The hydrologic and thermal states of foundation soils have an important influence on subgrade stability in degrading permafrost regions. However, thawing settlement remains a problem in the permafrost regions of Northeast China, because there are few relevant research results in this area, and the foundation deformation mechanism caused by the hydrologic and thermal changes of foundation soils is not clear. Therefore, a subgrade structure with large block stones as embankment filler was proposed to improve the foundation thawing settlement, and it has been successfully implemented in the road project (Yiershi–Chaiqiao section of 308 Provincial Highway) in Inner Mongolia Autonomous Region. To study the action mechanism of this structure on the subgrade deformation, a numerical model of the hydro–thermal–mechanical interaction process in unsaturated frozen soil was established, and the calculation of the water content, temperature, and deformation conditions of the subgrade within 20 years of the highway were carried out. The results show that the embankment block-stone layer can reduce the total heat of the deep foundation, and reduce the migration of unfrozen water from the deep foundation to the active layer. It can also increase the hydrologic and thermal convective flux inside and outside the block-stone layer, thus raising the permafrost table and reducing the subgrade deformation. The block-stone layer thickness has an important influence on the subgrade stability. By comparing the hydrologic, thermal change, and deformation of the block-stone structures with different thicknesses (5.0 m, 4.0 m, 3.5 m, 3.0 m, and 2.0 m), we found that when the thickness of the block-stone layer is 4.0 m, the subgrade stability is the best. In this case, the maximum uneven settlement and the maximum transverse deformation difference of the top surface of the subgrade are +0.521 cm and +0.462 cm, respectively. The subgrade stability is less optimal when the block-stone layer thickness is greater or less than 4.0 m. Thus, there exists an optimal embankment filling height related to the hydrologic and thermal conditions of the foundation soils. This study helps to elucidate the effect of unfrozen water content change on subgrade deformation during permafrost degradation and provides an important reference for solving the problem of thawing settlement of subgrade in permafrost regions.

Affected by global warming, permafrost thawing in Northeast China promotes issues including highway subgrade instability and settlement. The traditional design concept based on protecting permafrost is unsuitable for regional highway construction. Based on the design concept of allowing permafrost thawing and the thermodynamic characteristics of a block–stone layer structure, a new subgrade structure using a large block–stone layer to replace the permafrost layer in a foundation is proposed and has successfully been practiced in the Walagan–Xilinji section of the Beijing–Mohe Highway to reduce subgrade settlement. To compare and study the improvement in the new structure on the subgrade stability, a coupling model of liquid water, vapor, heat and deformation is proposed to simulate the hydrothermal variation and deformation mechanism of different structures within 20 years of highway completion. The results show that the proposed block–stone structure can effectively reduce the permafrost degradation rate and liquid water content in the active layer to improve subgrade deformation. During the freezing period, when the water in the active layer under the subgrade slope and natural ground surface refreezes, two types of freezing forms, scattered ice crystals and continuous ice lenses, will form, which have different retardation coefficients for hydrothermal migration. These forms are discussed separately, and the subgrade deformation is corrected. From 2019 to 2039, the maximum cumulative settlement and the maximum transverse deformation of the replacement block–stone, breccia and gravel subgrades are –0.211 cm and +0.111 cm, –23.467 cm and –1.209 cm, and –33.793 cm and –2.207 cm, respectively. The replacement block–stone subgrade structure can not only reduce the cumulative settlement and frost heave but also reduce the transverse deformation and longitudinal cracks to effectively improve subgrade stability. However, both the vertical deformation and transverse deformation of the other two subgrades are too large, and the embankment fill layer will undergo transverse deformation in the opposite direction, which will cause sliding failure to the subgrades. Therefore, these two subgrade structures cannot be used in permafrost regions. The research results provide a reference for solving the settlement and deformation problems of subgrades in degraded permafrost regions and contribute to the development and application of complex numerical models related to water, heat and deformation in cold regions.

China’s largest high-latitude permafrost distribution zone is in Northeast China. With the intensification of global warming and engineering construction, the carbon stored in permafrost will gradually thaw and be released in the form of methane gas. However, research on the changes in methane concentration and emission sources in this area is still unclear. In this paper, the AIRS (Atmospheric Infrared Sounder) data carried by the Aqua satellite were used to analyze the distribution and change trends in the overall methane concentration in the near-surface troposphere in Northeast China from 2003 to 2022. These data, combined with national meteorological and on-site monitoring data, were used to study the methane emission characteristics and sources in the permafrost area in Northeast China. The results show that the methane concentration in the near-surface troposphere of Northeast China is mainly concentrated in the permafrost area of the Da and Xiao Xing’an Mountains. From 2003 to 2022, the methane concentration in the near-surface troposphere of the permafrost area in Northeast China showed a rapid growth trend, with an average linear trend growth rate of 4.787 ppbv/a. In addition, the methane concentration in the near-surface troposphere of the permafrost area shows a significant bimodal seasonal variation pattern. The first peak appears in summer (June–August), with its maximum value appearing in August, and the second peak appears in winter (December–February), with its maximum value appearing in December. Combined with ground surface methane concentration monitoring, it was found that the maximum annual ground surface methane concentration in degraded permafrost areas occurred in spring, causing the maximum average growth rate in methane concentration, also in spring, in the near-surface troposphere of permafrost areas in Northeast China (with an average value of 6.05 ppbv/a). The growth rate of methane concentration in the southern permafrost degradation zone is higher than that in the northern permafrost stable zone. In addition, with the degradation of permafrost, the geological methane stored deep underground (methane hydrate, coal seam, etc., mainly derived from the accumulation of ancient microbial origin) in the frozen layer will become an important source of near-surface troposphere methane in the permafrost degradation area. Due to the influence of high-permeability channels after permafrost degradation, the release rate of methane gas in spring is faster than predicted, and the growth rate of methane concentration in the near-surface troposphere of permafrost areas can be increased by more than twice. These conclusions can provide a data supplement for the study of the carbon cycle in permafrost areas in Northeast China.

The spatiotemporal changes in surface water area (SWA) in the basins of Northeast China have far-reaching impacts on their economic, agricultural, and social development and ecological sustainability. However, the long-term variation characteristics of water bodies in the Northeast basin and its main driving factors are still unclear. Based on the global surface water dataset, combined with the Meteorological and Vegetation Normalized Index (NDVI) datasets, this study used linear regression and correlation analysis to investigate the temporal and spatial variation characteristics of surface water in Northeast China and its driving factors from 1988 to 2020. The results show that (1) the seasonal surface water area (SSWA) and permanent surface water area (PSWA) in Northeast China increased at the rates of 58.408 km²/ year and 169.897 km²/ year, respectively, from 1988 to 2020. Taking 2000 as the node, PSWA and SSWA showed a trend of first decreasing and then increasing. (2) Changes in surface water types in each basin have significant space–time differences, and the transition between water bodies is dominated by the addition and reduction of seasonal water bodies. PSWA decreased significantly in western basins such as the Ulagai River Basin, the Otindag Desert, and the Liao River Basin, but increased significantly in the Songhua River Basin. (3) The driving forces of surface water change in different basins are different. Temperature and NDVI play a leading role in the change of SWA in the western arid region; permafrost degradation under the condition of air temperature rise is an indispensable factor affecting SWA change in the Argun River Basin; the eastern basin with a larger surface water area responded more strongly to changes in precipitation and evapotranspiration. Land-use conversion and water conservancy project construction were the main reasons for the increase of SWA in the Songhua River Basin under reduced precipitation. This research provides a reference for the in-depth study of the characteristics of surface water resources in Northeast China and has important practical significance for the scientific management of water resources in the basin.

There are few types of research on the occurrence mechanism and deformation characteristics of landslides induced by permafrost degradation. With the aggravation of climate warming, landslides are more and more common in permafrost regions. A slow landslide (the K178 + 530 landslide) in the permafrost region of the Xiao Xing'an Mountains in Northeast China was monitored for a long time. The deformation characteristics and occurrence mechanism of the landslide were studied using field investigation, on-site drilling, sensor monitoring, laboratory test, Google satellite image, unmanned aerial vehicle (UAV) photogrammetry, high-density electrical method, and ground-penetrating radar. In addition, a hydro-thermal-mechanical coupling model of frozen soil under saturated conditions was established to simulate the deformation process, pore water pressure change, and effective stress distribution of the slope, and the simulation results were verified according to the monitored data. The results show that the meltwater recharge caused by permafrost degradation reduced the cohesion and internal friction angle of the soil near the trailing edge of the landslide, thus providing dynamic and mechanical conditions for slope deformation. The melting of the continuous segregation ice in the active layer contributed to the formation of a sliding surface and provided deformation conditions for the start of the landslide. The combination of these two factors finally led to the occurrence of the landslide. According to its deformation mechanism, it can be judged that the landslide is a thrust-type landslide. In addition, the melting of ice lenses in the seasonally frozen layer is the main source of soil strength damage, and the landslide sliding rate reached the maximum when the ice lens melted completely. The K178 + 530 landslide is a typical case of landslides caused by permafrost degradation. This study provides a reference for the identification, early warning, and prevention measures of this type of landslide.