Rock Mechanics on a Geological Base

495 Pages·1995·12.594 MB·1-498\495

by  R. Pusch (Eds.)

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Introduction
In my mind there is nothing more fascinating than watching a rocky landscape
and speculating over how it evolved from endogenic and exogenic processes, and
it becomes really exciting if there is a construction site around where one can find
out if and how the excavation design has been adapted to the local rock structure
that resulted from these processes. Some 30 years ago, when I got engaged in
rock engineering, such adaption was generally very poor and this is still often the
case as illustrated by the great surprise shown by designers and constructors in
the performance of certain recent big excavation projects when quite logically
appearing structures were met with. The structure of rock is by far the most
important parameter for assessing risk and cost in any rock construction project
and rock structure is therefore a major issue in the present book. It also deals with
other matters that are essential for the performance of rock in applied geology,
like the stress conditions, which are equally important at both shallow and large
depths, and the initiation of failure and disintegration at rock excavation.
In the years 1967-74 I served as lecturer in soil mechanics at the Chalmers
Technical University in Gothenburg, Sweden, and as consultant in engineering
geology to the Swedish company Jacobson & Widmark, which was major con-
sultant in a number of difficult rock excavation and foundation projects in south-
ern Sweden, and these two activities took place in the period when numerical
calculation methods were introduced in rock mechanics and when the importance
and meaning of discrete weaknesses in crystalline rock had been recognized.
Since then the development of numerical stress/strain calulation techniques has
been tremendous as documented by a large number of rock mechanical text-
books, while rather tittle has been made to understand and quantify the behavior
of discontinuities. A major idea in writing this book is therefore to deal with the
occurence and properties of structural features in rock.
Early 1978, a few years after my academic and professional careers in soil
mechanics and geology had brought me to the professor's chair in soil mechanics
Rock Mechanics on a Geological Base
2 Chapter 1
at the University of LuleL I became engaged in the work that had just been initi-
ated by the Swedish Nuclear Fuel and Waste Management Co (SKB), Stockholm,
for development of a technical solution of the problem of safe disposal of radio-
active waste from the Swedish nuclear reactors. This engagement has continued
through the years and a number of major findings have been collected in the book
"Waste Disposal in Rock" published by Elsevier some time ago. In preparing that
book it became obvious that rock mechanical aspects were so numerous and
important that they could not be covered by this first book. Instead, they fit in the
present one, which therefore serves to give the reader with tittle basic insight in
rock mechanics and excavation techniques the required background to appreciate
the possibilities and difficulties offered by rock for effective isolation of hazarde-
ous waste.
Now, for what category of readers is this book intended and what knowledge is
necessary in order to digest it? In my mind it is sufficient to have a basic educa-
tion in geology, hydrology or environmental sciences to profit from it, but a tech-
nical background as civil or construction engineer with some acquaintance with
methods for numerical calculation makes it easier to fully comprehend the con-
tent. A major aim is to use only little space for derivation of mathematical expres-
sions and lengthy calculations of stress and strain, and instead outline the
principles for solving problems related to them, and also to minimize or avoid
traditional and conventional geological issues, which are treated in numerous
textbooks that the interested reader will be referred to. Instead, focus is on a
number of practically very important matters, which deserve to be more thor-
oughly treated than in common literature, like characterization, visualization, and
performance of structural features. Many of them represent major current ideas,
results and findings evolved in the work in which the author and his collaborators
at the company Clay Technology AB, Lund, Sweden, have been involved.
Finally, I think that the fascinating but complex subject of rock mechanics on a
geological base still has so many undetected features that everyone dealing with
it should consider himself a humble student. This applies to the author of this
book as well as to many readers, which is the reason why the predicament "we"
is used throughout the book.
Rock Mechanics on a Geological Base
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Rock Nature
1-2 Geology
1.1-2 Why is geology important?
"Ask a geologist and you get an esoteric, impractical and academic answer: he
hides his lack of technical understanding by using descriptive terms of value only
to fellow geologists".
This sort of reasoning is very common among engineers and it is explained by
the fact that many geologists have an education that does not make them able to
fully understand technico/geological issues. The engineer should therefore have
an own insight in geology that is sufficient to take geological features into con-
sideration and to interpret the rock decription given by geologists.
From a strict technical viewpoint the composition of the rock material is not of
major importance, but it is essential that all the components that affect the physi-
cal and chemical properties of the rock mass are considered. Hence, why terms
like mica shale or amphibolite do not give valuable information per se on the
mechanical properties of a rock mass, the conventional petrographic description
gives indirect information on the mode of formation and thereby on the minerals
and fabric and consequently on the technically important properties, and it should
therefore be specified as a basis of any rock engineering project.
A matter that is of great importance for rock mechanical analysis, and that has
become increasingly importance in recent time, is that of structural discontinui-
ties, of which faults and fractures are wellknown examples. The definition of a
complete hierarchy of discontinuities of rock mechanical importance is a major
issue in the present book. Since experience shows that the most difficult condi-
tions for excavation work, particularly in underground projects, are related to the
Rock Mechanics on a Geological Base
4 Chapter 2
presence of clay seams and weathered zones in crystalline rock, we will examine
their origin, composition and performance in some detail in the book. Clay min-
erals, especially smectites, will therefore be in focus of the present chapter, and
the behavior of clay in rock examined rather deeply in forthcoming chapters on
strength and stability.
2-2 Mineralogy
2-2.1 Why are minerals and mineral compositions of impor-
tance?
The mineral composition, i.e. the representation of crystals of different atomic
constituents and strength, is of certain importance to the engineer. Geologists
often claim that it is necessary for the engineer to be able to recognize all the
individual minerals of a rock, but true need for this is usually only for five partic-
ular purposes, namely:
.1 For estimating the wearing of driilbits (richness in quartz)
2. For structural characterization of rock material with respect to presence of zones of
weak minerals (mica, chlorite bands)
3. For judging the sensitivity of a rock to chemical degradation by heat or dissolution
(sulphates, chlorides, feldspars), or to mechanical degradation at compaction of rock-
fill (richness in mica)
4. For identification of weathering that causes slaking or expansion on exposure to water
(clay minerals)
5. For estimating the mechanical properties of discontinuities that are commonly coated
or filled with minerals with special properties (chlorite, graphite and clay minerals)
The shape and size and arrangement of the individual mineral constituents are
fundamental microstructural parameters which, together with their physical prop-
erties determine the bulk physical properties of rock. We will see in the chapter
on rock strength that they control initiation of failure at critical stress constella-
tions. It is true that for most practical rock mechanical projects macroscopic
structural features, like fractures, are of greater interest because they cause the
wellknown scale-dependence of rock behavior, but this effect is largely control-
led by the coatings of the fracture surfaces or fillings in the fractures and the min-
erals that make up the coatings and fillings are therefore still important. The
Rock Mechanics on a Geological Base
Rock Nature 5
subject is in fact very complex and would have to occupy the entire book to be
given fair space. We need to confine ourselves here to specify and discuss those
minerals that are most important with respect to the five subjects listed above.
2-2.2 Mineral types
One commonly distinguishes between the six major groups of rock-forming min-
erals of different chemical composition, crystal lattice type and physical proper-
ties that are specified in Table 1 and Figure .1
Table 1 Common rock-forming minerals
Group Mineral Species Elements
Silicates (S) Quartz (Q) Si,O
Feldspars (F) Microcline O,K,1A,iS
Orthoclase O,K,1A,iS
Plagioclase ,aN,1A,iS Ca,O
Pyroxene (P) Si,Fe,Mg,O
Amphibole (A) Si,A1,Fe,Mg,O,OH
Mica (M) Muscovite Si,A1,K,O,OH
Biotite Si,A1,K,Mg,Fe, O,OH
Epidote (E) Si,AI,(Fe),Ca,O,OH
Chlorite (Ch) Si,A1,Mg,Fe,O,OH
Oxides (O) Magnetite Fe,O
Hematite Fe,O
Carbonates Calcite Ca, O
(C) Dolomite Ca,Mg,O
Sulphates (S) Gypsum Ca, S,O
Barite Ba, S,O
Chlorides )1C( Halite Na, 1C
Elements Graphite (G) C
Rock Mechanics on a Geological Base
Chapter 2
A A A
a = 8.4.~,
)
Pattern of iS atoms in ~ and ~ quartz,
projection on (0001)
Basic structural feature of aft feldspars projected
on (001). ehT stoat black circles are iS and AI; the
large ones .0 ehT 0 atoms projecting to right and
left form the means of linking this chain to neighbors.
2K
(b)
6o
3 Si + AI
o</ i T

(e)
| z / o-~i,"-..,
IA
/ ! '~\, .')
I i o'2.;/j
I ~ ~
~( I ~ r
< 5i IA
b axis
O-iS chains in pyroxenes and amphiboles Structure of muscovite, IA2K iS(4 )HO(02O)2IA6 .4
(a)(c)(b), the pyroxene chain as seen in Pyrophyllite layers with one aluminum substi-
plan, in elevation, and end on, respec- tuted for one out of four silicones in each
tively. (d)(c)(e), the amphibole chain tetrahedral layer, linked together by potassium
from the same three aspects. (After atoms in twelvefold coordination with oxygen.
Bragg & Claringbull).
Figure 1 Crystal structure of rock-forming minerals
Clay minerals are defined in Table 2 and their crystal constitution is illustrated
in Figure 2. Virtually all of them are phyllosilicates with a structure composed of
well-defined sheets of linked silica tetrahedra and aluminum/magnesium octahe-
dra. It is the combination of these sheets into various types of layers that distin-
Rock Mechanics on a Geological Base
Rock Nature 7
guishes the major clay mineral groups and gives them their characteristic
chemical and physical properties. Tetrahedral sheets (T) share the three basal
oxygens (or hydroxyls) with nearest neighbors, and the linked networks of the
trigonal basal units of Si tetrahedra produces a hexagonal framework. Octahedral
sheets (O) contain linked octahedral coordination groups in which divalent and
trivalent cations are enclosed by an octahedral network of oxygens and hydrox-
yls.
Table 2 specifies di-and trioctahedral species, of which the firstmentioned have
two out of three octahedral positions occupied by cations, while the lastmen-
tioned have all sites occupied. Another distinction is in the form of different pro-
portions of tetrahedral and octahedral layers: 1:1 means assymetric arrangement
of T and O in each lamella - also called sheet or flake - leading to regular group-
ing of T/O/T/O in rather thick particles with different surface properties of oppo-
site basal surfaces of the particles, while 1:2 implies symmetric T/Off crystals
with hydrate interlayers and thin particles. These differences give the particles
different charge distributions and cation exchange capacities, and thereby differ-
ent hydration properties and expandabilities.
There are in fact reasons to make further speciation of the smectites since
some of them react differently on stabilizing activities and carry names that may
be not be easily decoded by non-specialists. Table 3 lists common smectites and
their distinctive features.
Table 2 Clay minerals
Mineral group Crystal structure Crystal habit Clay properties
Kaolinite (K) 1:1, Dioctahedral Stacks of hexago- Silt-like behavior
nal flakes
Halloysite (Hy) 1:1, Dioctahedral Tubular Silt-like behavior
Vermiculite (V) 2:1, Trioctahedral Stacks of macro- Expansive
scopic crystals
Illite (I) 2:1, Dioctahedral Stacks of 10 A Slightly expansive
flakes
Smectite (S) 2:1, Di- or triocta- Stacks of 12-25 A Expansive
hedral flakes
Mixed-layer (I/S, Di-or trioctahedral Interlayered I/S or Expansive
K/S) K/S flakes
Rock Mechanics on a Geological Base
Chapter 2
<O
~ ,~, --?.. : . . . . .. 6(0.)
2(o.).~o
4.$i
6O
Lattice characteristics of the tnclinic
mineral kaolinite (Berry & Mason)
ETILLEDIEB
_AI .3 6
i + ,A
~~-"~-"~= IA "3 ~oF Sj .4 "".'," V ~M5
6 (o.)
"l" ...... ~ E T I N O L L I R O M T N O M
N~ ~ - - , , ~-''" si "4
Mg z" ROF AI 3~
( ~-"6 0
/ i b ax;s
H(oC 2 "~)0 r E T I N O R T N O N
Chlorite is a trioctahedral three-layer
p yro ph yllite-lik e structure.
~eF3 Brucite layers MgsAI(OH 21) serve as
.3 ROF iS ~4 ligands (Berry & Mason)
Three common monoclinic smectite
minerals. They have different cations in
tetrahedral and octahedral positions and
take up interlamellar cations like the
hydrated calcium ion ((Brindley &
MacEwan)
Figure 2 Crystal structure of clay minerals. Upper: Kaolinite. Right:
Chlorite (classified as rock-forming minerals by certain disci-
plines). Lower: Common smectite minerals
Rock Mechanics on a Geological Base
Rock Nature 9
All clay minerals have a large specific surface area, which - together with the
characteristic net negative lattice charge- yield physico/chemical properties that
are typical of colloidal systems. The charge distribution of each particle, which
consists of a number of flakes, is not uniformly distributed, however, which
means that at normal pH the edge charge is usually positive and the charge of the
flat surfaces negative. At low bulk densities this creates edge-to-face aggregation
with a void size distribution and gel strength that depend on the salt content of
the porewater. At high bulk densities the expansive clay minerals are more or less
in parallel arrangement and exert a swelling pressure on the surroundings
because of the physical nature of the interlamellar water and of the electrical dou-
ble-layers at the outer surfaces of stacks of flakes (Figure 3). This pressure is of
great practical importance and we will consider it in detail in the chapters dealing
with rock strength and stability.
Table 3 Smectite minerals
Expandability,
Smectite mineral Cations in T and O interlamellar
hydrate layers
Montmorillonite Si in ,T 1A and Mg in O Strong, 1-3
hydrate layers
Beidellite Si and 1A in ,T 1A in O Medium, 1-2
hydrate layers
Nontronite Si and 1A in ,T Fe in O Medium, 1-2
hydrate layers
Saponite Si and 1A in ,T Mg and Strong, 1-3
1A in O hydrate layers
Figure 3 Electron micrograph of stacks of montmorillonite lamellae
Rock Mechanics on a Geological Base
01 Chapter 2
2-2.3 Mineral identification
Rock-forming minerals
For many purposes it is sufficient to recognize quartz, feldspars, micas, chlorite,
clay minerals, carbonates, chlorides, sulphates, and graphite and this can be made
by use of their hardness, luster and color of macroscopic crystals. Table 4 serve
as a guide for identification of these minerals, but a basic course in mineralogy is
recommended to any student and engineer who wants to be acquainted with rock
mechanics for getting more substantial training in mineral diagnostics.
Diamond is the hardest species with the reference hardness .01 7 means that
scratching can be made with hard steel, 6 with ordinary steel, 3 with bronze, 2
with nails, while 1 is easily crushed between the finger nails.
Table 4 Characteristic properties of major rock-forming minerals
Relative Disintegration in
Mineral Color Luster
hardness water
Quartz (Q) None Oily None
Feldspars (F) Red, white Shiny None
Micas (M) Black, Shiny, None
white transp.
Chlorite (Ch) Black, Shiny, None
green transp.
Clay min. Strong
(CM)
Carbonates (C) White Shiny Weak
transp.
Chlorides )1C( White Shiny Strong
transp.
Sulphate (S) Yellow, Metallic Some
white
Graphite (G) Grey Shiny None
Rock Mechanics on a Geological Base