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The thermal performance of an experimentally built expressway embankment with a combined cooling measure at Beiluhe Basin in the Qinghai-Tibet Plateau was investigated based on long-term field monitoring. Six-year records of soil temperatures showed that the embankment cooled by a measure combining hollow concrete bricks and ventilation ducts had a significant cooling effect on the subgrade soil. Over the investigated time period, the permafrost table beneath the embankment moved upward to the natural ground surface, and the subgrade soil layers down to a depth of −15 m underwent a considerable cooling. Then, the long-term thermal performance of this kind of cooled embankment with different surface widths (13 and 26 m) is comparatively studied using numerical simulations. The simulated results show that both embankments perform well in cooling the subgrade soil. However, the annual heat loss of the subgrade soil beneath the wider cooled embankment is greater than that of the narrow ones. Via the wider cooled embankment, more cold accumulates in the subgrade soil after the embankment's construction, which we term as the "cold accumulative" effect. This cold accumulative effect can ensure the underlying permafrost thermal stability better in the context of climate warming and strength the long-term embankment stability. • Cooling effect of embankment cooled by a combining method were tested in the field. • Thermal budget of soils under cooled embankment with different widths was compared. • A cold accumulative effect of cooled embankment with a greater width was proposed. [ABSTRACT FROM AUTHOR]

In a permafrost environment, supra-permafrost water is an important local factor affecting the shallow ground thermal regime and also plays a significant role in geotechnical problems. In this study, the impacts of supra-permafrost water ponding and drainage on the thermal regime and settlement characteristics of a railway embankment in continuous permafrost zone, the interior of the Qinghai-Tibet Plateau were investigated using 13-year records (2003–2016) of field measured data. Results showed that, because of the great latent heat of the ponded water alongside the embankment, a zero curtain layer (ZCL) as much as 5.5 m thick developed in and beneath the embankment after embankment construction in 2003. A three-layered thermal regime developed in the embankment and the subgrade. Although crushed rock revetments were installed 5 years after embankment construction, no soil cooling occurred beneath the ZCL and the ZCL remained stable in thickness. Meanwhile, embankment settlement developed quickly and nearly linearly from 2003 to 2012, with a cumulative settlement approaching approximately 160 mm at both the sunny and the shady embankment shoulders. Compression and creep of warm and ice-rich permafrost layer (WIPL) beneath the ZCL are considered as the main contributors to this significant settlement. Following drainage of the ponded water during 2012, the ZCL cooled quickly and completely frozen after two cold seasons. The underlying permafrost table moved upwards into the embankment and the WIPL cooled with time. Embankment settlement also slowed significantly. However, an asymmetrical temperature field developed in the embankment and the subgrade because of the sunny-shady embankment slope effect, leading to differential settlement between the sunny and the shady embankment shoulders. [ABSTRACT FROM AUTHOR]

Two-phase closed thermosyphons are extensively used for cooling foundation soils of tower footings in permafrost regions along the Qinghai-Tibet Power Transmission Line (QTPTL). In this paper, the working period of these thermosyphons is firstly determined by field observations. Then, a 3-D numerical heat transfer model is constructed to study the long-term cooling effects of thermosyphons and thermal performance of foundation soils. The numerically simulated results show a significant cooling of foundation soils near thermosyphons in cold season approximately from late October to early May, and a rapid shift of the thermosyphon cooling to the footing side due to greater thermal conductivity of the concrete footing. As a consequence, a colder bulb would develop under the footing, increasing the bearing capacity of foundation soils. In warm season from the mid-May to mid-October, the maximum thaw around the footing would be greater than that in the natural ground due to better heat transfer through the concrete footing. During a 50-year operational period, foundation soils under the footing would roughly go through about four thermal stages under a combined effect of the thermosyphon cooling and climate warming: a rapid cooling stage in the first to 5th year, a stable stage in the 6th–15th year, a rapid warming stage in the 16th–35th year and a slow warming stage after the 35th year. During the operational period, the maximum thaw around the footing would be less than the embedding depth of the footing (5.8 m), meeting the design criterion for thermal stability of foundation soils. [ABSTRACT FROM AUTHOR]

Preliminary field observations along the Qinghai–Tibet Power Transmission Line (QTPTL) show that foundation soils of some shallow footings (with an embedding depth of 3.7 m) with thermosyphons in permafrost regions still suffer substantial and rapid thaw settlement in warm seasons. In this paper, a series of numerical simulations on the long-term thermal performance of foundation soils of these shallow footings are carried out. The simulated results show that thermosyphons could do effectively cool foundation soils at depths 2.5 to 8.0 m under the footing. However, in warm season from mid-May to mid-October, the rapid warming of shallow foundation soils near the footing caused by heat transfer through the concrete footing could not be prevented. The maximum thaw around the footing could be as much as 1.0 m deeper than that in the natural ground. Under a warming climate, the maximum thaw around the footing would go deeper than the embedding depth of the footing with four thermosyphons in very warm (≥− 0.5 °C) permafrost regions and with two thermosyphons in warm (≥− 1.0 °C) permafrost regions during a 50-year operational period. To retard thaw penetration around the footing, a combined cooling method of thermosyphons and insulation boards is proposed for foundation soils. Numerically simulated results show that the additional placement of the boards on ground surface could prevent the rapid warming of shallow foundation soils and effectively reduce the maximum thaw around the footing. The method could also effectively delay permafrost warming under the footing. Thus, it is recommended to be used at shallow footings in very warm, ice-rich permafrost regions along the QTPTL. [ABSTRACT FROM AUTHOR]

Taking the sandstone subjected to pH = 1 and 3 HCl solutions as the research object, a dynamic chemical damage model was established based on mineral dissolution theory and chemical kinetics principles. A mechanical damage model of sandstone was also established by continuous damage mechanics theory. Moreover, a coupled chemical and mechanical damage model was deduced through microstress analysis, and a damage constitutive model for acid-corroded sandstone was constructed based on the Weibull distribution and the D–P criterion. The results showed that the acid–rock reaction showed evident time dependence, and the reaction rate decreased with the prolongation of corrosion time. The chemical damage variables of corroded sandstone subjected to pH = 1 and 3 HCl solutions reached 0.34 and 0.12 after 180 days, respectively. The damages caused by chemical and mechanical actions are nonlinear with the generalized damage. The chemical action can slow down the deterioration rate of sandstone under load, which was more obvious as the immersion time extended and the pH values of the solution declined. The stress state could change the deterioration process of sandstone, and the damage rate declined as the confining pressure increased. Finally, the uniaxial and triaxial compression test results verified the theoretical model, and the two agreed well, which verified that the coupled damage model is reasonable and applicable. [ABSTRACT FROM AUTHOR]

Abstract: Crushed rock embankment (CRE) was most prevalent embankment structure with cooling measures along the Qinghai–Tibet Railway in permafrost regions. Ground temperature dataset from embankment construction (in 2002) to 2010 at nine monitoring sites along the railway were compiled to study long term cooling processes and cooling effects of CRE. Monitoring results indicated that the heat stored in filling materials could dissipate out in two cold seasons after embankment construction. Under cooling effect of CRE, permafrost tables beneath CRE moved upwards significantly during the first three years after embankment construction and then maintain almost constant. But underlying permafrost experienced different thermal process in relative warm and cold permafrost regions. In cold regions, underlying permafrost up to 14m in depth experienced considerable cooling trends during the first five to sixth years after embankment construction. While in warm regions, the uppermost permafrost layer warmed after upwards movement of permafrost table, and underlying deep permafrost had no obvious cooling trends with time. Finally, differences in freezing and thawing seasons of, and differences in ground thermal regimes of cold and warm permafrost regions were investigated based on ground temperature from natural borehole. Then, the different cooling effects of CRE in two regions were interpreted based on these differences. [Copyright &y& Elsevier]

Abstract: Thermal regime of conventional embankments (CEs) was investigated by compiling ground temperature data from 13 monitoring sites along the Qinghai–Tibet Railway in permafrost regions. The investigation included shallow ground temperatures within embankment, variations of permafrost table and dynamic changes of deep permafrost temperature beneath embankment. The results indicated that the shallow ground temperatures within two embankment shoulders differed from each other; the difference at depth of 50cm was less than 1°C in warm seasons but more than 2.7°C in cold seasons. Statistic analyses of permafrost table variations beneath CEs confirmed existence of a reasonable height range of CE in regions with mean annual ground temperature (MAGT)<−0.6 to −0.7°C. When the height of CE lay in the range, underlying permafrost tables all had obvious upward movements and could be maintained well with time. By contrast, in regions with MAGT>−0.6°C, permafrost tables either declined (beneath sunny shoulders) or maintained near original level (beneath shady shoulders) at majority sites no matter what the height of CE was. As these permafrost tables were not stable; ground temperatures near them increased slowly with time. Analyses of ground temperature profiles indicated that the deep permafrost beneath CEs all warmed in the contexts of climate warming and construction activities, and this warming trend was more pronounced in warm permafrost regions. Supra-permafrost talik developed and/or permafrost degraded from both permafrost table and permafrost base beneath embankment at some sites with MAGT>−0.5°C. [Copyright &y& Elsevier]

Abstract: Based on field monitoring datasets, characteristics of embankment deformation were summarized along the Qinghai–Tibet Railway in four permafrost regions with different mean annual ground temperatures (MAGTs). Then, further analyses were carried out at some typical monitoring profiles to discuss mechanisms of these embankment deformations with consideration of detailed information of thermal and subsurface conditions. The results indicated that in regions with MAGT <−1.5°C, embankments only experienced seasonal frost heaves, and of which the magnitudes were not significant. So, the embankments in the regions performed satisfactorily. Whereas in regions with MAGT ≥−1.5°C, both traditional embankment and crushed rock embankment experienced settlements, but characteristics and mechanisms of the settlements were different for the two kinds of embankment. For crushed rock embankment, the magnitudes of settlement and differential settlement between right and left embankment shoulders were not significant and increased slowly. In respect that upwards movements of permafrost tables and better thermal stability of permafrost beneath embankment, mechanism of settlements on the embankment was inferred as creep of warm and ice-rich layer often present near permafrost table. While for traditional embankment, particularly in warm and ice-rich permafrost regions, the magnitudes of settlement and differential settlement between right and left embankment shoulders were significant and still increased quickly. Considering underneath permafrost table movements and permafrost temperature rises, mechanisms of settlements on the embankment included not only creep but also thawing consolidation of underlying permafrost. Therefore, some strengthened measures were needed to ensure long-term stability of these traditional embankments, and special attention should be paid on temperature, ice content and applied load within the layer immediately beneath permafrost table since warming and thawing of the layer could give rise to considerable settlement. [Copyright &y& Elsevier]

Abstract: The cooling effect and thermal stability of air-convection crushed-rock embankment, including crushed-rock revetment embankment (CRRE) and U-shaped crushed-rock embankment (USCRE), were comparatively investigated by analyzing the ground temperature data collected by a long-term monitoring system along the Qinghai–Tibet Railway. The performances of the CRRE and the USCRE were examined by temperature profiles of six monitoring sites located in permafrost regions with different mean annual ground temperatures (MAGTs). The results indicated that the grounds beneath the USCRE experienced better thermal stability than those beneath the CRRE due to enhanced cooling effect over winter time and insulating effect over summer time of the USCRE. The results also indicated that the thermal stabilities of the air-convection crushed-rock embankments varied with MAGT: in regions with lower MAGT, the air-convection crushed-rock embankments generated stronger cooling effects, and the cooling processes of the shallow grounds beneath the embankments were obvious while the deep ground temperatures kept almost constant; whereas in regions with higher MAGT, especially for the CRRE, the permafrost tables under the embankments had been elevated but the deep permafrost experienced considerable warming. In addition, the differences between ground temperatures beneath right and left shoulders of the embankments were obvious at some monitoring sites, and those thermal differences were closely related to the embankment orientation. The factors determining those thermal differences include solar radiation and may also include wind-forced convection within crushed-rock layers. [Copyright &y& Elsevier]

The shear behavior of a frozen soil-structure interface plays a vital role in analyzing the performance of engineered structures in cold regions. This study investigates the freeze-thaw cycling impact on the shear behavior of a frozen soil-concrete interface by direct shear tests. Test specimens consisting of a Portland cement concrete disk and frozen loess with an initial moisture content ranging between 9.2% and 20.8% were prepared and conditioned at various sub-zero temperatures (i.e., −1 °C, −3 °C, and − 5 °C) and freeze-thaw (F-T) cycles (i.e., 0, 5, 10, and 20). Experimental results, including shear stress - horizontal displacement curves, the displacements and shear strengths at both peak and residual status, are presented and analyzed. In particular, the influence of F-T cycling on shear behavior is examined. The peak displacement is found to increase linearly with increasing initial moisture content, but their correlation weakens as the number of F-T cycling increases. However, the residual displacement is found to be insensitive to the initial moisture content, temperature, normal stress, and F-T cycling. While the residual friction angle is slightly larger than the peak one before F-T cycling, both gradually increase with an increasing number of F-T cycling. Large cohesion in the peak strength, especially at lower temperatures, completely diminishes in the residual strength due to damage of ice bondage, resulting in a significant shear strength loss between the peak and residual status. The impact of F-T cycling on the interface shear behavior can be attributed to moisture migration toward, formation and accumulation of ice films at the interface. The findings from this study including the shear strength parameters and the displacements at both peak and residual status, can be used to simulate the performance of engineered geotechnical assets such as pile foundations, retaining walls, and earth dams or irrigation channels with concrete linings in cold regions. • Investigate the shear behavior of a frozen soil-concrete interface. • Focus on both shear strength and displacement characteristics. • Examine F-T cycling impact on the shear displacement and strength parameters. • Present key parameters for the simulation of the interface shear behavior. [ABSTRACT FROM AUTHOR]

Surface water is an important local factor in engineering geological conditions in permafrost regions. In this paper, the impacts of surface water ponding on the long-term thermal and settlement characteristics of an air convection embankment (ACE) were investigated based on 13 years of data records from field observations along the Qinghai-Tibet Railway (QTR). The time series of the ground temperatures and the settlement of two ACEs with and without adjacent surface water ponding were investigated and compared. The results indicated that surface water ponding could significantly affect the thermal performance of the ACE. Without surface water ponding, the ACE showed a satisfactory cooling effect; the underlying permafrost table rose, and the shallow permafrost layer cooled significantly after construction of the embankment. However, with surface water ponding, the great heat capacity and latent heat of the ponded water and related capillary action impeded the thermal interaction between the ACE and the underlying permafrost. Before the drainage of the ponded water, no permafrost cooling occurred beneath the embankment. The settlement of the two ACEs also differed significantly. The creep of the ice-rich permafrost layer immediately beneath the permafrost table was inferred as the main contribution of the settlement. The temperature of the layer was the main factor determining the rate of settlement. The results can help the hydro-thermal analysis of engineering foundations built over permafrost and provide a reference for embankment construction and maintenance in permafrost regions with rich surface water. • Water ponding can exert significant impacts on thermal performances of the ACE. • The ACE without water ponding showed satisfactory cooling on underlying permafrost. • Settlement of the ACE was mainly induced by creep of ice-rich permafrost layer. • Temperature of ice-rich permafrost layer determined settlement rate of the ACE. [ABSTRACT FROM AUTHOR]

In winter, cold air flowing into tunnels undergoes heat exchange with the surrounding rock and lining, rendering frost protection a critical concern. In this paper, a practical procedure for integrating the FEM–CFD coupled method into probabilistic analysis is presented for determining the thermal insulation length demand of tunnels in cold regions, in which uncertainties in the air temperature and wind speed are considered. The results indicate that with ventilation implemented in the coldest month, tunnel temperatures gradually rise along the longitudinal direction, exhibiting a distinct temperature variation near the entrance. Based on the numerical simulation results obtained with 100 air temperature and wind speed samples, generated using the Monte Carlo method, the thermal insulation length demand of the tunnel ranges from 1000 to 1500 m, showing a distribution that follows the characteristics of a normal distribution. Additionally, a corresponding surrogate model is developed, and a thermal insulation failure probability model is proposed to analyze the sensitivity of meteorological parameters, revealing that the air temperature is the primary factor influencing the thermal insulation length demand. The findings of this research provide novel insights for the engineering community in designing resilient and sustainable tunnels in cold regions, mitigating the potential risks associated with freezing events and ensuring safe and efficient transportation infrastructure. • Probabilistic models for air temperature and wind speed were proposed. • Uncertainties of air temperature and wind speed affect thermal insulation length. • A surrogate model of the thermal insulation length demand was developed. • An analysis of the thermal insulation failure probability was conducted. [ABSTRACT FROM AUTHOR]

The interfacial frictional resistance of anchor solids, bolt, and the surrounding soil of soil nails in the flexible support structures of frozen soil slopes is a key parameter for calculating and evaluating its stability. Based on a series of direct shear tests under the condition of a constant normal load boundary, the shear mechanical behavior of the interface between the soil and a cement slurry block under different soil temperatures, water contents, and normal pressures was studied, and the interfacial deformation mechanism was analyzed. The results show that in the thawing state, with the increase of water content, the shear surface moves from the side close to the soil to the side close to the cement slurry block. However, the shear plane is determined in the direct shear test, which makes the effect of the change of the soil's own strength on the interfacial shear strength smaller. In the frozen state, the behavior of the interfacial shear stress–shear displacement behavior at the same water content varies from strain-hardening type to strain-softening type with the temperature decreasing. The normal displacement is affected by temperature, normal pressure, and water content. At the same temperature, the maximum normal displacement gradually decreases with the increase of water content and normal pressure. At the same normal pressure and water content, the maximum normal displacement tends to increase and then decrease as the temperature drops. Still, the maximum normal displacement in the frozen state is more significant than in the thawing state. At the same water content and normal pressure, the interfacial peak shear strength shows a nonlinear increase with the decrease in temperature. At the same normal pressure and temperature, the interfacial peak shear strength delivers a linear increasing law with increasing water content. The peak interfacial cohesion increases with the decreasing temperature and increasing water content, and this phenomenon is more evident at lower temperatures. The interfacial friction angle does not change significantly with the rise in water content at the same temperature, but as temperature decreases, its average value increases significantly. The research results can provide a reference for the design of slope support structures in cold regions. [ABSTRACT FROM AUTHOR]

The Loess Plateau is one of the most tectonically and seismically active areas in the world. Observations from past strong earthquakes, particularly the Minxian–Zhangxian and Wenchuan earthquakes, have shown distinctive evidence of seismic site effects in the mountainous area of southeastern Gansu province. In this study, seismic damage in the loess areas of southeastern Gansu province induced by these earthquakes was investigated and briefly described. Different types of ground motion were selected, and the one-dimensional equivalent linear method was used for numerical analysis of the ground motion effects in the loess regions. Moreover, seismic response analysis of a typical loess tableland was conducted. The results showed that the seismic responses of a typical loess tableland under different seismic excitations have totally different dynamic characteristics. Moreover, the seismic damage in loess regions was more serious under far-field seismic excitation compared with near-field seismic excitation with the same peak acceleration. Through this study, the quantitative assessment of ground motion effects can be approximately estimated and the mechanism of site amplification effects on ground motion is further explained. [ABSTRACT FROM AUTHOR]

In cold and arid regions, the maintenance of roadways and railways faces significant challenges due to serious spalling and sheet erosion, rill erosion, and shallow sliding of loess slopes. Moreover, the frequency and severity of these shallow failures, as well as the vegetation cover, differ greatly between sunny and shady slopes, further complicating slope maintenance. This study focuses on field observations conducted over two consecutive years along a highway at the western end of Chinese Loess Plateau, specifically examining soil temperatures and moisture content within two cut slopes. By analyzing the differences in hydro-thermal regimes within a depth of 100 cm between the sunny and shady slopes, and considering the meteorological data collected at the study site, the research aims to provide insights into these failures of loess slopes. The results indicate that the shady slope undergoes significant seasonal freeze-thaw cycles from the surface down to a depth of 100 cm. The freezing duration ranges from 96 to 120 days, with a freezing index ranging from −250 °C·d to −850 °C·d. Additionally, the shallow layer on the shady slope experiences significant daily freeze-thaw cycles in late autumn and early winter. In contrast, the sunny slope experiences minimal seasonal freeze-thaw processes. Although the shallow surface layer of the sunny slope undergoes some daily freeze-thaw cycles in late winter and early spring, the amplitudes of soil temperature fluctuations are significantly smaller compared to those on the shady slope. Given that the precipitation is concentrated in summer and autumn, the moisture content of the soil on the shady and sunny slopes exhibits noticeable seasonal changes. The soil layers on the shady slope tend to be relatively dry, while those on the sunny slope remain relatively moist. The moisture content within the observed depth on the sunny slope remains relatively consistent, while on the shady slope, a clear gradient is evident. The moisture content of the shallow slope layers within a depth25 cm primarily responses to heavy rainfall events, with a greater response observed on the shady slope compared to the sunny slope. The field-observed hydro-thermal data collected in this study serve a valuable basis for implementing revegetation strategies and enhancing the stability of loess slopes in cold and arid regions. • Hydro-thermal regimes of sunny and shady cut slopes were observed in loess regions. • Seasonal and daily freeze-thaw of sunny and shady cut slopes were compared. • Dry-wet cycles of sunny and shady cut slopes were compared. • Response of soil moisture content of slopes to rainfall events were discussed. [ABSTRACT FROM AUTHOR]

Freeze–thaw cycles can deteriorate the expansive soil performance. Phase change materials (PCMs) absorb and store heavy latent heat from the environment, which can be utilized to mitigate freeze–thaw deteriorations in expansive soil. In this study, an environmentally friendly paraffin-based PCM was selected, which included two different forms: liquid (pPCM) and microcapsules (mPCM). The volume change, mechanical properties, thermal properties, and microstructure of the improved soil were studied. These results showed that PCMs can release considerable thermal energy during phase transitions, which improves the thermal stability of the soil. The pPCM enhanced the plasticity of soil and prevented brittle failure; mPCM reduced the damage to the soil microstructure caused by freeze–thaw cycles. Macroscopically, it weakened the swell–shrink behavior and limited the degradation of the resilient modulus, unconfined compressive strength, and failure strain of expansive soil. The optimal PCM content was determined based on the mechanical and thermal properties of the improved soil. This study provides a new idea for the engineering treatment of expansive soil in cold regions. [ABSTRACT FROM AUTHOR]

Using the triaxial shear or compressive strength as a single index of the resistance of frozen soils to failure does not always meet frozen soil engineering requirements for the comprehensive evaluation of the resistance. In this study, triaxial compression experiments were carried out on undisturbed ice-rich frozen clay samples with various levels of water content under different confining pressures to study the characteristics of the failure strain energy density of the samples. The results indicate that as the confining pressure increased, the failure strain energy density first increased and then decreased. The failure strain energy density reached a maximum at a critical confining pressure of 2.00 MPa for 13.25–25.76% water content and 1.00 MPa for 26.02–45.82% water content. The failure strain energy density increased as the water content increased at low confining pressures (0.05–0.50 MPa) but then declined slightly at intermediate confining pressures (1.00–2.00 MPa). At a high confined pressure of 3.00 MPa, the failure strain energy density decreased overall as the water content increased. There were similarities and differences between the change characteristics of the compressive strength and the failure strain energy density. The failure strain energy density can be used as a supplementary reference index of the resistance of frozen soils to damage. The variation characteristics of the failure strain energy density of undisturbed frozen clay are essentially consistent with those of remolded frozen sandy soils. However, there are also clear differences between the characteristics of the failure strain energy density of these two types of frozen soil. [ABSTRACT FROM AUTHOR]

The thermal state of permafrost in the present and future is fundamental to ecosystem evolution, hydrological processes, carbon release and infrastructure integrity in cold regions. In 2011, we initiated a permafrost monitoring network along the China–Russia crude oil pipeline (CRCOP) route at the eastern flank of the northern Da Xing'anling Mountains in Northeast China. We compiled an integrated dataset of the ground thermal state along the CRCOP route consisting of meteorological data near the southern limit of latitudinal permafrost, ground temperature (GT) data in 20 boreholes with depths of 10.0–60.6 m, soil volumetric liquid water contents (VWCs) and 2D electrical resistivity tomography (ERT) data at different sites. Results demonstrate a permafrost warming during 2011–2020 in the vicinity of the southern limit of latitudinal permafrost, as manifested by rising GTs at almost all depths in response to climate warming. Local thermal disturbances triggered by the construction and operation of CRCOPs have resulted in significant permafrost warming and subsequent thawing on the right-of-way (ROW) of the pipelines. This permafrost thaw will persist, but it can be alleviated by adopting mitigative measures, such as an insulation layer and thermosyphons. The in situ observational dataset is of great value for assessing the variability of permafrost under the linear disturbances of the CRCOPs and related environmental effects, for understanding hydro–thermal–mechanical interactions between the buried pipelines and permafrost foundation soils, and for evaluating the operational and structural integrity of the pipeline systems in the future. The dataset is available at the National Tibetan Plateau/Third Pole Environment Data Center (10.11888/Cryos.tpdc.272357; Li, 2022). [ABSTRACT FROM AUTHOR]

As a major parameter in the energy balance of the ground surface, temperature represents the level of exchange of energy and moisture between the ground and air. The Qinghai-Tibet Plateau (QTP) has the permafrost region with the highest altitude and the largest area in low–middle latitude of the world. The variation in ground surface temperature has an impact on the existence and development of the permafrost. Therefore, the analysis of the ground surface temperature in the QTP is significant to reflect the energy exchange in permafrost regions. This paper collected solar radiation data and calculated the conversion coefficient from total solar radiation to long-wave radiation of the ground surface on different underlying surfaces. The ground surface temperature was inversely calculated and modified based on the reception of solar radiation on different underlying surfaces. A simplified calculation model of ground surface temperature was built to reflect the ground surface temperature on different underlying surfaces of the QTP. The calculation results were compared with MODIS and showed good fitness, providing a systematic and reliable method for calculating the ground surface temperature on the QTP. The above model plays a significant role in the estimation of soil moisture, ground surface energy and water balance. [ABSTRACT FROM AUTHOR]

Thermal insulating materials (TIMs) are widely used in roadways, tunnels, airfields, pipeline systems, and buildings in cold regions to prevent infrastructure from freeze damages and reduce energy consumption. During the period of use in cold regions, however, seasonal freeze-thaw actions can cause degradation of thermal and mechanical properties of TIMs and even malfunctions. Lots of research has been carried out on the durability of TIMs under cyclic freeze-thaw (CFT), but comprehensive reviews are scarce at present. Based on a literature review, the state and development of the test apparatus, temperature controls for the CFT test, and changes in the thermal and mechanical characteristics of TIMs during the CFT operations were summarized in this study. The review shows that the CFT can damage the TIMs' microstructure, which will result in an increase in the materials' thermal conductivity and water absorption rate with a decrease in their mechanical strength. However, since the application scenarios of TIMs are various, there are significant differences in previous researches, especially the test conditions. These differences limited the applicability of the test results and make a barrier among researchers and engineers. Thus, a unified test method for the durability of TIMs under the CFTs should be established in the future. [ABSTRACT FROM AUTHOR]

To carry out engineering construction in a permafrost area, it is of great importance to grasp the spatial distribution of permafrost and its hydrothermal characteristics within the scope of engineering activities. Compared with the continuous permafrost area, the permafrost distribution in discontinuous permafrost area is complicated due to existence of island talik, which causes highly poor continuity and uniformity of ground hydrothermal conditions and brings great difficulty and uncertainty to the engineering geological investigation. In the present study, the spatial distribution of permafrost and the freeze-thaw process of shallow ground at the I-stage terrace of a river on the Qinghai-Tibet Plateau were studied based on field investigation including drilling, electrical resistivity tomography, and borehole temperature measurement. The results show that the relict permafrost, patchy permafrost, and island talik are distributed in the limited range of the study area. Among them, the buried depth of relict permafrost layer is in the range of 7 ~ 14 m, while the patchy permafrost layer is in the range of 3.0 ~ 17.0 m depth, and the mean annual ground temperature of both is close to -0.1°C, which belongs to extremely high-temperature permafrost. The seasonal freezing/thawing depths of shallow ground within the relict permafrost, patchy permafrost, and island talik range from 2.5 to 3.0 m. The freezing period of the seasonal freezing layer is 5.5 months, while the thawing period of the seasonal thawing layer is 6 months. The complex plane and spatial distribution of permafrost in the field are closely related to local factors such as topography, geomorphology, groundwater, and lithology of shallow ground. It is believed that the formation of the island talik in the study area is controlled by solar radiation and precipitation infiltration. [ABSTRACT FROM AUTHOR]

The lakes distributed in permafrost areas on the Tibetan Plateau (TP) have been experiencing significant changes during the past few decades as a result of the climate warming and regional wetting. In September 2011, an outburst occurred on an endorheic lake (Zonag Lake) in the interior of the TP, which caused the spatial expansion of three downstream lakes (Kusai Lake, Haidingnor Lake and Salt Lake) and modified the four independent lake catchments to one basin. In this study, we investigate the changes in surficial areas and water volumes of the outburst lake and related downstream water bodies 10 years after the outburst. Based on the meteorological and satellite data, the reasons for the expansion of downstream lakes were analyzed. Additionally, the importance of the permafrost layer in determining hydrological process on the TP and the influence of from lake expansion on engineering infrastructures were discussed. The results in this study showed the downstream lakes increased both in area and volume after the outburst of the headwater. Meanwhile, we hope to provide a reference about surface water changes and permafrost degradation for the management of lake overflow and flood on the TP in the background of climate warming and wetting. [ABSTRACT FROM AUTHOR]

Frost cracking is one of the primary causes of deterioration in frozen soil structures, yet few relevant numerical studies have been reported, and the simulation of frost cracking in soils remains challenging due to inadequate consideration of reasonable simulation algorithms. Numerous experimental studies have identified frost heave and desiccation shrinkage as the principal cause of frozen soil cracking. On this basis, this study presents a peridynamic (PD) model that considers the coupled effects of frost heaving and desiccation shrinkage for simulating frost cracking in soils during freezing process. The heat conduction equation is reformulated using the peridynamic differential operator (PDDO). The variation of thermal parameters for soils is addressed using the thermal enthalpy method, equivalent homogeneous method, and linear release assumption of latent heat. The frost-heaving load induced by pore water is represented using an equivalent displacement load. The multiphysics solution using PDDO and bond-based peridynamics (BBPD) considering freezing heave and desiccation shrinkage is developed for the first time. By simulating the frost cracking of a two-dimensional soil strip after model validations, the resulting crack pattern closely resembles the experimental observation. It indicates that the present model can capture the phase transition interface (PTI) and cracking behaviors of frozen soils. [ABSTRACT FROM AUTHOR]

Owing to the limitations of the apparatus, the influence of its principal stress direction on the anisotropic behavior and non-coaxiality of frozen soil has not been fully considered in previous studies. At a temperature of -10°C, a series of hollow cylinder tests for frozen standard sand (FSS) was conducted under different directional angles of major principal stress and mean principal stresses in this study. The experimental results indicate that the stress-strain-strength anisotropy and non-coaxiality of the FSS are highly dependent on the principal stress direction. The stress components of the FSS vary linearly with increasing shear stress at different directional angles of the major principal stress and mean principal stresses. With a linear increase in shear stress, the strain components of the FSS exhibited a nonlinear increasing trend. The FSS strength gradually decreased as the directional angle of the major principal stress and the mean principal stresses in the test range increased. Under the different principal stress directions, the non-coaxiality of the FSS, non-coincidence of the direction of the principal strain increment and the principal stress direction, were observed. The directions of the principal strain increment and principal stress gradually tended to be coaxial as shear stress increased. Although the non-coaxial angle of the FSS increased gradually with an increase in the directional angles of the major principal stress, it did not change with the change in the mean principal stress. The non-coaxial angle of the FSS was observed to be as large as 35° in the early stage of shearing under different mean principal stresses. [ABSTRACT FROM AUTHOR]

The China-Russia Crude Oil Pipeline (CRCOP), one of the four key national energy strategic channels, plays an important role in ensuring national energy security, diversifying oil supplies and transportation patterns, promoting the China-Mongolia-Russian economic corridor, and deepening Sino-Russian strategic cooperation. Aiming at permafrost engineering problems along the CRCOP, the principles of thaw control are put forward. This study develops a set of innovative techniques for mitigating and controlling the thaw of permafrost foundation soils of the pipeline, which provides an important guarantee for the green construction and safe operation of the CRCOP and can protect the permafrost environment along the pipeline route. The research results can offer important references and basis for design, construction, and maintenance of the similar projects in permafrost regions. [ABSTRACT FROM AUTHOR]

Linear transportation infrastructure constructed in permafrost zones is severely affected by permafrost degradation and the consequent thaw settlement. Considering the strong heat absorption of wide asphalt pavement, constructing an expressway with an integral embankment or two separated embankments in permafrost zones is under discussion at present. In this paper, thermal regime of permafrost beneath expressways constructed with an integral embankment (IE, 3 m in thickness) and two separated embankments (SEs, 3 m in thickness and with an 20 m wide separating strip) is investigated by numerical simulation with consideration of climate warming effect firstly. The results show that permafrost degradation beneath the IE is more significant than that beneath the SEs. Till the 50th operational year, the simulated permafrost table beneath the centerlines of the IE and the SEs decline by 6.5 m and 4.6 m, respectively, with a difference up to 1.9 m. The underlying permafrost warming beneath the IE is also severer than that beneath the SEs. Then, numerical simulations are carried out to investigate thermal regime and potential thermal interaction beneath the SEs with different widths of separating strip. The results show that when the separating strip is narrow than 20 m, thermal interaction between the SEs will lead to an asymmetric thermal regime beneath the two embankments. Compared with the outer side of the SEs, the permafrost table beneath the neighboring side of the SEs is deeper and the underlying permafrost is also warmer. Thermal interaction between the SEs can also lead to considerable permafrost degradation beneath the separating strip. Thus, the separating strip width should be considered during design of an expressway constructed with the SEs in permafrost zones. • Comparative analysis on thermal performances of the IE and the SEs is carried out. • Potential thermal interaction between the two SEs with different separating strip widths is investigated. • The asymmetry of permafrost table and soil temperature beneath the SEs is discussed. [ABSTRACT FROM AUTHOR]

Most red beds demonstrate inferior geotechnical properties in natural conditions and need to be improved when used as construction material. In this study, a serious of triaxial tests, permeability tests, and scanning electron microscopy (SEM) analysis were carried out on lime-stabilized and untreated red bed soil after experiencing different wetting-drying (W-D) cycles. The test results showed that, with the increase in the added lime, the shear strength, strength parameters (including the cohesion and the internal friction angle), and the shear modulus of red bed soil increased gradually. For the untreated specimens, the four parameters decreased considerably after experiencing W-D cycles, while for the lime-stabilized specimens, they generally increased with an increase in the W-D cycles. Without experiencing the W-D cycles, the permeability coefficient increased by two times after it was stabilized with 10% lime. But with an increase in the W-D cycles, the permeability coefficient of the untreated and lime-stabilized specimens continuously increased and significantly decreased, respectively. Finally, variations in microstructure of the red bed soil under the effects of the lime stabilization and W-D cycles were discussed based on the SEM analysis. The results may contribute to improvement of red bed soil when used as roadbed and airfield fillings. [ABSTRACT FROM AUTHOR]

Over the last several decades, a variety of methods have been developed for treating contaminated sites. Which technique is applied depends on the type of contaminant, the extent of dispersion, and the likelihood of additional contamination during the cleanup. Among the selection of potential techniques is the artificial freezing method, in which soil is frozen to significantly lower the permeability and prevent further diffusion of pollutants. While this method appears to have merit in theory, few studies have investigated the migration of pollutants during the soil freezing process. In this paper, a series of artificial freezing purification tests were performed and the influences of different freezing rates on purification efficiency were investigated. The experimental results show that the freezing rate is an important factor affecting the freezing purification efficiency for heavy metal copper and the purification rate decreases rapidly with increasing freezing rate. A soil freezing rate of 0.3 °C/h results in a heavy metal copper purification rate of only 27%; far less than that of the freezing rate of 0.05 °C/h, where the purification rate is 85%. This study demonstrates that a strong correlation exists between the purification rate of heavy metal copper and the soil freezing rates. We also demonstrate that the purification rate can be expressed as a cubic of the soil freezing rate. Therefore, the optimal purification of contaminated sites can be obtained by determining the proper freezing rate. [ABSTRACT FROM AUTHOR]

Abstract The permafrost in the Da Xing'anling Mountains Northeastern China has experienced rapid and significant degradation due to climate warming and human activities during the last few decades. In this study, the permafrost degradation and freezing/thawing characteristics of the active layer at Mo'he airport were investigated through a ground temperatures data set collected over 10 years of measurements. During the monitoring period, thawing permafrost under the airport was observed, induced by climate warming, and thermal disturbance of airport construction and operation. The permafrost table under the blast pad declined considerably and varied from −9 m to −14 m, and from −4.5 m to −12 m under the natural site. The maximum depth of frost penetration at this airport shows a decreasing trend. The freezing duration of soil under the blast pad is shorter comparing with the natural ground surface, while its thawing duration is longer. The warming rates of permafrost at −10 m depth under the blast pad were 0.25–0.33 °C/a, and those under the natural ground were 0.03 °C/a and 0.15 °C/a. The results provide valuable data for designing, construction and operation of the airport, and a reference for other airports construction in permafrost regions. Highlights • The permafrost under the Mo'he airport degraded and disappeared during the past 10 years. • The maximum depths of frost penetration under the runway exhibited a decreasing trend. • The freezing duration of foundation soil under the concrete blast pad was shorter and its thawing duration was longer. • The deep ground temperature under the blast pad showed a higher warming rate. [ABSTRACT FROM AUTHOR]

In this paper, a series of triaxial cyclic loading-unloading (L-U-C), triaxial cyclic loading-unloading-holding (L-U-H-C) and triaxial cyclic stress relaxation tests (L-R-C) were performed for frozen China standard sand (FCSS) under laboratory conditions varying with confining pressure, temperature and holding time. A composite system synchronizing the real-time computed tomography (CT) device and the triaxial testing device was used to explore the internal structure change of FCSS during L-U-H-C. The strengthening behavior was clearly observed by comparing the L-U-C and L-U-H-C tests. While this behavior is generally caused by particle compaction as occurred in common soil, pressure melting of ice crystal and recrystallization followed may additionally contribute to strengthening behavior of frozen soil. The increase in mean CT number during the holding stage further confirms the effects of pressure melting of ice crystal and recrystallization on the strengthening of FCSS from an internal structure aspect. [ABSTRACT FROM AUTHOR]

Abstract The China-Russia Crude Oil Pipeline (CRCOP) traverses 441-km-long permafrost zones in northeastern China. Since the pipeline operation in 2011, significant ground surface subsidence occurred within the trench of the pipeline in warm and ice-rich permafrost zones. Oil temperatures from three pump stations were obtained and the soil temperatures around the pipe at two cross sections along the CRCOP in Jiagedaqi, northeast China were monitored during the period from 2011 to 2017. With the date records, permafrost thawing and thaw bulb development around the pipe were investigated in this study, as well as the cooling effect of thermosyphons experimentally installed around the pipe. Results showed that the mean monthly oil temperature ranged from 0.4 to 17.9 °C and increased slowly with operation time increase. Due to thermal effect of the warm oil, the artificial permafrost table near the pipe was approximately 5.9 m greater than that in the natural ground 5 years after pipeline operation. A thaw bulb developed around the pipe and expanded at a rate of more than 1.0 m/a in depth. Following the permafrost thawing, ground subsidence around the pipe was significant. The buried depth of the pipe declined with a rate of approximately 0.35 m/a during the study period. To mitigate the permafrost thawing and the pipe settlement, a pair of thermosyphons were experimentally installed at the two sides of the pipe in March 2015. After 2 years of installation, the shallow foundation soils were cooled down and the thaw bulb was partially re-frozen. However, there was still a thawed layer beneath the pipe. Long-term thermal regime of foundation soils under the cooling effect of the thermosyphons needs further investigation. Highlights • Warm oil caused significant permafrost thaw around the oil pipe. • A thaw bulb developed around the oil pipe and expanded rapidly with time. • Thermosyphons near the pipe cooled shallow grounds but the thaw bulb still existed. [ABSTRACT FROM AUTHOR]

A physical calculation model for the content of unfrozen water of silty clay was proposed with a consideration of freezing point. The freezing point of bulk water, capillary water and bound water was calculated. The freezing point of bulk water was assumed to be constant in the calculation. The Kelvin equation and the theory of water activity were used to calculate the freezing-point changes of the capillary and bound water during the volume decrease, respectively. The influence of soil-particle size on the freezing point was taken into account in the model. In the calculation, the unfrozen water content at a negative temperature was defined as the sum of the volume of capillary and bound water whose freezing point was lower than the given temperature. By using silty clay from the Qinghai-Tibet Plateau as an example, the results of the model calculations were validated by using the measured data from time-domain reflection and nuclear magnetic resonance. The calculations for four existing models as proposed by other scholars were listed for comparison with our model. The results show that the calculated results of our model are precise and reflect the hysteresis effect of the unfrozen water content during freezing and thawing. The model validated an order of freezing in soil: bulk water froze first, most capillary water froze next, and most bound water froze last. This paper explains the existence of unfrozen water from the perspective of freezing-point change and provides a theoretical basis for the calculation. [ABSTRACT FROM AUTHOR]

A series of multistage triaxial compression, creep, and stress relaxation tests were conducted on frozen loess at the temperature of −6℃ in order to study the damage evolution and recrystallization enhancement of mechanical properties during deformation process. The effect of strain rate, confining pressure, and hydrostatic stress history in the degradation laws of mechanical properties is investigated further. The strain rate has a significant influence on the stress–strain curve which dominates the evolution trend of mechanical properties. The mechanical behaviors (strength, stiffness, and viscosity) of frozen loess all exhibit evident response for the consolidation and pressure melting phenomenon caused by the confining pressure. The multistage loading tests under different hydrostatic stresses are capable of differentiating the development characteristics of mechanical properties during axial loading and hydrostatic compression process, respectively. The testing results indicated that the recrystallization of the ice particle in the frozen soils is an important microscopic factor for enhancement behaviors of mechanical parameters during the deformation process. This strengthening degree of mechanical properties is determined by temperature, duration time, deformation degree, and stress state during the recrystallization process. The phase transformation led by pressure melting and ice recrystallization is a nonnegligible changing pattern of frozen soils microstructure, which has apparent role in the damage evolution of mechanical properties. [ABSTRACT FROM AUTHOR]

Densely compacted loess on which many man-made infrastructures are built are often exposed to strong weathering processes such as repeated wetting-drying (WD) and freeze-thaw (FT) cycles. These weathering processes in arid and seasonally frozen ground regions result in loose structure and strength deterioration of loess soils, and they cause serious engineering problems and expensive maintenance costs of infrastructures built on densely compacted loess. In this study, specimens of densely compacted loess were exposed to different intensities of weathering processes under controlled laboratory environment to quantitatively assess their deterioration effects on geotechnical properties. According to the laboratory results, the wetting-drying weathering has a stronger impact on the geotechnical properties of densely compacted loess than the freezing-thawing weathering. The unconfined compressive strength, elastic modulus, and cohesion of the loess specimens decrease with increasing number of WD and FT cycles while the vertical compression strain and collapse deformation increase. The densely compacted and non-collapsible loess specimens exhibited collapsible again (secondary collapse) after 5 WD cycles. The deterioration mechanisms of densely compacted loess induced by wetting-drying and freezing-thawing weathering processes have been presented and were strongly recommended to be taken into account for the prediction of long-term stability and serviceability of man-made infrastructures built on densely compacted loess. [ABSTRACT FROM AUTHOR]

A series of triaxial compression, creep and stress relaxation tests at temperature of − 6 °C were conducted on frozen loess, experienced different freeze-thaw numbers (0–12times), in order to study the thermal cycling effect in mechanical behaviors. The freeze-thaw process has a strong influence in the mechanical behaviors of frozen loess. The strength, stiffness and viscosity properties of frozen loess weaken gradually with the increase of the freeze-thaw procedure until the cycle number gets to the critical value for steady state of soil sample. The freezing low temperature ranged from − 6 °C to − 12 °C has no evident effect in the testing results of same freeze-thaw numbers. The strength distribution loci of frozen loess in the equivalent stress-mean stress space exhibits a unidirectional shrinkage over the range of the freeze-thaw process studied. Triaxial strength characteristics of frozen loess, experienced different freeze-thaw numbers, are more in agreement with the Ma's failure criterion for frozen soils (Ma et al., 1993). The stiffness properties (elastic modulus, elastoplastic coupled degree and damage degree) and the viscosity properties (relaxing stress, stable relaxation time, stable creep rate and viscoelastic modulus) of frozen loess both depend on the freeze-thaw process. The microstructure change of frozen loess induced by the thermal cycling effect is analyzed detailly so as to explore the micro-mechanism of property degeneration. Finally, a developed constitutive model, considering the freeze-thaw and confining pressure effect, is proposed to research and predict the triaxial strength and deformation characteristics of frozen loess experienced different thermal cycling process. [ABSTRACT FROM AUTHOR]

This paper presents the Kuyu tunnel as an example to study the deformation characteristics and collapse mechanisms of Water-Rich Soil Tunnels (WRST) by means of field monitoring and numerical simulation. The obtained results indicated that deformation of the surrounding soil occurs mainly when the excavating face is between 2.0 and 0.5 of tunnel's diameter from the observation section. The deformation and collapse of WRST are controlled by the geological conditions (poor lithology and seepage-induced decrease in soil strength) and construction factors (overlong cyclic footage and delayed supporting enlarge the disturbance area). This study provides some useful information for the design and construction of WRST. [ABSTRACT FROM AUTHOR]

In the permafrost areas, the frozen salinized sandy soil is a unique geotechnical component whose mechanical properties will influence the stability of engineering. Thus, formulating a constitutive model for frozen salinized sandy soil has significance scientifically for the design of engineering in cold regions. Combining the theoretical framework of breakage mechanicals for materials, micromechanical analysis method, and homogenization method, a meso-macro constitutive model for frozen salinized sandy soil is put forward in this research. In the proposed model, considering the structure characteristics, the frozen salinized sandy soil is abstracted as a type of inhomogeneous binary materials, the bonded elements composed of the unbroken ice crystals, salt crystals, and soil particles, behaving elastic-brittle mechanical properties, and the frictional elements composed of the broken ice crystals, salt crystals, and soil particles, with elastic-plastic mechanical properties. Due to the disparity in the mechanical characteristics of the constituent materials, the impact of salt concentration on the mechanical parameters of the bonded elements is also taken into consideration. Moreover, the nonuniform distribution of stress and strain with the representative volume element is described by introducing the breakage ratio with the derivation of the strain concentration tensor. The comparison with the tested results demonstrates that the proposed mese-mace constitutive model could simulate the stress-strain and volume law of frozen salinized sandy soil under different confining pressures, which has satisfactory adaptability to reflect the physical and mechanical properties of frozen salinized sandy soil. • A meso-macro constitutive model is derived for frozen salinized sandy soil. • The nonuniform distribution is described by the derivation of the strain concentration tensor. • The mechanical differences of the components for the frozen saline sandy soil are considered in proposed model. [ABSTRACT FROM AUTHOR]

The damage variable is very important to study damage evolution of material. Taking frozen loess as an example, a series of triaxial compression and triaxial loading-unloading tests are performed under five strain rates of 5.0 × 10−6–1.3 × 10−2/s at a temperature of −6°C. A damage criterion of frozen loess is defined and a damage factor Dc is introduced to satisfy the requirements of the engineering application. The damage variable of frozen loess is investigated using the following four methods: the stiffness degradation method, the deformation increase method, the dissipated energy increase method, and the constitutive model deducing method during deformation process. In addition, the advantages and disadvantages of the four methods are discussed when they are used for frozen loess material. According to the discussion, the plastic strain may be the most appropriate variable to characterize the damage evolution of frozen loess during the deformation process based on the material properties and the nature of the material service. [ABSTRACT FROM AUTHOR]

Rapid permafrost thawing triggered by heat release from the buried warm-oil pipeline would result in the instability of the pipeline, posing a potential risk of an oil spill in permafrost regions. Therefore, how to precisely control the ground temperature to mitigate pipeline foundation permafrost from thawing is of significant importance. Thermosyphon is a widely-accepted and extensively-used countermeasure against permafrost thawing in permafrost engineering. This paper presents 3-year (2015–2018) monitored data of ground temperature and geophysical survey results conducted by electrical resistivity tomography in April 2018, to investigate the thermal stabilizing effect of thermosyphons installed nearby the China-Russia crude oil pipeline (CRCOP) and the influencing factors. Furthermore, numerical simulation tests were conducted to evaluate their long-term cooling applicability and to optimize their layout parameters. Field observations show that the thermosyphons can cool down and even freeze the thawed soil layers around the CRCOP, prohibiting the infiltration of water into the thawing front to some extent and effectively mitigating the rapid thawing of permafrost beneath the pipeline. The performance of the thermosyphons is greatly influenced by the method and time of thermosyphon installation, as well as layout parameters including the number and longitudinal spacing. Two pairs of thermosyphons with a shorter spacing are unable to control the thawing of the underlying permafrost due to the non-anticipated higher oil temperature, the thermal erosion of water ponding, and climate warming. The simulated results indicate that thermosyphons perform well in the first 5 years after installation. Thermosyphon lowers the mean annual temperature of soils surrounding it by about 2 and 1.4 times when the evaporator section length is increased by 50% and the longitudinal spacing is decreased by 0.5 m, respectively. The application of thermosyphons has proven its strength for thermally stabilizing the pipeline foundation permafrost along the CRCOP. • 3-year ground temperature data and geophysical survey results are presented. • Thermosyphon can mitigate the rapid thawing of permafrost beneath the pipeline. • Thermosyphons performs most significantly in the first five years. • Lengthening thermosyphon and decreasing spacings are favorable for the better cooling effect. [ABSTRACT FROM AUTHOR]

Multiple studies demonstrate that narrow-linear strong disturbances triggered by pipeline engineering activities have resulted in rapid permafrost degradation. However, the extent, duration, and severity of permafrost disturbances induced by two parallel buried warm-oil pipelines remain unclear. Here, we examine the thermal disturbances of the China-Russia crude oil pipelines (CRCOPs I and II) on the surrounding permafrost. Ground temperature measurements on and off the right-of-way of the CRCOPs and an electrical resistivity tomography survey were conducted at a selected high-latitude wetland site in northeastern China. The observations showed that warm oil flow in pipelines dissipated heat to the surrounding soil, resulting in the thawing of underlying permafrost accompanied by the formation and development of thaw bulbs around the pipes. Currently, the thaw bulb of CRCOP-I was about four times as large as that of CRCOP-II, mainly due to the longer 7-year working duration of CRCOP-I. To quantify the permafrost disturbances around the pipeline, a conductive heat transfer model was established. A numerical simulation of the soil thermal regime around the pipeline suggested the initial growth of the thaw bulb was quite fast but gradually slowed with time, of which the shape was modified to ellipse from a semicircle. The existence of CRCOP-II would accelerate the warming of permafrost on the CRCOP-I right-of-way due to the small separation distance between them. We expect that the thaw bulbs of the two pipelines will expand larger than previously estimated due to the adverse effects of pipeline settlement and surface water flow, sparking a wider permafrost disturbance. • Thermal permafrost disturbances caused by two parallel buried warm-oil pipelines in a high-latitude wetland were analyzed. • Initial growth of thaw bulb around the warm-oil pipeline was fast but gradually slowed with time. • Thaw bulbs will expand and eventually overlap partially as the operational period of the two pipelines proceeds, sparking a wider permafrost disturbance. [ABSTRACT FROM AUTHOR]

A series of triaxial compression and creep tests were conducted on frozen loess under different confining pressure and temperature conditions in order to study the rate-dependent mechanical behaviors. Experimental results illustrate that uniaxial instantaneous strength and long-term strength both follow the same variation rule with change of temperature over the range studied. The frozen loess exhibits prominent dilatancy behavior under low confining pressures. When the stress level exceeds the long-term strength, the mean dilatancy coefficient has a tendency to constant value in stable creep stage. The parabola-shape strength loci of frozen loess in mean stress-effective stress space were determinated experimentally. This testing result is more in agreement with the Ma's yield criterion for frozen soils (Ma et al., 1993). In order to establish the rate-dependent constitutive model of frozen loess, the hyperplasticity theory with multiple internal variables is applied and further extended to rate-dependent materials in this investigation. The predictive capability of this theoretical model, considering the effect of the pressure crushing and melting phenomena, is verified by the stress–strain-time behavior of the frozen loess in triaxial creep tests. [ABSTRACT FROM AUTHOR]

Many of the unprotected tower footings along the Qinghai–Tibet Power Transmission Line (QTPTL) were buried by open excavation in thaw-unstable permafrost. Their thermal stability was of concern due to climate warming and human-induced thermal disturbances. On the basis of ground temperature measurements during the last three years, the tower foundation soils were stable during the studied period from 2011 to 2013 in spite of appreciable impacts of engineering construction of tower footings on the thermal regimes of the surrounding and underlying permafrost. The thaw front near a typical concrete footing was 0.67 m to 1.74 m deeper than that at adjacent natural, undisturbed sites in summer, and ground temperatures near such footings were lower than those at nearby natural sites in winter. This thermal enhancement (TE) effect of concrete footings lengthened the duration of freezing and thawing of the surrounding soils, deepened the permafrost table near the tower foundations, and subsequently adversely affected the thermal stability of the tower foundation soils. Numerical modeling was used for simulating the long-term thermal stability of the foundations soils and impacts of the TE effect, ice contents, mean annual ground temperature (MAGT), embedding depths of tower footings, and climate warming on ground thermal regime. Under the scenario of climate warming by 2.6 °C within the next 50 years, the active layer thickens and permafrost temperatures at depths rise progressively with time. The ice contents and MAGT of permafrost remarkably would affect the degradation rate of permafrost around the footings. The TE effect of the footings thickened the active layer by at least 1 m relative to natural sites 20-m away from the footings. The thaw settlement of tower foundation soils could present higher risk for the unprotected footings with a shallow (< 5.8 m) embedment in warm and ice-rich permafrost soils under a warming climate. In the end, some recommendations and suggestions were made. [ABSTRACT FROM AUTHOR]

The freezing behaviour of soil in deep alluvium is one of the important issues in artificial freezing engineering. In this paper, an apparatus was developed for studying the freezing characteristics of soil under high loading conditions and the related test procedures were also suggested. Based on this apparatus, a series of one-dimensional freezing tests were also carried out on silty clay under various loading conditions and pseudo temperature gradients. The results show that all soil samples exhibit compression under high loading conditions, and it consists of four stages in which the strain rate is I) very low, II) rapidly increasing, III) decaying and IV) relatively constant. During freezing tests, the freezing front moves downward rapidly during the initial period, and it tends to be stable after 10 h. As the increase of axial load or pseudo temperature gradient, the freezing front moves downward more quickly and lies closer to the warm end at the thermostatic stage. Even under high loading conditions, there is an obvious water redistribution phenomenon, which shows that the water migrated to the frozen part and gathered at the freezing front in the soil sample under the existence of a pseudo temperature gradient. [ABSTRACT FROM AUTHOR]

Abstract: The China–Russia Crude Oil Pipeline (CRCOP) faces significant challenges due to differential frost heaving and thaw settlement resulting from significant variations of oil temperatures along the pipeline. Oil temperature distribution along the pipeline during the long-term operation period is a very important factor in pipeline foundation design under future climate warming and various frozen soil conditions. It is important for the assessment and prediction of differential frost heave and thaw settlement of the pipeline foundations soils, forecasting the development of the seasonal and inter-annual frozen and thawed cylinders around the operating pipeline, stress–strain analysis of the pipeline, and mitigation of subsequent frost hazards. A quasi three-dimensional computational model was developed to predict the oil temperature along the pipeline. It was verified by analytic solutions of the minimum oil temperatures along the route provided by the Daqing Oilfield Engineering (DOE) Co. The oil temperatures were predicted and analyzed for two proposed annual oil flow rates of 15million tons (0.3mbpd) and 30million tons (0.6mbpd) with and without mitigative measures (only pipe insulation was considered here) during the operation period. Also, the inter-annual variations of oil temperature at key typical locations were investigated to understand the impact of climate warming. The results indicated that the maximum oil temperature cools southwards, but the minimum oil temperature warms southwards (with the inlet oil temperatures from −6 to +10°C). However, the average annual oil temperature decreases southwards in the northern part of the pipeline, then it starts to slowly increase. The amplitudes of oil temperature change will decrease southwards. Oil temperatures will slightly increase with elapsing time due to the imposed boundary conditions of climate warming. The oil temperatures with a lower flow rate vary more significantly than that with a higher flow rate because the oil temperature with a low flow rate is more affected by the thermal regime of the surrounding soils and the external environments. Insulation around the pipeline tends to reduce the oil temperature variations along the pipeline during pipeline operation period. Therefore, pipe insulation can effectively reduce the development of frozen and thawed cylinders in the permafrost zone. The phase change of water in soils around the pipeline has a distinct influence on the oil temperature during the freeze–thaw transition periods. The oil temperature tends to be equal to the ambient ground temperature around the pipeline with southward distance and with elapsing operation time. The pipeline oil temperature is controlled by the incoming oil temperature and the surrounding ground temperature before the equalization. It would be mainly controlled by the ground temperature around the pipeline afterwards. [Copyright &y& Elsevier]

Abstract: The proposed China–Russia Crude Oil Pipeline (CRCOP) will be subjected to strong frost heave and thaw settlement of the surrounding soil as it traverses permafrost and seasonally frozen ground areas in Northeastern China. The freezing–thawing processes, the development of the maximum frozen cylinder in taliks and thawed cylinder in permafrost areas, and the variations in the maximum freezing depths under the pipeline in taliks and thawing depths in different permafrost regions near Mo''he station, the first pumping station in China, were studied in detail using numerical methods in this paper. The inlet oil temperature at Mo''he station was assumed to vary from 10 to −6°C in a sine wave form during the preliminary design phase. Research results showed that the freezing–thawing processes of soils surrounding the buried pipeline had distinct differences from those in the undisturbed ground profile in permafrost areas. In summer, there was downward thawing from the ground surface and upward and downward thawing from the pipeline''s surface once the temperature of the oil rose above 0°C. In winter, downward freezing began from the ground surface but upward and downward cooling of the cylinder around the pipeline didn''t begin until the temperature of the oil dropped below 0°C. However, in the undisturbed ground profile, downward thawing from the ground surface occurred in summer and downward freezing from the ground surface and upward freezing from the permafrost table occurred in winter. The maximum thawing depths and thawed cylinder around the pipeline in warm permafrost enlarged with elapsing time and decreasing water content of the soils. In taliks, the maximum freezing depths and frozen cylinder around the pipeline kept shrinking with elapsing time and increasing water content of the soils. The freezing–thawing processes and development of the thawed and frozen cylinders around the pipeline were muted by any insulation layer surrounding the pipeline. Insulation had better thermal moderating on the heat exchange between the pipeline and the surrounding soils during the early operating period. But its role slowly weakened after a long-term operating. Research results will provide the basis for assessment and forecasting of engineering geological conditions, analysis of mechanical stability of the pipeline, foundation design, and pipeline construction and maintenance. [Copyright &y& Elsevier]

The China–Russia crude oil pipeline (CRCOP) traverses rivers, forests, and mountains over permafrost regions in northeastern China. Water accumulates beside the pipe embankment, which disturbs the hydrothermal balance of permafrost underlying the pipeline. Ground surface flows along the pipeline erode the pipe embankment, which threatens the CRCOP's operational safety. Additionally, frost heave and thaw settlement can induce differential deformation of the pipes. Therefore, it is necessary to acquire the spatial distribution of water features along the CRCOP, and analyze the various hazard probabilities and their controlling factors. In this paper, information regarding the permafrost type, buried depth of the pipe, soil type, landforms, and vegetation were collected along the CRCOP every 2 km. Ponding and erosive damage caused by surface flows were measured via field investigations and remote sensing images. Two hundred and sixty-four pond sites were extracted from Landsat 8 images, in which the areas of 46.8% of the ponds were larger than 500 m2. Several influential factors related to freeze–thaw hazards and erosive damage were selected and put into a logistic regression model to determine their corresponding risk probabilities. The results reflected the distributions, and forecasted the occurrences, of freeze–thaw hazards and erosive damage. The sections of pipe with the highest risks of freeze–thaw and erosive damage accounted for 2.4% and 6.7%, respectively, of the pipeline. Permafrost type and the position where runoff encounters the pipeline were the dominant influences on the freeze–thaw hazards, while the runoff–pipe position, buried depth of the pipe, and landform types played a dominant role in erosive damage along the CRCOP. Combined with the geographic information system (GIS), field surveys, image interpretation and model calculations are effective methods for assessing the various hazards along the CRCOP in permafrost regions. [ABSTRACT FROM AUTHOR]

To study long-term strength characteristics under complex stress states, a series of hollow cylinder creep tests were conducted on frozen clay under different stress levels at a temperature of −6 °C. Experimental results showed that the creep characteristics of frozen clay were significantly affected by the mean principal stress and the intermediate principal stress coefficient. Most generalized shear creep curves of frozen clay presented typical non-attenuation creep under axial load and torque. A long-term strength criterion for frozen soil, exhibiting the strength characteristics of multiple stress components under complex stress states, was obtained based on activation energy theory. In addition, a modified elliptical function and a generalized nonlinear strength theory (GNST) were introduced to describe the evolution of the strength envelope with time. Finally, a new strength criterion for frozen clay in three-dimensional principal stress space, considering the effect of time, was established by combining the modified elliptical equation and the GNST shape function. The proposed strength criterion has great significance for predicting the strength characteristics of frozen clay under complex stress states. • The long-term strength characteristics of frozen clay were obtained under axial load and torque. • The long-term strength criterion for frozen clay under complex stress states was established. • The experimental failure surfaces of frozen clay with time changes had been shown in 3D principal stress space. • A new strength criterion for frozen clay was proposed considering the influences of time in 3D principal stress space. [ABSTRACT FROM AUTHOR]

A series of triaxial shear, creep and relaxation tests with different stress paths were conducted on frozen loess at a temperature of −6 °C in order to experimentally study the path-dependent strength and deformation behaviours of the loess. Testing results indicate that the strength, the stiffness and the stress-strain relationships of frozen samples all rely on consolidation pressure and the shear stress path. The effects of K0 (0.625–1) consolidation path in the mechanical behaviours of frozen loess are not significant. Triaxial creep and relaxation tests were performed by seven pre-set paths. The rheological behaviours of frozen loess are also sensitive to the stress path. The initial loading process before rheological tests leads to the microstructure damage and anisotropic behaviours of frozen samples, and further dominates the strengthening of the viscosity properties during the creep (stress relaxation) process. Three types of stress history (consolidation, pure shear and couple history) were selected for preloading on frozen samples before triaxial tests to study the influences of the stress history on the mechanical behaviours in detail. The weakening effects of the pure shear history in the soil properties were experimentally verified. The stiffness behaviours of frozen loess are more susceptible to the damage process (prestress history) than the strength behaviours. Due to the competitive influences of the consolidation and shear history on the soils properties, the rheological behaviours of samples are dependent on the specific path history and maximum stress history of the preloading procedure. • Testing methods for different stress paths were applied in the frozen soils. • Triaxial shear, creep and stress relaxation tests were conducted for studying path-dependent mechanical behaviours. • Strength, stiffness, stress-strain relationships and rheological properties under different stress paths were investigated. [ABSTRACT FROM AUTHOR]