This study focused on the coupling heat transfer mechanism and the cooling efficiency of L-shaped two-phase closed thermosyphons (L-shaped TPCTs) in the wide subgrade of permafrost regions. Considering the fact that time–space dynamics change the effects of the air temperature, wind speed, and geotemperature, a coupled air temperature–L-shaped TPCT–subgrade soil heat transfer model was established using the ANSYS 15.0 software platform, and the rationality of the model was verified through measured data. The heat-transfer characteristics of the L-shaped TPCTs and the long-term thermal stability of the subgrade were studied under different inclination angles of the evaporator (α = 15°, 30°, 50°, 70°, and 90°). Then, the cooling effectiveness of a composite subgrade with TPCTs and an XPS insulation board was numerically calculated. The results show that the heat flux of the L-shaped TPCT was the greatest when α = 50°, and the heat flux reached the maximum value of 165.7 W·m⁻² in January. The L-shaped TPCT had a relatively good cooling effect on the subgrade as a whole when α = 50° and 70°, but the thawing depth at the center of the subgrade with L-shaped TPCTs reached 9.0 m below the ground surface in the 30th year. The composite subgrade with L-shaped TPCTs/vertical TPCT/XPS insulation board is an effective method to protect the permafrost foundation and improve the long-term thermal stability of the wide subgrade. The maximum heat flux of evaporation section of the L-shaped TPCT is 18.8% higher than that of the vertical TPCT during the working period of the TPCTs of the composite subgrade.

Pile foundations are widely adopted in the Qinghai–Tibet Railway. The pile’s bearing capacity must be influenced the trends of global warming, rising foundation soil temperatures, expanding active layers, ground ice melting, etc. Thus, the pile’s long-term bearing capacity must be understood, and methods for evaluating its stability in permafrost regions must be developed. The bearing capacity of a pile foundation in a permafrost region is caused by the freezing strength of the pile–soil interface and support force at the pile bottom. Based on the formation mechanism of the bearing capacity of pile foundations in permafrost regions, the stability ratio of the freezing area and the freezing strength between the pile and soil are proposed to evaluate the thermal stability of the pile’s bearing capacity in permafrost. Using the method, a bearing capacity study on a single pile in typical moist permafrost and two piles in a permafrost region where warm groundwater is underground is performed. Compared with the field-measured value, the evaluation method proves to be correct and effective. Using the two parameters dependent on the soil–pile interface temperature, the effect of the thermal stability of pile bearing capacity may be described intuitively. Thus, both parameters are conducive to the analysis of pile bearing capacity formation mechanism in permafrost.

To investigate the engineering performance and microstructural evolution of magnesium chloride–cement composite-stabilized loess, laboratory tests were conducted on loess treated with different cement contents and MgCl₂ dosages. Compaction tests, unconfined compressive strength (UCS) tests after 7 and 28 days of curing, X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) analyses were performed to establish the relationship among hydration products, pore evolution, and strength development. The results showed that MgCl₂ increased the maximum dry density and reduced the optimum moisture content, thereby improving the compactability of cement-stabilized loess. The UCS exhibited a non-monotonic response to increasing MgCl₂ dosage, with an initial decrease followed by an increase and a subsequent decline. The optimal MgCl₂ content was 1.7% for 8% cement and 1.8% for both 10% and 12% cement. Integrated XRD, SEM, and NMR analyses revealed that an appropriate MgCl₂ dosage regulates the hydration and microstructural evolution of cement-stabilized loess through the formation of Mg-bearing reaction products, enhanced interparticle bonding, and refinement of the pore structure. The resulting reductions in total porosity and the proportion of medium and large pores were closely associated with the observed strength enhancement, establishing a clear hydration–pore structure–strength relationship. Although the 12% cement–1.8% MgCl₂ mixture achieved the highest strength, the 10% cement–1.8% MgCl₂ mixture provided a more favorable balance between mechanical performance and cement consumption. These findings provide mechanistic insight into MgCl₂-regulated hydration and pore-structure evolution and support the optimized use of MgCl₂ as an auxiliary modifier for cement-stabilized loess.

This study focuses on loess in the Lanzhou region and employs a self-developed LM-1 ionic cementitious curing agent (mainly sodium silicate) combined with cement for loess improvement. Through mechanical tests, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) analysis, the mechanical properties, microstructure, and mineralogical evolution of composite-improved loess were systematically investigated, with emphasis on revealing the reinforcement mechanism. The results show that combining LM-1 and cement significantly enhances the unconfined compressive strength (UCS) of loess. The optimal proportion of 1.5% LM-1 + 10% cement achieves 7-day and 28-day UCS values of 3.43 MPa and 4.12 MPa, representing increases of 69.0% and 54.7% over 10% cement alone, and outperforming 12% cement-only specimens. This formulation develops a uniform, dense C-S-H gel network with optimized pore structure and reduced microcracks. LM-1 generates hydrated silicate products through alkali-activated reactions, which together with cement hydration C-S-H gel form a dual-gel cementation system, while effectively accelerating cement hydration. This provides a theoretical basis and technical support for balancing cement reduction and performance enhancement in subgrade reinforcement and slope protection in loess regions.

The combined effects of global warming and increasing rainfall threaten the stability of railway embankments in permafrost regions of the Qinghai–Tibet Plateau. An indoor embankment model at a geometric scale of 1:17 was constructed based on a typical section of the Qinghai–Tibet Railway on the Chumar River High Plain. Experiments were conducted in a temperature-controlled environmental chamber using four rainfall classes with different durations, prescribed according to observed seasonal precipitation characteristics. Temperature and moisture variations were monitored at different depths beneath the embankment center, shoulder, and slope toe under rainfall and rain-free conditions. The study quantifies three regulatory effects of rainfall on the hydrothermal field: (1) The “seasonal differentiation effect” on the temperature field—rainfall generally reduces shallow embankment temperature (from April to October, average cooling of 0.8 °C, 0.6 °C, and 0.4 °C at 10 cm depth below the pavement, shoulder, and slope toe, respectively). Maximum cooling occurs in summer (1.8 °C beneath the embankment center in July); in autumn, cooling is observed beneath the embankment center and shoulder; in spring, shallow ground temperatures increased with seasonal warming, while rainfall generally reduced their values relative to the corresponding rain-free condition. (2) The “spatial heterogeneity effect” on the moisture field—rainfall significantly increases shallow moisture content (average increases of 3.1%, 4.3%, and 4.9% at the same locations), with the slope toe showing the largest increase due to surface runoff, while beneath the pavement the “pot-cover effect” intensifies vapor condensation. (3) Rainfall-associated temperature and moisture differences generally decreased with depth. Under the prescribed experimental conditions, moisture differences were relatively small at monitored model depths of 50 cm and greater. The results reveal the hydrothermal response mechanism of permafrost embankments under coupled rainfall and freeze–thaw cycles, providing refined experimental evidence for disease prevention and long-term stability evaluation.