Analysis of dynamic compressive stress induced by passing trains in permafrost subgrade along Qinghai-Tibet Railway

To investigate dynamic compressive stress characteristics and related influencing factors in the permafrost site along Qinghai-Tibet Railway (QTR), a numerical analysis was carried out using self-compiling train-rail-subgrade-site dynamic coupling program ZL-TNTLM and ZL-RNTLM, by which the effect of the vibrating load generated by passing trains on the natural permafrost table was analyzed quantitatively. The results indicate that: (1) the additional dynamic compressive stress depend directly on axle load, the vibration energy at subgrade top surface is relatively larger when vibration frequencies range from 0.5 to 10 Hz, and the dominant frequencies at the top of the natural permafrost table are confined to be less than 3 Hz; (2) the vibration-affected subgrade area is mainly limited to the oval zone of 6 m below sleepers, stress concentration phenomena occur in the vicinity of rail-supporting zone in ballast, the maximum dynamic compressive stress at the bedding superface presents a saddle-type distribution, and certain negative exponential function can be adopted to fit the attenuation relationship of the dynamic stress vs. depth; (3) the dynamic compressive stress of subgrade top surface is the largest in winter, then followed by spring and autumn, and the least in summer, while the corresponding trend is quite the opposite in the interior of subgrade; (4) the dynamic compressive stress surrounding the wide gap decreases with the rise of train speed, however vibration response tends to be stable at certain critical speed, and the critical speed decreases with depth below subgrade surface; (5) the additional dynamic stress at the natural permafrost table can be notably reduced by appropriately increasing the embankment height. The drawn conclusions can, believed by the authors, provide certain theoretical insight for the safety evaluation of QTR operation, vibratory subsidence prediction, as well as the construction of permafrost subgrade.