Experimental study of the anisotropic flow deformation and critical state of sand

Abstract

The optimization of the geotechnical design and the realistic analysis of the geotechnical problems require the deep understanding of the mechanical behaviour of soils subjected to complex loading. The aim of the present study is to conduct complex loading tests in the hollow cylinder apparatus for investigating the effect of fabric anisotropy on the mechanical behaviour of sand. Specifically, the way fabric anisotropy affects the sand’s vulnerability to flow deformation is placed under investigation. Monotonic loading tests were conducted on isotropically consolidated sand with fixed direction angle, α, of the axis of the major principal stress, σ1, with respect to the vertical direction of deposition, constant intermediate principal stress ratio, b=(σ2-σ3)/(σ1-σ3), and constant mean total stress, p. In a different set of experiments the effect of the continuous rotation of the stress principal axes on the mechanical behaviour of anisotropically consolidated sand was investigated. The parameters b and p were kept constant during the rotation of the stress principal axes while the deviatoric stress, q, was either kept constant or increased. By these means the influence of both the rate of principal stress rotation and consolidation stress ratio, Kc=σ’3c/σ’1c, was studied. It was proved experimentally that the deviatoric stress ratio, η=q/p’, at the triggering of flow deformation is not a unique function of α, as stated in the literature, but instead depends on the stress – strain history. Τhe second part of the present study aims at giving an answer to a very important question raised recently in the literature: should the Critical State Theory (CST) be revisited in order to take into account the effects of fabric anisotropy? The CST constitutes the main framework for developing constitutive models that simulate the mechanical behaviour of geomaterials and other granular materials. However, the CST is incomplete in the sense that it does not take into consideration the effects of fabric anisotropy on the behaviour of granular materials at critical state, which are quite influential according to the results of contemporary studies. For this reason, the Anisotropic Critical State Theory (ACST) was recently developed, and a thought experiment was proposed proving that ACST enhances and completes the classical CST. The thought experiment requires that a sand specimen is brought to critical state by means of monotonic loading with fixed stress principal axes. Thereafter, the stress principal axes are rotated while keeping the stress principal values constant. The question raised is whether the sand will remain at or abandon the critical state. In the present study, the rotation of the stress principal axes was conducted at stress states progressively closer to the critical state determined for the sand under examination. The experimental results corroborate the claim that the ACST constitutes a necessary revision of the classical CST. Moreover, it is shown that both the dilatancy and non-coaxiality of sand depend on the stress – strain history. Finally, it is worth noting that the hollow cylinder apparatus of the National Technical University of Athens was upgraded for the needs of the present study and that the loading tests presented in the thesis were performed for the first time in Greece.