【摘要】 全球气候变暖导致多年冻土层温度升高，进而显著改变桩周土体的物理和力学性质，这对铁路桥梁桩基础的竖向承载特性产生了深远影响。为量化分析多年冻土层对既有铁路桥梁桩基础竖向承载特性的影响，以青藏铁路桥梁广泛使用的高承台桩基础为研究对象，通过室内缩尺模型试验对比研究了竖向荷载作用下非冻土（对比组）与多年冻土（多年冻土层厚度为140 cm）条件下桩基础承载性能及桩周土体的破坏特征。试验结果表明：在非冻土条件下，桩周土体表面出现近似矩形的闭合裂缝，且从矩形四角向外延伸，0.5倍桩径以内的土体出现明显沉陷，土体表面仅有一条主裂缝；当有多年冻土层存在时，桩周土体虽然出现未闭合近似矩形裂缝，但表层土未发生明显沉降。此外，多年冻土层的存在显著提高了桩基础竖向极限承载力，多年冻土条件下桩基础的极限承载力约为非冻土条件下的4.5倍。分析发现，桩基础承载力的提升主要源于多年冻土层中桩侧摩阻力的显著增加，多年冻土层存在时最大桩侧摩阻力约为非冻土条件下的7.1倍。相对而言，多年冻土层对桩基础端承力的影响并不明显，多年冻土条件下桩基础最大端承力相较于非冻土条件提高了8.8%。因此，多年冻土区既有铁路桥梁桩基础承载性能评估时应当充分考虑多年冻土层的影响。本文研究结果可为多年冻土区铁路桥梁桩基础的设计及运营维护提供科学参考和理论支撑。更多还原

【Abstract】 Frozen soil is widely distributed in China, including the eastern monsoon region, the arid northwestern regions, and the Qinghai-Xizang Plateau, while permafrost is mainly distributed on the Qinghai-Xizang Plateau. In recent years, the implementation of national strategies such as the “Belt and Road Initiative” and “Western Development Strategy” has accelerated the construction of infrastructure, including railways across the Qinghai-Xizang Plateau. Considering the sensitivity and vulnerability of permafrost, the “bridge-for-embankment” approach has been widely implemented in railway construction to ensure the quality of the project and minimize thermal disturbance to the surrounding permafrost. Pile-supported bridges are widely used in railway construction in permafrost regions due to the advantage of minimal thermal impact on the permafrost. However, in permafrost regions, the thickness of the permafrost layer and the seasonal active layer are greatly affected by local temperature variations. Seasonal variations and global warming change the thermal and mechanical properties of foundation soils, consequently affecting the vertical bearing characteristics of pile foundations in permafrost regions. These changes introduce substantial uncertainties in the bearing capacity of pile foundations, which directly affect the long-term stability and safety of bridges. To quantitatively analyze the effect of permafrost layer on the vertical bearing characteristics of existing railway bridge pile foundations, this study took the high-cap pile foundations widely used in Qinghai-Xizang Plateau as the research objects, and investigated the load-bearing performance of pile foundations under non-frozen soil conditions(control group) and permafrost conditions(with a 140 cm thickness). Additionally, small-scale indoor model tests were conducted to examine the failure characteristics of soil around the piles. The results showed that under the condition of non-frozen soil, a nearly rectangular closed crack formed on the soil surface around the pile, radiating from its four corners. Significant surface displacement occurred within a range of 0.5 times the diameter of the pile, accompanied by a single primary crack on the soil surface. In contrast, under permafrost conditions, no noticeable surface displacement was observed, although an open rectangular crack occasionally appeared around the pile. In addition, the presence of permafrost significantly improved the vertical ultimate bearing capacity of pile foundations. The maximum bearing capacity under non-frozen soil conditions was approximately 40 kN, while it reached approximately 178 kN under permafrost conditions, representing a 4.5-fold increase. This increase in bearing capacity of pile foundations primarily resulted from a substantial rise in pile side friction resistance within the permafrost soil layer. The maximum side friction resistance under non-frozen soil conditions was 106.39 kPa, and increased to 752.20 kPa under permafrost conditions, demonstrating a sevenfold enhancement. In contrast, the influence of permafrost on the end bearing capacity of pile foundations was relatively minor. The maximum end bearing capacity of pile foundations was 12.5 kN under non-frozen soil conditions and 13.6 kN under permafrost conditions, representing an 8.8% increase. Overall, the presence of permafrost layer significantly changes the vertical bearing characteristics of pile foundations, including the failure characteristics of soil around pile, the bearing capacity of pile foundations and the exertion of side friction resistance. Therefore, it is essential to fully consider the effects of permafrost when evaluating the bearing performance of pile foundations for railway bridges in permafrost regions.更多还原