Research On Damage Effect And Energy Evolution Mechanism Of Compacted Clayey Soil Used For Subgrade In Seasonal Frozen Region

The distribution area of seasonal frozen region in China is 5.137×106 km2,accounting for 53.5% of the country's land area.With the continued development of China's “Western Development” and “rejuvenating the old industrial base in Northeastern China”,the steady implementation of the “One Belt and One Road”,and the rapid economic and social development,the scale of transportation infrastructure construction in the seasonal frozen region will continue to expand,and construction and operating standards will be greatly improved.Under the comprehensive influence of freeze-thaw,moisture and driving load,the subgrade in the seasonal frozen region will undergo deformation and strength changes due to the attenuation of the compacted clayey soil's performance,which will bring about a series of road diseases,such as slope slumps,cracks,slab staggering and frost boiling.The process of deformation and failure of compacted clayey soil in the subgrade in the seasonal frozen region is essentially a comprehensive process of continuous damage accumulation,and it's also a a comprehensive process of energy absorption,accumulation,dissipation and release.Understanding and mastering the damage effect and energy evolution mechanism of the compacted clayey soil under the influence of freeze-thaw,moisture and load are of great significance for analyzing the evolution law of the compacted soil properties of the subgrade in the seasonal frozen region,as well as guiding the subgrade engineering practice and catastrophe prevention.At the same time,because of the complexity of soil properties and other factors,different regions and different types of soils show different engineering properties.Some research results have not reached a consensus yet,research on the different soil in the different region is needed to be carried out.The thesis relied on the National Natural Science Foundation of China “Theoretical and Experimental Research on the Transformation between Static and Dynamic Parameters of Subgrade Materials after Freeze-thaw Cycles”.In view of the widespread distribution of clayey soil and the universality of its application in the subgrade engineering,the low liquid limit clay in the borrow area of the CP05 contractual section of the reconstruction and expansion project from Siping to Changchun of Jingha Express was selected as the research object.Based on the characteristics of the subgrade engineering work in the seasonal frozen region,the mechanical properties of the soil were tested by the indoor freeze-thaw cycles and triaxial compression tests.Based on the analysis,a constitutive model of damage under freeze-thaw-moisture-load coupling was constructed by using the continuum damage mechanics and strain equivalence principle,and soil damage detection and evaluation methods based on elastic wave theory and ultrasonic non-destructive testing techniques were explored.And the energy characteristics in the process of deformation and destruction was also analyzed.The specific research work carried out in the thesis is as follows:(1)Based on laboratory tests,the mechanical properties' evolution of compacted clayey soil in the seasonal frozen zone were analyzed.Firstly,based on the test of basic properties of the subgrade soil,test specimens were molded and subjected to freeze-thaw cycles;then the development of stress-strain relationship,elastic modulus and shear strength of the subgrade compacted clay were studied by triaxial compression test under the influence of freeze-thaw cycles,moisture,and confining pressure,obtained stress-strain relations,as well as mechanical parameters such as elastic modulus,shear strength parameters,failure strength,and the relationship between them and freeze-thaw cycles,moisture and confining pressure;Then used analysis of variance to explore the significance of the influence of freeze-thaw cycles,moisture,confining pressure and the interaction between them on the mechanical properties of compacted clayey soil used for subgrade,and obtained the degree of significance of the influencing factors and their sequencing.(2)The damage variable equation and the damage constitutive model of the subgrade compacted clay under freeze-thaw-moisture-load coupling were established,and the damage evolution characteristics of the compacted clayey soil in the subgrade were also discussed in this paper.Firstly,based on the heterogeneity of the compacted clayey soil material and the randomness of the damage propagation,it is assumed that the failure of soil micro-elements obeys the Weibull distribution and the Mohr-Coulomb criterion,the damage constitutive model of subgrade compacted clayey soil under triaxial compression was established by using the continuum damage mechanics theory combined with the strain equivalence principle and generalized Hooker law,and proposed a method to determine the damage model parameters based on the stress-strain characteristics of subgrade compacted clayey soil under triaxial compression conditions;After the freeze-thaw-moisture-load coupling effect of the compacted clayey soil was analyzed by using the extended strain equivalence principle,a damage variable evolution equation that can comprehensively consider the effect of freeze-thaw cycles,moisture and load was proposed and embeded into the damage constitutive model of compacted clayey soi under the triaxial compression conditions,thus the damage constitutive model of compacted clayey soil used for subgrade was established that can consider the freeze-thaw-moisture-load coupling effect.Finally,the test results of mechanical properties of the compacted clayey soil are quoted to verify the rationality of the damage constitutive model,and explored the damage propagation characteristics,obtained the expansion of the subgrade compacted clayey damage with strain,freezing,moisture and confining pressure.(3)The detection and evaluation mothering of soil damage based on ultrasonic technology was researched and proposed in the thesis,and the damage characteristics of compacted clayey soil used for subgrade in the seasonal frozen region were analyzed by using the method.First of all,based on elastic wave theory,considered the effects of freeze-thaw cycles and moisture changes on the compacted clayey soil,and the damage variable calculation formula including three parameters of longitudinal wave velocity,water content and geometric deformation parameters was derived through rigorous theoretical derivation.Based on the formula,a method for damage detection and evaluation of the compacted clayey soil in seasonal frozen region based on ultrasonic technology was proposed;then,the geometric deformation and ultrasonic longitudinal wave velocity of subgrade compacted clayey soil under the influence of freeze-thaw cycles and moisture were tested to evaluate the damage characteristics of subgrade compacted clay by using the method proposed in the thesis,and the damage evolution characteristics of subgrade compacted clay in in seasonal frozen region under the influence of freeze-thaw cycles and moisture were obtained;And then the feasibility and reliability of the damage detection and evaluation method proposed in the thesis was verified by the damage evaluation index defined by the elastic modulus and ultimate strength.Finally,based on the data of damage detection and evaluation,a damage variable prediction model considered the effect of freeze-thaw cycles and moisture was established and verified.It showed that the fitting effect was ideal and can reflect the damage evolution characteristics of compacted clay under the influence of external environmental factors.It also can provide a basis and reference for the damage prediction and evaluation of subgrade engineering in seasonal frozen region without relevant data.(4)Based on the theoretical analysis and the data of triaxial compression test,the energy evolution rule and characteristics of failure strain in the deformation and failure process of compacted clayey soil used for subgrade in seasonal frozen region were analyzed.Firstly,based on the analysis of energy types and energy conversion in the process of deformation and the failure of compacted clayey soil,the energy calculation formula under the condition of triaxial compression was deduced by mathematical and physical analysis method,and the conversion rate of strain energy and the failure strain energy and their corresponding determination method were proposed;And then based on the triaxial compression test data of compacted clayey soil,the evolution law of strain energy and strain energy conversion rate with deviatoric stress and axial strain were calculated and analyzed,and the influence of freeze-thaw cycles,moisture and confining pressure on them were analyzed and obtained.Finally,the evolution characteristics of the failure strain energy of compacted clayey soil in the seasonal frozen region were analyzed as well as the significance of the influencing factors to obtain the effect of freez-thaw cycles,moisture and confining pressure on the failure strain energy of compacted clay used for subgrade,as well as the degree of significance of the influencing factors and their sequencing.