Permafrost Subgrade Research Articles

Numerical simulation of the vibration response of the permafrost subgrade under harmonic irregularity in the Qinghai-Tibet Railway

Abstract Harmonic excitation can significantly increase the dynamic response of wheel-rail interactions when a train passes. In this study, based on the train-track vertical coupled dynamics model (ZL-TNTLM) and ABAQUS, a unified train-track-subgrade coupled dynamics model is established. Field measurement data corroborated its accuracy and reliability. The analysis focused on the dynamic response of the permafrost subgrade of the Qinghai-Tibet Railway (QTR) to harmonic excitation, including the track-sleeper force and vertical dynamic stress. This study examined the influences of the wavelength, wave depth, and railhead depression on the dynamic response of a subgrade. The results indicated that single-harmonic excitation amplifies the track-sleeper forces and the vertical dynamic stress in the subgrade, particularly near the harmonic centre. Additionally, axle load variations and seasonal thawing broaden the dynamic stress range on the subgrade, with harmonic excitation exacerbating this impact, thereby increasing the risk to the stability of the ice-rich permafrost layer. A decrease in wavelength and increase in wave depth result in a greater number of affected sleepers, intensifying the vertical vibrations of the subgrade, especially in the medium- and high-frequency ranges of vertical dynamic stress. Notably, wavelengths between 1 and 2 m significantly exacerbate the adverse effects on the ice-rich permafrost layer. Shorter wavelengths also induce higher-frequency vibrations. These findings offer valuable insights for the design, operation, and maintenance of permafrost subgrade systems under irregular excitation.

Performance and reinforcement of air-cooled embankments traversing degrading permafrost of the Qinghai-Tibet Plateau

The reliable operation of railway embankments traversing degrading permafrost regions is challenged by climate warming. This study examines performances of four main types of railway embankments on the Qinghai-Tibet Plateau in thermally stabilizing permafrost foundation over warm permafrost using numerical modelling and 10-year monitoring data. Then, a reinforcement measure that combines a thermal conductivity variable system (TCVS) was designed to improve the cooling capacity of the crushed-rock sloped embankment (CRSE) by countering the heat absorption of slopes during summers. A coupled thermal-fluid-solid model was built to simulate and assess the cooling performance and reinforcing capacity of the new design. Results show that the crushed-rock embankments can produce convection cooling on the permafrost subgrade but the performances vary with different structures. The CRSE has insufficient cooling capacity to withstand the underlying permafrost degradation in warm permafrost regions. The optimized CRSE that combines the TCVS can effectively cool the underlying warm permafrost and decrease the shady-sunny slope effect under a warming climate, and can be used as an effective reinforcement measure. This study confirms the application of air-cooled embankments in protecting permafrost subgrade and provides guidance for structural design of embankment traversing degrading permafrost.

Impact of surface temperature changes on subgrade settlement of permafrost regions railway: a case study of the Qinghai-Tibet railway, China

ABSTRACT Parts of sections in permafrost regions of the Qinghai-Tibet Railway (QTR) have been deformed due to permafrost degradation, the global warming and human activities. Therefore, monitoring the subgrade settlement of the QTR in permafrost regions has been a major working in QTR maintenance. For this reason, this paper investigates the subsidence characteristics and derivative mechanisms of railway subgrades in the permafrost regions of the QTR. Additionally, it analyses the impact mechanisms of surface temperature changes on the subsidence of permafrost subgrades. Building on this investigation, a C-I-GWO-BP neural network prediction model was developed to forecast subgrade settlement in permafrost regions. The findings reveal a substantial correlation (R2 = 0.834) between the deformation of railway subgrades in permafrost regions and changes in surface temperature. Meanwhile，with the intensification of global warming and human activities, the subsidence of permafrost subgrade has shown an increasing trend year by year. The primary areas of deformation are predominantly concentrated in ice-rich permafrost regions, including rivers, lakes, and snowmelt runoff areas. Additionally, when compared to traditional BP neural network models, the C-I-GWO-BP neural network prediction model proposed in this article showcases diminished prediction errors and heightened accuracy in identifying optimal weights and thresholds. This model offers technical support for the efficient, accurate, and automated prediction of railway subgrade subsidence in permafrost regions.

Color and reflective properties of magnesium phosphate cement grouting material for cooling pavement

In order to reduce the absorption of solar radiation by asphalt pavement in plateau permafrost regions and protect the underlying permafrost subgrade, the aim of this research was to develop a dark high near-infrared (NIR) reflective material using magnesium phosphate cement (MPC) and chrome black T, and to construct reflective grouting compound pavement (RGCP) by grouting the material into large void asphalt mixture. The optimal magnesium (M)/phosphate (P) ratio of MPC was determined to be 10 through solar spectral reflectance test, with a NIR reflectance of 71.74 %. A series of experiments, including chromaticity characteristics, reflectance test, SEM-EDS analysis, and strength test, were conducted to characterize the impact of chrome black T content on the reflection and mechanical properties of MPC. The results indicated that the chrome black T particles were uniformly distributed on the surface of MPC and filled in its pores, which not only effectively colored the MPC, but also improved its mechanical strength. Since the reflectivity of chrome black T was lower than that of MPC, its incorporation also reduced the NIR reflectivity of MPC to a certain extent. Overall, MPC with 1 % chrome black T content had a color similar to asphalt and a NIR reflectance of 43.91 %, making it a suitable dark NIR reflective material. An indoor irradiation experiment verified that RGCP can maintain a lower temperature under solar radiation. Compared with the control group, the surface temperature of the RGCP specimen was reduced by 4.5 °C.

Phase-change pavement structures with gradient low-thermal conductivity for permafrost protection

Currently available permafrost protection techniques do not pay enough attention to the properties of asphalt pavement, and the problem of strong heat-absorbing asphalt pavement and permafrost constraining each other is a global challenge that needs to be urgently addressed. Phase change materials (PCMs) can regulate the operating temperature of pavement structures by storing and releasing large amounts of heat during phase changes. In this paper, composite shaped phase change material (CSPCM) was applied to the permafrost region of Qinghai-Tibetan Plateau, and a new method of phase-change pavement structures with gradient low-thermal conductivity was proposed to reduce the heat absorption of the pavement and realize the active protection of permafrost. The structures were designed based on finite element software, and the heat flux and temperature distribution within the structure were analyzed and verified with indoor and outdoor thermal regulation tests. Results showed that the preferred gradient low-thermal conductivity structure reduced the annual net heat absorption at the top of the pavement by 8.18 %, the heat absorption in summer by 7.65 %, and the heat release in winter by 6.75 % compared with the control group. The heat flux and the temperature distribution within the pavement structures were significantly altered by the phase-change asphalt pavement structures with gradient low-thermal conductivity, and the simulation results of the finite element model agreed well with the results of the indoor and outdoor thermal regulation tests. The temperature regulation factor (RT) was proposed as an evaluation index to quantitatively analyze the temperature regulation performance of CSPCM on the pavement structure, and the results showed that the flexible base layer had the greatest influence on the temperature regulation performance of the pavement structure. The phase-change asphalt pavement structures with gradient low-thermal conductivity proposed in this paper is a structure that has an efficient cooling effect on the interior of the pavement, and provides a new idea for improving the stability of permafrost subgrade and the integrated design of materials and structures in permafrost regions.

Prediction of Permafrost Subgrade Thawing Settlement in the Qinghai–Tibet Engineering Corridor under Climate Warming

Prediction of Permafrost Subgrade Thawing Settlement in the Qinghai–Tibet Engineering Corridor under Climate Warming

Model test on cooling performance of a new method for mitigating permafrost thaw around buried oil pipeline

Buried pipelines have been widely used to transport petroleum-based products over long distances in permafrost regions. Field observations reveal that they are endangered by rapid permafrost thawing and resulting massive soil movements, despite adopting some measures to protect pipeline foundation permafrost. A new mitigation technique is proposed to slow down the permafrost thawing ulteriorly. With this technique, a seasonal air-cooled embankment (SACE), mainly composed of a crushed rock layer, supports the pipeline and transfers the heat from the pipeline to ambient air in the cold seasons. A scale model test with the controlled air and oil temperatures was carried out to evaluate the proposed measure. The temperature and volumetric unfrozen water content of the permafrost subgrade beneath both the SACE and the direct-buried pipeline, as well as ground surface displacements, were measured during the whole testing process. A comparison of the hydrothermal process of the subgrade permafrost under the SACE and the direct-buried warm pipeline indicates that the proposed measure can substantially mitigate the rapid thawing of permafrost by controlling the geothermal regimes of the pipeline.

Cooling Effects of Interface Heat Control for Wide Permafrost Subgrades

Quantitative studies of the heat transfer mechanism of permafrost subgrades and its effect on the permafrost under the subgrade are crucial for the study of permafrost subgrade disposal measures; however, few studies have been conducted in this area. In the present work, by quantitatively analyzing the permafrost subgrade heat transfer mechanism and the variations in the underlying permafrost, the preliminary parameters of the interface heat control method—such as the application period, position, and imported cold energy quantity—are determined. The cooling effects of the ideal interface heat control method for different application schemes are analyzed. Finally, by determining the optimized temporal inhomogeneous interface energy control strategy, the required inlet velocity and artificial permafrost table for a mechanical ventilation permafrost subgrade are calculated and compared. The results show that (1) the suitable cold energy application position and period are a 0.5 m interface above the subgrade bottom and the lower thaw season, respectively, and that the imported cold energy needs to vary within the subgrade service life; (2) by adopting interface heat control measures, the maximum difference between the artificial permafrost table under the subgrade and the nearby natural ground table is only 0.097 m, and the temperature of the underlying permafrost and the area of the thawing bowl are significantly reduced; and (3) the mechanical ventilation subgrade employing the cold energy importing strategy of the interface heat control parameter also achieves a protection effect for permafrost, but as the cold air inside the ventilation pipe is gradually heated, it is necessary to amplify the inlet air speed to a certain extent for a better cooling effect.

Numerical simulation for vibration-induced settlement and permanent deformation accumulation in permafrost subgrades of the Qinghai-Tibet Railway

Train-induced vibrations critically influence permafrost subgrade deformation. This study used ABAQUS secondary development to establish a finite element model of vibration-induced settlement for permafrost subgrades in the Qinghai-Tibet Railway (QTR) based on a cumulative plastic strain model. A numerical simulation method for cumulative vibration-induced settlement prediction considering seasonality, vibration frequency, train types, and global warming was proposed. The scientificity of the simulation was verified by monitoring data. Then, the influences of embankment height, ice-rich permafrost thickness, and initial mean annual ground temperature (MAGT) of the site on cumulative vibration-induced settlement and permanent deformation accumulation in permafrost subgrades were analysed. The study further quantitatively evaluated cumulative dynamic and static deformation in the DK1136 section under various global warming conditions. The results indicate that cumulative vibration-induced settlement primarily occurs near the ice-rich permafrost layer and is significantly influenced by ground temperature. Permanent deformation accumulation increases with increasing embankment height, ice-rich permafrost thickness and initial MAGT of the site. Permanent deformation accumulation primarily results from thaw consolidation of the permafrost layer, with vibration-induced settlement contributing approximately 20 %. For permafrost regions, an optimal railway embankment height of 3–3.5 m and effective mitigation of settlement issues involve lowering the ground temperature and the embankment weight. These findings provide essential insights for subgrade challenges in permafrost regions.

Numerical Simulation of Heat Transfer of Porous Rock Layers in Cold Sandy Regions

The heat transfer characteristics of porous rock layers (PRLs) have significant seasonal differences. This feature has been used to protect the permafrost subgrade under highways and railways from degeneration. However, in cold sandy environments, the transformation law of heat transfer characteristics of PRLs on account of climate warming and aeolian sand filling needs to be solved. This work developed a coupled heat transfer model for the soil–PRL system aimed at analyzing the convective heat transfer process and mechanism of a closed PRL. Furthermore, the impact of climate warming and sand filling on the cooling performance of the PRL under different mean annual air temperatures (MAATs) of −3.5, −4.5, and −5.5 °C was quantified. The numerical results indicated that the natural convection of the closed PRL occurred only in winter, and the effective convective height of the rock layer decreased with the sand-filling thickness. As the thickness of sand filling increased, the critical temperature difference for the occurrence of natural convection increased, accompanied by decreases in the Rayleigh number, the duration, and intensity of natural convection. When the sand-filling thickness exceeded 80 cm, natural convection would not occur in the PRL. Under a warming scenario of 0.052 °C·a−1, the cooling performance of the PRL could offset the adverse impact of climate warming and raise the permafrost table in the first 20 years. Moreover, the closed PRL can be more effective in permafrost regions with colder MAATs. For cold sandy permafrost zones, sand-control measures should be taken to maintain the long-term cooling performance of the PRL. This study is of great significance in guiding porous rock embankment design and road maintenance along the Qinghai–Tibetan Railway.

Wind-driven device for cooling permafrost

Preserving permafrost subgrade is a challenge due to global warming, but passive cooling techniques have limited success. Here, we present a novel wind-driven device that can cool permafrost subgrade by circulating coolant between the ambient air and the subgrade. It consists of a wind mill, a mechanical clutch with phase change material, and a fluid-circulation heat exchanger. The clutch engages and disengages through freezing and melting phase change material, while the device turns off when the outside air temperature exceeds a certain threshold, preventing heat from penetrating the subgrade. Two-year observations demonstrate that the device effectively cooled permafrost measuring 8.0 m in height and 1.5 m in radius by 0.6–1.0 °C, with an average power of 68.03 W. The device can be adapted for cooling embankments, airstrip bases, pipe foundations, and other structures. Further experimentation is required to evaluate its cooling capacity and long-term durability under various conditions.

Thermal characteristics and optimization of phase change energy storage subgrade in permafrost regions

In permafrost regions, the thawing of permafrost will cause serious damage to the road structure, and delaying the degradation of permafrost is the key to ensure the road performance. Based on the energy storage characteristics of phase change material (PCM) and the anti-seepage performance of geotextile, a phase change geotextile (PCG) with heat absorption and waterproof functions is prepared in this study. PCG is applied to the subgrade structure, and the phase change energy storage subgrade (PCESS) is proposed. This subgrade structure can effectively reduce the temperature and increases the artificial permafrost table in permafrost regions. In order to study the temperature regulation mechanism of PCESS, the effects of thermal properties of PCM and epoxy resin mixture (PCMERM) and structural parameters of PCG were analyzed by numerical models. The mean absolute errors (MAEs) were 0.88 and 1.08 with experiments conducted to validate numerical model. The analysis of influencing factors showed that the phase change temperature of PCMERM exhibits the best value to improve the regulation effect of PCESS under given conditions. Increasing the heat of fusion and dosage of PCMERM can enhance the regulation effect. When PCG was laid on the top of the subgrade, the regulation effect was the best. Compared with the traditional permafrost subgrade, PCESS showed long-term thermal stability. In addition, the prediction model for the artificial permafrost table change was established. The model predicted the artificial table in three cases, and the MAEs were 0.048, 0.019 and 0.039, respectively. These results suggest that the model can accurately predict the changes of the artificial permafrost table.

Risk Zoning of Permafrost Thaw Settlement in the Qinghai–Tibet Engineering Corridor

The Qinghai–Tibet Plateau is the highest and largest permafrost area in the middle and low latitudes of China. In this region, permafrost thaw settlement is the main form of expressway subgrade disaster. Therefore, the quantitative analysis and regionalization study of permafrost thaw settlement deformation are of great significance for expressway construction and maintenance in the Qinghai–Tibet region. This paper establishes a thaw settlement prediction model using the thaw settlement coefficient and thaw depth. The thaw depth was predicted by the mean annual ground temperatures and active-layer thicknesses using the Radial Basis Function (RBF) neural network model, and the thaw settlement coefficient was determined according to the type of ice content. Further, the distribution characteristics of thaw settlement risk of the permafrost subgrade in the study region were mapped and analyzed. The results showed that the thaw settlement risk was able to be divided into four risk levels, namely significant risk, high risk, medium risk and low risk levels, with the areas of these four risk levels covering 3868.67 km2, 1594.21 km2, 2456.10 km2 and 558.78 km2, respectively, of the total study region. The significant risk level had the highest proportion among all the risk levels and was mainly distributed across the Chumar River Basin, Beiluhe River Basin and Gaerqu River Basin regions. Moreover, ice content was found to be the main factor affecting thaw settlement, with thaw settlement found to increase as the ice content increased.

The Influence of Vegetation Cover on the Settlement Behavior of Permafrost Subgrade in the Greater Khingan Mountains Forest Region

The subgrade construction in the permafrost forest region will aggravate the degradation of frozen soil, which will lead to the settlement of the subgrade. Based on the road project of National Highway 332 in the Great Khingan Mountains, by means of field observation, experimental testing, and numerical simulation, a thermo-hydro-mechanical coupling numerical model of the permafrost subgrade considering vegetation cover was established to analyze the influence of vegetation cover on the settlement behavior of the permafrost subgrade. The study indicates that vegetation cover mainly influences the seasonal active layer temperature of permafrost, and its cooling effect on permafrost in the warm season is more significant compared with the warming effect in the cold season. The volumetric water content of the subgrade with vegetation cover is greater than that without vegetation cover in the cold season. The situation is just the opposite in the warm season. The damage to the subgrade is mainly reflected in the settlement caused by the thawing of frozen soil. The maximum settlement of the subgrade with and without vegetation cover is 8.3 mm and 9.5 mm at the foot of the subgrade slope. After construction, the settlement behavior of the permafrost subgrade will undergo a degradation period of 3 years, a restoration period of 2 years, and finally, tend to be stable.

Study on the Effect of Non-Linear Ventilation Pipe Composite Measures on the Slope Permafrost Subgrade

In the alpine mountain area, the stability of the steep-slope permafrost subgrade is mainly affected by the temperature variation in the frozen soil layer at the base of the road. Under the conditions of climate warming and engineering-related disturbance, the cooling of the subgrade is an urgent problem to be solved in the field of construction. In this paper, the numerical calculation method, combined with data monitored in situ, is used to study the cooling effects of several subgrade cooling measures, in which the non-linear ventilation pipe and rubble layer are combined. The results show the following: (1) the temperature field of the steep-slope subgrade is clearly different in lateral terms—after 20 years of operation, the maximum difference in the melting depth between the left shoulder and the subgrade center is 3 m; (2) the maximum melting depth of the gravel subgrade center is 3.85 m, while the maximum difference in the melting depth between the left shoulder and the subgrade center is 2.3 m; (3) the cooling effect of the composite measures is noteworthy, and a −2.2 °C freezing area appears under the subgrade.

Thermal effect of the accumulated water with different depths on permafrost subgrade in cold regions

In cold regions, the thermal effect of accumulated water on underlying permafrost and permafrost subgrade remains a significant hazard causing engineering risks. Water depth of accumulated water may be an important influence factor of permafrost thermal stability, but there is lack of qualitative and quantitative research about that. In this study, equivalent thermal conductivity theory and solid heat transfer theory have been used to establish the calculation model for simulating heat transfer in water and soil. Thereafter, the accuracy and reliability of the calculation model are checked by monitored data and subsequently used to analyze the thermal erosion of water on underlying permafrost and permafrost under the embankment. These simulation results show that shallow water can protect permafrost and deeper water disrupts the thermal stability of underlying permafrost. The thermal effect extent of water is primarily determined by its depth, and the concept of critical depth and stable depth of accumulated water has been proposed. Moreover, the temperature field of permafrost under embankment can be changed by the slope toe water. In addition, the thermal effect range of the slope toe water is limited by the thermal influence radius, which increases with the depth of standing water. These findings provide support as well as a fundamental base for environmental issues arising from the accumulated water. These observations will, thus, also be valuable to further engineering environment studies in cold regions.

Train-induced vibration and subsidence prediction of the permafrost subgrade along the Qinghai-Tibet Railway

Railway track design and maintenance researchers have been pursuing reliable methods for predicting the long-term performance of the Qinghai-Tibet railway (QTR) subgrade. Based on theoretical analysis, numerical simulation, field monitoring and laboratory tests, this paper studied the train-induced vibration response characteristics of the Beiluhe subgrade of the QTR to evaluate the impact of railway vibration loads on the ice-rich permafrost layer. The results show that increasing the embankment height can significantly reduce the additional dynamic stress of the ice-rich permafrost layer. Moreover, increasing the mean annual ground temperature (MAGT) and decreasing the embankment height will aggravate permafrost table vibration-induced subsidence. This study also analysed the relationships between the train-induced permafrost table vibration subsidence and the MAGT, the ice-rich permafrost layer thickness and the embankment height and predicted the permafrost table vibration subsidence in the DK1136 test section under different global warming conditions. The analyses and prediction demonstrate that the appropriate embankment height of the DK1136 section is 3–3.5 m, and the application of engineering measures for actively cooling the permafrost foundations is the best choice for reducing vibration-induced subsidence, thawing settlement and compression deformation.

Variation of Permafrost Upper Limit in Permafrost Subgrade Covered by Snow on Steep Slope of Alpine Mountains

The thermal insulation effect of snow on the active layer thickness of permafrost cannot be ignored. Under the dual influence of snow cover and roadbed construction, this paper studies the influence of snow cover on the temperature field and permafrost upper limit under the permafrost roadbed in the alpine mountains. Through the application of PDE module in Comsol software, the hydrothermal coupling analysis of roadbed is carried out by means of numerical analysis. The results show that the permafrost degradation is accelerated by the snow on the subgrade surface, the temperature increases significantly, the thickness of the active layer increases, and the permafrost upper limit moves downward. With the increase of seasonal snow thickness, the permafrost temperature under the subgrade increases, the low-temperature permafrost under the subgrade has the risk of changing to high-temperature permafrost. The melting groove is formed under the subgrade on the right side, and with the increase of snow thickness, the area of the melting groove expands and develops towards the center of the subgrade. Permafrost upper limit on the left and right shoulder is lower than that on the right shoulder, and snow cover has a greater impact on permafrost upper limit on the right shoulder.

Thermal and deformational repairing effect of crushed rock revetment acting as reinforcement along Qinghai–Tibet railway in permafrost regions

Due to the particularity and complexity of permafrost subgrade, research on its long-term maintenance and reinforcement under climate warming and engineering activities is of great significance. To mitigate subgrade diseases caused by thermal disturbance during the engineering construction and operation in the initial stage, the crushed-rock revetment (CRR) was additionally paved with a thickness of 1.5 m and 1.0 m on some sunny and shady shoulders of the traditional embankments along the Qinghai–Tibet railway, respectively. The improving effects for thermal and deforming stability are evaluated based on observation data of ground temperatures and embankment deformations at two sites from 2002 to 2014. The results show that a larger uplifting magnitude in the artificial permafrost table (APT), greater ground temperature decreasing amplitudes and reduction ranges of settling rate appear under the shady embankment shoulder in warm permafrost region, and both sides in the cold permafrost region when reinforcing with CRR. However, in warm permafrost region, the laying of CRR on the sunny slope of subgrade may cause considerable thermal disturbance to the underlying permafrost foundation, combined with the resulting additional stress, induce the further expansion of differential settlement. Moreover, the thermal stability strengthening effect of the CRR is closely related to the variation of the APT thickness in the earlier stage, convection intensity inside the CRR, ‘cold energy reserve’ in the deeper permafrost, and amount of solar radiation received by the CRR. More effective reinforcements should be implemented to alleviate the potential threaten beneath sunny embankment slope in warm permafrost regions.

A New Mitigation Measure to Counter Thermal Instability of Air‐Cooled Embankment in Sandy Permafrost Zones of Tibet Plateau

A crushed‐rock revetment (CRR) with high permeability that can be paved on embankment slopes is widely used to cool and protect the subgrade permafrost. In this study, a traditional CRR over warm permafrost was selected to investigate its cooling characteristics based on the ground temperature observed from 2003 to 2014. A new mitigation structure (NMS) was designed to improve the cooling capacity of the CRR and to counter the pore‐filling of the rock layer. Numerical simulations were conducted to evaluate the cooling performance and reinforcing capacity of the NMS based on a developed heat and mass transfer model. The results indicate that the traditional CRR can improve the symmetry of the permafrost subgrade and decrease the ground temperature of shallow permafrost. However, the CRR cannot generate strong enough cooling to influence the deep (below 10 m depth) and warm permafrost with a mean annual ground temperature above −1.0°C. The wind‐blown sand can further weaken the cooling of the CRR and cause significant permafrost warming and thawing beneath the slopes, posing a severe threat to the long‐term safe operation of the embankment. The proposed NMS can produce a significantly superior cooling performance to the CRR. If the CRR is reinforced by the new structure, it can not only effectively cool the underlying warm permafrost but also elevate the permafrost table. The new structure can also protect the rock layer on the slopes from sand‐filling. The NMS can be used as an effective method for roadbed design or maintenance over warm permafrost.