高海拔多年冻土区砂石路面公路的路基温度场特征

包卫星; 刘亚伦; 毛雪松; 李伟; 秦川; 郭强; 陈锐

doi:10.19818/j.cnki.1671-1637.2023.04.004

高海拔多年冻土区砂石路面公路的路基温度场特征

doi: 10.19818/j.cnki.1671-1637.2023.04.004

包卫星^1,,
刘亚伦¹,
毛雪松¹,
李伟¹,
秦川¹,
郭强^{1, 2},
陈锐¹

1.
长安大学公路学院，陕西西安 710064
2.
新疆维吾尔自治区交通建设管理局，新疆乌鲁木齐 830000

基金项目:

国家自然科学基金项目 51878064

新疆维吾尔自治区重大科技专项项目 2020A03003-7

陕西省自然科学基础研究计划项目 2021JM-180

详细信息

作者简介:
包卫星(1979-), 男, 新疆乌鲁木齐人，长安大学教授，工学博士，从事特殊土路基工程研究

中图分类号: U416.168
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- 被引次数: 0
出版历程
- 收稿日期: 2023-03-15
- 网络出版日期: 2023-09-08
- 刊出日期: 2023-08-25

Characteristics of subgrade temperature field of gravel road in high altitude permafrost region

1.
School of Highway, Chang'an University, Xi'an 710064, Shaanxi, China
2.
Transportation Construction Administration of Xinjiang Uygur Autonomous Region, Urumchi 830000, Xinjiang, China

Funds:

National Natural Science Foundation of China 51878064

Major Science and Technology Projects of Xinjiang Uygur Autonomous Region 2020A03003-7

Natural Science Basic Research Project of Shaanxi Province 2021JM-180

More Information

Author Bio:
BAO Wei-xing(1979-), male, professor, PhD, baowx@chd.edu.cn

摘要

摘要: 为了研究修筑公路对高海拔多年冻土层热状态的影响，开展了新藏公路多年冻土区路段沿线病害调查，在海拔5 400 m地带修筑了冻土地温监测断面与气象监测站点；对气温、地温、辐射强度进行了监测，依据监测结果计算了冻土上限处的热流通量，分析了多年冻土层地温变化特征；基于热传导和热扩散理论，建立了天然地基及普通路基下部多年冻土地温-深度理论预测模型。研究结果表明：多年冻土区公路病害主要由于沥青路面大量吸热导致，热棒、隔热层等主动、被动保护的手段虽有一定效果，但不能改变多年冻土的快速退化；研究区域天然地基与路基中心一天内温差最高达19.66 ℃，左、右路肩一天内温差最高为4.94 ℃，天然地基下深层多年冻土温度稳定在-6.0 ℃左右，路基中心下部深层多年冻土温度稳定在-5.6 ℃左右，路基下部相较天然地基温度变化更为剧烈，且等温层温度更高；研究区域的辐射强度在一天的10:00~18:00显著增强，在一年的3~6月为辐射强度的顶峰期，浅层地温主要受辐射强度的年周期变化影响；天然地基、路基中心、阴坡路肩与阳坡路肩下部多年冻土层年热流通量依次为-4 001、-14 649、-4 487与58 303 kJ·m^-2，路基中心散热速率大于天然地基，阳坡路肩处大量吸热；天然地基的等温层出现在9.79 m深度处，而路基中心等温层出现在9.61 m深度处，路基中心等温层位置更浅，路基土的换填使路基下部浅层冻土温度变化更明显，短期内对下部多年冻土的散热有正向作用；在阴阳坡效应下，阳坡下部多年冻土温度升高，路基热稳定性降低，并产生不均匀沉降。
- 路基工程 /
- 多年冻土路基 /
- 地温预测模型 /
- 现场监测 /
- 热流通量 /
- 热扩散理论
Abstract: In order to study the effect of highway construction on the thermal state of permafrost layer in high altitude area, the investigation on the road diseases in the permafrost regions along the Xinjiang-Xizang Highway was conducted, a temperature monitored cross-section of the ground and meteorological monitoring sites were built in the area with an altitude of 5 400 m, and the air temperature, ground temperature and radiation intensity were monitored. According to the monitoring result, the heat flux at the upper limit of the frozen soil was calculated, and the changing characteristics of ground temperature in permafrost layer were analyzed. Based on the heat conduction theory and heat diffusion theory, a ground temperature-depth theoretical prediction model of the permafrost under natural subgrade and normal subgrade was proposed. Research results show that the road diseases in the permafrost region are mainly caused by the large amount of heat absorption of asphalt pavement. Although the active and passive protection measures such as hot rods and thermal insulation layers have some positive effects, they cannot change the rapid degradation of the permafrost. The largest temperature difference between the natural foundation and the subgrade center in the study area is up to 19.66 ℃ in a day, and the temperature difference between the left and right shoulders is up to 4.94 ℃ in a day. The temperature of the deep permafrost under the natural foundation maintains at about -6.0 ℃, and the temperature of the deep permafrost in the lower part of the subgrade center maintains at about -5.6 ℃. The temperature of the lower part of the subgrade is more drastic than that of the natural foundation, and the temperature of the subgrade of the isothermal layer is higher. The radiation intensity in the study area increases significantly at 10:00-18:00 in a day, the peak radiation intensity is between March and June in a year, and the shallow ground temperature is mainly affected by the annual cycle variation of radiation intensity. The annual heat fluxes of the permafrost layer in the lower part of the natural foundation, subgrade center, shady slope shoulder, and sunny slope shoulder are -4 001, -14 649, -4 487 and 58 303 kJ·m^-2, respectively. The heat dissipation rate of the subgrade center is greater than that of the natural foundation, and a large amount of heat is absorbed at the shoulder of the sunny slope road. The isotherm of the natural foundation appears at a depth of 9.79 m, while the isotherm of the subgrade center appears at a depth of 9.61 m, indicating the isotherm of the subgrade center is shallower. The compaction of the subgrade soil makes the temperature change of the shallow part of the permafrost more obvious, which has a positive effect on the heat dissipation of the lower permafrost in the short term. Under the sunny-shady slopes effect, the increase of the permafrost temperature in the lower part of the slope reduces the thermal stability of the subgrade and results in uneven settlement.
- subgrade engineering /
- permafrost subgrade /
- ground temperature prediction model /
- in-site monitoring /
- heat flux /
- heat diffusion theory

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图 1 研究区域地理位置

Figure 1. Geographic location of research region

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图 2 试验路基结构断面

Figure 2. Structure sections of test subgrades

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图 3 试验路段病害

Figure 3. Diseases of test road sections

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图 4 监测点布设

Figure 4. Monitoring points deployment

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图 5 监测断面

Figure 5. Monitoring section

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图 6 浅层地温变化曲线

Figure 6. Varing curves of shallow ground temperature

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图 7 天然地基孔、路基中心孔月均化温度-深度变化曲线

Figure 7. Monthly average temperature-depth varing curves of natural foundation pore and subgrade center pore

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图 8 护道孔月均化温度-深度变化曲线

Figure 8. Monthly average temperature-depth varing curves of berm pores

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图 9 路肩孔月均化温度-深度变化曲线

Figure 9. Monthly average temperature-depth varing curves of road shoulder pores

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图 10 天然地基孔各深度温度随时间变化曲线

Figure 10. Varing curves of different depths temperature of natural foundation pore with time

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图 11 各时刻的辐射强度在一年周期内的变化曲线

Figure 11. Radiation intensity variation curves at different moments over a one-year period

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图 12 一天中各时刻年均辐射强度

Figure 12. Average annual radiation intensities at all times in a day

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图 13 各孔位3.5~4.0 m深度热流通量

Figure 13. Heat fluxes of different pores at depths of 3.5-4.0 m

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图 14 天然地基孔与路基中心孔实测和拟合温度曲线

Figure 14. Measuring and fitting temperature curves of natural foundation pore and subgrade center pore

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图 15 天然地基孔与路基中心孔实测和预测温度曲线

Figure 15. Measured and predicted ground temperature curves of natural foundation pore and subgrade center pore

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图 16 各深度处长期实测与预测地温

Figure 16. Long-term measured and predicted ground temperatures at different depths

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图 17 多年冻土地温-深度预测等温线

Figure 17. Temperature-depth prediction isotherms of permafrost

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表 1 多年冻土参数

Table 1. Permafrost parameters

冻胀等级	标准冻深/m	最大冻深/m	含冰量/%	融沉系数
Ⅰ、Ⅱ	1.9~3.0	3.8	8~15	0.9~1.7

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表 2 路基下3.5~4.0 m深度土体的热收支

Table 2. Heat budgets of soil at depths of 3.5-4.0 m under subgrade

位置	天然地基	阴坡路肩	路基中心	阳坡路肩
土体年热收支/(kJ·m^-2)	-4 001	-4 487	-14 649	58 303

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参考文献(32)

[1]	WU Qing-bai, ZHANG Zhong-qiong, LIU Yong-zhi. Long-term thermal effect of asphalt pavement on permafrost under an embankment[J]. Cold Regions Science and Technology, 2010, 60(3): 221-229. doi: 10.1016/j.coldregions.2009.10.007
[2]	LUO Xiao-xiao, YU Qi-hao, MA Qin-guo, et al. Study on the heat and deformation characteristics of an expressway embankment with shady and sunny slopes in warm and ice-rich permafrost regions[J]. Transportation Geotechnics, 2020, 24: 100390. doi: 10.1016/j.trgeo.2020.100390
[3]	权磊, 田波, 牛开民, 等. 青藏高原高等级道路路基路面温度变化特征[J]. 交通运输工程学报, 2017, 17(2): 21-30. https://transport.chd.edu.cn/article/id/201702003 QUAN Lei, TIAN Bo, NIU Kai-min, et al. Temperature variation properties of pavements and subgrades for high-grade roads on Qinghai-Tibet Plateau[J]. Journal of Traffic and Transportation Engineering, 2017, 17(2): 21-30. (in Chinese) https://transport.chd.edu.cn/article/id/201702003
[4]	马勤国, 赖远明, 吴道勇. 多年冻土区高等级公路路基温度场研究[J]. 中南大学学报(自然科学版), 2016, 47(7): 2415-2423. https://www.cnki.com.cn/Article/CJFDTOTAL-ZNGD201607032.htm MA Qin-guo, LAI Yuan-ming, WU Dao-yong. Analysis of temperature field of high grade highway embankment in permafrost regions[J]. Journal of Central South University (Science and Technology), 2016, 47(7): 2415-2423. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZNGD201607032.htm
[5]	孙志忠, 马巍, 温智, 等. 青藏铁路多年冻土区普通路基地温监测及其预测分析[J]. 铁道学报, 2010, 32(3): 71-76. https://www.cnki.com.cn/Article/CJFDTOTAL-TDXB201003017.htm SUN Zhi-zhong, MA Wei, WEN Zhi, et al. Experimental and numerical analyses on traditional embankment of Qinghai-Tibet Railway[J]. Journal of the China Railway Society, 2010, 32(3): 71-76. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-TDXB201003017.htm
[6]	常晓丽, 金会军, 何瑞霞, 等. 大兴安岭北部多年冻土监测进展[J]. 冰川冻土, 2013, 35(1): 93-100. https://www.cnki.com.cn/Article/CJFDTOTAL-BCDT201501011.htm CHANG Xiao-li, JIN Hui-jun, HE Rui-xia, et al. Review of permafrost monitoring in the Northern Da Hinggan Mountains, Northeast China[J]. Journal of Glaciology and Geocryology, 2013, 35(1): 93-100. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-BCDT201501011.htm
[7]	房建宏, 刘建坤. 国道214沿线路基下伏多年冻土热状况长期变化分析[J]. 中国公路学报, 2016, 29(11): 25-32. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201611006.htm FANG Jian-hong, LIU Jian-kun. Analysis of long-term changes for thermal regine of permafrost underlying subgrade along National Highway G214[J]. China Journal of Highway and Transport, 2016, 29(11): 25-32. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201611006.htm
[8]	汪双杰, 崔福庆, 陈建兵, 等. 基于地气耦合模型的多年冻土区宽幅路基温度场数值模拟[J]. 中国公路学报, 2016, 29(6): 169-178. doi: 10.3969/j.issn.1001-7372.2016.06.003 WANG Shuang-jie, CUI Fu-qing, CHEN Jian-bing, et al. Numerical simulations of temperature field for wide subgrade in permafrost regions under earth-atmosphere coupled system[J]. China Journal of Highway and Transport, 2016, 29(6): 169-178. (in Chinese) doi: 10.3969/j.issn.1001-7372.2016.06.003
[9]	李宁, 徐彬, 陈飞熊. 冻土路基温度场、变形场和应力场的耦合分析[J]. 中国公路学报, 2006, 19(3): 1-7. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL200603000.htm LI Ning, XU Bin, CHEN Fei-xiong. Coupling analysis of temperature, deformation and stress field for frozen soil roadbed[J]. China Journal of Highway and Transport, 2006, 19(3): 1-7. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL200603000.htm
[10]	戚春香, 李瑶, 杨简, 等. 青藏高原机场跑道多年冻土地基温度场特征[J]. 交通运输工程学报, 2019, 19(1): 33-47. doi: 10.19818/j.cnki.1671-1637.2019.01.005 QI Chun-xiang, LI Yao, YANG Jian, et al. Characteristics of temperature field of airfield runway permafrost subgrade in Qinghai-Tibetan Plateau[J]. Journal of Traffic and Transportation Engineering, 2019, 19(1): 33-47. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2019.01.005
[11]	刘德平, 汪双杰, 王彩勤. 国道214线鄂拉山至清水河段多年冻土地温预测模型研究[J]. 公路交通科技, 2016, 33(5): 53-60. https://www.cnki.com.cn/Article/CJFDTOTAL-GLJK201605009.htm LIU De-ping, WANG Shuang-jie, WANG Cai-qin. Research of prediction model of permafrost temperature of National Road 214 from Ela Mountain to Qingshui River[J]. Journal of Highway and Transportation Research and Development, 2016, 33(5): 53-60. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-GLJK201605009.htm
[12]	刘德平, 汪双杰, 金龙. 共和至玉树公路多年冻土地温拟合模型[J]. 中国公路学报, 2015, 28(12): 100-105. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201512016.htm LIU De-ping, WANG Shuang-jie, JIN Long. Permafrost ground temperature fitting model of Gonghe-Yushu Highway[J]. China Journal of Highway and Transport, 2015, 28(12): 100-105. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201512016.htm
[13]	汪双杰, 王佐, 袁堃, 等. 青藏公路多年冻土地区公路工程地质研究回顾与展望[J]. 中国公路学报, 2015, 28(12): 1-8, 32. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201512002.htm WANG Shuang-jie, WANG Zuo, YUAN Kun, et al. Qinghai-Tibet highway engineering geology in permafrost regions: review and prospect[J]. China Journal of Highway and Transport, 2015, 28(12): 1-8, 32. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201512002.htm
[14]	李金平, 张娟, 陈建兵, 等. 高寒冻土区路基变形演化规律与破坏特征[J]. 交通运输工程学报, 2016, 16(4): 78-87. doi: 10.19818/j.cnki.1671-1637.2016.04.008 LI Jin-ping, ZHANG Juan, CHEN Jian-bing, et al. Evolution laws and failure characteristics of subgrade deformation in alpine permafrost region[J]. Journal of Traffic and Transportation Engineering, 2016, 16(4): 78-87. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2016.04.008
[15]	刘戈, 汪双杰, 金龙, 等. 多年冻土区热棒路基应用效果[J]. 交通运输工程学报, 2016, 16(4): 59-67. doi: 10.19818/j.cnki.1671-1637.2016.04.006 LIU Ge, WANG Shuang-jie, JIN Long, et al. Applicable effect of thermosyphon subgrades in permafrost regions[J]. Journal of Traffic and Transportation Engineering, 2016, 16(4): 59-67. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2016.04.006
[16]	金龙, 汪双杰, 穆柯, 等. 青藏公路热棒路基降温效能[J]. 交通运输工程学报, 2016, 16(4): 45-58. doi: 10.19818/j.cnki.1671-1637.2016.04.005 JIN Long, WANG Shuang-jie, MU Ke, et al. Cooling effect of thermosyhpon subgrade for Qinghai-Tibet Highway[J]. Journal of Traffic and Transportation Engineering, 2016, 16(4): 45-58. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2016.04.005
[17]	ZHANG Zhen-yu, YU Qi-hao, YOU Yan-hui, et al. Cooling effect analysis of temperature-controlled ventilated embankment in Qinghai-Tibet Testing Expressway[J]. Cold Regions Science and Technology, 2020, 173: 103012.
[18]	LUO Xiao-xiao, YU Qi-hao, MA Qin-guo, et al. Evaluation on the stability of expressway embankment combined with L-shaped thermosyphons and insulation boards in warm and ice-rich permafrost regions[J]. Transportation Geotechnics, 2021, 30: 100633.
[19]	汪双杰, 熊丽, 张驰, 等. 多年冻土区公路病害模糊专家预测方法[J]. 交通运输工程学报, 2016, 16(4): 112-121. doi: 10.19818/j.cnki.1671-1637.2016.04.012 WANG Shuang-jie, XIONG Li, ZHANG Chi, et al. Fuzzy expert prediction method for highway diseases in permafrost region[J]. Journal of Traffic and Transportation Engineering, 2016, 16(4): 112-121. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2016.04.012
[20]	韩龙武, 蔡汉成, 程佳, 等. 莫斯科-喀山高速铁路沿线季节性冻土冻融特征[J]. 交通运输工程学报, 2018, 18(3): 44-55. doi: 10.19818/j.cnki.1671-1637.2018.03.005 HAN Long-wu, CAI Han-cheng, CHENG Jia, et al. Freezing and thawing characteristics of seasonal frozen soil along Moscow-Kazan High-Speed Railway[J]. Journal of Traffic and Transportation Engineering, 2018, 18(3): 44-55. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2018.03.005
[21]	李顺群, 陈之祥, 夏锦红, 等. 冻土导热系数的聚合模型研究及试验验证[J]. 中国公路学报, 2018, 31(8): 39-46. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201808005.htm LI Shun-qun, CHEN Zhi-xiang, XIA Jin-hong, et al. Aggregation model research and experimental verification of frozen soil thermal conductivity[J]. China Journal of Highway and Transport, 2018, 31(8): 39-46. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201808005.htm
[22]	XIAO Wei, LIU En-long, YIN Xiao, et al. Numerical simulation of coupled liquid water, stress and heat for frozen soil in the thawing process[J]. Engineering Computations, 2022, 39(4): 1492-1510.
[23]	陈建兵, 刘志云, 崔福庆, 等. 青藏高原工程走廊带多年冻土辨识及年平均地温预估模型[J]. 中国公路学报, 2015, 28(12): 33-41, 56. https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201512006.htm CHEN Jian-bing, LIU Zhi-yun, CUI Fu-qing, et al. Permafrost identification and annual mean ground temperatures prediction model for Qinghai-Tibet Engineering Corridor[J]. China Journal of Highway and Transport, 2015, 28(12): 33-41, 56. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-ZGGL201512006.htm
[24]	CHEN Xing, JEONG S, PARK C E, et al. Different responses of surface freeze and thaw phenology changes to warming among Arctic permafrost types[J]. Remote Sensing of Environment, 2022, 272: 112956.
[25]	MA Shen, YANG Bin, ZHAO Jing-yi, et al. Hydrothermal dynamics of seasonally frozen soil with different vegetation coverage in the Tianshan Mountains[J]. Frontiers in Earth Science, 2022, 9: 806309.
[26]	GONZÁLEZ-ROUCO J F, BELTRAMI H, ZORITA E, et al. Borehole climatology: a discussion based on contributions from climate modeling[J]. Climate of the Past, 2009, 5(1): 97-127.
[27]	陈友明, 王宇航, 莫志姣. 土壤初始温度模型[J]. 湖南大学学报(自然科学版), 2007, 34(7): 27-29. https://www.cnki.com.cn/Article/CJFDTOTAL-HNDX200707005.htm CHEN You-ming, WANG Yu-hang, MO Zhi-jiao. Modeling for initial soil temperature[J]. Journal of Hunan University (Natural Sciences), 2007, 34(7): 27-29. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-HNDX200707005.htm
[28]	何报寅, 刘杰, 徐贵来, 等. 浅层地温剖面曲线拟合法估算热扩散系数[J]. 华中师范大学学报(自然科学版), 2012, 46(1): 104-108. https://www.cnki.com.cn/Article/CJFDTOTAL-HZSZ201201023.htm HE Bao-yin, LIU Jie, XU Gui-lai, et al. Estimation of thermal diffusivity using subsurface temperature profile curve fitting algorithm[J]. Journal of Central China Normal University (Natural Sciences), 2012, 46(1): 104-108. (in Chinese) https://www.cnki.com.cn/Article/CJFDTOTAL-HZSZ201201023.htm
[29]	符进, 姜宇, 彭惠, 等. 多年冻土区大直径钻孔灌注桩早期回冻规律[J]. 交通运输工程学报, 2016, 16(4): 104-111. doi: 10.19818/j.cnki.1671-1637.2016.04.011 FU Jin, JIANG Yu, PENG Hui, et al. Early refreezing law of large-diameter cast-in-place piles in permafrost regions[J]. Journal of Traffic and Transportation Engineering, 2016, 16(4): 104-111. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2016.04.011
[30]	ZHAO Shang-min, CHENG Wei-ming, YUAN Ye-cheng, et al. Global permafrost simulation and prediction from 2010 to 2100 under different climate scenarios[J]. Environmental Modelling and Software, 2022, 149: 105307.
[31]	SEMIN M, LEVIN L, BOGOMYAGKOV A, et al. Features of adjusting the frozen soil properties using borehole temperature measurements[J]. Modelling and Simulation in Engineering, 2021, 2021: 1-17.
[32]	穆柯, 袁堃, 金龙, 等. 高寒高海拔多年冻土区拓宽路基差异沉降[J]. 交通运输工程学报, 2016, 16(4): 68-77. doi: 10.19818/j.cnki.1671-1637.2016.04.007 MU Ke, YUAN Kun, JIN Long, et al. Differential settlement of widened subgrade in cold and high-altitude permafrost regions[J]. Journal of Traffic and Transportation Engineering, 2016, 16(4): 68-77. (in Chinese) doi: 10.19818/j.cnki.1671-1637.2016.04.007