X.S. Li

Noncoaxial Behavior of Sand under Various Stress Paths

AbstractIn this paper, the results of three series of drained tests carried out on sands using hollow cylinder apparatus are presented. The noncoaxiality, defined as the difference between the major principal stress direction and the corresponding principal strain increment direction, is investigated. In the first series of tests, the sand was isotropically consolidated before being sheared with the fixed principal stress direction. In the other two series of tests, the sand specimens were isotropically consolidated and then sheared by rotating the major principal stress axes while the deviator stress level was either fixed (pure stress rotation) or increased continuously (combined shear loading). The experimental results provide clear evidence for material noncoaxiality when the rotation of principal stress direction is involved. The results from these series of tests show that the degree of noncoaxiality depends on the level of deviatoric stress and the stress increment direction. It tends to decrease w...

Particle-Scale Insight into Deformation Noncoaxiality of Granular Materials

AbstractThis paper explores the mechanism of deformation noncoaxiality from the particle scale. A multiscale investigation has been carried out with the particle-scale information obtained from discrete element simulation on granular materials. The specimens were prepared anisotropically and sheared in various loading directions. The deformation noncoaxiality, i.e., noncoincidence between the principal stress direction and the principal strain increment direction, was observed. The directional statistical theory has been used to study the anisotropies in material fabric and particle interactions, and to characterize them in terms of direction tensors. Based on the stress-force-fabric relationship, the stress direction was determined from these direction tensors. In monotonic loading, it was observed that the force anisotropy is always coaxial with the loading direction, i.e., the strain increment direction, while the fabric anisotropy only gradually approaches the loading direction. This noncoincidence be...

Numerical Simulation of the Influence of Initial State of Sand on Element Tests and Micropile Performance

This paper presents a state-dependent constitutive model for sand formulated within the critical-state framework and its implementation into a numerical analysis (FLAC3D) program. The implemented model was verified by using drained triaxial results on sands. The proposed model is shown to capture the stress path dependent behavior of sand over a wide range of densities and confining pressures well based on a unique set of parameters. Numerical simulations of the behavior of a micropile under vertical loading shows that the side and tip resistance, and thus the total resistance of the pile, are functions of the “in situ state” of soil as defined by the state parameter ψ=e-ec in which e is the void ratio and ec the void ratio at the critical state.

Constitutive modeling of flow liquefaction and cyclic mobility

Flow liquefaction is due to contractive response of loose granular soils, while cyclic mobility is related to both contractive and dilative response of granular soils. These two failure mechanisms may occur in a single soil, depending on the density and confining pressure applied. However, due mainly to the lack of a unified framework to describe the contractive and dilative response of granular soils, these two failure mechanisms are considered separately in current practice. The key issue in unified treatment of these failure mechanisms is correct modeling of dilatancy. The classical stress dilatancy theory in its exact form is incapable of doing this. The dilatancy must be a function of stress state as well as of the internal material state. Such a state dependent dilatancy in conjunction with the critical state framework can effectively model both flow liquefaction and cyclic mobility over a wide range of material and stress states, using a single set of material constants. This paper describes this unified approach and a model that follows this approach.

A Flexible Strain Gage for Soil Testing

A novel strain gage for the measurement of soil deformation is introduced. The gage is based on the principle that the change in the length of a coil is linearly related to the change in its inductance. The gage measures a wide range of strains and can be easily installed on a soil specimen in any direction; therefore, it is particularly suitable for the measurement of deformations of in situ subsurface soil. This paper reports the principle and the characteristics of the gage. A discussion on further improvements is also given.