| Global warming and human engineering constructions accelerate the degradation of permafrost, thus, it will pose more problems to carry out construction in permafrost regions. In order to protect permafrost and keep the stability of highway embankment in permafrost regions, block stone embankment is used widely as a proactive cooling countermeasure. However, firstly block stone embankment slope is exposed so far, it wenkens the protective effects of block stone embankment on permafrost; secondly, the work of large-scale in-situ testing of block stone embankment for the cooling effect is less than others, especially lacks of long-term and effective monitoring. In this thesis, the writer made a model test of block stone embankment, combined with temperature field observational data of field test road embankment, and selected the appropriate model of block stone embankment to simulate its temperature field. Then, this paper discusses cooling principle and temperature field variation of block stone embankment. This can provide scientific basis in roadbed designing, construction and maintenance in permafrost regions, besides, the underlying permafrost can be protected better, and also it can reduce the disease of roadbed. The research has high academic values and important engineering significance. Specific research work and the conclusions achieved are as follows:(1) In order to explore the heat transfer characteristics of block stone embankment, a research about the cooling mechanism and effect of block stone has been made through laboratory test, as well as block stone layer has same grain size but with different overlying conditions. The results show that block stone layer with closed boundary expresses insensitivity about outside temperature changing, while the cooling rate is slower than open boundary situation. Block stone with closed boundary mainly relies on natural convection to achieve cooling performance on its bottom soil, but under open boundary conditions, cooling performance mainly relies on forced convection inside.(2) In this study, based on the observational data collected from the in-situ test section of block stone embankment at Wudaoliang along the Qinghai-Tibetan Highway, the changing processes of block stone and ordinary embankments were compared, as well as the proactive cooling effect of the block stone embankment was analyzed. The observational data showed that the block stone performed a “thermal diode†effect, meaning that the block stone increased “cold energy inflow†in cold seasons while restricting heat inflow in warm seasons. This is effective for preventing permafrost from thawing and proactively protecting permafrost.(3) Based on the convection theory of incompressible fluid in porous media, a numerical model of convective heat transfer about block stone embankment has been built in permafrost regions, then compared with the observational data collected from the in-situ test section to prove that the model is reasonable and reliable. Using this model, numerical simulation on block stone embankment of the velocity field and temperature field is presented. The numerical results show that the most intense of air convection in block stone layer is discovered in the cold seasons, while the presence of convective vortices inside causes striking cooling effect, and at the same time large scale cold energy is introduced from the external environment. Air natural convection inside block stone layer disappears in warm seasons and also block stone layer can stop heat energy to enter into the roadbed. Long thermal stability study on block stone embankment suggests that cooling effect of block stone embankment is distinct in short term, but from a long-term analysis, considering global warming and strong endothermic of asphalt, block stone embankment in permafrost regions is difficult to play a better role in reducing ground temperature, protecting permafrost and maintaining thermal stability of roadbed, so it must be reinforced. |
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