On the structural anisotropy of physical and mechanical properties of a Bunter Sandstone

Abstract

Studies of structural anisotropy of rocks contribute to the understanding of their mechanical behaviour variation in a broad spectrum of geological settings. In this work we characterise the lithological variation and the mechanical behaviour of the Trendelburg beds, a fine-grained subarkose from the Bunter Sandstone, with moderate effective porosity (10%) and low permeability (0.5 mD). Traditional triaxial compression tests were carried out with varying confining- and pore pressures in water saturated specimens (7 cm diameter ×14 cm length). Ultrasonic velocity, permeability, deformability, compressive strength, mechanical work and fracture pattern were determined in two directions of anisotropy (0° and 90°) with respect to bedding. Changing the angle of anisotropy leads to different reactions to the influence of lithological heterogeneities, which reaches a maximum when arranged parallel to σ1. The Trendelburg beds has significant anisotropy effects, which tend to increase with effective pressure. The effects of a structural anisotropy due to bedding are associated with an anisotropy of physical properties, stress state, pore pressure and likely pore space distribution. Mechanical properties are direction dependent, however, the influence of lithological factors may diverge between both directions of anisotropy as pressure increases.