Research On Constitutive Law And Mathematical Model Of Anisotropic Sands Under Rotation Of Principal Stress Axes

The study on the stress-strain characteristics and constitutive model for anistropic sands subjected to cyclic rotation of principal stress axes is one of the most important and difficult problems in the practical projects, in order to predict mechanical responses of the artificial fill structures, such as rochfill dam and subgrade. Based on the previous research results, Presented in this study are experimental facts, physical interpretation, microscopic mechanism and the theoretical formulations which were made systematically for the purpose of investigating deformation behavior of anisotropic sands under cyclic rotation of principal stress axes. The main achievements could be summarized as follows. 1. The basic stress-strain characteristics and physical mechanism for anisotropic micaous sands were investigated based on two kinds of drained tests including fixed principal stress axes and cyclic rotation of principal stress axes with constant magnitudes of principal stresses. The main mechanical laws of anisotropic sands were observed and found in the following:1) When the orientation angle of major principal stress changes from 0 to 90 ,the shear strength of micaous sands at first tends to reduce, reaches its minimum value at the orientation angle of60 , and then increases gradually. The coefficient of intermediate principal stress has a great impact on the shear strength and deformation of anisotropic sands. The dilatancy of anisotropic sands is obviously affected by the major principal stress direction angle.2) The volumetric strain induced due to cyclic rotation of principal stress axes can produce in the same magnitude as that due to shear with fixed principal stress axes. It is also composed of a reversible dilatancy component and an irreversible dilatancy component, which are affected by anisotropy.3) There exists obvious non-coaxiality of the direction of the principal stress with that of the principal strain increment. Its degree depends mainly on the change of shear stress component. Non-uniform change in the shear strain increment occurs in the condition of cyclic rotation of the principal stress axes and always appears two peak values for each cycle.2. Based on the idea that the same microscopic mechanism has to induce the similar macroscopic responses, the DEM method is applied to this research where anistropic sands are simplified as elliptic granular materials. The deformation behavior and microscopic mechanism of elliptic particles are investigated through three kinds of numerical tests including compression, fixed principal stress axes and cyclic rotation of principal stress axes. The numerically-obtained results are compared to the laboratorily-tested mechanical responses of micaous sands in the same stress paths, and consequently the similar macroscopic mechanical responses are much beneficial to reveal and explain the microscopic reasons of the constitutive response of anisotropic micaous sands.3. A new cyclic constitutive model which can describe the deformation behavior of anisotropic sands under combined variations of principal stress axes and magnitudes is developed based on the understanding of the constitutive facts and laws obtained from the above physical fundamentals. Its effectiveness is verified by a comparison of model simulation with various test results for the micaous sands under different conditions such as variation of stress amplitude and cyclic rotation of principal stress axes.