Study On Deformation Characteristics Of Saturated Sand Under Cyclic Spherical Stress

The soil in embankment dams and slopes may accumulate irreversible deformation due to variation of water level.During that process,soil skeleton experiences a stress path of cyclic loading and unloading in effective spherical stress at constant deviator stress.To study the deformation characteristics of soil under such stress paths,a series of stress-path triaxial tests under cyclic spherical stress at a low rate were carried out.The development law of volumetric strain and shear strain during the monotonic increase or decrease of spherical stress was studied.The influence of initial deviatoric stress,spherical stress and density on the soil deformation was discussed.The PFC numerical tests under such stress path were carried out.A mathematical description method of incremental form was proposed to describe the stress-strain relationship of soil under cyclic spherical stress,and the method was preliminarily verifed.The following research results have been obtained:1.The stress-path triaxial tests under cyclic spherical stress at a low rate with constant deviatric stress were carried out on Fujian standard sand.The results showed that the cyclic loading of spherical stress led to significant volumetric strain and shear strain accumulation in soil.(1)The volumetric strain increased/decreased with the increase/decrease of spherical stress in each cyclic process.The relationships between volumetric strain and the logarithm of spherical stress could be approximately described as linear.(2)The shear strain progressively increased with the increase of spherical stress in each cycle,but the growth was all less than 0.1%,and could be neglected.(3)When spherical stress decreased,the development of shear strain was closed related to the stress ratio.The shear strain changed slowly at relative small stress ratio,and occured steeply increasing with the increase of stress ratio.However,the variation of shear strain under different stress ratio was on the same line with the increase of stress ratio.2.The influence of different consolidation deviator stress,consolidation spherical stress and relative density on soil defoamation characteristics was discussed.(1)The influence of consolidation deviator stress on the volumetric strain was little during the initial loading stage,and became obvious during the subsequent reloading and unloading stage.The accumulative volumetric strain under different deviator stress were almost uniform.The consolidation deviator stress had great influence on the accumulative rate of shear strain.The accumulative shear strain under high consolidation deviator stress(900 kPa)is about 26 times of that under low consolidation deviator stress(300 kPa).The conspicuous initial deviator stress usually determines the development direction of accumulative shear strain.(2)When consolidation spherical stress increased,the amplitude of volumetric strian decreased obviously,while the accumulation of volumetric strain was almost invariable.The volumetric strain curve under different consolidation spherical stress was nearly coincided,after spherical stress was normalized by the consolidation spherical stress.The growth and accumulation of shear strain were greatly reduced,under the same stress ratio increment,when the consolidation spherical stress increased.The development of accumulative shear strain was closely related to the consolidation stress ratio,under the condition of the same stress path length.(3)The amplitude and accumualtion of volumetric strain decreased slightly,after the initial relative density was increased.The influence of relative density on shear strain was enhanced with the increase of consolidation deviator stress.When consolidation deviator stress was less than 700 kPa,the relative density has little effect on the growth and accumulative rate of shear strain.When consolidation deviator stress was larger than 700 kPa,the shear strain growth and accumulation of dense sand were greatly reduced.3.The conventional triaxial tests were carried out by PFC numerical simulation software,and a group of microscopic parameters were obtained by comparing with the laboratory test results.The numerical experiments under the stress path of cyclic loading and unloading with spherical stress were further simulated,and the development trend was in agreement with the overall trend of laboratory test results.However,because the numerical test failed to reflect the initernal heterogeneity of the triaxial specimen and microcosmic change of sand,the strain under simulation test was much less than that of laboratory test.4.Considering the possible internal changes of soil density and fabric,two internal variables corresponding to volumetric and shear deformation were defined.A mathematical description method of incremental form was proposed to describe the stress-strain relationship of soil under cyclic spherical stress.The feasiblity of the model was preliminarily verified by the comparison and analysis between the experimental results and the model calculation results.