Experimental Analysis On Particle Size Effect Of Soil Strength And Deformation Characteristics Based On Effective Stress

Soil is a product of rocks in nature,that continually undergoes development by way of weathering,erosion,geological action and deposition.The soil particles size includes about 6 scales,in the range of less than micron to tens of centimeters,and the soil particles in different scales form different structures.For the interaction of soil structures in several particle scales,the macroscopic mechanical properties of the soil are great influenced.In addition,the mechanical properties of the soil particle interface are completely different from the internal properties.Under the external force,the sliding and separating can be easily occurred during the interface,which leads to the inhomogeneous and discontinuous micro deformation in soil,and will cause discontinuity and inhomogeneity at the macroscopic level.Now,the traditional soil mechanics is based on continuity of soil,which is not consistent with actual situation.If the model that based on multi-scales and multilevel coupling mechanical characteristics can be set up to simulate the soil discontinuity caused by particles size effect,it will be able to describe the stress state of the soil more accurately.In this paper,based on the strain gradient of equivalent effect,a series of osmosis tests,direct shear tests and triaxial compression tests are carried out to study the law of particle size effect on the strength and deformation of soil medium based on effective stress,and it is compared with the law of particle size effect based on the total stress..The samples are made up of clay with particles size less than 0.075 mm,sand with particle size larger than 0.075 mm and pore water,finally,the applicability of the model is discussed by comparing the test results with the model predictions.The main researches and achievements in this paper are as follows:(1)The penetration test of a series of samples with differentvolume fraction ratio and particle size is made.It is found that the permeability of the soil element is increased with the increase of the particle ratio,and the permeability of the soil element is reduced with the increase of the particle size,but the effect is not obvious.(2)A series of samples with different volume fraction ratio and particle sizes were prepared by quick shear test,consolidated-quickly shear test and slow shear test.The test results show that,the mass of the soil cell shows a strong particle size effect: the shear strength of the soil element increases with the increase of the volume fraction ratio,and decreases slightly with the increase of the particle size.The effect of particle size on cell strength is less than that of volume fraction ratio.(3)In the direct shear test,the cohesion and friction angle of the sample increases with the increase of the volume fraction ratio when the volume fraction ratio is less than 0.3.When the volume fraction ratio is greater than 0.3 less than 0.4,the cohesion decreases with the increase of the volume fraction ratio,and the internal friction angle increases with the increase of the volume fraction ratio.(4)Preparing a series of samples with different volume fraction ratio and particle size,and carrying out three axis test(UU test,CU test and CD test).Whether or not the effect of pore water pressure is considered,the particle size effect exists in the soil medium,and with the increase of the volume fraction ratio,the(effective)yield stress increases gradually.With the increase of particle size,the(effective)yield stress of the samples decreases gradually.The rule of effective cohesion and effective friction angle in three axis test is similar to that in direct shear test.(5)In the three axis test,the(effective)stress strain curve of the soil basically shows the strain hardening phenomenon,and the larger the volume fraction ratio,the earlier the soil enters the yield state.When the volume fraction ratio is relatively large(alpha =0.4),the sample appears softening in the UU test.In the CD test,under the high confining pressure state(P=300kPa),the shear strain reaches 15%,and the effective shear stress decreases.(6)The relationships of the strain gradient of equivalent effect,density of coordinated micro-cracks,the intrinsic scale with theparticles size and volume fraction ratio of reinforcing particles respectively.The comparisons of the tests results and the predicted soil strength of soil cell element model show that the relative error is smaller and prove that the soil cell element model can be used to analyze multi-scale coupling mechanical properties applicably.