A new ripraped-rock slope for high temperature permafrost regions

Cold Regions Science and Technology 45 (2006) 42 – 50 www.elsevier.com/locate/coldregions

A new ripraped-rock slope for high temperature permafrost regions Xiaojuan Quan a,b , Ning Li a,c,⁎, Guoyu Li a a

State Key Laboratory of Frozen Soil Engineering, CAREERI, Chinese Academy of Science, Lanzhou, Gansu 730000, China b School of Civil Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China c Institute of Geotechnical Engineering, Xi'an University of Technology, Xi'an, Shanxi 710048, China Received 21 March 2005; accepted 9 January 2006

Abstract Considering the strong wind condition in the Qinghai–Tibet plateau, the cooling effects of three traditional embankments were simulated taking into account the forced convection of air in porous media. The results showed that the traditional ripraped-rock slope cannot exploit the advantages of the thermal diode effect of the ripraped-rock slope to the full, and is not satisfactory in protecting the permafrost. Then a sunshade-blocking-wind ripraped-rock slope was suggested to gain better cooling effects and to overcome some specific problems that might occur in the Qinghai–Tibet Railway practice. The numerical simulations on the effect of the new measure proved that it would cool the underlying permafrost effectively and satisfactorily maintain the thermal stability of the Qinghai–Tibet Railway, and it would not encounter the problems that might happen to the traditional embankment, such as blocking of ripraped-rock by blown sands and snow, and the temperature difference between the south and north slopes. © 2006 Published by Elsevier B.V. Keywords: Ripraped-rock; Thermal diode effect; Cooling effect; New-type of ripraped-rock slope

1. Introduction The Qinghai–Tibetan Railway passes through high temperature, high ice content permafrost regions, where it is important to protect permafrost to insure the stability of railway roadbed (Cheng, 2003). Two most effective protecting methods adopted in constructing the railway are the cracked rock embankment and the ripraped-rock slope, whose thermal diode effect has been proven by laboratory experiments and in situ measurements (Cheng et al., 1981; He and Zhang, 2000;

⁎ Corresponding author. Tel.: +86 931 4967290; fax: +86 931 8271054. E-mail address: [email protected] (N. Li). 0165-232X/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.coldregions.2006.01.003

Goering, 1998; Yu et al., 2004; Sun et al., 2004a,b; Feng, 2002). Goering and Kumar (1996) simulated the winter-time convection in open-graded embankment, and found that the temperature at the roadbed bottom was lowered 5–6 °C compared to normal embankments. Lai et al. (2003) studied the long-term effect of the ripraped-rock and found that the permafrost under traditional ballast embankment in the range of 5 m will be thawed in the future 50 years, while the riprapedstone mass embankment can protect the permafrost from thawing, and at the same time make it cooler. In most of the recent numerical simulations on the effect of the protecting measures applied in the Qinghai–Tibet Railway, the cracked rock embankment and the ripraped-rock slope are generally regarded as closed systems, and it is assumed that natural convection

X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50

occurs in them in winter when the temperature at the top is lower than that at the bottom, otherwise, heat conduction caused the major heat exchange in summer when the temperature at the top is higher than that at the bottom. However, it should be noted that the riprapedrock slope in the Qinghai–Tibet Railway is mostly open to the atmosphere (Ma et al., 2002; Sun et al., 2004b), therefore, the wind can blow through the ripraped-rock slope, and the temperature will be strongly influenced by the wind (Zhang et al., 1994; Gao et al., 2003). Therefore, it may not be appropriate to ignore the forced convection in the ripraped-rock slope, which helps to remove heat from the roadbed and is advantageous to cool the roadbed in winter, but will also heat the roadbed in summer. To study the heat transfer mechanism and the cooling effects on the permafrost of the riprapedrock slope under different boundary conditions, here we study by numerical simulation three kinds of embankment: normal embankment (consists of sandy soil, without ripraped-rock), open ripraped-rock slope, insulation slope (the ripraped-rock slope is covered with insulation material). It is concluded that the traditional ripraped-rock slope cannot exploit the advantages of the thermal diode effect of the ripraped-rock slope to the full, and is not satisfactory in maintaining the permafrost stability. Then a new measure, the sunshade-blockingwind ripraped-rock slope (SBWRRS), is put forward, and the efficiency of the method in maintaining permafrost is verified through numerical simulations. Two-dimensional temperature characteristics of embankment models are performed shown as Fig. 1; the first layer is the embankment which is filled with sandy gravel, the slope is composed of ripraped-rock with a diameter of 10 cm, the second layer is sub-clay which is the active layer of the frozen soil, and the third layer is gravel sub-clay formed permafrost.

9m

5m

0.8m

I 2.5m

slope

A 2m

E

B

C

Sub-clay (active layer)

3m

J

K

sandy gravel (embankment)

M

G

2. Governing equations The mass of ripraped-rock or cracked rock can be regarded as porous media saturated with air which is viewed as a single fluid. It is assumed that the pore air is in thermal equilibrium with the solid media and Darcy's law can be used to relate pore air velocity to pressure. The Boussinesq approximation is applied here. With these assumptions, the following equations are obtained for conservation of mass, momentum, energy and state equation of air (Goering and Kumar, 1996; Bear, 1972): Mass equation: YY jd m ¼ 0

ð1Þ

Momentum equation: lY Y m ¼ j P þ q Y g K

ð2Þ

Energy equation: C

∂T þ Cf Y m djT ¼ kj2 T ∂t

ð3Þ

In order to close the above equation, the equation of state of the air is needed: q ¼ q0 ½1  bðT  T0 Þ

ð4Þ

where Y m is the velocity, K is intrinsic permeability of porous media, μ is dynamic viscosity, ¯P is pressure of the air, Y g is acceleration of gravity, ρ is density of the air, T is temperature, λ is equivalent thermal conductivity of the media, C is equivalent heat capacity of the medium, Cf is specific capacity of the air, β is thermal expansion of air, ρ0 and T0 are reference values for the density and the temperature. Substituting Eq. (4) into Eq. (2), and redefining ¯ in order to remove the hydrostatic compopressure P nent yields the following equation for pressure:  K
Y Y m ¼ j P þ q0 Y g bT ð5Þ l

D

where

F

g y P ¼ P þ q0 ð1 þ bT0 Þ Y

ð6Þ

Through Eqs. (1) and (5), we get the control equation  K
Y  j j P þ q0 Y g bT ¼ 0 ð7Þ l

Gravel sub-clay permafrost

20m

43

L

Fig. 1. The traditional embankment profile.

H

Eqs. (3), (5) and (7) show the relation between temperature and velocity. The problem is nonlinear and should be solved by numerical analytic method. Using

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X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50 40

air temperature 30

natural ground temperature embankment temperature

Temperature (°C)

20

slope temperature 10 0 -10 -20 -30 -40 0

1000

2000

4000

3000

5000

6000

7000

9000

8000

Time (hour) Fig. 2. Boundary conditions of model.

Galerkin's method, the following finite-element formulae are obtained. ½Kp fPg ¼ fFv g þ ½KPT fT g  ½C 

∂Tj ∂t



ð8Þ

3. Boundary conditions and material parameters     þ ½ K  Tj ¼ Fq :

ð9Þ

Where, ½KP  ¼

XZ

½KPT  ¼

½C ¼

½K ¼

K jNi jNj dX X l

XZ

ð11Þ

XZ K q0 bNi Y g jNj dX X l

ð12Þ

C

XZ X

XZ

fFq g ¼

ð10Þ

Ni Y mn dC

fFv g ¼ 

Using Crank–Nicolson method, we can solve the Eqs. (8) and (9) on every time interval Δt to obtain the numerical solution of the present problem.

X

ð13Þ

CNi Nj dX

kðjNi jNj þ jNi Nj ÞdX

XZ C

Ni kjT dC

ð14Þ

ð15Þ

Ni and Nj are the shape functions of the element.

3.1. Boundary conditions Most numerical data are taken from the Beiluhe Test field along the Qinghai–Tibet Railway, where a section of embankment is built on high temperature high ice content permafrost. Temperature boundaries are shown in Fig. 2 which were obtained from in situ data of the whole year and those of the following 9 years will be calculated according to the climatic warming by 0.4 °C within 10 years. For the normal embankment and the insulated slope cases, the normal speed of air at each boundary is set as zero, because there is no air flow across any of the physical boundaries. 1 Open ripraped-rock slope: On the natural ground surface AB and embankment surface IK shown in the Fig. 1, the corresponding in situ data are added which are taken from the Beiluhe Test field. For the ripraped-rock layer, it is assumed that the main process is the forced convection caused by the outside wind. Therefore, it is assumed that wind can flow through it because of its thin thickness of 80 cm.

Table 1 Thermal parameter of the material Physical variable Ripraped-rock slope Sandy gravel (embankment) Sub-clay (active layer) Gravel sub-clay permafrost

λu/W · m− 1 · K− 1 0.38 1.13 0.84 0.87

λf/W · m− 1 · K− 1 0.38 1.58 1.2 1.4

Cu/J · m− 3 · K− 1 6

1.01 × 10 2.17 × 106 2.2 × 106 2.6 × 106

Cf /J · m− 3 · K− 1 6

1.01 × 10 1.7 × 106 1.9 × 106 2.5 × 106

K (m2) 1.58 × 10− 6 1.0 × 10− 9 1.0 × 10− 9 1.0 × 10− 9

X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50

45

2 1 0

Insulation slope -1

0 0

-2

Normal slope

-3

Ripraped slope

-4 -5 5

6

7

8

9

10

11

12

13

14

15

16

17

Fig. 3. Comparison of the 0 °C isotherms of the three embankments when the mean boundary temperature takes its highest value in summer in the 10th year.

The in situ air temperature is imposed on the inner boundary CJ without the ripraped-rock layer. The geothermal heat flux through the bottom boundary GH is q = 0.06 W/m2. 2 Normal embankment: The corresponding in situ temperatures are imposed on all the boundaries: natural ground, slope and embankment surface. 3 Insulation slope: The boundary conditions are set the same as the normal embankment.

capacity and thermal conductivity coefficient are C = 5.0 × 104J·m− 3·K− 1, λ = 0.03 W m− 1·K− 1, respectively. The other thermal parameters are listed in the Table 1, in which λ is thermal conductivity coefficient and C is the volumetric heat capacity, the suffixes u and f stand for unfrozen soil and frozen soil, and K is the intrinsic permeability of the media. These parameters are determined by experiments and calculations (Xu et al., 2001; Bear, 1972).

3.2. Thermal parameters in embankment

3.3. Numerical method

The specific heat of air at the elevation of 4500 m is Cf = 1.004 kJ/kg, the thermal conductivity coefficient of the air is λf = 2.0 × 10− 2 W m− 1·K− 1, the air density is ρ = 0.641 kg m− 3, the kinematic viscosity coefficient is μ = 1.75 × 10− 5 kg m − 1 ·s − 1 . Insulating material is expanded polystyrene (EPS), whose volumetric heat

The temperature simulation is carried out using the 3G2001 modeling package developed by Chen (2001) to solve the heat convection problem in porous media. The temperature field in this paper is a nonstatic phase changing conduction problem, so the method of the sensible heat capacity is used (Lai et al., 2003).

2 1 0

Insulation slope -1

Normal slope

-2 -3

Ripraped slope -4 -5 5

6

7

8

9

10

11

12

13

14

15

16

17

Fig. 4. Comparison of the −1 °C isotherms of the three embankments when the mean boundary temperature takes its lowest value in winter in the 10th year.

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X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50 Temperature (°C)

4. Results on three traditional embankments

-20

-15

-10

-5

0

5 0

Temperature (°C) -2

0

2

4

6

8

10

12

14

0

Depth (m)

2

2

Depth (m)

The 0 °C isotherms of the three embankments when the mean boundary temperature takes its highest value in summer of the 10th year are shown in Fig. 3. The three curves go distinctively in depth from high to low as insulation slope; normal embankment; and open ripraped-rock embankment. The height difference between the highest and the lowest ones underlying the slope is about 1.7 m, while that underlying the natural ground is not so distinct. From the distribution of the curves, it can be concluded that the open ripraped-rock slope and the normal embankment absorb too much heat on the slope, while the insulated layer can hold back the flowing of heat effectively in summer. Fig. 4 is the temperature field of the three types of the embankment in winter in the 10th year when the boundary temperature gets its lowest value. Here we are interested in the − 1 °C isotherm, which reflects the cooling effect of the measures in winter. The − 1 °C isotherm is arranged in Fig. 4 from high to low as insulation slope, normal embankment, and open ripraped-rock embankment. The maximum difference between the − 1 °C isotherm of the open ripraped-rock slope and that of the insulation slope is 2.5 m. The − 1 °C isotherm of the open ripraped-rock slope is the deepest and that of the insulation slope is the highest, which means that in cold seasons the open ripraped-rock slope exchanges heat with outside constantly to cool the embankment, while the insulation slope cannot send out heat easily. That is to say the slope should be opened in winter to make it easier to release heat from embankment. Fig. 5 is the temperature curve of the section ML, which is located under the slope as shown Fig. 1, in summer in the 10th year when the boundary temperature is the highest. The arrangement of the three temperature

Ripraped-rock slope

4

Normal slope 6

Closed slope

8

Fig. 6. The temperature curve of the section ML at the lowest temperature in winter in the 10th year.

curves from left to right is insulation slope, normal embankment, open ripraped-rock embankment. The most left curve is that of the insulation slope whose temperature is lower than the other two. In the 0 to 3 m depth range, the temperature difference between the insulation slope and the ripraped-rock slope is 2–6°C. From the phenomenon that the ripraped-rock slope has the highest temperature while the insulation slope has the lowest temperature, it can be concluded that the ripraped-rock slope is not satisfactory in keeping out heat, while the insulation slope is satisfactory. This may be explained by the fact that the ripraped-rock slope allows the warm air to pass through it, while the insulation layer prevents heat from entering the slope by its low heat conductivity. Fig. 6 shows the temperature along section ML when the boundary temperature reaches its lowest value in winter of the 10th year. Located at the most left is that of the open ripraped-rock slope, which takes the lowest temperature among the three embankments. This reveals that the open ripraped-rock slope has the most satisfactory cooling effect in winter, because it allows the cold air to flow through it, which cools the underlying permafrost constantly by exchanging heat with it. The difference between the temperatures at 2.5 m depth underlying the insulation slope and the open ripraped-rock slope is about 2.5 °C. The positive value in Table 2 means that heat is conducted upward, that is, the underlying permafrost is

Normal slope 4

Ripraped-rock slope Insulation slope

6

Table 2 The heat budget of the embankment locating at the 4.5 m depth/(J/m2) (3months of summer and winter in the 10th year) Time

Ripraped slope

Normal slope

Insulation slope

Winter Summer Net annual

2158.3 −2293.5 −135.2

2102.6 − 2153.2 − 140.3

21.3 − 73.0 − 51.7

8

Fig. 5. The temperature curve of the section ML at the highest temperature in summer in the 10th year.

X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50 awning board

47

2 k

rip

ra

pe

oc dr

embankment

0

active layer

-2

-4

permafrost -6

-8 0

2

4

6

8

10

12

14

16

Fig. 7. Diagrammatic sketch of the sunshade-blocking-wind riprapedrock slope.

Fig. 9. Distribution of the embankment temperature in the 5th year (September 19) with awning.

releasing heat. It is seen that the frozen soils at 4.5 m below the three types of embankments are all in the heat releasing state in winter (lasting 3 months), while in summer (lasting 3 months) they are all in the heat absorbing state. For the three embankments, the total heat of winter and summer is negative, the heat quantities are −140.3 J/m2, −135.2 J/m2, and −51.7 J/m2 for normal embankment, open ripraped slope and insulation slope, respectively. Calculation on the total quantity of heat absorbed in summer by the frozen soil underlying the three embankments shows that the heat absorbed by that underlying the open ripraped-rock slope is second to that underlying the normal slope, which absorbed the most heat. The reason is that the warm air can flow through the open riprap slope, and so make it easier to transfer heat into it, while the frozen soil underlying the insulation slope absorbs the least heat because the insulation layer makes it more difficult for heat to pass through it in summer. On the

other hand, because the open ripraped slope makes it easier, while insulation layer makes it more difficult for the underlying soil to release heat, the amount of heat released from the open ripraped-rock slope is the most, and that from the insulation slope is the least in winter. From the above comparison, it can be concluded that some effective measures should be applied to protect the slope in summer, otherwise the slope will absorb too much heat, and the slope should be opened so that it can exchange heat with the environment in winter. To meet both the requirements, which is impossible by either of the traditional measures, a new type of ripraped-rock slope, the sunshade-blocking-wind ripraped-rock slope (SBWRRS), is suggested and its thermal effects are analyzed in the following, which has many functions and will not encounter the problems, such as the temperature difference between the south and the north slope, sand blocking problem for the sand transported as “bed load”,

2

2

0

0

-2

-2

-4

-4

-6

-6

-8

-8 0

2

4

6

8

10

12

14

16

Fig. 8. Distribution of the embankment temperature in the 10th year (September 19) with awning.

0

2

4

6

8

10

12

14

16

Fig. 10. Distribution of the embankment temperature in the 5th year (January 19) with awning.

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X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50

simulations, the air temperature shown in Fig. 2 is imposed on the slope BI, because the slope is kept out of solar radiation by the shading board, which makes its temperature similar to that of louvers, from which air temperature is measured. Figs 8 and 9 are the temperature distribution on September 19th in the 5th year and that in the 10th year, respectively. The coordinates of the permafrost table under the embankment in the 5th year are: X = 9.95 m, Y = − 0.35 m; X = 11.89 m, Y = − 0.21 m; X = 16.4 m, Y = − 0.78 m. The coordinates of the permafrost table under the embankment in the 10th year are: X = 9.89 m, Y = − 0.22 m; X = 10.96 m, Y = − 0.15 m; X = 16.24 m, Y = − 0.61 m. From the difference in the permafrost table between the 5th year and 10th year, we note that the permafrost table is rising slowly with a maximum value of 13 cm in 5 years. However, the − 0.2 °C isotherm raises distinctly, which means that the zero temperature zone, the region whose temperature ranges from − 0.2 °C to 0 °C and in which the soil may be either thawed or frozen, shrinks significantly, and turns from a thick stratum in the 5th year into a very thin slip in the 10th year. The temperature distribution on January 19th in the 5th year and in the 10th year is shown in Figs. 10 and 11. It is seen that the freezing depths underlying the embankment in the 5th year are: X=7.72 m, Y=−1.17 m; X = 10.8 m, Y = − 0.36 m; X = 13.27 m, Y = 0.3 m. The coordinates of the − 2 °C isotherm underlying the embankment in the 10th year are: X =7.69 m, Y = −1.31 m; X = 9.22 m, Y = − 2.14 m; X = 13.63 m, Y = −0.086 m. Comparison between these two groups of data reveals that the − 2 °C isotherm dropped during the five years with the maximum value of 1.78 m, from which it can be

2

0

-2

-4

-6

-8 2

4

6

8

10

12

14

16

Fig. 11. Distribution of the embankment temperature in the 10th year (January 19) with awning.

which exist in the practice of traditional open riprapedrock slope in the Qinghai–Tibetan Railway. 5. Cooling effect of the new ripraped-rock slope The new ripraped-rock slope proposed is sketched in Fig. 7, where the size is the same as in Fig. 1, the thickness of the sun shading board is about 2 cm. There is a clearance of 70 cm between the sunshade and ripraped-rock slope. The awning is advantageous to keep out warm wind in summer and avoid the effect of solar radiation on the slope, which influences the stability of the embankment significantly. The effectiveness of such an arrangement is confirmed by the in situ data that the temperature difference between the outside and inside of an awning constructed at Fenghuoshan Mountain along the Qinghai–Tibetan Railway is 4–8 °C (Feng, 2002). In the following

Temperature (°C) -1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

0

-2

-4

Depth (m)

0

1st year 4th year 6th year 8th year 10th year

-6

-8

Fig. 12. Temperature distribution along the depth in September from 1st year to 10th year.

1

X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50

49

Temperature (°C) -7

-6

-5

-4

-3

-2

-1

0 0

-2

1st year 6th year 8th year

-4

10th year

Depth (m)

4th year

-6

-8

Fig. 13. Temperature distribution along the depth in January from 1st year to 10th year.

concluded that the new embankment has a better cooling effect in winter. Shown in Fig. 12 is the temperature along depth at the centerline of the embankment in September of various years. The most left curve is that of the 10th year and the most right one is that of the 1st year. In the depth from 0 m to 8 m the temperature is decreasing with time, and that at the 4 m depth decreases from − 0.1 °C in the 1st year to − 0.6 °C in the 10th year. Although the temperature decreasing amplitude in summer is small, it is certain that the new slope has some cooling effect in summer, which proves the successfulness of the new measure in avoiding the solar radiation and blocking warm wind out of the slope. Fig. 13 shows the temperature along the depth in January. The arrangement of the curves is similar to that of Fig. 12, the temperature in the depth from 0 m to 8 m range is also decreasing with the time, and the maximum temperature difference between the 1st year and 10th year is 2.5 °C, which occurs at 4 m depth. The decreasing trend of the temperature shows that the freezing action of the new types of the slope is satisfying. Table 3 The maximum thawing depth of the embankment in September (unit: m) Permafrost table

2.0 m (got from experimental data)

Time (year)

Slope base

Road shoulder

Center of embankment

1st 4th 6th 8th 10th

1.89 1.50 1.37 1.26 1.13

0.5 0.27 0.25 0.23 0.21

0.81 0.70 0.63 0.58 0.54

From Table 3, it can be seen that the raising of the permafrost table is different at the slope base, road shoulder and center of the embankment. During the 10 years, the permafrost table at the slope base raises 76 cm, and the rises of the permafrost table at the road shoulder and center of the embankment are respectively 29 cm and 27 cm. The difference in the raised height of the permafrost table shows that the sunshade optimizes the cooling effect of the slope and makes cooling effect of the embankment more effective. 6. Conclusions The cooling effects of three types of traditional embankments to protect permafrost are studied by numerical simulations and the simulated temperature profiles are compared to investigate the efficiency of these measures. It is found that the − 1 °C isothermal line of permafrost underlying the open ripraped-rock slope is 2.0 m, which is the deepest among the three measures, and means that this measure strengthens the cooling effect on permafrost in winter. While, the underlying location of the permafrost table is about 2.5 m in summer, which is also the lowest among the three embankment types, and means that the open riprapedrock slope absorbs heat, and thaws the permafrost severely in summer. In addition, the quantity of heat given out by underlying permafrost in winter is less than that absorbed in summer, so, annually, the permafrost is heated. Therefore, the cooling effect of the traditional open ripraped-rock slope is not satisfying and new engineering measures should be sought to maintain permafrost.

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X. Quan et al. / Cold Regions Science and Technology 45 (2006) 42–50

To exploit the thermal diode effect of the riprapedrock slope to the full, a new type of ripraped-rock slope, SBWRRS, is put forward, which combines the advantages of both the ripraped-rock slope and the sunshade measures. The numerical simulation on the effect of the new slope shows that the − 1 °C isothermal line of the underlying permafrost gets deeper with time, and the zero temperature zone almost disappears in 10 years. The raising of the permafrost table in 10 years is 76 cm at the slope base, and those at the road shoulder and the embankment center are 29 cm and 27 cm, respectively. From the simulation it is concluded that the new type of ripraped-rock slope has satisfying cooling effect, and should be adopted to guarantee the thermal stability of the Qinghai–Tibetan Railway embankment and other engineering constructions in cold regions. Acknowledgements This work was supported by the grant of the Knowledge Innovation Program of the Chinese Academy of Sciences, No. KZCX1-SW-04. The authors also appreciate the helpful comments made by Professor Ma Wei, Wu Qingbai from State Key Laboratory of Frozen Soil Engineering, CAREERI, CAS. References Bear, J., 1972. Dynamics of Fluids in Porous Media. Elsevier, San Diego, USA. Chen, Feixiong, 2001. The fully coupled modeling of the thermalmoisture-deformation behavior for the saturated freezing soils. Xi'an University of Technology dissertation for PhD. Cheng, Goudong, 2003. The impact of local factors on permafrost distribution and its inspiring for design Qinghai–Xizang railway. Science in China, Series D: Earth Sciences 33 (6), 602–607. Cheng, Goudong, Tong, Boliang, Luo, Xuebo, 1981. The most important of two problems in embankment construction of the

thick underground ice. Journal of Glaciology and Geocryology 3 (2), 6–11. Feng, Wenjie, 2002. An Application Study of the Riprap and Awning for the Roadbed in Qinghai–Tibet Plateau Permafrost Regions. Chinese Academy of Sciences for degree of master of philosophy. Gao, Rong, Wei, Zhigang, Dong, Wenjie, et al., 2003. Variation of the snow and frozen soil over Qinghai–Tibetan Plateau in the late twentieth century and their relations to climatic change. Plateau Meteorology 22 (2), 191–196. Goering, D.J., 1998. Experimental investigation of air convection embankments for permafrost-resistant roadway design. Proceedings of the 7th International Conference on permafrost, pp. 319–326. Goering, D.J., Kumar, P., 1996. Permeability effects on winter-time natural convection in gravel embankments. Cold Regions Science and Technology 24, 57–74. He, Guisheng, Zhang, Luxin, 2000. Analysis on climate change and thermal stability of experimental railway embankment in Fenghuuoshan region of Qinghai–Tibet Plateau. Journal of Glaciology and Geocryology 22, 20–25 (Suppl.). Lai, Yuanming, Zhang, Luxing, Zhang, Shujuan, et al., 2003. Cooling effect of ripped-stone embankments on Qing–Tibet railway under climatic warming. Chinese Science Bulletin 48 (6), 598–604. Ma, Wei, Cheng, Guodong, Wu, Qingbai, 2002. Preliminary study on technology of cooling foundation in permafrost regions[J]. Journal of Glaciology and Geocryology 24 (5), 579–587. Sun, Binxiang, Xu, Xuezu, Lai, Yuanming, et al., 2004a. Experimental researches of thermal diffusivity and conductivity in embankment ballast under periodically fluctuating temperature. Cold Regions Science and Technology 38, 219–227. Sun, Zhizhong, Ma, Wei, Li, Dongqing, 2004b. Experimental study on cooling effect of air convection embankment with crushed rock slope protection in permafrost regions. Journal of Glaciology and Geocryology 26 (4), 435–439. Xu, Xuezu, Wang, Jiacheng, Zhang, Lixinm, 2001. Thermophysics of Frozen Soil. Science Publishing Company, Beijing. Yu, Wenbing, Lai, Yuanming, Zhang, Xuefu, Zhang, Shujuan, et al., 2004. Laboratory investigation on cooling effect of coarse rock layer and fine rock layer in permafrost regions. Cold Regions Science and Technology 38, 31–42. Zhang, Yinsheng, Pu, Jianchen, Ohate, T., 1994. The climatic feature at the Tanggula Mountain pass on the center of Tibetan Plateau. Journal of Glaciology and Geocryology 16 (1), 41–48.