Geotechnical Modelling

David Muir Wood

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9780419237303
• eBook ISBN: 9781351990042
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

Modelling forms an implicit part of all engineering design but many engineers engage in modelling without consciously considering the nature, validity and consequences of the supporting assumptions.

Derived from courses given to postgraduate and final year undergraduate MEng students, this book presents some of the models that form a part of the typical undergraduate geotechnical curriculum and describes some of the aspects of soil behaviour which contribute to the challenge of geotechnical modelling.

Assuming a familiarity with basic soil mechanics and traditional methods of geotechnical design, this book is a valuable tool for students of geotechnical and structural and civil engineering as well as also being useful to practising engineers involved in the specification of numerical or physical geotechnical modelling.

1

Introduction to modelling

1.1 Introduction

In the same way that we may be surprised to find that prose is what we have been speaking all our lives, so scientists and engineers are often unaware that almost everything that they do is concerned with modelling. This book is concerned with the application of principles of modelling to soil mechanics and geotechnical engineering.

A model is an appropriate simplification of reality. The skill in modelling is to spot the appropriate level of simplification—to recognise those features which are important and those which are unimportant. Very often engineers are unaware of the simplifications that they have made and problems may arise precisely because the assumptions that have been made are inappropriate in a particular application.

Engineering is fundamentally concerned with modelling. Engineering is concerned with finding solutions to real problems—we cannot simply look around until we find problems that we think we can solve. We need to be able to see through to the essence of the problem and identify the key features which need to be modelled—which is to say those features of which we need to take account and include in the design. One aspect of engineering judgement is the identification of those features which we believe it safe to ignore.

In this chapter the theme of modelling is introduced by reference to modelling activities that are familiar from early and standard courses in soil mechanics and geotechnical engineering within degree programmes in civil engineering and which form the basis for the development of geotechnical design. The scope of subsequent chapters of the book is then defined.

1.2 Empirical models

Although the preference in this book is for models which have a sound analytical or theoretical basis there is a long history of empirical modelling in geotechnical engineering. The dictionary tells us that empiricism rests solely on experience and rejects all prior knowledge (and defines an empiric as a ‘quack’). Precisely because soils are tricky materials to deal with, a lot of geotechnical engineering has had to be based on experience—because the more rigorous modelling tools have tended to lag behind the demands of the industry. Many of the techniques have been semi-empirical rather than purely empirical. A few examples are given here. It may be objected that these are empirical procedures rather than empirical models: the distinction is somewhat semantic. The key is that these procedures have been found to provide satisfactory answers even though the logical thread cannot always be continuously traced. (The prescription of many medicines is based on knowledge that they work without necessarily being able to state exactly why they work.)

Figure 1.1 Bearing capacity of shallow foundation on clay

1.2.1 Vane strength correction

Much of geotechnical design has hitherto relied upon ultimate limit state calculations which are driven by estimates of soil strength hoping, thus, to guard against geotechnical collapse. (Classically, a factored design based on an ultimate limit state calculation, with the factor chosen from experience, might be used to guarantee satisfactory serviceability without performing a separate serviceability calculation.) Thus the ultimate bearing capacity (ζ_u of a footing on a clay of undrained strength c_u might classically be written as

ζu=Nccu+ζs⁢(1.1)

where N_c is a so called bearing capacity factor and ζ_s is the surcharge on the surface of the clay at the level of the foundation—which might simply be due to the weight of overburden at this level (Fig 1.1). Then, if we can find values of undrained strength, we can estimate capacities of footings; similar calculations can be performed for other classes of geotechnical structure, such as embankments and excavations.

Given a strength model it needs to be populated with values of soil strength determined from laboratory or in-situ testing. Most of the widely used strength models lack the subtleties of, for example, rate effects and anisotropy with which the ground itself is certainly familiar. Particular tests measure soil strengths in particular ways: if short term undrained strength of clay is of concern then the in-situ vane is commonly used to estimate the undrained strength. The vane (Fig 1.2) measures a mixed strength, combining shearing on horizontal and vertical surfaces in the soil. A strength model is required to extract an estimate of undrained soil strength from the actual measurement of the torque required to rotate the vane and hence to generate a failure mechanism through the clay. A simple assumption of uniform soil strength on all surfaces of the failing block of clay of height h and diameter d indicates that the torque T is given by

Figure 1.2 Shear vane

Figure 1.3 Shear strength mobilised along slip surface in clay

T=16πcud3(1+3hd)⁢(1.2)

Any actual failure mechanism of the geotechnical structure will require the clay to shear along surfaces having completely different alignments (Fig 1.3).

The vane measures the strength in a matter of seconds—in practice a geotechnical structure may take weeks or months to complete and yet the permeability of the ground may be sufficiently low for the behaviour still to be described as undrained.

Comparisons of estimates of failure conditions (or margin of safety against collapse) of embankments and excavations in soft clay (Bjerrum, 1972) indicate that a ‘correction’ factor, μ, dependent on soil plasticity (section §1.8) must be applied to the strength emerging from the vane test (Fig 1.4). That is:

Figure 1.4 Correction factor for ...

Table of contents

• Cover
• Half title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1 Introduction to modelling
• 2 Characteristics of soil behaviour
• 3 Constitutive modelling
• 4 Numerical modelling
• 5 Physical modelling
• 6 Centrifuge modelling
• 7 Theoretical modelling
• 8 Soil-structure interaction
• 9 Envoi
• Bibliography
• Index