Undrained anisotropy and rotational shear in granular soil

Abstract

The impact of fabric anisotropy on the behaviour of granular soil remains a subject of great interest. In particular, the effects of principal stress rotation on the undrained response of saturated sand are not fully understood. This paper describes an experimental investigation conducted in an automated hollow cylinder apparatus into the undrained anisotropic behaviour of saturated sand in rotational shear, which is defined as a class of non-proportional loading with a continuous rotation of the principal stress directions but a constant deviatoric stress. Special attention in this investigation was placed on the influence of the relative magnitude of the intermediate principal stress, characterised by the parameter b = (σ2 − σ3)/(σ1 − σ3), on the pore pressure response and deformation characteristics. The experimental observations indicate that soil specimens, even in very dense state, were weakened by the build-up of pore water pressure in rotational shear. The intermediate principal stress parameter b was not a negligible factor for the behaviour of saturated sand in the rotation shear, but had significant impact. Under otherwise identical conditions, the soil specimen rotationally sheared under the condition b = 0 exhibited a much stronger resistance to pore pressure build-up and a less dramatic stiffness degradation than that sheared under the condition b = 1·0. This observation is considered to be directly associated with the soil fabric anisotropy.

Macro deformation and micro structure of 3D granular assemblies subjected to rotation of principal stress axes

“…As previously reported, the specimen approaches a denser state at the higher stress ratio. This is similar to the experimental observation reported on the drained response of sand under rotational shear [9,35]; and shares the same mechanism with the larger pore pressure build-up at higher stress ratio in the undrained rotational shear [36,37].…”

“…And the principal direction of strain increment aligns more in the stress increment direction, as shown in Fig. 20, consistent with experimental study [15,28].…”

“…21b. Sample DEB00Y09 is less contractive than sample DEB00Y09, consistent with the experimental observation [15,28]. It is interesting to note that the sample sheared at low b value (b = 0.0) experience a dilative strain in the out-of the plain with ε yy being negative while sheared at a higher b value (b = 0.5), ε yy is highly contractive.…”

“…It is known that the intermediate principal stress has a significant effect on the material deformation to stress rotation [15,28]. To investigate this effect, the specimen has been sheared at different b values (b = 0.0, b = 0.5 and b = 1.0), and then subjected to principal stress rotation with η = 0.9.…”

“…Extensive experimental observations on sand responses to principal stress rotation have been published since the 1980s' [5][6][7][8][9][10] using the hollow cylindrical device which offers independent control of the magnitudes and directions of principal stresses. Non-coaxial deformation and volume contraction are typical deformation characteristics observed when rotating the principal stress axes [6,7,[11][12][13][14][15].…”