Anisotropic shear strength of a residual soil of sandstone

This paper discusses results of laboratory tests carried out with a residual soil originated from the weathering of eolian sandstone from southern Brazil. Parent rock features, like microfabric and particle bonding, are remarkably well preserved within this residual soil. Stiffness and shear strength properties were evaluated with consolidated drained (CID) and consolidated undrained (CIU) triaxial compression tests. Undisturbed specimens were tested with two different orientations between the specimen axis and bedding surfaces (i.e., parallel (delta = 0 degrees) or perpendicular (delta = 90 degrees)) to investigate the effect of anisotropy. When CID triaxial tests were performed with delta = 0 degrees, the yield surface associated with the structure was much larger than when tests were performed with delta = 90 degrees. Coincidently, CIU tests with delta = 0 degrees showed peak shear strengths much greater than for delta = 90 degrees at comparable test conditions. Once the peak shear strength was surpassed, CIU tests followed collapse-type effective stress paths not shown by corresponding tests with remolded specimens. A near coincidence was observed between the yield surface determined with CID tests and the envelope of collapse-type effective stress paths for delta = 0 degrees and delta = 90 degrees.