Undrained behaviour of two silica sands and practical implications for modelling SSI in liquefiable soils

The undrained behaviour of cohesionless soils represents a crucial aspect that has to be considered when modelling soil-structure interaction in liquefiable soils. As a consequence, this paper is devoted to study the undrained behaviour of two silica sands by means of multi-stage triaxial tests. The multi-stage loading condition consisted of cyclic loading applied to cause liquefaction of the samples, followed by undrained monotonic loading that aimed to investigate their the post-liquefaction behaviour. To consider both liquefaction and cyclic mobility phenomena, the resistance to liquefaction was defined in terms of cyclic stress ratio (CSR) required to develop 5% in double-amplitude axial strain in 20 cycles. It was found that the resistance to liquefaction of both silica sands increased considerably with increasing in relative densities. The data obtained from second stage of testing demonstrated that the post-liquefaction response was always dilative regardless of the initial relative density of the sample. It was found that post-liquefaction behaviour can be exhaustively characterised by three parameters: (a) take-off shear strain, γto, i.e. shear strain required to mobilise 1kPa of shear strength; (b) initial secant shear modulus, G1, defined as 1/ γto; (c) post-liquefied shear modulus at large strain, G2 (γ>>γto). It was further noted that these parameters were strongly influenced by the initial relative densities of the samples. As expected it was found, γto decreased with increasing relative density, whereas, both shear moduli (G1 and G2) increased with increasing relative density. Practical implications for the construction of reliable p-y curves for liquefiable soils are discussed.