Study On The Stress-strain Characteristics Of Soils With Triaxial Test Based On Local Deformation Measurement

The application of digital image processing technique for local deformation measurement in triaxial test leads to much advantages comparing to traditional measuring method. It improves the accuracy of deformation measurement, and provides a new, more reasonable and effective method in specimen deformation measurement. By using the digital image processing technique, stress-stain characteristics of soils are studied based on local deformation measurement of soil specimen.Firstly, the thesis analyzes the errors caused by different factors and proposes corresponding methods to eliminate or lessen the errors. Some items for minimizing measurement errors and noticed testing conditions are listed. The precision of digital image processing technique can reach the theoretic precision and satisfy high precision requirement of specimen deformation measurement under steady condition.Secondly, with the help of local deformation measurement in triaxial deformation test, the author studies the membrane penetration of triaxial specimen, and discusses the characteristics of soil stress-stain property in local deformation measurement using Duncan-Chang model along with Cam-clay model and Nanshui model. By studying on the determination method of model parameters, the thesis compares the parameters of the two methods, and discusses the influence on the calculation results. Because of the difference of measurement methods and measurement area, the parameters are different from each other between local deformation and whole deformation measurement of the three models. In Duncan-Chang model, the difference between K, n and K_b , m is notable, particularly K and K_b. In the Nanshui model, there are great differences among R_d, c_d and d. Similarly, there are times of difference between's parameters λ and κ in Cam-clay model. The calculation results based on these inaccurate parameters are also inaccurate.Thirdly, the soil's Poisson's ratio is an important parameter for transverse deformation. The law of Poisson's ratio in this study is obtained from the experimental results. The axial strain-radial strain variation trend is similar under different confining pressure. The tangent Poisson's ratio changes from 0.2 to 0.3 at initial loading, and reaches 0.5 at very small strain. The tangent Poisson's ratio of local deformation changes faster than that of traditional whole deformation measurement and the value of the former is larger than thatof the latter. The soil tangent Poisson's ratio increases as the principal stress ratio increases. When the principal stress ratio of the soil reaches a certain value, the tangent Poisson's ratio will be larger than 0.5. It is reasonable to establish the relationship between the Poisson's ratio and principal stress ratio.Finally, the thesis analyzes the results of triaxial cyclic loading experiment of sandy soil. Due to the attributes of soil's particle-framework, if loading arrives at a certain extent, the volume will shrink in unloading as long as the specimen expands in loading. With the increasing of density and the load scope, the phenomena of expanding in loading and contraction in unloading become more obvious, and recovering of the contraction of volume need more times of cyclic loading. After several times of cyclic loading, the structural deformation of particles, such as slippage, rotation and crushing, will fade away eventually, and the structure deformation will arrive at a stable state.