Failure characteristics of two porous sandstones subjected to true triaxial stresses

Abstract

We conducted an extensive suite of true triaxial experiments in two porous sandstones, Bentheim (porosity ≈ 24%) and Coconino (17.5%). Our experiments demonstrate that failure of both sandstones is not only a function of σ3 but also of σ2. For a given σ3, σ1 at failure (σ1,peak) increases as σ2 is raised above σ3 between tests. The σ1,peak reaches a peak as σ2 is about halfway between σ3 and σ1 and then gradually decreases such that when σ2 ≈ σ1,peak, it approaches its initial magnitude when σ2 = σ3. For a constant σ3, failure‐plane angle increases with σ2 by a maximum of less than 10° as σ2 rises from σ2 = σ3 to σ2 = σ1,peak. The effect of σ2 on both failure level and failure‐plane angle is stronger in the lower‐porosity Coconino sandstone than in the Bentheim sandstone. The σ2 dependence of failure mode in the Bentheim is different than Coconino over the same σ3 range. Both sandstones failed dilatantly at low σ3 magnitudes. However, at high σ3 (100–120 MPa), Bentheim sandstone developed shear‐enhanced compaction bands, followed by pure compaction bands at σ3 = 150 MPa. Compaction bands were not observed in the Coconino. Microscopic observations via SEM reveal that tensile microcracking is dominant when shear banding occurs (under low σ3), while pervasive grain crushing and pore collapse inside compaction bands are observed at high σ3.

Failure characteristics of two porous sandstones subjected to true triaxial stresses: Applied through a novel loading path

“…The inclusion of test conditions at Lode angles other than ± 30° is necessary to formulate an adequate three‐dimensional failure description [e.g., Makhnenko et al ., ; Labuz et al ., ]. Furthermore, our novel true triaxial tests confirmed that the dispersion of τ oct,f ‐ σ oct,f trend based on the common true triaxial tests [ Ma and Haimson , ] is largely due to the dependence on the Lode angle Θ (or the stress ratio b ) [ Haimson et al ., ].…”

“…A summary of the physical properties of both sandstones can be found in Ma and Haimson []. The specimens tested in this study were prepared from the same blocks as those in the series of tests reported also by Ma and Haimson []. Good consistency in physical and mechanical properties was found in specimens prepared from these blocks.…”

“…As illustrated in Figure , loading procedure begins with the application of all three principal stresses hydrostatically up to the desired level, and then maintaining σ 3 constant while raising σ 1 and σ 2 simultaneously at a fixed stress ratio b (=Δ σ 2 /Δ σ 1 ), where Δ σ 1 = σ 1 − σ 3 and Δ σ 2 = σ 2 − σ 3 [ Jimenez and Ma , ]. The novel loading path differs from the common loading path [ Ma and Haimson , ] in the way σ 2 is controlled. The novel loading path maintains a constant Lode angle Θ (equation ), which can be conveniently expressed in terms of the stress ratio b : where b = 0, σ 2 = σ 3 and Θ = +30°, corresponding to axisymmetric compression; where b = 1/2, σ 2 = ( σ 1 + σ 3 )/2 and Θ = 0°, corresponding to deviatoric pure shear state; and where b = 1, σ 2 = σ 1 , and Θ = −30°, corresponding to axisymmetric extension (Table ).…”

“…It was introduced by Rudnicki [] and used by Haimson and Rudnicki [] as a yield description. We use it here as such, but it has been used as a failure condition by Ma [], Ma et al . [], Rudnicki [], and Ma et al .…”

“…They led to the determination of frictional coefficient ( μ ) and dilatancy factor ( β ) as described by equation (11). For Coconino sandstone, Poisson 's ratio ν = 0.25 [ Ma and Haimson , ] and parameter A = 0.6 [ Ma , ] for failure description F = 0 (equation ) were used. For Bentheim sandstone, Poisson 's ratio ν = 0.36 [ Ma and Haimson , ] and A = 0.33 [ Ma , ].…”