Site Response
Motivation & Objectives
The objective of this work package is to reduce uncertainties on ground motion at surface level by a proper consideration of the site effect. It aims at taking into account the realistic non-linear soil behavior for strong motions as well as the evaluation of the reliability of site characterizations based on geotechnical measurements.
The work package uses different approaches:
- a numerical approach by calculation using physical models – (e.g. finite elements, spectral elements) or analytical resolution - to solve the wave propagation equations, implying many simplifications of the reality
- an experimental approach by comparison of actual recordings, either between two surface sensors - one at the site of interest and the other on a nearby rock reference site (SSR), or between a deep sensor at substratum and one at the surface level.
Coordination
Evelyne FOERSTER (CEA) & Cyril SIMON (EDF)
Actions
Analysis of the site term for stations likely to present 2D or 3D effects
Type:
Collaboration: tbd
Status: still in defining outlines
Topographic site effects
Collaboration: tbd
Status: still in defining outlines
Using Coda and seismic ambient vibration
Collaboration: tbd
Status: still in defining outlines
Aleatory variability for site effects
Collaboration: tbd
Status: still in defining outlines
Increasing the reliability of degradation curves for engineering uses
The sensitivity of the site response analysis to ground non-linearities is well known. Characterization of the non-linear behaviour of the ground is mainly based on laboratory tests, that raises the usual questions of representativeness and uncertainties associated with these tests.
- bibliographic synthesis of the published G-g and D-g expressions proposed by various authors
- potentially implement some specific lab tests
- characterization of the variability of the curves for a given material
- representativeness of the lab tests; adjustment on in-situ tests and on actual records in bore-hole and free field
- relevance of degradation curves expressed as a function of a shear strain ratio g/gr
Characterizing variability and model uncertainties
Up to now, site response calculations usually considered a uniform Vs per layer and the ground propertie variability was accounted for by shifting the modulus value of the whole layer by 2 coefficients around the best estimate value (typically 2/3 and 3/2). This approach poorly represents the reality nor the effects that a spatial variability have on the ground motion. The objective of this task is to develop and/or improve methodologies to account for 1D/2D/3D variability and to assess modelling uncertainties
- Ground variability into 1D soil column calculations.
- Characterizing 2D variability and calibrating attenuation at high frequencies
- Numerical strategy to properly account for the variability in 2D and possibly in 3D models
- Methodology to properly assess the uncertainties of the models and predictive capabilities of the models, especially when few field data is available
- Quantifying uncertainties related to empirical and numerical approaches for 2D site effect evaluation in low seismicity areas
Improving numerical modelling strategies for site effects and SSI
Engineers face numerical issues when they need to take non-linearities into account in their simulations.
- Numerical strategies to properly account for nonlinear site effect in SSI computations
- Improving the absorbing boundary conditions (non linear soil behaviour; wave inclination; pore water pressure in saturated zone)
- non-linear constitutive behaviour laws (parameter calibration strategies; optimizing the formulation of existing models; improving damping capabilities at low and high strains)
Note: PHD Thesis planned (outside of SIGMA-2) at EDF R&D to progress on constituve laws.
Working group, synthesis of the findings of this Work Package
The aim is to produce a guideline for the engineer, to perform 1D and 2D site response calculations; some perspectives regarding the 3D calculations could also be included.
- This guideline should address (non-comprehensive list):
- The details of the modelling strategies (boundaries, mesh, signal input…)
- How to account for the variability (spatial and parameters…)
- How to express the results (where in the model, mean values, envelope…?)
- How to validate the model with actual records of instrumented sites?
- How to express result confidence and limitation.
