The characterisation of the
ground motion at the surface of a typical soil deposit is pursued via
truly nonlinear dynamic analyses of the soil column subject to real
rock ground motion recordings. A finite element code performs
dynamic response analyses of horizontally layered sites. The procedure
is formulated on the basis of effective stress principle, vector
motion, transient pore fluid movement and generalised material
stiffness. The multi-directional formulation and the use of plasticity
models for soil behaviour allow handling of shear wave and compression
wave simultaneously. For simplicity, however, only one horizontal
acceleration time history is usually applied in the analyses.
Example
of Geological
map with computation grid, borehole
locations
and
fundamental periods estimated from microtremors
The non-linear effect of
the soil layers on the intensity
of the ground motion at the surface is captured by a site-specific,
frequency-dependent amplification function, AF(f), where f is a generic
oscillator frequency. The median AF(f) can be accurately predicted by a
small number of ground motion records (as few as ten or less) driven
through the finite element model of the soil deposit with uncertain
properties. A specific software is used to define the hazard at the
ground surface by combining the seismic hazard at the bedrock and the
amplification functions obtained by site amplification analyses.
We used this methodology
as a building block for computing the surface hazard at each grid
point. However, when the purpose of the study is to provide hazard maps
for relatively large areas and for many different geotechnical
conditions, carrying out a large number of finite element analyses is a
cumbersome task. In addition, the limited amount of available
geological/geotechnical information may complicate the definition of
detailed input stratigraphies and soil parameters for the numerical
model. For these reasons, we addressed this problem by developing a
response surface that yields the AF(f) of a soil column at a grid
location (or of multiple soil columns that are plausible for the site
based on the available information) based on a family of pre-computed
AF(f) for a large number of soil columns.
Example of Response Surface
For more
information on Geodeco's seismic risk services please
contact
Paolo Bazzurro,
Ph.D.
