The critical state behaviour of granular material in triaxial and direct simple shear condition: A DEM approach

The critical state (CS), the anchor concept in the critical state soil mechanics (CSSM) framework, has been comprehensively evaluated in triaxial stress conditions, where all the principal stresses are known but the pure shear stresses in the shear directions are absent. Therefore, the evolution of first and second invariants of the stress tensor (σ′) i.e. the mean effective stress (p′) and deviatoric stress (q) along with volumetric strain (ε_v) is often used to identify the CS. However, the same does not apply for a direct simple shear (DSS) test, which is more representative of the real ground condition, as in most cases the minor principal stresses are not measured. This leads to the challenge of determining the CS line in the e-log(p^') space and thus the concept of state parameter (ψ) cannot be used to characterise soil behaviour. Using DEM, this study evaluates CS for DSS, using evolution σ′ and ε_v, in the e-σ'_N space, compare its location with triaxial CS lines in the e-log(p^') space. The CS line for DSS in e-σ'_N space was then used to define a modified state parameter, ψ, (ψ_m). It was found that both ψ and ψ_m showed good correlations with characteristic behaviour e.g. onset of liquefaction, phase transformation state, fabric anisotropy etc. Therefore, ψ_m maybe a good alternative of ψ for DSS test where the minor principal effective stresses are not measured to define a CSL in e-log(p^') space.