Study Of Frost Heaving Effect And Service Performance Of Permafrost Retaining Structures

As a kind of unstable special soil, permafrost has properties closely depended on temperature variations and freeze-thaw processes, which brings up a lot of engineering problems. The most serious problem is the deformation and instability of structures caused by frost heave and thaw settlement. Researches of deformation and stability of retaining structures built on permafrost are not only instructive for the design, construction and maintenance works, but also valuable references for researches on stress and interaction of other structures on permafrost.Supported by the Major State Basic Research Development Program of China (973 Program), gravity retaining wall was chosen as the object in this study. Interaction mechanism of frozen soil-concrete interface and Multi field coupling model of frozen soil freezing thawing process were taken as two basic starting points. With the methods of Laboratory testing, theoretical analysis and numerical simulation, the three major problems of frost heaving and thaw settlement, interaction of frozen soil and retaining structures, deformation and stability of retaining walls were thoroughly studied. The final object was to establish an effective theory and method which can guide the design works of retaining structures in permafrost. The main achievements were as follows:(1) Both of the dynamic and static direct shear apparatus for shear tests of frozen soil-concrete interface were developed and modified. The dynamic direct shear system was created independently and had a high innovation degree. The apparatus were proved applicable and reliable by interface tests.(2) A serious of multi factor controlled direct shear tests on frozen soil-concrete interface were accomplished. The strength and stress-strain relationship of interface under both dynamic and static loads were tested and the influences of factors were studied. Based on the method of multiple linear regression, the strength and stress-strain relationship by multi factors were analyzed. Explicit relationships between mechanical parameters and factors were presented. Based on the tests, a mechanical model of frozen soil-concrete interface was finally established.(3) A coupling model for freezing and thawing process of frozen soil was built based on moisture and heat transfer. Ice-water phase transition in pore, unfrozen water content curve by temperature, thermodynamic parameters connected with phase composition of soil, a proper soil-water characteristic curve, diffusion rate and specific water capacity connected with ice-water composition, blocking effects of moisture transfer by ice were all considered in this model. To analyze the deformation and stress, a proper frost heave model was established. Segmented relationship between ice content and frost heave ratio was considered. Isotropous free frost heave was determined by ice content obtained in the moisture and heat coupling model.(4) Base on development of multi field coupling software COMSOL Multiphysics, a method to analyze the freezing and thawing process and interaction of frozen soil and structures was established. A serious of different working conditions of gravity retaining walls were set up. Results of different soil replacement measures, different insulation measures, different groundwater tables were obtained, including fields of temperature, total water content, ice content, stress, displacement and interface state. These results described the physical fields in different times and different working conditions. Variation trends of deformation on retaining walls were discussed and frost heaving measures were compared. By the conception of efficiency cost ratios, different design parameters of retaining walls were discussed. An optimal frozen-preventing design of retaining wall was brought out.(5)Four displacement indexes were used to describe the frost heave effect of retaining walls. Based on the requiments of serviceability and stability, methods of clausal evaluations and general evaluations were built. Long-term deformation effects of a retaining wall during its design life were predicted. Evaluation system was applied in the model and evolution of service performance was discussed. Methods to improve the service performance were brought out.