Study On Mechanical Properties Of Coarse-grained Soil Based On Large-scale True Triaxial Test

Coarse-grained soil is more and more used as bearing layer or filling material in engineering.Many properties of coarse-grained soil are similar to sand,but they are quite different,first of all,the particle size or gradation range of coarse-grained soil is much larger than that of sand.Therefore,higher requirements are put forward for the test equipment.In addition,particle breakage is an important feature of coarse-grained soil which is different from other geotechnical materials.In order to better reveal the engineering properties of coarse-grained soil,it is necessary to use large-scale true triaxial test instrument to study the particle breakage of coarse-grained soil,and then study its influence on the strength and deformation of coarse-grained soil.True triaxial apparatus is a key equipment to study the variation of soil deformation strength under three-dimensional principal stress.Based on the small-scale true triaxial apparatus developed by Xi'an University of technology,a large-scale true triaxial servo loading mechanism with vertical rigidity and orthogonal two-way flexibility in horizontal plane is further developed.The sample size is 300 mm?300 mm?600 mm.The problem of specimen preparation and installation caused by the increase of instrument size is overcome.The stability problem of automatic control system is optimized by PID closed control algorithm.Based on large scale true triaxial apparatus,a series of shear tests with different confining pressures and different medium principal stresses were carried out in this paper.The unloading resilience characteristics,stress cross properties,shear dilatancy and shear strength characteristics of coarse-grained soil are studied.The influence of particle breakage on the shear strength of coarse-grained soil is revealed through particle screening test.Finally,an elastic-plastic constitutive model considering particle breakage is established in the framework of critical state theory.The main achievements of this paper are as follows:(1)A prediction model of resilient modulus of coarse-grained soil with small principal stress and spherical stress as variables is established.When one of the variables remains unchanged,the modulus of resilience varies nonlinearly with the other variable and has a linear relationship in double logarithmic coordinates.The establishment of the model is conducive to better understanding of the unloading characteristics of coarse-grained soil,and reflects the influence of stress path and material response on the resilience deformation characteristics of coarse-grained soil.(2)The dilatancy factor begins at a very high positive value(compression),decreases rapidly with the increase of stress ratio,and finally in the state of dilatancy.In the same medium principal stress state,with the increase of mean principal stress,the stress ratio corresponding to the characteristic point of phase transformation state increases,and the dilatancy decreases when the failure stress ratio is reached.It indicates that the larger the mean principal stress is,the more obvious the inhibition of dilatancy deformation is.Under the same mean principal stress,the failure stress ratio under different intermediate principal stress states varies with the specific test,and the shear dilatancy trend increases with the increase of the intermediate principal stress coefficient.(3)Under the true triaxial stress state,the shear strength of coarse-grained soil is nonlinear,which conforms to the power function curve on the p-q plane,and the internal friction angle decreases with the increase of mean principal stress.(4)Under the action of load,the particle breakage of granular geotechnical materials changes the particle composition,thus affecting its mechanical properties.The microcosmic mechanism of different fragmentation pattern s of particles is expounded,and it is revealed that particle breakage is an important factor affecting the nonlinear change of shear strength of coarse-grained soil.When the particles are broken,the stress between them is redistributed,and the deformation caused by the rearrangement of particles is irreversible.The more serious the particle breakage is,the more obvious the nonlinear change of shear strength is.(5)The constitutive relation of soil is an important basic theory of geotechnical engineering,and also an important means to solve practical engineering problems.Based on the large-scale true triaxial test of coarse-grained soil,this paper reveals the influence of particle breakage on the dilatancy and shear strength of coarse-grained soil.It is considered that particle breakage not only affects the dilatancy equation,but also the strength envelope,hardening criterion or hardening parameter.Therefore,based on the concept of critical state and plastic theory,an elastic-plastic constitutive model of coarse-grained soil is established by using the kinematic yield locus of uncorrelated flow.The elliptical plastic potential surface,similar to the modified Cam-clay model,is adopted in this model.The difference is that the concepts of stress ratio in phase transition state and particle breakage parameter are introduced,and the dilatancy and particle breakage of materials are considered.(6)In the current model,the p–q plane is divided into two different regions.Any stress state lower than the phase transformation stress ratio(M_c)will produce plastic volume compression,while the stress state higher than M_c will induce plastic volume dilation.In the existing models,the effect of particle breakage on plastic shear deformation and volumetric strain is properly considered.The overall elastoplastic behavior of coarse-grained soil is described by10 model parameters.According to the true triaxial test results,the rationality of the model is verified.The results show that the model can accurately evaluate the influence of stress-strain,dilatancy and particle breakage of granular crushable coarse-grained soil,and the physical meaning of model parameters is clear,which can provide valuable reference for engineering design and construction.