Creep Characteristics Of Q₂ Compacted Loess In Yan'an New Area

With the development of cities and the growth of population in China,land is extremely scarce and tense on the Loess Plateau,and the problem of insufficient urban space in the region is becoming increasingly prominent.To solve this problem,the city of Yan 'an decided to launch the project of "cutting hills and filling ditches to build a city",in order to expand the urban construction area.Due to the influences from mechanical compaction,backfill height and location factors,the dry density of the soil at different locations of the high-fill project varied greatly,which easily induced uneven settlement of the foundation and then affected the long-term stability of the high-fill project.Therefore,the study of stress,strain versus time and microstructural characteristics of loess in this area is of guiding significance for the construction of high fill projects.In this paper,taking the Yan'an New Area high fill project as an example,the Q₂ loess with different dry densities and different confining pressures was selected as the research object,and its shear strength and shear strength parameters laws were explored by triaxial shear test;the stress,strain and time changing laws were analyzed by triaxial creep tests;taking the fitting results of Burgers creep model and Singh-Mitchell creep model were compared with the test results,and then selecting a creep model applicable to the Yan'an Q₂ remodeled loess;the microstructure changes of specimens with different dry densities before and after creep were analyzed by scanning electron microscope test.The main conclusions are as follows:(1)In general,the shear strength of Yan'an Q₂ remodeled loess increases with the increase of dry density and confining pressure.The shear strength parameters increased exponentially with the increase of dry density.(2)The strain-time curve of Yan'an Q₂ remodeled loess has an obvious three-stage characteristic.Under the condition of a constant confining pressure,the smaller the dry density of the specimen is,the more obvious the creep phenomenon is.Under the condition of a constant dry density,as the confining pressure increases,the stress applied to the specimen to reach the yield damage state increases,and the yield strength of the specimen also increases.(3)The stress-strain isochronous curve of Yan'an Q₂ remodeled loess has obvious inflection points.Under the condition of a constant confining pressure,when the level of deviator stress is low,the slope of the curve gradually increases with the increase of dry density,and the stress-strain isochronous curve is linear in general;when the deviator stress is high,the curve bends and the slope of the curve gradually becomes smaller.Under the condition of a constant dry density,with the increase of the confining pressure,the deviator stress corresponding to the inflection point of the isochronous curve,the axial stress required to reach the yield damage of the specimen,and the yield strength,increase respectively.(4)For the Yan'an Q₂ remodeled loess with different dry densities,the fitting values of Burgers creep model are closer to the experimental results at different deviator stress levels;the fitting values of Singh-Mitchell creep model are better only at deviator stress levels of 0.2 and 0.4,and at deviator stress levels of 0.6 and 0.8,the fitting values of Singh-Mitchell creep model with time increase gradually larger than the test values.Therefore,for the Yan'an Q₂ remodeled loess,the Burgers creep model is more fittable.(5)By qualitative and quantitative analysis of the microstructural changes of the Yan'an Q₂ remodeled loess with a some dry density before and after the creep test,it can be found that the content of large and medium pores is greatly reduced and the number of small and micro pores is significantly increased after the creep tests.The pore fractal dimension is lower,the orientation is weakened,and the particle abundance is increase after creep test.With the increase of the dry density,the magnitude of the change of the orientation of the particles before and after the creep test gradually decreases.