Nonlinear finite element analysis for seismic site amplification assessment of urban slopes showing surface geology and topography irregularities

In urban areas, buildings close to the crest of a slope can undergo serious seismic disturbances due to seismic site amplifications, often induced by local site conditions. In this context, this paper aims to assess how combined surface geology and topography irregularities can amplify the site response to the ground surface of the Ciloc slope, subject to the reference earthquake that occurred in Constantine province on October 27, 1985. Therefore, a two-dimensional nonlinear dynamic finite element analysis was conducted on various geometric models, in which both slope angle and lithology were simultaneously varied. To describe properly the earthquake frequency content, an analytical solution based on the interpolation method of the general equation of the dynamic motion was programmed under MATLAB language. Fast Fourier Transform (FFT) was also programmed to compute the Fourier spectral amplitude of a selected accelerogramme. Likewise, the computer software SeismoSignal has been used to compare analytical outcomes. Four slope configurations were simulated namely, an actual slope, a layered slope, a homogeneous marl slope and a homogeneous alluvial slope with various slope angles. It was found that maximum site amplifications had occurred behind the crest and not at the exact crest of the slope as commonly expected. Due to the dilatancy phenomenon, alluvial homogenous formations as cohesionless soils, show high spectral parameter amplifications than the other cases. It was also concluded that a seismic response analysis of these sedimentary deposits is strongly recommended as a preliminary study for the seismic analysis of the neighbouring buildings, to avoid serious seismic destructions.