Geomechanical behaviors of shale after water absorption considering the combined effect of anisotropy and hydration

Abstract

Geomechanical behaviors of shale in many geotechnical engineering areas, such as shale gas extraction, tunnel excavation, slope engineering, are highly influenced by anisotropy and hydration. In this work, a number of ϕ25 mm × 50 mm shale specimens were prepared with three levels of moisture contents and seven bedding plane orientations (β, in degrees) relative to the axial direction. A series of compression tests for these specimens was then conducted under four different confining pressures to investigate their geomechanical behaviors considering the combined effect of anisotropy and hydration. The results demonstrate the following points. 1) During the water absorption process, the shale specimens swell and suffer damage, which are primarily observed along the bedding plane. 2) The curve of compressive strength (peak deviatoric stress) vs. β is “U-shaped,” with the lowest value at β = 30°. The apparent elastic modulus decreases with the increase in β and then remains nearly unchanged when β > 45°. As β increases, the strains at peak stress and average apparent Poisson's ratio increase first and then decrease when β > 30°. 3) The plastic deformation stage and residual stress stage, which negatively relate to the elastic-brittle properties of shale, become more obvious with increasing moisture content, and the specimen for which β is closer to 30° exhibits a more significant change of that. 4) With increasing moisture content, the compressive strength and axial strain at peak stress decrease, apparent elastic modulus decreases nonlinearly, and average apparent Poisson's ratio increases approximately linearly. 5) The effect of anisotropy on the compressive strength decreases with increasing confining pressure and this weakened effect by confining pressure becomes less with increasing moisture content. 6) With the increase in β, the effect of water absorption on the compressive strength first strengthens a little and then weakens when β > 30° or 45°, and this change with β becomes more significant under higher confining pressure. 7) The effect of confining pressure on the compressive strength strengthens a little when β < 30° and then weakens with the increase in β, and this change with β weakens with increasing moisture content. 8) The anisotropy of shale could be the major controlling factor for geomechanical behaviors, and water absorption along the bedding plane can enhance the effect of anisotropy. The results can not only guide the design and construction of geotechnical engineering structures related to shale, but can also be used as a reference for geological research on the deformation and failure of shale formations affected by water intrusion and geological stresses.