## 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

future.

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

model.

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

future.

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

model.

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.

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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