Geotechnology. An Introductory Text for Students and Engineers

352 Pages·1977·21.156 MB·English

by  A. Roberts (Auth.)

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G E O T E C H N O L O GY
An Introductory Text for
Students and Engineers
by
A. ROBERTS
Mackay School of Mines, The University of Nevada,
Reno, Nevada89507, U.S.A.
PERGAMON PRESS
OXFORD NEW YORK TORONTO SYDNEY PARIS FRANKFURT
U.K. Pergamon Press Ltd., Headington Hill Hall,
Oxford OX3 OBW, England
U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park,
Elmsford, New York 10523, U.S.A.
CANADA Pergamon of Canada Ltd., 75 The East Mall,
Toronto, Ontario, Canada
AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street,
Rushcutters Bay, N.S.W. 2011, Australia
FRANCE Pergamon Press SARL, 24 Rue des Ecoles,
75240 Paris, Cedex 05, France
WEST GERMANY Pergamon Press GmbH, 6242 Kronberg-Taunus,
Pferdstrasse 1, Frankfurt-am-Main, West Germany
Copyright© 1977 Pergamon Press Ltd.
All Rights Reserved. No part of this publication may be reproduced, stored in a
retrieval system or transmitted in any form or by any means: electronic, electrostatic,
magnetic tape, mechanical, photocopying, recording or otherwise, without
permission in writing from the publishers
First edition 1977
Library of Congress Cataloging in Publication Data
Roberts, Albert F 1911-
Geotechnology.
1. Engineering geology. I. Title.
TA705.R561976 624'.151 76-45440
ISBN 0-08-019602-0 Hard Cover
ISBN 0-08-021594-7 Flexi Cover
Printed in Great Britain by A. Wheat on & Co., Exeter
Preface
Geotechnology is applied geology in the context of engineering. It deals with engineering
in and on earth materials, and it is a prime concern of the geophysicist, the geological, the
mining, and the civil engineer. The author's interest in the subject stems from his training
and professional experience as a mining engineer, as a university lecturer both in engineer-
ing geology and in mining engineering, and as the director of a university postgraduate school
which, over the years in which it was active, made a significant contribution to the emergence
of rock mechanics as a science in its own right. As founder of the International Journal of
Rock Mechanics and Mining Sciences, and its Editor-in-Chief for the past decade, it has been
the author's privilege to have been in close contact with research workers, institutes, and
industrial concerns interested in geotechnology, all over the world. During this period, two
matters have become increasingly in evidence. First, a growing awareness, on the part of the
community-at-large, of the effects of engineering and technology on the human environment,
and second, a widening antipathy between many of those who are pursuing the development
of geotechnology, particularly rock mechanics, as an academic field of study and research,
and the engineers who are concerned with its practical application.
As to the first matter, there are signs that emphasis in the activities of the environmentalists
is changing from talk to action, as it must do if the environment is to be protected or improved.
But wise action is usually a matter of compromise, based on knowledge and understanding
of the issues involved. This is what the education and training of the engineer must fit him
to do, and if "environmental geology" is ever to be anything more than a name, then the
geologist must also be an engineer.
As to the second matter, it is true that the great bulk of published material in rockmechanics
consists of scientific papers written by experts for other experts in the subject. It is com-
paratively seldom that a paper with obvious practical application is published, although many
of the scientific papers have a practical application and it is unfortunate that the investigator
seldom takes it upon himself to show what that application might be. It is also necessary
to dispel the impression, sometimes given, that the geotechnical expert spends half his time
before the event telling the world what might happen, and another half of his time describing
what happened and how, after the event, leaving the site engineer to shoulder the responsi-
bility of attempting to deal with the event itself.
Those who are involved with the teaching of geotechnology at college level sometimes
may have, in one class, students whose background covers only part of the broad spectrum
of interests concerned. At one extreme there will be geologists with little or no knowledge
of the basic engineering sciences, such as materials science or applied mechanics. At the other
extreme there will be civil engineers, knowledgeable in structural design but to whom geology
has hitherto been a closed book. Another group may be geophysicists, specialized only in
a particular aspect of applied geology and physics, and in the middle may be geological and
mining engineers with a broader, albeit a somewhat superficial background, over a wider
range. There was a time when universities and senior institutes of learning were permitted
to be organized in such a way that these various groups could be taught separately, and some-
times even individually, in tutorial fashion, but those days, alas!, are almost gone. Now even
our universities must be geared to mass production.
XVI
Preface
Faced with such a situation the lecturer is posed a problem in how besttobring his diversified
audience to a common level of participation in his subject and to maintain their interest while
doing so. It is the purpose of the author, in this text, to pick up the various threads in geo-
physics, materials science, soil mechanics, rock mechanics, applied geology, civil and mining
engineering, and attempt to weave them into a coherent and interrelated fabric of common
interest.
The work is essentially an introductory text and it is suggested that it can form the basis
of one year's or two semesters' classwork for first-year students at university or engineering
college. Much of the content deals with fundamental matters, and with systems of instru-
mentation and measurement. It is intended that this will be followed by a second volume,
dealing with applications to specific problems in geotechnology, which will form the basis
for second-year student work. The two volumes will also be useful to practising engineers
who work with earth materials, but whose college studies did not include geotechnology
and who may hence have a need for a simplified review treatment of the subject, in relation
to their everyday professional duties. If the gap between theory and practice in rock mechanics,
for example, is to be bridged then the bridge must be built from both sides, the research spe-
cialist and the site engineer working together with a common aim.
The treatment throughout is descriptive and non-mathematical, and makes no assumptions
as to the reader's previous background of knowledge. It may therefore be read and under-
stood by the layman. At the same time the subject matter is pointed towards the numerous
sources of reference used in compiling the text, and details of these are given at the end of
each chapter. If, while he is reading this text, the serious student is also concurrently acquir-
ing a basic knowledge of engineering science and geology, he will be able to pick up these
references, together with the many more specialized texts that already exist in various aspects
of geotechnology. In this way he will be able, should he so desire, to develop his interest
in whatever branch of the subject he may choose to take up, in more advanced study.
xvii
CHAPTER 1
Engineering in Earth Materials
The Scope of Geotechnology
Geotechnology is a field of study and engineering in which the applied geologist, the civil
engineer, and the mining engineer have a common interest. It is only during the past 35 years
or so that the subject has been pursued systematically with the aim of developing it as a science
in its own right. Traditionally mining has always been regarded as an art rather than a science,
and while applied geology in the context of geophysics and geochemistry has made substantial
advances in analytical treatment, engineering geology has remained very largely descriptive in
its approach. Soil mechanics and foundation engineering emerged as a specialist field of ex-
pertise in the late 1930s, as a branch of civil engineering, to be followed more recently by
rock mechanics. The three allied fields, Engineering Geology (now more appropriately termed
Geological Engineering), Mining Engineering, and those aspects of Civil Engineering concerning
earthworks and tunneling, comprise what may now be termed Geotechnology: engineering
in earth materials.
Soil Mechanics
Soil mechanics is the science of unconsolidated material. It deals with the behavior of soils
consisting of discrete particles, with intervening voids or pore spaces, and with the interaction
of the particles and the pore fluids. It is the task of soil mechanics to predict the effect on
the soil of a given system of forces such as might be produced, for example, by the construction
of a heap of material standing at its own angle of repose, or controlled by the erection of
retaining walls designed to restrict the space occupied by the heap.
Soil mechanics is also applied in foundation engineering to provide a basis for rational
design of the foundations of engineering structures. The increasing size of bridges, dams,
tower blocks, etc., requiring deep and extensive foundations, brings civil and structural
engineers into more frequent contact with some earth materials which, in their undisturbed
state, are rocks rather than soils, and with other materials that are unquestionably rocks,
in any environment. Confidence in the engineering properties of material that conventional
methods of site exploration have indicated apparently to be solid, hard, rock has sometimes
been rudely shattered by the occurrence of failures of catastrophic proportions. These
include, during recent years, the Malpasset Dam (where the cause of failure was found to
lie in a material weakness of a key section of the rock foundations) and also the collapse
of large rock and earth masses (such as occurred during the Alaskan and Peruvian earthquakes)
or which may occur as the result of some change in the ground water characteristics (which
is thought to have been partly responsible for the Vaiont catastrophe). One result of these
and other occurrences of a like nature, has been that engineers previously concerned with
soils are now also giving their attention to rocks, and rock mechanics, or the study of the
engineering properties of rocks and rock masses, is now included with soil mechanics as the
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Geotechnology
fundamental basis of foundation engineering.
Rock Mechanics
Rock mechanics is the theoretical and applied science of the mechanical behavior of rocks,
as materials and in the mass. Interest in the science comes from many different directions,
from Geophysics in relation to seismic phenomena, from Structural Geology in relation to
earth structures and geotectonics, from Civil Engineering as a logical extension of soil mechanics
and in connection with tunneling and earthworks, from Applied Geology in the context of
geological engineering, and from Mining Engineering in relation to the control of ground move-
ment, the control and support of excavations, and in relation to techniques of rock and earth
excavation by mining equipment. To all these special interests must be added the general
growth of public concern at the despoliation of amenities by open-pit mining and the dis-
posal of surface waste materials, the periodic occurrence of dam failure, problems of slope
stability in large rock cuts and open-pits, the distress and damage caused by natural disasters
such as landslides and earthquakes, and the obvious need for more knowledge as to their
causes, prediction and limitation. Other factors include the increasing scope and scale of under-
ground civil and military engineering works. The advent of large diameter rock-boring machines
has brought a new dimension to tunneling operations in rock, in which rock mechanics must
take a leading role, both in relation to excavation and in support. In the 30 years that have
elapsed since the advent of atomic energy an entirely new technology of nuclear engineering
construction has evolved. In so doing it has promoted the use of conventional explosives,
on a scale never before contemplated, to rock excavation and geological engineering. Another
new technology, dealing with excavation and construction in frozen earth materials, is emerging
as the result of the economic development of Siberia, Alaska, and Northern Canada.
There is much going on in geotechnology today that should stimulate the interest and
imagination of high school and university students. Many of these young people are frequently
heard to express concern at the state of our society as they see it, with their desire to be
in some way, in the movement to promote change directed towards more idealized conditions.
Amongst these groups, none are more vociferous than those who wish to limit the pollution
and destruction of amenities that, all too often, can be seen to have resulted from uncontrolled
or careless industrialization and urban development. But while it is right that the social scien-
tists and the humanists should raise these questions, and prod the politicians into legislative
action where this is necessary, in the end it is the economist and the engineer who, between
them, will have to produce practicable solutions to the many problems that exist. It is the
aim of this book to introduce the reader to some of the engineering problems involved when
dealing with the soils and rocks that are part of mankind's natural environment.
The Classification of Earth Materials
At the onset we must define what it is that we are to discuss. By earth materials is meant
soils and rocks, and we may look at existing classification systems to see how they are listed
and described. One such listing can be made on the basis of the mode of formation. It des-
cribes the parent rock as being either igneous, sedimentary, or metamorphic in origin, and
the derived soil as the products of weathering, disintegration, and decomposition of the parent
rock, either residual at the parent site, or transported from that site by the processes of erosion:
wind, water, ice, and gravity.
Such a classification tells us nothing about the constituents of the materials concerned.
2
Engineering in Earth Materials
For this we must look at the mineralogical and petrological characteristics, on the basis of
which another classified subdivision may be made. This separates the igneous rocks into groups,
either acid or basic, depending upon their relative content of silica, the aluminium silicates,
and the ferromagnesian silicates. The same form of grouping has the sedimentary rocks as
being either sands, clays, or limestones, while the metamorphic rocks fall into two broad
classes: (a) quartzite and marble, both crystalline, massive, and non-foliated, and (b) the foli-
ated slates, phyllites, and schists. (The classification as given here is not complete, but the
reader will now have the broad outline of the system, and is referred to any introductory
text on geology for a detailed description.)
When we look at the soil products of weathering and erosion that are derived from our
new subdivision of parent rocks we see that while the acid constituent quartz breaks down
to form sands, and while the felspars and the ferromagnesian silicates break down to form
clay minerals, other factors come into play to determine the character of the residual
soil type. These factors are climate, topography, and time.
For example, an arid climate will allow decomposition products such as the carbonates
of calcium and magnesium to accumulate and so produce an alkaline soil, whereas the break-
down products of the same parent material in a wet and humid climate would be character-
ized by a concentration of iron and aluminium, the soluble calcium and magnesium products
being leached out, resulting in an acid soil. The same parent material in a hot climate would
produce the red latérite soil that is characteristic of tropical regions, but this would not happen
in a cold climate, where the siliceous breakdown products would accumulate, together with
the clay silicate minerals, to produce sticky soils of a predominantly blue-grey color. A cold
climate, too, would slow down the rate of decay of organic materials, and promote the accum-
ulation of mosses and peat.
Our classification has now extended beyond that possible on the basis of mode of occurr-
ence, to include mineral composition and some chemical and physical properties. From an
engineering viewpoint, however, we need also to know something about those characteristics
of the soil that will affect its engineering properties. Basically, what we must know is: how
will the soil behave when it is loaded by the weight of a structure erected upon it?, or how
will it respond to the gravitational loads imposed by its own mass, if, for example, we cut
a trench through it or build a soil embankment with it? Will the structure stand? Will the
sides of the trench and the slopes of the embankment remain stable?, or will they collapse?
Many factors must be explored before a reasonably assured answer may be given to such
questions. We must know something of how the soil compacts under the weight of an applied
load, how porous it is, and how permeable it is to water flow when compacted, and, since
the water content of a soil has a critical bearing upon its strength, what is its shear strength
and compressibility when compacted.
The Classification of Soils for Engineering Purposes
The engineering strength properties of an earth material are not always constant. It is poss-
ible for a material which under a given set of circumstances may be solid, to be changed, in
a very short space of time, into a fluid mass, simply because during that short time interval
one or more of the basic controlling factors has changed in order of magnitude. The factor
that is most often charged with the responsibility for such transformations is the pore fluid
pressure within the material, but there are also other influences. It could be, for example
the presence of a particular type of mineral material, combined with the effect of dynamic
3
Geotechnology
shock, such as could result from earthquake, or in mining technology, a rock burst, which is
the sudden fracture of a large rock mass, or it could be the result of blasting.
Consistency Limits
The critically controlling influence of pore fluid pressure can be included in a soils classi-
fication in terms of the consistency limits, in which four states of consistency are recognized:
(1) liquid, (2) plastic, (3) semi-solid, and (4) solid.
Liquid limit
Moisture in an unconsolidated earth material, in a limited amount, promotes intergranular
cohesion, but at the same time it builds up the pore pressure and, in a greater amount, it may
turn the whole mass into a fluid. Such a fluidized mass has no shear strength, that is, it cannot
resist deformation. If, however, the water content is reduced and the material is dried out. a
point will be reached when the granular mass begins to exert resistance against the change of
shape produced by an applied force, and if that force is removed the mass is seen to have
suffered permanent deformation. It is then acting as a plastic solid. The moisture content
at which the mass ceases behaving as a liquid and starts to behave as a plastic is termed its
liquid limit (LL).
Plastic limit
With further drying the mass becomes a harder material, and its shear strength increases,
ultimately until the material displays brittle failure characteristics in response to increase of
imposed load. The moisture content at the point where the material ceases to display plasticity
and begins to behave as a brittle solid is the plastic limit (PL) and the range of water contents
over which the material is in the plastic state is called its plasticity index.
The Unified Soil Classification
The Unified Soil Classification classifies soils on the basis of the factors texture,
and liquid limits (see Table 1.1). The system is comprised of fifteen soil groups, each identified
by a two-letter symbol. Soils are classified in terms of particle size, coarse-grained soils being
sands and gravels, while fine-grained soils are silts and clays. Gravel is defined as having a parti-
culategrain size ranging from 76.2 mm to 4.76 mm, and sand from 4.76 mm to No.200 sieve
size, while clay and silt have a component grain size less than 200 sieve, which is about the
minimum individual grain size recognizable by the unaided human eye.
The first letter in each two-letter symbol indicates into which of these soil types a given
material belongs. The second letter indicates the general gradation and plasticity of the soil,
clay being more plastic than silt. Thus, W represents clean, well-graded, materials with a regular
grain size, while P represents clean, but poorly graded, materials. M represents fine materials
of a silty character and C represents clay fines. The three types of fine-grained soils — inorganic
silts, inorganic clays, and inorganic silts/clays — are further subdivided in terms of their liquid
limits. L classifies soils with LL less than 50, having low to medium compressibility and plasti-
city. H indicates materials with LL greater than 50 and comprises soils of high compressibility
and plasticity.
The great merit of the Unified Soil Classification is its flexibility and its adaptability to
cover materials of intermediate characteristics between the classified groups, for which purpose
4
Engineering in Earth Materials
TABLE 1.1. The Uni fled Soil Classification System
Major divisions Group Typical names
Symbols
Coarse- Gravels Clean GW Well-graded gravels, gravel-sand
grained (More than mixtures, little or no fines
soils half of gravels GP Poorly graded gravels, gravel-
(More than coarse fraction sand mixtures, little or no
half the above No. 4 fines
material above sieve size)
Gravels GM Silty gravels, poorly graded
No. 200 sieve with gravel-sand—silt mixtures
size) fines GC Clayey gravels, poorly graded
gravel-sand-clay mixtures
Sands Clean SW Well-graded sands, gravelly
(More than sands, little or no fines
half of sands SP Poorly graded sands, gravelly
coarse fraction sands, little or no fines
less than No. 4 Sands SM Silty sands,
sieve size with sand—silt mixtures
fines SC Clayey sands
sand—clay mixtures
Fine- Silts and clays ML Inorganic silts and very fine
grained sands, rock flour, silty or
soils clayey fine sands with
(More than slight plasticity
half the (Liquid limit less than 50) CL Inorganic clays of low to
material less medium plasticity, gravelly
than No. 200 clays, sandy clays, silty
sieve size) clays, lean clays
OL Organic silts and organic
silt-clays of low plasticity
Silts and clays MH Inorganic silts, micaceous or
diatomaceous fine sandy
or silty soils, elastic silts
(Liquid limit greater than 50) CH Inorganic clays of high
plasticity, fat clays
OH Organic clays of medium
to high plasticity
Highly organic soils PT Peat and other highly
organic soils
Boundary classifications: Soils possessing characteristics of two groups are designated
by combinations of group symbols.
For example, GW-GC: well-graded gravel-sand mixture with clay binder.
dual symbols may be used. When used in reports to describe materials encountered in the field,
say in a borehole or a trench section, it is more meaningful than is descriptive nomenclature
alone, since it conveys information about some of the essential factors that are important
from an engineering viewpoint. It enables the engineer to make a preliminary assessment of
the suitability of the soil for the engineering project concerned, before a detailed site investi-
gation is commissioned.
5