Impact of roadside water on sloped subgrade stability along the Qingzang Railway with two-phase closed thermosyphon and crushed rock revetment

In permafrost regions of the Qinghai-Xizang Plateau, embankments of the Qinghai-Xizang Highway and Qinghai-Xizang Railway experiencing roadside water accumulation exhibit more pronounced engineering deteriorations. A widely accepted view is that the accumulated water adjacent to the embankment possesses substantial thermal energy, which accelerates the degradation-even disappearance-of the underlying permafrost. Moreover, the presence of roadside water keeps the embankment soil in a persistently high-moisture state, thereby making the frozen-soil embankment more susceptible to deformation under traffic loading. However, in the permafrost regions of the Qinghai-Xizang Plateau, deteriorations of embankments affected by roadside water are more commonly manifested as undulating pavement surfaces, and extensive crack networks appear on the embankment crest even where thermosyphons are installed. These manifestations are not fully consistent with the deterioration mechanisms proposed by existing viewpoints. We propose the hypothesis that temperature gradients, formed due to the freezing and thawing processes between the roadside water-affected soil and the roadbed soil, lead to moisture migration under the influence of temperature gradients, resulting in frost heave and thaw settlement in the roadbed soil. To validate this hypothesis, we conducted the following investigations sequentially. Initially, we selected a roadbed with a thermosyphon (TPCT) system, which has a significant cooling effect, as the study object. By analyzing the temperature monitoring data of the roadbed section, the temperature variance was calculated to identify the time nodes where the temperature gradient of the roadbed soil was maximum and minimum. Subsequently, corresponding roadbed temperature distribution maps were drawn, illustrating the changes in the temperature and position of the low-temperature core near the TPCT over time. Furthermore, using small-scale indoor model experiments, we qualitatively concluded that moisture in the soil migrates toward the TPCT due to the temperature gradient. Thereafter, combining borehole water content data and precipitation data from the sloped terrain construction site, the formation mechanisms and timing characteristics of roadside water accumulation were analyzed. Ultimately, by integrating the ground temperature data, air temperature data, roadside water formation mechanisms, and the operating characteristics of the TPCT, it was concluded that roadside water, while in a thawed state during TPCT operation, acts as a supplementary source for moisture migration in the roadbed soil. This migration leads to cracking in the TPCT roadbed. Therefore, this study reveals a novel damage mechanism: asynchronous freeze-thaw processes induce temperature gradients, which drive the migration of roadside water into the roadbed and are responsible for the cracking damage.