Noname manuscript No. (will be inserted by the editor) Effect of coupling excess pore pressure and deformation on nonlinear seismic soil response Silvana Montoya-Noguera · Fernando Lopez-Caballero Received: July 13, 2015/ Accepted: date Abstract The excess pore pressure (∆p w ) generation and consequent reduction in effective stress leads to the softening of a liquefiable soil deposit that can alter ground motions in terms of amplitude, frequency con- tent and duration. However, total stress models, which are the most currently used, do not take into account coupling of excess pore pressures and soil deformations. To assess this effect two analyses were made: i) A Biot hydraulic and mechanical computation of a saturated soil deposit with coupling pore pressures and soil de- formations and ii) a mechanical computation of a de- coupled model with same initial behaviour. Both analy- ses were performed with a fully nonlinear elastoplastic multi-mechanism model. As ∆p w depends on the soil properties, two soils were analysed: loose-to-medium and medium-to-dense sand. The results regarding the profile of maximum accelerations and shear strains, the surface accelerations and their corresponding response spectra are analysed. The mean values of the normal- ized response spectra ratio of surface accelerations be- tween the coupled and decoupled model show a deam- plification of low and high frequencies (i.e. at frequen- cies lower than 1.0Hz and higher than 10Hz) that tend to increase with the liquefaction zone size. Coupling of ∆p w and soil deformation is therefore of great impor- tance to accurately model the ground motion response. On the contrary, while peak acceleration predictions could be conservative, the amplification on the low fre- S. Montoya-Noguera Ecole Centrale Paris Tel.: +331-41-131705 Fax: +331-41-131442 E-mail: silvana.montoya-noguera@ecp.fr F. Lopez-Caballero Ecole Centrale Paris quencies could be largely underestimated which could be highly prejudicial for flexible buildings. Keywords Earthquake engineering · Site response · Soil nonlinearity · Numerical modelling · Liquefaction 1 Introduction The performance of structures during earthquakes is strongly influenced by the behaviour of the soils that support them. Local site effects can influence structures performance in two primary ways: by imposing addi- tional deformations on the structure through ground failure and by influencing the ground motions that ex- cite them. The reliable and economical seismic design of structures requires that local site effects on the ground motions be accurately predicted [1, 2]. Liquefaction is a phenomenon commonly observed in earthquakes in which the strength and stiffness are significantly reduced by the generation of excess pore water pressure in saturated cohesionless soils. In loose sands, the tendency of densification causes the excess pore water pressure to increase and the effective stresses to decrease. Some consequences are excessive vertical settlement, lateral spreading, flooding of low-lying land and even soil failure. Additionally, liquefaction also af- fects the dynamic response of soil deposits. As the soil stiffness decreases so does the effective shear-wave ve- locity and the predominant frequency shifts to lower values. Similarly, as the strains increase, the energy dis- sipation increases and the soil amplification will be re- duced causing a “strong-motion deamplification effect” [3]. Usually, total stress models, like the one employed in SHAKE [4] program, are used to quantify the effect