Multiscale mechanical characterisation of intermediate soils: experiments and modelling

Tailings dams are made of waste material left over from mining operations. These geo-structures have accumulated recent attention in the geotechnical community due to the 2019 sudden failure of an iron-ore tailings dam in Brumadinho (Brazil), resulting in 270 fatalities from an avalanche of 9.7 million cubic meters of waste (Robertson et al., 2019). With over 3,000 active tailings facilities globally and numerous inactive ones, safety concerns are heightened. Static liquefaction, identified as the direct cause of the Brumadinho dam failure, is a common factor in such geo-environmental disasters. Rather than under unsaturated conditions, liquefaction primarily occurs in saturated, loosely deposited, contractive soils, characterized by undrained strength brittleness. Static liquefaction in tailings is not well understood because of the complicated structure of these materials involving a mixture of sands, silts and clays. These mixtures are often referred to as "intermediate soils" and new constitutive models able to characterise their mechanical response are urgently needed. Some researchers have used constitutive models within the Critical State Soil Mechanics framework (e.g., CASM; Nor-Sand) to explain how static liquefaction led to failure of various tailing dams while others have used a Mohr-Coulomb constitutive framework with undrained softening. However, these soil models are designed for either clays or sands, neglecting the complex microstructure of intermediate soils and its evolution during shearing (e.g., anisotropy). This oversight leads to inaccuracies in modelling and potential underestimation of the liquefaction risk.

X-ray Computed Tomography (XRCT) has significantly advanced in-situ characterization of geomaterials. The valuable information of the grain arrangement at the micro scale that this technique can provide has been key in the development of more realistic constitutive frameworks to represent the behaviour of clean sands. Based on these advances, the major goal of this project is the development of a new constitutive model for intermediate soils combining current state of knowledge with findings from newly generated XRCT data for intermediate soils.