Page 187 - Numerical Analysis and Modelling in Geomechanics

Chapter 6
Back analysis of geotechnical problems

Annamaria Cividini and Giancarlo Gioda

Abstract
Some back analysis techniques applicable in geotechnical engineering are
summarised, considering both their deterministic and probabilistic aspects. The
discussion is limited to static conditions. First, two alternative back analysis
approaches are described in a deterministic context, referred to as “direct” and
“inverse” techniques. Subsequently, a probabilistic Bayesian technique is
discussed which aims at investigating the influence of the experimental errors on
the results of back calculations. Finally, two applications to actual tunnelling
problems are summarised.

Introduction
Back analysis techniques are nowadays often used in geotechnical engineering
practice for determining the mechanical characteristics of soil/rock masses on the
basis of field measurements performed during excavation or construction works
[1–4].
In this context, a back analysis consists in finding the values of the parameters
of the geotechnical medium that, when introduced in the stress analysis of the
problem under examination, lead to results (e.g. displacements, stresses, etc.) as
close as possible to the corresponding in situ measurements.
In general terms, two “tools” are necessary to perform a back analysis. The
first is a stress analysis procedure for determining the stress, strain and
displacement distributions for the problem at hand. The second is a suitable
optimisation algorithm which minimises, with respect to the “unknown”
mechanical parameters of the soil/rock mass, a non-linear function representing
the discrepancy between the quantities measured in the field and the
corresponding data obtained by the stress analysis.
Here some aspects of the back analysis procedures in the field of geotechnical
engineering are illustrated, adopting the finite element method as a suitable
solution technique and limiting the discussion to static conditions.