The influence of stress-induced anisotropy in undrained yield and ultimate shear strengths in brittle loose deposited silts

¹ CONSTRUCT-GEO, Universidade do Porto (FEUP), Rua Dr. Roberto Frias s/n, Porto, Portugal
² Escuela Superior Politecnica Del Litoral (ESPOL-FICT), Campus Gustavo Galindo, Guayaquil, Equador
³ Faculdade de Engenharia, Universidade do Porto (FEUP), Rua Dr. Roberto Frias s/n, Porto, Portugal

^* Corresponding author: viana@fe.up.pt

Abstract

The typical undrained behaviour observed in brittle non-plastic soil is ruled by the combination of density and stress levels. Some specific silty and sandy mining waste with particles morphologies that generate high smallstrain stiffness to strength ratios when increasing deviatoric stress (q) in stress paths that tend to decrease mean effective stress (p´) may drop down the deviatoric stress before reaching the frictional critical sate. The ratio between the peak (or yield) value (S_u=q/2) and the corresponding p´ is usually associated with a locus in q-p´ that is commonly associated to a straight Instability Line (I_L) with a unique ratio (η_L) and for an initial state parameter. However, this is not the case if an induced anisotropy is installed differently while the at rest stress ratio (hereby defined as initial, K₀) is achieved by continuous rate the principal stresses consolidated in lab prior to loading with distinct values. This fabric effect is decisive for design and in stability assessment of earth structures, like dams or piles of mine tailings where non-plastic fines are dominant, even if the prevailing stress-path is in compression. In a thorough and quite complete program varying these conditions on iron ore tailings from Minas Gerais state in Brazil, reconstituted in lab with differentiated state parameters, and its relation to the induced anisotropy effect.