The differential settlement of warm permafrost foundations significantly impacts the safe operation of highway and railway embankments. The use of geotextile encased lime energy columns (GELECs) has been proven to be effective in pre-thawing shallow layers of warm permafrost as a novel method to reduce the post-construction settlement of embankments. Understanding the interaction between the GELECs and the soil is crucial in illustrating the load transfer mechanism. This study conducted a series of large-scale direct shear tests on GELEC-soil in degraded permafrost environments using an improved temperature-controlled direct shear test apparatus with assembled large shear boxes. The effects of different shear rates, water contents, and types of geotextiles on the mechanical behavior of the interface were analyzed. The strength development of the interface under various curing times was studied in detail. The experimental results indicate that the interface strength increases significantly during the initial stage of curing while the rate of strength increase diminishes over time. The improvement in peak shear strength is primarily attributed to the increase in interfacial cohesion, and the increasing trend of the cohesion follows an exponential decay function. And the microscopic strengthening mechanism of the interface was analyzed through SEM tests. Finally, a nonlinear elastic model incorporating a parameter to represent the variation of cohesion was developed to describe the shear stress-strain relationship at the GELEC-soil interface under different curing times.

In Interior Alaska, a slope underlying the Trans Alaska Pipeline System has recently experienced downslope movement, which is attributed to a buried frozen, ice-rich peat layer. We performed a field investigation of the site, including coring and sampling, and conducted a suite of laboratory tests, including mechanical tests at temperatures between -0.56 and -5 degrees C to quantify the secondary creep behavior and to estimate the impact of soil cooling on the creep deformation. We tested a variety of soils, including ice-rich silt, silty peat, and peat with the majority having an organic content of 10% or greater. The results indicated that temperature has a strong control on the resulting time-dependent mechanical properties. Here we provide secondary creep power law relationships for these soils. The analysis indicates that cooling the soils can be effective in reducing creep movement; for example, cooling by 1.1 degrees C from -0.56 to -1.67 degrees C results in an order of magnitude reduction in the shear deformation rates. These results are significant as they add to the limited amount of work done on the time-dependent mechanical behavior of ice-rich peat and organic soils at warm sub-freezing temperatures.

The temperature state of warm permafrost is in the negative temperature near-phase-transition interval and thus is extremely sensitive to small fluctuations of the heat and stress environment. The dynamic load induced by the vehicle operation not only changes its magnitude cyclically, but also its principal stress direction (PSD) consistently changes rotationally. However, the existing conventional dynamic research methods can only simulate the cyclic stress environment with constant PSD, which is very different from the stress environment induced by locomotive operation, and thus the conventional dynamic research results are bound to be difficult to accurately reflect the influence of the stress field induced by vehicle operation on the development law of warm permafrost permanent settlement deformation. Therefore, in this paper, a series of dynamic studies were carried out for the Qinghai-Tibet silt at -2 degrees C by simulating the cardioid-shaped stress path induced during the operation of a heavy-duty locomotive using a frozen hollow cylinder apparatus. It is found that the rotational effect of the principal stress direction under the conditions of the cardioid-shaped stress path (CSSP) action accelerates the development of vertical permanent deformation in the frozen soil. The main reason that caused the axial strain increases advantage is the rotation of principal stress direction can accelerate the viscous energy dissipation during dynamic loading process. According to the law of axial strain development in the high-temperature frozen soil under the conditions of different CSSP action, an empirical model of permanent deformation development in high-temperature frozen soil considering the PSD rotation effect was established, and it was found by this model calculation that under the general condition of heavy-load locomotive operation, compared with the conventional research method without considering the PSD rotation, the axial cumulated strain in high-temperature frozen soil increased by 33.3% after considering the rotation effect in the PSD.

In order to study the dynamic response of the warm permafrost subgrade under traffic loads, the low temperature dynamic triaxial test was conducted on remolded soil which extracted from Yakeshi highway in Inner Mongolia Autonomous Region and gained the hysteretic loop curves, while the deviatoric stress changes from 0 to 50 kPa. The cyclic loading frequency is 6 Hz and the test temperature is - 1.5 degrees C. The result shows that each deviatoric stress-strain hysteresis loop can be divided into two parts, and data points can be described by sine function separately for each part. The phase angle of sine function is changing with the number of loading cycles and the sample always keeps stable. The warm permafrost hysteresis loop area linearly reduces and the dynamic modulus exhibits a pattern of initial increase, followed by a decrease, and then a subsequent increase.

Permafrost is mostly warm and thermally unstable on the Tibetan Plateau (TP), particularly in some marginal areas, thereby being susceptible to degrade or even disappear under climate warming. The degradation of permafrost consequently leads to changes in hydrological cycles associated with seasonal freeze-thaw processes. In this study, we investigated seasonal hydrothermal processes of near-surface permafrost layers and their responses to rain events at two warm permafrost sites in the Headwater Area of the Yellow River, northeastern TP. Results demonstrated that water content in shallow active layers changed with infiltration of rainwater, whereas kept stable in the perennially frozen layer, which serves as an aquitard due to low hydraulic conductivity or even imperviousness. Accordingly, the supra-permafrost water acts as a seasonal aquifer in the thawing period and as a seasonal aquitard in the freezing period. Seasonal freeze-thaw processes in association with rain events correlate well with the recharge and discharge of the supra-permafrost water. Super-heavy precipitation (44 mm occurred on 2 July 2015) caused a sharp increase in soil water content and dramatic rises in soil temperatures by 0.3-0.5 degrees C at shallow depths and advancement thawing of the active layer by half a month. However, more summer precipitation amount tends to reduce the seasonal amplitude of soil temperatures, decrease mean annual soil temperatures and thawing indices and thin active layers. High salinity results in the long remaining of a large amount of unfrozen water around the bottom of the active layer. We conclude that extremely warm permafrost with T-ZAR (the temperature at the depth of zero annual amplitude) > 0.5 degrees C is likely percolated under heavy and super-heavy precipitation events, while hydrothermal processes around the permafrost table likely present three stages concerning TZAR of 0 degrees C.

Long-term thermal effects of air convection embankments (ACEs) over 550-km-long permafrost zones along the Qinghai-Tibet railway were analyzed on the basis of 14-year records (2002-2016) of ground temperature. The results showed that, after embankment construction, permafrost tables beneath the ACEs moved upward quickly in the first 3years and then remained stable over the next 10years. The magnitude of this upward movement showed a positive correlation with embankment thickness. Shallow permafrost temperature beneath the ACEs decreased over a 5-year period after embankment construction in cold permafrost zones, but increased sharply concurrent with permafrost table upward movement in warm permafrost zones. Deep permafrost beneath all the ACEs showed a slow warming trend due to climate warming. Overall, the thermal effects of ACEs significantly uplifted underlying permafrost tables after embankment construction and then maintained them well in a warming climate. The different thermal effects of ACEs in cold and warm permafrost zones related to the working principle of the ACEs and natural ground thermal regime in the two zones. (c) 2018 American Society of Civil Engineers.