J. Cent. South Univ. (2020) 27: 2082-2093

DOI: https://doi.org/10.1007/s11771-020-4432-9

In-situ experiment investigations of hydrothermal process of highway in deep seasonal frozen soil regions of Inner Mongolia, China

ZHANG Hong-wei(张洪伟)^{1, 2}, WANG Xue-ying(王学营)^{1, 2}, ZHAO Xin(赵鑫)^{1, 2}, LIU Peng-fei(刘鹏飞)³

1. Key Laboratory of Transport Industry of Management, Control and Cycle Repair Technology for

Traffic Network Facilities in Ecological Security Barrier Area, Hohhot 010051, China;

2. Inner Mongolia Transport Construction Engineering Quality Supervision Bureau, Hohhot 010051, China;

3. Institute of Highway Engineering, RWTH Aachen University, Aachen 52074, Germany

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: To reveal the influencing factors and changing rules for the hydrothermal interaction process of highway subgrade, the field measurements of Shiwei-Labudalin Highway in Inner Mongolia, China was conducted for 3 years, based on which the freezing-thawing rules and water content changing characteristics were analyzed. The main results show the subgrade presents a frequent freezing-thawing alternation, and the water content of subgrade exhibits an obvious seasonal alternation. The subbase has the maximum water content, while the base has the minimum water content. The change of water flux is concentrated in the thawing period and consistent with the change of temperature gradient. The subbase layer has the most active water flux due to the heat absorption and impermeability of pavement that easily causes the water accumulation in this layer. Therefore, the prevention and treatment for the freezing-thawing disease should be started from heat insulation and water resistance.

Key words: subgrade engineering; hydrothermal process; field observation; seasonal frozen soil regions; freezing-thawing disease

Cite this article as: ZHANG Hong-wei, WANG Xue-ying, ZHAO Xin, LIU Peng-fei. In-situ experiment investigations of hydrothermal process of highway in deep seasonal frozen soil regions of Inner Mongolia, China [J]. Journal of Central South University, 2020, 27(7): 2082-2093. DOI: https://doi.org/10.1007/s11771-020-4432-9.

1 Introduction

The area of seasonal frozen soil in China is about 5.13×10⁶ km² [1], which accounts for 53% of the country’s land area, with significant regional characteristics [2]. Unlike permafrost, the seasonal frozen soil freezes in cold season and melts in warm season, which is greatly affected by the external environment. The alternating state of freezing and thawing makes the mechanical properties of the seasonal frozen soil significantly different. In the cold season, the water migration and the ice-water phase change cause the soil to heave. But in the warm season, the soil settles after thawing. Under the action of freezing-thawing cycles, the alternating occurrence of frost heaving and thawing settlement is a prominent feature of such special soils, which plagues the construction in seasonal frozen soil areas. Furthermore, the disasters of road engineering in seasonal frozen soil areas frequently occur, causing serious problems for the normal operation of roads and railways. The frost heave, settlement, mud-pumping and cracks of road are major disease forms in the seasonal frozen soil [3-7]. The occurrence of such diseases not only seriously affects the safety and health operation of the road, but also consumes a large amount of maintenance costs. For example, the area of frost boiling in Shen-Dan Highway was more than 600000 m² during the period from 1967 to 1999, and the freezing-thawing diseases accounted for 1/3 of the city’s roads of Changchun City, China in 2004, with remediation and maintenance costs nearly 200 million Yuan [1].

The development of the freezing-thawing disease in seasonal frozen soil areas is mainly controlled by water freezing and ice accumulation [8]. Water migration and accumulation are the source that induces frost heaving, and temperature change is an external influence factor. The coupling effect of hydrothermal state is the direct cause of the occurrence of freeze-thaw disease [9, 10]. At present, it is much needed to deeply understand the cause and to properly deal with the disaster. In order to explain the frost heaving mechanism of subgrade in the seasonal frozen soil, some scholars have developed theoretical models of the water migration and frost heaving in freeze-thaw environments [11-13], which reveal the process of water migration and the mechanism of ice accumulation under hydrothermal coupling, and also indicate that the prevention and control of frozen diseases in the seasonal frozen soil should start from the two aspects of water and heat [14]. Studies have shown that using replacement and insulation measures can effectively delay the freezing time of the subgrade and alleviate the frozen disease [15, 16]. In addition, the improvement of the drainage system, the use of new water and drainage measures are also effective means to alleviate the frost heaving of the subgrade in the seasonal frozen soil areas [17-20].

Moreover, as for the field measurement, ANDREW et al [21] investigated the moisture, temperature, and frost penetration at the Ohio Strategic Highway Research Program Test Road and the results were analyzed to develop the general expressions for the seasonal variations of average daily air temperature and variations of temperature and moisture in the fine-grained subgrade soil. OLIVIER et al [22] reported the formation of ice lens is the main mechanism to cause the frost heave of the subgrade soils involved in the high degradation rate of the flexible pavement, and they further developed the flexible pavement damage models, associated long-term roughness performance to frost heave and degradation mechanisms. BILODEAU et al [23] tested the flexible pavement structural response in thawing season for different loading conditions, which analyzed how warm temperature affect the pavement. YANG et al [24] presented soil freezing and pavement cracking due to frost action, which were the prevailing types that cause heavy damage to roads, and also founded that pavement damage was closely related to the saline soil distribution, freeze-thaw cycles, water migration and embankment geometry. JEAN et al [25] founded the increase of water content in the pavement environment was significant during spring thaw. KLENE et al [26] reported the winter-time convection could lower foundation soil temperatures beneath open-graded embankments by as much as 5 °C on an annual average. GOERING [27] showed that construction of surface facilities in cold regions presented large engineering challenges because of the alteration of the thermal regime at the ground surface. However, on the basis of above mentioned literatures, up to now, the understanding of the coupling effect of hydrothermal performance for highway has not been clear enough [28-31]. It is urgent to conduct field measurement of hydrothermal process for highway in the deep seasonal frozen soil regions.

Hulunbuir plateau is located in the western part of the Greater Khingan Mountains of Inner Mongolia, China. It is a typical high-latitude and deep seasonal frozen soil area in China, which has a cold climate, abundant rainfall and large seasonal freezing depth, with the freeze-thaw disease particularly developed in highway subgrade. Due to the entropic effect after freezing [32], the surface water content of the soil in this area is large, and the water migration under freezing and thawing is significant, which has a great impact on freezing damage in the highway. The foundation of the area is dominated by fine-grained soil, and the characteristics of frost heaving under the combined effects of water, soil, temperature and pressure are obvious [33]. Under such conditions, the prevention and control of road freezing and thawing diseases have become the emphasis and difficulty in the construction of highway projects. Under the influence of the climatic environment, the hydrothermal feature and interaction process of highway asphalt pavement are the keys to solve the above problems. At present, the research on the hydrothermal process of highways in the seasonal frozen soil area mostly focuses on numerical simulation and laboratory experiment, which lacks the verification comparison of physical engineering. Based on this, this paper takes the Shiwei- Labudalin Highway of S201 Line in Inner Mongolia Autonomous Region as the research object. The observation data of temperature and moisture in the test section from 2016 to 2019 are collected, and the spatial and temporal distribution of water and heat in the structural layer of asphalt pavement and subgrade have been analyzed. It is of great significance to deeply understand the mechanism of highway subgrade freezing and thawing in high-latitude and deep seasonal frozen soil regions, which scientifically guides highway construction in this region.

2 Test observation situation

2.1 Site situation

This study selected the Shiwei-Labudalin Highway of the S201 Line in Inner Mongolia Autonomous Region, China as the in-site experiment site. The project is located in the Hulunbuir Plateau, the northern part of Ergun, which is a warm cool humid, semi-humid forestry and animal husbandry climate zone in the northwestern forest edge of the Great Khingan. The annual average temperature of this region is lower than 0, and the average frost-free period is 95 d. From the end of November to March of the next year, the minimum temperature is lower than -30 °C. The temperature difference between day and night is large, and the freezing and thawing is severe (Figure 1). The annual rainfall varies significantly, mainly in summer (200-300 mm), accounting for more than 65% of the annual rainfall. The S201 Line was originally a three-level highway and was put into operation in 2005, with the subgrade width of 7.5 m (pavement surface width of 6.0 m). After years of operation, the existing projects are difficult to meet the increasing demand of local traffic. In the original engineering corridor, a new first-level highway was built, and the framing and staging construction model was adopted, the first phase project was newly built with a subgrade width of 12.25 m. The original road site investigation shows that the severe environment effect with strongly freeze-thaw cycles, extreme low temperature and alternate drying-wetting in the region, leads to the roads diseased seriously. The main forms of disease include longitudinal subgrade crack, frost boil, subgrade slope and sliding and so on.

Figure 1 Air temperature curves of observation site:

2.2 Observation site arrangement

The test project is located at the S201 Line K109+700, and the observation site is flat and wide (Figure 2). The geographic position of observation section is 119°58′50″, 50°35′25″, with an altitude of 612.2 m. It is an important part of the “Field Scientific Observation and Research Station of Geological Environmental System Permafrost Region of Northeast China”. Observing the development of surface vegetation near the observation section, the site drilling revealed that the surface above 0.50 m is humus soil, which is loose and wet, and is rich in plant roots; the surface above 6.0 m is silt clay, soft plastic and nearly saturated. The main physical parameters of the foundation soil obtained through the drilling machine to the field sampling (sampling depth 3.20 m) are shown in Table 1. The height of the observation section is 2.3 m, which is filled with gravel soil, and the thickness of the pavement structure is 82 cm, with the specific structure shown in Figure 3.

In order to analyze the inferior hydrothermal process of the high-grade highway asphalt pavement in the deep seasonal frozen soil area, water and temperature observation devices were buried in the test section. The temperature observation used a high precision thermistor probe (MYT-N01), and the precision of measurement was ±0.02 °C, which was drilled and buried in the middle and shoulder of the subgrade, respectively. In the pavement structure layer, the temperature sensor buried at each junction of the surface layer, the base layer and the subbase layer, and buried at every 50 cm along the depth in the subgrade. The moisture observation used MYMAS-1 moisture sensor, and the volumetric water content (VWC) was obtained by measuring the dielectric constant of the soil, and the test accuracy was ±6%. Similar to the ground temperature observation, moisture sensors were buried in the middle-road and bilateral shoulders to observe the water changes of each site of the pavement structure layer and the subgrade, with the specific layout shown in Figure 3. All data collection adopted MYJK-106 automatic data acquisition instrument, which recorded data every 6 h. In addition, a miniature automatic weather station was set up near the observation section to provide local meteorological data. Up to now, the observation section has obtained observation data for nearly three years from October 2016 to July 2019.

Figure 2 Layout of monitoring section:

Table 1 Physical parameters of foundation soil

Figure 3 Pavement structure layer and monitoring plan

3 Analyses of freezing-thawing process

Figure 4 illustrates the temperature versus time curves at different depths of the asphalt pavement in the observed section. It can be seen that during the whole monitoring period, the temperature change law of bottom subgrade at different depths of pavement structure layer are basically consistent, and the ground temperature is sinusoidal with the operating time. During the operation period, due to the direct effect of ambient temperature, the maximum temperature of the surface layer (0.10 m) can reach 28.6 °C, and the minimum temperature is -27.4 °C, with the annual range is the largest. The annual range of other layers is decreased due to the thermal resistance of the subgrade. Moreover, the initial freezing time and thawing time at different layers are also delayed in different degrees. According to the observation data of Figure 4 and based on Eqs. (1) and (2), the freezing-thawing process of different parts of the pavement structure layer and the subgrade was further analyzed. Various indicators such as the freezing-thawing index of each layer were statistically obtained, as shown in Table 2. The results show that the highest and lowest temperatures in all layers of the subgrade decrease with increase of the depth. Similarly, the freezing index and the thawing index at each layer also decrease with increase of the depth. However, the freezing-thawing index ratio increases with increase of the depth of subgrade, and reaches the maximum near the base of the road, which shows that the subgrade is in a state of rapid thawing, much higher than the natural surface, and outclass than the average level in this area (0.89) [34]. It further reflects that the construction of the subgrade projects in this area and the endothermic action of the asphalt pavement have a large thermal disturbance on the original foundation, which destroys the freeze-thaw state of the original foundation.

Figure 4 Temperature change at different depths of asphalt pavement

                         (1)

                         (2)

where I_F is the freezing index; I_T is the thawing index; T_i is the mean daily temperature below 0; T_j is the mean daily temperature above 0.

Figure 5 reflects the freezing-thawing process at the center of the subgrade. During the operation period, both the pavement structural layer and the subgrade show a significant freezing and thawing alternative state. The initial freezing time and thawing time lag with the increase of depth, i.e., the initial freezing time of the pavement structure layer is roughly in the early November of each year, and the initial thawing time is around in the mid-April of each year, and the difference from surface layer to cushion layer is about 10 d. The initial freezing and thawing time near the bottom of the subgrade is about one month later than that of the pavement structure layer, which is consistent with the reflection laws in Figure 4. In addition, it can be seen from Figure 5 that the complete freezing period of the subgrade is about 150 d. From the first freezing period to the third freezing period of the subgrade, the total freezing time is reduced by about 10 d, and the thawing period is extended. It further illustrates that the endothermic and heat accumulation of the asphalt pavement in this area have a significant impact on the thermal regime of the subgrade.

Table 2 Temperature and freezing-thawing indexes at different depths of subgrade

Figure 5 Freezing-thawing process at central of subgrade

4 Water variation of subgrade

Freezing-thawing cycle and dry-wet alternation are obvious in this area, and water migration is active in subgrade. Mastering the spatial-temporal distribution characteristics of water content in subgrade is of great significance for understanding the hydrothermal energy process and revealing the freezing-thawing diseases of permafrost subgrade. Figure 6 shows the variation curve of the volume water content in different layers in the subgrade. On the whole, the law of water content change in each layer is basically the same, which shows the significant seasonal change characteristics. During the freezing period, the unfrozen water content in the subgrade decreases obviously due to the ice-water phase change. The water content in the base (0.20 m) is the smallest (about 5%), and the water contents in other layers are also about 5% to 10%. In the thawing period, the subgrade thaws gradually with increase of air temperature, and the water content (liquid water) increases accordingly. The subbase (0.46 m) is the highest (40%), and the change range of this layer is the largest. And the water content in the base (0.20 m) is still the smallest in warm season (below 10%); the range of change in cold and warm season is not big. Some studies have shown that there is no hydraulic relationship between the middle surface of asphalt pavement and the base layer, and the impermeability of asphalt pavement structure blocks the infiltration of external meteoric precipitation. Therefore, the moisture accumulation at the base (0.46 m) mainly comes from the moisture migration under the subgrade.

Figure 7 shows the spatiotemporal change of the water content at the center of the subgrade, while Figure 6 only shows the changes of the water content at several typical depths at the center of the subgrade. Therefore, in order to reflect the entire process of the spatiotemporal change of the water content at the center of the subgrade during freezing and thawing, the triangulation with linear interpolation method was used to obtain the water content change at different depths over time. Meanwhile, the 0 °C line is superimposed in the Figure 7 for joint analysis with freezing-thawing process. Similar to Figure 5, water content presents significant seasonal variation characteristics on the time scale. In the warm season, the water content of each layer increases with the increasing temperature, and decreases gradually after the soil is completely thawed. From early November of each year, the subgrade starts to freeze and the unfrozen volume water content in the subgrade decreases sharply.

Figure 6 Water content change at different depths

Figure 7 Water content change process under asphalt pavement

After the subgrade is completely frozen, the unfrozen volume water content in the subgrade is less than 10%. In the direction of depth, the distribution of the unfrozen volume water content is discontinuous. In the warm season, the water content is the largest at the subbase layer is 40%-45%. The water content of the surface layer and the base layer is the smallest, generally keeping below 10%, which indicates that the two layers cut off the hydraulic channel with the surroundings and have no water communication with the atmospheric environment. In the subgrade (below 0.82 m), the water content remains between 20% and 25%, while near the bottom of the subgrade (2.30 m), the water content increases to 40%.

5 Hydrothermal interaction in subgrade

The structural characteristics of the strong heat absorption and imperviousness of the asphalt pavement superimposed on the severe freezing and thawing cycle and the external environment of alternating wet and dry, which makes the hydrothermal process of the highway subgrade in the deep seasonal frozen soil area particularly complicated, and further causes the occurrence of the highway subgrade freezing-thawing disease in this area. Therefore, an in-depth analysis of the relationship between water and heat under asphalt pavement is of great significance for understanding the hydrothermal process of highway subgrade in seasonal frozen soil region. Figure 8 shows the distributions of different layers with the temperature and water content of the pavement. Generally, as the temperature increases, the water content increases. At the subbase layer(Figure 8(b)), the water content shows a distinct stepwise process with temperature changes. When the temperature is less than 0°C, the water content is between 10% and 15%. When the temperature is higher than 0 °C, the water content jumps sharply to around 40%. The base layer and the subgrade also have similar phenomena, but they are not as obvious as the subbase layer. Moreover, the statistical results show that the water accumulation at the subbase layer of the asphalt pavement is significant and the phase transition is obvious.

Figure 8 Correlation between heat and water under asphalt pavement:

It should be noted that the heat absorption and water sealing characteristics of the asphalt pavement hinder the exchange of hydrothermal energy between the subgrade and the external environment, and the water vapor migration in the subgrade is also a cause of water accumulation in the subbase [35]. This phenomenon has been confirmed in airports and roads in arid and semi-arid regions in the west [36]. In this paper, the regional freeze-thaw cycles are severe, and the alternating hot and cold occur not only in the cold and warm seasons, but also in the temperature difference of about 20 °C (Figure 1). During the daytime, the surface of the asphalt pavement absorbs heat and causes the temperature to rise. The water vapor in the subgrade migrates toward the road surface, and is blocked by the base layer and the surface layer, which gathers at the subbase layer. The accumulation of water vapor increases the vapor pressure and reduces the phase transition temperature of moisture condensation. At night or during rainfall, the surface temperature of the asphalt pavement and the base layer decrease, and the water vapor condenses into condensed water, which causes the water content of the subbase to increase. This phenomenon is also known as the “pot lid effect” [36]. Due to the difficulty of water vapor transmission observation, quantitative data cannot be obtained from on-site observations for the time being, but some studies have initially confirmed its mechanism by means of numerical analysis and laboratory tests [35, 37].

In order to further analyze the relationship between temperature and water under asphalt pavement, the temperature gradient and water flux at the base and subbase layer were calculated. The temperature gradient is calculated by using Eq. (3):

                       (3)

whereis the temperature gradient; z is the depth direction of the subgrade; T_h1 and T_h2 are the ground temperatures at the depths of h₁ an h₂, respectively.

The calculation of water flux is calculated by using Eq. (4) [38, 39]:

                     (4)

where is the water flux at depth z_i and time t_j; θ is the water content; is the upper boundary flux at time t_j. Considering the impermeability of the surface layer, and the lower part of the surface layer is selected as the upper boundary flux,

The central difference is used to discretize Eq. (4), and it can be obtained:

          (5)

where k is the cross-section number from z_i to z_m.

From Eqs. (3) to (5), the temperature gradient and water flux at different positions at different time can be obtained, and the calculation results are shown in Figure 9.

It can be seen from the calculation results in Figure 9 that the change of water flux is mainly concentrated on the thawing period and is in good agreement with the change of temperature gradient. When the temperature gradient is large, the water flux variation in the pavement structure layer is also active. From the time scale, the change of water flux is mainly concentrated in the second half of the thawing period, that is, from mid-July to mid- late-October of every year. And in the second year after the subgrade is built, the water change is significantly more active than that in the first year. It shows that with the increase of operation time, the migration and accumulation of water will gradually increase. In terms of depth, although the temperature gradient of the base layer is larger than that of the subbase layer, but the water flux at the subbase layer is more significant, which indicates that this layer is the main part of water accumulation.

Observation results and mechanism analysis indicate that the temperature change is an important driving force for water migration and accumulation, which also provides a certain idea for the prevention and treatment of freezing-thawing diseases in the highway subgrade of this region. The prevention measures and principles need to start from two aspects, which are temperature and moisture [40-45]. Firstly, the water in the subgrade should be controlled. For example, drainage measures should be taken in the subgrade, and drainage channels should be set in the pavement structure layer, with the adhesive layer, with water stable layer and cushion course should be added, to prevent moisture from being supplied through the upper and lower layers of the water stable layer. Secondly, the insulation measures of the subgrade should be well done, such as adding insulation layer under the pavement, effectively reducing the freezing period time and the temperature gradient. Finally, the diffusion of water vapor inside the subgrade is also an important factor in the enrichment of water near the surface, and necessary measures should be taken to block the water vapor diffusion channel.

Figure 9 Correlation between temperature gradient and water flux under asphalt pavement:

6 Conclusions

1) The temperature of different layers of highway subgrade at in-situ experiment site in the Hulunbuir plateau shows a significant seasonal variation. The complete freezing period of the subgrade is about 150 d, and the freezing-thawing index ratio near the subgrade is more than 1.6 times of that at the natural surface. As the operating time increases, the thawing period of the subgrade gradually increases, indicating that the thermal effect of the asphalt pavement has a significant disturbance on the thermal state of the natural foundation.

2) The water migration in the subgrade is active under the influence of freezing and thawing. Due to the impervious characteristics of the asphalt pavement, the water content at the base layer is the lowest and at the subbase layer is the highest; and the seasonal alternation of water content is obvious with the change of the freeze-thaw cycle.

3) The temperature and water content of the subgrade show a significant linear correlation. The change of water flux is consistent with the change of temperature gradient, and it is active in the second half of the annual thawing period, which indicates that the temperature gradient is an important driving force for water migration. In the asphalt pavement structure layer, the water flux at the base layer changes significantly, further indicating that the layer is the main part of water migration and accumulation. The interaction analysis between water and temperature of asphalt pavement also provides ideas for the prevention and treatment of freezing-thawing diseases of subgrade in similar areas. The thermal insulation of pavement structure and drainage within the layer are effective measures for the aforementioned purpose.

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(Edited by ZHENG Yu-tong)

中文导读

内蒙古深季节冻土区公路水热过程的实体工程试验

摘要：季节冻土区道路工程冻融病害频繁，路基的水热耦合作用是病害发育的控制因素。为揭示该地区公路路基的水热作用过程，分析其影响因素和变化规律，基于内蒙古深季节冻土区室韦—拉布达林公路实体工程3年的观测数据，分析了沥青路面结构层和路基的冻融规律、含水率变化特征以及水热相互作用过程。观测数据表明，该地区公路路基冻融频繁，面层处的最高、最低温度和融化指数、冻结指数均为最大，其他部位的地温随着路基深度的增加有所减小，路基底部附近的融冻指数比约为天然地表的1.6倍以上。路基的完全冻结时间在150 d左右，且随着运营时间的延长，融化期有所延长。路基内含水率表现出显著的季节交替特性，底基层处的含水率最大，暖季时达到40%左右，基层处的含水率最小，在10%以下，路基体内的含水率在20%～25 %。地温与含水率存在显著的线性相关性，水分的波动受温度影响显著。沥青路面的温度梯度和水分通量的统计关系表明，水分通量的变化主要集中在融化期内，且与温度梯度的变化吻合较好，底基层内的水分通量最为活跃，说明温度梯度是水分迁移和聚集的重要驱动力，沥青路面吸热和封水的结构特性是诱发路基下部水分在底基层处聚集的主要原因，提出此类地区道路工程的冻融病害防治需从保温和隔水两方面入手。

关键词：路基工程；水热过程；现场观测；季节冻土区；冻融病害

Foundation item: Project(2018-MSI-018) supported by the Key Science and Technology Project of the Ministry of Transport of China; Project(NJ-2018-28) supported by the Construction Science and Technology of the Department of Transport of Inner Mongolia Autonomous Region of China; Project(2019MS05029) supported by the Natural Science Fund Project of Inner Mongolia Autonomous Region of China; Project(2020MS05077) supported by the Natural Science Fund Project of Inner Mongolia Autonomous Region of China; Project(NJ-2020-05) supported by the Research on Complete Survey Technology of Highway Road Area in High-latitude Permafrost Region, China

Received date: 2020-03-25; Accepted date: 2020-05-10

Corresponding author: ZHAO Xin, PhD Candidate; Tel: +86-471-6536719; E-mail: Zhaoxin2016@yeah.net; ORCID: 0000-0002-4233- 2121; LIU Peng-fei, PhD, Senior Research Engineer; Tel: +49-2418020389; E-mail: liu@isac.rwth-aachen.de; ORCID: 0000-0001-5983-7305