Patrick Dangla

Numerical analysis of seismic wave amplification in Nice (France) and comparisons with experiments

The analysis of site effects is very important since the amplification of seismic motion in some specific areas can be very strong. In this paper, the site considered is located in the center of Nice on the French Riviera. Site effects are investigated considering a numerical approach (Boundary Element Method) and are compared with experimental results. The experimental results are obtained thanks to real earthquakes (weak motion) and microtremor measurements. The investigation of seismic site effects through numerical approaches is interesting because it shows the dependency of the amplification level on such parameters as wave velocity in surface soil layers, velocity contrast with deep layers, seismic wave type, incidence, damping... In this specific area of Nice, experimental measurements obtained for weak motion lead to strong site effects. A 1D-analytical analysis of amplification does not give a satisfactory estimation of the maximum reached levels. A boundary element model is then proposed considering different wave types (SH, P, SV) as the seismic loading. The alluvial basin is successively assumed as an isotropic linear elastic medium and an isotropic linear viscoelastic solid with Zener type behaviour (standard solid). The influence of frequency and incidence is analyzed. The thickness of the surface layer, its mechanical properties, its general shape as well as the seismic wave type involved have a great influence on the maximum amplification and the frequency for which it occurs. For real earthquakes, the numerical results are in very good agreement with experimental measurements for each motion component. The boundary element method leads to amplification values very close to the actual ones and much larger than those obtained in the 1D case. Two dimensional basin effects are then very strong and are well reproduced numerically.

Seismic Site–City Interaction: Main Governing Phenomena through Simplified Numerical Models

This work focuses on the analysis of the multiple interactions between soil layers and civil-engineering structures in dense urban areas submitted to a seismic wave. To investigate such phenomena, called site–city interaction (sci) herein, two simplified site–city configurations are considered: a homogeneous, periodically spaced city and a heterogeneous, nonperiodically spaced city, both on a constant- depth basin model. These 2D boundary-element method models are subjected to a vertically incident plane SH Ricker wavelet. A parametric study of the city parameters (density of buildings and their natural frequencies) and the thickness of the basin is carried out to characterize the sci and to investigate its sensitivity to some governing parameters. The following parameters are analyzed: building vibrations, induced ground motion, ground-motion perturbations inside and outside the city, spatial coherency, and kinetic energy of the “urban wave field.” A so-called site–city resonance is reached when the soil fundamental frequency and structure eigenfrequencies coincide; building vibrations and ground motion are then significantly decreased and the spatial coherency of the urban field is also strongly modified. Building density and city configuration play a crucial role in the energy distribution inside the city.

Seismic site effects in a deep alluvial basin: numerical analysis by the boundary element method

The main purpose of the paper is the numerical analysis of seismic site effects in Caracas (Venezuela). The analysis is performed considering the boundary element method in the frequency domain. A numerical model including a part of the local topography is considered, it involves a deep alluvial deposit on an elastic bedrock. The amplification of seismic motion (SH-waves, weak motion) is analyzed in terms of level, occurring frequency and location. In this specific site of Caracas, the amplification factor is found to reach a maximum value of 25. Site effects occur in the thickest part of the basin for low frequencies (below 1.0 Hz) and in two intermediate thinner areas for frequencies above 1.0 Hz. The influence of both incidence and shear wave velocities is also investigated. A comparison with microtremor recordings is presented afterwards. The results of both numerical and experimental approaches are in good agreement in terms of fundamental frequencies in the deepest part of the basin. The boundary element method appears to be a reliable and efficient approach for the analysis of seismic site effects.

A Simple and Efficient Regularization Method for 3D bem: Application to Frequency-Domain Elastodynamics

An efficient and easy-to-implement method is proposed to regularize integral equations in the 3D boundary element method (bem). The method takes advantage of an assumed three-noded triangle discretization of the boundary surfaces. The method is based on the derivation of analytical expressions of singular integrals. To demonstrate the accuracy of the method, three elastodynamic problems are numerically worked out in the frequency domain: a cavity under harmonic pressure, diffraction of a plane wave by a spherical cavity, and amplification of seismic waves in a semispherical alluvial basin (the second one is also investigated in the time domain). The numerical results are compared to semi-analytical solutions; a close agreement is found for all problems, showing the accuracy of the proposed method.

Poroelastic Analysis of Partial Freezing in Cohesive Porous Materials

We revisit the poromechanics set up by Olivier Coussy for better understanding of the mechanical effect of partial freezing in cohesive porous materials. This approach proves to be able to quantitatively predict swelling even if the in-pore liquid does not expand when solidifying. In this case, dilation results from the so-called cryosuction process that dominates thermal shrinkage under cooling, as shown in our analysis conducted on the historical experiment run by Beaudoin and MacInnis (1974, “The Mechanism of Frost Damage in Hardened Cement Paste,” Cem. Concr. Res., 4, pp. 139–147) on benzene saturated 24-h old cement paste. Both mechanisms are also at work when freezing water saturated early age cement paste with air voids. In this case, the cryosuction process results in shrinkage and should be added to the thermal shrinkage, their respective amplitudes being temperature dependent but, a priori, of the same order of magnitude.

Water Removal by Freeze-Drying of Hardened Cement Paste

This paper investigates the water removal extent of freeze-drying (F-drying) technique on hardened cement pastes. Two cement pastes with w/c = 0.5, 0.3 were prepared, and the authors measured the hydration degree, specific surface area, porosity, and solid skeleton density of the material samples at curing ages from 7 days to 180 days. From these results, the C-S-H gel volumes of samples are predicted from Jennings model and also evaluated from drying data. The good agreement between the results of two approaches confirms that F-drying removes only partially the water in gel pores of LD C-S-H structure. At the age of 180 days, the residual porosity saturated by water after F-drying is evaluated as 28% (w/c = 0.5) and 55% (w/c = 0.3) with dried C-S-H estimated as 28% (w/c = 0.5) and 13% (w/c = 0.3).

Evaluation de la perméabilité à l'eau liquide des bétons à partir de leur perte de masse durant le séchage

ABSTRACT One of the objectives of the LCPC research project “Transfers in concretes and durability of structures” is the development of methods of determination of transport properties. These parameters play a key role in the evaluation of the durability of cement-based materials. In this paper, a method to assess the liquid water permeability of weakly permeable concretes is proposed. It is about an indirect method combining model and experiment. A precise analysis of the moisture transfers has also been carried out on the basis of this modelling. The conclusions obtained, relative to the mechanisms involved during the drying of weakly permeable materials, are also presented here.

Modal Superposition Method for the Analysis of Seismic-Wave Amplification

Seismic site effects involve both wave propagation phenomena and vibratory resonance processes. In this article, modal approaches are then considered to investigate site effects in alluvial basins through the latter aspect. Some simplified modal methods are briefly recalled first. For a shallow alluvial site in Nice, France, standard eigenmode estimation is then proposed to make a qualitative analysis and preliminary comparisons with other numerical results obtained by the boundary element method (BEM). The influence of the bedrock on these results is discussed. One considers afterward the modal superposition method to analyze site effects from a quantitative point of view (effective modal mass). For a translational excitation (assumption of rigid base motion) and a frequency range acceptable for such vibratory investigations, a strong amplification appears around 1.6 and 1.8 Hz, giving a very high value of the cumulated effective modal mass (80%). These results are shown to be in good agreement with experimental ones and other numerical ones (BEM). The modal superposition method, accounting for the seismic excitation, then appears as an efficient tool, not only for the vibratory characterization, but also for simplified analyses of site effects in alluvial basins.

Sound propagation above a porous road surface with extended reaction by boundary element method.

Acoustic impedance of an absorbing interface is easily introduced in boundary element codes provided that a local reaction is assumed. But this assumption is not valid in the case of porous road surface. A two-domain approach was developed for the prediction of sound propagation above a porous layer that takes into account the sound propagation inside the porous material. The porous material is modeled by a homogeneous dissipative fluid medium. An alternative to this time consuming two-domain approach is proposed by using the grazing incidence approximate impedance in the traditional single-domain boundary element method (BEM). It can be checked that this value is numerically consistent with the surface impedance calculated at the interface from the pressure and surface velocity solutions of the two-domain approach. The single-domain BEM introducing this grazing incidence impedance is compared in terms of sound attenuation with analytical solutions and two-domain BEM. The comparison is also performed with the single-domain BEM using the normal incidence impedance, and reveals a much better accuracy for the prediction of sound propagation above a porous interface.

A vanishing diffusion process in unsaturated soils

A model for simulating water flow and air flow in unsaturated soils is presented herein. Drainage from a one-dimensional soil column is specifically investigated. A mathematical analysis of the equations reveals that, due to a vanishing diffusion process, the gradient of the water degree of saturation is infinite at the water table. Moreover, this discontinuity propagates at a velocity that satisfies a local non-linear equation involving only the properties of the material. A numerical simulation of this problem serves to confirm the results obtained.