Fernando Lopez-Caballero

International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases

PREdiction of NOn‐LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strong‐motion recordings obtained at well‐known sites. The benchmark presently involves 21 teams and 23 different computational codes. nWe present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression “input motion” in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log_(10) scale) for a peak ground acceleration of 5u2009u2009m/s^2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation.

PRENOLIN: International Benchmark on 1D Nonlinear Site‐Response Analysis—Validation Phase Exercise

This article presents the main results of the validation phase of the PRENOLIN project. PRENOLIN is an international benchmark on 1D nonlinear (NL) site‐response analysis. This project involved 19 teams with 23 different codes tested. It was divided into two phases; with the first phase verifying the numerical solution of these codes on idealized soil profiles using simple signals and real seismic records. The second phase described in this article referred to code validation for the analysis of real instrumented sites. n nThis validation phase was performed on two sites (KSRH10 and Sendai) of the Japanese strong‐motion networks KiK‐net and Port and Airport Research Institute (PARI), respectively, with a pair of accelerometers at surface and depth. Extensive additional site characterizations were performed at both sites involving in situ and laboratory measurements of the soil properties. At each site, sets of input motions were selected to represent different peak ground acceleration (PGA) and frequency content. It was found that the code‐to‐code variability given by the standard deviation of the computed surface‐response spectra is around 0.1 (in log10 scale) regardless of the site and input motions. This indicates a quite large influence of the numerical methods on site‐effect assessment and more generally on seismic hazard. Besides, it was observed that site‐specific measurements are of primary importance for defining the input data in site‐response analysis. The NL parameters obtained from the laboratory measurements should be compared with curves coming from the literature. Finally, the lessons learned from this exercise are synthesized, resulting also in a few recommendations for future benchmarking studies, and the use of 1D NL, total stress site‐response analysis.

Numerical Evaluation of Earthquake Settlements of Road Embankments and Mitigation by Preloading

AbstractThis article presents an assessment of the effects of pore water pressure generation of the soil foundation on the seismic road embankment response. Numerical simulations were carried out to study the preloading technique as an improvement method for reducing the liquefaction potential and the induced settlements in a sandy soil profile. The analyses showed that the use of preloading reduces the induced settlements mostly because of the increase in lateral confinement in the superficial soil layers that results from an increase of the coefficient of lateral earth pressure at rest (ko). The research also showed that the efficiency of the countermeasure method was limited to cases in which earthquakes produced a liquefaction zone lower than the depth of the overconsolidated soil.

PHYSICS-BASED SCENARIO OF THE 2007 CHUETSU-OKI EARTHQUAKE

This study presents a preliminary calibration of a large-scale seismological model of the M W 6.6 Niigata-Ken Chuetsu-Oki (Japan) earthquake of July, 16 th 2007. The strong ground motion affected a wide area in the surrounding of the Kashiwazaki-Kariwa Nuclear Power Plant (KKNPP). Due to the relative small source-to-site distance and shallow hypocenter depth, this seismic scenario resulted very interesting and well documented (a consistent database of seismic recordings is available). In this context, the test-case represents a suitable benchmark for the work-packages of the SINAPS@ project. SINAPS@ is the first French research project with the objective to quantify the uncertainty of the procedures to estimate the seismic risk. In this context, an omni-comprehensive approach is followed, modelling the wave-propagation from the fault to the structural components. This study is intended to describe the steps to build up and calibrate a reliable seismic scenario, capable to provide a synthetic wave-field at the regional scale. This objective is pursued by (1) assessing a stratified geological model for the Niigata region, (2) testing the effect of the source parameters in a kinematic approach (e.g. the rise time and the shape of the Source Time Function) and (3) checking the topography effect. A series of 3D source-to-site numerical simulations of the M W 4.4 NCOEQ2007 aftershock was therefore carried out with this regard (performed by means of the Spectral Element Method, SEM). The seismic scenario was calibrated with the aid of the semi-analytical solution provided by the Wave-Number Integration Method (WNI) and by comparing the synthetics with the recordings at several locations (i.e. the KNET-Kik-Net stations nearby and the KKNPP site). Finally, the forward physics-based analysis ended up by providing broad-band synthetic time-histories to be exploited in further studies as a reliable regional incident wave-field for the engineering bedrock

SEISMIC EVALUATION OF EMBANKMENT-TYPE STRUCTURES WITH COUPLED HYDRO-MECHANICAL MODEL

The response of geotechnical structures under earthquake loading is highly nonlinear and often leads to problems of slope stability, foundation settlement and soil liquefaction. The prediction of these failure modes is a topic of great interest in geotechnical earthquake engineering, particularly for structural integrity assessment, which requires estimation of structural behavior during and after collapse. In this study, the earthquake response of a road embankment resting on a soil foundation is investigated. Both fully drained and coupled effective stress analysis are performed. Moreover, two different soil types are used for the embankment (looseto-medium and medium-to-dense sand). In the fully drained case, no failure of the embankment is observed for the medium-to-dense sand and for a certain level of ground motion. However, in the coupled effective stress simulation, for the same material and input motion, liquefaction of the foundation soil can lead to the generation of a thick shear band which produce large settlements and accelerate the failure of the embankment. The response of the loose-to-medium sand is fundamentally different, as several localization zones are generated, at the foundation part and inside the embankment body, as well. In both simulations fully drained and coupled effective stress analyis the embankment fail can occur due to shear band generation inside the embankment.

EFFECT OF COUPLING EXCESS PORE PRESSURE AND SOIL DEFORMATION ON THE NONLINEAR RESPONSE

Abstract. The development of excess pore pressure (∆p w) 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 content, and duration. To assess these effects in a 1D medium to loose sand model, two analysis were made: (1) A BIOT hydraulic and mechanical computation of a saturated soil deposit and (2) a mechanical computation of a dry soil with equivalent behavior. The results regarding the profile of maximum accelerations and shear strains, the surface accelerations and their corresponding response spectra ratio are analyzed. The mean values of the normalized response spectra ratio between the wet and dry surface acceleration show a deamplification of high frequencies (i.e. f > 1.0Hz) and an amplification of low ones that tend to increase with the liquefaction zone size. Coupling of ∆pw and soil deformation is therefore of great importance to accurately model the ground motion response. On the contrary, while peak acceleration predictions could be conservative, the amplification on the low frequencies could be largely underestimated which would be highly prejudicial for tall buildings.