Thermokarst lakes are widespread and increasing due to climate change on the Qinghai-Tibet Plateau （QTP）. They were formed by the ground subsidence in ice-rich permafrost landscapes and have strong impacts on permafrost thermal states. To gain a deep understanding of lake evolution and its thermal impacts on permafrost， here， a one-dimensional numerical model coupled lake and subsidence processes was developed. This model was validated using the field measurements of lake depth， lake ice and ground temperatures in Beiluhe Basin. Impact simulations for thermokarst lake with various depths were performed under current climate conditions as well as under a strong climate warming scenario. The simulation results indicated that in current climate， the shallow lakes （<1.0 m） were stabilized and lake water could freeze back to the bottom in winter， while the deep lakes （≥1.0 m） could deepen constantly and not freeze to their base. These deep lakes rose the bottom temperature and accelerated thawing of the underlying permafrost as well as developing of taliks in 50 years. Under a strong climate warming， all lakes deepened and warmed the underlying permafrost. The ice thickness increased for the shallow lakes， but decreased for the deep lakes. An open talik was likely to form under the deep lakes. Such changes could significantly alter the landforms and hydrology in permafrost regions on the QTP.
Bedded rock has a significant bedding plane structure. The direction of bedding plane determines the anisotropy of rock， and the strength of anisotropy is determined by the difference of rock physical properties and the connection strength of adjacent layers， its bedding structure nature of the vulnerability to the impact of weathering effects such as freeze-thaw （FT） cycles. In this research， bedded sandstone specimens with different bedding dip angles were tested by rock triaxial test system （GCTS） after 0， 10， 20， 30 and 40 FT cycles and under confining pressures of 0， 5， 15， 25 and 40 MPa. The test results show that the FT cycles causes cracks in the rock bedding plane， and shear slip failure along the bedding plane is more likely to occur. After 40 times of FT cycles， the uniaxial compressive strength of the specimen with β=0° decreased by 14.5%， while the specimen with β=67.5° decreased by 57.9%， which enhanced the inherent anisotropy of the bedded rock. The influence of confining pressure on rock anisotropy is related to its inherent anisotropy， which shows that with the accumulation of freeze-thaw cycles， the stress-induced anisotropy of rock under confining pressure changes from initial enhancement to inhibition. This study provides theoretical basis for the design， construction and safe operation of bedded rock engineering in cold regions.
The freeze-thaw process of active layer significantly affects or controls the ground-air energy exchange， surface hydrology， periglacial landform and the engineering construction in permafrost regions. The spatial distribution of active layer thickness is one of the core issues in the study of permafrost regions， and its accurate spatial distribution simulation is also one of the most tough problems yet to be resolved. The Tanggula Mountains is the largest mountain range in the central part of the Qinghai-Tibet Plateau， where permafrost wildly distributes. On the basis of field investigation， this paper analyzed the spatial differentiation characteristics of active layer and its main influencing factors. There are huge spatial differences in the thickness distribution of active layers in Tanggula area， with the minimum active layer thickness is 1.2 m， and maximum is 5.6 m. The variation law of average active layer thickness of different vegetation types is： marsh meadow < alpine meadow < alpine desert < alpine steppe. Although there are significant differences in air temperature and precipitation between the south slope and the north slope， there is no significant difference in the distribution of active layer thickness. The partial derivatives of Stefan equation show that the largest changing rate of active layer thickness comes from soil moisture， followed by soil thermal conductivity， and the smallest changing rate comes from ground surface temperature. Combining with the relationship analysis between active layer thickness and measured soil moisture， test pits data and surface thawing index in the study area， the results showed that the most important factor affecting the spatial distribution of active layer thickness is soil moisture， followed by soil thermal conductivity. While the change of ground surface temperature has no significant effect on the active layer thickness， and even can be ignored.
Permafrost peatlands in the Greater Higgnan Mountains are one of the areas sensitive to global warming response. In the context of global warming and permafrost degradation， to investigate the spatial-temporal variations of inorganic nitrogen in permafrost peatlands during the autumn freeze-thaw， this study was conducted in situ field experiments from September to November 2019 in the three permafrost peatlands in the Greater Higgnan Mountains， and analyzed the spatial and temporal variations of inorganic nitrogen in the shallow and deep layers of permafrost peatlands during the prophase， metaphase and anaphase of the autumn freeze-thaw. A multiple linear regression model was established for the relationship between soil inorganic nitrogen content and soil hydrothermal factors. The results of the study showed that the ranges of soil ammonium nitrogen （NH4+-N） concentration in the permafrost peatlands（Calamagrostis angustifolia peatland， XY）， （Larix gmelina-Sphagnum peatland， XA）， and （Eriophorum vaginatum peatland， BM） were （1.00±0.00）~（20.60±0.20） mg?kg-1， and the ranges of nitrate nitrogen （NO3--N） concentration were （0.02±0.01）~（14.64±1.11） mg?kg-1. The inorganic nitrogen was dominated by soil NH4+-N， and inorganic nitrogen concentrations were significantly higher during the anaphase of the autumn freeze-thaw than that of the prophase. Although hydrothermal interactions did not significantly affect inorganic nitrogen during this freeze-thaw， the BM inorganic nitrogen concentrations in the shallow layers were most responsive to 10~20 cm water content throughout the freeze-thaw period （R2=0.344， P<0.01）， and the inorganic nitrogen concentrations in the shallow layers was only related to soil moisture and temperature in the shallow layers during the prophase， metaphase and anaphase of the autumn freeze-thaw. The study showed that the autumn freeze-thaw could affect the inorganic nitrogen dynamics in the permafrost peatlands， and that the variations of the soil moisture and temperature in the shallow layers affected the inorganic nitrogen concentrations. This study could complement the data on the effect of autumn freeze-thaw on soil inorganic nitrogen in permafrost peatlands in the Greater Higgnan Mountains and provide basic data for the study of greenhouse gas emissions in response to global warming.
Soil temperature and moisture content are important factors affecting the change of dissolved organic carbon （DOC）. However， the response of soil DOC to soil hydrothermal during the autumn freezing period is not clear. Three types of permafrost peatlands ［Calamagrostis angustifolia （CP）， Larix gmelini-Sphagnum swamp （LP） and Eriophorum vaginatum （EP） peatland］ in the Greater Higgnan Mountains were selected as research objects in the study. Field in situ experiments were carried out to explore the effect of soil hydrothermal variations on soil DOC changes in permafrost peatland during autumn freezing period. The results showed that soil DOC content was in order： EP > CP > LP，with an average of 84.01，44.42 and 43.19 mg?L-1 respectively during the autumn freezing period. The content of soil DOC in the three types of permafrost peatlands fluctuated and decreased in the early autumn freezing period， while the variations of soil DOC content in CP and LP were gentle in the middle and late autumn freezing period. In the early autumn freezing period， the DOC content in the whole soil layer in CP decreased with the decrease of shallow soil temperature； however， the content of DOC in the shallow and the whole soil layer in CP increased with the increase of soil moisture content in the shallow layer in the late period. In the middle autumn freezing period， the shallow soil temperature increased and soil moisture content decreased in LP， which reduced soil DOC content； the content of DOC in the whole soil layer in LP decreased with the increase of the shallow temperature. In the late autumn freezing period， the content of DOC in the deep and whole soil layer in EP increased with the increase of soil moisture content in the deep soil layer. Besides， throughout the autumn freezing period， the shallow soil DOC content in LP was mainly driven by surface temperature， and deep soil DOC content in LP was mainly affected by soil moisture content in the deep soil layer； the whole soil DOC content was significantly affected by surface temperature. The study indicated that the variations of soil hydrothermal in the permafrost peatlands during autumn freezing period drove the changes of soil DOC content. The results of this study could provide basic scientific data for the study of carbon cycles in permafrost region and carbon emissions under the “double carbon” background.
Freeze-thaw cycle is one of the important factors affecting rock strength and environment change in cold region engineering. Annual freeze-thaw frequency， namely the annual accumulated amount of freeze-thaw cycle， can directly determine the impact of freeze-thaw cycle on cold region engineering and environment. Thus， it has a great significance to investigate annual freeze-thaw frequency in cold region. Xing’anling （Khingan） region has the largest intact primeval forest in China， with a forest coverage rate of 80.95%. The region covers northern part of Heilongjiang Province and northeast part of Inner Mongolia Autonomous Region. It belongs to the temperate continental monsoon climate， and due to the high latitude， there is seasonally frozen soil， island permafrost and discontinuous permafrost. In this paper， Xing’anling region of Northeast China is taken as the research object， and surface temperature data of 18 meteorological stations in Xing’anling region in the past 30 years from 1990 to 2017 are selected for collation， and annual freeze-thaw frequency of each station is obtained. Then， the variation of annual freeze-thaw frequency of each station is studied and analyzed in spatial distribution and time series. The results show that annual freeze-thaw frequency increases with the increase of latitude， decreases with the increase of longitude， and increases with the increase of altitude in Xing’anling region of Northeast China， and all have good linear trend. In the long-term dimension， annual freeze-thaw frequency showed an obvious downward trend from 1990 to 2017， and a sudden change occurred around 2004. Further analysis shows that among the three factors affecting the spatial distribution of annual freeze-thaw frequency， latitude and longitude have a greater influence， while altitude has a relatively small influence. There was a good negative correlation between annual freeze-thaw frequency and NDVI in Xing’anling region of Northeast China， indicating that annual freeze-thaw frequency was related to local vegetation coverage to some extent， and decreased with the increase of vegetation coverage.
The distribution of permafrost and taliks is very complex in the Tuotuo River Basin （TRB）， which is located in interior of the Qinghai-Tibet Plateau. Characterizing the spatial distribution and the thermal stability of permafrost and taliks is of great significance to community activities and engineering construction in TRB. Based on the zonation of permafrost and talik distribution around TRB conducted in the 1980s， the soil temperature and its variation process of permafrost and taliks in the south and north banks of the Tuotuo River were analyzed by using the observation data of five boreholes （N1~N5） along the Qinghai-Tibet Railway in the north bank and five boreholes （S1~S5） on the first terrace in the south bank. The results showed that， under the climate warming， permafrost and taliks in the north banks experienced significant degradation and warming process. From 2005 to 2020， the permafrost at the N1 borehole has undergone a significant down-draw degradation process， from extremely unstable and high-temperature permafrost to thawed zone. From 2005 to 2013， the annual average ground temperature of the talik at N2 increased at a rate of 0.3~0.4 °C·（10a）-1. At Maqutang on the south bank， permafrost prevails from the first-class terrace to the gentle slope of the Kaixinling Mountain， with both through and non-through taliks on the first-class terrace. The spatial distribution and the thermal stability of permafrost and talik in the TRB are further promoted by analyzing the changes in temperatures at boreholes in the basin. However， to meet the requirements of mapping and engineering construction of permafrost and taliks in the TRB， it is still necessary to carry out geological investigation with multiple methods and in-depth research on development mechanism of taliks in the future.
Frozen soil is generally regarded as a strongly rheological geomaterial. The strength attenuation of frozen soil is an important inducement for disease and instability in subgrade engineering， pile engineering and artificial freezing construction. Few efforts have been made to investigate the attenuation characteristics of strength envelope surface for frozen soil under complex stress states experimentally and theoretically. Considering this， at a temperature of -6 ℃， a series of triaxial stress relaxation tests under various confining pressures were carried out on the frozen subgrade soil specimens at strength points. The degeneration of strength parameters and stress attenuation process of frozen soil under complex stress states were systematically studied. The degradation law and mechanism of cohesion and internal friction angle are synchronously revealed in the stress relaxation process. Testing results indicate that the stress relaxation process of compacted frozen soil is significantly influenced by confining pressure. The stress relaxation ratio is increasing linearly with the rise of confining pressure if the confining pressure is beyond 1.5 MPa. The anti-relaxation ability of frozen soil is greatly reduced during high confining pressure conditions： the stress relaxation ratio of frozen soil is only 41.94% under 1.5 MPa， but exceeds 90.30% under 16 MPa. The strength of frozen soil attenuates linearly with time in the semi-logarithmic coordinate system. When the confining pressure is higher than 1.5 MPa， the strength attenuation rate of frozen soil increases with the rise of confining pressure. As the development of stress relaxation of frozen soil， cohesion decreases linearly but internal friction angle increases linearly with time in the semi-logarithmic coordinate system. It manifested that the cementation in frozen soil shows evident rheological features and it is a key inducement for strength attenuation. Moreover， the attenuation law and value of cohesion in frozen soil which is measured by triaxial stress relaxation test are similar to the spherical template indenter test results. This may provide a new test method for obtaining the long-term strength and cohesion of frozen soil. On the basis of test results， the stress states of frozen soil in all stress relaxation curves at 12 relaxation durations were captured， and the rate-dependent variation characteristics of strength envelope in p-q stress space were analyzed in detail. Under high confining pressures， the strength envelope of frozen soil shows different geometric features as time goes on. In addition to the decline of level， the strength surface exhibits clockwise rotation with time， and the third stage sharply decreases at first and then becomes flat. Based on the analysis of characteristics of experimental strength surface and evolution law of strength parameters during the stress relaxation process， a rate-dependent strength theory for frozen soil considering the stress relaxation effect is established in this paper.
Deep studying the dynamic strength of frozen soil is of great significance to the construction and stability evaluation of cold region engineering and artificial freezing engineering. In order to reveal the effect of rotation of principal stress axis on dynamic strength characteristics of frozen clay， dynamic triaxial and hollow torsional shear tests of frozen clay under different confining pressures were carried out by using a frozen hollow cylinder （FHCA-300）. And the effects on dynamic strength， dynamic cohesion and dynamic internal friction angle of frozen clay were discussed. The results show that rotation of principal stress axis reduces the dynamic strength of frozen clay specimens， and the lower the confining pressure is， the more obvious the effect is. Under the condition of rotation of principal stress axis， the attenuation rate of dynamic cohesion of frozen clay with number of vibrations is faster than that with fixed principal stress axis direction. Different from the characteristics that the dynamic internal friction angle decreases with the increase of number of vibrations under the fixed direction of principal stress axis， the dynamic internal friction angle increases with the increase of number of vibrations under the condition of rotation of principal stress axis. In addition， it is found that the dynamic strength， dynamic cohesion and dynamic internal friction angle have a good linear relationship with the logarithm of number of vibrations under the condition of rotation of principal stress axis. The linear equations were used to fit them respectively， and the fitting coefficients and determination coefficients were given.
Thermal diffusivity is a prime influencing factor of permafrost thermal response sensitivity to external heat disturbances， and it is also the key basic data for engineering design and construction in cold regions. In present work， thermal diffusivity of typical soil samples along the Xidatan-Tanggula Mountain of the Qinghai-Tibet engineering corridor were calculated using the thermal conductivity test results of the transient plane heat source method and the theoretical value of specific heat calculated by mass weighting method. Then， the distribution characteristics and the parameter influence law of thermal diffusivity of frozen and unfrozen soil in the corridor zone had been analyzed and compared. Finally， the prediction models of thermal diffusivity of frozen and unfrozen soil based on empirical fitting formula method and RBF neural network method had been developed and compared. The research results showed that： （1） The thermal diffusivity of the soil in the Qinghai-Tibet engineering corridor is positively correlated with the particle size. The increasing order of thermal diffusivity value of unfrozen soil is cohesive soil， silt， fully weathered rock， sandy soil and gravel soil， and order is cohesive soil， fully weathered rock， silt， gravel soil and sandy soil for frozen soil. （2） The correlation among thermal diffusion coefficient， volume weight and natural moisture content varies with soil types and freezing/thawing states， and the thermal diffusivity of frozen and unfrozen soil has a significant positive linear relationship. （3） Using thermal diffusivity of unfrozen soil as fitting parameter， the prediction accuracy of ternary fitting prediction model is significantly higher than that of binary fitting model. （4） The RBF neural network model has the greatest prediction accuracy for both frozen and unfrozen soils， which is the best soil thermal diffusivity prediction model.
In permafrost regions， road infrastructures have thermal impacts on adjacent permafrost， but the diversities of the feedback of permafrost under different surface conditions from road thermal impact is not totally understood. Based on the monitoring data of two monitoring sites along the Qinghai-Tibet Highway， this work studies the diversities of thermal impact of Qinghai-Tibet Highway on adjacent permafrost under different surface conditions. The results show that the Qinghai-Tibet Highway has a significant thermal impact on the permafrost on both sides， but the feedbacks from the thermal impact are different due to different surface conditions. Compared to the site with higher vegetation coverage， the mean annual ground temperature as well as the active layer thickness are respective greater， along with a wider horizontal thermal impact in the site with lower vegetation coverage. In addition， for the site with lower vegetation coverage， at the slope foot of the embankment， because of its bare surface， the topsoil is more susceptible to external disturbances， leading to form a thermal exchange pattern differing from other positions， and this may be a reason causes the different thermal states between the two sites. In present， there are many linear projects in the Qinghai-Tibet Engineering Corridor which makes it necessary to take into account the interactions between projects and their relationships with permafrost. This work can provide references for the setting of reasonable distance between linear projects in the Qinghai-Tibet Engineering Corridor in the permafrost region of the Qinghai-Tibet Plateau as well as for the reasonable distance arrangement between bidirectional roadbed of the Qinghai-Tibet Expressway to be built， so as to reduce mutual disturbances and ensure the operational safety of the them.
The distribution of frozen soil in our country is very broad， and the area of permafrost alone accounts for 22.4% of the total land area. As a special kind of soil， frozen soil has many properties that thawing soil does not have due to the influence of ice cement in the soil. Among the many properties of frozen soil， the deformation and strength of frozen soil are the basic problems affecting engineering construction in frozen soil areas. The spherical template indenter test is widely used in the test of the mechanical properties of frozen soil because of its simple test process and relatively accurate test results. Compared with the conventional triaxial test or direct shear test， the test process of the spherical template indenter test is simple and easy to implement， the test period is short， and the sample preparation requirements are low. The advantage of effective cohesion is more significant. Therefore， based on the spherical template indenter test of the frozen soil， this paper estimates the strength and mechanical index of the soil through the indentation depth of the spherical template indenter test， and establishes the relationship between the force of the sample and the indentation depth of the indenter test. The specific test method is as follows： take the water-saturated frozen sandy soil made of different particle size groups （the moisture content of the sample is affected by the particle size in the saturated state） as the research object， study the variation law of the depth of the frozen soil sample pressed into the soil by the spherical indenter with time under the conditions of different fixed loads. By comparing and referring to the frozen sands of each particle size group， the long-term equivalent cohesion of the frozen sands of different particle size groups is summarized. The change law of force （long-term shear strength） with time， and the research method of elastic mechanics to solve space problems， summed up the mutual conversion between the depth
The electric double layer structure is of great significance for studying the mechanical properties of clay and the water migration of frozen soil. In order to explore the influence of different influencing factors on the potential distribution of the diffusion electric double layer of clay particles， referring to the Gouy-Chapman-Stern electric double layer theory， based on the Nernst-Planck equation and the Poisson-Boltzmann equation， the numerical software COMSOL was used to quantitatively analyze the temperature， concentration， particle size， particle shape and the relative dielectric constant of the solution on the potential distribution of the diffusion electric double layer. The research shows that the effect of temperature on the potential distribution is not obvious， but with the increase of temperature and the thickness of the Stern layer， the surface potential and Stern potential of clay particles both increase； and as the solution concentration and relative permittivity decrease， the surface potential value increases； when the mineral composition， surface charge density and particle shape are determined， the effect of size on the potential distribution of the diffusion electric double layer is not significant； however， the irregular shape of the particles has an obvious effect on the potential distribution， when the particle shape has an included angle， the surface potential at the included angle is much larger than other positions， and the smaller the included angle， the larger the surface potential value at the included angle.
To study the problem of reclaimed asphalt pavement （RAP） and iron tailing sand （ITS）， they are added to cement stabilized macadam as gravel and sand. The road performance of cement stabilized macadam is studied through unconfined compressive test， flexural-tensile test and temperature shrinkage test， and the effects of RAP and ITS content on cement stabilized macadam is analyzed. The results show that： When RAP content is fixed （25%）， the increase of ITS content is beneficial to improve compressive strength and flexural-tensile strength of cement stabilized macadam， but it will increase temperature shrinkage strain and temperature shrinkage coefficient of the material， and reduce temperature shrinkage property. Among them， ITS45 （ITS content is 45%） has better temperature shrinkage performance. When ITS content is fixed （60%）， the increase of RAP content will not be conducive to compressive strength of cement stabilized macadam， but it can increase flexural-tensile strength， and reduce temperature shrinkage strain and temperature shrinkage coefficient. Among them， RAP70 （RAP content is 70%） has the best temperature shrinkage performance. Compared with ITS45， RAP70 has a wider construction temperature range.
Expansive soil is a kind of special clay， which has obvious swell-shrink characteristics and multiple fissures. It is very easy to induce various freezing damages in channel engineering in cold regions. Uniaxial compression is an important branch of physical and mechanical characteristics of frozen soil. In order to investigate the uniaxial compression characteristics of frozen expansive soil， a series of uniaxial compression tests considering different dry densities and temperatures were carried out， and the effects of dry density and temperature on the stress-strain relationship， failure mode， compressive strength and elastic modulus of frozen expansive soil were discussed. The test was carried out on low temperature dynamic testing machine in the frozen soil laboratory of Hohai University. The compression test was carried out at a constant rate. When the axial strain reached 30%， the test was stopped （except for the brittle failure sample）. The test results show that the stress-strain relationship curve of the sample under various temperatures gradually changes from weak strain softening to strain hardening with the increase of dry density. The higher the temperature， the more significant the softening characteristics of the stress-strain curve. The failure mode of samples under different temperatures are obviously different. Obvious local collapse and spalling appear on the surface when the sample is damaged at -2 ℃， while the final failure mode of samples at -5 ℃， -10 ℃ and -15 ℃ is “drum-like” failure， accompanied with few obvious cracks and shear planes on the sample surface. The uniaxial compressive strength of frozen expansive soil samples increases linearly with the increasing dry density. Besides， the uniaxial compressive strength of the samples also increases with decreasing temperature， however， there are some differences in its amplification within each temperature range， which is closely related to the ice content in samples. In addition， the elastic modulus of the sample increases linearly with the increasing dry density and decreasing temperature. The research results are expected to provide reference for the freezing damage mechanism of channel engineering and the treatment of channel foundation soil in the expansive soil section in the cold region.
To investigate the dynamic properties （including the dynamic deformation and dynamic strength） of frozen mixed soils subjected to the freeze-thaw cycles， a series of cyclic dynamic experiments were conducted through a cryogenic triaxial apparatus. With the variation of freeze-thaw cycles， a conspicuous influence on the dynamic stress-strain behaviors， volumetric strain curves， hysteresis loop， accumulative axial strain， dynamic resilient modulus， residual deformation， and dynamic strength of frozen mixed soil was analyzed. The results show that： （1） With the increase of the freeze-thaw cycles， the dynamic stress-strain curves and dynamic volumetric strain curves of frozen mixed soil gradually become looser， and the number of failures decreases linearly； （2） The action of freeze-thaw influences the variation of the hysteresis loop， the accumulative axial strain， the dynamic resilient modulus， and the residual deformation of frozen mixed soil， leading an unacceptable deformation； （3） The dynamic resilience modulus and the dynamic strength of frozen mixed soil are reduced with the number of freeze-thaw cycles increased.
Based on predecessors in the soil structure and the surface roughness affect the soil-structure of contact area between the tangential frost heaving force research is less， this study starts from the problem of tangential frost heaving force between soil-lining contact surfaces of canal foundation in western Sichuan seasonal frozen area， and focuses on the influence law and effect of surface roughness of lining on shear strength， cohesion and internal friction angle between contact surfaces. Combined with environmental temperature， moisture content and freezing time， the correlation and aboriginality of four factors on peak shear strength between contact surfaces are comprehensively explored by orthogonal analysis. The results show that the peak shear strength， cohesion and internal friction angle between the contact surfaces show the same law with the change of lining surface roughness， and the more rough the lining surface is， the three indexes will increase. The orthogonal analysis reveals that the most significant factor affecting the peak shear strength between the contact surfaces is the roughness of the lining， followed by environmental temperature and moisture content， and the effect of freezing time is not obvious. At the same time， the peak shear strength under low temperature， low moisture content， long freezing time and high roughness of the lining is larger. This result can provide theoretical support for the frost heaving hazard prevention of canal system engineering in seasonal frozen region.
The water-salt phase transformation characteristic of pore solution in mortar is the key theoretical basis for studying the water-thermal-salt coupling model and the damage mechanism of cementitious materials. In order to study the variation of the pore solution in saline soil area of Hexi Corridor， the thermal parameters such as heat flow and phase transition temperature were measured by Differential Scanning Calorimetry. According to the conservation principle of mass and heat， the separation of water and salt was preliminarily realized. Ice， salt crystal， and unfrozen water contents at different temperatures were calculated to reveal the mechanism of water and salt phase transformation of the pore solution at low temperature. Moreover， the internal degradation mechanism of mortar was clarified by Scanning Electron Microscopy and Energy Dispersive Spectroscopy. The results show that the freezing point of the pore solution decreases and phase transition time delays， as salt concentration increases. Meanwhile， the sequence of water-salt phase transformation reverses and the content of unfrozen water moves towards low temperature as the concentration increases. At the same salt concentration， the effect of sodium chloride， mixed salt and sodium sulfate on the freezing temperature of the pore solution decreases in turn. Mortar suffers physical-chemical coupling action under low temperature and salt erosion environment. Sodium chloride is the most destructive to mortar， and the effects of mixed salt and sodium sulfate on failure of mortar are in decreasing order.
With the rapidly increasing railway operating length in the permafrost regions of China， dynamic analysis of permafrost embankment under random train loads has become an urgent engineering problem to be solved. Taking a cross-section of the embankment of Qinghai-Tibet Railway as an example， this paper adopts the explicit time-domain method to analyze the statistical characteristics of its responses to random train loads. In order to obtain the train load samples， which serve as the inputs in the stochastic analysis， an efficient procedure is proposed incorporating the random parameters of track irregularity and train speed. The numerical integration method is used to discretize the equation of motion in the time domain， and an explicit expression of the dynamic responses of permafrost embankment is established. Based on the established explicit expressions， Monte Carlo simulation can be efficiently conducted， delivering the statistics of the random system outputs， including the mean values， standard deviations and mean peak values of the embankment dynamic responses. Using this method， the random dynamic responses of the permafrost embankment in summer and winter are analyzed. It is found that the displacement and velocity responses in summer are more severe than those in winter. Numerical results indicate that the explicit time-domain method has high accuracy and efficiency in analyzing the random vibration of permafrost embankment.
This paper presents a series of triaxial dynamic tests under constant and variable confining pressure to explore the characteristics of cumulative deformation and resillient deformation for frozen coarse-grained fillers. Testing results indicate that axial cumulative plastic strain increases with the increase of freezing negative temperature and with the decrease of the content of coarse particles. The volumetric strain first decreases and then increases with the decrease of temperature at five cyclic stress amplitudes. The three-stage development features of volumetric strain with the increasing of the content of coarse particles were experimentally determinated. The accumulative plastic strains of subgrade fillings were further analyzed and classified based on three existing shakedown criteria. The assessment capabilities of three shakedown criteria were examined by testing results. Based on Werkmeister criterion， the cumulative permanent strain of frozen sample has a tendency of arriving to the plastic creep area under variable confining pressure. According to Ma criterion， similar evaluation results can be found under constant confining pressure. The cumulative permanent strain of all testing samples distribute in the plastic shakedown area that corresponding to the Chen criterion. The defining method for demarcation point between the post-compaction compression stage and secondary cyclic compression stage is non-ignorable factor for reliably evaluating the cumulative plastic deformation of frozen coarse-grained fillers based on three criteria.
For freezing sinking in deep alluvium， the deep clay experiences freeze-thawing under high ground stress due to the formation and thawing of the frozen wall， which can lead to changes in its engineering properties. In this paper， loaded freeze-thaw tests were conducted on deep remodeled clay by indoor tests， and triaxial shear tests were conducted on deep freeze-thaw remodeled clay under different conditions of dry density， surrounding pressure， freezing temperature and thawing temperature to explore the changes of shear strength of deep remodeled clay under the effect of loaded freeze-thaw. The results showed that the freeze-thaw effect changed the bias stress-strain relationship of deep remodeled clay from soft type to hardened type； meanwhile， the shear strength and cut-line modulus both decreased to a certain extent after the freeze-thaw effect， and the maximum shear strength decreased by 48.8% and the maximum cut-line modulus E0.5 decreased by 72.0% under the present test conditions. Through the significance analysis of the influencing factors， it was found that the initial dry density had the most significant effect on the shear strength and the cut line modulus， and the perimeter pressure had the weakest effect on both.
Freezing-thawing and initial water content are the main factors to influence the water migration characteristics， frost heaving and thaw settlement of coarse-grained soil. To clarify the water migration characteristics and micro mechanics of coarse-grained soil with different initial water content under freezing and thawing， fluorescein is taken as the tracer and computerized tomography （CT） is adopted as the micro mechanics method， a series of experiments were carried out to study the temperature field， frost depth development， water supply， final water distribution， water migration images and CT values of the coarse-grained soils with different initial water content under freezing and thawing. Results indicate that the frost depth increase continually with the increasing numbers of the freezing-thawing cycles basically. The external water supply and the migration height of the liquid water are negatively correlated with the initial water content. CT scanning results indicate that after several times of freezing-thawing， the water migration results in the changes of the soil pore and particle structure， then the changes of the samples density increase and the porosity decreases， then the related deformation occurs in the soil.
The accumulated deformation of saline soil caused by the special temperature and humidity environment is the main cause of numerous engineering problems in seasonally frozen region. However， the deformation failure mechanism of saline soil is not well defined. The freeze-thaw cycle experiments are carried out by silt with different salt contents to investigate the variation characteristic of temperature， unfrozen water content， pore water pressure， matric suction and displacement during the test. It is found that the pore water pressure and matric suction are sensitive to soil temperature and have important effects on soil deformation. By analogy with the effective stress principle of unsaturated soil， the effective stress equation of frozen saline soil is given. The soil deformation is divided into temperature strain， salt expansion， frost heave， dissolve collapse， thaw settlement and residual strain， which completely explains the deformation mechanism of frozen saline soil. Moreover， the degree of influence of salt content on soil deformation is discussed， which indicates that frost heave and thaw settlement are the main strains of saline soil with low salinity. With the increase of salt content， the contribution of salt expansion and dissolve collapse becomes more and more significant. The soil deformation is minimum when the salt content is 1%， which indicates that proper control of salt content can effectively suppress soil deformation.
Freeze-thaw cycle is one of the main factors cause engineering damage in cold regions. In order to investigate the effect of freeze-thaw cycles on the mechanical and thermal properties of lignin fiber stabilized loess， light compaction test， freeze-thaw cycles test， unconsolidated undrained triaxial shear test， thermal constant analysis test and X-ray diffraction test are used in this study. lignin fiber admixture， number of freeze-thaw cycles and envelope pressure are used as variables for this study. The results showed that the maximum dry density of the lignin fiber stabilized loess tend to decrease and the optimum moisture content tend to increase with increased admixture； As the number of freeze-thaw cycles increase， the stress-strain curve of the specimens change from strain-hardening to strain-weak softening. In addition， the rate of mass loss， break strength， modulus of elasticity， cohesion， angle of internal friction and thermal conductivity of the specimens all tended to decrease with the number of freeze-thaw cycles， with the highest decay rate after the first freeze-thaw cycle and always reach a maximum at a admixture level of 5%； the break strength and cohesion of the specimens stabilize after 6~9 freeze-thaw cycles； the loess and the composition of the lignin fiber-stabilized loess with 5% admixture was similar in the X-ray diffraction analysis and no new material was found to be produced； therefore， the lignin fiber is a green physically cure material. The results of this study can provide new ideas and methods for research related to soil stabilization and consolidation in cold regions.
Vegetation concrete （VC） ecological protection technology is an effective solution for the vegetation recovery of bare steep slopes， which has been increasingly applied in cold regions in recent years. When the technology is implemented， the nutrient retention ability of VC substrate is essentially concerned. Under the actions of freeze-thaw cycles， fertility of the VC substrate as well as natural soil is thought to degrade gradually. It has been recognized that the nutrient retention ability of soil is significantly correlated with its physical structure. Similarly， the nutrient retention ability of VC substrate could be supposed to be physical structure-dependent. To enhance the comprehensive performance of VC substrate in cold regions， the investigation of nutrient retention ability is required， which nevertheless is still little identified. In this study， a series of freeze-thaw cycle experiments for VC substrate were conducted， and the effects of initial water contents and freeze-thaw cycles on characteristic parameters of water-stable aggregates and leaching loss rates of major nutrient substances were studied. A freeze-thaw cycle for specimen treatment， performed by a fast air freeze-thaw test machine， was defined as the freezing process of 12 hours at -20 ℃ in addition to the thawing process of 12 hours at +20 ℃. Except for the non-treatment， namely without freeze-thaw cycle， 7 treatments were considered to prepare the specimens， including 1 cycle， 2 cycles， 4 cycles， 8 cycles， 16 cycles， 32 cycles and 64 cycles. According to the field experience in practice， the lower and upper initial water contents of specimens were designated to be 18% and 24%， respectively. The results showed that the water-stable aggregates of the VC substrate were mainly composed of the particles with size ranging from 0.05 mm to 0.25 mm， which contained the proportions over 50% of total mass for all specimens. With increasing initial water content， the water-stable micro-aggregates transformed into the macro-aggregates， among which the particles of ≥1~2 mm were found to hold the maximum increase rate in proportion. Other parameters， which could quantitatively represent the characteristics of aggregate structure， also showed that the aggregate stability increased with initial water content. In addition， the contents of particles smaller than 0.25 mm were positively related to freeze-thaw cycles， while that of the particles larger than 0.25 mm showed the inverse trend. This indicated that the average value of aggregate particle sizes decreased with freeze-thaw cycles. It was noticed that the dispersion rates of aggregate increased with initial water content， which showed that destructive action to aggregate caused by freeze-thaw cycles was greater than the reinforcement provided by the increasing cement hydration products. Furthermore， the freeze-thaw cycles required for the aggregate characteristic parameters of VC to reach the stable state were more than that for natural soil. It may be due to that natural soil would go through the repetitive process of decomposition and aggregation， while destruction process of cement hydration products was irreversible. For the fertility， a high initial water content was associated with the increasing contents of major nutrient substances. Contents of organic matter， ammonium nitrogen （NH4+-N）， available phosphorus （PO43--P） and potassium （K+） still increased with freeze-thaw cycles， while content of nitrate nitrogen （NO3--N） decreased. Moreover， the leaching losses of these nutrient substances increased with freeze-thaw cycles obviously. From the Pearson correlation analysis， the leaching loss rates of major nutrient substances were found to correlate closely with the aggregate characteristic parameters. In consideration of significance levels and absolute values of correlation coefficients， geometric mean diameter （GMD） could be suggested as the reasonable index to describe the nutrient retention ability of VC substrate. The results may contribute to illustrate the underlying reason for VC substrate fertility degradation under freeze-thaw cycles and provide theory basis for countermeasure.
The dielectric constant is one of the key parameters for measuring soil moisture content through the frequency domain reflection method. To obtain the influence of temperature on soil dielectric constant， the vector network analyzer was used to test the dielectric constant of soil samples with different water contents and salt contents at different temperatures. By analyze the change law of soil dielectric constant with different factors， a step function was introduced to describe the relationship between dielectric constant and temperature. The results show that through the water-ice phase change， the temperature controls the soil dielectric constant. When the temperature is lower than a certain temperature or higher than freezing point， the dielectric constant is less affected by temperature. The dielectric constant has a great change in the severe phase of ice-water transformation. The dielectric constant of soil increases with the increase of salt content. The increase of initial water content will increase the dielectric constant. A three-stage model based on step function can be well describe influence of temperature on the soil dielectric constant.
Due to the effect of global warming on the temperature amplification in high-altitude mountain areas， ice and snow geological disasters tend to increase， and generally show the characteristics of chain development. They are characterized by dynamic transformation of disaster types， large impact range and high damage degree， which seriously threaten the safety of major engineering constructions in the region. In this paper， the common types of ice and snow geological disasters in the alpine regions are summarized into six types： ice rockfall， snow rockfall， ice and snow debris flow， moraine accumulation landslide， glacial lake outburst， glacier debris flow， and their typical development characteristics are analyzed respectively； Then， the three typical chain combination relationships of ice and snow geological disasters are analyzed. It is found that the ice and snow geological disaster chain generally has a dynamic evolution process of “high-level initiation → dynamic collapse → phase transformation”； Based on this， the phased characteristics and internal mechanism of high-level initiation， dynamic collapse and phase transformation of ice and snow geological disasters are analyzed， and the cumulative chain amplification mechanism of “high-level initiation → potential energy transformation → erosion scraping → slip accumulation” of ice and snow geological disasters is defined. Finally， the evaluation models of each evolution stage of ice and snow geological disaster chain are combed. It is considered that the new fracture diffusion model and Flores contact force model can be used to evaluate the dynamic collapse process from high-level initiation to dynamic collapse of ice and snow geological disasters； Plough cutting model can better describe the process of erosion， scraping and volume enlargement； Particle flow model， air lubrication model and excess pore water pressure model can better reveal the characteristics of high-speed slip accumulation of this kind of disaster chain； In addition， Voellmy Rheological model and Bingham fluid seepage model can better explain the phase transformation characteristics in the disaster chain. The research has basic reference value for the scientific evaluation of the dynamic evolution process of ice and snow geological hazard chain.
River runoff is an important water resource in the arid region of northwest China. Under the background of climate change， the exploration on change characteristics and influencing factors of river runoff is of great significance for understanding the law of river hydrological change. Based on the daily discharge of Changmabao hydrological station， the observation materials from meteorological stations and radiosonde stations， and the first and second Chinese glacier inventory， the variation characteristics and possible influencing factors of runoff in the upper Shule River were systematically analyzed by using the linear trend， empirical mode decomposition and hierarchical multiple regression. The results show that the annual runoff of the upper Shule River showed a significant increasing trend during 1954—2016， with a rate of 1.00×108 m3·（10a）-1. Both runoff in flood period and non-flood period also showed a similar increasing trend. The runoff suddenly changed in 1999 and had two oscillation periods of about 15a and 7a， among them the 15a oscillation period was the most significant. Both cumulative positive temperature and precipitation were the main climatic factors affecting the runoff in the upper Shule River， which can explain more than 80% of the runoff change. There was a significant positive correlation between the summer runoff in the upper Shule River and the height of 0 ℃ layer. It means that the height changes of 0 ℃ layer can be used to predict the runoff change of the river in flood period， which provides an important reference for evaluating the runoff change of glacial melt recharge rivers in the arid region of northwest China. From 1966 to 2006， the ice volume in the upper Shule River basin decreased by about 5.77 km3， indicating that glacier changes play a crucial role in the change and regulation of runoff in the basin.
The dynamic change of unfrozen water during freezing-thawing process is closely related to the ice-water phase transition， which is an important basis for the study of unsaturated soil. In this paper， the unfrozen water content of unsaturated sand during freeze-thaw process is tested by on-line temperature control and layered scanning NMR technology. The peak size and peak area data of the T2 distribution curve （different T2 values on the curve correspond to the characteristics of pore water category， and the area below the curve corresponds to the moisture content of the sample） in the freezing and thawing process inverse the size and location of water content in soil， and the peak shape and relaxation range of the curve （the initial value and termination value of each peak） inverse the distribution of different types of water （adsorbed water and capillary water） and soil structure. When the test results are processed， the frozen area and unfrozen area of the sample are firstly divided according to the freezing point obtained by the test. There are obvious differences in unfrozen water content and pore changes between frozen and unfrozen areas， which are caused by ice water phase transition and water migration. The ice-water phase transformation is dominant in the freezing area of soil samples， and the water mainly migrates from the unfrozen area to the e and f layers near the freezing front. Firstly， capillary water migration in medium and large pores is dominant， followed by adsorbed water migration in small pores. According to the analysis of the volume increase of water phase into ice and the proportion of pore volume， it can be seen that the tiny pores in the freezing zone will connect to form medium and large pores during the freezing process. Water migration is dominant in the unfrozen area. The large pores in the unfrozen area are compressed into small pores under consolidation. During the test， the pore changes of e and f layers near the freezing front are the most dramatic.
Stability of foundations supporting power transmission lines in permafrost regions is closely related to the surrounding active layer thickness and ground ice variations. Thermal disturbance during the construction and heat effect of the concrete foundations tend to thaw the surrounding permafrost， endangering stability of the towers. The electrical resistivity tomography （ERT） method is frequently used in engineering and environmental research in permafrost regions. The reliability and limitation of ERT investigation are related to geoelectrical structures and array used for data collection. We reduced the uncertainty by forward and inverse modeling the geoelectrical models， which represent the permafrost around tower foundations. The results show that the active layer thickness can be effectively detected using different types of arrays， but the spatial distribution of ice-rich and ice-poor permafrost is barely identified when the active layer thaws out. The reliability of ground ice identification improves significantly if the ERT data are collected when the active layer is frozen. Under this condition， the dipole-dipole array shows a better ability to explore configurations of ice-poor and ice-rich permafrost. Effectiveness of the method was verified for tower foundations investigation in permafrost regions along Qinghai-Tibet Power Transmission Line. Degradation of ground ice due to the construction disturbance and heat effects of the concrete foundations were effectively identified. The results indicate that forward and inverse modelling can provide essential insights in designing experiment and collecting data. In this way， the reliability and resolution of ERT in investigating the permafrost characteristics can be significantly improved.
Thermal insulation materials （TIMs） are widely used in cold regions engineering. During period of use， TIMs usually suffer cyclic multi-fields coupling actions such as moisture intrusion， freeze-thaw and salt erosion， which can cause degradation of their thermal insulation， waterproof and strength performances. At present， there are many kinds of available TIMs in the market， but their thermal and mechanical properties and durability differ considerably. Thus， for different cold regions engineering， a reasonable selection of TIMs does not only affect their insulation performance but also the long-term stability of the engineering infrastructure. With regard to cyclic freeze-thaw （CFT） actions， four kinds of TIMs widely used in cold regions engineering， including FLK FLOLIC FOAM （FLK）， polyurethane（PU）， polystyrene extruded board （XPS） and polystyrene foamed boards （EPS）， were selected to conducted CFT in dry conditions and after immersed in water and salt solution. After experiencing different numbers of CFT， apparent density， water absorption rate， thermal conductivity， compressive strength， bending strength and micro-structure of the four kinds of TIMs were tested following the related specifications. The results showed that， after immersion， the thermal and mechanical performances of hydrophilic FLK degraded considerably， while these of the other three hydrophobic TIMs changed slightly. In dry conditions， the CFTs caused considerable changes in the water absorption rates of FLK， XPS and EPS and the thermal conductivity of FLK. While for specimens immersed in water and salt solution， after 30 CFTs， the thermal conductivity and bending strength of FLK increased by 50% and decreased by exceeding 0.3%， respectively， while the compression strength of EPS decreased more than 10%. After the CFTs， there was considerable difference among changes in physical and mechanical properties of the TIMs immersed in water and salt solution. Through scanning electron microscopy images， the cell scale and density and the cementation mode of solids of the TIMs can be observed. However， changes in micro-structure of the TIMs after CFTs cannot be identified. Based on the test results， the frost durability of the four kinds of TIMs were discussed with regard to their application scenery. It is hoped that this study can provided references for application of TIMs in cold regions engineering.
In order to study the influence of cold-end temperature， soil group and boundary hydraulic pressure on soil frost heave and the applicability of the current methods on assessing frost heave susceptibility of soils. Three factors and three levels of orthogonal experiments are carried out. The sequence of influencing frost heave ratio of soils is obtained by using the gray correlation method. The results show that the correlation among boundary hydraulic pressure， cold-end temperature and frost heave ratio is relatively larger， and the correlation between soil group and frost heave ratio is relatively smaller. Therefore， the factors affecting the frost heave ratio of soil from strong to weak are boundary hydraulic pressure， cold-end temperature and soil group. The ice lenses are formed in sandy soil samples during freezing under boundary hydraulic pressure， and the frost heave ratio of some sandy soil samples were larger than that of silty clay samples. Therefore， it can be concluded that it is not reasonable to assess the frost heave sensitivity of soil under the condition of boundary hydraulic pressure by fine particle content， and the frost heave susceptibility of soils should be assessed according to the actual situation. It is generally believed that the influence of soil group on frost heaving is strong， while it is found that the effect of soil on frost heaving is the weakest under boundary pressure. The relationship among frost heave susceptibility， thaw weakening susceptibility and frost susceptibility were discussed. It is found that although the sandy soil can frost heave under boundary hydraulic pressure， it can rapidly discharge water during thawing， which can reduce the frost damage caused by soil thawing. Therefore， coarse-grained soil should be replaced with fine-grained soil when treating the frost damage. In order to ensure the safe operation of engineering structures in cold regions under boundary hydraulic pressure， it is proposed that the effect of boundary hydraulic pressure on soil should be solved firstly. Moreover， constructing drains to drain water and employing replacement method.
The acoustic velocity of deep soils subjected to freeze-thaw is the basis of application of acoustic technology in deep artificial ground freezing engineering. For the deep soils， the in-situ freeze-thaw process is under high earth stress. However， acoustic test devices are not currently available for testing under high pressure freeze-thaw conditions. To investigate the shear wave velocity of deep soils under the conditions， a freeze-thaw test device with side-mounted bending element was developed. The test device is mainly composed of a specimen bearing chamber， loading system， temperature control system， data acquisition system and bending element test system. The specimen bearing chamber is a double-layer cylinder， the inner cylinder is a Teflon cylinder with the wall thickness of 8 mm， and the outer cylinder is a steel cylinder with the wall thickness of 15 mm. The bending element probe is mounted on the side wall of the inner cylinder. This structure can meet the requirements of deformation resistance， thermal insulation and vibration isolation under high stress. The bending element test system was used to investigate the method of determining the acoustic travel time and the test parameters. According to the test results， the acoustic travel time is determined by the initial waveform method， and the transmitting probe is excited by square wave. The excitation frequencies of 4 kHz and 5 kHz were chosen for high-density frozen clay and unfrozen clay， respectively. Finally， the shear wave velocities of deep clay at different negative temperatures under high stress were tested using the device， and the applicability of the device was verified.
There is a severe phenomenon of blowing snow by wind in winter in Mayitas area of Xinjiang. Road traffic interruptions and casualties caused by wind and snow disasters often occur. The existing engineering measures can alleviate the problems of low visibility and problems with accumulating snow on pavement caused by wind and snow to a certain extent， but cannot completely eliminate the impact. Aiming at the problem that the blowing snow disaster difficult to be completely cured under strong wind conditions， a shed tunnel engineering treatment scheme is proposed， and a wind tunnel simulation experiment is carried out on the shed tunnel form and snow prevention scheme at the entrance of the tunnel. The wind tunnel experimental section is 8 m long， 1.3 m horizontally wide and 1 m high. The shed tunnel model is made by 1：60 scale 3D printing method， the snow barrier model is made of wooden strips， and the simulation medium is made of fine sand， refined salt and sawdust. Firstly， three kinds of shed structures， including fully enclosed， overhanging and ventilated， were simulated under the condition of 90° wind direction， and compared with the form of blowing snow stacking. Three parameters for similarity were used： density， particle size and stacking shape. The similarity is determined by weighted Euclidean distance between simulation medium and blowing snow， the similarity of the slump angle of the predecessors is verified， and the most suitable medium for simulating blowing snow is deduced. According to the experimental results， combined with the actual situation of wind blowing and snow disasters in Mayitas， flow field of the closed shed tunnel by a wind direction angle of 30° has been separately conducted with or without protective measures. The results show that when the wind tunnel stacking experiment is used to study the deposition state of blowing snow under non-low temperature conditions， the sawdust has good similarity with the blowing snow in the process of wind and snow movement. A large amount of medium was poured into the ventilated shed during the experiment， which proved that its anti-wind and snow performance was not ideal. By contrast， the closed shed and the overhanging shed have better wind and snow prevention effects. The snow barrier has a good blowing snow inhibition function at the entrance. From the stacking experiment， in the comparison with and without snow barrier， volume of the medium in the shed is very different， which proves that the snow barrier can effectively prevent the blowing snow from entering the shed. Among them， the difference in wind speed inside and outside the shed is obvious， which proves that snow barrier can significantly reduce the wind speed outside the shed and has the function of inhibiting the formation of weak wind areas inside the shed. The study has confirmed that the shed tunnel project with suitable snow barrier layout plan for tunnel entrance protection is an effective means to control wind and snow disasters. For highway traffic in Xinjiang， it is a feasible solution to use shed tunnel engineering to control blowing snow disasters. After dealing with the snow deposition at the entrance of shed tunnel， the shed tunnel engineering form of low-cost environment integration and the vehicle operation safety guarantee measures of long-distance shed tunnel are the problems that need to be solved in the future.
