Engineering Behaviour of Rocks Engineering Behaviour of Rocks Second Edition IAN FARMER LONDON NEW YORK CHAPMAN AND HALL First published 1968 by E. & F. N. Spon Ltd Second edition 1983 Published by Chapman and Hall Ltd 11 New Fetter Lane, London EC4P 4EE Published in the USA by Chapman and Hall 733 Third Avenue, New York NYlO017 © Ian W. Farmer Softcover reprint of the hardcover 1s t edition 1968 Phototypeset by Cots wold Typesetting Ltd, Gloucester ISBN-13: 978-94-009-5980-4 e-ISBN-13: 978-94-009-5978-1 DOl: 10.1 007/978-94-009-5978-1 This title is available in both hardbound and paperback editions. The paperback edition is sold subject to the condition that it shall not, by way of trade or otherwise, be lent, re-sold, hired out, or otherwise circulated without the publisher's prior consent in any form of binding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent purchaser. All rights reserved. No part of this book may be reprinted, or reproduced, or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or in any information storage and retrieval system, without permission in writing from the Publisher. British Library Cataloguing in Publication Data Farmer, I. W. Engineering behaviour of rocks. - 2nd ed. 1. Rock mechanics I. Title 624.1 '5132 TA706 Library of Congress Cataloging in Publication Data Farmer, I. W. (Ian William) Engineering behaviour of rocks. Rev. ed. of: Engineering properties of rocks. 1st ed. 1968. Bibliography: p. Includes index. 1. Rock mechanics. 2. Rocks-Testing. I. Title. TA706.F36 1983 624.1'5132 82-19931 Contents Preface vii CHAPTER 1 ENGINEERING DESCRIPTION OF ROCKS 1.1 Rock testing 3 1.2 Uniaxial or unconfined strength 7 1.3 Empirical field and laboratory tests 14 1.4 Porosity and permeability 18 1.5 Discontinuous rock 24 CHAPTER 2 STRESS AND STRAIN 2.1 Stress at a point 33 2.2 Pore pressure and effective stress 37 2.3 Strain at a point 42 2.4 Representation of stress and strain 44 2.5 Relation between stress and strain 45 2.6 Geostatic stresses 51 2.7 Measurement of in situ stress 54 CHAPTER 3 ROCK DEFORMATION 3.1 Rock tests in compression 59 3.2 Rock deformation in compression 65 3.3 Mechanics of micro fracture 69 3.4 Rock macrofracture 74 3.5 The complete rock deformation curve 77 vi Contents CHAPTER 4 ROCK STRENGTH AND YIELD 4.1 Rock strength criteria 81 4.2 Yield criteria 85 4.3 The critical state concept 89 4.4 Triaxial testing 94 4.5 Axial and volumetric strain data 97 4.6 The Hvorslev surface in rocks 111 CHAPTER 5 TIME DEPENDENCY 5.1 Creep strain 120 5.2 Phenomenological models of creep 125 5.3 Time-dependent deformation 128 5.4 Time-dependent strength reduction 131 5.5 Cyclic loading 135 5.6 Rapid loading 139 CHAPTER 6 DISCONTINUITIES IN ROCK MASSES 6.1 Discontinuity measurement 145 6.2 Discontinuity orientation data 148 6.3 Shear resistance of a rock containing a discontinuity 151 6.4 Shear resistance of a discontinuity 158 6.5 A critical state model for rock discontinuity strength 165 6.6 Measurement of discontinuity shear resistance 167 CHAPTER 7 BEHA VIOUR OF ROCK MASSES 7.1 Discontinuity frequency 169 7.2 Rock mass classification systems 172 7.3 Rock mass strength criterion 184 7.4 The relevance of rock mass strength 189 REFERENCES 193 AUTHOR INDEX 201 SUBJECT INDEX 204 Preface The first edition of this book was received more kindly than it deserved by some, and with some scepticism by others. It set out to present a simple, concise and reasonably comprehensive introduction to some of the theoretical and empirical criteria which may be used to define rock as a structural material. The objectives - reinforced by the change in title - remain the same, but the approach has been changed considerably and only one or two sections have been retained from the first edition. The particular aim in this edition is to provide a description of the mechanical behaviour of rocks, based firmly upon experimental data, which can be used to explain how rocks deform, fracture and yield, and to show how this knowledge can be used in design. The major emphasis is on the behaviour of rocks as materials, although in the later chapters the behaviour of discontinuities in rocks, and the way in which this can affect the behaviour of rock masses, is considered. If this edition is an improvement on the first edition it reflects the debt lowe to numerous people who have attempted to explain the rudiments of the subject to me. I should like to thank Peter Attewell and Roy Scott in particular. I should also like to thank Tony Price and Mike Gilbert whose work at Newcastle I have used shamelessly. I. W. Farmer Newcastle upon Tyne, September 1982 1 Engineering Description of Rocks Geologists recognize only one naturally occurring earth material called rock. Engineers differentiate between rocks and soils, although sometimes the dividing line is unclear. In particular, engineers differentiate between the reactions of rocks and soils to the forces imposed on them or in them by construction. The study of the reaction of soils to these forces is called soil mechanics and the study of the reaction of rocks is called rock mechanics. Both rocks and soils are made up of mineral and organic particles. In the former, the particles are generally bonded or cemented together and an initial yield resistance must be overcome before they shear in an unconfined state. Soils exhibit no real resistance to shear in an unconfined state, and a very small energy input is required to precipitate breakdown. The test behaviour of soils can often be quite closely related to their mineralogy. Although there are about 2000 minerals in the earth's crust, silicates make up about 99% ofthe total rock volume. The basic silicate structure is such that only one form - quartz - can easily resist the weathering processes to which rocks in the earth's environment are subjected. Most of the others, originally constituents of igneous rocks, are subject to chemical change during the weathering process to form clay minerals, mainly hydrous aluminium silicates produced by alteration of micas and feldspars. Thus sediments - the end product of the weathering process - can be viewed most simply as comprising two main materials, quartz and clay mineral. Quartz particles tend to be blocky and equidimensional and are generally larger than silt size (greater than 0.06 mm). A sediment made up mainly of quartz particles would be described as 1 2 Engineering Behaviour of Rocks silt, sand or gravel. Clay mineral particles tend to be small (less than 0.002 mm), flat and platy and a sediment containing a large proportion of these would be referred to as a clay. Sedimentary rocks have greater engineering interest because they contain the weaker rocks. They result mainly from the compaction and cementation of sediments, although other processes including recrystallization, replacement, differential solution and alteration may occur. These processes are described by the general term diagenesis and occur during changes in pressure and temperature as a bed of sediments is buried beneath later sediments. The major changes resulting from diagenesis may be summarized after Krumbein (1942) as: (a) Particle size - particularly in fine-grained sediments, may in crease through cementation, recrystallization or alteration. (b) Particle shapes - may become more or less rounded through solution or recrystallization. (c) Particle orientations - may alter during compaction or re crystallization; usually this will be controlled by particle shape and water content. (d) Porosity and permeability - will usually be reduced through compaction, cementation, solution and recrystallization. (e) Structure - will be changed as the material changes from a free flowing sediment to a more brittle solid. The mineralogical composition of a sediment may also change during diagenesis but, it is generally true to say that the mineralogy of sedimentary rocks is similar to that of the original sediments. Thus shales and mudstones comprise predominantly clay mineral, and sandstones and gritstones are usually a quartz aggregate cemented together with a carbonate or clay mineral matrix. Depending on the conditions of deposition, sandstones may of course have a high clay mineral content and shales a high quartz content. It is always important to define terminology correctly, and in the case of soils and rocks this is confused by the different concepts ofthe material and the mass. The material form - which is usually how the soil or rock is delivered to the laboratory - may comprise either discrete particles in the case of a soil, or an intact specimen held together by interparticle bonding in the case of a rock. This is the basic difference between rocks and soils as materials. A soil sample may be held together by suction or other forces but it is inherently a particulate system. The particles in a rock are cemented or bonded Engineering Description of Rocks 3 together - in other words a rock has real rather than apparent cohesion. The massive form of rock or soil differs radically from the material form. Soils are usually layered and their mechanical reactions and ability to transmit water vary from layer to layer. Laboratory test results can, however, usually be extrapolated, albeit with caution (see for instance Rowe 1968) to the mass and it is not necessarily incorrect to treat soil as a continuum. Rocks are often layered, but more importantly they are .fissured and jointed and this means that rock masses may sometimes be controlled more in their reaction to forces by the discrete nature of the fissured mass than by the properties ofthe material. Rock mechanics must therefore be defined as the study of rock deformation and fracture in both its intact material form and as a discontinuous mass. Nevertheless, through convention or otherwise, rocks are usually described for engineering purposes through their action as materials, and it is useful to start by considering some ofthe simple tests to which rocks are subjected and which can be used to define and compare their engineering reactions. 1.1 Rock testing Since the title of the first edition of this book was Engineering Properties of Rocks, it is important to start by qualifying the use of the word 'properties'. The Shorter Oxford English Dictionary defines a property as a 'characteristic quality of a person or thing'. It is therefore correct to refer to the physical make-up or to the mechanical reactions of rocks under test as properties. But it is also important to remember that the properties of rocks obtained under laboratory test conditions are related to the test conditions. For instance, a rock cylinder tested in uniaxially unconfined compression will behave differently if its length/diameter ratio is 0.3 than if its length/diameter ratio is 3 (see Fig. 1.1). There are therefore no fundamental mechanical properties of rocks in the sense of material constants characteristic of a particular rock. There are standard tests of various types which give useful indices of rock properties for comparison with other rocks tested under similar conditions. There are also some fundamental physical properties which include mineralogical composition and phase relationships,