True triaxial tests in two porous sandstones: Experimental failure characteristics and theoretical prediction

True triaxial tests have been carried out in two quartz-rich, high porosity, sandstones, Coconino (n = 17.5%) and Bentheim (n = 24%) by maintaining constant but different σ₂ and σ₃ and raising σ₁ until failure occurred (at σ₁,_peak). For each constant σ₃ level, σ₂ was varied from test to test between σ₂ - σ₃ and σ₂ = σ₁, σ₁,peak for a given σ₃ increased with σ₂, reached a maximum (up to 15% higher than when σ₂ = σ₃), and then declined so that when σ1 approached σ₁, σ₁,peak was about equal to its base value when σ₂ = σ₃. A separate series of tests was carried out using a novel loading path by maintaining constant Lode angle θ (= 0°). This series of tests characterized the dependence of the octahedral shear stress at failure τ_oct,_f on the octahedral normal stress at failure σ_oct,_f when θ = 0°. The latter tests were used to obtain the necessary parameters employed in a three-invariant failure theory proposed by Rudnicki (2013). The theory was then applied to predicting the variation of σ₁,_peak with σ₂ for a given σ₃. The prediction reasonably replicated the typical ascending-then-descending σ₁,_peak vs. σ₂ trend observed experimentally in both sandstones, confirming (with some limitations) the applicability of the Rudnicki's (2013) theory.