Monitoring and analysis of ground temperature and deformation within Qinghai-Tibet Highway subgrade in permafrost region

YaHu Tian， YuPeng Shen， WenBing Yu， JianHong Fang

1. School of Civil Engineering， Beijing Jiaotong University， Beijing 100044， China

2. Qinghai Research Institute of Transportation， Xining， Qinghai 810008， China

3. State Key Laboratory of Frozen Soil Engineering， Cold and Arid Regions Environmental and Engineering Research Institute， Chinese Academy of Sciences， Lanzhou， Gansu 730000， China

Monitoring and analysis of ground temperature and deformation within Qinghai-Tibet Highway subgrade in permafrost region

YaHu Tian1，2*， YuPeng Shen1， WenBing Yu3， JianHong Fang2

1. School of Civil Engineering， Beijing Jiaotong University， Beijing 100044， China

2. Qinghai Research Institute of Transportation， Xining， Qinghai 810008， China

3. State Key Laboratory of Frozen Soil Engineering， Cold and Arid Regions Environmental and Engineering Research Institute， Chinese Academy of Sciences， Lanzhou， Gansu 730000， China

In order to study the stability of the Qinghai-Tibet Highway embankment at Chumaerhe in the permafrost region of northwest China， the ground temperature and deformation at different depths were monitored under the left and right shoulders of the embankment where thermosyphons were set up only on the left shoulder. Based on the monitored data，characteristics of ground temperature and deformation of the left and right shoulders are analyzed and discussed. The results show that the start time of freezing or thawing of the seasonal active layer was about one to two months later than that of the embankment body itself. The stability of each shoulder was mainly controlled by the settlement of different soil layers， whereas frost heave of soil had scarcely any effect on the stability of the embankment. For the left shoulder， the settlement was mainly influenced by the seasonal active layer and then by the embankment body itself， due to freeze-thaw cycles which may change the soil properties； however， the permafrost layer remained fairly stable. For the right shoulder，creep of the warm permafrost layer was the main influence factor on its stability， followed by settlement of embankment body itself， and finally settlement of the seasonal active layer. Compared with the deformation of the left shoulder， the permafrost layer under the right shoulder was less stable， which indicates that the thermosyphons had a significantly positive effect on the stability of warm permafrost.

Qinghai-Tibet Highway； permafrost； deformation； ground temperature； monitoring

1　Introduction

Subgrade hazards， such as uplifting， subsidence，and cracking of the subgrade surface， frequently interfere with the operation of highways in permafrost regions， to which close attention has been paid by researchers for many years. Ever since the construction of the Qinghai-Tibet Highway in 1954， these subgrade diseases still have not been thoroughly solved. More than 80% of the diseases of the Qinghai-Tibet Highway in permafrost regions have resulted from thaw settlement of the ground， and have mainly occurred in high-ice-content permafrost and warm permafrost regions. In these areas larger consolidations have often emerged from the permafrost layer due to subgrade fill gravity and vehicle loads， even when the permafrost table did not vary （Zang and Wu，1999； Liu et al.， 2002； Wang， 2005； Chen et al.， 2006）.Regarding the thaw settlement of subgrade in permafrost， laboratory tests and analyses of the causes and characteristics of thaw settlement of clay by Zhu and Zhang （1982）， He et al. （2003）， and Liang et al. （2006）indicated that freeze-thaw cycles， water content， external loads， and soil density have obvious effects on the thaw settlement coefficient and the elastic modulus of clay. Based on in-situ monitored data， Zhang et al.（2007） and Ma et al. （2008） verified that embankment settlement consists of four parts： settlements of the embankment body itself and the original active layer，consolidation of thawed permafrost， and compression of warm permafrost. Qi et al. （2009） and Yu et al.（2011） pointed out that embankment settlement in permafrost regions results from many physical and mechanical processes of different soil layers， such as creep of unfrozen layers， thaw settlement due to the degradation （thaw） of permafrost underneath， creep of warm frozen layers， and freeze-thaw cycling of the active layer.

At present， disease treatments are conducted in the most serious hazard areas of the Qinghai-Tibet Highway every two to three years， and thermosyphons are among the most common control measures. In this study， both the ground temperature and embankment settlement at different soil layers were monitored within a typical section of Qinghai-Tibet Highway embankment， at Chumaerhe in the permafrost region，where thermosyphons were set up on only the left shoulder of the embankment. Based on the monitored data， the characteristics of ground temperature and embankment deformation are discussed and the effect of the thermosyphons is analyzed here. It is hoped that this study may provide help， to some degree， for the disease prevention of subgrade in permafrost regions in the future.

2　Description of in-situ tests

The monitoring section is located in the subgrade of the Qinghai-Tibet Highway （K2961+200） at Chumaerhe， which is located in Qinghai Province， China. The width of the embankment surface there is 10.5 m and heights of the left and right shoulders of the embankment are 2.3 m and 3.0 m， respectively. Because the sunny slope is at the left side of the embankment，thermosyphons with a 15-degree angle are situated on that embankment at spacing intervals of 4 m （Figure 1）. The total length of each thermosyphon is 12 m； the lengths of the evaporator sections and the condenser sections are 6.0 m and 4.0 m， respectively， and the inner and outer diameters are 0.08 m and 0.1 m. In-situ drilling exploration indicated that the subgrade fill is sand-clay with gravels with a water content of about 5%-13.6%. The top layer， about 2-5 m beneath the natural ground surface， is silt sand followed by sub-clay； the weight water contents of the silt sand and sub-clay were both about 12.6%-24.9% during the study period. Permafrost tables under the natural ground surface and the left and right shoulders of the embankment were about 2.2 m， 4.1 m， and 9.0 m，respectively， and the mean annual ground temperature was about -0.6 °C （Figure 2）.

In the monitoring section， three layered settlement monitoring points， the L1 point， the L2 point， and the L3 point， were embedded at different depths under the left shoulder of the embankment； their depths were 0.5 m， 2.3 m， and 4.1 m under the shoulder surface， respectively （Figures 2 and 3）. Likewise， the depths of three monitoring points under the right shoulder， the R1 point， the R2 point， and the R3 point， were 0.5 m，3.0 m， and 9.0 m under the shoulder surface， respectively. The monitoring datum point， which was 75 m from the left slope toe of the subgrade， was 15 m below the natural ground surface. Every monitoring point was made of PVC pipe， steel pipe， steel plate， and Vaseline. The PVC pipe enclosed each steel pipe and the Vaseline filled the space between the PVC pipe and the steel pipe. The deformation of each monitoring point was observed manually by an electronic level every month or two for one full year.

Ground temperature was monitored by thermistors located every 0.5 m， and they were set into 15-m-deep holes located in the left and right shoulders of the subgrade. The ground temperature under the left shoulder was collected by a CR3000 data logger（Campbell Scientific， USA） once an hour， and the ground temperature under the right shoulder was monitored by a QSY300Z data logger （Stone Edge Science and Technology Co. Limited， China） manually.

The L1 and R1 points were fixed at 0.5 m under the shoulder surface， the L2 and R2 points were fixed at the bottom of the subgrade， and the L3 and R3 points were at the permafrost table under each shoulder. Thus， the deformations of different soil layers， the embankment body itself， the seasonal active layer， and the permafrost layer， could be obtained by measuring the height of the steel pipe relative to the datum point.

Figure 1　The monitored subgrade with thermosyphons set in the left embankment

Figure 2　Schematic diagram of cross section of the monitored embankments

Figure 3　Plan view of the monitoring site

3　Discussion of the monitoring data

3.1Characteristics of the ground temperature

Figure 4 shows the ground temperature changes at different depths under the left shoulder from June 2013 to June 2014. It can be seen that temperatures of soil within the 4.5-8.5-m depth began to descend obviously after November 2013， reaching the minimum of -2.0 °C during January and February， which was due to the cooling effect of the thermosyphons in the cold season. Those temperatures increased gradually after April 2014 when the climate began to warm and the thermosyphons stopped operation；they finally stabilized at -0.3 °C. However， the permafrost temperature below 9.0 m scarcely varied during the year.

Figure 5 shows ground temperature changes versus time at different depths， 0.5 m， 2.3 m， and 4.1 m under the left shoulder. In June 2013 the temperature of the soil at the 0.5-m depth increased gradually and reached the maximum of 13.1 °C in August. Afterwards， with the air temperature lowering， the ground temperature started to descend and became negative in mid-October， and reached its minimum in January. The soil layer began to thaw from the surface in late April of the next year. With respect to the soil at the 2.3-m depth， its maximum temperature， 4.8 °C，occurred in September 2013 and became negative after early December 2013； it first started to thaw in early June 2014. The temperature of soil at the 4.1-m depth remained negative during the monitoring period， ranging from -0.1 °C to -0.4 °C from June to November 2013； its minimum temperature， -1.2 °C，occurred in March.

Figure 4　Temperature changes with time at different depths at the left shoulder

Because the ground temperature under the right shoulder was monitored manually， data was acquired only three times in 2013. Figure 6 shows the ground temperature changes at different depths on June 21，October 15， and November 26， 2013. It can be seen that the ground reached the maximum thaw depth in mid-October， and the soil layer between 1.5 m and 9.0 m depth maintained a positive temperature， although the shallow soil layer at the 1.5-m depth had frozen on November 26. The permafrost temperature below 12.0 m hardly varied.

According to previous in-situ drilling exploration in 2012， the permafrost tables under the left and right shoulders were 4.1 m and 9.0 m under the subgrade surface， respectively， which indicates that the monitored data and methods in the current study were reliable.

The above analysis shows that the thermosyphons could make the permafrost table uplift obviously， especially in the warm permafrost region， and that the seasonal active layer began to freeze or thaw about one to two months later than the embankment body itself.

Figure 5　Temporal ground temperature change at 0.5 m， 2.3 m， and 4.1 m depths under the left shoulder

Figure 6　Temporal temperature change at different depths at the right shoulder

3.2Characteristics of deformations at different depths

Figure 7 shows the deformation at different layers under the left shoulder from November 2012 to November 2014. The ground deformations at the 0.5-m and 2.3-m depths fluctuated during the monitoring period， which was due to the frost heave of soil in winter and the thaw settlement in summer. However，the start times of frost heave or thaw settlement at those two depths were not simultaneous； the active layer started to freeze or thaw later than the embankment body itself. From November 2012 to mid-October 2014， the total settlements of soil at the 0.5-m， 2.3-m， and 4.1-m depths were 24 mm， 23 mm，and 3 mm， respectively. There was little deformation of soil at the 4.1-m depth， which indicated that the permafrost was quite stable without the effect of external heat.

The maximum frost heave of ground at the 0.5-m depth occurred in April （9 mm） and the maximum thaw settlement occurred in October （21 mm）. At the same time， the mean annual frost heave and thaw settlement at the 2.3-m depth were 5 mm and 13.5 mm，respectively. There was virtually no deformation of permafrost under the 4.1-m depth. Based on the location of each monitoring point （Figure 3）， the deformation differences between the 0.5-m and 2.3-m depths and between the 2.3-m and 4.1-m depths are therefore considered as the deformation of embankment body itself and the deformation of seasonal active layer， respectively. Thus， when the road surface suffered its most serious deformation in April， the frostheaves resulting from the embankment body itself and seasonal active layer were 4 mm and 5 mm， respectively. Likewise， the thaw settlements produced by the embankment body itself and the seasonal active layer were 7.5 mm and 13.5 mm， respectively. This demonstrates that the seasonal active layer was the main influence factor of left shoulder stability， followed by the embankment body itself， and finally the permafrost. This was specifically due to freeze-thaw cycles，which may change soil properties.

Figure 7　Deformation at different layers under the left shoulder

Figure 8 shows the deformation at different layers under the right shoulder. The ground deformation characteristics at the 0.5-m and 3.0-m depths under the right shoulder were similar to those under the left shoulder， namely， the frost heave and thaw settlement of the former began later than the latter's. Significantly， during the whole monitoring period the ground deformation at the 9.0-m depth was greatest， meaning there was more settlement as time went on. From November 2012 to mid-October 2014， the total ground settlements at the 0.5-m， 3.0-m， and 9.0-m depths were 25.5 mm， 24 mm， and 18 mm， respectively.

Figure 8　Deformation at different layers under the right shoulder

Figure 8 shows that the mean annual frost heaves of ground at the 0.5-m， 3.0-m， and 9.0-m depths were 6.5 mm， 3.5 mm， and 0.5 mm， respectively. When the maximum deformation arose on the right shoulder surface in the cold season， 3 mm of the height of the frost heave originated from the embankment body itself， 3 mm came from the seasonal active layer， and 0.5 mm came from the permafrost layer. The mean annual settlements were 19 mm， 12 mm， and 9 mm，respectively， when the settlement reached the maximum in October. Correspondingly， the settlements resulting from the embankment body itself， the sea-sonal active layer， and the permafrost layer were 7 mm， 3 mm， and 9 mm， respectively. Therefore， the stability of the right shoulder surface was mainly affected by the settlements of the different soil layers，especially the warm permafrost layer. Compared with the deformation of the left shoulder， the permafrost under the right shoulder was less stable， which indicates that the thermosyphons had a significantly positive effect on the stability of warm permafrost.

4　Conclusions

The ground temperature and deformation at different depths of a studied section of the Qinghai-Tibet Highway embankment， where thermosyphons were set up only on the left shoulder， indicated that the start time of freezing or thawing of the seasonal active layer was about two months later than that of the embankment body itself. The stability of each shoulder was mainly controlled by the thaw settlements， whereas frost heave had little effect on the stability of the embankment. For the left shoulder， the settlement was mainly influenced by the seasonal active layer and then by the embankment body itself， due to the freeze-thaw cycles which may change the soil properties. However， the permafrost layer under the left shoulder remained nearly stable. For the right shoulder， creep of the warm permafrost layer was the main influence factor of shoulder stability， followed by settlement of the embankment body itself， and finally settlement of the seasonal active layer. Compared with the deformation of the left shoulder， the permafrost under the right shoulder was less stable，which indicated that the thermosyphons had a significantly positive effect on the stability of warm permafrost.

Acknowledgments：

The authors wish to acknowledge the support provided by the National Natural Science Foundation of China（No. 41271072）， the national 973 Project of China（No. 2012CB026104）， the Fundamental Research Funds for the Central Universities （No. 2011JBZ009），and Open Fund of the Qinghai Research Institute of Transportation （No. 20121208）.

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*Correspondence to： Ph.D.， YaHu Tian， School of Civil Engineering， Beijing Jiaotong University， No. 3， Shangyuan Cun， Haidian District， Beijing 100044， China. Tel： +86-10-51684077； E-mail： yhtian@bjtu.edu.cn

February 21， 2015 Accepted： April 21， 2015