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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.

Freeze-thaw interface in unsaturated coarse-grained soil (CGS) is a weak plane which can cause slope failures in cold regions. This study presents a series of large-scale direct shear tests on freeze-thaw interface in CGS through a temperature control system. The tested soil was taken from a high slope in the Qinghai-Tibet Plateau. It was remolded with three dry densities (1.9, 2.0, and 2.15 g/cm3) and three moisture contents (9.0%, 11.5%, and 14.0%). With testing results, direct shear curves mainly performed as hardening deformation, and they were affected considerably by specimen conditions. The shear strength increased with both the increasing dry density and normal stress, but it was opposite with moisture content changed. The cohesion and internal friction angle increased with the increase in dry density but decreased with the moisture content. The particle movement and water migration of freeze-thaw interface in CGS during the test were significant, and they had close relations with the shear properties of specimens. And, an empirical model was produced to express the effect of pore ice on the shear strength of interface during the shear test. The tests and analysis in this study may provide useful references for CGS slope stability analysis in high cold regions.

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.

The buried China-Russia Crude Oil Pipeline (CRCOP) traverses 441-km discontinuous permafrost zone and has been operating at positive oil temperature since 2011. The underlying permafrost is degrading and thaw settlement occurs in the trench. An instrumented site was established to monitor ground temperature and water content under the CRCOP to evaluate permafrost degradation and cooling performance of the thermosyphons installed near the pipe. Field observations show that: (1) mean annual oil temperatures are higher than 0 °C and show a gradual warming trend (average increase by 2 °C during the observation period from 2012 to 2016; (2) the active layer thickness (ALT) increases by 2.7 m and the deep (15–20 m) permafrost temperature, 2 m away from the uninsulated pipe, rises 0.2 °C from 2014 to 2017; (3) thermosyphon can cool the soils surrounding the pipe and effectively mitigate thawing of underlying permafrost depending on its number, spacing and working duration; and (4) a thaw bulb surrounding the pipe exists even in winter due to a higher oil temperature. Field observations provide a better understanding of permafrost degradation, cooling effect and design parameters of thermosyphons, and basic data for numerical validation, implications for other similar cold regions pipeline engineering.

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.

The formation of thawed interlayer beneath embankment can result in embankment settlement in permafrost regions. Based on the data on ground temperatures and deformations beneath the embankment, observed in-situ along the Qinghai-Tibet Railway in permafrost regions from 2006 to 2013, characteristics of the thawed interlayer beneath the embankment and its influence on the embankment settlement are studied. The results indicate that the thawed interlayer hardly forms beneath the natural field, and beneath the embankments from the Qinghai-Tibet Railway the thawed interlayer develops widely, and it can be refrozen totally in the regions with lower mean annual ground temperature, and developed further in the regions with higher mean annual ground temperature. The thawed interlayer is closely related to the embankment settlement. The ice content of permafrost underlying the thawed interlayer influences the settlement of embankment. The higher the ice content is, the larger the settlement is, and vice versa. The increase in thickness of thawed interlayer mainly results from the decline of artificial permafrost table in high-temperature permafrost regions.

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.

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.

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.

In seasonal frost regions, tunnels are affected by frost damage after excavation. At entrance and exit section of tunnels, thermal insulation layer (TIL) is widely used to mitigate this problem. In this study, a moisture-temperature coupled mathematical model was established. Based on field observed air temperature at entrance section of Altun mountain tunnel, the thermal regime of surrounding rocks was investigated. Moreover, a comprehensive evaluation method was established to evaluate the thermal insulation effects of seven thermal insulation materials. The results showed that, with increase in thickness of TIL, maximum frozen depths of surrounding rocks decreased almost linearly. At the vault (H1), arch hance (H2), foot (H3), and inverted arch (H4) of the tunnel, the thermal insulation effect was different. When the thickness of TIL reached to 5 cm, there would be no seasonal freezing process at the four points. Seven types of TIL were considered for the thermal insulation of the tunnel. The optimal thickness of them was 4.2, 11.0, 5.5, 6.3, 10.2, 12.8 and 8.5 cm. To ensure the surrounding rocks do not experience freezing process, rigid polyurethane foam would be the best choice. Considering the cost, dry process aluminum silicate fiber would be a better one.

Compacted loess is widely used as fills of road embankments in loess regions of northern China. Generally, densely-compacted loess can satisfy the requirements of embankment strength and post-construction deformation. I lowever, uneven subsidence, pavement cracks and other related damages can affect the integrity of loess subgrade after several years of operation, and even cause some hazards, especially in North China, where the strong freeze-thaw erosion occurs. In this study, cyclic freeze-thaw tests for both densely and loosely compacted loess samples were performed to determine the variation in engineering properties such as volume, void ratio, collapsible settlement, microstructure, and the related mechanisms were addressed. The experimental results showed that an obvious water migration and redistribution occurred within the samples during freeze-thaw cycles. Ice lenses and fissures could be identified in the upper frozen layers of the samples. After freeze-thaw cycles, the dry densities of the nipper layers of samples changed significantly due to strong freeze-thaw erosion. The dry densities decreased for the dense sample and increased for the loose sample. It can be found that dense samples become loose, while loose samples became dense with the increasing number of freeze-thaw cycles. Their related void ratios changed reversely. Both void ratios tended to fall into a certain range, which verified the concept of a residual void ratio proposed by Viklander. The loosening process of densely compacted samples involves the formation of large pores, volume increase and density reduction as well as the related changes in mechanical properties because freeze-thaw cycles may be important contribution to problems of loess road embankments. Adverse effects of freeze-thaw cycles, therefore, should be taken into account in selecting loess parameters for the stability evaluation of road embankment in seasonally frozen ground regions.

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 factorDcis 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.

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.

The crushed rock layers (CRLs) with a satisfactory cooling effect has been widely used to mitigate permafrost thaw beneath road embankments in cold regions. However, whether the CRLs can still work as well for an expressway constructed with an integral embankment is still under discussion, it is generally accepted that the expressway with wide asphalt pavement should be constructed with two separated embankments rather than one integral embankment in permafrost zones. In that case, wind field around the two embankments would be of great importance as it would impact the work conditions of the CRLs considerably. Here, a coupled computational fluid dynamics and heat transfer model was developed to numerically investigate and quantify the characteristics of wind field and thermal performances of an expressway constructed with two separated crushed-rock embankments (SCREs) with different separating strip widths (SSWs). The model was validated by the field observed data of wind flows and ground temperatures around an experimental expressway. The simulated results indicated that the two SCREs can ensure the long-term thermal stability of permafrost subgrade in the projected climate warming scenario. However, due to different distribution of wind field around the two SCREs, permafrost thermal regimes beneath them differed considerably under different SSWs. The SSW exerted a significant impact on the cooling effect of the rear embankment, while less significant to that of the front embankment. To ensure that the two SCREs have close thermal performances, the SSW between them should be not less than 15 times of the embankment height. In addition, the local warming effect, which is caused by low wind speed, can result in severe permafrost degradation when the separating strips were narrow.

In the context of climate warming, damage to road pavements and embankments in permafrost regions caused by thawing of underlying permafrost severely reduces the serviceability of roads. In...

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.

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 (

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.

Ground temperatures under a buried warm China-Russia oil pipeline were monitored to evaluate permafrost thawing and cooling performance of thermosyphons installed near the pipe in sporadic permafrost regions. Field observations demonstrated that warm oil thawed the underlying permafrost and increased the active layer thickness. Thermosyphons can cool the soils surrounding the pipe and effectively mitigate the permafrost thawing depending on their number and working duration. But now the heat dissipated from the warm pipeline and construction disturbance were not completely removed by two pairs of thermosyphons after two winters of operation. There was still a thawed layer beneath the thermosyphons even in winter. Further long-term monitoring of the cooling performance of thermosyphons is needed. This study provided basic data and analytic references for other similar cold region pipelines.

Based on long-term monitoring data, the relationships between permafrost degradation and embankment deformation are analyzed along the Qinghai-Tibet Highway (QTH). Due to heat absorbing effect of asphalt pavement and climate warming, permafrost beneath asphalt pavement experienced significant warming and degradation. During the monitoring period, warming amplitude of the soil at depth of 5 m under asphalt ranged from 0.21 °C at the XD1 site to 0.5 °C at the KL1 site. And at depth of 10 m, the increase amplitude of ground temperature ranged from 0.47 °C at the NA1 site to 0.07 °C at the XD1 site. Along with ground temperature increase, permafrost table beneath asphalt pavement decline considerably. Amplitude of permafrost table decline varied from 0.53 m at the KL1 site to 3.51 m at the NA1 site, with mean amplitude of 1.65 m for 8 monitoring sites during the monitoring period. Due to permafrost warming and degradation, the embankment deformation all performed as settlement at these sites. At present, those settlements still develop quickly and are expected to continue to increase in the future. The embankment deformations can be divided into homogeneous deformation and inhomogeneous deformation. Embankment longitudinal inhomogeneous deformation causes the wave deformations and has adverse effects on driving comfort and safety, while lateral inhomogeneous deformation causes longitudinal cracks and has an adverse effect on stability. Corresponding with permafrost degradation processes, embankment settlement can be divided into four stages. For QTH, embankment settlement is mainly comprised of thawing consolidation of ice-rich permafrost and creep of warming permafrost beneath permafrost table.

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.

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.

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.

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.

Generally, construction for pile foundation in permafrost has to be carried out in winter to minimize the thermal distribution to the underlying or surrounding permafrost. Thus, there exists a problem that it is hard to meet the stipulated requirement to the compaction degree of the backfilled frozen soil blocks around the pile foundation excavated quickly. In order to study the effect of froze soil blocks on the heat transfer process between pile and permafrost during the construction of the Qinghai-Tibet electrical transmission line in winter, some laboratory tests were carried out for the highly porous frozen soil blocks and the naturally compacted thawed soil body, respectively. In addition, the thermal conductivities were calculated under different temperature gradient according to the measured thermal data. Results show that the convective heat transfer occurs in the highly porous frozen soil blocks at negative temperature corresponding to winter time, which is favorable for refreezing the pile foundation and lowering permafrost temperature. However, backfilling the highly porous frozen soil blocks hardly meet the requirement of compaction degree. It has dual effect on the stability of tower foundation depending on the specific site conditions such as permafrost temperature, ice content, soil type, permeability, hydraulic condition, and embedded depth of pile. Results also show that the equivalent thermal conductivity of the frozen soil blocks is over five times more than that of the thawed soil body on average. This is because the convective heat transfer occurs in frozen soil blocks in winter, which has stronger heat exchange effectiveness and can diminish refreezing time. Tests have revealed the process of heat transfer of frozen soil blocks used as fills around the pile foundation in permafrost, verified its thermal semiconductor effect, and accumulated and expanded data of the thermal conductivity.

In permafrost zones, the ground thermal regime beneath and around foundations is of great importance to long-term stability and integrity of engineering infrastructures. In this study, ground temperatures beneath and around embankments of the Qinghai-Tibet Highway (QTH) and the Qinghai-Tibet Railway (QTR) were observed at two profiles in continuous permafrost zones on the Qinghai-Tibet Plateau. The lateral thermal influences of the embankments on natural ground in the vicinity were analyzed based on the field observed data. The results indicated that the lateral thermal influence of the QTH and QTR embankments on shallow ground in the vicinity was very slight. The permafrost table depth or freeze-thaw process in the shallow ground near the embankments was mainly determined by the ground surface conditions. Different from the case of shallow ground, the lateral thermal influence of the QTH embankment on the permafrost in the vicinity was significant. Based on the permafrost temperature distribution at −10 m depth, the scopes of the lateral thermal influence of the QTH embankment calculated from the embankment shoulder were as much as 30–40 m at the two observation profiles. Whereas, the lateral thermal influence of the QTR embankment on the permafrost in the vicinity was slight. Because the QTR was constructed using cooling method of crushed rock embankment at the two profiles, the underlying permafrost either maintained well in thermal regime or experienced a slight cooling. The factors affecting the scope of the lateral thermal influences of embankments on permafrost in the vicinity were discussed at the end of the paper, as well as the related mitigation method. This study provides information that could aid engineering design of linear transportation projects and the assessment of environmental impacts in permafrost zones.

Dynamic response characteristics and stability of embankment are of primary importance for railroad operation in permafrost regions. The strong motion tests are carried out on a traditional sand gravel embankment at the Beilu River segment along the Qinghai–Tibet Railroad, and the acceleration waveforms at the shoulder and the slope toe of the embankment, when passenger train and freight train pass, are collected through strong motion tests. There is an obvious attenuation effect during the waveform transfer process from the shoulder to slope toe, and the natural frequency of the embankment is between 30–40Hz. Based on the tests in situ, the nonlinear dynamic finite element analysis is applied for numerical simulations on dynamic response of the traditional sand gravel embankment to train load, and the influences of underlying active layer on the dynamic response of the embankment at different seasons are analyzed. The results show that the vibration attenuation of the train load is obvious at different seasons, which presents a linear decrease tendency in summertime, but a nonlinear decrease tendency in wintertime. Both of the two decrease tendencies mainly occur within the soil layer above the permafrost table, but the attenuation effect in summertime, when the active layer is thawed, is slightly greater than that in wintertime when the active layer is frozen. Soil deformation induced by train vibrations occurs mainly above the permafrost table in summertime, but in wintertime, it mainly occurs above the natural surface. Meanwhile, the amount of deformation at the same location in summertime is far more than that in wintertime.

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 14 m 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.

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

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.

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 15 million tons (0.3 mbpd) and 30 million tons (0.6 mbpd) 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.

Permafrost (perennially frozen ground) appears widely in the Golmud-Lhasa section of the Qinghai-Tibet railway and is characterized by high ground temperature (≥–1°C) and massive ground ice. Under the scenarios of global warming and human activity, the permafrost under the railway will gradually thaw and the massive ground ice will slowly melt, resulting in some thaw settlement hazards, which mainly include longitudinal and lateral cracks, and slope failure. The crushed rock layer has a thermal semiconductor effect under the periodic fluctuation of natural air. It can be used to lower the temperature of the underlying permafrost along the Qinghai-Tibet railway, and mitigate the thaw settlement hazards of the subgrade. In the present paper, the daily and annual changes in the thermal characteristics of the embankment with crushed rock side slope (ECRSS) were quantitatively simulated using the numerical method to study the cooling effect of the crushed rock layer and its mitigative ability. The results showed that the ECRSS absorbed some heat in the daytime in summer, but part of it was released at night, which accounted for approximately 20% of that absorbed. Within a year, it removed more heat from the railway subgrade in winter than that absorbed in summer. It can store approximately 20% of the “cold” energy in subgrade. Therefore, ECRSS is a better measure to mitigate thaw settlement hazards to the railway.

In permafrost regions, buried oil pipelines have a substantial thermal impact on the surrounding permafrost due to the large temperature difference between the oil and ground. The change in the thermal regime of the permafrost may trigger frost heave and thaw settlement of the pipeline, and can even destroy the pipeline. In this paper, an example of parallel-buried pipelines is performed, and a numerical method is employed to simulate the thermal impact of pipelines on permafrost foundations for both ice-poor and ice-rich permafrost with various kinds of distances between parallel pipelines. The results show that the thermal effect caused by the operation of pipelines can increase the temperature of permafrost, as well as cause the underlying permafrost foundation to thaw gradually. The thermal impact of parallel pipelines on the permafrost foundation depends on the distance between the parallel pipelines, ice content of the permafrost, initial permafrost temperature, and so on. The degree of influence on each other decreases as the distance between parallel pipelines increases. With an insulated pipeline that has been in operation for 50 years, the thaw depths under the parallel pipelines, with distances of 5 m, 10 m, and 15 m, increase by 1.4 m, 0.4 m, and 0.1 m compared to the single pipeline in the ice-rich permafrost district with the average ground temperature being −0.5°C and the ice content being 35%. Nevertheless, there is no difference between the single and parallel pipelines if the distance is more than 20 m. The recommended distance between parallel pipelines in this district should be greater than 15 m. Different permafrost districts should set separate recommended distances, and the corresponding protective measures should be made outside the recommended distance.

The Qinghai-Tibet Railway traverses 550 km of permafrost region. After embankment construction in 2003, considerable detrimental permafrost-related embankment deformations developed along the railway. Based on field monitoring data collected from 2005 to 2012, typical embankment deformations, as well as their mechanisms along the railway were analyzed, taking into account the thermal regime and ice content of the subgrade soil layers. Results showed that frost heave of the embankment was slight and only occurred on a few in-cut roadbeds and air convection crushed rock embankments (ACCEs) in colder permafrost regions ( −0.7 °C), thaw consolidation of the underlying permafrost resulted in significant settlement on TSGEs, some of which reached 40 cm.

A large amount of work has been conducted to study the effect of freeze-thaw cycles on the geotechnical properties of various soils. But less laboratory work has been focused on the effect of number of freeze-thaw cycles and on quantitative relationship between mechanical behavior and freeze-thaw cycles. This study undertook a series of tests including freeze-thaw (FT) (after 0, 2, 5, 11, 21, and 31 freeze-thaw cycles respectively), unconfined compression (UC) and unconsolidated-undrained triaxial compression (UUTC) tests. These tests aim to assess the influence of freeze-thaw cycles on the mechanical behavior of compacted fine- grained soil, to establish correlation between the mechanical behavior and freeze-thaw cycles, and to facilitate the prediction of changes in geotechnical properties of soil. Freeze-thaw cycles notably influence the stress-strain curve reducing the UC strength by 11%, elastic modulus by 32%, and cohesion by 84% after 31 freeze-thaw cycles in comparison with those soils unexposed to freeze-thaw. The angle of internal friction is slightly increased by 1 to 2°. The weakening and deteriorating effects of freeze-thaw on the compacted fine-grained soil are confirmed. They can provide a scientific basis for design consideration of cold-region infrastructures, and for countermeasures against frost heave and thaw settlement.