True Triaxial Compression Tests on a Medium-High Porosity Sandstone and the Effect of the Intermediate Principal Stress on Strength and Fault Angle

摘要

True triaxial tests have been carried out on the quartz-rich Coconino sandstone, (porosity 17.5%) to investigate the effect of the intermediate principal stress (σ_2) on strength and fault angle. In all tests the least and intermediate principal stresses (σ_3 and σ_2) were maintained constant, while the largest stress (σ_1) was raised until failure (at σ_(1, peak)), and beyond. For each constant σ_3, σ_2 was varied between σ_3 and σ_1. Rock strength for a given σ_3 increases with σ_2, reaches a peak (up to 10% higher than when σ_3 = σ_2), and then gradually drops, but when σ_2 =σ_1, it is still higher than the σ_(1,peak) when σ_2 =σ_3. Brittle failure eveloped a single fault at lower σ_3, to multiple parallel and conjugate faults at σ_3≥80 MPa. Fault angle under σ_3 = σ_2 loading decreased with the rise in σ_3 from ~80° at σ_3 =0 to ~50° at σ_3 = 150 MPa. However, fault angle increased by up to 15° as σ_2 was raised above σ_3. Both strength and fault angle dependence on σ_2 was lower than in tested crystalline rocks. The results of these tests contradict the Mohr-Coulomb criterion, which neglects σ_2 effect on strength, and predicts a unique fault angle for a given rock.