Study On Triaxial Shearing Characteristics Of Medium Quartz Sand Under Normal To High Pressure

With the rapid development of China's economy,large-scale earth-rock dams,large-scale hydropower stations,deep-water tunnels and other large-scale projects have been constructed,which promotes the continuous development of geomechanics,especially sand shearing characteristics under high pressure.The strength and deformation of sand under high pressure is different from the state of normal pressure.Moreover,grain breakage of sand often occur for these large projects,which can further cause the change of soil mechanical parameters.The characteristics of sand under loading are influenced by the state variables of sand,such as relative density,confining pressure and the structure of soil itself,and other factors related to the properties of sand,such as particle shape,particle gradation,particle surface characteristics and mineral composition.The diversity of state variables and other factors related to sand properties results in different mechanical properties and particle breakage.In order to study the influences of relative compactness of sand,confining pressure and stress path on the strength and deformation characteristics of soil under high confining pressure,the high-pressure triaxial shearing tests have been carried out under the drainage consolidation for medium grained quartz sand with different relative compactness under two different stress paths and the wide range confining pressure(0.2MPa-8MPa).Meanwhile,the influences of initial relative density,confining pressure and stress path on particle breakage and its mechanical properties have been investigated.The main research results are as follows:(1)The stress-strain curves of the specimens under conventional triaxial tests(CTC)show strain softening to different degrees in the normal to medium-pressure range.The volume of the specimen begins to expand with the increase of axial strain,and its dilatancy increases with the increase of relative density and the decrease of confining pressure.The stress-strain curve gradually changes to strain hardening type and the volume of the sample gradually tends to shear contraction when entering the high confining pressure range(?3>2MPa).And the transition value of confining pressure will increase for denser sand.For the test of reduced triaxial compression path,the change characteristics of strain-stress curves under different confining pressure are similar to those of CTC tests under normal and medium pressure.(2)The particle breakage rates of sand samples increase with the increase of confining pressure for both CTC and RTC paths.In CTC tests,particle breakage rate increases gradually with the increase of initial confining pressure p0 when p0 is below a certain value,then the particle breakage rate increases slowly or tends to be stable.After that,as the confining pressure continues to increase to a higher value,the particle breakage begins to increase again.The transition confining pressure is 4MPa for loose sand samples and 3MPa for medium and dense sand samples.At high confining pressure,secondary phase transformation occurs during shear process due to the obvious particle breakage in the late stage of medium dense and dense sand shear.(3)The development of the relationship between strength and stress dilatancy is reviewed.The failure internal friction angle and peak dilatancy angle of sand are calculated.The relationship between the failure internal friction angle and logarithmic confining pressure of soil with different compactness is approximately linear.The attenuation rate of the failure internal friction angle with the increase of logarithmic confining pressure is determined by fitting.After that,the rationality of the determination method of the critical state internal friction angle is discussed.And the critical friction angle of the test sand is determined based on the Bolton stress-dilatancy relationship.The relationship between the dilatancy index and the initial relative density and the mean effective stress is established,and the relevant parameters are determined.