Study On The General Linear Failure Criterion And Elastic Parameters Of Geomaterials

Geomaterial is a natural porous medium with complex mechanical properties.Few general theories are able to physically describe the characteristics of strength and deformation.The research is concentrated on the two fundamental problems in geomechanics,namely strength and deformation.Strength and elastic parameters of geomaterials under complex stress states are experimentally investigated.Firstly,the so-called Paul-Mohr-Coulomb(PMC)failure criterion is developed for geomaterials,containing all three principal stresses and three recognizable material parameters:friction angles for compression ?c and extension ?e,and isotropic tensile strength V0.A plane fitting method is proposed to determine the three material parmaters so that PMC can be applied to soil for the first time.The effect of intermediate principal stress on strength can be quantitatively described by the difference between friction angles for compression and extension.An ideal elasto-plastic model based on PMC is built in FLAC 3D,and the model is applied in the simulation of a three dimentional tunnel to explore the effect of intermediate principal stress on stability.PMC can be modified with six parameters to describe the nonlinear failure surface for geomaterials.The modified PMC is able to approximately capture the nonlinear and convex nature of the failure surface,characterizing the shape evolution of failure surface in the ?-plane and exhibiting a nonlinear variation of deviatoric stress q with mean stress p,all by piecewise linear segments.Furthermore,in order to measure the strength of rock under multi-axial stress states,a true triaxial apparatus is developed from the plane-strain apparatus with the loading configuration that ?1 is appled by the rigid platens,?2 is appled by a pair of piston assemblies and ?3 is appled by the confining hydraulic pressure.Uniaxial compression,triaxial compression and extension,true triaxial and uniaxial tension tests are performed to investigate the strength of Dunnville sandstone under different stress states.In the true triaxial test,a unique stress path is achieved by keeping mean stress constant during loading,making it possible to observe the failure surface in a certain ?-plane.Additionally,a novel method for uniaxial tension test is proposed:the specimen is machined as the same geometry as the standard mental specimen for uniaxial tension test by using water jet;then the specimen is directly clamped to the load frame such that tensile force can be applied.The method removes two major limitations in conventional uniaxial tension test,i.e.,eccentric tension and stress concentration.The six-parameter PMC agrees well with the strength data of Dunnville sandstone,however,tension cut-off is required for the tensile strength.Finally,the elastic parameters of geomaterials were meatured in the stable state of deformation,achieved in cyclic triaxial test.Moreover,the effects of stress state and porosity on the elastic parameters are investigated.It is illustrated that cohesionless soil and rock specimens gradually tend to the stable state of deformation under a constant cyclic deviatoric stress amplitude.As the specimen shows a completely elastic behavior in the stable state,it is reasonable to measure the elastic parameters based on the stress-strain curve of stable state.The elastic parameters of cohesionless soil and rock are significantly affected by the stress state and porosity.The corresponding relationships are given by the suggested equations in the thesis.