Analysis of thermal characteristics and thermal accumulation effect of integrated rigid pile-raft subgrade applied in the permafrost region of Qinghai-Xizang Plateau

To verify the applicability of the integrated rigid pile-raft subgrade (PRS) in permafrost regions of the Qinghai-Xizang Plateau and to clarify its thermal characteristics and thermal disturbance effects on the permafrost foundation, temperature field data during the construction and the first year after completion of the first highway PRS test section in the permafrost region were monitored, and comparative analyses were conducted with adjacent block-stone subgrade (BSS) and natural ground (NG). Research results show that the seasonal freeze-thaw depths of the upstream borehole without water, the midstream borehole with less-water, and the downstream borehole with more water in the PRS are 6.8, 10.3, and 8.5 m, respectively. The seasonal freeze depths of the BSS is 6.7 m, and that of the NG is 2.9 m. A non-freezing interlayer is formed in the midstream borehole of the PRS within the depth range of 3.80–8.25 m. The monthly average temperature-depth curve clusters of the PRS present an obvious right-skewed distribution, which is manifested by a longer duration and greater depth of positive temperatures. The BSS shows a similar but weaker trend, while the temperature distribution of the NG is approximately symmetrical around 0 ℃. The heat conduction capacity, heat storage capacity, and cold conduction capacity of the PRS are higher than those of the semi-rigid base asphalt pavement combined with BSS. A significant thermal accumulation effect is exhibited by the PRS, followed by the BSS, while the natural ground shows an approximately balanced thermal state. The thermal accumulation effect of the midstream borehole of the PRS is significantly higher than that of other boreholes due to the scale effect formed by the continuous pile-raft structure. Within one annual cycle, the cumulative heat inflow at the bottom surface of the raft reaches 139.2 MJ·m^-2. Strong fluctuations of the annual cumulative heat in shallow foundation layers are related to the combined effects of water content variation and freezetthaw phase change in the seasonal freeze-thaw layer, while fluctuations in deep foundation layers are related to heat transfer driven by groundwater flow. It is suggested that thermal insulation materials be installed between the raft and the subgrade fill to mitigate heat inflow and protect the underlying permafrost. The analysis results provide a reference for structural optimization and engineering applications of the integrated rigid PRS in the permafrost region of the Qinghai-Xizang Plateau.