Study On The Size Effect And Shear Band Of Soil Material

The soil is a special and complex particle collection.The size of soil has a very important effect on the strength and deformation characteristics.The size effect of soil material has always been one of the most important problems in geotechnical field.In engineering design,it is often use the theory and method based on the test results of small size in the laboratory,so the conclusion will be the actual differences with the engineering.Only by systematically studying the size effect can we accurately describe the mechanical properties and deformation characteristics of the soil material in the actual engineering.In this paper,sand soil was used for the research,use the triaxial test and particle flow code,get the soil sample size change corresponding to the stress strain,volume strain,shear strength index and other characteristics of the impact,it is proved that the size effect of soil material is objective.At the same time,the formation and development of the shear band are described from the viewpoint of the micro scale,and the microscopic displacement law of the particles and the variation of local porosity are revealed,and analyzed the reason of the soil size effect.The main work and research results of this paper are as follows:(1)Making Xiamen standard sand dense sand sample,respectively to test the size of? 39.1 mm and ?101 mm consolidation drainage triaxial shear test,get its mechanical properties,deformation characteristics and shear strength index,the effects of specimen size on the test results.The results show that the size effect of soil material is objective.At low confining pressure(100kPa),the change of sample size has a significant effect on the mechanical properties of the specimens.The larger the sample size is,the larger maximum axial deviatoric stress difference is.With the increase of confining pressure,the size effect of strength is not obvious.The samples showed the trend of the volume change of dense sand,but the volume shrinkage of the large scale sample was more obvious in the volume contraction stage,the small size specimen has stronger dilatancy effect.(2)With the particles flow code(PFC2D),compiled procedure of simulating biaxial test mode.By changing the sample size,four kinds of samples with the same aspect ratio were obtained,and by changing the particle porosity,respectively to simulate the size effect of loose sand and dense sand,further supplement to the triaxial shear test.The results show that the particle flow simulation is similar to that of the three axial test,and the size effect of specimen can be observed more clearly when the sample size difference is large enough.The effect of sample size effect on strength decreases with the increase of confining pressure,when the confining pressure is greater than 900kPa,the size effect of strength is not obvious,and the stress-strain curves of the specimens are almost coincident.For the dense sand,the same confining pressure,the larger the size of the sample,the more unobvious the phenomenon of volume expansion,according to this,it can be concluded that in the engineering practice,it is not necessarily cause the volume expansion of the large soil size.The effect of sample size on the internal friction angle is not significant,but it has a great influence on the c value.When the sample is loose sand,the body size effect is more obvious than the dense sand.(3)the formation and development of the shear band of the biaxial test of PFC were studied by using the displacement field and measurement circle program of the PFC.Through the displacement field of PFC,the inclination and thickness of the shear band are measured by visual observation.The results show that the inclination of shear band has little influence on the sample size,but has a great influence on the thickness of shear band.L1 sample(600mm×300mm)shear band width and sample width ratio is about 10%?15%,while for S1 sample(60mm×30mm),the ratio is about 20%?30%.With the increase of specimen size,the proportion of shear band is reduced,and the volume expansion of the sample decreases,therefore,the cause of the influence of the change of the sample size on the volume change is explained..At the same time,it is the first time to use the measurement circle to monitor the porosity inside and outside the shear band and get the position of the shear band,it provides a new way for the study of shear bands in the future.