The Xinjiang-Tibet Highway （XTH） is a transportation corridor of manpower and supplies from inland regions to Tibet. The XTH has the highest altitude in China， which also has complex geological conditions and high distress ratio. The pavement distress was investigated completely to analyze the spatial distribution of pavement distress along the XTH and reveal the relationship among different types of distress. The distress ratio of 7 types of pavement distress was obtained and the relationship of pavement distress and ground surface temperature， altitude and vegetation cover was quantitatively fitted along the XTH. The result shows that， distress occurrence ratio of racking is has the largest percentage， which is the main reason to reduce the road condition. Pavement distress has a positive correlation and interaction with each other. One type of distress appears with the formation and development of other types of distress. The pavement cracking of the roadway with high altitude was obviously influenced by solar radiation and freeze-thaw cycle， which has a larger occurrence scale and development ratio than other regions. This paper reveals the relationship of each pavement distress and environmental factors， and provides a scientific basis and theoretical reference for the operation， maintenance and distress treatment of the XTH and planning Xingjiang-Tibet transportation corridor.
Snow avalanches， which are widely and frequently developed at high elevations， seriously threatens the built traffic corridors in the Tibetan Plateau. Susceptibility evaluation of snow avalanche via machine learning model with a high forecast accuracy can be appled to quickly and effectively assess the regional avalanche risk. This paper took the central Shaluli Mountain region as the study area， in which the snow avalanche inventory was established through remote sensing interpretation and field investigation verification. We quantitatively extracted 17 evaluation factors via GIS-based analysis， and these factors were selected through the variance expansion factor （VIF）. Four machine learning models containing SVM， DT， MLP and KNN were used to compile the susceptibility index map of snow avalanches， and kappa coefficient and ROC curve were used to verify the accuracy. The results suggested that the susceptibility indexes obtained from SVM， DT， MLP and KNN were in the range of ［0，0.964］， ［0，815］， ［0，0.995］ and ［0，1］， respectively. The accuracy test results show that these four models all have good prediction accuracy. Among them， the SVM model is the best. The results also indicated that the areas with the high snow avalanche susceptibility mainly distributed in Genie Mountain and Rigong Mountain， most of which were above the planation surface of the Tibetan Plateau. The average altitude of the extremely high snow-avalanche-prone areas is 4 939 m， while the average altitude of the high snow avalanche-prone areas is 4 859 m. The snow avalanche has low perniciousness on the Sichuan-Tibet Highway and the Sichuan-Tibet Railway in the study area. This study can provide theoretical basis and method reference for disaster prevention and mitigation of snow avalanche along Sichuan-Tibet Railway and other major projects across Shaluli Mountains region.
The orthoimage and Digital Surface Model （DSM）， acquired by the unmanned aerial vehicle （UAV） with high resolution sensors， could reflect the fine terrain accurately. In this study， the terminus of Yanong Glacier in southeastern Tibetan Plateau was chose as the study area. The orthoimage and the DSM with the resolution of 1.5 cm were acquired by the UAV of M300 RTK with M6P camera. Compared to Planet image and TanDEM， the orthoimage and the DSM present the surface feature and fine terrain in glacier area accurately. Based on the methods of slope threshold and Red band threshold， the special morphologies of glacier surface， such as ice cliff and glacial lake， were extracted automatically， and the uncertainties of extraction were lower than 10%. Hence， the fine terrain in glacier area， acquired by UAV with high resolution sensors， has great potential in the future glacier research.
Glacier surface albedo impacts the energy budget on the glacier surface， the variability of which strongly affects glacier melting and mass balance. Therefore， it is crucial to investigate the spatial and temporal variations of glacier surface albedo. Taking this into account， the spatial and temporal variations of albedo and the relationship between glacier surface albedo and mass balance were investigated using Landsat images， MODIS daily albedo products and the in situ measured albedo from Automated Weather Station installed on Muz Taw Glacier， Sawir Mountains during 2011 to 2021. The results indicated that the surface albedo derived from Landsat and MOD10A1 were consistent with the measured albedo for the same period with the coefficient of determination reaching to 0.95 （P<0.05） and 0.62 （P<0.01）， respectively. The significant spatial and temporal characteristics of albedo on Muz Taw Glacier were existed. On the spatial scale， the surface albedo increased with the altitude rising along the glacier central flowline. However， the albedo decreased with the altitude rising at an altitude of more than 3 600 m a.s.l. due to the topographic differences. At the same altitude， the albedo gradually increased from the glacier boundary to the middle part. During 2011 to 2021， the average surface albedo increased at the lower rate of 0.0024 a-1 in the ablation period from May to August， and the annual average increase rate was 0.0017 a-1 in the same period. The monthly average albedo changed obviously in the different seasons. It was lower from June to August with an average value of 0.330 and higher from December to February in the next year with an average value of 0.586. The albedo in the ablation zone decreased faster than that in the accumulation zone during the ablation period. The study further confirmed that there was a significant positive correlation between the average surface albedo in summer from June to August and the annual glacier mass balance with the coefficient of determination reaching to 0.84 （P<0.01）. Air temperature， solid precipitation， cloud amount， solar incident angle and light absorbing impurities were important factors affecting the albedo variation. This study will provide basic support for studying glacier ablation process and mechanism glacier energy and mass balance simulation， etc.
Glacier flow is a key factor in understanding the nature of glaciers， and it is also one of the main research contents of glacier dynamics， which can provide basic support for rational utilization of glacier resources and early warning of glacier disasters. There are many mountain glaciers located in the west of China. The study on the spatiotemporal variation of surface velocity of glaciers also has great significance for the social and economic development of the western China. Koxkar Baxi Glacier， locates on the southern slopes of the Tomur Peak， is a typical dendritic glacier. In order to obtain the conditions of Koxkar Baxi Glacier’ flow rates and its variation to further reveal the future of the variation of glacier， the spatiotemporal variability of glacier velocity was surveyed using correlation （COSI-Corr） method on Landsat imagery from 2014 to 2020. The results show that： （1） The average annual flow velocity of the Koxkar Baxi Glacier was 0.04~0.05 m·d-1 during 2014 to 2020. （2） The glacier reaches its maximum flow velocity near the center part， and the velocity decreased towards both lateral margins. In a longitudinal profile， ice flow velocity in the accumulation area increased down to the equilibrium line， while decreased towards the glacier terminal. The maximum velocity with 0.17~0.20 m·d-1 was found near the equilibrium line altitude. （3） The glacier flow velocity in warm seasons were 16.67% faster than that in cold seasons. （4） The glacier flow velocity from 2014 to 2020 showed a slight decreasing trend， and the average flow velocity decreased 0.01 m·d-1. （5） Temperature and precipitation had certain influence on the seasonal fluctuation and interannual variation of the flow velocity of the glacier.
Snow cover is of great hydrological， ecological， and climatic significance in the Tibetan Plateau. MODIS snow products are widely used at present but are seriously affected by clouds. Scholars at home and abroad have developed a variety of cloud removal products for raw MODIS daily snow products， but the accuracy of these products in the Tibetan Plateau has not been evaluated comprehensively. Therefore， this paper uses Landsat-8 data with high resolution as the reference value to conduct systematic verification of three datasets of cloud-free snow products released on a daily basis. The results show that compared with the two sets of products （M*D10A1GL06 and MODIS_Dysno_Cloudfree）， which are produced based on raw MODIS daily snow cover product realized by NSIDC （National Snow and Ice Data Center）， the MODIS CGF SCE product produced based on MODIS surface reflectance data， has a great advantage in snow identification accuracy. The MODIS CGF SCE product optimized the NDSI threshold for different land cover types. Although the accuracy of snow identification was significantly improved， the problem of large snow identification error in forest areas was still not effectively resolved， and there was a high underestimate error.
Extremely cold weather has an important influence on winter production and life in the Greater Khingan Mountains region. This paper uses the daily minimum temperature data of ground observation stations during extreme cold weather from 1974 to 2021 in the Greater Khingan Mountains region， monthly circulation index data， the spatial distribution and temporal variation characteristics of extreme cold days and extreme minimum temperature were analyzed by climate statistical method；The abrupt changes and periods of extreme cold days and extreme minimum temperature were tested by Mann-Kendall method and Morlet wavelet analysis；calculating the recurrence period of extreme minimum temperature by empirical frequency method；correlation method was used to analyze the circulation factors which had significant influence on the number of extremely cold days.The results are followed： （1） The spatial distribution of extreme cold days in the Greater Khingan Mountains region was not uniform，and gradually decreasing from northwest to south. The extreme cold days was at most 717 d in Huzhong，and at least 29 d in Gagadaki，the extreme cold days in the whole region mutated in 1979，and the average annual extreme cold days decreased 14.2 d after the mutation compared with that before the mutation，and the annual extremely cold days have a significant cycle of 2 to 4 years.（2） The extreme minimum temperature in the whole region mutated in 1990，before the mutation the extreme minimum temperature was low and after the mutation began to rise，the significant cycle of annual extreme minimum temperature was 4 to 5 years，the extreme lowest temperature was -49.6 ℃ in Mohe， followed by -49.2 ℃ in Huzhong；the extreme lowest temperature occurs once every 2 years，once every 5 years and once every 10 years in Huzhong，while the extreme lowest temperature occurs once in 20 years，once in 50 years and once in 100 years in Mohe.（3） SCAND teleconnection patterm has a good correlation with extreme cold days in winter （January，February and December） in the Greater Khingan Mountains region.Positive growth of the circulation mode，it has great influence on the extreme cold weather in winter in the Greater Khingan Mountains region.
Based on the daily minimum temperature data in China from 1961 to 2018， using n-order polynomial fitting， sliding t-test， empirical Orthogonal Function analysis， Morlet wavelet transform and other methods， this paper reveals the temporal and spatial distribution characteristics of the number of cold surge （CS） day， CS frequency and CS intensity by time and region. The results are as follows. On a national level， the number of CS day， CS frequency， and CS intensity decreased from 1962 to 2000. Specifically， the trend of every element of CS has changed from a previous decrease trend to an increase trend， and the inflection point was around 2000. The CS activity occurred more frequently， became more strengthen， and last for longer time after 2000. There are obvious spatial differences in the number of CS days， CS frequency， CS intensity and their changing trends in China. The number of CS days and CS frequency reach the maximum value in northeast China and northern Inner Mongolia. Meanwhile， the CS intensity value is low in the southeast China and high in the northwest China， with the exception of southern Xinjiang. The changes in the number of CS day and the CS frequency are mainly manifested as the “Northeast， Southwest Reverse Pattern”， the CS intensity is mainly manifested as the “Uniform Change Pattern”. The number of CS day， CS frequency， and CS intensity did not change significantly after， but a significant transition point was detected in 1980. On a regional scale， from 1962 to 2018， the number of CS day， frequency and intensity of cold surge in all regions showed a decreasing trend， while they increased after 2000. In the northern and northeastern of China， mean of three elements of cold surge increased after 2000. The transition year was different in different regions. The transition year of three elements of cold surge in Northeast China were the earliest. Mean value of the number of CS day， CS frequency， and CS intensity have an oscillation period of 3~5 years in total China and all regions. In addition， the periodic oscillations of the number of CS day and CS frequency in all regions are basically the consistent.
Based on the data observed from the meteorology gradient tower， eddy covariance system and the active layer from 2005 to 2016 at the Tanggula （TGL） and Xidatan （XDT） sites in the permafrost region of the Qinghai-Tibet Plateau， the long-term variations of surface energy fluxes and its impact on the active layer under the background of climate change were analyzed by using the eddy covariance method， the gradient method and the Simultaneous Heat and Water （SHAW） model. The results were as follows： From 2005 to 2016， the air temperature and the air-ground temperature difference increased， while the annual precipitation， the 10 cm soil moisture and the wind speed decreased at the Tanggula and Xidatan sites. The net radiation and sensible heat flux showed an increasing trend， while the latent heat flux showed a decreasing trend， the surface soil heat flux had no obvious change. The seasonal variations of surface energy fluxes were obvious in Tanggula and Xidatan， but there are regional differences in energy fluxes due to the influence of altitude， latitude， aspect， surface freezing-thawing process， precipitation and underlying surface conditions. In Tanggula and Xidatan， the surface freezing indices were negatively correlated with soil heat flux， while the thawing indices and the active layer thickness were positively correlated with soil heat flux. The surface soil heat flux accumulation was linearly increasing with the thawing depth.
Using the long-term ground temperature monitoring data of the permafrost zone along the Qinghai-Tibet Railway from 2006 to 2020， three types of typical roadbed structures were analyzed. Traditional embankment （TE）， U-shaped crushed rock embankment （UCRE） and crushed rock revetment embankment （CRRE） were included the three types of typical roadbed， which were selected to the long-term monitoring sections within the warm permafrost zones. The evolution of ground temperature field， mean annual ground temperature （MAGT） and annual maximum ground temperature （AMGT） in the depth range of 20 m under the embankment were analyzed and studied since 15 years of operation. The monitoring and analysis results show that： the growth rate of MAGT under the left and right shoulders of the TE is always higher than that of the same depth in the natural site. The MAGT under the UCRE is always lower than the natural site and always maintains a certain difference， whereas， the difference in ground temperature under the left and right shoulders is also not negligible. The MAGT of the left shoulder in the CRRE is not much different from that of the natural hole， while the MAGT of the right shoulder is always lower than that of the natural hole， and the differ in ground temperature between the left and right shoulders is smaller than that of the UCRE. The artificial permafrost table （APT） under the TE is always lower than that of in the natural site. Both the UCRE and CRRE， the APT in the left and right shoulders of them has been elevated into the embankment， and the differ of APT between the left and right shoulders is about 1.0~1.5 m. the differ of APT between the left and right shoulders in the CRRE is slightly lower than that of UCRE. Overall， because of the influence of thermal disturbance about engineering and climate warming， the TE in the warm permafrost zones cannot keep the thermal stability of permafrost under the embankment. Some active-cooling and reinforcement measures need to be taken. Both of the UCRE and CRRE， have a certain active-cooling effect on the permafrost under embankment， but the differ in ground temperature between the left and right shoulders still needs to be taken seriously.
In order to study the effect of freeze-thaw cycles on the type I fracture toughness of granite under ultra-low temperature conditions， semi-circular bending （SCB） specimens were used in this study， and different freeze-thaw times （1， 2 and 3 times） were selected. The granite in the natural state was treated with -160 ℃ ultra-low temperature freeze-thaw cycles， and the three-point bending test was carried out on the granite after the freeze-thaw cycle. and microstructure effects. The results show that with the increase of freeze-thaw cycles， the localized damage of I-type crack tip of granite is intensified， the fracture toughness is decreased， the number of microcracks and pores in the rock is increased， the length of cracks is increased， and the pore size is increased. Finally， the changes of rock frost heaving force and fracture toughness under low temperature and ultra-low temperature conditions are compared and analyzed. Compared with low temperature conditions， the frost heaving force produced by ultra-low temperature freezing and thawing is larger. When fracture toughness decreases by approximately the same amplitude， rocks need more cycles of freezing and thawing at low temperature. The research results can provide theoretical reference for underground storage of liquefied natural gas （LNG） in ultra-low temperature environment.
A comprehensive grasp of the research status of tensile strength of frozen soil is the basis for further research. Firstly， the typical methods that can be used to test the tensile strength of frozen soil are introduced， and the test conditions， sample forms and stress mechanism of different test methods are described in detail. The advantages and disadvantages of typical tensile strength test methods are compared and listed. Secondly， the research work and shortcomings based on different test methods are summarized. Then， the latest research progress of the influence of temperature， water content， loading （deformation） rate， soil quality and sample size on the change law of frozen soil tensile strength is comprehensively analyzed. Finally， it is proposed to develop and improve the research method and system of frozen soil tensile strength， and increase the testing research of warm frozen soil tensile strength， so as to obtain the prospect of more accurately simulating the tensile failure behavior of frozen soil. It is pointed out that the internal cause of the formation of the tensile strength and the tensile failure mechanism of frozen soil should be thoroughly revealed by combining the research methods of microstructure and digital image technology of frozen soil. Based on the multi-factor test， a more perfect prediction method of frozen soil tensile strength is explored. Meanwhile， expand the in-situ test research on the tensile strength of frozen soil， and strengthen the parallel research ideas of indoor and outdoor double tracks. Through the analysis of the research status and development trend at home and abroad， it provides reference and guidance for the experimental study of frozen soil tensile strength， the improvement of theoretical model of frost heave， geotechnical engineering design in cold regions and artificial freezing reinforcement engineering.
Seasonally frozen soils are widely distributed in China in terms of area， and the freeze-thaw cycle effect generated by the alternation of cold and warmth is one of the causes of engineering damage in cold areas during construction， and it is particularly important to restore the nature and state of the soil when it is subjected to freeze-thaw action. Therefore， sandy soil specimens with different numbers of freeze-thaw cycles were prepared， and the long-term strength of frozen sandy soil was tested using a spherical template indenter. Using fractal theory and the microstructure image processing software ImageJ， the change law of grain group and long-term strength of two frozen sandy soils under different numbers of freeze-thaw cycles were studied. The results show that： for fine sand （FS）， the fractal dimension DB has a highly significant positive correlation with the long-term strength variation， among which ≥0.15~0.20 mm and ≥0.25~0.40 mm have the best fit with the long-term strength， and are the dominant grain classes of FS. For medium sand （MS）， the fractal dimension DB is slightly positively correlated with the long-term strength， and the variation shows a “vertical N” trend， in which the grain size content of ≥0.30~0.40 mm and ≥0.40 mm fits better with the long-term strength， and is the dominant grain class for MS. The content of other grain groups did not correlate significantly with the long-term strength change. The freeze-thaw action changed the content ratios of coarse and fine grain agglomerates in the soil. With the increase of the overall particle size interval， the dominant particle size also increases， which shows that the long-term strength of frozen sandy soil tends to decrease and then increase with the increase of the content of some particle sizes. The results of the study can provide theoretical reference for the determination of long-term strength in areas subject to freeze-thaw action.
It is an economical solution to replace slag using lime mortars as subgrade filler. However， lime mortars has low initial strength， slow hardening rate， and long carbonization time， which is not conducive to rapid construction. In this paper， the pozzolanic reaction between metakaolin and lime is used to improve lime mortars. Through uniaxial compression testing and triaxial compression testing， the mechanical parameters of improved lime mortars under freeze-thaw cycles is analyzed. The surface porosity of improved lime mortars image is extracted by image processing technology， and the relationship between porosity and strength is established. The mechanism of improved lime mortars in metakaolin is analyzed by studying the age， lime contents and moisture contents. The results show that： metakaolin can effectively increase the reaction rate and mechanical properties of lime mortars； metakaolin can restore the loss of mechanical properties of lime mortars caused by freeze-thaw cycle to a certain extent， and reduce the pores caused by ice crystals； after reaching the optimum strength， the strength of lime mortars will decrease with the increase of lime， while it will continue to increase after being improved by metakaolin； the pozzolanic reaction consumes more water than the carbonation process of lime mortars， and effectively improves the frost resistance of lime mortars.
Pile foundation is one of the most commonly used and suitable foundations to support transmission line structure， especially in seasonally frozen soil regions and permafrost regions. Axial compression is the controlling condition in the design of foundations for such structures as bridges and buildings， while uplift and overturning will control the design of transmission line structure foundations. This paper presents an extensive overview of previous studies including experimental （e.g.， laboratory model test and full-scale field load test）， analytical/theoretical （e.g.， limit equilibrium and limit analysis based on plasticity） and numerical （e.g.， finite difference and finite element methods）. The review indicates that study on the uplift behavior of pile foundation in frozen soil is relatively limited， particularly in the case of combined effect of axial uplift and lateral loading. Interaction between pile and frozen soil and mechanism of load transfer along the pile shaft and around the pile tip still remain unclear. Therefore， this paper implements finite difference analysis within FLAC3D to investigate the behavior of pile foundation in frozen silty clay and gravelly sand under axial uplift behavior and the effect of ground condition and lateral loading on the uplift behavior. Because of the axisymmetric condition of the problem studied， only half of the model is simulated. The chosen domain of the medium is discretized into a set of quadrilateral elements and the pile is discretized by the cylinder element. The interaction between the soil and pile is considered according to interface elements. Mohr-Coulomb criterion is adopted to model the soil behavior （perfectly elastic-plastic）， while the pile is simply considered as a rigid body. The soil parameters such as Young’s modulus， cohesion and internal friction angle used for numerical analyses are determined by laboratory tests and estimated according to the empirical correlations with in-situ tests. The present numerical modeling is verified with the results from field loading tests on pile foundations in Qinghai-Tibet ±550 kV transmission line project. On this basis， parametric studies are carried out to uncover the behavior of pile in frozen soil. It is observed that pullout is the dominant failure mechanism of pile and the uplift load-displacement curve clearly exhibits an asymptote， consisting of initially linear elastic， nonlinear transition， and finally linear regions. These results are consistent with the observations in a few previous studies. In addition， larger uplift capacity of pile foundation in freezing period and gravelly sand is gained （about 20%）. Lateral loading increases the deflection and therefore， decreases the uplift capacity of pile foundation. For the convenience of using the results obtained in practice， the values of uplift factor for pile foundation in silty clay and gravelly sand are provided. Finally， it should be noted that the method used， and the results obtained in the current work could be useful for engineers and designers， at least providing them some qualitative evidence for pile design in seasonally frozen soil regions and permafrost regions. This is important and necessary to ensure the safety of construction in such regions. Meanwhile， numerical analyses in the current work can be a benchmark example for subsequent research studies.
At present， artificial freezing method has become one of the effective methods for coal mine shaft to pass through water-rich soft rock strata， which can stop the movement of groundwater and limit the deformation of surrounding rock. In order to study the frost heaving characteristics of sandstone under different freezing conditions， frost heaving tests of saturated and dry Cretaceous red sandstone samples under different freezing rates （10 ℃·h-1， 5 ℃·h-1， 2 ℃·h-1， 1 ℃·h-1） and different confining pressures （5 MPa， 10 MPa， 15 MPa， 20 MPa， 25 MPa） were carried out by using GCTS （Geotechnical Consulting & Testing Systems） servo-controlled low temperature and high pressure triaxial rock testing system. In this paper， based on the existing theory of physical and mechanical properties of frozen soil， we studied the frost heaving law of sandstone under different freezing conditions and explored the frost heaving mechanism. The result shows that in the process of cooling， the dry rock sample always produce cold shrinkage deformation， while the saturated rock sample first produce cold shrinkage deformation， then produce frost deformation， and finally the deformation tends to be stable. The deformation of saturated rock samples is much larger than that of dry rock samples. The larger the stress level of rock samples at the same temperature is， the smaller the frost deformation is， which shows a linear negative correlation， mainly because the high confining pressure limits the volume expansion of the water phase in the pore inside the rock samples when it becomes ice. The frost deformation of rock samples is mainly affected by confining pressure and water content， while the frost heaving rate is mainly affected by cooling rate. Under this test condition， the higher the cooling rate of sandstone is， the higher the frost heaving rate is， and the relationship between them is approximately linear. For saturated rock samples， the confining pressure reduces the rock frost heaving by limiting the expansion during the phase transformation of ice water， and the temperature affects the rock frost heaving by affecting the freezing rate of pore water and the thermal expansion and cold contraction of rock skeleton. For dry rock samples， the deformation is mainly due to the volume contraction of rock mineral particles caused by thermal expansion and cold contraction effect， and the greater the temperature change， the greater the deformation. Based on the experimental results and theoretical analysis method， a calculation formula of rock frost heaving considering the influence of confining pressure was established. By calculating the frost heave of sandstone samples under different confining pressures， it is found that the calculated values are in good agreement with the experimental results. Moreover， according to the calculation formula of frost heaving， the influence factors of rock frost heaving during freezing can be divided into two categories： internal cause and external cause. The internal cause includes porosity， saturation， volume modulus of ice and rock skeleton， and the external cause includes temperature and confining pressure. For saturated rock， the frost heaving is mainly affected by factors such as confining pressure， temperature and porosity. When the saturation， porosity and freezing rate are low， the rock may only produce shrinkage deformation， because these indicators determine whether the rock produces frost heave or freeze shrinkage. The mechanism of rock frost heaving is very complicated due to the interaction and restriction between the internal and external factors and the dynamic changes of rock microstructure and mechanical properties during the process of frost heaving. The research results can provide theoretical reference for freezing construction scheme design of deep coal seam mine construction， and also provide a theoretical basis for the study of physical and mechanical properties and engineering application of soft rock in frozen soil area.
Calcium lignosulfonate， as an environmentally-friendly improved material， has been applied to the field of soil reinforcement in recent years. In order to explore the solidification effect of calcium lignosulfonate on loess， indoor tests such as confined immersion compression test， freeze-thaw cycle test， unconsolidated undrained triaxial shear test， scanning electron microscope test and X-ray diffraction test were carried out to analyze the effects of freeze-thaw cycle times， content and confining pressure on the mechanical properties and microscopic mechanism of calcium lignosulfonate improved loess. The results show that： adding calcium lignosulfonate can effectively eliminate the collapsibility of loess. In addition， with the increase of calcium lignosulfonate content， the stress-strain curve of improved loess changes to general hardening type， while with the increase of freeze-thaw cycles， the stress-strain curve changes to weak hardening type. The failure strength of improved loess increases first and then decreases with the increase of calcium lignosulfonate content， and the failure strength is the highest when the calcium lignosulfonate content is 1%. With the increase of freeze-thaw cycles， the failure strength of improved loess decreases first and then tends to be stable， while the shear strength index shows a downward trend. Through scanning electron microscope test and X-ray diffraction test， it is found that cement is formed in the improved loess and fills the pores among soil particles， which improves the compactness of the soil and the strength of the soil. However， the freezing and thawing effect causes the contact mode of soil particles to change from surface-to-surface contact to point-to-point contact and point-to-surface contact. In addition， no new mineral components were found in the loess improved by calcium lignosulfonate， and the freezing and thawing did not cause the dissolution of mineral components in the soil.
Natural damage such as fissures and pores make the rock microstructure show strong heterogeneity， which influences the failure process and mode. In this paper， the numerical test of freeze-thaw sandstone splitting failure with natural damage was carried out based on CT non-destructive identification technology， combined with digital image processing technology and CASRock numerical simulation software. The analysis of splitting failure mode， deformation localization and crack evolution process of freeze-thaw sandstone with natural damage reveals the failure mechanism of sandstone with natural damage under freeze-thaw and load. The results show that the expansion of primary pores （cracks） and the formation of new pores in sandstone are the main forms of freeze-thaw rock failure evolution. The failure of rock containing natural damage under freeze-thaw and load is related to the degree and distribution of natural damage. The generation of secondary cracks mostly occurs in natural damage-intensive areas. During the loading process， the stress in the localized damage zone is far greater than sandstone’s overall stress， and rock’s failure in the localized damage zone is synchronized with the energy release and stress release in the region. Localized damage reflects the evolution of cracks in rocks and helps to predict the direction of sandstone crack development. The failure mode of rock is related to the number of freeze-thaw cycles. The freeze-thaw cycles make the sandstone with natural damage gradually change from brittle failure to ductile failure， and the change of the overall strength of the rock is a gradual deterioration process.
Snowdrift is one of the most typical snow disaster forms of road traffic in Xinjiang area. Its impact on road traffic is mainly reflected in reducing driving visibility and causing traffic interruption induced by a large amount of snow on the road surface. The snowdrift disaster along the road in Xinjiang area shows the characteristics as many points， long lines and wide spread occurring with uneven distribution in time and space. Snowdrift question involves wind field， snow distribution field， temperature field and humidity field coupling， which is a very complex scientific problem. At the same time， the prevention and control of snowdrift disasters is also a practical problem that needs to be solved urgently in the construction and operation of road traffic infrastructure in cold areas. For expressways in areas of frequent snowdrift occurring， its snowdrift disaster caused by the w-beam barrier is becoming more and more obviously. But there is still a lack of in-depth research on the w-beam barrier， which is one kind of auxiliary facilities of the highway deepening the snowdrift disaster. In view of the status mentioned above and in order to explore the causes and prevention methods to snowdrift disaster by w-beam barrier， on the background of Beijing-Urumqi Expressway （G7） engineering， the field model tests of the w-beam barrier， the cable barrier and the no barrier embankment were designed. The study also combined with numerical simulation and highway field investigation and other methods in order to verify each other. In the meantime， the influence of the w-beam barrier and cable barrier on the road snowdrift disaster of the highway is studied， and the distribution law of wind field and snow field on the embankment surface with w-beam barrier and cable barrier is obtained. The results show that the snowdrift velocity increases from the foot of the embankment on the upwind side till to the w-beam barrier installed at the shoulder of or the central divider of the road. Then it moves to the road surface through the gap in the lower part of the w-beam barrier. At this time the w-beam barrier disturbs the wind field near the ground， causing a weak wind area to appear behind the beam， resulting in a large number of snow particles deposited. Especially when the lower gap of the w-beam barrier is filled with snow， the road area behind the w-beam barrier covers the driving lane with snow， which seriously affects the road capacity. While the section without setting up a barrier or setting a cable barrier is conducive to the passage of snowdrift flow. Considering the uncertainty of future climate change， in order to better prevent the impact of snowdrift on traffic operation safety， it is proposed to replace the w-beam barrier in some areas with serious wind and snow disasters with cable barrier that meet the safety requirements of expressways. Moreover， other snowdrift protection measures such as snow fences， snow barriers and snow walls can also be adopted. The research results can provide useful reference for snow removal of the expressway and new expressway construction in the windy and snow areas of Xinjiang.
Under the background of climate warming， the occurrence time， frequency， intensity， and impact of snowmelt flood disasters have changed significantly. Thus， establishing a global snowmelt flood disaster database is particularly important for disaster risk management. With the help of a web crawler， and based on multiple data sources such as natural disaster databases， documents， books， government agency websites， and news media， this study collected relevant information of snowmelt floods and mixed floods and established standards for identifying snowmelt flood events and their disaster impacts based on data from the different sources. Following the screening， sorting， fusion， and integration of snowmelt flood events， a global snowmelt flood disaster dataset containing 579 pieces of data with strong pertinence and reliability was constructed. The temporal and spatial distribution characteristics of global snowmelt flood disasters from 1900 to 2020 were preliminarily analyzed. The results showed that the snowmelt floods were mainly distributed between 30° N and 60° N， with more mixed floods south of 50° N and more snowmelt floods north of 50° N. Spring was the period of highest incidence of snowmelt flood disasters， followed by winter， summer， and autumn， respectively. The snowmelt floods that occurred in spring， autumn， and winter were mainly at 40°~50° N， and the snowmelt floods that occurred in summer were mainly at 30°~40° N. Compared with the snowmelt floods， the mixed floods were more frequent and more destructive， and their frequency increased with climate warming. The results provide a scientific basis for risk prevention and loss assessment of global snowmelt flood disasters.
The groundwater system in the middle and lower reaches of Heihe River basin is replenished by the upper cryosphere meltwater and precipitation. The stability of the groundwater system in the middle and lower reaches of Heihe River basin faces more risks due to the cryosphere shrinkage caused by climate warming. Groundwater models are efficient tools for researchers in accessing the stability of groundwater system. However， the uncertainty of groundwater model parameters is an important issue. In this paper， an uncertainty analysis method based on data assimilation algorithm which reduces parameters uncertainty by including observations is proposed. The proposed method is then used to analyze the uncertainty of 13 parameters in a groundwater model of the middle reaches of Heihe River Basin （which is constructed based on MODFLOW）. The influence and optimal values of hyper-parameters are discussed. The uncertainty of groundwater model parameters is analyzed using the proposed algorithm. The results show that the data assimilation algorithm can effectively reduce the uncertainty of groundwater model parameters. The diversity and quantity of observations play an important role in reducing the uncertainties. The parameters in subzones with frequent interaction between surface water and groundwater are more uncertain. The hydraulic conductivity， specific yield and irrigation backflow coefficient have significant influence on the groundwater level， while streambed hydraulic conductivity shows great influence on the streamflow. This study will provide a more reliable modelling method for groundwater research and an important support for researching the stability of groundwater system in the northwestern area.
Beiluhe basin lies in a permafrost region where is located in the interior of Tibetan Plateau. Ecosystem in the area is subjected to the freeze-thaw process of the active tjaele， and there is conspicuous correlation between soil moisture （SM） and vegetation coverage. To retrieve the soil moisture content of Beiluhe basin with a total area of 2 037.94 km2， a synergistic method， which combined improved water cloud model， Oh， Dubois and Topp model， was presented in this paper base on Sentinel-1A multi-polarization SAR and Landsat-8 time series images data. The accuracy was validated with the in-situ point SM data： Adjusted-R2 of the regression equation is 0.6848， and RMSE is 0.039 cm3·cm-3. The analysis of correlation among freeze-thaw process， SM and vegetation cover from macro watershed scale manifests： Vegetation coverage has a significant delayed effect on the freeze-thaw process of the active tjaele， that is， the higher vegetation coverage， the more lagging freeze-thaw time； These study results are basically consistent with predecessors in-situ observation data， verifying the feasibility of studying correlation among soil freeze-thaw process， SM， and vegetation coverage from the macro watershed scale based on Sentinel-1A annual time series data.
As the most important part of the global carbon cycle， soil carbon pool is the largest carbon pool in terrestrial ecosystems. Soil carbon pool in permafrost regions is the most sensitive carbon pool to climate change. Weak climate change will have a huge impact on the organic carbon production in the shallow soil， and then affect the regional landscape and ecology. As an indicator reflecting the antioxidant capacity of soil organic carbon， oxidation stability affects the quantity and quality of soil organic carbon， and its variation has a certain regularity in the alpine permafrost region under the influence of climatic factors. In order to explore the distribution characteristics of soil organic carbon and its oxidation stability in frozen soil， based on the experimental data and the climatic data from 2011 to 2019， the random forest model was used to conduct multi-factor digital mapping on soil organic carbon content， soil organic carbon components with different oxidation difficulty degrees， and soil organic carbon oxidation stability coefficient and environmental variables （average annual precipitation， average annual sunshine hours， average annual air temperature， and altitude） and analyze the controlling factors. The results showed that the model had an interpretation degree of more than 54% for the shallow soil organic carbon in frozen soil area of Three River Source Region， and the digital mapping could reflect the distribution of soil organic carbon well. Soil organic carbon was mainly affected by precipitation and sunshine duration， and temperature took second place. The spatial distribution of components with different oxidation difficulty is different， but the oxidation stability has the distribution characteristics of high in the north and low in the south. Cold and dry are conducive to improving the oxidation stability of organic carbon in shallow soil of frozen soil area.
Soil freeze-thaw cycles have important effects on surface water and energy balance， and then affect vegetation growth， soil water content， carbon cycle and terrestrial ecosystem. Passive microwave plays an important role in monitoring global and regional surface freeze-thaw processes due to its high temporal resolution， abundant data and sensitivity to soil moisture. With the launch of passive microwave sensors at home and abroad， it provides conditions for the study of permafrost interannual variation， seasonal variation， diurnal variation and long time series of near-surface soil freeze-thaw cycle. In recent years， the study of surface freeze-thaw cycle using passive microwave data has gradually increased. Based on previous studies， this paper summarizes the types of passive microwave remote sensing data and the characteristics of the bands contained in them. Expounded the principle of passive microwave monitoring data used for freezing and thawing， focus on passive microwave data in five categories in the study of freezing and thawing monitoring algorithms， including double index algorithm， the decision tree algorithm， freeze-thaw discriminant algorithm， seasonal threshold algorithm and based on the freezing L-band relative factors discriminant algorithm threshold， and analysis of 5 kinds of algorithms are compared； The freeze-thaw products based on different algorithms and passive microwave data were combed. Finally， the problems and future research directions of passive microwave remote sensing in surface freeze-thaw applications are summarized. In the acquisition of passive microwave data， it is found that the passive microwave data is missing due to the physical characteristics of the sensor， the shape and orbit of the earth， and the low resolution of passive microwave data leads to the low precision of freeze-thaw discrimination. For the problem of missing passive microwave data， it is proposed to use the average value of passive microwave data before and after two days to fill the missing brightness temperature data， or establish statistical function to complement the missing data. For the problem of low passive microwave resolution， the current development trend is to scale down based on passive microwave data and combine with multiple data products， such as ground temperature and active microwave data， or perform probability discrimination on surface freezing-thawing state in pixels， so as to better describe surface freeze-thaw state. In terms of the algorithm for discriminating surface freezing-thawing， based on the problem that dual-index algorithm， decision tree algorithm， freezing-thawing discriminant algorithm and seasonal threshold algorithm cannot accurately distinguish snow and frozen soil， this paper proposes to adopt the method of data assimilation or start from the snow radiation and frozen soil dielectric model. Optimization of the algorithm for the snow covered surface can further improve the accuracy of freeze-thaw classification. Based on existing freeze-thaw products， Although SMAP freeze-thaw products continue to be updated， SAMP satellite was launched late， and SAMP freeze-thaw products have a short time series. In the future， the time span of this algorithm for freezing-thawing products can be extended by combining L-band data provided by SMOS satellite. The problems mentioned above and the direction of further research are of great significance for improving the accuracy of freezing and thawing discrimination and improving the understanding of the variation law of freezing and thawing cycles， and also have certain research space.
There are 14 internationally recognized snow mountains above 8 000 meters on the Earth. The author of this article， Mr. Mi Desheng， recalls the discovery of three snow mountains of more than 8 000 meters through direct surveying and cartographic surveys， bringing the number of snow mountains above 8 000 meters in the world to 17. In order of elevation， the 15th Mount is the East Annapurne in the Himalayas with elevation of 8 013 m a.s.l.， located at 28°35′42.51″ N， 83°49′17.63″ E within Nepal. The 16th Mount is the Central Peak in the Karakoram with elevation of 8 011 m a.s.l.， located at 35°48′44.05″ N， 76°33′56.80″ E on the border between China and Pakistan. The 17th Mount is the Northwest Shishapangma Peak in the Himalayas with elevation of 8 008 m a.s.l.， located at 28°21′16.54″ N， 85°46′40.52″ E within China.
