The foundation layer thickness and stiffness impact the site response by influencing the fundamental frequencies
and vibration modes in soil structure interaction (SSI) problems. From a practical perspective, the geotechnical characterisation of earthfill dams is typically focused on the borrow materials comprising the dam, while the foundation materials are often under-characterised, with the depth to the bedrock commonly only approximately estimated. In the seismic response of dams, these unknowns may also impact the deformation patterns affecting the overall stability of the dam. A back-analysis of seismic recorded data for an existing tailings sand dam is performed, to determine the thickness and stiffness of the soil foundation layer by
finite element analysis. A cyclic non-linear model (CNL) is employed in the Finite Element analyses which consider different depths to bedrock and soil stiffness profiles. The results suggest satisfactory agreement with the recorded data in terms of acceleration response spectra and amplification ratios and highlight the impact of the foundation layer characteristics on the overall dam response.