Numerical Simulation On Coupling Water,Water Vapor,Heat And Stress Fields For Subgrade Modifying Silty Clay By Oil Shale Residue And Fly Ash In Cold Region

Numerical Simulation on Coupling Water,Water Vapor,Heat and Stress Fields for Subgrade Modifying Silty Clay by Oil Shale Residue and Fly Ash in Cold RegionIn China,about 70% of the earth's surface is covered by permafrost and seasonal frozen soil.In permafrost and seasonal frozen regions,subgrade must undergo freezingthawing cycles,which leads to serious subgrade diseases and shortens the service life of subgrade and pavement structure.The subgrade is mostly unsaturated soil.In the non-saline soil area,the diseases are the result of the combined action of water,temperature and stress fields.The water field shows the distribution and change of liquid water,ice and vapor water,the temperature field shows the distribution and change of temperature,and the stress field shows the distribution and change of stress and strain.At present,there is no the numerical calculation theory and case of coupling water,water vapor,heat and stress considering freezing-thawing cycles,which cannot provide a numerical calculation basis for describing the distribution of water,ice,water vapor,temperature,stress and strain in the subgrade in the cold region;as a result,when analyzing the freezing-thawing failure mechanism of subgrade in cold region,there is no theory and quantitative analysis method for the interaction of water,ice,water vapor,temperature,stress and strain in freezing-thawing cycles.With the development of oil shale and the increase of national energy demand,the accumulation of oil shale residue and fly ash has caused serious environmental pollution,and a large number of oil shale residue and fly ash need to be treated.In order to reduce the adverse effects of freezing-thawing cycles on permafrost subgrade and seasonal permafrost,some methods have been used,such as reducing the groundwater level,using anti-freezing subgrade materials,and setting cold resistance or heat insulation layer,and have achieved certain beneficial effects.At present,there are few reports about the combined use of oil shale waste residue and fly ash as subgrade heat insulation materials,and the only research is limited to the evaluation of their respective or combined static and dynamic characteristics,permeability and other functions.Based on the project of national natural science foundation of China(NSF),“Study on the mechanism of cold resistance layer to cure subgrade frost damage(51578263)” and National key research and development program “Theory and method of intelligent perception of road infrastructure(2018YFB1600200)”,two works are carried out in this paper.First,the mechanism of freezing-thawing failure of subgrade in cold region is analyzed theoretically,and the mathematical model of coupling water-water vaporheat-stress(HVTM)fields of subgrade in cold region is constructed and verified by experiments,and then the application of HVTM coupling model in cold subgrade is presented;the research results are of great significance for predicting the distribution changes of water,water vapor,ice,temperature,stress and strain of subgrade in cold region,and quantitatively analyzing the freeze-thaw failure mechanism of subgrade in cold region.Second,based on the research group's preliminary study on the mechanics of modifying silty clay(SC)by oil shale residue and fly ash(modified SC),oil shale residue from Wangqing county,Jilin province and fly ash from Changchun no.2 power plant were selected to modify the SC widely distributed in Jilin province,and through experiments and HVTM coupling numerical simulation,the beneficial effects of using modified SC(MC)as the thermal insulation material of subgrade in cold region are explored;the research results are of great significance for the improvement of the service performance of subgrade and the reuse of solid waste in cold region.Based on the above research purposes,the research contents of this paper are as follows:(1)Based on the theoretical analysis of the multi-field coupling mechanism of subgrade freeze-thaw failure in cold region,combing with continuity equation,energy conservation equation,balance differential equation,constitutive equation,and the boundary conditions,the mathematical model of HVTM for unsaturated subgrade soil in cold region is constructed to describe the liquid water migration,vapor water migration,ice content distribution,heat distribution,stress and strain states of subgrade in cold region.(2)Under different density,moisture content,freezing-thawing cycle times,the thermal conductivity and specific heat capacity of MC and SC were tested,and the calculation equations of thermal conductivity and specific heat capacity,considering changes of temperature and freezing-thawing times,were established to improve the accuracy of HVTM model.Based on the micro and meso experiments of MC,the main controlling factors of the thermal conductivity of MC during the freezing-thawing cycles were analyzed.(3)Using the water-heat-stress coupling test system of soil column designed by the research group,the water-heat-stress coupling test of unsaturated SC and MC during freezing was carried out to analyze the thermal insulation capacity of MC;the water vapor migration test box and low temperature constant temperature tank were used to carry out the water vapor migration test of unsaturated SC.Based on the above tests of water-heat-stress coupling and water vapor migration of unsaturated SC,data support is provided for the validation of HVTM coupling mathematical model of subgrade in cold region.(4)Based on HVTM coupling mathematical model and mathematical module of COMSOL Multiphysics,the secondary development of COMSOL Multiphysics was carried out to realize the numerical calculation of HVTM coupling mathematical model;combined with the data of water-heat-stress coupling test and water vapor migration test for unsaturated SC,the HVTM coupling mathematical model was verified.(5)Based on the structure of the experimental section of the expressway,the MC is used as the thermal insulation layer of the experimental section,and the environmental assessment method is used to evaluate its impact on the environmental background of the experimental section;the HVTM coupling mathematical model was applied to the experimental section(the MC as the thermal insulation layer)and the comparison section(no the thermal insulation layer)to provide a case of application of HVTM coupling mathematical model for subgrade in cold region;the distribution changes of water,water vapor,ice,heat,stress and displacement in test section and comparison section after 3 years of operation were simulated,and the beneficial effects of MC were analyzed.