QUANTITATIVE PREDICTION OF SITE EFFECTS IN SEDIMENTARY VALLEYS BY AN OPTIMISED 2D HYBRID METHOD

The response of a site to a seismic solicitation depends on local topography and geotechnical characteristics. Current building codes, based on 1D models of soil columns, do not take 2D phenomena into account to estimate these site effects. This work aims at quantifying the combined effects of geometry and stratigraphy in sedimentary basins subjected to synthetic SV waves of vertical incidence. The main purpose of this paper is to elaborate a predictive method to estimate horizontal ground movements (ux) in two-dimensional irregular configurations. Extensive parametric studies are done by means of an optimised hybrid numerical technique combining finite and boundary elements. Following a reasoning used in the construction of safety coefficients, the ratio ux/ux(ref) is expressed as a product of various factors. The reference station, giving the displacement ux(ref), is taken on the outcrop. Each coefficient represents a site effect parameter: offset (x/L); filling (H1/H); depth (H/L); slope (a) and shape (ellipsoid or trapezium). The topographical and geotechnical variables controlling 2D site effects are evaluated separately in order to identify the predominant parameters. Specific 2D effects are also characterized by a simplified breakdown requiring a unique “2D Combined Amplification” coefficient (2DCA coefficient).