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To analyse the damage characteristics of thermosyphon embankments in a permafrost region, the Qingshui River section along the Qinghai-Tibet Highway (QTH) was used as a case study in which a field investigation and drilling were performed. The field soils were sampled, and their water content, dry density and compaction degree were tested in the laboratory. Based on the measured temperature data of the thermosyphon embankment, damage characteristics and possible associated causes were analysed. The major damage found in thermosyphon embankments was a longitudinal crack, which developed 1.0 to 2.0 m away from the thermosyphon. In partial sections, waves and other damage also occurred. The damage was primarily attributed to the non-uniform temperature distribution, which resulted in a non-uniform distribution of the mechanical properties of the embankment filling. Under a heavy traffic load, stress concentration phenomena occurred at the freezing-thawing interface that gradually developed into a longitudinal crack with increased highway operation. The embankment damage history, rainfall seepage, ponding near the embankment and freezing-thawing cycles further accelerated the development of damage along the thermosyphon embankments. According to the characteristics and causes of thermosyphon embankment damage, precisely computing the cooling efficiency of thermosyphon in the design stage is necessary to prevent freezing-thawing interface waves resulting from excessive cooling effectiveness, to reduce thermal erosion from ponding near the embankment by improving embankment drainage systems, and to set necessary structural measures at thermosyphon embankments that can improve their strength. [ABSTRACT FROM AUTHOR]

Abstract: The two-phase closed thermosyphon (TPCT) is a passive high-performance thermal transfer device that can efficiently decrease the ground temperature. However, only a few studies focus on the quantization of working efficiency of thermosyphons. Thus, in this study, based on data obtained from an experimental TPCT embankment on the Qinghai–Tibet Highway from 2004 to 2012, the ground temperature fields of the TPCT embankment were obtained, and the horizontal and vertical ground temperature characteristics were analysed. The results showed that the TPCT embankment exhibited better thermal stability than that of a traditional embankment. The artificial permafrost table was elevated to or maintained at the original natural level due to the cooling effect of the TPCT. To quantitatively analyse the cooling effect of the TPCT embankment, a thermal budget evaluation method is proposed. The calculated results revealed the dynamic working state of the thermosyphon during its service period. The annual average energy of the TPCT transferred in a year was generally determined to be between 1500MJ and 2000MJ. The results of this study could provide a method to evaluate the TPCT embankments and provide technological support in designing the TPCT embankments in permafrost regions. [Copyright &y& Elsevier]

Abstract: The two-phase closed thermosyphon is an effective thermal semiconductor which can decrease the ground temperature of the underlying permafrost only in cold seasons, but does not transfer thermal energy in warm seasons. In this paper, based on the monitoring data of experimental embankment of the Qinghai–Tibet Highway from 2004 to 2011, the working state, temperature distributions and freeze–thaw process of embankment armed with two-phase closed thermosyphon were analyzed. It was found that thermosyphon embankment could play a rapid cooling effect after construction, which can weaken the sunny side-slope thermal effect and ensure the stability of embankment effectively. Compared with traditional embankment, the thermosyphon embankment offered a relatively long freezing period in a year for the underlying permafrost. For this reason, a relatively stable low-temperature zone around thermosyphon was formed. The monitoring data indicated that the artificial permafrost table beneath the thermosyphon embankment elevated to or maintained at the natural level. The thermosyphon can help to enhance the capability of defending against climate warming and further to increase the long-term stability of the embankment in permafrost regions. [Copyright &y& Elsevier]

Abstract: Thaw settlement is the main embankments distresses of highway in permafrost regions, according to survey data of the Qinghai–Tibet Highway (QTH). It can be effectively mitigated or even controlled by raising the embankment height. In view of this engineering problem, this study proposes the concept of the Height Effect of Embankment in Permafrost (HEEP). The concept represents the deformation and failure rules of embankment resulting from height variations. A thermal-elastic-plastic thaw settlement computational model is used to simulate the settlement processes of embankment, considering scenarios of different mean annual ground temperatures (MAGTs) and different heights. The model is validated by the field monitored data from a specific embankment section along the QTH. It is found that the total deformation of embankment is of considerable value and comes primarily from the thaw settlement of permafrost. Some special structures are recommended to supplement the built embankment, to ensure the stability of embankment in warm permafrost regions. The research results could provide essential theoretical and technological support for the transversal section design of highway embankments in permafrost regions. [Copyright &y& Elsevier]

Testing the unfrozen water content ( θ u ) based on frozen soil resistivity ( ρ ) is considered to be a common and convenient method. However, the reliability of the theoretical model for testing unfrozen water content requires further study; hence this paper proposes a new resistivity model for testing the unfrozen water content. By considering the variable characteristics of unfrozen water in the soil freezing process and based on the three-element electrical conduction model, four types of micro unit representing different unfrozen water contents were proposed, and the resistivity model was established on the basis of microscopic analysis. The resistivity model showed that a power function can be used to describe the relationship between the soil resistivity and unfrozen water content of frozen soil. Silty clay was used as a research object. The temperature, unfrozen water content and resistivity of soil samples with different initial water content and same soil density were tested by Nuclear Magnetic Resonance (NMR) technology and resistivity testing equipment. The results of the experiments performed on frozen silty clay verified the reasonableness of the proposed model for the resistivity of frozen soils. The conductive mechanism of frozen soil was discussed, as well as the resistivity model and its in-situ application for unfrozen water testing. [ABSTRACT FROM AUTHOR]

Ice nucleation active (INA) bacteria are capable of triggering ice formation close to 0 °C, but their ability of increasing ice content in warm permafrost remain unknown. Ice content is vital because it determines the bearing capacity of warm permafrost. Through nuclear magnet resonance and direct shear device, we found that adding INA bacterium Pseudomonas syringae with a concentration of 1 g/L in warm frozen soil can result in 64% increase in the shear strength, 113% increase in cohesion and 27% increase in ice content. The internal friction angle of warm frozen soil is less affected by P. syringae. Warm frozen soil with P. syringae exhibits brittle failure under normal stresses of 100 kPa to 300 kPa and plastic failure under 400 kPa. The shear strength increment can be regulated by the concentration of P. syringae which exponentially relates to ice content and linearly to shear strength. This emerging strategy reveals the importance of INA bacteria in cooling permafrost, and provides a sustainable and environment-friendly method for confronting permafrost degradation and the subsequent infrastructure instability. • INA bacteria are introduced firstly to improve the shear strength of warm frozen soil. • Using INA bacteria in frozen soil is a sustainable and environment-friendly strategy. • INA bacteria can effectively increase the ice content in warm frozen soil. • Bacterial concentration determines the degree of INA bacteria enhancing frozen soil. • INA bacteria have great potential dealing thawing-related problems in permafrost. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a two-dimensional unsteady model for analyzing the roadbed temperature in permafrost regions of the Qinghai-Tibet Plateau (QTP) is developed and the heat transfer processes of comprehensive earth-atmosphere coupled system are simulated by the numerical method. In the computation, the surrounding air environment of ground surface is brought into the research model and the corresponding unsteady and non-uniform influence factors of earth-atmosphere coupled system are also considered to obtain reasonable simulation results. Meanwhile, the model is validated with monitored data at a depth of 0.5m in different roadbed positions. Based on the above model, the spatial and temporal temperature distributions of the roadbed surface in Wudaoliang region are determined. Then the influences of the system parameters, such as the ambient temperature, wind speed, roadbed height and route trends, on the local temperature distribution are investigated. The results show that, temperature of surface regions exhibits a periodic sinusoid trend and always higher than the ambient air temperature. For different surface regions, the temperature of asphalt pavement keeps the maximum value and the natural ground surface has an abrupt step change in temperature at beginning of April and end of October. The variation of ambient air temperature has little influence on annual average temperature of permafrost roadbed surface. The deviations between the surfaces'' annual average temperature increase and their average values are less than 1%. Annual average temperature of various surfaces decreases as the wind speed and the roadbed height increase. Furthermore, the annual average temperature of leeward slope (also sunny side) along EW direction is the highest one among all three route directions and such temperature decreased systematically from SW45° direction to SN direction. However, the annual temperature of upwind slope (also shady side) is just contrary to the leeward slope. [Copyright &y& Elsevier]

The strength deterioration of soil-rock mixtures (SRM) subjected to freeze–thaw (F–T) cycles leads to instability and failure of upper engineering structures in cold regions. However, the mutual feedback response mechanism pertaining to the changes of pore and strength in SRM under F–T cycles are rarely addressed. Nuclear magnetic resonance and triaxial tests were carried out to study the pore structure characteristics and strength response patterns of samples. A correlation model of SRM porosity and strength deterioration was first proposed under F–T cycles, and the model rationality was verified by test data. The results demonstrated that the pore connectivity and porosity increased throughout the F–T process, with the T 2 spectral distribution curves exhibiting three peaks. Among these peaks, the main peaks underwent slight changes, while the secondary and micro peaks presented significant changes. Before 3 F–T cycles, the pore distribution evolved to small pores uniformly, followed with the large pores increasing and the micropores disappearing. With increasing of F–T times, the strength and cohesion of SRM experienced a drastic decline, while the internal friction angle demonstrated a slight decrease accompanied by fluctuations. Based on the analysis of test results, a correlation model regarding the porosity and strength deterioration was proposed through the relationship between the micro-structure evolution and the macro–mechanical response during F–T cycles. Furthermore, intrinsic mechanism of SRM strength deterioration under F–T cycles was revealed by considering the pore structure characteristics. The results can provide theoretical insights for the analysis of F–T disaster mechanism and prevention of SRM in cold regions. • The macro and micro characteristics of SRM during F–T cycles are obtained. • Qualitative and quantitative analysis of pore structures are performed based on NMR. • The changes in pore connectivity of samples under F–T cycles are evaluated with MnCl 2 solution. • A correlation model of SRM porosity and strength deterioration are first proposed. • The feed-response mechanism between pore and strength in SRM under F–T cycles are revealed. [ABSTRACT FROM AUTHOR]

The creep properties of soil-rock mixture (SRM) usually exhibit high variations after suffering harsh field conditions in cold regions such as freeze-thaw cycles, which can easily trigger landslides and threat the long-term stability of SRM slopes. However, the creep properties of SRM induced by freeze-thaw cycles have not been studied, especially in terms of the evolution of creep deformation and failure types under multiphase transitions within SRM material. Therefore, the shear and creep properties of SRM specimens under different freeze-thaw cycles (0 to 15 cycles) and rock contents (15% to 55%) were investigated to reveal the creep failure mechanisms of SRM using large-scale direct shear and direct shear creep tests. The shear stress-displacement curves, shear strengths, the creep curves and long-term strengths of SRM specimens were analyzed. Results showed that increasing freeze-thaw cycles tended to decrease the shear strength of the SRM specimen. The final creep deformation of SRM generally decreased with increasing rock content, and the first 3 to 5 freeze-thaw cycles resulted in significant creep displacement and the decrease of long-term strength. • The creep of soil-rock mixture (SRM) under freeze-thaw cycles was first studied. • SRM creep evolution and failure types under multiphase transitions were revealed. • The effect of rock content on creep properties of SRM was investigated. • The shear strength of SRM under freeze-thaw cycles was analyzed. • The first 3 to 5 freeze-thaw cycles resulted in significant creep deformation. [ABSTRACT FROM AUTHOR]

To establish a more realistic and comprehensive algorithm to assist decision makers in optimizing a route in a permafrost regions, the optimal design should be determined and funds should be used more efficiently. In this paper, we used the permafrost subgrade treatment, the permafrost distribution, geological engineering conditions, interference of facilities along the route, route length and 10 other indicators to reflect the factors affecting the selection of route in permafrost regions. An optimization model for highway route scheme was established based on matter element theory by comparing the optimal value of each evaluation program. The weight of each index was determined by an analytical hierarchy process. A case study shows that the optimal model was simple and intuitive, providing a new comprehensive evaluation method for route scheme selection considering the unique geological conditions in permafrost regions. [ABSTRACT FROM AUTHOR]

The growth and melting of internal ice crystals reorganizing the pore structure and internal skeleton of soil-rock mixture (S-RM), which leads to the deterioration of the S-RM shear strength after freeze–thaw cycles and its strength characteristics after the cycles are different from the normal temperature one. Due to unclear S-RM strength deterioration mechanism after freeze–thaw cycles, stability of S-RM cutting slopes in cold regions cannot be effectively evaluated. In this paper, particle flow code (PFC) simulation as well as direct shear and nuclear magnetic resonance (NMR) tests were carried to study strength degradation behaviors and pore structure changes of S-RMs containing various amounts of rock after freeze–thaw cycles. The results show that the shear strength and internal friction angle of S-RM are no longer positively correlated with the rock content after the cycles, and the strength parameters will decrease at rock contents of above 45%. Based on the simulation test, shear band thickness variation after the cycles was quantitatively evaluated, and an innovative method to obtain the fluctuation value of shear failure surface was proposed. It is found that the change laws of shear band thickness and failure surface fluctuation value with rock content after the cycles are consistent with that of shear strength, and they all reach the maximum value when the rock content is 45%. Fractal theory was introduced for quantitative evaluation of changes in S-RM pore structure after the cycles, and combined with the strength parameter attenuation, the strength deterioration mechanism of S-RM was revealed: The deterioration of shear strength in samples containing low rock content is mainly due to changes in the contact form between particles caused by internal inclusion structure formation after the cycles. The deterioration of S-RM with rock content of 55% and 65% is mainly due to attenuation of the internal skeleton effect caused by the appearance of overhead structures. The internal pores and skeleton structure of the sample with 45% rock content have little change, so the attenuation changes of the strength parameters and the undulation value of the failure surface are minimal. • Macro- and meso-degradation behaviors of S-RM under freeze–thaw cycles are obtained. • Shear strength is not positively correlated with the rock content after the cycles. • A method to obtain the fluctuation value of shear failure surface by PFC is proposed. • Qualitative and quantitative analyses of pore structures are performed based on NMR. • The strength degradation mechanisms of S-RM after freeze–thaw cycles are revealed. [ABSTRACT FROM AUTHOR]

Highways/railways often pass across slope areas and their embankments are often built on the slopes in permafrost regions. It is difficult to ensure the thermal stability of the embankments at the slopes due to the effect of slopes. To protect the underlying permafrost, the crushed-rock embankments are often used in the slope areas. Therefore, it is very necessary to explore the thermal state of crushed-rock embankments located on the slopes. In this study, we studied numerically the temperature characteristics of three kinds of crushed-rock embankments located on a slope under global warming, i.e. crushed-rock interlay embankment, crushed-rock interlayer-revetment embankment and crushed-rock base embankment. Numerical results indicate that the crushed-rock interlayer embankment and the crushed-rock interlayer-revetment embankment, located on a slope with a ratio of 1:3.73 (about 15° from the horizontal), cannot effectively eliminate the negative effect of climate warming and construction-induced warming, and the effect of slope is still obvious on the thermal stability of permafrost under the crushed-rock interlayer embankment. However, the crushed-rock base embankment can significantly reduce the temperature of underlying permafrost and keep the underlying permafrost table stable for a long term; furthermore, the ground temperatures under the long side slope are far lower than those under the short side slope, and this will be more advantageous to control the slide of the embankment located on a slope and increase its stability. We also find that the three kinds of embankments cannot all remove the thermal effects of construction from themselves in a short term. Generally speaking, the crushed-rock base embankment structure can be very advantageous to the thermal stability of the embankment on a slope. [ABSTRACT FROM AUTHOR]

Abstract: For the multi-lane expressway embankment constructed in permafrost regions, the width is always over 20m and asphalt-concrete pavement is adopted, both of which would largely increase the heat absorption of underlying permafrost compared with ordinary highway embankment. The increased heat absorption would result in more severe warming and thawing in underlying permafrost and threat the stability of the embankment. So, traditional cooling measures would not be effective enough to protect the underlying permafrost or to ensure the thermal stability of multi-lane expressway embankment. Based on this, a combined cooling measure composed of crushed-rock interlayer and ventilated ducts at its top was presented by the authors, the cooling mechanism of which was validated under laboratory condition. However, it is of necessity to further investigate the cooling effect of the new combined cooling measure under actual condition and its long-term cooling effect under global warming background. In this study, firstly, the observed temperature data from an in-situ test embankment of the new combined cooling measure were collected and analyzed. The observed data illustrated that the new cooling measure was effective in protecting underlying permafrost under actual condition of the Qinghai-Tibetan Plateau. Secondly, a numerical model was validated by the in-situ test data. Finally, the temperature distributions of ordinary multi-lane embankment, multi-lane embankment with crushed-rock interlayer and multi-lane embankment with combined crushed-rock interlayer and ventilated ducts after 20years ofconstruction were numerically simulated under global warming background. According to the numerical study result, the long-term cooling effect of single crushed-rock in multi-lane embankment was limited. After the ventilated ducts were added at the top of the crushed-rock interlayer, the cooling effect was greatly enhanced and was effective enough to protect the underlying permafrost. [Copyright &y& Elsevier]

Two-phase closed thermosyphons (TPCTs) are widely used as heat transfer devices in engineering structures in permafrost regions to prevent freeze–thaw damage. In this paper, we reported on a laboratory experiment to study the heat transfer characteristics and working state of a TPCT installed in the soil based on the typical temperature conditions in the permafrost regions of the Qinghai–Tibet Plateau. The results indicated that when the temperature of the condenser section was lower than that of the evaporator section, and the negative temperature difference between the condenser and evaporator sections was beyond a critical temperature difference, the TPCT was active; otherwise, the TPCT was inactive. The critical temperature difference between the condenser and evaporator sections that was required for the TPCT to begin to work was defined as the “startup temperature difference”. Furthermore, we found that the efficiency of the TPCT improved linearly with the negative temperature difference between the condenser and evaporator sections when the TPCT was working. In this study, the startup temperature difference of the TPCT was about −0.20°C and the total thermal resistance was about 0.31°C/W. These results provide a basis for future work evaluating and improving the performance of TPCTs used for embankment design in permafrost regions. [ABSTRACT FROM AUTHOR]

Abstract: As far as the railway embankment in permafrost regions is concerned, its mechanical properties are closely related to soil temperature. Especially for the embankment in the warm permafrost regions (mean annual ground temperatures range from 0–−1.5 °C), its stability will change greatly with minor temperature variations. The Qinghai–Tibet Railway (QTR) is approximately 1142 km long, of which 275 km are underlain by warm permafrost. Furthermore, the Qinghai–Tibet Plateau (QTP) is in an active seismic zone. Thus, the seismic stability, liquefaction, and deformation of the embankment of QTR are critical issues on safe operation. However, there are few papers concerning about the seismic response of railway embankment in permafrost regions. This paper will propose a thermal-dynamic coupled model, and the model and its specific analytical procedure can be described as follows. Firstly, based on the governing differential equations of transient temperature field with phase change, the temperature distributions of railway embankment are computed and analyzed by numerical method. Then, since the mechanical properties of frozen soil are effected by temperature, the dynamic consolidation equations of Biot, whose parameters are relative to temperature, are deduced in detail by adopting dynamic consolidation theory. Lastly, by using this numerical model, the seismic response of the QTR embankment is simulated, and the characteristics of seismic response, such as excess pore water pressure, stress and displacement, are analyzed in detail. Meanwhile, the seismic stability of this embankment is evaluated. The results show that this numerical model is able to simulate seismic response of embankment in permafrost regions. [Copyright &y& Elsevier]

Separate embankment is an important subgrade structure in the future Qinghai-Tibet expressway (QTE) construction. In this work, the comprehensive earth-atmosphere coupled heat transfer numerical model of the 26 m width embankment and the different spacing separate embankment had been built. To make the analysis more general, 12 typical computation cases which consider the distribution pattern of upper soil type and external environment conditions along the QTE have been developed. In the computation, the spatial and temporal temperature distribution characteristics of 26 m width embankment and the separate embankment under the given working condition are calculated and compared. Then heat transfer characteristic, freeze-thaw feature, and reasonable interval of separate embankment are investigated. The computation results show that: compared with the prominent heat absorption and heat collection effect of the 26 m width embankment, the separate embankment contributes greatly to reducing the subgrade thermal disturbance and protecting the underlying permafrost, thus the separate embankment should be a reasonable choice for the future Qinghai-Tibet expressway. The maximum thaw depth and thaw plate are influenced by parameters such as ambient temperature and water content of frozen soil. The increase of subgrade interval also reduces the influence of subgrade on underlying permafrost and the maximum thaw depth. And above decreasing trend becomes negligible when the interval of separate embankment reaches to a certain value, therefore there is an optimal spacing between the permafrost subgrade on the heat transfer perspective. • Complex heat transfer process of 26 m amplitude embankment and the different spacing separate embankment are simulated. • Full-width subgrade has lower basement thermal absorption, but more serious thermal disturbance. • The increase of subgrade interval reduces the influence of subgrade on underlying permafrost. • The influence of spacing becomes insignificant when the amplitude separation is more than 9 m based on the analyze model. [ABSTRACT FROM AUTHOR]

Cooling technologies adapting to expressway embankment in permafrost regions have been drawing increasing attention due to its greater permafrost degradation risk; however, none of them has been put into practice. In adaption to the main permafrost-associated challenges of the construction of the Gonghe-Yushu Expressway (GYE) embankment, some new combined cooling technologies were presented. In order to investigate the cooling performance of the newly-presented combined cooling technologies under the conditions of warm and degrading permafrost and wide expressway embankment, an 8.06 km long field embankment test was built along the GYE. In this paper, the configuration and in-situ monitoring systems of 2 newly-presented combined cooling technologies were introduced. Then the working state and cooling performances of the cooling technologies were analyzed through field monitoring data. The first 3 years' monitoring data showed that the ventilation slab (VS) and the heat-induced pavement (HIP) prolonged the lasting period of the natural convection inside the crushed-rock layer (CRL) respectively through decreasing the temperature at CRL top. The CRL embankment, the VS-CRL embankment and the HIP-CRL embankment performed efficiently in preventing an intense permafrost thawing. The field embankment test is expected to supply technical support for construction of other expressways in permafrost regions. • Configurations and design principles of some new cooling technologies were introduced. • The ventilation slab and the heat-induced pavement intensified the natural convection of CRL. • The cooling technology adapted well to the thermal effect of the wide GYE embankment. [ABSTRACT FROM AUTHOR]

Global warming causes a large area of permafrost degradation, which leads to increasingly engineering problems related to pile foundations in Tibetan Plateau, particularly in warm permafrost. The present research investigated the effects of permafrost degradation on the axial behaviour of concrete piles in warm permafrost. Quick axial compression tests were conducted on concrete piles installed in frozen soils in the cold room laboratory. Results showed that the behaviours of pile gradually degenerated with the increase of active layer thickness (z a) and ground temperature. The development of shaft resistance as the pile settled was compared at various cases. Adfreeze strength on the pile shaft was reduced with increased ground temperature. The adfreeze strength for concrete piles was obtained in silt and clay at temperature of −1.10 °C to −2.35 °C. Adfreeze strength was not reached near the pile tip. Piles in frozen soils could still gain the tip resistance at a displacement as much as 10% pile diameter and did not reach the plunging failure at such a displacement. • The influence of permafrost degradation on the pile behaviours were studied. • Physical model tests were conducted to analyze axial loading behaviour of piles. • The adfreeze strength was obtained at temperature of −1.10 °C to −2.35 °C. [ABSTRACT FROM AUTHOR]