DEM investigation of strength and critical state behaviours of granular materials under true triaxial loadings

This paper investigates the strength and critical state (CS) behaviours of granular materials via DEM simulations of true triaxial drained tests under three different loading modes including constant b (intermediate stress ratio) tests with constant σ1′ (major principal stress), constant p′ (mean pressure) and constant σ3′ (minor principal stress) respectively. To this end, a series of samples are generated with the same particle size distribution, and with the confining stresses ranging from 100 kPa to 900 kPa. The CS is achieved for all samples. Both the macroscopic behaviours and the microscopic behaviours are examined and compared considering different loading modes, confining stresses and intermediate stress ratios (b). The critical state lines (CSLs) are found to be unique and independent of the loading modes, but dependent on the b values. The CSLs with b=0 and b=1 form the two boundaries of CSLs respectively beyond which CSLs under all other b tests cannot go beyond. Six different strength criteria are examined and compared in terms of both peak and CS failures. The Mohr–Coulomb strength criterion is found to be only suitable for axisymmetric loading conditions. The Lade-Duncan criterion is only suitable for describing peak strengths, which is dependent on the loading modes and confining stresses. The Satake criterion and Matsuoka-Nakai criterion are the more appropriate strength criteria for describing CS failures, indicating that the CS values of both the Satake parameter and the Matsuoka-Nakai parameter describe an inherent property that characterizes the CS failure for a given type of soil. The CS mechanical coordination number is fitted by a curved line for a given b value, which is unique regardless of the loading modes. The peak and CS values of both major and minor principal fabric tensors decrease with increasing b values, while they increase with increasing b values for the intermediate principal fabric tensor.