Study On Permanent Deformation Of Subgrade Low Liquid Limit Clay Under Freeze-thaw Cycles

There is a large overlap between the distribution of seasonal frozen soil areas in China and the planning of comprehensive transportation hubs,which means that the development of transportation infrastructure in China cannot be separated from the focus on freezing and thawing.The freeze-thaw cycle,as a natural weathering force,causes a series of special diseases(such as frost heave,thaw subsidence,pulping,and uneven settlement)in the subgrade,which not only directly affects the stability of subgrade,but also damages the service level of pavement structure and threatens driving safety,resulting in the inability of transportation facilities to perform their due functions such as convenience,safety and comfort.Based on this,this paper selects the widely distributed clay in the seasonal frozen soil area as the research object,and conducts static triaxial tests considering the number of freeze-thaw cycles and the level of confining pressure.Then,based on the static triaxial test results,a dynamic triaxial test was carried out considering the loading stress,the number of cyclic load,the number of freeze-thaw cycles and the confining pressure level.Determine the influence law of different factors on the critical failure stress and permanent deformation of subgrade soil.Then,according to the dynamic triaxial test results,establish the judgment criteria for the permanent deformation type of subgrade soil,and establish the permanent deformation prediction of subgrade soil considering the number of freeze-thaw cycles and other factors.The regression relationship between the model parameters and the basic physical performance parameters of the subgrade soil is established,and the applicability and rationality of the model under long-term cyclic loading are verified.The main findings of this study are as follows:(1)The stress-strain curve gradually transitions from the strain-softening type to the strain-hardening type curve with the increase of the confining pressure,and gradually develops from the strain-hardening type to the strainhardening type with the increase of the number of freeze-thaw cycles.Strain softening curve.At the same time,the elastic modulus and failure strength have a positive correlation with the confining pressure,and a negative.correlation with the number of freeze-thaw cycles.In addition,the cohesion force gradually attenuated with the increase of the number of freeze-thaw cycles until it became flat.The effect of the internal friction angle on the number of freeze-thaw cycles was not sensitive,and the value of the internal friction angle fluctuated repeatedly around 23°.(2)The critical failure stress increases gradually with the increase of the confining pressure,and gradually decreases with the increase of the number of freeze-thaw cycles until it becomes stable.The final value of permanent deformation increases gradually with the increase of the loading stress,decreases with the increase of the confining pressure,and increases at first and then stabilizes with the increase of the number of freeze-thaw cycles.At the same time,the permanent deformation under cyclic loading increases rapidly in the early stage of loading,and can reach more than 70%of the final value of the permanent deformation.With the continuous progress of the loading process,the accumulation rate of permanent deformation gradually decreases,and the deformation tends to a constant value.(3)Based on the grey relational analysis method,the grey relational degree of each influencing factor was calculated.The degree of influence on permanent deformation from large to small is the number of freeze-thaw cycles,octahedral shear stress,net body stress,and number of cyclic loadings.There was no significant difference in the grey relational degree of the factors.(4)A judging criterion for different development types of permanent deformation is proposed,which has good applicability to different subgrade soil types and can be extended to other subgrade engineering designs.At the same time,a permanent deformation prediction model that comprehensively considers the number of freeze-thaw cycles,confining pressure,loading stress,and cyclic loading times,and an empirical relationship between model parameters and basic physical performance indicators are established.Applicability and rationality under load are verified.