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Sun sheds were once used as an effective roadbed cooling technique in permafrost regions. Numerical and indoor-experiment results have confirmed the cooling effects of the sun-shaded embankments, but field-monitored data are still necessary to evaluate their long-term performance. This paper discusses the cooling effects of a sun shed over a section of the Qinghai-Tibet Railway embankment, by examining near-surface air and ground temperatures and the permafrost table depth at the shaded section, at an unshaded embankment section, and in surrounding natural ground. Monitoring results from 2006 to 2010 indicate that the sun shed has lowered near-surface air temperatures and underlying permafrost temperatures beneath the embankment, and promoted permafrost aggradation. The estimated annual heat budget of the permafrost beneath the sun-shaded embankment section has been negative over the past nine years, indicating the sustained cooling performance of the sun shed. This study confirms the utility of sun sheds as an effective design and maintenance concept for infrastructure embankments in warm permafrost regions, provided that the structure can resist harsh environmental conditions. [ABSTRACT FROM AUTHOR]

Abstract: The retrogression and thermal regime of a thaw slump affecting the stability of a slope along the Qinghai–Tibet Highway in China were studied over the last twenty years. The thaw slump was initiated in 1992 and the retrogression rate at the headwall slowed down somewhat in the recent years following the thermal stabilization of the affected area. According to field observation and monitoring, while the ground temperature was still not stabilized after the occurrence of the thaw slump, the depth of the permafrost table was usually shallower in the recently disturbed area than in the undisturbed area away from the thaw slump. However, due to a positive heat budget in the disturbed area, the ground temperature was increasing and the permafrost table was deepening. Moreover, since the ground temperature is higher close to the headwall than in the undisturbed area, lateral heat flux occurs in the thaw slump area inducing more retrogression at the headwall. However, as soon as the vegetation invaded the bare ground surface of the disturbed area, the ground temperature was decreasing in the disturbed area over the past two years and stabilization of the disturbed area was occurring. Following the stopping of the headwall retrogression and the decrease of the lateral heat flux, the ground temperature near the headwall is also decreasing. [Copyright &y& Elsevier]

Ground temperatures from the surface to 15 m depth were measured for nine years (2008–2016) at two upland and sloping sites, to characterize the ground thermal regime. The two sites were on south and north slopes of Kunlun Mountain, Qinghai-Tibet Plateau, respectively, and were not disturbed by anthropogenic activity. Therefore, changes in ground thermal conditions are indicative of natural variations in permafrost in mountainous terrain. The mean annual air temperatures (T a) from October 2014–2016 were −4.2 (north slope) and −5.6 °C (south slope), but the corresponding mean annual surface temperatures (T s) were −2.8 and −1.5 °C. Thawing season n -factors were 1.3 at the north site and 2.1 at the south site, resulting in warmer permafrost on the southern slope, which highlights the significance of air and ground surface temperature in determining the thermal regime of permafrost. The mean annual ground temperature in the active layer (0.5 m depth) at the north site was approximately 0.9 °C lower than at the south site, and the active-layer thickness was ~1.0 m greater on the south slope. The mean annual ground temperature at the top of permafrost (T 2.0m) was about 0.6 °C lower at the north site than that at the south site. Rapid warming at the north site during the study period caused the ground temperature at the depth of zero annual amplitude to increase and reach near-parity with the south site. Numerical simulations suggested that permafrost in mountainous regions on QTP will warm by about 0.01 °C/y in the next 50 years. • Ground temperatures at two sloping sites were measured, to characterize the permafrost thermal regime at mountainous regions. • Ground temperature is opposite to air temperature, highlighting the importance of local factors to determine thermal regime. • The permafrost is warming as a rate of ~0.01 °C/y next 50 years and it is rapider at north slope than that at south slope. [ABSTRACT FROM AUTHOR]

Embankment deformation is a key consideration in high speed railway construction. Frost heave due to temperature and moisture variation is critical to consider in areas of seasonally frozen ground. This paper examined ground temperatures, frost heave, and moisture content in 2013–2014 at two sites along the embankment of the Harbin-Dalian Passenger Dedicated Line (HDPDL) railway. The railway is in seasonally-frozen ground of Jilin Province, China. The embankment at one site (K977) was built on the undisturbed ground surface while the other was in a cut section (K1004). Displacement measurements over one year indicated that frost heave of 14 mm at K977 and 25 mm at K1004 occurred in ballast during the freezing season. During the annual freeze-thaw period, soil moisture content varied drastically in the upper 0.6 m. The frost depth and freezing index were strongly and positively correlated during the freezing season. Soil moisture content was regarded as the primary control on the amount of frost heave, while frost depth was secondary. Furthermore, water vapor diffusion may have been an important contributor to the formation of near-surface ice. At K1004 site, the maximum recorded deformation was large and exceeded the 15 mm Chinese standard for maximum allowable heave. This suggested that high ground water level increased the amount of frost heave. [ABSTRACT FROM AUTHOR]

Freeze–thaw (F–T) weathering of rocks can degrade macroscopic properties through changes in the pore structure, which may have effects on the triggering mechanism of geological hazards in cold regions. In this study, sandstone specimens of 50 mm diameter and 205 mm height were subjected to immersion and F– t- tests, computed tomography (CT) scanning, and uniaxial compression tests. The CT slice areal porosity was used to divide the bottom portion of specimens into a non-immersed part, water migration transition region, and submerged part. The pore-throat networks of these parts were established, and the distribution characteristics of the pores were investigated. The results showed that sandstone weathering can lead to the deterioration of uniaxial compressive strength, accompanied by a change in pore structure. Specifically, distribution of the specimens' areal porosity changed dramatically along the axial direction under the F–T cycles, however, the heterogeneity coefficient of the submerged part changed slightly. The equivalent radius of dominant pores in the non-immersed part was primarily concentrated within 100 μm, while the counterparts of the transition region and submerged part trend towards 200–400 μm under F–T cycles, with a proportion of these pores showing a 3.2- to 6.4-fold increase. The uniaxial compressive strength of the bottom portion of the specimen infiltrated by water gradually decreased as the F–T cycles progressed. The absolute permeability simulation demonstrated that the permeability of the specimens gradually increased as the number of F–T cycles increased. In this particular case, it was assumed that the region far away from the submerged part with lower water content first froze and blocked the pathway of water migration, profitably affecting the F–T deterioration. The conclusions summarised herein will benefit descriptions of the effect of freeze–thaw action under immersion conditions to guide the prevention of geological engineering disasters in cold regions. • Areal porosity and associated heterogeneity coefficient of sandstone were impacted by water infiltration. • The effects of moisture on the distribution of pore equivalent radius subjected to freeze–thaw were analysed. • Based on the PNM, the permeability is calculated, and freeze -thaw cycles mainly affect the range of water infiltration. [ABSTRACT FROM AUTHOR]

Numerous cold region tunnels have been constructed in recent decades. To prevent frost damage, problem of frost heaving force calculation is to be solved urgently. During the freezing process of cold region tunnels, temperature gradient occurs mainly in radial direction, whereas the temperature gradient in circumferential and axial directions is negligibly gentle. Hence, the surrounding rock of cold region tunnels freezes unidirectionally along the radial direction, which results in the transversely isotropic frost heave of surrounding rock. Therefore, an analytical elasto-plastic solution of frost heaving force is proposed, in which an anisotropic frost heave coefficient k is introduced to consider the transversely isotropic frost heave. And the surrounding rock is considered as ideal elasto-plastic material conforming to Mohr-Coulomb yield criterion in the solution. Moreover, the method to determine the elastic or plastic state of the surrounding rock after frost heave is proposed. For the condition that the frozen surrounding rock doesn't reach the plastic state, an elastic solution of frost heaving force considering the transversely isotropic frost heave is also established. Then, the analytical elasto-plastic solution is verified with the existing solutions and a model test, and proved to be reasonable and applicable. Finally, the influence of the related factors, such as the frost heaving property (the anisotropic frost heave coefficient and the volumetric frost heaving strain) of surrounding rock, the initial ground stress, and the mechanical parameters of surrounding rock on the frost heaving force and plastic zone radius is analyzed. Results show that as the anisotropic frost heave coefficient k increases, the frost heaving force increases significantly, which demonstrates that the transversely isotropic frost heave of surrounding rock has considerable impact on the frost heaving force and should be considered. • Frost heave of surrounding rock is transversely isotropic in cold region tunnels. • An analytical elasto-plastic solution for frost heaving force in cold region tunnels is derived. • Use a coefficient to consider the transversely isotropic frost heave of surrounding rock in the solution. • Propose a method to determine the elastic or plastic state of the frozen surrounding rock. [ABSTRACT FROM AUTHOR]

Abstract Icing of substation equipment seriously threatens power system security and stability. Existing manual and mechanical deicing methods require that equipment be powered off first. Because hot air has strong insulating properties, it is a very suitable deicing method for live substation equipment. However, the parameters of existing hot-air deicers are selected by experience, resulting in suboptimal deicing efficiency. In this study, a mathematical model and a simulation model of hot-air live deicing have been developed. The influence of hot-air velocity, temperature, pipeline length, and diameter on deicing efficiency is examined. The underlying causes of suboptimal deicing are analyzed, and its influencing rules are revealed. Then optimization models for these factors were constructed, and the optimal value of each factor was obtained. Finally, based on the research results, a set of hot-air deicing devices for live substation equipment was developed. Tests showed that deicing efficiency was high. The research reported in this paper can provide a theoretical basis for parameter selection of hot-air deicing devices for live substation equipment. Highlights • A mathematical model and a simulation model of hot-air live deicing have been developed. • The influence of hot-air velocity, temperature, pipeline length, and diameter on deicing efficiency is examined. The underlying causes of suboptimal deicing are analyzed, and its influencing rules are revealed. • The optimization models based on these factors were constructed, and the optimal value of each factor was obtained. • The research can provide a theoretical basis for parameter selection of hot-air deicing devices for live substation equipment. [ABSTRACT FROM AUTHOR]

Frost-heave of the subgrade significantly influences the stability of transportation infrastructure. In particular, small amounts of frost heave in coarse-grained subgrade fill may pose challenges to the safe operation of high-speed railway lines in cold regions. In this study, frost heave, ground temperatures, and soil moisture contents were examined at 15 test sections along Lanzhou-Xinjiang high-speed railway (LXHR), and the factors controlling the observed heave are discussed. The development of frost heave in three subgrade layers (0–0.5, 0.5–1.5, and 1.5–2.7 m depth) was examined at each section over two freezing seasons (October 2015 to September 2017). The frost depth in the subgrade reached 220–400 cm, significantly deeper than in surrounding natural ground. The frost penetrated 2.3 m of A/B group fill, a special fill material for high-speed railways. The frost heave in some sections is greater than the allowable total deformation limit of 19 mm. Most of the total frost heave (40–50%) occurred in the 50 cm thick subgrade surface layer. Residual deformation developed over the two years, which has likely resulted in increased porosity and decreased compaction and strength. Frost heaving is mainly controlled by supplies of water from the surface, from below, and laterally from adjacent ground, and the fines content of the material. The data presented provide valuable information on the construction and performance of high-speed railways in regions with seasonally-frozen ground. [ABSTRACT FROM AUTHOR]

A series of 1-D freezing experiments were conducted to simulate frost heave in saturated coarse-grained soil (CGS) with variable fines content. Visible frost heave occurred in CGS that contained a considerable fraction of fines when supplied by an external water source. Near the beginning of the experiments, a small amount of water was expelled, likely due to consolidation of the thawed portion of the sample. As time elapsed, the total amount of frost heave was proportional to the intake of water. Furthermore, frost heave increased linearly as the fines content increased. At higher fines content, frost heave occurred mainly due to water migration and segregated ice lensing. Finally, a simple finite element model that combined thermal and mechanical analyses based on Takashi's equation was developed. The model accurately reproduced experimental results and therefore can be used to simulate and predict frost heave in CGS. [ABSTRACT FROM AUTHOR]

Abstract: By analyzing temperature characteristics of tested embankments at the Changchun site of Harbin to Dalian Passenger Dedicated Line (HDPDL), we found that the temperature changing process is different in parts of the roadbed and foundation under seasonal freeze–thaw cycles. In cold seasons, the temperature under the reinforced concrete component was higher than that under the shoulders. This difference decreases with the depth of roadbed. In warm seasons, these phenomena appear as a reverse trend, and also the temperature difference decreases with the depth of roadbed as usual. In different parts of the roadbed, the maximum seasonal frozen depths were all higher than that in the natural ground, because of the roadbed materials, that changed the heat exchange process between the air and the ground surface. It should be multiplied by an appropriate correction factor for the standard depth of seasonal freezing to allow the setting design of the anti-frost layer thickness of the roadbed. By changing the roadbed filling materials, a series of computer simulations were carried out. The thermal states of roadbeds were simulated 50years under the climate warming. It was indicated that the modified common A/B group fills do not improve the thermal state, though the filled layers were designed to increase the roadbed bearing capacity. The maximum frozen depth was 1.5–1.6m under the track plates, and 1.7–1.8m under the shoulders. The reinforced concrete component made the isothermals appear as a saddle shape, and change to be flatter with the depth of roadbed. [Copyright &y& Elsevier]

The crushed-rock interlayer (CRI), as typical cooling technique used in the Qinghai-Tibet Railway, cannot be directly applied in expressways traversing permafrost areas because of the intense heat-absorption asphalt pavement. Sufficient cooling of permafrost foundation to meet the demand of engineering stability is an important issue. In this study, a CRI embankment combined with vertical ventilation ducts (VVDs) was designed to strengthen the convective cooling of wide expressways. To assess the performance of the combination design, a field experiment was conducted in the Tibet Plateau by building a full-scale expressway section. A numerical model was then developed and validated to analyze the heat transfer process and to predict its long-term thermal regimes considering climate warming. Results show that the new structure can produce an obviously increased convection cooling on permafrost foundation by enhancing the winter cold-energy utilization. The VVDs at the centerline play an effective role in dissipating heat accumulation in embankment center and can eliminate thermal asymmetry caused by plateau's prevalent wind direction. The combination of CRI and VVDs is effective to control ground temperature to improve expressway embankment stability. This new design could be considered for application in expressways built on permafrost stratum. • A new combined structure for cooling permafrost foundation of expressway was presented. • A full-scale field experiment and series of numerical simulations were performed to evaluate its cooling performance. • Convection cooling enhancement was observed in porous rock layer of the new design. • The new combined structure is effective to maintain the thermal stability of permafrost expressway. [ABSTRACT FROM AUTHOR]

A poor cooling effect on warm permafrost (> −1.0 °C) was observed in the crushed-rock revetment embankment (CRRE), which is widely used in the Qinghai-Tibet Railway. The filling of the porous rock layers caused by wind-blown sand and rock-weathering in the Qinghai-Tibet Plateau can further weaken the cooling capacity of the CRRE and may cause instability of the railway vehicles. This study describes a novel slope-cooling structure (NSCS), which combines a CRRE and a slope-warming prevention measure. A full-scale embankment was built using the new design in a warm permafrost region of the plateau. Ground temperatures and air speeds in the pore space of rock layer of the slopes were monitored. The thermal characteristics of the NSCS were evaluated along with its cooling performance. The results indicated that the new slope structure produced an effective cooling process on the side-slopes and the subgrade permafrost. The permafrost table was elevated and the ground temperatures at the embankment shoulders showed a decreasing trend after construction, particularly under the shady side. Asymmetrical convection process, which exhibits a significantly higher air speed and frequency in winters than those in summers, was observed to occur in the rock layers of the new structure. The NSCS decreases the heat absorption during summers and thus is beneficial for the heat-dissipating of the side-slopes. Moreover, the NSCS can reduce the sunny-shady slope temperature difference, which is induced by embankment orientation; therefore, the symmetry of the embankment temperature distribution is maintained. In general, these cooling characteristics contribute to the enhancement of thermal stability of the embankment and confirm the application of the NSCS as an effective method for the design and maintenance of embankments formed on warm permafrost. • A novel slope-cooling structure (NSCS) for permafrost embankment was presented. • The thermal characteristics and cooling performance of the NSCS were evaluated. • The NSCS effectively cooled the side-slopes and underlying subgrade permafrost. • The NSCS reduced the sunny-shady slope temperature difference. • The NSCS maintained the symmetry of the embankment temperature distribution. [ABSTRACT FROM AUTHOR]

The Harbin–Dalian Passenger Dedicated Line (HDPDL) is the first high-speed railway in deep seasonally frozen ground in China. Frost heave is among the most difficult problems in roadbed deformation control in such area. In this study, the ground temperatures and frost heave in a road cut section (K865) and an embankment section (K866) along the HDPDL were analyzed based on monitoring data in situ. From 2012 to 2018, the maximum frost heave in the cut section ranged from 4.52 mm to 9.18 mm, while in the embankment section this value ranged from 1.86 mm to 5.28 mm. The data clearly show that frost heave in the cut section was much more serious than that in the embankment section, although the frozen depth of the road cut was approximately 20 cm shallower than that of the embankment. The frost heave ratios of the roadbed filling materials in the well-graded gravel layer (0–0.5 m), non-frost susceptibility A/B group fill layer (0.5–1.5 m), and A/B group layer (1.5–2.7 m) were 1.20%, 0.14%, and 0.08% on average in the cut section and generally higher than 0.63%, 0.04%, and 0.02% in the embankment over six freezing seasons, respectively. The investigation of frost heave in different soil layers at 24 monitoring sites indicated that most frost heave occurred in the top well-graded gravel layer (0–0.5 m), account for averagely 66% of the total deformation. The frost heave of the subgrade along the HDPDL mainly developed during the early period of each freezing season and an abrupt increase typically occurred during the initial period of the thawing process. • Long-term monitored data are presented for the cut and embankment sections along the Harbin-Dalian Passenger Dedicated Line. • The difference of ground temperature and frost heave between cut and embankment sections is analyzed. • The mechanism of frost heave of high speed railway subgrade in seasonally frozen regions is discussed. [ABSTRACT FROM AUTHOR]