Study On Stability Of Permafrost Subgrade Under Train Loading And Freeze-Thaw

With the construction and operation of Qinghai-Tibet railway in permafrost regions, the surface temperature condition under subgrade has been changed, and the equilibrium state of the water and heat process in perennial-frost which under the natural surface has also been broken, which resulting the migration of upper limit for permafrost under subgrade. Under the effect of long term train load and freezing and thawing cycle, the stability of subgrade will be reduced. As a result, it is important to analysis the water and heat process in subgrade under freeze-thawing cycle and evaluates the thermal stability of subgrade. On this basis, the effect from long term train load to the subgrade stability should be studied and the steady state of subgrade in the future will be predicted. All of these play a guiding role in the design, construction and maintenance for subgrade engineering, and have great significance on the operation safety of railway.In support of National Natural Science Foundation of China (50778012), the effect of freezing and thawing cycle on the properties of frozen soils was studied in this paper. Both laboratory dynamic triaxial test and on-site observation for frozen soils were performed in this study. Based on the experimental results, a computational model was built to analysis the temperature and water distributions under freezing. After that, the influence of train load and freezing and thawing cycle on the stability of frozen soil subgrade was analyzed. The study contained the following main parts:The physical mechanics and thermodynamics characterizing quantities for frozen soils were summarized, and the influence rules of these characterizing quantities on the engineering properties of frozen soils were presented. The following quantities are greatly influenced by the freeze-thawing cycle:dry density, water content, elasticity modulus, coefficient of heat conductivity, slip resistance, cohesive strength, etc. The forming mechanism for the special properties of frozen soils was studied, that is to say, the changing law for the physical and mechanical behavior of frozen soils in the freezing and thawing process was proposed. Based on the existing experimental results, theoretical research basis and empirical achievements, the selection rule of physical mechanics parameters which used in the numerical calculations was also presented.To provide a basis for selecting calculating parameters, an experimental program has been developed. A series of controlled laboratory tests were carried out on silty clay samples from Qinghai-Tibet railway in permafrost regions. It is shown that the physical quantities, such as the dynamic modulus of elasticity, dynamic damping ratio, dynamic friction angle and cohesive strength, are dependent of temperature, water content, confining pressure and vibration frequency. The changing law was proposed at the same time.Based on the on-site observation results at experimental section, a computational model for frozen soil subgrade was established, incorporating soil dynamics, heat transfer theory and physics of frozen soils. On this basis, the effect of freezing and thawing cycle on the temperature and water distributions of subgrade was studied, and a evaluation method for analyzing subgrade stability which taking temperature and water content as indicator was proposed. Using this proposed method, the steady state for the subgrade of the established model in the next fifty years was evaluated.With the above results, and taking the Qinghai-Tibet railway as the object of this study, a 3D FEM model for vehicle-track-subgrade interactions was established. The effects of moving load on the frozen soil subgrade which is freezing, thawing and at iced state were analyzed. The simulations for predicting the stability of frozen soil subgrade influenced by long term train load were carried out, and the results were taken to make evaluations from both the soil strength and subgrade deformation, which providing a reference frame for the operation and maintenance of Qinghai-Tibet railway.