This research proposal is focused on soil anisotropy. It encompasses different scales, from the soil element to typical of engineering boundary value problems, and is aimed to shed some light on this crucial aspect of soil mechanics. The key ingredients of soil anisotropy will be first highlighted based on experimental observations; the following step will be focused on the continuum mechanics-based constitutive modelling of such features, leading to the final stage of the activity, which will be devoted to the evaluation of the potential impact of anisotropy on the solution of relevant boundary value problems, as numerically analysed in Finite Element simulations.
The proponents will work on this rather fundamental aspect of the mechanics of soil, trying not to miss the final objective of providing useful tools to be adopted in engineering practice.
In detail, the scientific question at the base of the proposal concerns the link between elastic anisotropy and what occurs at the microscale as an effect of by irreversible strains. This issue will be investigated from a theoretical perspective in light of a new unifying framework, in which both reversible and irreversible anisotropic pattern of soil behaviour can be accounted for through the use of a single fabric tensor. Its engineering impact will be examined with reference to two challenging engineering problems, namely the prediction of settlements induced by the excavation of shallow tunnels in urban areas and the propagation of shear waves within anisotropic soil deposits in seismic prone sites.
The main expected scientific outcomes are the following:
OS1) New and original interpretations of existing experimental results on both clays and sands: these will aim at highlighting the intensity and evolving character of the elastic anisotropy as detected by up to date laboratory techniques;
OS2) New phenomenological constitutive models, developed at the continuum level in light of the above analysis of the experimental results: they are expected to quantitatively account for elastic and plastic anisotropy within a unique framework in which all the directional properties of the material and their evolution (i.e. hardening) will be expressed as a function of a unique fabric tensor, this latter accounting for the microstructural features of the soil.
OS3) Development of a new form of elasto-plastic coupling, differing from the known ones for its tensorial nature: this is expected to clarify some related constraints that characterise the models, including their non-associativity and non-coaxiality.
The main expected applied outcomes are the following:
OA1) Identification of the classes of engineering problems where anisotropy can play a role in the performance of an engineering structures interacting with the ground.
OA2) Quantitative evaluation of the impact of anisotropy on two common and, at the same time, challenging engineering problems: the prediction of the settlements induced by the excavation of shallow tunnels and the evaluation of the seismic site effects in multidimensional wave propagation conditions: it is expected that the proposed understanding of the anisotropic behaviour of soils will provide a guidance to properly tackle such complex geotechnical engineering boundary value problems.
OA3) Triggering a more widespread use of advanced constitutive models in engineering practice: in fact, the plasticity-based models we intend to formulate, characterised by rotational/distortional hardening, will be amenable for a simple initialisation procedure based on routine laboratory or in situ tests, allowing to easily establish the amount of rotation/distortion of the yield surfaces at different depths and locations of a soil deposit. This information is barely available in practical applications, thus representing a major limitation to the use of advanced models in the design of civil engineering structures, whose response depends crucially on such initial condition.