Finite element-based non-linear dynamic soil-structure interaction.

The modelling of unbounded domains is an important consideration in many engineering
problems, for example in fluid flow, electro-magnetics, acoustics and solid
mechanics. This thesis focuses on the problem of modelling elastic solids to infinity,
with the specific purpose of modelling dynamic soil-structure interaction (DSSI).
However, the reader should be aware that the techniques presented may also be
adapted to address those other physical phenomena.
The need for techniques to model the soil domain to infinity and a qualitative
introduction into the problems associated with dynamic soil-structure interaction
are outlined in chapter 1. This is done to illustrate why such an abstract mathematical
concept of modelling infinite domains has an important role to play within
the design process of large, safety critical, civil engineering structures.
A brief review of a number of alternative ways of addressing this problem is given
in chapter 2. Their relative strengths and weaknesses along with the typical applicability
of the techniques is discussed. A consequence of this review is the
identification of a very promising rigorous approach [59] which is singled-out for
further study. A detailed explanation of this (Consistent Infinitesimal Finite Element
Cell Method, CIFECM) method is then given in chapter 3. Attention is
restricted to the use of the technique for solving the 3-D vector wave equation in
the time domain.
The features of the non-linear dynamic finite element code, into which the CIFECM
has been incorporated, is highlighted in chapter 4. The non-linear (microplane) material
model for quasi-brittle materials is described along with the solution strategy
employed. It should be mentioned that the soil is treated within this thesis
as drained linear elastic medium. The method of coupling the CIFECM into the
dynamic equation of force equilibrium for both directly applied and transmitted
loading regimes is detailed.
Application of the code follows in chapter 5; firstly by introducing the simplest test
problem of one finite element coupled with one CIFECM element to model a surface
foundation. Comparisons are made between the dynamic displacements resulting
from the method and standard FE solutions obtained from the use of extended
meshes and fixed boundary conditions, along with a study of the influence input
variables. Following these examples a larger (more realistic) engineering problem
is tacked involving the simulation of an aircraft impact on a reinforced concrete
nuclear containment vessel. This represents the first use of the method in a 3-D nonlinear
structural analysis problem. The results illustrate the practical implications
of including DSSI in the analysis.
III
In chapter 6, a series of general observations on the method are made with an
assessment of its value together with a discussion on its wider application to other
engineering fields. Possible future developments to make the method more computationally
efficient are finally suggested.