Devastating earthquakes, such as those that recently affected Turkey and Morocco in 2023, are among the deadliest natural disasters. In addition, liquefaction-induced damage induced by strong shakings further exacerbates economic losses and elongates the repairing time for structures and infrastructures. Consequently, improving foundation soils and strengthening structural elements are possible ways to enhance the resilient capacity of a building to resist seismic forces. The main goal of this study is to numerically investigate the behaviour of innovative synthetic materials, made by a mixture of gravel and granulated rubber, used as geotechnical seismic isolation (GSI) systems relevant to the mitigation of seismic risk. To this aim, numerical simulations of a well-known and instrumented gravelly site affected by liquefaction during the 1995 Kobe earthquake are performed using a pore water pressure model implemented in one of the most popular nonlinear software adopted for dynamic analyses. The analyses were carried out with and without soil treatment made by a mixture of gravel and rubber, showing the efficacy of the proposed intervention. The result of the study contributes to expanding the knowledge about mixture-composite materials and assessing the gravel-rubber mixtures (GRMs) as an effective mitigation treatment of seismic risk.