The Study On Frozen Soil Thaw Consolidation Behavior With Periodical Thermal Boundary

Thaw consolidation is one of the main causes triggering instability of engineering constructions in permafrost regions, such as railways, highways, petroleum pipelines and buildings. Take the Qinghai-Tibetan railway and highway for instances, recently-monitored data indicated that various instabilities or even damages were mainly induced by thaw settlement of permafrost layers, such as cracks of asphalt pavement and culverts, uneven subsidence of embankments and failures of slopes. To address this issue, numerous experimental and numerical analyses have been carried out. However, most of them adopted constant thermal boundaries in the thaw consolidation theory, which were inapplicable for the engineering practices. Consequently, little work has been done on seasonally-variational thaw settlement induced by periodical changes of thermal boundaries, namely air temperatures. From previous engineering practices, it has been recognized that embankment subsides obviously faster in warm seasons than cold seasons due to the closes of drainage boundaries when ground surfaces are frozen. Therefore, it is urgent to further improve and then use the thaw consolidation theory with periodical thermal boundaries. For this purpose, both experimental and numerical work concerning thaw consolidation has been carried out in this thesis, as follows:(1) Thaw consolidation tests with different styles of periodical boundaries were conducted for soil samples with different initial water/ice content. The developing processes of both thaw line and thaw consolidation were particularly investigated. It was indicated that the thaw consolidation induced settlement showed a step-like developing pattern under the effect of periodical thermal boundaries. This is the most significant characteristic of thaw consolidation in permafrost regions, which is usually ignored when using constant thermal boundaries. It is worth mentioning that the thaw depths and deformation values under the two different thermal boundaries were getting close with the increase of freeze-thaw cycles.(2) Thaw consolidation theory with periodical thermal boundaries is implemented using FLAC. Thaw consolidation is calculated in the thawed permafrost layers, which is defined as "post-thawed zone". In addition, different magnitudes of permeability were employed to analyze the effect of freeze-thaw cycles on thaw consolidation in post-thawed zones.(3) One-dimensional thaw consolidation tests were conducted to verify the numerical results. Based on this theory, the development of thaw consolidation degree was calculated. It was indicated that the thaw consolidation degree changed periodically under the effect of periodical thermal boundaries, decreased continuously within every freeze-thaw cycles, and then showed a tendency to the same deformation value under constant thermal boundaries. A decaying deformation characteristic was also identified in the process of thaw consolidation.(4) The numerical analyses indicated that the periodical. characteristic of thaw consolidation was mainly resulted from the thawing of deep permafrost layers underneath road embankments, usually taking place from August and November every year. Through comparing the calculated and monitored data, the numerical results were proven to be reliable. Therefore, the established simulation platform can be taken as a useful tool to analyze the relative engineering problems in permafrost regions.(5) It can also be seen from the numerical results that thaw consolidation degrees firstly increased and then decreased continuously. This may imply that thaw consolidation may end within the first couple of operating years for the road sections. Decreasing of thaw consolidation degree often comes from thawing of the deep post-thawed permafrost layers. Regression analyses showed that the decreasing rate of thaw consolidation degree was positively related to the thaw consolidation ratio, and a power function can well describe their relationship.In sum, the results indicated that the newly-developed simulation platform could reasonably capture the periodical characters of thaw consolidation under the periodical thermal boundaries. This feature of thaw consolidation is of great importance to alleviate the deformation problems of highways or other related engineering constructions, which should be taken into consolidation in calculating thaw consolidation in permafrost regions.