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Cooling performance of two-phase closed thermosyphons installed at a highway embankment in permafrost regions
Applied Thermal Engineering 98 (2016) 220–227
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Cooling performance of two-phase closed thermosyphons installed at a highway embankment in permafrost regions Fan Yu a, Jilin Qi b,*, Mingyi Zhang a, Yuanming Lai a,c, Xiaoliang Yao a, Yongzhi Liu a,d, Guilong Wu a,d a State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China b Beijing High Institution Research Center for Engineering Structures and New Materials, Beijing University of Civil Engineering and Architecture, Beijing 100044, China c School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China d Beiluhe Observation Station of Frozen Soil Environment and Engineering, Cold and Arid Region Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
H I G H L I G H T S
• • • •
Long-term monitored data are presented for a road section of the Qinghai-Tibet Highway before and after installing the TPCTs. Cooling scope and period of the TPCTs are analyzed; cooling effects for the soil layers are examined. Cooling performance of the TPCTs is discussed from the perspectives of embankment deformation and crack formation. Some suggestions are proposed for a better design of the TPCTs in highway constructions in permafrost regions.
A R T I C L E
I N F O
Article history: Received 10 August 2015 Accepted 25 November 2015 Available online 24 December 2015 Keywords: Two-phase closed thermosyphon Ground temperature Permafrost degradation Embankment deformation Crack formation Thaw consolidation
A B S T R A C T
Two-phase closed thermosyphon (TPCT) is a popular way to prevent permafrost layers from degrading, and consequently ensure the stabilities of engineering constructions in permafrost regions. Although TPCTs have been numerically and experimentally investigated for many years, long-term field monitored data concerning the cooling performance of TPCTs are limited. This paper presents the ground temperatures, embankment deformations and some related meteorological factors for a road section of the Qinghai– Tibet Highway before and after installing the TPCTs in permafrost regions. Based on the monitored data, three main aspects are analyzed: 1) cooling scope and period of the TPCTs; 2) cooling effects for the soil layers, especially for the permafrost layers; and 3) remedying effects with respect to embankment deformation and crack formation. Some corresponding suggestions are proposed for a better design of TPCTs in the construction of roadways in permafrost regions. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction In permafrost regions, engineering constructions have been suffering different levels of deformations caused by permafrost degradation [1,2]. Taking the Qinghai–Tibet Highway as an example, 31.7% of the road sections from Golmud to Lhasa (520 km) in the permafrost regions faced roadbed diseases in 1999 [3], among which 85% of them were caused by thaw settlement [4]. Under global warming and the thermal effect caused by road embankments, the underlying permafrost layers are degrading [5]. Thaw consolidation has been considered to be the main cause of embankment
* Corresponding author. Tel.: +8613811703629; fax: +86 931 4967292. E-mail address: [email protected] (J. Qi). http://dx.doi.org/10.1016/j.applthermaleng.2015.11.102 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
deformation for a long time [2]. In recent years, creep of warm permafrost layers has also been proved to be another main cause [6,7]. It is found that the two deformation causes are exactly the main reason why the Qinghai–Tibet Highway was reconstructed and repaired for many times [8,9]. In addition, 79% of the permafrost regions on the Qinghai–Tibet Plateau were characterized by “warm permafrost”, where the mean annual ground temperatures (MAGTs) were higher than −1.5 oC [10]. The easy-to-thaw characteristic determines that some cooling methods should be taken to protect the permafrost layers from degrading, and thus to ensure the stabilities of the engineering constructions. Two-phase closed thermosyphon (TPCT) is one of those cooling methods, which has already been used in permafrost regions for many decades. For instances, the TPCTs were successfully used in the Trans-Alaska Pipeline System between 1974 and 1977 [11]. The TPCTs were also proved by in-situ geothermal observation between
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1990 and 1997 to be a reliable technology for rehabilitating a housing with unacceptable settlement on the Ellesmerer Island, Canada [12]. In China, the TPCTs have been frequently employed to insure the stabilities of the Qinghai–Tibet Highway [13,14], the Qinghai– Tibet Railway [15,16], and the Chaidaer–Muli Railway [17]. In addition, other cooling methods were combined with the TPCTs for better cooling effects, such as crushed rock revetment [18,19], forcedair ventilation [20] and insulating material [21]. In these engineering constructions, the evaporator sections were embedded in the soil layers, while the condenser sections were exposed in the air. The TPCTs only work when the temperature difference exceeds the threshold, i.e. the start-up temperature difference. Zhang et al. [22,23] found that it was about −0.2 °C by means of indoor physical model test and a self-designed experimental apparatus. In order to enlarge the cooling range underneath the embankments, inclined TPCTs have been numerically and experimentally investigated [23,24]. Besides the inclining angle, there are three other influencing factors of the cooling performance of the TPCTs, including aspect ratio (the ratio between evaporator length and internal diameter), filling ratio (the ratio between the volumes of working fluid and evaporator section) and working fluid [23]. The influencing factors have been extensively investigated by previous researchers [23,25–28]. With respect to the previous numerical modeling work, for a simplified calculation, the thermal resistance of the TPCTs were usually ignored and the heat-convention coefficient between air and the radiating fins was taken as a constant when the TPCTs were working [21,29–31]. On the other hand, the heat transfer coefficients for every part of the TPCTs were introduced by Pan and Wu [32], based on which a coupled air-TPCT-soil model was put forward [33]. For ease of calculation, however, some empirical formulas were adopted for calculating the heat resistances in the coupled model, which may lead to some unavoidable errors. Under this circumstance, a fullycoupled model is urgently needed. The governing equations in the three zones have already researched respectively. The three governing equations in the air zone are continuity, momentum and energy equations [34,35]. Also, the three equations are usually used for simulating two-phase flows in TPCTs [36–38]. In the soil layers,
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the heat transfer equation with phase-change considered is usually adopted [39–41]. From the above analyses, it can be found that the TPCTs have been considered to be a popular and efficient way to ensure the stabilities of the engineering constructions in permafrost regions, especially in warm permafrost regions. There are many influencing factors for the cooling performance of the TPCTs, which needs further investigation. A fully-coupled air-TPCT-soil model is urgently needed for a better prediction. To date, long-term field monitored data about the cooling performance of the TPCTs are currently lacking. This would be favorable for understanding the cooling performance of the TPCTs, and for establishing the fully-coupled model for the stabilities of the engineering constructions in permafrost regions. In this paper, more than 10-year monitored data including ground temperatures, embankment deformations and some related meteorological factors are presented for a road section installed with TPCTs in permafrost regions. The main objective is to examine the cooling performance of the TPCTs from the perspectives of thermal and mechanical (deformation) stabilities.
2. Monitoring program The monitoring program was carried out at a road section of the Qinghai–Tibet Highway. The highway mileage is K3187 + 000, while the place is named as Kaixinling. The longitude and latitude are N33°57′29′′ and E92°20′58′′ respectively, and the geographic position can be seen in a previous paper [42]. An illustration of the instrumented road section is shown in Fig. 1. Ground temperatures and embankment deformations were started to monitor in 2003. Due to an unacceptable embankment deformation, TPCTs were installed around September 2009. The working fluid is anhydrous ammonia, while the container is made by carbon steel. As shown in Fig. 1, the lengths of the three parts of the TPCTs, condenser, adiabatic and evaporator, are 4, 3 and 5 m respectively. Some other parameters of the TPCTs are summarized in Table 1. A meteorological station was established in Aug. 2008. Detailed information about these three monitoring items is introduced in the following.
Fig. 1. Schematic cross-section (a) and plan (b) of the instrumented road section installed with TPCTs.
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Table 1 Some parameters of the TPCTs. Parameters
Values (mm)
Inner diameter of pipe Outer diameter of pipe Fin height Fin space Fin thickness
79 89 25 10 1
2.1. Ground temperatures Two thermistor cables were installed at the left shoulder and in the middle of the road section on September 2003, and another one was added at the right shoulder on April 2013. As shown in Fig. 1, the three cables reached the depth of 20 m below the asphalt pavement. The thermistors have an accuracy of ±0.05 °C, which were developed by State Key Laboratory of Frozen Soil Engineering, Chinese Academy of Sciences. The data were collected by a CR3000 datalogger (Campbell Scientific Inc., USA) twice a month. The depths of permafrost tables are obtained by interpolating calculation based on the monitored ground temperatures. 0 °C is taken as the thawing temperature for ease of discussion, since it varies with soil type, load, water and salt contents, etc. [43]. 2.2. Embankment deformations Twenty steel nails and a 15-m-deep steel pipe were embedded into the asphalt pavement on October 2003, as shown in Fig. 1b. It is assumed that the deformation of the soil layers deeper than 15 m can be negligible, and thus the steel pipe can be taken as a bench-
mark. The elevation differences between the steel nails and the benchmark are the embankment deformations. An electronic total station was used to monitor the deformations once or twice a month. It was suspended from August 2008 to August 2009 due to the replacement of asphalt pavements and the installation of the TPCTs. 2.3. Metrological factors A meteorological station was installed nearby the road section on August 2008. Air temperatures and wind speeds at the heights of 2 m and 10 m were monitored using HMP45C-L11 and 05103L11 sensors (Vaisala, Finland). In addition, air humidity and radiation and precipitation were also continuously measured. Since air temperatures and wind speeds are the top two influence factor for the heat transfer coefficient of the TPCTs [44], only they are adopted in this paper. 3. Results and discussions The monitored data including ground temperatures, embankment deformations, air temperature and wind speeds at the height of 2 m are shown in Fig. 2. Fig. 2a shows the monthly average values of air temperatures and wind speeds at the height of 2 m. The accumulated values of embankment deformations (Fig. 2b) correspond with the ground temperatures below left/right shoulders and in the middle of the road section (Fig. 2c–e). The Kriging interpolation method is adopted to obtain the continuous ground temperatures based on the monitored data at different depths. The embankment deformations at left shoulder refer to the average values of the monitoring points 10# and 15#, that in the middle refer to the average values of the monitoring points 8# and 13#, while that at
Fig. 2. Monitored data including air temperatures and wind speeds (a), embankment deformations (b) and ground temperatures below the left shoulder (c), in the middle (d) and below the right shoulder (e).
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Fig. 3. Average ground temperatures in the middle (a) and below the left shoulder (b) at the road section of the Qinghai–Tibet Highway in 2004, 2009, 2010 and 2013.
the right shoulder refer to the average values of the monitoring points 6# and 11# (Fig. 1b). It is worth mentioning that right/left shoulders are determined when facing the direction to Lhasa (Fig. 1b).
3.1. Cooling scope of the TPCTs The cooling scope indicates the soil layers cooled by the TPCTs horizontally and vertically, which is closely related with the embankment deformation. Since the ground temperatures were missing during the period of installing the TPCTs in July and August of 2009, the ground temperatures in 2008 is taken as the initial state to examine the cooling scope. Because of lacking sufficient thermistor cables from the left shoulder to the middle of the road section (Fig. 1), it is only possible to get some information about the horizontal cooling radius from the thermistor cable in the middle. Fig. 3 presents the average ground temperatures in the middle and below the left shoulder in 2004, 2009, 2010 and 2013. In the middle of the road section, the ground temperatures of the upper 4-m soil layers firstly decreased from 2004 to 2008 and then increased from 2008 to 2013. The similar variational trend was found for the upper 2-m soil layers below the left shoulder. This phenomenon indicates that changes in the ground temperatures of the upper soil layers may be mainly resulted from climate warming and thermal influence of the road embankment. Small changes were found for the ground temperatures of the soil layers below the depth of 4 m in the middle, indicating that the horizontal cooling radius may be less than 5 m. Thus, it is suggested that inclined TPCTs should be better for cooling the permafrost layers in the middle. It can be seen from Fig. 3b that the vertical cooling range below the left shoulder was 2–9 m from 2008 to 2010 (area (1)), and was 4–12 m from 2010 to 2013 (area (2)). Thus, the maximum cooling depth reached 12 m during the monitoring period. Another interesting phenomena was that the cooling range moved downward. The upper limit moving downward from 2 m to 4 m may be mainly caused by climate warming and the thermal effect of the road embankment. The lower limit moving downward from 9 m to 12 m may be resulted from the long-term cooling effect of the TPCTs neutralizing the upward geothermal flows. Therefore, it is found that the cooling scope of the TPCTs is under the thermal influences from climate warming, the road embankment and upward geothermal flows.
3.2. Cooling period of the TPCTs For the road embankments installed with TPCTs in permafrost regions, it is well known that TPCTs start to work when the temperature at condenser section is a little bit lower than that at evaporator section [21,23]. Because the temperatures at evaporator and condenser sections were not measured directly in this paper, the working period can only be estimated according to the air temperature and ground temperatures around the TPCTs. Therefore, it is assumed that the TPCTs started to work once the air temperature was colder than the ground temperatures. Since the evaporator section ranged from 2 to 8 m below the road surface, the ground temperatures of these soil layers are employed. Fig. 4 presents the air temperature and the ground temperatures at the depth of 2, 4, 6 and 8 m below the left shoulder. Due to the different thermal states at different depths, the cooling period may vary from part to part at the evaporator section. In 2010, the cooling period at the depth of 2 m ranged from January to April and then from August to December, while it ranged from January to April and then from October to December at the depths of 4, 6 and 8 m. Therefore, the cooling period was eight months at 2 m, which was one month longer than that at 4, 6, and 8 m. This indicates that some errors may be
Fig. 4. Air temperature and ground temperatures at the depths of 2, 4, 6 and 8 m below the left shoulder in 2010.
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triggered when only one value of soil temperature was usually employed for determining the cooling period of the TPCTs in previous literatures [21,30].
that the ground temperature was slightly decreased at 4 m from 2010 to 2013. The possible reason is that the ground temperature may also be influenced by the heat transferred from the asphalt pavement.
3.3. Cooling effects of the TPCTs 3.3.1. Cooling effect around the adiabatic and evaporator sections of TPCTs The adiabatic section ranged from 0 to 3 m, while the evaporator section ranged from 3 to 8 m below the road surface. Fig. 5 illustrates the ground temperature at the depths of 0.5, 1.5, 2, 4, 6 and 8 m in the years of 2008, 2010 and 2013. The ground temperature in 2008 is also taken as the initial thermal state to examine the cooling effects. For the soil layers around the adiabatic section (Fig. 5a–c), the ground temperatures were greatly influenced by climate warming and the thermal influence caused by the road embankment. Thus, it is difficult to estimate the cooling effects taken by the TPCTs. As for the soil layers around the evaporator section (Fig. 5d–f), the ground temperatures were mainly influenced by the TPCTs. Thus, the black shadows may indicate the cooling effects caused by the TPCTs at different depths. The larger the area is, the greater the cooling effect was. Though there was a larger temperature difference between the air temperature and the ground temperature at 4 m during the cooling period (Fig. 4), it seems that the strongest cooling effect was found at 6 m (Fig. 5e). From 2008 to 2010, the cooling amplitudes of the ground temperatures at 4, 6 and 8 m were −0.70, −0.74 and −0.26 °C respectively, while they were −0.85, −1.14 and −0.62 °C from 2008 to 2013. Of note is
3.3.2. Cooling effect for the permafrost layers The main purpose of the TPCTs was to prevent the underlying permafrost layers from thawing or warming. Permafrost thawing can be directly reflected by the raising/dropping of the permafrost table, while mean annual ground temperature (MAGT) at the depth of 20 m can be taken as an indicator for permafrost warming. Fig. 6 presents the variations of the depths of permafrost tables and MAGTs below the left and right shoulders and in the middle of the road embankment. At the left shoulder, the permafrost table firstly dropped down from 2003 to 2008 without TPCTs, and then rose up from 2008 to 2013 under the cooling influence of the TPCTs. The increasing rate of the depth of permafrost table was 0.30 m/a, while the decreasing rate was 0.51 m/a. The MAGT kept rising during the monitoring period, but different rising rates were observed. The temperature rising rate was 0.24 °C/10a from 2003 to 2008, while it was 0.06 °C/10a from 2008 to 2013. An obvious cooling effect was found below the left shoulder, since the thermistor cable was so close to the TPCTs. The permafrost table rose up faster in the first two years (2008–2010) than that in the latter years (2010–2013), while MAGT kept increasing in the first two years but then showed a tendency to decrease in the latter years. A possible reason is that the bottom of the evaporator section was just placed at the permafrost table in 2008. Thus, the upper permafrost layers would be firstly cooled with a result of the permafrost table rising fast, but the rising rate
Fig. 5. Ground temperatures at the depths of 0.5 m (a), 1.5 m (b), 2 m (c), 4 m (d), 6 m (e), and 8 m (f) below the left shoulder in 2009, 2010 and 2013.
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Fig. 6. Depths of the permafrost tables and mean annual ground temperatures at the depth of 20 m below the left and right shoulders and in the middle of the road section.
would slow down with the heat transferring from the lower permafrost layers in the later years. In the middle of the road section, a similar cooling effect as that below the left shoulder was observed. Before installing the TPCTs, the increasing rate of the depth of permafrost table was 0.18 m/a, while the rising rate of MAGT was 0.13 oC/10a. After installing the TPCTs, the decreasing rate of the depth of permafrost table was 0.11m/a, while the rising rate of MAGT was 0.02 °C/10a. It is worth mentioning that the permafrost table fell down from 2009 to 2011, and then rose up from 2011 to 2014. This indicates that it would take a few years for the TPCTs to case a cooling effect on the permafrost layers in the middle. As for the permafrost layers below the right shoulder, it is impossible to determine the cooling effect due to the insufficient geothermal data. The depth of the permafrost table in 2013 was about 3.5 m, which was much lower than that below the left shoulder or in the middle of the road section. This was because that the right slope was on the shady side, which would be favorable for cooling the underlying permafrost layers.
As for the different deformation points, the deformation rates were 4.0 and 1.2 cm/a at the left shoulder before and after installing the TPCTs respectively, 4.4 and 0.2 cm/a in the middle, and 5.5 and 0.9 cm/a at the right shoulder. The deformation rate at left shoulder was 6 times faster than that in the middle after installing the TPCTs. It seems that the cooling effect at the left shoulder was better than that in the middle (Fig. 6), but a better remedying effect was observed in the middle with respect to embankment deformation. Three possible reasons are 1) creep of the warm permafrost layer was the main deformation source after installing the TPCTs, since the geotemperatures of the permafrost layers below the left
3.4. Remedying effects of the TPCTs To remedy the embankment with unacceptable deformations or cracks was the purpose of installing the TPCTs. In another words, the embankment stability can be taken as an indicator to examine the cooling performance of the TPCTs. In this section, the cooling performance will be discussed from the perspectives of embankment deformation and crack formation. 3.4.1. Remedying effect with respect to embankment deformation Embankment deformation is closely related with the degradation of permafrost layers underneath the road embankment [42]. Thaw consolidation was associated with permafrost thawing, while the creep of permafrost layers was mainly influenced by permafrost warming [6,9]. Therefore, the installation of the TPCTs was supposed to ease the embankment deformation. The accumulated deformations before and after installing the TPCTs at the right and left shoulders and in the middle are presented in Fig. 7. The overall characteristic is that the deformation rates with TPCTs were obviously slower than that without TPCTs. The average deformation rate before installing the TPCTs was about 6 times larger than that after installing the TPCTs. From this point, the TPCTs may be a reliable way to alleviate embankment deformation.
Fig. 7. Embankment deformations at the left/right shoulders and in the middle of the road section before and after installing the TPCTs.
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Fig. 8. Two photos showing the cracks on the asphalt pavement of the road embankment.
shoulder were higher than that in the middle; 2) consolidation of the thawed permafrost layers in the middle can be negligible, since the ice content was very low; 3) permafrost conditions were different below the left shoulder and in the middle, leading to different deformation rates. 3.4.2. Remedying effect with respect to crack formation Fig. 8 shows the cracks on the road surface on July 2013 (a) and April 2015 (b). The road surface was repaired by filling with asphalts before April of 2015. It can be seen that 1) the cracks were mainly longitudinal; 2) the cracks formed earlier at the left shoulder than the right shoulder; 3) the cracks at the left shoulder were presented more severe than that at the right shoulder. If taking the embankment deformation in the middle as a reference, deformation differences between the middle and the left/right shoulders after installing the TPCTs are shown in Fig. 9. The annual deformation difference was about 1 cm, which would be insufficient to induce the cracks. Therefore, the longitudinal cracks may not be caused by the deformation differences. From Fig. 8a, it can be seen that a longitudinal crack between the curb and the asphalt pavement appeared firstly with the maximum width of about 30 cm. Thus, the cracks may be resulted from the different levels of permafrost degradation below the asphalt pavement and the left slope (or the neighboring natural ground surface). An intense thermal effect at the left slope (sunny slope) may be the cause of more and severe cracks appearing at the left shoulder than the right shoulder. 4. Conclusions and suggestions In order to examine the cooling performance of TPCTs, ground temperatures, embankment deformations and some metrological
Fig. 9. Deformation differences between the middle and left/right shoulders after installing the TPCTs.
factors have been monitored for a road section of the Qinghai– Tibet Highway before and after installing the TPCTs since 2003. Frequently-concerned topics are analyzed including cooling scope, cooling period, cooling effects and remedying effects with respect to both embankment deformation and crack formation. The major conclusions and some suggestions can be summarized as follows: 1. The cooling depth of the TPCTs reached the maximum depth of 12 m during the monitoring period, which may be reliable to prevent the lower permafrost layers from warming. However, small changes were observed for the ground temperatures of permafrost layers in the middle of the road section, indicating that inclined TPCTs should be suggested. The cooling scope of the TPCTs is mainly influenced by climate warming, thermal effect of road embankments, and upward geothermal flows. These three main influencing factors should be taken into consideration when designing the layout of TPCTs in the construction of roadways. 2. The cooling period was eight months at 2 m, which was one month longer than that at 4, 6, and 8 m. In previous literatures, only one value of soil temperature was usually employed for determining the cooling period of the TPCTs, which may induce some errors. Thus, the variation of soil temperatures along the evaporator section should be taken into consideration for improving the calculation accuracy. 3. The best cooling effect was found around the middle of the evaporator section. This should be considered when designing the embedding depth of the TPCTs. Different cooling effects were found for the permafrost layers below the left/right shoulders and in the middle of the road section. The permafrost tables below the two shoulders were cooled fast with the permafrost tables raising up and the MAGTs decreasing. However, it would be taken a long period for the permafrost layers in the middle to cool down. The large differences among the depths of the permafrost tables below the left/right shoulders and in the middle may be unfavorable for the stability of the road embankment. Thus, different embedding depths, inclining angles and longitudinal intervals of the TPCTs are recommended for the sunny and shady shoulders. 4. The embankment deformation slowed down very obviously after installing the TPCTs. However, creep of the warm permafrost layers may be a potential deformation source under the cooling effect of the TPCTs. The deformation differences at different places of the road embankment was not the cause of the longitudinal cracks. Instead, different thermal states of the permafrost layers were presented below the asphalt pavement, the embankment slopes and the neighboring natural ground surfaces. This may be the reason for the formation of the cracks. Thus, preventing or controlling the longitudinal cracks would be an important topic in both engineering construction and scientific research in the
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future. Under this circumstance, combining other cooling methods with TPCTs may be a solution for solving this problem. There are limitations for any approach; some improvements and additional work should be done in the future. The cooling permafrost of TPCTs needs to be tested using more road sections with different geological conditions. Also, some improvements should be made for the field experiment, such as direct measurement of pipewall temperatures of TPCTs, and increase in the thermistor cables for measuring ground temperatures. At present, the major challenge is to establish a fully-coupled air-TPCT-soil model, which is just our ongoing research. For this purpose, laboratory model tests are planned to be conducted to obtain some key input parameters, such as the heat-convection coefficient between air and the radiating fins of TPCTs. These three kinds of research would benefit to an optimal design of TPCTs for the engineering constructions in permafrost regions. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 41572268), the National Key Basic Research Program of China (973 Program No. 2012CB026102), the National Natural Science Foundation of China (Grant Nos. 41230630 and 41471063), the 100-Talent Program of the Chinese Academy of Sciences (Granted to Dr. Mingyi Zhang), the National Natural Science Foundation of China (Grant No. 41501084), and the Foundation for Excellent Youth Scholars of CAREERI, CAS (Grant No. Y451091001). The authors greatly appreciate the two anonymous reviewers for their constructive comments and suggestions, and we would also like to thank Dr. Wansheng Pei for providing some field photos. References [1] F.E. Nelson, O.A. Anisimov, N.I. Shiklomanov, Subsidence risk from thawing permafrost, Nature 410 (6831) (2001) 889–890. [2] Z.W. Wu, Y.Z. Liu, Frozen Subsoil and Engineering, Ocean Press, Beijing, 2005. [3] G.D. Cheng, W. Ma, Frozen soil engineering problems in construction of the Qinghai-Tibet Railway, Chin. J. Nat. 28 (6) (2006) 315–320 (in Chinese). [4] Q.B. Wu, Y.Z. Liu, J.M. Zhang, C.J. Tong, A review of recent frozen soil engineering in permafrost regions along Qinghai-Tibet Highway, China, Permaf. Perig. 13 (3) (2002) 199–205. [5] Q.B. Wu, X.F. Dong, Y.Z. Liu, H.J. Jin, Responses of permafrost on the Qinghai-Tibet Plateau, China, to climate change and engineering construction, Arct. Antarct. Alp. Res. 39 (4) (2007) 682–687. [6] J.L. Qi, S. Yu, J.M. Zhang, Z. Wen, Settlement of embankments in permafrost regions in the Qinghai-Tibet Plateau, Norw. J. Geogr. 61 (2) (2007) 49–55. [7] B. Zheng, J.M. Zhang, Y.H. Qin, Investigation for the deformation of embankment underlain by warm and ice-rich permafrost, Cold Reg. Sci. Technol. 60 (2) (2010) 161–168. [8] S.J. Wang, M. Huo, W.J. Zhou, Subgrade failure of Qinghai-Tibet Highway in permafrost area, Highway 5 (2004) 22–26 (in Chinese). [9] F. Yu, J.L. Qi, X.L. Yao, Y.Z. Liu, In-situ monitoring of settlement at different layers under embankments in permafrost regions on the Qinghai-Tibet Plateau, Eng. Geol. 160 (2013) 44–53. [10] G.D. Cheng, Research on engineering geology of the roadbed in permafrost regions of Qinghai-Xizang Plateau, Qual. Sci. 23 (2) (2003) 134–141 (in Chinese). [11] C.E. Heuer, The application of heat pipes on the Trans-Alaska pipeline, United States Army Crops of Engineers, Cold Regions Research and Engineering Laboratory Special Report 79-26, 1979, 1–27. [12] J. McKenna, K. Biggar, The rehabilitation of a passive-ventilated slab on grade foundation using horizontal thermosyphons, Can. Geotech. J. 35 (4) (1998) 684–691. [13] Y. Song, L. Jin, J.Z. Zhang, In-situ study on cooling characteristics of two-phase closed thermosyphon embankment of Qinghai–Tibet Highway in permafrost regions, Cold Reg. Sci. Technol. 93 (2013) 12–19. [14] D. Wu, L. Jin, J.B. Peng, Y.H. Dong, Z.Y. Liu, The thermal budget evaluation of the two-phase closed thermosyphon embankment of the Qinghai–Tibet Highway in permafrost regions, Cold Reg. Sci. Technol. 103 (2014) 115–122. [15] W.D. Pan, S.C. Zhao, W.Z. Xu, S.S. Yu, W.D. Ma, Application of probe to enhance thermal stability of roadbed in Plateau permafrost areas, J. Glaciol. Geocryol. 25 (4) (2003) 433–438 (in Chinese).
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Applied Thermal Engineering j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / a p t h e r m e n g
Research Paper
Cooling performance of two-phase closed thermosyphons installed at a highway embankment in permafrost regions Fan Yu a, Jilin Qi b,*, Mingyi Zhang a, Yuanming Lai a,c, Xiaoliang Yao a, Yongzhi Liu a,d, Guilong Wu a,d a State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China b Beijing High Institution Research Center for Engineering Structures and New Materials, Beijing University of Civil Engineering and Architecture, Beijing 100044, China c School of Civil Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China d Beiluhe Observation Station of Frozen Soil Environment and Engineering, Cold and Arid Region Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
H I G H L I G H T S
• • • •
Long-term monitored data are presented for a road section of the Qinghai-Tibet Highway before and after installing the TPCTs. Cooling scope and period of the TPCTs are analyzed; cooling effects for the soil layers are examined. Cooling performance of the TPCTs is discussed from the perspectives of embankment deformation and crack formation. Some suggestions are proposed for a better design of the TPCTs in highway constructions in permafrost regions.
A R T I C L E
I N F O
Article history: Received 10 August 2015 Accepted 25 November 2015 Available online 24 December 2015 Keywords: Two-phase closed thermosyphon Ground temperature Permafrost degradation Embankment deformation Crack formation Thaw consolidation
A B S T R A C T
Two-phase closed thermosyphon (TPCT) is a popular way to prevent permafrost layers from degrading, and consequently ensure the stabilities of engineering constructions in permafrost regions. Although TPCTs have been numerically and experimentally investigated for many years, long-term field monitored data concerning the cooling performance of TPCTs are limited. This paper presents the ground temperatures, embankment deformations and some related meteorological factors for a road section of the Qinghai– Tibet Highway before and after installing the TPCTs in permafrost regions. Based on the monitored data, three main aspects are analyzed: 1) cooling scope and period of the TPCTs; 2) cooling effects for the soil layers, especially for the permafrost layers; and 3) remedying effects with respect to embankment deformation and crack formation. Some corresponding suggestions are proposed for a better design of TPCTs in the construction of roadways in permafrost regions. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction In permafrost regions, engineering constructions have been suffering different levels of deformations caused by permafrost degradation [1,2]. Taking the Qinghai–Tibet Highway as an example, 31.7% of the road sections from Golmud to Lhasa (520 km) in the permafrost regions faced roadbed diseases in 1999 [3], among which 85% of them were caused by thaw settlement [4]. Under global warming and the thermal effect caused by road embankments, the underlying permafrost layers are degrading [5]. Thaw consolidation has been considered to be the main cause of embankment
* Corresponding author. Tel.: +8613811703629; fax: +86 931 4967292. E-mail address: [email protected] (J. Qi). http://dx.doi.org/10.1016/j.applthermaleng.2015.11.102 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
deformation for a long time [2]. In recent years, creep of warm permafrost layers has also been proved to be another main cause [6,7]. It is found that the two deformation causes are exactly the main reason why the Qinghai–Tibet Highway was reconstructed and repaired for many times [8,9]. In addition, 79% of the permafrost regions on the Qinghai–Tibet Plateau were characterized by “warm permafrost”, where the mean annual ground temperatures (MAGTs) were higher than −1.5 oC [10]. The easy-to-thaw characteristic determines that some cooling methods should be taken to protect the permafrost layers from degrading, and thus to ensure the stabilities of the engineering constructions. Two-phase closed thermosyphon (TPCT) is one of those cooling methods, which has already been used in permafrost regions for many decades. For instances, the TPCTs were successfully used in the Trans-Alaska Pipeline System between 1974 and 1977 [11]. The TPCTs were also proved by in-situ geothermal observation between
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1990 and 1997 to be a reliable technology for rehabilitating a housing with unacceptable settlement on the Ellesmerer Island, Canada [12]. In China, the TPCTs have been frequently employed to insure the stabilities of the Qinghai–Tibet Highway [13,14], the Qinghai– Tibet Railway [15,16], and the Chaidaer–Muli Railway [17]. In addition, other cooling methods were combined with the TPCTs for better cooling effects, such as crushed rock revetment [18,19], forcedair ventilation [20] and insulating material [21]. In these engineering constructions, the evaporator sections were embedded in the soil layers, while the condenser sections were exposed in the air. The TPCTs only work when the temperature difference exceeds the threshold, i.e. the start-up temperature difference. Zhang et al. [22,23] found that it was about −0.2 °C by means of indoor physical model test and a self-designed experimental apparatus. In order to enlarge the cooling range underneath the embankments, inclined TPCTs have been numerically and experimentally investigated [23,24]. Besides the inclining angle, there are three other influencing factors of the cooling performance of the TPCTs, including aspect ratio (the ratio between evaporator length and internal diameter), filling ratio (the ratio between the volumes of working fluid and evaporator section) and working fluid [23]. The influencing factors have been extensively investigated by previous researchers [23,25–28]. With respect to the previous numerical modeling work, for a simplified calculation, the thermal resistance of the TPCTs were usually ignored and the heat-convention coefficient between air and the radiating fins was taken as a constant when the TPCTs were working [21,29–31]. On the other hand, the heat transfer coefficients for every part of the TPCTs were introduced by Pan and Wu [32], based on which a coupled air-TPCT-soil model was put forward [33]. For ease of calculation, however, some empirical formulas were adopted for calculating the heat resistances in the coupled model, which may lead to some unavoidable errors. Under this circumstance, a fullycoupled model is urgently needed. The governing equations in the three zones have already researched respectively. The three governing equations in the air zone are continuity, momentum and energy equations [34,35]. Also, the three equations are usually used for simulating two-phase flows in TPCTs [36–38]. In the soil layers,
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the heat transfer equation with phase-change considered is usually adopted [39–41]. From the above analyses, it can be found that the TPCTs have been considered to be a popular and efficient way to ensure the stabilities of the engineering constructions in permafrost regions, especially in warm permafrost regions. There are many influencing factors for the cooling performance of the TPCTs, which needs further investigation. A fully-coupled air-TPCT-soil model is urgently needed for a better prediction. To date, long-term field monitored data about the cooling performance of the TPCTs are currently lacking. This would be favorable for understanding the cooling performance of the TPCTs, and for establishing the fully-coupled model for the stabilities of the engineering constructions in permafrost regions. In this paper, more than 10-year monitored data including ground temperatures, embankment deformations and some related meteorological factors are presented for a road section installed with TPCTs in permafrost regions. The main objective is to examine the cooling performance of the TPCTs from the perspectives of thermal and mechanical (deformation) stabilities.
2. Monitoring program The monitoring program was carried out at a road section of the Qinghai–Tibet Highway. The highway mileage is K3187 + 000, while the place is named as Kaixinling. The longitude and latitude are N33°57′29′′ and E92°20′58′′ respectively, and the geographic position can be seen in a previous paper [42]. An illustration of the instrumented road section is shown in Fig. 1. Ground temperatures and embankment deformations were started to monitor in 2003. Due to an unacceptable embankment deformation, TPCTs were installed around September 2009. The working fluid is anhydrous ammonia, while the container is made by carbon steel. As shown in Fig. 1, the lengths of the three parts of the TPCTs, condenser, adiabatic and evaporator, are 4, 3 and 5 m respectively. Some other parameters of the TPCTs are summarized in Table 1. A meteorological station was established in Aug. 2008. Detailed information about these three monitoring items is introduced in the following.
Fig. 1. Schematic cross-section (a) and plan (b) of the instrumented road section installed with TPCTs.
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Table 1 Some parameters of the TPCTs. Parameters
Values (mm)
Inner diameter of pipe Outer diameter of pipe Fin height Fin space Fin thickness
79 89 25 10 1
2.1. Ground temperatures Two thermistor cables were installed at the left shoulder and in the middle of the road section on September 2003, and another one was added at the right shoulder on April 2013. As shown in Fig. 1, the three cables reached the depth of 20 m below the asphalt pavement. The thermistors have an accuracy of ±0.05 °C, which were developed by State Key Laboratory of Frozen Soil Engineering, Chinese Academy of Sciences. The data were collected by a CR3000 datalogger (Campbell Scientific Inc., USA) twice a month. The depths of permafrost tables are obtained by interpolating calculation based on the monitored ground temperatures. 0 °C is taken as the thawing temperature for ease of discussion, since it varies with soil type, load, water and salt contents, etc. [43]. 2.2. Embankment deformations Twenty steel nails and a 15-m-deep steel pipe were embedded into the asphalt pavement on October 2003, as shown in Fig. 1b. It is assumed that the deformation of the soil layers deeper than 15 m can be negligible, and thus the steel pipe can be taken as a bench-
mark. The elevation differences between the steel nails and the benchmark are the embankment deformations. An electronic total station was used to monitor the deformations once or twice a month. It was suspended from August 2008 to August 2009 due to the replacement of asphalt pavements and the installation of the TPCTs. 2.3. Metrological factors A meteorological station was installed nearby the road section on August 2008. Air temperatures and wind speeds at the heights of 2 m and 10 m were monitored using HMP45C-L11 and 05103L11 sensors (Vaisala, Finland). In addition, air humidity and radiation and precipitation were also continuously measured. Since air temperatures and wind speeds are the top two influence factor for the heat transfer coefficient of the TPCTs [44], only they are adopted in this paper. 3. Results and discussions The monitored data including ground temperatures, embankment deformations, air temperature and wind speeds at the height of 2 m are shown in Fig. 2. Fig. 2a shows the monthly average values of air temperatures and wind speeds at the height of 2 m. The accumulated values of embankment deformations (Fig. 2b) correspond with the ground temperatures below left/right shoulders and in the middle of the road section (Fig. 2c–e). The Kriging interpolation method is adopted to obtain the continuous ground temperatures based on the monitored data at different depths. The embankment deformations at left shoulder refer to the average values of the monitoring points 10# and 15#, that in the middle refer to the average values of the monitoring points 8# and 13#, while that at
Fig. 2. Monitored data including air temperatures and wind speeds (a), embankment deformations (b) and ground temperatures below the left shoulder (c), in the middle (d) and below the right shoulder (e).
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Fig. 3. Average ground temperatures in the middle (a) and below the left shoulder (b) at the road section of the Qinghai–Tibet Highway in 2004, 2009, 2010 and 2013.
the right shoulder refer to the average values of the monitoring points 6# and 11# (Fig. 1b). It is worth mentioning that right/left shoulders are determined when facing the direction to Lhasa (Fig. 1b).
3.1. Cooling scope of the TPCTs The cooling scope indicates the soil layers cooled by the TPCTs horizontally and vertically, which is closely related with the embankment deformation. Since the ground temperatures were missing during the period of installing the TPCTs in July and August of 2009, the ground temperatures in 2008 is taken as the initial state to examine the cooling scope. Because of lacking sufficient thermistor cables from the left shoulder to the middle of the road section (Fig. 1), it is only possible to get some information about the horizontal cooling radius from the thermistor cable in the middle. Fig. 3 presents the average ground temperatures in the middle and below the left shoulder in 2004, 2009, 2010 and 2013. In the middle of the road section, the ground temperatures of the upper 4-m soil layers firstly decreased from 2004 to 2008 and then increased from 2008 to 2013. The similar variational trend was found for the upper 2-m soil layers below the left shoulder. This phenomenon indicates that changes in the ground temperatures of the upper soil layers may be mainly resulted from climate warming and thermal influence of the road embankment. Small changes were found for the ground temperatures of the soil layers below the depth of 4 m in the middle, indicating that the horizontal cooling radius may be less than 5 m. Thus, it is suggested that inclined TPCTs should be better for cooling the permafrost layers in the middle. It can be seen from Fig. 3b that the vertical cooling range below the left shoulder was 2–9 m from 2008 to 2010 (area (1)), and was 4–12 m from 2010 to 2013 (area (2)). Thus, the maximum cooling depth reached 12 m during the monitoring period. Another interesting phenomena was that the cooling range moved downward. The upper limit moving downward from 2 m to 4 m may be mainly caused by climate warming and the thermal effect of the road embankment. The lower limit moving downward from 9 m to 12 m may be resulted from the long-term cooling effect of the TPCTs neutralizing the upward geothermal flows. Therefore, it is found that the cooling scope of the TPCTs is under the thermal influences from climate warming, the road embankment and upward geothermal flows.
3.2. Cooling period of the TPCTs For the road embankments installed with TPCTs in permafrost regions, it is well known that TPCTs start to work when the temperature at condenser section is a little bit lower than that at evaporator section [21,23]. Because the temperatures at evaporator and condenser sections were not measured directly in this paper, the working period can only be estimated according to the air temperature and ground temperatures around the TPCTs. Therefore, it is assumed that the TPCTs started to work once the air temperature was colder than the ground temperatures. Since the evaporator section ranged from 2 to 8 m below the road surface, the ground temperatures of these soil layers are employed. Fig. 4 presents the air temperature and the ground temperatures at the depth of 2, 4, 6 and 8 m below the left shoulder. Due to the different thermal states at different depths, the cooling period may vary from part to part at the evaporator section. In 2010, the cooling period at the depth of 2 m ranged from January to April and then from August to December, while it ranged from January to April and then from October to December at the depths of 4, 6 and 8 m. Therefore, the cooling period was eight months at 2 m, which was one month longer than that at 4, 6, and 8 m. This indicates that some errors may be
Fig. 4. Air temperature and ground temperatures at the depths of 2, 4, 6 and 8 m below the left shoulder in 2010.
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triggered when only one value of soil temperature was usually employed for determining the cooling period of the TPCTs in previous literatures [21,30].
that the ground temperature was slightly decreased at 4 m from 2010 to 2013. The possible reason is that the ground temperature may also be influenced by the heat transferred from the asphalt pavement.
3.3. Cooling effects of the TPCTs 3.3.1. Cooling effect around the adiabatic and evaporator sections of TPCTs The adiabatic section ranged from 0 to 3 m, while the evaporator section ranged from 3 to 8 m below the road surface. Fig. 5 illustrates the ground temperature at the depths of 0.5, 1.5, 2, 4, 6 and 8 m in the years of 2008, 2010 and 2013. The ground temperature in 2008 is also taken as the initial thermal state to examine the cooling effects. For the soil layers around the adiabatic section (Fig. 5a–c), the ground temperatures were greatly influenced by climate warming and the thermal influence caused by the road embankment. Thus, it is difficult to estimate the cooling effects taken by the TPCTs. As for the soil layers around the evaporator section (Fig. 5d–f), the ground temperatures were mainly influenced by the TPCTs. Thus, the black shadows may indicate the cooling effects caused by the TPCTs at different depths. The larger the area is, the greater the cooling effect was. Though there was a larger temperature difference between the air temperature and the ground temperature at 4 m during the cooling period (Fig. 4), it seems that the strongest cooling effect was found at 6 m (Fig. 5e). From 2008 to 2010, the cooling amplitudes of the ground temperatures at 4, 6 and 8 m were −0.70, −0.74 and −0.26 °C respectively, while they were −0.85, −1.14 and −0.62 °C from 2008 to 2013. Of note is
3.3.2. Cooling effect for the permafrost layers The main purpose of the TPCTs was to prevent the underlying permafrost layers from thawing or warming. Permafrost thawing can be directly reflected by the raising/dropping of the permafrost table, while mean annual ground temperature (MAGT) at the depth of 20 m can be taken as an indicator for permafrost warming. Fig. 6 presents the variations of the depths of permafrost tables and MAGTs below the left and right shoulders and in the middle of the road embankment. At the left shoulder, the permafrost table firstly dropped down from 2003 to 2008 without TPCTs, and then rose up from 2008 to 2013 under the cooling influence of the TPCTs. The increasing rate of the depth of permafrost table was 0.30 m/a, while the decreasing rate was 0.51 m/a. The MAGT kept rising during the monitoring period, but different rising rates were observed. The temperature rising rate was 0.24 °C/10a from 2003 to 2008, while it was 0.06 °C/10a from 2008 to 2013. An obvious cooling effect was found below the left shoulder, since the thermistor cable was so close to the TPCTs. The permafrost table rose up faster in the first two years (2008–2010) than that in the latter years (2010–2013), while MAGT kept increasing in the first two years but then showed a tendency to decrease in the latter years. A possible reason is that the bottom of the evaporator section was just placed at the permafrost table in 2008. Thus, the upper permafrost layers would be firstly cooled with a result of the permafrost table rising fast, but the rising rate
Fig. 5. Ground temperatures at the depths of 0.5 m (a), 1.5 m (b), 2 m (c), 4 m (d), 6 m (e), and 8 m (f) below the left shoulder in 2009, 2010 and 2013.
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Fig. 6. Depths of the permafrost tables and mean annual ground temperatures at the depth of 20 m below the left and right shoulders and in the middle of the road section.
would slow down with the heat transferring from the lower permafrost layers in the later years. In the middle of the road section, a similar cooling effect as that below the left shoulder was observed. Before installing the TPCTs, the increasing rate of the depth of permafrost table was 0.18 m/a, while the rising rate of MAGT was 0.13 oC/10a. After installing the TPCTs, the decreasing rate of the depth of permafrost table was 0.11m/a, while the rising rate of MAGT was 0.02 °C/10a. It is worth mentioning that the permafrost table fell down from 2009 to 2011, and then rose up from 2011 to 2014. This indicates that it would take a few years for the TPCTs to case a cooling effect on the permafrost layers in the middle. As for the permafrost layers below the right shoulder, it is impossible to determine the cooling effect due to the insufficient geothermal data. The depth of the permafrost table in 2013 was about 3.5 m, which was much lower than that below the left shoulder or in the middle of the road section. This was because that the right slope was on the shady side, which would be favorable for cooling the underlying permafrost layers.
As for the different deformation points, the deformation rates were 4.0 and 1.2 cm/a at the left shoulder before and after installing the TPCTs respectively, 4.4 and 0.2 cm/a in the middle, and 5.5 and 0.9 cm/a at the right shoulder. The deformation rate at left shoulder was 6 times faster than that in the middle after installing the TPCTs. It seems that the cooling effect at the left shoulder was better than that in the middle (Fig. 6), but a better remedying effect was observed in the middle with respect to embankment deformation. Three possible reasons are 1) creep of the warm permafrost layer was the main deformation source after installing the TPCTs, since the geotemperatures of the permafrost layers below the left
3.4. Remedying effects of the TPCTs To remedy the embankment with unacceptable deformations or cracks was the purpose of installing the TPCTs. In another words, the embankment stability can be taken as an indicator to examine the cooling performance of the TPCTs. In this section, the cooling performance will be discussed from the perspectives of embankment deformation and crack formation. 3.4.1. Remedying effect with respect to embankment deformation Embankment deformation is closely related with the degradation of permafrost layers underneath the road embankment [42]. Thaw consolidation was associated with permafrost thawing, while the creep of permafrost layers was mainly influenced by permafrost warming [6,9]. Therefore, the installation of the TPCTs was supposed to ease the embankment deformation. The accumulated deformations before and after installing the TPCTs at the right and left shoulders and in the middle are presented in Fig. 7. The overall characteristic is that the deformation rates with TPCTs were obviously slower than that without TPCTs. The average deformation rate before installing the TPCTs was about 6 times larger than that after installing the TPCTs. From this point, the TPCTs may be a reliable way to alleviate embankment deformation.
Fig. 7. Embankment deformations at the left/right shoulders and in the middle of the road section before and after installing the TPCTs.
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Fig. 8. Two photos showing the cracks on the asphalt pavement of the road embankment.
shoulder were higher than that in the middle; 2) consolidation of the thawed permafrost layers in the middle can be negligible, since the ice content was very low; 3) permafrost conditions were different below the left shoulder and in the middle, leading to different deformation rates. 3.4.2. Remedying effect with respect to crack formation Fig. 8 shows the cracks on the road surface on July 2013 (a) and April 2015 (b). The road surface was repaired by filling with asphalts before April of 2015. It can be seen that 1) the cracks were mainly longitudinal; 2) the cracks formed earlier at the left shoulder than the right shoulder; 3) the cracks at the left shoulder were presented more severe than that at the right shoulder. If taking the embankment deformation in the middle as a reference, deformation differences between the middle and the left/right shoulders after installing the TPCTs are shown in Fig. 9. The annual deformation difference was about 1 cm, which would be insufficient to induce the cracks. Therefore, the longitudinal cracks may not be caused by the deformation differences. From Fig. 8a, it can be seen that a longitudinal crack between the curb and the asphalt pavement appeared firstly with the maximum width of about 30 cm. Thus, the cracks may be resulted from the different levels of permafrost degradation below the asphalt pavement and the left slope (or the neighboring natural ground surface). An intense thermal effect at the left slope (sunny slope) may be the cause of more and severe cracks appearing at the left shoulder than the right shoulder. 4. Conclusions and suggestions In order to examine the cooling performance of TPCTs, ground temperatures, embankment deformations and some metrological
Fig. 9. Deformation differences between the middle and left/right shoulders after installing the TPCTs.
factors have been monitored for a road section of the Qinghai– Tibet Highway before and after installing the TPCTs since 2003. Frequently-concerned topics are analyzed including cooling scope, cooling period, cooling effects and remedying effects with respect to both embankment deformation and crack formation. The major conclusions and some suggestions can be summarized as follows: 1. The cooling depth of the TPCTs reached the maximum depth of 12 m during the monitoring period, which may be reliable to prevent the lower permafrost layers from warming. However, small changes were observed for the ground temperatures of permafrost layers in the middle of the road section, indicating that inclined TPCTs should be suggested. The cooling scope of the TPCTs is mainly influenced by climate warming, thermal effect of road embankments, and upward geothermal flows. These three main influencing factors should be taken into consideration when designing the layout of TPCTs in the construction of roadways. 2. The cooling period was eight months at 2 m, which was one month longer than that at 4, 6, and 8 m. In previous literatures, only one value of soil temperature was usually employed for determining the cooling period of the TPCTs, which may induce some errors. Thus, the variation of soil temperatures along the evaporator section should be taken into consideration for improving the calculation accuracy. 3. The best cooling effect was found around the middle of the evaporator section. This should be considered when designing the embedding depth of the TPCTs. Different cooling effects were found for the permafrost layers below the left/right shoulders and in the middle of the road section. The permafrost tables below the two shoulders were cooled fast with the permafrost tables raising up and the MAGTs decreasing. However, it would be taken a long period for the permafrost layers in the middle to cool down. The large differences among the depths of the permafrost tables below the left/right shoulders and in the middle may be unfavorable for the stability of the road embankment. Thus, different embedding depths, inclining angles and longitudinal intervals of the TPCTs are recommended for the sunny and shady shoulders. 4. The embankment deformation slowed down very obviously after installing the TPCTs. However, creep of the warm permafrost layers may be a potential deformation source under the cooling effect of the TPCTs. The deformation differences at different places of the road embankment was not the cause of the longitudinal cracks. Instead, different thermal states of the permafrost layers were presented below the asphalt pavement, the embankment slopes and the neighboring natural ground surfaces. This may be the reason for the formation of the cracks. Thus, preventing or controlling the longitudinal cracks would be an important topic in both engineering construction and scientific research in the
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future. Under this circumstance, combining other cooling methods with TPCTs may be a solution for solving this problem. There are limitations for any approach; some improvements and additional work should be done in the future. The cooling permafrost of TPCTs needs to be tested using more road sections with different geological conditions. Also, some improvements should be made for the field experiment, such as direct measurement of pipewall temperatures of TPCTs, and increase in the thermistor cables for measuring ground temperatures. At present, the major challenge is to establish a fully-coupled air-TPCT-soil model, which is just our ongoing research. For this purpose, laboratory model tests are planned to be conducted to obtain some key input parameters, such as the heat-convection coefficient between air and the radiating fins of TPCTs. These three kinds of research would benefit to an optimal design of TPCTs for the engineering constructions in permafrost regions. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No. 41572268), the National Key Basic Research Program of China (973 Program No. 2012CB026102), the National Natural Science Foundation of China (Grant Nos. 41230630 and 41471063), the 100-Talent Program of the Chinese Academy of Sciences (Granted to Dr. Mingyi Zhang), the National Natural Science Foundation of China (Grant No. 41501084), and the Foundation for Excellent Youth Scholars of CAREERI, CAS (Grant No. Y451091001). The authors greatly appreciate the two anonymous reviewers for their constructive comments and suggestions, and we would also like to thank Dr. Wansheng Pei for providing some field photos. References [1] F.E. Nelson, O.A. Anisimov, N.I. Shiklomanov, Subsidence risk from thawing permafrost, Nature 410 (6831) (2001) 889–890. [2] Z.W. Wu, Y.Z. Liu, Frozen Subsoil and Engineering, Ocean Press, Beijing, 2005. [3] G.D. Cheng, W. Ma, Frozen soil engineering problems in construction of the Qinghai-Tibet Railway, Chin. J. Nat. 28 (6) (2006) 315–320 (in Chinese). [4] Q.B. Wu, Y.Z. Liu, J.M. Zhang, C.J. Tong, A review of recent frozen soil engineering in permafrost regions along Qinghai-Tibet Highway, China, Permaf. Perig. 13 (3) (2002) 199–205. [5] Q.B. Wu, X.F. Dong, Y.Z. Liu, H.J. Jin, Responses of permafrost on the Qinghai-Tibet Plateau, China, to climate change and engineering construction, Arct. Antarct. Alp. Res. 39 (4) (2007) 682–687. [6] J.L. Qi, S. Yu, J.M. Zhang, Z. Wen, Settlement of embankments in permafrost regions in the Qinghai-Tibet Plateau, Norw. J. Geogr. 61 (2) (2007) 49–55. [7] B. Zheng, J.M. Zhang, Y.H. Qin, Investigation for the deformation of embankment underlain by warm and ice-rich permafrost, Cold Reg. Sci. Technol. 60 (2) (2010) 161–168. [8] S.J. Wang, M. Huo, W.J. Zhou, Subgrade failure of Qinghai-Tibet Highway in permafrost area, Highway 5 (2004) 22–26 (in Chinese). [9] F. Yu, J.L. Qi, X.L. Yao, Y.Z. Liu, In-situ monitoring of settlement at different layers under embankments in permafrost regions on the Qinghai-Tibet Plateau, Eng. Geol. 160 (2013) 44–53. [10] G.D. Cheng, Research on engineering geology of the roadbed in permafrost regions of Qinghai-Xizang Plateau, Qual. Sci. 23 (2) (2003) 134–141 (in Chinese). [11] C.E. Heuer, The application of heat pipes on the Trans-Alaska pipeline, United States Army Crops of Engineers, Cold Regions Research and Engineering Laboratory Special Report 79-26, 1979, 1–27. [12] J. McKenna, K. Biggar, The rehabilitation of a passive-ventilated slab on grade foundation using horizontal thermosyphons, Can. Geotech. J. 35 (4) (1998) 684–691. [13] Y. Song, L. Jin, J.Z. Zhang, In-situ study on cooling characteristics of two-phase closed thermosyphon embankment of Qinghai–Tibet Highway in permafrost regions, Cold Reg. Sci. Technol. 93 (2013) 12–19. [14] D. Wu, L. Jin, J.B. Peng, Y.H. Dong, Z.Y. Liu, The thermal budget evaluation of the two-phase closed thermosyphon embankment of the Qinghai–Tibet Highway in permafrost regions, Cold Reg. Sci. Technol. 103 (2014) 115–122. [15] W.D. Pan, S.C. Zhao, W.Z. Xu, S.S. Yu, W.D. Ma, Application of probe to enhance thermal stability of roadbed in Plateau permafrost areas, J. Glaciol. Geocryol. 25 (4) (2003) 433–438 (in Chinese).
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