Study On Stress-strain Properties Of Sand In Prior-failure State

The constitutive model of soil describes the relationship between stress and strain of soil.Based on the result of the geotechnical triaxial test,the model usually introduces certain assumptions to band the load and deformation with mathematical expressions.The constitutive relationship is used to solve the problems in geotechnical engineering,such as foundation bearing capacity,deformation,and stability of some structures.Therefore,the study of the constitutive model roses one of the most significant and energetic projects in soil mechanics and geotechnical engineering.With the clear stress conditions,the conventional triaxial compress test has become the primary method for establishing new models and determining parameters,where the state of the sample is close to the practical state of soil in geotechnical engineering.Due to the limitation of deformation measurement technology or the simplification of the constitutive model,the soil samples are regarded as unity in traditional triaxial tests.A series of complex nonlinear elastic models and elastoplastic models are created with the overall stress-strain curve,which is used to characterize the deformation of the entire specimen.With the help of the whole surface deformation measurement system,the deformation of samples in triaxial compress tests shows the inhomogeneous characteristics after localized progressive failure.The traditional constitutive model with the soil unity isn’t consistent with the experimental results.The new constitutive model is constructed as follows: according to the periodic deformation characteristic of the sample,the constitutive model includes three stages: prefailure stage,failure stage,and post-failure stage.The whole sample follows the same stressstrain relationship before localized failure.The second stage is the asymptotic failure of the sample and finally forms a shear band.The state of deformation after failure is similar to the sliding friction model of rigid blocks.Based on this phenomenon,this paper puts more focus on the stress-strain characteristics in pre-failure state and completes the main tasks as follows:(1)A brief digital model of the whole surface deformation measurement system is constructed in the same process,which realizes the bidirectional calculation between the 3D coordinates of the feature points on the sample surface and the corresponding 2D coordinates of the imaging plane is realized.That model can be used to reconstruct the 3D morphology of the sample and analyze the local deformation and experiment error.(2)A series of conventional triaxial compress(CTC)tests and triaxial cyclic tests were conducted for ISO standard sand and emery sand.The deformation characteristics of the soil sample in these stages were analyzed from the overall and local perspectives.The influence of test conditions on deformation and strength was discussed based on the overall stress-strain curve,which further demonstrates the rationality of the new constitutive model.(3)On the results of the triaxial cyclic load tests,the loading and unloading equivalent moduli in the prior-failure state are defined,which both form the hyperbolic model with the number of cycles.And the parameters in the expression are on behalf of the reciprocal of elastic modulus and the initial loading or unloading plastic modulus.Based on the result of all the tests,the influence of test conditions on elastic modulus is discussed by the particle interaction.That provides an elastoplastic analysis method based on the new constitutive model.