Study of anisotropic shear strength of granular materials using DEM simulation

Abstract

SUMMARYThis paper investigates shear strength of granular materials with inherent fabric anisotropy. Most previous studies have described strength of these materials in the principal stress space, and the orientation of the bedding plane with respect to the principal stress directions was used as the reference geometrical descriptor of inherent fabric. The present study has found that it is theoretically more convenient and practically more useful to use instead the inclination angle of the bedding plane with respect to the shear plane for the same purpose. Direct shear tests and biaxial compression tests with different loading directions with respect to the bedding planes were simulated with discrete element method (DEM) models consisting of ellipse-shaped particles. Key mechanical behaviors of natural sands reported in the literature were successfully captured in the numerical simulation. A shear failure criterion was determined as a function of the inclination angle based on the direct shear simulation results, and was used to successfully predict the results of the biaxial compression simulations. Microstructural inspection of deformation and strain localization of the biaxial compression simulations found that the proposed shear failure criterion can reasonably predict the orientations of the initial failure planes. It was also discovered that shear bands in directions conjugate to the initial failure plane orientations can develop and dominate specimen deformation at larger strain levels. Considering the availability of biaxial compression test equipment and historical data, two methods for back-calculating inclination angle-dependent shear strength from biaxial compression results were proposed, and validated using DEM simulation results.

DEM study of fabric features governing undrained post-liquefaction shear deformation of sand

“…However, 2D and 3D particles share many common microscopic mechanisms, particularly those related to particle interactions, which govern macroscopic behavior of granular materials in 2D and 3D alike. The fact that ''A-class predictions'' made using DEM [11] have been validated in later experimental observations [32] on actual sand samples provides strong confidence in 2D DEM's capabilities in qualitatively capturing behaviors of sand.…”

“…The samples consist of circular particles ranging from 0.30 to 1.00 mm in diameter, with a median particle size D 50 of 0.72 mm, and a uniformity coefficient D 60 /D 10 of 2.13. ''Master packs'' of particles are fabricated with different initial void ratios through a pluviation process described in detail by Fu and Dafalias [11]. Specimens for undrained cyclic biaxial simulations are then trimmed out of the master packs and isotropically consolidated under initial effective mean stress p in (Fig.…”

“…A DEM simulation package PPDEM that has been successfully applied in a number of sand fabric-related studies [11][12][13][14]33] is employed. Although PPDEM has the ability to model arbitrarily shaped 2D particles, circular particles are used to avoid the complication of strong anisotropic behavior and enable us focus solely on liquefaction.…”

Identifying and following particle-to-particle contacts in real granular media: An experimental challenge

“…Furthermore, full characterisation of the evolution of contacts within a granular assembly under load might support and encourage the development of more micromechanically-based constitutive models for granular materials (in the style of [10,11]), and help feed these models with data coming from experiments on real materials.…”

Discussion of ‘Experimental investigation of shear strength of sands with inherent fabric anisotropy’ by Tong et al. (doi:10.1007/s11440-014-0303-6)

“…The authors also mentioned that it is desirable to develop theoretical modelling of the observed soil response. It is emphasized by the authors in this paper and a previous one [1] that it is practically more convenient to formulate the anisotropic failure criterion of sand in terms of the shear plane (or failure plane) orientation, as some existing geotechnical analysis methods which rely on shear strength of soils such as slope stability analysis and foundation failure analysis explicitly assume a potential failure plane without referring to the loading direction. The discusser thinks that a failure criterion formulated this way would be useful for special cases in which the failure planes are known but may not be suitable for general cases in which the failure planes cannot be easily determined.…”