Shear banding in torsion shear tests on cross-anisotropic deposits of fine Nevada sand

Abstract

A series of torsion shear experiments was performed on large hollow cylinder specimens of Fine Nevada sand with major principal stress directions relative to vertical, α, varying from 01 to 901 and with the intermediate principal stress, σ 2 , varying from σ 3 to σ 1 as indicated by b ¼ (σ 2 -σ 3 )/(σ 1 -σ 3 ). The Fine Nevada sand was deposited by dry pluviation, thus producing a sand fabric with horizontal bedding planes and crossanisotropic characteristics. The various stress conditions were achieved by varying the pressures inside and outside the hollow cylinder specimen relative to the shear stress and the vertical deviator stress according to a pre-calculated pattern. All stresses and all strains were determined from careful measurements so that analysis of the soil behavior could be made reliably. The soil behavior was determined for a pattern of combinations of α varying with increments of 22.51 from 01 to 901 and b varying with increments of 0.25 from 0.0 to 1.0. Thus, 25 test locations were established, but many tests were repeated to study the consistency of the results. The friction angles varied considerably with α and b, thus indicating the importance of the intermediate principal stress and the principal stress directions relative to the horizontal bedding planes. The observed shear bands essentially followed the expected directions, but due to the cross-anisotropy shear bands were also observed in the direction of the major principal stress in regions with high b-values. The strength variation was also influenced by the flexibility of the boundaries in these regions.

Anisotropy and Non-coaxial Behaviour of Sand Along Different Strain Paths

“…However, the stress tensor is no longer coaxial with strain (and its increment) when the loading direction deviates from the current stress and the axis of symmetry, i.e., 𝛼 𝜀 = 30 • , 45 • and 60 • . Complementary results are reported for tests along constant stress directions in the literature (Cai, 2010;Gutierrez and Ishihara, 2000;Gutierrez et al, 1991;Lade et al, 2014b;Yang, 2013).…”

Strength anisotropy of granular material consisting of perfectly round particles

“…The strength of granular materials, such as sand, has long been known to be loading direction‐dependent. Such strength anisotropy has been observed in triaxial tests, plane‐strain compression tests, direct shear tests, and torsional shear tests (e.g., ). Sand's strength anisotropy has commonly been attributed to inherent fabric anisotropy caused by elongated particles' preferred orientation when deposited under gravity, with long axes aligned parallel to the bedding plane (normal to the deposition direction) as shown in Figure (e.g., ).…”