Mechanical Characteristics Of Natural Soil Considering Stress Paths And Micro Damage

In practical engineering,the stress paths is complex,and different stress paths will change microstructure.There is a correlation between the changes of microstructure and macroscopic mechanical properties of soil.The change of macroscopic mechanical properties of soil under external load can be characterized by the damage of microstructure.Considering the influence of stress path,it is of great significance to study the relationship between the macroscopic mechanical properties of soil and the damage of microstructure.The undrained triaxial shear tests under different stress paths were carried out on the 7th layer loess,the 8th and the 9th layer silty clay in Jinan city.Field Emission Scanning Electron Microscopy was used to analyze the 7th loess,the 8th and 9th layer silty clay under different stress paths,and the influence of different stress paths on the strength and deformation characteristics of natural soil and their macroscopic and microscopic damage characteristics were also studied.It is found that the stress-strain curve of soil under different stress paths is basically presented by strain hardening.The deviatoric stress at failure point advanced with the increasement of consolidation pressure.The peak strength of deviatoric stress under loading stress path is obviously higher than that under unloading stress path.The pore pressure under the unloading stress path is lower than the loading stress path.The stress path has obvious influence on the strength of soil.It is pointed out that different stress paths,axial strains and positions of the scanning layers have a great influence on the characteristics of the microstructure.The void ratio increases with the increasement of the axial strain,and void ratio in the middle part of samples is higher than that intwo ends during a shear process.The shape coefficients of particles and pores gradually decrease with increasement of axial strain,and the shape coefficients of the middle scanning layers are slightly less than those of the two ends.The distribution fractal dimension of the loess basically exhibits a linear positive correlation with the axial strain.The distribution fractal dimension of the 8th silty clay shows an exponential correlation with the axial strain.When the axial strain ranges from 0 % to 2 %,the distribution fractal dimension of soil under CTC stress path does not change,while the distribution fractal dimension increases slightly under the stress path of RTC.When the axial strain is more than 5 %,the increasing rate of the distribution fractal dimension under CTC stress path is faster.The morphological fractal dimension of the loess and the 8th silty clay showed an exponential increase with strain,however,in the latter part,the growth rate slowed down.Based on the damage theory,the damage variable was defined in this thesis,and it was coupled to the elastoplastic model.Combined with the related yield criterion,the elastoplastic constitutive model considering microscopic damage is obtained,and the applicability of this model is analyzed by a metro station foundation pit engineering example.The results show that this constitutive model can effectively reflect the change of internal force of retaining structure,the lateral displacement of retaining pile and the settlement of surface during the process of excavation.The simulated values are basically close to the measured data,the calculation results can provide scientific guidance for the design and construction of metro stations.