The behaviour of granular material was investigated by simulating an undrained triaxial compression test for three-dimensional assembly of ellipsoid particles over a wide range of void ratio, e, and mean confining stress, p′. The assembly was either isotropically consolidated or K₀ consolidated prior to undrained shearing up to 30% axial strain to reach steady-state conditions. A unique steady-state line was obtained, irrespective of the consolidation path. The micro-mechanical quantities, such as coordination number and von Mises fabric in terms of second invariant of deviatoric fabric, F_vM, also reached steady-state values. The normalised anisotropic fabric variable, A, and the trace of the joint stress–fabric tensor, K_F, evolved during undrained simulation and reached the steady-state value at the end of shearing. This forms a unique relationship in K_F–p′–e space and the projection of this relationship in e–log(p′) space is the classical steady-state line. This underpins the concept of fabric evolution and steady-state fabric in anisotropic critical state theory. The relationships of state parameter, ψ, and stress ratio at instability, η_IS = (q/p′)_IS, were dependent on consolidation path, and the difference was not related to coordination number, but to F_vM. However, a new phenomenon was observed that stress ratio, η, at the end of K₀ consolidation and η_IS may reach beyond the steady-state stress ratio, M. Experimental data are needed to verify this; until then such behaviour should be regarded as unproven.

The discrete-element method (DEM) has gained popularity for developing a qualitative understanding of soil behaviour under a critical state soil mechanics (CSSM) framework. Most studies with a three-dimensional assembly of particles have used spheres as representative granular material to reduce computational demands. However, most granular materials – for example, sands – are not rounded, but possess features of angularity. Therefore, ellipsoid and cluster particles with different degrees of eccentricity were used in this study to evaluate the effect of the particle shape on the drained and undrained triaxial loading behaviour after isotropic and K₀ consolidation. The particle numerical properties and grain size distributions were kept the same for all specimens, irrespective of particle shape. The critical state data points for spheres and ellipsoids plotted on almost the same critical state line (CSL) in e–log(p′) space, whereas the CSLs of clusters plotted above them. Additionally, M lines shifted downward with increasing sphericity. It was also found that the stress ratio at the triggering of static liquefaction (η_IS = q/p′) in η_IS–ψ space was affected by particle shape and consolidation path. The dilatancy (d = dε_v^p/dε_q^p) was also affected by particle shape. It was found that dilatancy parameters for the SANISAND constitutive model are affected by particle shape, which may contribute to an improved understanding of particle shape in constitutive modelling.

Undrained and drained triaxial compression tests were simulated using the discrete element method (DEM). It was found that the transition states between contractive and dilative behavior of dense specimens, that is, the characteristic (Ch) and phase transformation (PT) states for drained and undrained conditions, respectively, are the same state in terms of stress ratio and fabric quantity ($F_{vM}$ ). The transition was not due to the change in number of particle contacts, that is, coordination number ($CN$ ). A unique critical state (CS) line was obtained for a large range of mean effective stresses ($p^{\prime }$ ) for both drained and undrained simulations, which facilitates further analysis in term of the state parameter ($\psi$ ). Because PT, Ch states, and CS were representative of sand behavior, the SANISAND model was adopted to predict DEM simulations. Despite some limitations, the model can predict overall trends of shearing response and produce adequate qualitative behavior such CS, PT, or Ch states of DEM simulations.

Discrete-element method (DEM) simulations of three-dimensional (3D) assemblies of ellipsoid particles were used to evaluate the critical state (CS) for both drained and undrained (constant volume) conditions. A series of conventional triaxial cyclic liquefaction tests with symmetrical cyclic deviatoric stress (${\sigma }_d$ ) with initial $q=0\mathrm{kPa}$ were simulated to develop a relationship between the cyclic stress ratio ($CSR={\sigma }_d/2{\sigma }_0^{\prime }$ ) and the number of cycles required for initial liquefaction ($N_L$ ), where ${\sigma }_0^{\prime }$ is the mean effective normal stress at the end of consolidation. Both cyclic mobility and instability type behaviors were observed depending on the initial void ratio ($e_0$ ) and ${\sigma }_0^{\prime }$ . The micromechanics quantities, i.e., the coordination number ($CN$ ), von Mises fabric ($F_{vM}$ ), fabric anisotropy intensity (${\alpha }_c$ ), and stress-strain behavior, suggested that cyclic mobility and instability may depend on the phase transformation and instability state, respectively. The cyclic resistance ratio ($CR{R}_{15}$ ), i.e., $CSR$ at $N_L=15$ , showed a unique relation with the initial state parameter (${\psi }_0$ ), irrespective of $e_0$ and ${\sigma }_0^{\prime }$ . Two series of postliquefaction monotonic simulations with and without reconsolidation exhibited a unique CS, which perfectly matched with the original CS line. The $F_{vM}$ also reached a unique, narrow range at the CS. The postliquefaction settlement during reconsolidation also showed a linear relation with ${\psi }_0$ .

The influence of particle shape was evaluated under drained and undrained (constant volume) condition using three-dimensional (3D) cubical assemblies of spheres, ellipsoids, and cluster of spheres (a combination of seven spheres with two different degrees of overlap) with same particle size distribution. It was found that the peak deviatoric stress, the minimum dilatancy ($d={\mathrm{d}\epsilon }_v^p/{\mathrm{d}\epsilon }_q^p$ ), corresponding stress ratio (${\eta }_{\mathrm{dmin}}$ ), the bounding surface dilatancy model, and the location of the critical state line (CSL) both in the $e\text{-}\log (p^{\prime })$ and the $q\text{-}{p}^{\prime }$ space were influenced by particle shape. Therefore, four corresponding sets of constitutive parameters for four different particle shapes were implemented in a bounding surface model to predict both drained and undrained (constant volume) discrete element method (DEM) simulation. Good prediction, irrespective of particle shape, indicates that the observed DEM behavior can be adequately captured by the theories of continuum mechanics. Importantly, the majority of the constitutive parameters were influenced by particle shape and can be correlated with simple shape descriptor of sphericity.

Lightweight structures built on expansive soils are susceptible to damage caused by soil movement. Financial losses resulting from the improper design of structures on expansive soils can be significant. The interactions and failure mechanisms of different geotechnical structures constructed on such soils differ depending on the structure type, site characteristics, and climatic conditions, as the behaviour of expansive soils is influenced by moisture variations. Therefore, the performance of different geotechnical structures (e.g., lightweight footings for residential buildings) is expected to be adversely affected by climate change (especially rainfall and temperature change), as geotechnical structures are often designed to have a service life of 50–100 years. Some structures may even fail if the effect of climate change is not considered in the present design. This review aims to provide insights into problems associated with expansive soils that trigger the failure of lightweight structures, including current investigations and industry practices. This review recognises that although the soil moisture conditions govern expansive soil behaviour, limited studies have incorporated the effect of future climate changes. In addition, this review identifies the need to improve the current Australian design practice for residential footings through the inclusion of more site-specific investigations and expected climate changes.