A cast-in-place pile foundation, widely utilized in the permafrost regions of the Qinghai-Tibet Plateau, boasts superior load-bearing capacity, effectively mitigating the seasonal freeze-thaw effects. In permafrost regions, substantial pile foundation load-bearing capacity is provided by freezing strength, with the freezing strength determined by the temperature of the surrounding permafrost. In modern times, global warming has been causing permafrost degradation, posing a risk to the safety of existing pile foundations. In order to maintain the stability of these foundations, it is crucial to release excess ground heat, considering the temperature-dependent freezing strength of the ground to pile shaft. Two-phase closed thermosyphons (TPCTs) have demonstrated strong performance in the realm of cooling permafrost engineering. In this study, TPCTs were utilized to mitigate the impact of permafrost degradation by installing them around a concrete pile in order to cool the foundation ground. Following this installation, a model experiment was carried out, which ingeniously focused on analyzing the cooling performance, the process of cold energy dissipation, and the cooling scope of the TPCT pile. The study's findings indicate that the operation time of the TPCT pile accounted for about 50% of the entire freeze-thaw cycle. This device could effectively cool the surrounding foundation soil within a specified area. The TPCT pile exhibited a low temperature advantage of 0.36 degrees C in comparison with the scenario without TPCT in terms of surrounding geotemperature, although it experienced significant cold energy dissipation. The conclusions drawn from this study have significant value for maintaining piles in permafrost regions.

Research in geocryology is currently principally concerned with the effects of climate change on permafrost terrain. The motivations for most of the research are (1) quantification of the anticipated net emissions of CO2 and CH4 from warming and thaw of near-surface permafrost and (2) mitigation of effects on infrastructure of such warming and thaw. Some of the effects, such as increases in ground temperature or active-layer thickness, have been observed for several decades. Landforms that are sensitive to creep deformation are moving more quickly as a result, and Rock Glacier Velocity is now part of the Essential Climate Variable Permafrost of the Global Climate Observing System. Other effects, for example, the occurrence of physical disturbances associated with thawing permafrost, particularly the development of thaw slumps, have noticeably increased since 2010. Still, others, such as erosion of sedimentary permafrost coasts, have accelerated. Geochemical effects in groundwater from trace elements, including contaminants, and those that issue from the release of sediment particles during mass wasting have become evident since 2020. Net release of CO2 and CH4 from thawing permafrost is anticipated within two decades and, worldwide, may reach emissions that are equivalent to a large industrial economy. The most immediate local concerns are for waste disposal pits that were constructed on the premise that permafrost would be an effective and permanent containment medium. This assumption is no longer valid at many contaminated sites. The role of ground ice in conditioning responses to changes in the thermal or hydrological regimes of permafrost has re-emphasized the importance of regional conditions, particularly landscape history, when applying research results to practical problems.

The long-term stability of the cast-in-place footings in permafrost regions has received much attention due to its climate sensitivity. The current research lacks long-term data validation, especially in the context of climate change. Based on the 13-year (2011-2023) temperature and deformation monitoring data from the Qinghai-Tibet Power Transmission Line, this study investigates the characteristics of permafrost variation and its impact on the stability of tower footings under the cooling effect from thermosyphons. The results reveal that the thermosyphons effectively reduce the ground temperature around the footings. After the first freeze-thaw cycle, the soil around the tower footings completed refreezing and maintained a frozen state. In the following 13 years, the ground temperature continued to decrease due to the cooling effect of thermosyphons. The duration notably exceeded the previously predicted 5 years. The temperature reduction at the base of the footings corresponded well with the frost jacking of the tower footings and could be divided into three distinct phases. In phase 1, the ground temperature around the footings rapidly reduced, approaching that of the natural field, while the footings experienced pronounced deformation. In phase 2, the ground temperature decreased at a faster rate, and the deformation rate of the footings slowed down. In phase 3, the frost jacking of the footings gradually retarded with the decrease in base temperature. Additionally, the ground temperature differences of over 1 degrees C were observed among different tower footings, which may lead to the differential deformation among the tower footings. The ground temperature differentiation is attributed to the difference in solar radiation intensity, which is shaded by the tower structure from different directions. This study provides theoretical support and empirical accumulation for the construction and maintenance of tower footings in permafrost regions.

Revegetation is an effective approach for restoring extremely degraded grassland (DG) in the Qinghai-Tibetan Plateau (QTP). However, little is known about its effects on permafrost stability. Our study investigated changes in the characteristics of DG and revegetated grassland (RG) in alpine permafrost regions of the QTP by means of in situ monitoring and sampling. Compared with DG, soil temperature was lower in warm months and slightly higher in cool months both at 2 and 10 cm depths after revegetation, while soil moisture generally decreased. Revegetation advanced the onset and increased the duration of completely frozen stage. The number of freeze-thaw days decreased at 2 cm but increased at 10 cm depth. The freeze-thaw strength weakened at 2 cm depth in spring and autumn, and at 10 cm depth in autumn, but increased at 10 cm depth in spring. The thawing index at the two depths and active layer thickness in RG were also significantly lower than those in DG. Revegetation significantly affected the particle size distribution and stability of soil aggregates by increasing the proportion of large macroaggregates. Thus, revegetation can effectively improve the permafrost stability of degraded grassland in the QTP and enhance the service functions of alpine grassland ecosystems.

As the basic units of soil structure, soil aggregate is essential for maintaining soil stability. Intensified freeze-thaw cycles have deeply affected the size distribution and stability of aggregate under global warming. To date, it is still lacking about the effects of freeze-thaw cycles on aggregate in the permafrost regions of the Qinghai-Tibetan Plateau (QTP). Therefore, we investigated the effects of diurnal and seasonal freeze-thaw processes on soil aggregate. Our results showed that the durations of thawing and freezing periods in the 0-10 cm layer were longer than in the 10-20 cm layer, while the opposite results were observed during completely thawed and frozen periods. Freeze-thaw strength was greater in the 0-10 cm layer than that in the 10-20 cm layer. The diurnal freeze-thaw cycles have no significant effect on the size distribution and stability of aggregate. However, 0.25 mm) and reduced aggregate stability. Our study has scientific guidance for evaluating the effects of freeze-thaw cycles on soil steucture and provides a theoretical basis for further exploration on soil and water conservation in the permafrost regions of the QTP.

With global warming and its amplified effect on the Tibetan Plateau, the permafrost on the Tibetan Plateau has been significantly degraded, manifested by decreased permafrost thickness, increased active layer thickness, thermokarst, and surface subsidence, causing severe damage to infrastructure. To better understand and assess the future stability of the Qinghai-Tibet Railway, we used a laterally coupled version of the one-dimensional CryoGrid3 land surface model to simulate the thermal regimes of the railway subgrade under current climate conditions. By modeling ground subsidence (i.e., by simulating the melting of excess ice) we provide estimates of future subgrade stability under low (Representative Concentration Pathway 2.6 [RCP2.6]) and high (RCP8.5) climate warming scenarios. Our modeled results reveal satisfactory performance with respect to the comparison of measured and modeled ground thermal regimes. Under current climate conditions, we infer that mostly thaw-stable conditions as maximum thaw depths do not reach the embankment base. The sunny side of the embankment (southeast-facing) reveals being more vulnerable to suffering from thaw settlement or thermal erosion than the shady side (northwest-facing). The extent of future railway failure due to thawing permafrost will depend on the magnitude of the warming. For conditions typical of Beiluhe (situated on continuous permafrost in the central Tibetan Plateau), the railway embankment might largely maintain safe operation until the end of the century under a scenario of climate stabilization. In contrast, under strong warming the railway subgrade is likely to destabilize from the 2030s onwards and embankment subsidence is initiated at mid-century through the melting of excess ice.

In recent years, with the global warming, the unfrozen water content of permafrost slope increases year by year. The decrease of slope stability is a great threat to the engineering construction in permafrost area. In this study, the south piedmont slope of Bayan Kara Mountain is taken as the research object. Through the field water and temperature monitoring of different positions and depths of the slope, the seasonal and interannual water change characteristics of the slope were analyzed. Combined with indoor shear strength test, numerical simulation and monitoring data, the moisture, temperature and stability of frozen soil slope in spring thawing period were analyzed. The analysis results show that: Water content and freeze-thaw cycles have great influence on the shear strength parameters at the interface. The slope moisture change in the region is divided into four stages, the water decline stage, the low water content stage, the water rise stage and the high water content stage. The freeze-thaw cycle and precipitation are the main reasons for the water change in each stage. From the middle of May to the middle of June is the high risk period of slope instability. The spring thaw landslide is dominated by shallow surface landslide, and the sliding surface is shallow.

The amount of rainfall varies unevenly in different regions of the Qinghai-Tibet Plateau, with some regions becoming wetter and others drier. Precipitation has an important impact on the process of surface energy balance and the energy-water transfer within soils. To clarify the thermal-moisture dynamics and thermal stability of the active layer in permafrost regions under wet/dry conditions, the verified water-vapour-heat coupling model was used. Changes in the surface energy balance, energy-water transfer within the soil, and thickness of the active layer were quantitatively analyzed. The results demonstrate that rainfall changes significantly affect the Bowen ratio, which in turn affects surface energy exchange. Under wet/dry conditions, there is a positive correlation between rainfall and liquid water flux under the hydraulic gradient; water vapour migration is the main form under the temperature gradient, which indicates that the influence of water vapour migration on thermalmoisture dynamics of the active layer cannot be neglected. Concurrently, regardless of wet or dry conditions, disturbance of the heat transport by conduction caused by rainfall is stronger than that of convection by liquid water. In addition, when rainfall decreases by 1.5 times (212 mm) and increases by 1.5 times (477 mm), the thickness of the active layer increases by 0.12 m and decreases by 0.21 m, respectively. The results show that dry conditions are not conducive to the preservation of frozen soil; however, wet conditions are conducive to the preservation of frozen soil, although there is a threshold value. When this threshold value is exceeded, rainfall is unfavourable for the development of frozen soil.

In this study, the instability of extreme temperatures is defined as the degree of perturbation of the spatial and temporal distribution of extreme temperatures, which is to show the uncertainty of the intensity and occurrence of extreme temperatures in China. Based on identifying the extreme temperatures and by analyzing their variability, we refer to the entropy value in the entropy weight method to study the instability of extreme temperatures. The results show that TXx (annual maximum value of daily maximum temperature) and TNn (annual minimum value of daily minimum temperature) in China increased at 0.18 degrees C/10 year and 0.52 degrees C/10 year, respectively, from 1966 to 2015. The interannual data of TXx' occurrence (CTXx) and TNn' occurrence (CTNn), which are used to identify the timing of extreme temperatures, advance at 0.538 d/10 year and 1.02 d/10 year, respectively. In summary, extreme low-temperature changes are more sensitive to global warming. The results of extreme temperature instability show that the relative instability region of TXx is located in the middle and lower reaches of the Yangtze River basin, and the relative instability region of TNn is concentrated in the Yangtze River, Yellow River, Langtang River source area and parts of Tibet. The relative instability region of CTXx instability is distributed between 105 degrees E and 120 degrees E south of the 30 degrees N latitude line, while the distribution of CTNn instability region is more scattered; the TXx's instability intensity is higher than TNn's, and CTXx's instability intensity is higher than CTNn's. We further investigate the factors affecting extreme climate instability. We also find that the increase in mean temperature and the change in the intensity of the El Nino phenomenon has significant effects on extreme temperature instability.

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 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.