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Affected by global warming, the permafrost in Northeast China (NEC) has been continuously degrading in recent years. Many researchers have focused on the spatial and temporal distribution characteristics of permafrost in NEC, however, few studies have delved into the field scale. In this study, based on the Optimal Parameters-based Geographical Detector (OPGD) model and Receiver Operating Characteristic (ROC) test, the spatial stratified heterogeneity of permafrost distribution and the indicating performance of environmental variables on permafrost in NEC at the field scale were analyzed. Permafrost spatial distribution data were obtained from the Engineering Geological Investigation Reports (EGIR) of six highways located in NEC and a total of 19 environmental variables related to heat transfer, vegetation, soil, topography, moisture, and ecology were selected. The H-factors (variables with the highest contribution in factor detector results and interaction detector results): slope position (γ), surface frost number (SFN), elevation (DEM), topographic diversity (TD), and annual snow cover days (ASCD) were found to be the major contributors to the distribution of permafrost at the field scale. Among them, γ has the highest contribution and is a special explanatory variable for permafrost. In most cases, interaction can improve the impact of variables, especially the interaction between H-factors. The risk of permafrost decreases with the increase of TD, RN, and SBD, and increases with the increase of SFN. The performance of SFN to indicate permafrost distribution was found to be the best among all variables (AUC = 0.7063). There is spatial heterogeneity in the distribution of permafrost on highways in different spatial locations. This study summarized the numerical and spatial location between permafrost and different environmental variables at the field scale, and many results were found to be informative for environmental studies and engineering construction in NEC., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Guo et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The current thermal balance of permafrost in northeastern China has been upset by human engineering construction disturbances and global warming. This has resulted in a rise in ground temperature and a fall in the permafrost table, which has a major impact on the stability, longevity, and operational safety of highway subgrades. To solve the issues above, the ground temperature monitoring data at K60+230 of the Shiwei–Labudalin highway were analyzed, and the numerical simulation of the temperature field change over 15 years was carried out for the ordinary subgrade as well as for sections of block-stone material subgrade with 1 m of straight-filled and different thicknesses of replacement fill (1 m, 2 m, 3 m, 4 m) by applying Comsol Multiphysis software. The results show that the temperature field of the subgrade exhibits significant asymmetry. There are variations in the rate of decline at different sites during the course of the 15 years when compared to where the permafrost table was located at the start of the study. Still, the rate of decline of the permafrost table is decreasing yearly. The straight-filled 1 m block-stone subgrade has a permafrost table 0.77 m higher in the bottom portion of its top surface than the ordinary subgrade. The replacement 1 m, 2 m, 3 m, and 4 m block-stone subgrade has a permafrost table in the lower portion of the top surface that is 1.05 m, 2.12 m, 3.32 m, and 4.75 m higher than the ordinary subgrade. The replacement block-stone subgrades, as opposed to ordinary subgrades, can strengthen the foundation, raise the permafrost table, and effectively reduce the impact of the upper boundary temperature on the lower permafrost. They can also increase the stability of permafrost subgrades. Of them, the block-stone filling with a thickness of 4 m and a particle size of 6–8 cm had the best impact. [ABSTRACT FROM AUTHOR]

Icing in cut slopes is a serious risk to transportation safety in cold regions. Research on the occurrence process and mechanism of icing is a prerequisite for proposing effective management measures. We took the cut slopes of the K162 section of the Beihei Highway as the research object. We used a combination of field investigation, geological exploration, monitoring, and simulation to study and analyze the power source, occurrence process, and triggering mechanism of icing in cut slopes. The results show that the geologic type of this cut slope is a mudstone–sandstone interaction stratum. Abundant shallow groundwater is the source of water for icing. The excavation of cut slopes extends the effect of negative temperatures on groundwater flow during the winter period. The process of ice formation in cut slopes can be described as follows: As the environmental temperature drops, the surface soil begins to freeze, resulting in a gradual narrowing of the water channel; then, the groundwater flow is blocked, so that the internal pressure begins to rise. When the internal pressure of the pressurized groundwater exceeds the strength of the frozen soil, groundwater overflows from the sandstone layer to the surface, forming icing. The high pore water pressure inside the cut slope is the precursor for the occurrence of icing. The dynamic pressure of the pore water pressure is the main driving force for the formation of icing in cut slopes. The obstruction of the water channel due to ground freezing is the triggering condition for ice formation in cut slopes. [ABSTRACT FROM AUTHOR]

The permafrost in Northeast China is experiencing rapid degradation due to the influence of climate change and human activities, profoundly impacting the local ecological environment and engineering construction. Understanding the spatiotemporal dynamics of long-term permafrost in this region is crucial; however, systematic research on this topic remains scarce. This study combines meteorological station data, MODIS land surface temperature (LST) datasets, and borehole locations to apply the surface frost number (SFn) model. This approach enables the simulation and estimation of the spatial distribution and changes in the area of the surface frost number without vegetation effects (SFnv) and permafrost temperature (PT) in Northeast China from 1971 to 2020. The area of the SFnv > 0.49 within the permafrost region decreased substantially from approximately 44.353 × 104 km2 to 19.909 × 104 km2 between 1971 and 2020, with a notable change in 1988. The area of permafrost calculated using PT < 0 was slightly smaller, declining from 39.388 × 104 km2 to 29.852 × 104 km2. There was also a significant increase in the area with PT ranging from −1 °C to 0 °C, indicating a decline in permafrost stability. Approximately 10.926 × 104 km2 of stable permafrost has been transformed into semi-stable and unstable permafrost. Moreover, from 1982 to 2020, the NDVI was negatively correlated with the area of stable permafrost and positively correlated with the area of transitional or unstable permafrost. Vegetation cover decreased as transitional or unstable permafrost degraded. These findings provide valuable information for permafrost research and engineering development in cold regions, as well as for future planning and adaptation strategies. [ABSTRACT FROM AUTHOR]

With the aggravation of climate warming, unstable soil slopes are more and more common in permafrost regions. The long-term monitoring of a slow earthflow (K178 + 530 landslide) in the Xiao Xing'an Mountains permafrost area in Northeast China was carried out. 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 photogrammetry, and high-density resistivity. To analyze the variation laws of pore water pressure and effective stress and their influence on slope deformation, a coupled hydro-thermo-mechanical model was established to reconstruct the deformation process of the slope. The results show that the groundwater recharge from the permafrost degradation and surface infiltration reduces the soil cohesion and internal friction angle near the main scarp and increases the soil gravity, thus providing dynamic and mechanical conditions for slope deformation. The melting of the continuous segregation ice in the active layer and surface infiltration reduces the soil strength of the sliding surface and provides 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, after the melting of the segregation ice, the upper soil slides along the slope under the action of gravity, causing the sliding surface to be parallel to the slope surface. The soil near the main scarp slides downward and accumulates near the toe to form several transverse ridges. The instability of the transverse ridges produces secondary sliding which causes the toe to advance continuously. The numerical simulation results can intuitively reflect the stage deformation characteristics of the slope, pore water pressure changes, and effective stress distributions, which provides a supplement for further understanding the formation mechanism and deformation process of the landslide. [ABSTRACT FROM AUTHOR]

In cold regions, there is a close relationship between vegetation and the underlying permafrost. While the vegetation protects the permafrost, the permafrost also provides the necessary water, nutrients, and physical support for the vegetation. As the most sensitive area of alpine vegetation to environmental changes, alpine timberlines respond rapidly to permafrost degradation. Based on the data from meteorological stations and remote sensing in Mount Wutai, the distribution and change in surface frost numbers under the influence of vegetation and alpine timberlines in Mount Wutai from 2001 to 2021 were produced. The results show that from 2001 to 2021, along with the continuous degradation of permafrost, the alpine timberline showed an upward followed by a slight downward trend. From 2000 to 2014, the alpine timberline consistently moved upward, with the melting of permafrost, which produces water playing a positive role. In 2014–2021, the permafrost near the timberline in the study area disappeared, and the upward trend in the alpine timberline was blocked, even downward in some areas. Analysis of the above phenomena shows that in the process of permafrost degradation, the functions of supporting and fixing roots as well as water storage for overlying forest vegetation by permafrost will be lost sequentially, which will have an impact on the growth of the vegetation and make the upward trend in NDVI in the growing season blocked. The process of permafrost degradation is beneficial to vegetation growth but is unfavorable at the end of the degradation period, which is reflected in the phenomenon of the alpine timberline rising and then falling. [ABSTRACT FROM AUTHOR]

In the permafrost zone, the vegetation growth condition is closely related to the permafrost occurrence state. Changbai Mountain is the highest peak in Northeast China and is also a permafrost distribution area, where the vegetation shows an obvious vertical distribution pattern, and the alpine timberline has a clear boundary. The intersecting zone of alpine timberline is an ecologically fragile area that can be used as an external signal amplifier and is an important site for monitoring climate change. In this study, the surface frost number and alpine timberline in the Changbai Mountain area were analyzed through combining ground and remote-sensing data, using the freezing number model and support vector machine method. The results show that the distribution characteristics of alpine timberline and permafrost at elevation are correlated, there is a response mechanism of alpine timberline to the degradation of permafrost, and the upward migration rate of alpine timberline has increased in the last 20 years. The continuous degradation of permafrost will change the environment of vegetation growth, which, in turn, will affect the global carbon cycle process. Focusing on the state of permafrost will help us to understand climate change in depth, and we can respond to a series of impacts caused by ecological changes in cold regions in advance. [ABSTRACT FROM AUTHOR]

High-resolution permafrost mapping is an important direction in permafrost research. Arxan is a typical area with permafrost degradation and is situated on the southern boundary of the permafrost region in Northeast China. With the help of Google Earth Engine (GEE), the maximum entropy classifier (MaxEnt) is used for permafrost mapping using the land surface temperature (LST) of different seasons, deviation from mean elevation (DEV), solar radiation (SR), normalized difference vegetation index (NDVI), and normalized difference water index (NDWI) as the characteristic variables. The prior data of permafrost distribution were primarily based on 201 borehole data and field investigation data. A permafrost probability (PP) distribution map with a resolution of 30 m was obtained. The receiver operating characteristic (ROC) curve was used to test the distribution results, with an area under the curve (AUC) value of 0.986. The results characterize the distribution of permafrost at a high resolution. Permafrost is mainly distributed in the Greater Khingan Mountains (GKM) in the research area, which run from the northeast to the southwest, followed by low-altitude area in the northwest. According to topographic distribution, permafrost is primarily found on slope surfaces, with minor amounts present in peaks, ridges, and valleys. The employed PP distribution mapping method offers a suggestion for high-resolution permafrost mapping in permafrost degradation areas. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Affected by global warming, permafrost degradation releases a large amount of methane gas, and this part of flammable methane may increase the frequency of wildfires. To study the influence mechanism of methane emission on wildfires in degraded permafrost regions, we selected the northwest section of Xiaoxing'an Mountains in China as the study area, and combined with remote sensing data, we conducted long-term monitoring of atmospheric electric field, temperature, methane concentration, and other observation parameters, and further carried out indoor gas–solid friction tests. The study shows that methane gas (the concentration of methane at the centralized leakage point is higher than 10,000 ppm) in the permafrost degradation area will release rapidly in spring, and friction with soil, surface plant residues, and water vapor will accelerate atmospheric convection and generate electrostatic and atmospheric electrodischarge phenomena on the surface. The electrostatic and atmospheric electrodischarge accumulated on the surface will further ignite the combustibles near the surface, such as methane gas and plant residues. Therefore, the gradual release of methane gas into the air promotes the feedback mechanism of lightning–wildfire–vegetation, and increases the risk of wildfire in degraded permafrost areas through frictional electrification (i.e., electrostatic and atmospheric electrodischarge). [ABSTRACT FROM AUTHOR]

Under the influence of climate change and human activities, the southern boundary of the permafrost region in Northeast China, which is located at the southern edge of the permafrost area of Eurasia, has moved north, the surface temperature has increased, and the thickness of the frozen layer has decreased. At present, there is a lack of classification standards or a map of the thermal state of permafrost that can reflect the dynamic change characteristics of permafrost in Northeast China. A vegetation impact factor consisting of normalized difference vegetation index and forest canopy closure was introduced into MODIS LST products, in order to improve the applicability of products in Northeast China. Based on the improved MODIS LST data, this study analyzed the distribution and change of the mean annual surface temperature and the surface frost number (SFnc) from 2003 to 2019. SFnc was used as the standard to classify the thermal state of permafrost, and a map of the thermal state distribution and changes of permafrost in Northeast China, with a spatial resolution of 1 km, was produced. Compared with the observation data of meteorological stations and field monitoring data, the reliability of classification results was nearly 95%. The map showed that there was no area of extremely stable permafrost (SFnc ≥ 0.667), the area of stable permafrost (0.55 ≤ SFnc< 0.667) changed from 14.9 × 104 km2 to 6.5 × 104 km2, the SFnc reduced from 0.564 to 0.557, the area of semi-stable permafrost (0.51 < SFnc < 0.55) changed from 17.68 × 104 km2 to 17.77 × 104 km2, the SFNc reduced from 0.529 to 0.528, and the area of transitional or unstable permafrost (0.49 ≤ SFnc ≤ 0.51) changed from 8.67 × 104 km2 to 9.56 × 104 km2. The thermal state of permafrost decreased and the distribution of stable permafrost shrank, due to continuous rising air temperature. The overall change characteristics of the thermal state distribution of permafrost were that the southern boundary of the permafrost region moved northward, the regional permafrost thermal state decreased, and there was an increased region of semi-stable, transitional or unstable permafrost from the frozen soil thawed at the edge of the permafrost region with higher stability. The permafrost region in Northeast China has lush vegetation, and the continuous degradation of permafrost will change the vegetation growth environment and affect the global carbon cycle process. This work will provide important data support for climate change feedback, natural disaster process research, and an early warning and prevention of terrestrial ecosystem response in the permafrost region of Eurasia. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We performed a regional geological survey in discontinuous permafrost (PF) area along the Bei'an to Heihe Expressway between September 2009 and October 2016 to investigate the state and change of PF in northeast China. Underground resistivity changes were periodically detected along the foot of the subgrade and soil temperatures were monitored under the road foundation. We combined local meteorological and average annual air temperature data to analyze changes in PF thickness. The climate data show that the average annual temperature has gradually increased in the study area since 1980, rising to 0 °C around 1990, and the frost number has decreased to less than 0.5 since 1988. The soil temperature results show that the PF temperature in this section is higher than − 1 °C and is in a high-temperature PF zone affected by changes in seasonal air temperature. The PF under the left foot of the subgrade (LPF) and under the road central separation zone of the road (CPF) both show a decreased PF table, increased PF base, and severe PF degradation. Owing to different cover thicknesses, as well as differences in heat transfer between frozen and unfrozen soil, the base of LPF degraded faster than its table, to CPF the opposite is true. Underground resistivity measurements to verify the accuracy of the PF degradation results. The ground temperature monitoring data and climate data show good consistency. Comprehensive analysis results show that PF in the study area is in a strong degradation stage and will soon disappear. [ABSTRACT FROM AUTHOR]

With global warming, the carbon pool in the degradation zone of permafrost around the Arctic will gradually be disturbed and may enter the atmosphere in the form of released methane gas, becoming an important factor of environmental change in permafrost areas. We selected the northwestern section of the Xiao Xing'an Mountains in China as the study area, located in the degradation zone on the southern margin of the permafrost region in Eurasia, and set up multiple study monitoring areas equipped with methane concentration sensors, air temperature sensors, pore water pressure sensors and soil temperature sensors for long-term monitoring of data changes using the high-density electrical method, ground penetrating radar and on-site drilling to survey the distribution of frozen soil and geological conditions in the study area, combined with remote sensing images of Sentinel-2 L1C and unmanned aerial vehicle photographs and three-dimensional image reconstruction, analysis of fire activities and related geological environmental factors. The results show that since 2004, the permafrost thickness of the marsh wetland in the study area has gradually reduced and the degradation rate obviously accelerated; the organic matter and methane hydrate (metastable methane hydrate and stable methane hydrate) stored in the permafrost under the marsh wetland are gradually entering the atmosphere in the form of methane gas. Methane emissions show seasonal changes, and the annual methane emissions can be divided into three main stages, including a high-concentration short-term emission stage (March to May), a higher-concentration long-term stable emission stage (June to August) and a higher-concentration short-term emission stage (September to November); there is a certain correlation between the change in atmospheric methane concentration and the change in atmospheric pressure and pore water pressure. From March to May every year (high-concentration short-term emission stage), with snow melting, the air humidity reaches an annual low value, and the surface methane concentration reaches an annual high value. The high concentration of methane gas entering the surface in this stage is expected to increase the risk of wildfire in the permafrost degradation area in two ways (increasing the regional air temperature and self-combustion), which may be an important factor that leads to a seasonal wildfire frequency difference in the permafrost zone of Northeast China and Southeast Siberia, with the peak in spring and autumn and the monthly maximum in spring. The increase in the frequency of wildfires is projected to further generate positive feedback on climate change by affecting soil microorganisms and soil structure. Southeastern Siberia and northeastern China, which are on the southern boundary of the permafrost region of Eurasia, need to be targeted to establish fire warning and management mechanisms to effectively reduce the risk of wildfires. [ABSTRACT FROM AUTHOR]

Under the influence of climate changing, permafrost in Northeast China (NEC) has been consistently degrading in recent years. Numerous scholars have investigated the spatial and temporal distribution patterns of permafrost in the NEC region. However, due to constraints in data availability and methodological approaches, only a limited number of studies have extended their analyses to the field scale. In this study, we established a particle swarm optimization (PSO)-based indicator composition algorithm (PSO-ICA) to obtain an indicator factor, η , that indicates the relative distribution probability of permafrost at the field scale. PSO-ICA screened and combined 12 high-resolution environmental variables to compose η. The spatial distribution data of permafrost with a length of 765.378 km provided by the engineering geological investigation report (EGIR) of six highways were used to train and validate the effectiveness of η in indicating permafrost. At the field scale, η was found to be similar to the surface freezing number (SFN) in its ability to indicate permafrost, with AUC values of 0.7046 and 0.7063 for the two by the ROC test. In addition, η has a good performance in predicting highway distresses in the permafrost region in the absence of survey data. This study also confirmed that the resolution and accuracy of permafrost mapping results can be improved by utilizing η. After downscaling the 1 km resolution SFN to 30 m resolution using η , the R2 of the linear relationship between SFN and permafrost temperatures from 43 monitoring boreholes was improved from 0.7010 to 0.8043. If η can help understand the distribution of permafrost at field scale, many engineering and environmental practices could potentially benefit. [Display omitted] • An indicator named η for permafrost distribution in Northeast China was found. • A PSO-based indicator composition algorithm was established for the indicator. • η was tested to be a good indicator of field-scale permafrost in several highways. • η was found to be able to predict the location of highway distress. • η can enhance the correlation between SFN and permafrost temperature. [ABSTRACT FROM AUTHOR]