
-0.5
-5
X/m
5
-0.5
X/m
-10
.8
-10
-10
-8
-0.7
-4
-0.5
-20
5
-4
-8
-1
-0.5
-1
-10
15
-10
-8
-5
X/m
0
Y/m
-20
-10
-0.7
-4
-0
.7
.7
-10
0
-4 -8
-0.5
-0.7
-5
(a)
-10
-1
Y/m
0
-10
Y/m
(a)
Y/m
5
-20
-15
-10
-5
0
5
10
15
20
25
X/m
Fig. 9. Thermal regimes of traditional CRIE on January 15 in 2nd (a), 5th (b), 20th
(c), and 30th (d) years after construction (unit: °C).
large-width pavement of the expressway. Under the effect of climate warming, the cooling performance of the traditional structure in winter is significantly reduced, and the 1.0 °C isotherm
disappears in the 30th year after construction (Fig. 9d) compared
with that in the 20th year. The permafrost temperature under
the traditional embankment becomes warmer, and reaches
approximately 0.7 °C as a result of its ineffective cooling process.
The warm permafrost at this temperature lacks thermal stability.
All of these facts indicate that the cooling of the traditional CRIE
for the expressway is relatively limited and cannot produce cooling
effects strong enough to influence the underlying permafrost, especially under the leeward side of the expressway.
In Fig. 10, after the thermal disturbance caused by the embankment construction dissipates, the new CRIE begins to show its cooling performance. In cold seasons, air convection occurs in the rock
layer, and cold energy is transferred from the cold air to underlying
soils. As a result, the permafrost beneath the centerline of the
embankment is cooled first, as shown by the decline of the 1.0
°C and 1.5 °C isotherms in Fig. 10b, which is significantly different
from the cooling characteristic of the traditional structure (Fig. 8b).
In the 20th year, the 1.0 °C and 1.5 °C isotherms both expand
-15
-25
-20
-15
-10
-5
0
5
10
15
20
25
X/m
Fig. 10. Thermal regimes of new CRIE on January 15 in 2nd (a), 5th (b), 20th (c), and
30th (d) years after construction (unit: °C).
beneath the embankment as a result of the cooling effect of the
new structure (Fig. 10c), which reveals that the underlying permafrost experiences an obvious cooling process in cold seasons.
Although the cooling capacity of the new structure is slightly
weakened by the effect of climate warming, the low-temperature
1.0 °C zone covers most of the area under the embankment in
the 30th year. This demonstrates the effectiveness of the new
design in cooling the permafrost under the expressway, especially
the embankment center. Furthermore, the geotemperature distribution is also symmetric after 30 years of operation. This implies
that the new CRIE can not only provide a good cooling performance, but also eliminate the thermal asymmetry induced by the
wind direction on the QTP.
To differentiate the cooling performances of the crushed-rock
interlayers in the new and traditional structures, Fig. 11a and b
show time series of the ground temperatures at depths of 6 and
15 m relative to the original ground surface beneath the centerline
and right shoulders of the two embankments. It can be found that
the cooling performance of the new CRIE significantly reduces the
soil temperatures at these two depths, and makes the fluctuation
of the temperature larger than that in the traditional CRIE,
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
1187
Fig. 11. Time series of ground temperatures at 6 and 15 m beneath (a) centerline and (b) right shoulders of two embankments, and (c) variation of ground temperature with
depth at centerlines of two embankments on October 15th in 30th year after construction.
revealing an obvious influence on the thermal status of the underlying permafrost of the new structure. Furthermore, the permafrost
temperature at the depth of 15 m beneath the centerline of the
new CRIE is obviously lower than that under the traditional structure, with a large difference of nearly 1.0 °C (Fig. 11a), which is significantly larger than the value of approximately 0.4 °C at the same
depth under the right shoulders of the two embankments
(Fig. 11b). This similarity could also be observed in the permafrost
at a depth of 6 m. It demonstrates the superior cooling effect of the
new design on the embankment center.
Fig. 11c shows the temperature profiles at the centerlines of the
two embankments on October 15, in the 30th year after construction. As shown, although the traditional CRIE can cool the permafrost to some extent, its cooling scope and magnitude are
limited, resulting in an obvious warming of the permafrost beneath
the embankment centerline under climate warming. The degeneration of the permafrost at temperatures higher than 1.0 °C results
in poor stability. However, if the new structure is adopted for an
expressway, the permafrost beneath the embankment experiences
considerable cooling processes after construction. In the 30th year,
the permafrost temperature at a depth of up to 15 m beneath the
centerline of the new embankment is still lower than 1.0 °C,
and the lowest temperature is nearly 1.7 °C at a depth of 5 m,
which is approximately 1.0 °C lower than that of the traditional
structure. The cold temperature condition implies that the effects
of climate warming and embankment construction cannot accelerate the permafrost degradation beneath the new CRIE because of
its significant cooling performance. The comparison of these temperature profiles further confirms the advantages of the crushedrock interlayer structure used in the new design, compared to
the traditional one, and demonstrates the effectiveness of the
new design in improving the thermal stability of expressways in
permafrost regions.
expressway in a permafrost region under the effect of climate
warming. The following conclusions can be drawn:
5. Conclusions
Acknowledgements
This paper presented a novel crushed-rock embankment structure for an expressway. A heat transfer model was developed to
investigate the cooling performance of the new design for an
This research was supported by the National Natural Science
Foundation of China (Grant No. 41701067, 41730640, 41630636),
the Science and Technology Service Network Initiative of the
(1) The cooling of the traditional CRIE for an expressway is relatively limited and cannot produce cooling effects strong
enough to influence the underlying permafrost. The largewidth embankment surface and significant heat absorption
caused by the asphalt pavement greatly weaken the cooling
capacity of the CRIE on the permafrost under the embankment center and leeward side, resulting in an obvious permafrost degradation and asymmetrical geotemperature
distribution. Thus, this structure cannot maintain the thermal stability of an expressway in permafrost regions.
(2) In contrast, the new CRIE produces a significant cooling performance and especially plays an effective role in lowering
the permafrost temperature beneath the centerline of the
expressway. Moreover, the new structure shows the effect
of improving the symmetry of the embankment temperature
distribution. Therefore, the new embankment structure can
effectively protect the underlying permafrost and ensure
its long-term thermal stability even under the climate
warming.
(3) Compared with the traditional structure, the new structure
is an effective method to prevent subgrade permafrost from
degenerating and enhance the embankment stability. This
structure has a low cost and low environmental impact,
and cannot affect the traffic safety. Therefore, it is suggested
that the new CRIE can be a candidate embankment structure
for the construction of expressways in permafrost regions.
Conflict of interest
The authors declare that they have no conflict of interest.
1188
M. Liu et al. / International Journal of Heat and Mass Transfer 127 (2018) 1178–1188
Chinese Academy of Sciences (Grant No. KFJ-STS-ZDTP-037), the
National Key Research and Development Program of China (Grant
No. 2017YFC0405101), the Postdoctoral Science Foundation of
China (2017M610662), and the Science Technology Research and
Development Plan of China Railway General Corporation (Grant
No. 2016G003-D).
References
[1] S.C. Kang, Y.W. Xu, Q.L. You, W.A. Flügel, N. Pepin, T.D. Yao, Review of climate
and cryospheric change in the Tibetan Plateau, Environ. Res. Lett. 5 (1) (2010)
015101.
[2] R. Fortier, A. LeBlanc, W.B. Yu, Impacts of permafrost degradation on a road
embankment at Umiujaq in Nunavik (Quebec), Canada, Can. Geotech. J. 48 (5)
(2011) 720–740.
[3] H. Batenipour, M. Alfaro, D. Kurz, J. Graham, Deformations and ground
temperatures at a road embankment in northern Canada, Can. Geotech. J. 51
(2014) 260–271.
[4] D.L. Kane, L.D. Hinzman, J.P. Zarling, Thermal response of the active layer to
climatic warming in a permafrost environment, Cold Reg. Sci. Technol. 19 (2)
(1991) 111–122.
[5] L.D. Hinzman, D.L. Kane, Potential response of an Arctic Watershed during a
period of global warming, J. Geophys. Res: Atmos. 97 (D3) (1992) 2811–2820.
[6] G. Doré, F.J. Niu, H. Brooks, Adaptation methods for transportation
infrastructure built on degrading permafrost, Permafrost Periglac. Process. 27
(4) (2016) 352–364.
[7] W. Ma, G.D. Cheng, Q.B. Wu, Preliminary study on technology of cooling
foundation in permafrost regions, J. Glaciol. Geocryol. 24 (5) (2002) 579–587.
[8] G.D. Cheng, Z.Z. Sun, F.J. Niu, Application of the roadbed cooling approach in
Qinghai-Tibet railway engineering, Cold Reg. Sci. Technol. 53 (2008) 241–258.
[9] Q.B. Wu, Z.J. Lu, T.J. Zhang, W. Ma, Y.Z. Liu, Analysis of cooling effect of crushed
rock-based embankment of the Qinghai-Xizang Railway, Cold Reg. Sci.
Technol. 53 (3) (2008) 271–282.
[10] Y.H. Mu, W. Ma, Y.Z. Liu, Z.Z. Sun, Monitoring investigation on thermal stability
of air-convection crushed-rock embankment, Cold Reg. Sci. Technol. 62 (2010)
160–172.
[11] F.J. Niu, M.H. Liu, G.D. Cheng, Z.J. Lin, J. Luo, G.A. Yin, Long-term thermal
regimes of the Qinghai-Tibet Railway embankments in plateau permafrost
regions, Sci. China Earth Sci. 58 (9) (2015) 1–8.
[12] Y.M. Lai, M.Y. Zhang, Z.Q. Liu, W.B. Yu, Numerical analysis for cooling effect of
open boundary ripped-rock embankment on Qinghai-Tibetan railway, Sci.
China Ser. D-Earth Sci. 49 (7) (2006) 764–772.
[13] D.J. Goering, S. Saboundjian, Design of passive permafrost cooling systems for
an interior Alaska roadway, in: Proceedings of the Cold Regions Engineering &
Construction Conference, 16-19 May, Edmonton, Alberta, Canada, Smith DW,
Lendzion C, Sego DC (Eds.), Construction Research Institute of Canada: Ottawa,
Ontario, Canada, University of Alberta, Edmonton, Alberta, Canada, 2004.
[14] J. Lepage, G. Doré, Experimentation of mitigation techniques to reduce the
effects of permafrost degradation on transportation infrastructures at Beaver
Creek experimental road site (Alaska Highway, Yukon), in: Proceedings of the
Join 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost
Conference, Calgary, Alberta, 2010.
[15] D.C. Esch, Road and airfield design for permafrost conditions, in: T.S. Vinson, J.
W. Rooney, W.H. Haas (Eds.), Roads and Airfields in Cold Regions: an ASCE
Monograph, American Society of Civil Engineers, New York, 1996.
[16] J.P. Li, Y. Sheng, Analysis of the thermal stability of an embankment under
different pavement types in high temperature permafrost regions, Cold Reg.
Sci. Technol. 54 (2) (2008) 120–123.
[17] M.Y. Zhang, Y.M. Lai, Q.B. Wu, Q.H. Yu, T. Zhao, W.S. Pei, J.M. Zhang, A full-scale
field experiment to evaluate the cooling performance of a novel composite
embankment in permafrost regions, Int. J. Heat Mass Tran. 95 (2016) 1047–
1056.
[18] Q.H. Yu, W. Gu, J. Qian, J.M. Zhang, X.C. Pan, Problem analysis of high grade
highway construction in permafrost regions, Highway 11 (2010) 74–80.
[19] Z.L. Feng, Y. Sheng, J. Chen, J.C. Wu, J. Li, Y.B. Cao, X.Y. Hu, X.M. Zhang, A
preliminary analysis of protective effect on permafrost of typical embankment
along Gonghe-Yushu highway, Chin. J. Rock Mech. Eng. 35 (3) (2016) 638–648.
[20] M.Y. Zhang, X.Y. Zhang, S.Y. Li, D.Y. Wu, W.S. Pei, Y.M. Lai, Evaluating the
cooling performance of crushed-rock interlayer embankments with
unperforated and perforated ventilation ducts in permafrost regions, Energy
93 (2015) 874–881.
[21] Y.M. Lai, H.X. Guo, Y.H. Dong, Laboratory investigation on the cooling effect of
the embankment with L-shaped thermosyphon and crushed-rock revetment
in permafrost regions, Cold Reg. Sci. Technol. 58 (2009) 143–150.
[22] M. Sheikholeslami, H.B. Rokni, Numerical modeling of nanofluid natural
convection in a semi annulus in existence of Lorentz force, Comput. Method
Appl. M 317 (2017) 419–430.
[23] M. Sheikholeslami, T. Hayat, A. Alsaedi, Numerical simulation of nanofluid
forced convection heat transfer improvement in existence of magnetic field
using lattice Boltzmann method, Int. J. Heat Mass Transf. 108 (2017) 1870–
1883.
[24] M. Sheikholeslami, H.B. Rokni, Simulation of nanofluid heat transfer in
presence of magnetic field: a review, Int. J. Heat Mass Transf. 2017 (115)
(2017) 1203–1233.
[25] D.H. Qin, Y.H. Ding, S.W. Wang, A study of environment change and its impacts
in western China, Earth Sci. Front. 9 (2) (2002) 321–328.
[26] M.Y. Zhang, Y.M. Lai, F.J. Niu, S.H. He, A numerical model of the coupled heat
transfer for duct-ventilated embankment under wind action in cold regions
and its application, Cold Reg. Sci. Technol. 45 (2) (2006) 103–113.
[27] W.Q. Tao, Numerical Heat Transfer, Xi’an Jiaotong University Press, Xi’an,
2004.
[28] Y.M. Lai, L.X. Zhang, S.J. Zhang, L. Mi, Cooling effect of ripped-stone
embankments on Qinghai-Tibet railway under climatic warming, Chin. Sci.
Bull. 48 (6) (2003) 598–604.
[29] D.A. Nield, A. Bejan, Convection in Porous Media, Springer Science & Business
Media, New York, 2006.
[30] W.D. An, Z.W. Wu, Interaction Among Temperature, Moisture and Stress Fields
in Frozen Soil, Lanzhou University Press, Lanzhou, 1990.
[31] X.Z. Xu, J.C. Wang, L.X. Zhang, Physics of Frozen Soils, Science Press, Beijing,
2001.
[32] M.Y. Zhang, Y.M. Lai, D.Q. Li, W. Chen, G.Q. Tong, Experimental study on
ventilation characteristics of a concrete-sphere layer and a crushed-rock layer,
Int. J. Heat Mass Transf. 59 (2013) 407–413.
[33] L.N. Zhu, Study of the adherent layer on different types of ground in
permafrost regions on the Qinghai-Xizang Plateau, J. Glaciol. Geocryol. 10 (1)
(1988) 8–14.
[34] M. Zhao, M.Q. Miao, Y.C. Wang, Boundary Layer Meteorology, China
Meteorological Press, Beijing, 1991.
[35] D.J. Goering, P. Kumar, Winter-time convection in open-graded embankments,
Cold Reg. Sci. Technol. 24 (1) (1996) 57–74.
Документ
Категория
Без категории
Просмотров
0
Размер файла
2 706 Кб
Теги
ijheatmasstransfer, 146, 2018
1/--страниц
Пожаловаться на содержимое документа