Seismic compression susceptibility in dry loose sandy and silty soil in a seismic microzonation perspective

Seismic microzonation studies (hereafter SMs or SM studies) have a strategic application in land use planning in the perspective of territory protection. At urban scale, SMs are finalized to forecast the spatial distribution of stratigraphic and topographic local amplification effects of ground motion and the co-seismic instabilities such as landslides or liquefaction phenomena. In this regard, the co-seismic instability due to the seismic compression (or densification) in dry loose sands, silty sands and sandy silts has been addressed in this paper through a numerical approach in order to provide simplified charts to evaluate the effect of the instability in terms of earthquake-induced settlements to be incorporated in SMs. In specific subsoil conditions, seismic cyclic loads can result in contractive volumetric strains inducing damages to structures, infrastructures and lifelines, accordingly with well documented post-earthquake damages of past events. In the present work, a procedure has been adopted to define the abovementioned simplified charts to predict the possible effect of the phenomenon in terms of soil permanent displacements. The proposed charts provide the estimation of settlements in function of three different PGA classes, the thickness of the soil layer susceptible to seismic compression/densification, its relative density and shear wave velocity. The proposed charts have been tested for the case study of Santa Clarita valley (California) during the 1994 Northridge earthquake, probably the best documented seismic compression case history available in the technical literature.