Analysis of local site effects on seismic ground response under Boumerdes earthquake scenario: case study of Zeralda site of Algeria

This paper presents the effects of geology and local soil conditions on the intensity of ground agitation with a moderate earthquake of magnitude M_w = 6.8 that struck Boumerdes, on May 21st 2003. The analysis of the earthquake response of the ground soil specific to the site of Zeralda, located west of Algiers was carried out. The first part consists in developing a database of the experimental site based on geophysical and geotechnical testing to estimate the local site conditions. In the second part, a numerical analysis is performed out using the FLAC^2D software to evaluate the dynamic characteristics of the sand and consolidated sand, in particular the determination of the evolution curves of the shear modulus (G) and the damping (D) versus the shear strain (γ). The nonlinear model developed by Ramberg–Osgood and limited by the Mohr–Coulomb criterion is used. In the third part, the earthquake response from the ground surface was analyzed using the FLAC^2D code, considering the nonlinear soil deposition behavior approach. The response spectrum resulting from this analysis of the nonlinear approach was compared with the response spectra of the Algerian seismic code (RPA 99, 2003) and Eurocode 8. A numerical evaluation of the G/G_max(γ) and D(γ) curves of soil deposits seems to be compatible and close to other experimental data in the literature. The model presented by Ramberg–Osgood is applicable to the surface layer composed mainly of sand and consolidated sand. It therefore appears that the response spectrum was relatively underestimated by the Algerian building code, particularly during the short periods where the maximum spectral accelerations occur. The obtained results also validate the ability of the Ramberg–Osgood model to simulate the seismic response of the overburden to ground motions.