Effect of planar structures on the anisotropy of Ranyah sandstone, Saudi Arabia

The anisotropy of metamorphic and sedimentary rocks is mainly due to the presence of a single set of discontinuity surfaces such as cleavage, foliation, schistosity, joints, microfissuring or bedding. The uniaxial compressive strength anisotropy curve is usually U-shaped. However, the presence of more than one discontinuity modifies the shape of the anisotropy curve. Therefore, Ranyah sandstone, having two sets of mutually perpendicular discontinuities, was selected and tested. The investigation includes the geological and engineering geological descriptions as well as the classification of the rock mass. It also includes the determination of the physical, mechanical and dynamic properties of the rock material. The most sensitive engineering property to the anisotropic behaviour for Ranyah sandstone was found to be the compressive strength. A W-shaped anisotropy curve was obtained. This shape is due the superimposed effects of the two directions of bedding and microfissures acting almost at right angles to each other, and is in line with Hoek and Brownʹs (Hoek, E., Brown, E.T., 1980. Underground Excavations in Rock. The Institution of Mining and Metallurgy, London, p. 165) hypothesis for two mutually perpendicular discontinuities. A comparison of the anisotropic behaviour of the Ranyah sandstone with same foreign sandstones of different origin was also made.