Modelling the influence of fines content on the instability of silty sands considering grain scale interactions

Experimental observations have shown the significant influence of silt grains on the instability and liquefaction characteristics of the host sands, but modelling such sand-silt mixtures remains a challenge, due to the complex interactions between fine silt grains and coarse sand grains. This paper presents a constitutive model based on the mechanism of grain-to-grain contacts to capture the behaviour of silty sands in both drained and undrained conditions. Grain scale contact behaviour is modelled using intrinsic void ratio and phenomenological constitutive relationships, and explicitly connected with the macro behaviour. This paves the way for the inclusion of the influence of silt grains on the stiffness and dilation/compaction tendency of the contacts, which in turn control the evolution of macro stress and strain. As a result, the effects of fines contents, stress states and initial densities on the liquefaction characteristics of silty sands can be captured in the proposed model based on phenomenological grain scale constitutive models. Validation against experimental data demonstrates reasonable predictive capabilities of the model in capturing the responses of silty sands with different fines contents. The proposed model is shown to be effective in predicting the onset and progressive development of liquefaction, as a result of changes caused by the addition of fine grains and its interaction with host sands.