Geoscience Reference
In-Depth Information
CHAPTER 6
MODELLING OF DYNAMIC SOIL PROBLEMS
David Muir Wood
Department of Civil Engineering, University of Bristol, Queen's Building,
University Walk, Bristol, UK
d.muir-wood@bristol.ac.uk
Abstract.
Somepossibilitiesforthemodellingofdynamicsoilproblemsaredescribed.Twobasic
features of constitutive models to be used in numerical analysis to simulate soil response under
non-monotonic loading are: a description of the small strain elastic behaviour which will evolve
and develop its own anisotropies as the stress or strain history unfolds; and provision for plasticity
on reversal of stress or strain path. Laboratory geophysical techniques provide one possibility for
tracking the evolution of stiffness anisotropy but have uncertainties of interpretation. Macroele-
mentmodellingisaformofnumericalsimulationusingsimpleelementalsystemswhichintroduce
important features from constitutive modelling. It lends itself to rapid parametric study of effects
of geotechnical and seismic nonlinearity. Physical modelling can provide controlled data for vali-
dation of numerical modelling: the design of such models requires consideration of scaling laws.
Results of some model tests on a single degree of freedom structure with a special isolating foun-
dation are shown.
1. Introduction
Geotechnical modelling (Muir Wood, 2004) is undertaken to increase understanding of
the mechanisms that occur in geotechnical systems in order to be able to predict the
performance of those or other related systems. Prediction involves extrapolation from
known behaviour towards an unknown region: the quality of the prediction will be as
good as thequality of the models underpinning the understanding of the system.
Theprincipaltypesofmodelusedtopredictperformanceofdynamicgeotechnicalprob-
lems are numerical and physical models. In general numerical models will be calibrated
against observations made on physical models. However, contained within the numeri-
cal modelling and implicit in extrapolations made on the basis of physical modelling is
some assumed constitutive model for the soil behaviour. Such constitutive models are
themselves inspired by direct observation of material behaviour at theelement level.
Webeginbydescribingaratherelegantclassofconstitutivemodelforsoilsandthenshow
how laboratory dynamic testing can feed into the selection of soil parameters. Physical
modelling using an earthquake simulator has been used to support studies of mitigation
of seismic response of existing foundations: some results will be shown as an example
of data generation for calibration of numerical models. The nature of soil nonlinearity is
