Research On Nonlinear Elastic Model Of Unsaturated Loess In Xinin

The primary origin of the native loess in the Loess Plateau of northwest China is aeolian deposition.This type of soil is a typical unsaturated soil,widely distributed in the area,and typically characterized by its collapsibility,sensitivity to water,and structural features.With the rapid development of social and economic activities in the region,such as large-scale construction,slope filling,highways,and railways,the unsaturation and structural characteristics of the loess give rise to special mechanical properties,which in turn directly affect the engineering construction and disaster prevention and mitigation in the region.Therefore,a clear understanding of the mechanical properties of unsaturated structured loess and the establishment of constitutive models that accurately describe its mechanical behavior are of great theoretical and practical significance for engineering construction in the loess area.In this paper,the original and remolded loess in the Chengzhong District of Xining City were selected as the research objects.The basic physical and mechanical parameters of the loess were obtained through basic geotechnical tests,and conventional triaxial drained shear tests and K₀consolidation tests were carried out on the original and remolded loess samples under different water content states.The distribution patterns of CSLs and NCLs of the unsaturated loess were obtained through conventional triaxial drained shear tests,and the theoretical equations for describing CSLs and NCLs,as well as the e-p-S_rin the triaxial shear process,were redefined based on the state variableρ_s(ρ_s=ρ_ss+ρ_st).Finally,the nonlinear elastic constitutive model of unsaturated loess based on the Duncan-Chang model was established to verify the stress-strain relationship curve of unsaturated loess.The main work carried out is summarized as follows:（1）The original and remolded loess were respectively prepared into triaxial specimens with five different water contents:0%,5%,10%,15%,and saturation.Conventional triaxial tests were conducted under four different confining pressures:100,200,300,and 400 k Pa.The test results show that there is a significant difference in the shear strength of the original and remolded loess with changes in water content（or saturation）,with the shear strength of the original loess being much higher than that of the remolded loess at low water content,but the difference is not significant at high water content.In addition,the normal consolidation line（NCLs）and the critical state line（CSLs）of unsaturated original and remolded loess are parallel in the v-lnp space,but the CSLs in the q-p space are distributed in a fan shape,rather than being a straight line as many people believe.Although the failure lines of saturated original and remolded loess basically coincide,the distribution range of the failure line of original loess is greater than that of remolded loess.（2）The linear relationship between void ratio e and saturation degree S_r of unsaturated loess under constant load was obtained through K₀ consolidation test,and an empirical formula was fitted between the humidification slopeλ_ωand constant load P.To separately describe the structural and unsaturated properties of loess,the state variableρ_s was redefined as the sum of the structural parameter(ρ_st)and the unsaturated parameter(ρ_ss).The CSLs and NCLs of unsaturated structural soil,as well as the relationship between e-p-S_r during triaxial shear process,were derived.Finally,the validation results of triaxial test data showed that the equation can accurately predict the distribution pattern of NCLs and CSLs of unsaturated structural loess.（3）Based on the critical state theory of unsaturated loess,all relevant parameters required for the model were derived and calculated.A nonlinear elastic constitutive model for unsaturated loess was established by combining each parameter with the relationship between water content（or saturation）.The model can well predict the strain hardening and strain softening curves of unsaturated structural loess before reaching the stress-strain curve of critical state stress.Although the inherent defect of the Duncan-Chang elastic model makes it unable to predict the stress-strain curve of the strain softening curve after the peak point,the evolution pattern of CSLs of unsaturated loess can be combined to predict the failure of unsaturated loess.