Advanced Geotechnical Analyses

Advanced Geotechnical Analyses

Geotechnical engineers have to deal with complex geometrical configurations as well as
enormously difficult materials which exhibit, strongly, a path-dependent mechanical
behavior. In addition, geological deposits display extensive inhomogeneities which are
often difficult to define quantitatively. As a result most geotechnical engineering design
problems require significant use of the engineer’s imagination, creativity, judgment,
common sense and experience. To many geotechnical engineers therefore the role of any
advanced analysis, particularly advanced computer based analyses, remains undefined.
The editors have therefore invited some outstanding engineers who are engaged not only
in developing advanced level geotechnical analyses, but are also in consulting practice to
write various chapters of this book. These chapters show that a careful blend of
engineering judgment and advanced principles of engineering mechanics may be used to
resolve many complex geotechnical engineering problems. It is hoped that these may
inspire geotechnical engineering practice to make more extensive use of them in the

Because of the difficulties associated with complex geometries and material behavior
it is not surprising that the advanced analyses described in this book make extensive use
of modern digital computers. Simplified hand calculations, although they have the
attraction of being very good teaching tools, are rarely able to quantitatively reproduce
the complete physical characteristics of the problem.
Chapter 1 deals with the complex interactions between fluid and solid skeletons for
both static and dynamic loading. The governing equations for the solid and fluid
constituents have been set out in a general manner and a nonlinear transient finite element
formulation for the problem developed. A centrifuge model test of a dike is then
simulated by the analysis, and the success of the developed analysis was demonstrated by
the ability of the analytical model to reproduce the physical observations in the centrifuge

The mechanical behaviour of saturated geomaterials in general, and of soils in particular,
is governed largely by the interaction of their solid skeleton with the fluid, generally
water, present in the pore structure. This interaction is particularly strong in dynamic
problems and may lead to a catastrophic softening of the material known as liquefaction
which frequently occurs under earthquake loading.

The two phase behaviour just described allows the solution of many problems of
practical interest, but is not adequate in others where semi-saturated conditions exist. In
particular, if negative fluid pressures develop, dissolved air is released from the fluid or
simply enters into the mixture via the boundaries and thus both air and water fill the
voids. Indeed it is this semi-saturated state that permits the negative pressures to be
maintained through the mechanism of capillary forces. Such negative pressures provide a

certain amount of ‘cohesion’ in otherwise cohesionless, granular matter and are necessary
to account for realistic behaviour of only partly saturated embankments 
under dynamic forces.

The saturated behaviour is fundamental and, though understood in principle for some
considerable time, can only be predicted quantitatively by elaborate numerical
computations, which fortunately today is possible due to the developments of powerful
computers. It is the aim of this chapter to present a full account of the development of
such numerical procedures and to extend such formulations to problems of semi-saturated
behaviour with a simplifying assumption concerning the air flow. The results of the
computations are validated by comparison with model experiments. Such validation is of
course essential to convince the sceptics and indeed to show that all stages of the
mathematical modelling are possible today. It is necessary to generate a predictive
capacity which in general, due to the scale of the phenomena, cannot be accurately tested

in the laboratory.


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