Numerical Modelling of One-Dimensional Wave Propagation in Deposits: Influence of the Effective Shear Strain Definition on the Equivalent Linear Method Solution

Seismic site response analysis plays a fundamental role in predicting earthquake ground motions at the ground surface by accounting for soil effects on wave propagation. Although advanced nonlinear methods can accurately simulate complex soil behaviour, their use in routine engineering practice is challenging. Consequently, the Equivalent Linear (EQL) method remains widely adopted despite relying on simplifying assumptions, particularly the definition of the effective shear strain (ESS), which governs the selection of strain-compatible soil properties. An inappropriate ESS may lead to over- or underestimation of soil nonlinearity and, consequently, of the seismic response. This issue is especially relevant for mine tailings, whose cyclic behaviour remains poorly understood despite the significant risks associated with earthquake-induced failures of tailings storage facilities. This study investigates the influence of the ESS assumption using an in-house EQL code through a detailed case study and a parametric analysis comprising 2106 simulations, in which seventeen surface ground-motion intensity measures are evaluated as a function of the ESS coefficient, Rγ. The results confirm general trends reported in the literature but also identify cases in which neither the magnitude nor the direction of the effects of varying Rγ can be inferred from Rγ alone, as they depend on the interaction between the deposit response and the frequency content of the input motion. Consequently, while the conventional value of Rγ=0.65 is generally adequate for routine analyses, parametric studies are recommended for critical structures such as tailings storage facilities.