| Supported by the national key basic research and development program(A.K.A.,the "973"Program)entitled "the basic research on major permafrost engineering of the Qinghai Tibet Plateau",this dissertation study is aimed at investigating the special subgrade structure that has been designed and constructed along the first expressway(i.e.,the Gonghe-Yushu expressway)in the permafrost region of the Qinghai-Tibet Plateau through field tests,laboratory tests,theoretical analyses and numerical simulations.Specifically,the temperature variations and the cooling effectiveness of this special cooling subgrade were analyzed and evaluated during the early stage of subgrade construction.The deformation characteristics and working mechanisms of this special subgrade structure were studied in the permafrost region,and the in-service performance evaluation system was proposed for practical applications.The research results could provide important guidance for the highway agencies to perform post-construction and operational maintenance work for this expressway subgrade,and provide valuable references and guidelines for the design and construction of similar expressway projects in permafrost regions.The major research findings of this dissertation study are summarized as follows:(1)The thermal conditions of three special cooling subgrade sections of the Gonghe-Yushu expressway were analyzed in the early stage of the construction.With the three-year geothermal data of those three special cooling subgrade sections,the geothermal characteristics of the embankments in cold and warm seasons were studied,and the degree of degradation of the permafrost table was analyzed.Through the comparative analysis warming rate and thawing rate of permafrost,the feasible subgrade type for expressway construction in this permafrost region was first proposed.The underlying mechanisms for the asymmetric subgrade temperature field were disclosed.(2)The basic characteristics of the major deformation sources(e.g.,freeze-thaw cycle,thawing sedimentation,warm permafrost creep,etc.)in the early period of the construction of the special subgrade structure in the permafrost region were revealed.By using the geothermal data,the influence of asphalt pavement surface on permafrost thermal stability was studied.Based on the deformation data,the deformation sources of different sub-layers in the subgrade during the expressway construction were revealed for the first time.The lagging effect of maximum frost heave and maximum thawing settlement was discussed,and the deformation characteristics of the special subgrade structure in the permafrost region were summarized.(3)The calculation models of frost heave rate and thaw settlement coefficient of sandy silt were established.Through the three-dimensional laboratory freeze-thaw tests,the main factors influencing the deformation of sand silt were analyzed,while the relationships between various factors and the stable frost heave rate(or stable thaw settlement coefficient)were revealed.By using the global optimization method,the prediction models for the stable frost heaving ratio and the thaw settlement coefficient were established respectively.(4)Based on the four-phase system of frozen soils and the theories of unsaturated soil seepage and heat conduction,the fully coupled hydro-thermal model of the frozen subgrade soil was established.With the use of this model,the degradation law of the permafrost table and the lateral thermal difference of the subgrade were studied,including the establishment of the artificial permafrost table calculation model for the special subgrade structure,the proposed new asymmetric subgrade structure,and the analysis of the influence of the expressway direction on the lateral thermal difference.With the effective thawing water content and temperature treated as influencing parameters,a calculation model of subgrade thaw settlement in the permafrost region was established,and the development trend of the long-term deformation of subgrade was studied.(5)The evaluation system of subgrade in-service performance in the permafrost region was established.According to the long-term subgrade performance and behavior,the deformation behavior of the special subgrade structure in the intended service life was predicted through example calculations.Further,the development and variation of subgrade in-service performance were analyzed and assessed by applying the developed evaluation system,from which recommendations and guidance were made for improving the in-service performance... |
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