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Lecture_Rock_1.pdf
1. 1
ROCK MECHANICS: INTRODUCTION
Prof. K. G. Sharma
Department of Civil Engineering
Indian Institute of Technology Delhi New Delhi India
Indian Institute of Technology Delhi, New Delhi, India
What is Rock Mechanics?
Introduction
What is Rock Mechanics?
Rock mechanics is a
discipline that uses the
principles of mechanics to
d ib th b h i f
describe the behaviour of
rock of engineering scale.
ISRM
2
2. 2
Why is Rock Mechanics Special?
Introduction
Why is Rock Mechanics Special?
Rock at engineering scale is Discontinuous,
Inhomogeneous, Anisotropic, and Non-linearly
Elastic, Plastic
Rock mechanics deals
with the response of
rock when the
boundary conditions
3
are disturbed by
engineering.
Origin of Rock
Rock Formation
Origin of Rock
Rock is a natural solid substance composed of
minerals.
R k f d b th i i i k
Rocks are formed by three origins: igneous rocks
from magma, sedimentary rock from sediments
lithification and metamorphic rocks through
metamorphism, as illustrated by the rock cycle.
4
4. 4
Igneous Rocks
Rock Formation
Igneous Rocks
Igneous rocks are formed when molten rock
(magma) cools and solidifies, with or without
crystallization.
They can be formed (i) below the surface as
intrusive (plutonic) rocks, or (ii) on the surface as
extrusive (volcanic) rocks. Intrusive is generally
coarse grained and extrusive fine grained.
7
They can also have different mineral contents.
Rock Formation
Granitic Andesitic Basaltic Ultramafic
Granitic
(acid) (felsic)
Andesitic
(intermediate)
Basaltic
(basic) (mafic)
Ultramafic
(ultrabasic)
Intrusive
(coarse grain) Granite Diorite Gabbro Peridotite
Extrusive
(fine grain) Rhyolite Andesite Basalt None
Silica Content >65% Silica 50-65% Silica 40-50% Silica <40% Silica
Main Mineral
Composition
Quartz
Orthoclase
N-Plagioclase
Amphibole
Plagioclase
Biotite
Ca-Plagioclase
Pyroxene
Olivine
Pyroxene
Minor Mineral
Composition
Muscovite
Biotite Pyroxene Olivine
Amphibole
Ca-Plagioclase
8
Composition
Biotite
Amphibole
Pyroxene
Amphibole
Ca Plagioclase
Colour Light Dark
5. 5
Sedimentary Rocks
Rock Formation
Sedimentary Rocks
Sedimentary rock is formed in three main ways:
(i) deposition of the weathered remains of other
rocks (known as 'clastic' sedimentary rocks);
(ii) d iti f th lt f bi i ti it d
(ii) deposition of the results of biogenic activity; and
(iii) precipitation from solution.
Clastic sedimentary rocks are commonly classified
by grain size.
9
Rock Formation
Particle size Comments Rock name
Particle size Comments Rock name
> 2 mm Rounded rock fragment Conglomerate
Angular rock fragment Breccia
1/16 - 2 mm Quartz with other minerals Sandstone
< 1/16 mm Split into thin layers Shale
< 1/16 mm Split into thin layers Shale
Break into clumps or blocks Mudstone
10
6. 6
Metamorphic Rocks
Rock Formation
Metamorphic Rocks
Metamorphic rock is a new rock
transformed from an existing rock,
through metamorphism – change due
t h t d
to heat and pressure.
Metamorphic rocks can have foliated
and non-foliated textures. Foliation is
due to the re-orientation of mica
11
minerals, creating a plane of cleavage
or visible mineral alignment feature.
Rock Formation
Rock Texture Metamorphic grade
Original parent
Rock Texture Metamorphic grade
rock
Slate Foliated Low grade
Shale (clay
minerals)
Phyllite Foliated
Low to intermediate
grade
Shale
Mica schist Foliated
Low to intermediate
grade
Shale
grade
Chlorite
schist
Foliated Low grade Basalt
Gneiss Foliated High grade
Granite, shale,
andesite
Marble Non-foliated Low to high grade
Limestone,
dolomite
12
Quartzite Non-foliated
Intermediate to high
grade
Quartz sandstone
7. 7
Rock Formation
Rock material strength is a structural strength of the
composition of the minerals. It is governed by
(i) the strength of the minerals, and
(ii) the structural bonding (integration) of the
i l
minerals.
13
Rock Joints
Rock Discontinuities
Rock Joints
Joints are the most common rock discontinuity.
They are normally in parallel sets.
Th ll id d t f th k
They are generally considered as part of the rock
mass. The spacing of joints is usually in the order of
a few to a few ten centimetres. For engineering,
joints are constant features of the rock mass.
14
8. 8
Rock Discontinuities
15
Faults
Rock Discontinuities
Faults
Faults are planar rock fractures which show
evidence of relative movement. Faults have different
scale and the largest faults are at tectonic plate
b d i F lt ll d t i t f
boundaries. Faults usually do not consist of a
single, clean fracture, they often form fault zones.
Large scale fault, fault zone and shear zone, are
large and localised feature. They are often dealt
16
separately from the rock mass.
9. 9
Faults
• Fractures that have been displaced:
Shea Displacement Fa lt Thickness
Shear Displacement, Fault Thickness
– Most faults are inclined at an angle
measured from horizontal
• Dip angle of the fault
• Dip angle of the fault
• Two blocks defined:
Folds
Rock Discontinuities
Folds
Fold is the bended originally flat and planar rock
strata, as a result of tectonic force or movement.
F ld ll t id d t f th k
Folds are usually not considered as part of the rock
mass. They are often associated with high degree of
fracturing and relatively weak and soft rocks.
Anticline Syncline
18
10. 10
Bedding Planes
Rock Discontinuities
Bedding Planes
Bedding plane is the interface between sedimentary
rock layers.
B ddi l i l t d l i l f t t
Bedding planes are isolated geological features to
engineering activities. It mainly creates an interface
of two rock materials. However, some bedding
planes could also become potential weathered
zones and groundwater pockets.
20
11. 11
21
Engineering Scale of Rock
Rock Material and Rock Mass
Engineering Scale of Rock
For civil engineering works, e.g., foundations,
slopes and tunnels, the scale of projects is usually a
few tens to a few hundreds metres.
Rock in an engineering scale is generally a mass of
rock at the site. This mass of rock, often termed as
rock mass, is the whole body of the rock in situ,
consists of intact rock blocks and all types of
22
discontinuities (joints, faults etc).
12. 12
A tunnel of 12 m diameter.
A borehole 10 cm.
23
An excavated quarry slope of about 30 m high.
Composition of Rock Mass
Rock Material and Rock Mass
Composition of Rock Mass
A rock mass contains (i) rock material, in the form of
intact rock blocks of various sizes, and (ii) rock
discontinuities that cuts through the rock, in the
f f f t j i t f lt b ddi l
forms of fractures, joints, faults, bedding planes,
and dykes.
Rock mass = Rock materials + Rock discontinuities
24
13. 13
Discontinuities
25
Rock material
Roles of Rock Joints in Rock Mass Behaviour
Rock Material and Rock Mass
Roles of Rock Joints in Rock Mass Behaviour
• Cuts rock into slabs, blocks and wedges, to be
free to fall and move;
• Acts as weak planes for sliding and moving;
P id t fl h l d t fl
• Provides water flow channels and creates flow
networks;
• Gives large deformation;
• Alters stress distribution and orientation;
26
Rock mass behaviour is largely governed by joints.
14. 14
Inhomogeneity of Rock Material
Inhomogeneity and Anisotropy
Inhomogeneity of Rock Material
Inhomogeneity represents property varying with
locations. Many construction materials have
varying degrees of inhomogeneity. Rock is formed
b t d hibit t i h it d t
by nature and exhibits great inhomogeneity, due to:
(i) different minerals in a rock,
(ii) different bounding between minerals,
(iii) existence of pores,
27
(iv) existence of microcracks.
Inhomogeneity of Rock Material
Inhomogeneity and Anisotropy
Inhomogeneity of Rock Material
Inhomogeneity is the cause of fracture initiation
leading to the failure of a rock material. If some
elements in the rock material matrix are very weak,
th ill t t t f il l d ll l d t l
they will start to fail early and usually lead to low
overall strength of the rock material.
28
15. 15
Inhomogeneity of Rock Mass
Inhomogeneity and Anisotropy
Inhomogeneity of Rock Mass
Inhomogeneity of a rock
mass is primarily due to the
existence of the various
di ti iti
discontinuities.
Rock masses are also
inhomogeneous due to the
mix of rock types,
29
interbedding and intrusion.
Anisotropy
Inhomogeneity and Anisotropy
Anisotropy
Anisotropy is defined as
properties are different in
different direction. It occurs
i b th k t i l d
in both rock materials and
rock mass.
Rock with obvious anisotropy
is slate. Metamorphic phyllite
30
and schist and sedimentary
shale also exhibit anisotropy.
16. 16
Anisotropy
Inhomogeneity and Anisotropy
Anisotropy
Rock mass anisotropy is
controlled by
(i) joint set, and
(ii) di t l
(ii) sedimentary layer.
31
Vertical Stress and Overburden
In Situ Stresses
Vertical Stress and Overburden
At depth, vertical stress in rock is the
overburden stress generated by
weight of the overlying material.
z
The average specific gravity of rocks
is about 2.7. The vertical stress at
depth can be estimated as
z
σv
σH
32
σv (MPa) ≈ 0.027 z (m)
σH
σh
17. 17
Horizontal Stress and Tectonic Stress
In Situ Stresses
Horizontal Stress and Tectonic Stress
Horizontal stresses in rock are primarily tectonic
stress.
H i t l t i k ll hi h
Horizontal stresses in rocks are generally higher
than vertical stress. The maximum horizontal stress
is usually in the same directions as tectonic
convergence movement. Tectonic stress has huge
variations in magnitude, and can be exceptionally
33
large.
In situ stress field can also
In Situ Stresses
In situ stress field can also
be altered by geological
factors and processes:
• Surface topography
E i z
• Erosion
• Intrusion
• Fault and faulting
z
σv
σH
34
σh
18. 18
In situ Stress Measurements
In Situ Stresses
In situ stress measurements show that vertical
stress is about 0.027z, the overburden pressure.
Ratio of average horizontal stresses (σH+σh)/2 to
vertical stress is between 0.5 to 3.0, mostly bounded
between (100/z +0.3) and (1500/z +0.5).
At common depth for civil engineering (<1000 m),
the variation of horizontal stress is wide.
35
the variation of horizontal stress is wide.
In Situ Stresses
36
19. 19
In rock, one horizontal stress is usually the major
principal stress, while the vertical stress or the
other horizontal stress represents the minor
In Situ Stresses
other horizontal stress represents the minor
principal stress, i.e.,
σH > σh > σv or σH > σv > σh
Vertical stress can be estimated from overburden.
Horizontal stresses should not be estimated If
Horizontal stresses should not be estimated. If
horizontal stress directions and magnitudes are
needed, in situ stress measurements must be
conducted.
σv = γz
37
σv > σH > σh in normal fault area
σH > σh > σv in thrust fault area
σH > σv > σh in strike-slip fault area
Effective Stress
In Situ Stresses
Effective Stress
In porous material, e.g., sandstone, effective stress
may be computed as total stress – pore pressure.
I f t d k
Pore
water
In fractured rock mass,
distribution of water is no
longer even and stress field
is no longer uniform. Hence,
the effective stress principle
38
p p
is no longer applicable.
20. 20
Re-distribution of Stress
In Situ Stresses
Re distribution of Stress
Rock engineering is an activity disturbing the
original stress field which is already in equilibrium.
Rock mechanics deals with stress re-distribution
d di t ib t d t d th h t t
and redistributed stresses, and the short term
response of rock during stress re-distribution and
long term behaviour in the redistributed stress field.
39
σV
σV
σV
σV
40
21. 21
Flow in Rock Material
Ground Water
Flow in Rock Material
Most of the igneous and metamorphic rocks are
very dense with interlocked texture. The rocks
therefore have extremely low permeability and
it S l ti di t k t i ll
porosity. Some clastic sedimentary rocks, typically
sandstones, can be porous and permeable.
Weathered rocks can also be porous and
permeable.
41
Flow in Fracture Network
Ground Water
Flow in Fracture Network
Rock masses are fractured. Fractures provide flow
paths and flow is governed by the apertures.
Fl i f t d k i l t ll d b
Flow in a fractured rock mass is also controlled by
the connectivity of fracture system or network.
Although a rock mass can be seen as highly
fractured, only a limited percentage of fractured are
interconnected and conduct flow. At site it is
42
common to see only a few fractured has water flow
while others are dry.
22. 22
Ground Water
43
Effects of Groundwater and Pressure
Ground Water
Effects of Groundwater and Pressure
Groundwater is important to rock mechanics:
(i) Water pressure contributes to the stress field;
(ii) W t h k t f i ti
(ii) Water changes rock parameters, e.g., friction;
(iii) When water is present, it increases the
complexity of rock engineering, e.g., more
difficult to tunnel with water inflow and high
water pressure.
44
23. 23
Weathering and Weathered Rocks
Special Rocks
Weathering and Weathered Rocks
All rocks disintegrate slowly as a result of:
(i) Mechanical weathering, breakdown of rock into
ti l ith t h i h i l iti
particles without changing chemical composition
of the minerals in the rock.
(ii) Chemical weathering, breakdown of rock by
chemical reaction, primarily by water and air.
45
Special Rocks
Fresh granite
46
Weathered granite
24. 24
Weathered Rock
Special Rocks
Weathered Rock
Weathering is progressive, between fresh rock and
completed material (soil), rocks can be slightly,
moderately and highly weathered. Those weathered
k till i t t d h t t d t t
rocks are still intact and have structure and texture
as rock. However, due to weathering, their
properties have been affected and altered.
Weathering causes significant reduction of rock
47
material strength.
Soft Rocks and Hard Soils
Special Rocks
Soft Rocks and Hard Soils
Sedimentary rocks are formed by sediments (soils)
through long processes of compaction and
cementation. The process could be stopped before
th di t b i l t l lidifi d Th
the sediments are being completely solidified. The
materials then could be highly consolidated but not
fully solidified. Typically, those materials have low
strength and high deformability, and when placed in
water, they often can be dissolved. When dry, they
48
behave as weak rock and when in water, it
collapses.
25. 25
Swelling Rock
Special Rocks
Swelling Rock
Some rocks have the characteristics of swelling,
that is when the rock is exposed with water (directly
in contact with water or in air), it expanse. This is
i il d th lli b h i f th i l
primarily due the swelling behaviour of the minerals
of the rock, typically the montmorillonite clay
mineral. Rock and soil containing considerable
amount of montmorillonite minerals will exhibit
swelling and shrinkage characteristics.
49 50
26. 26
Crushed Rock
Special Rocks
Crushed Rock
Characteristics of highly fractured and crushed
rocks are quite different from the massive rock
mass. They behave as granular and block materials,
d di th t d f i ti Wh h
depending on the geometry and friction. When such
materials are encountered in engineering, they need
to be addresses separately.
51
Special Rocks
52
27. 27
R k Di ti iti
Rock Mass Behaviour
Rock
ROCK MASS
Discontinuities
BEHAVIOUR
Ground Water
Some of the types of structures on, in or of rock
(after Brown, 1993)
Field of Application Types of structures on, in or of rock
Mi i S f i i l t bilit k
Mining • Surface mining- slope stability; rock mass
diggability; drilling and blasting;
fragmentation.
• Underground mining- shaft, pillar, draft and
stope design; drilling and
• blasting; fragmentation; cavability of rock
g g y
and ore; amelioration of rockbursts;
mechanized excavation; in situ recovery.
Energy Development Underground power stations (hydroelectric and
nuclear); underground storage of oil and gas;
energy storage (pumped storage or
d i t ) d f d ti
21 August 2017 54
compressed air storage); dam foundations;
pressure tunnels; underground repositories for
nuclear waste disposal; geothermal energy
exploitation; petroleum development including
drilling, hydraulic fracturing, wellbore stability.
28. 28
Some of the types of structures on, in or of rock
Field of Application Types of structures on, in or of rock
Transportation Highway and railway slopes, tunnels and
bridge foundations; canals and
bridge foundations; canals and
waterways; urban rapid transport tunnels and
stations; pipelines.
Utilities Dam foundations; stability of reservoir slopes;
water supply tunnels; sanitation
tunnels; industrial and municipal waste
tunnels; industrial and municipal waste
treatment plants; underground
storages and sporting and cultural facilities;
foundations of surface power stations.
Building Construction Foundations; stability of deep open
excavations; underground or earth sheltered
21 August 2017 55
homes and offices.
Military Large underground chambers for civil
defense and military installations; uses of
nuclear explosives; deep basing of strategic
missiles.
Rock versus Other Materials
• Rock differs from other engineering materials:
g g
Contains discontinuities such as joints, bedding
planes, folds, sheared zones and faults which
render its structure discontinuous.
• We cannot prescribe strength & modulus
We cannot prescribe strength & modulus
values. Whereas for concrete & steel we can
prescribe the values.
21 August 2017 56
29. 29
INTACT ROCK or ROCK MATERIAL
and ROCK MASS
• Intact rock may be considered as a continuum or
y
polycrystalline solid between discontinuities consisting of
an aggregate of minerals or grains.
Properties governed by the physical properties of the
materials of which it is composed and the manner in
hi h th b d d t h th
which they are bonded to each other.
• Rock mass is the in situ medium comprised of intact
rock blocks separated by discontinuities.
Discontinuous and often have heterogeneous and
i t i ti
anisotropic properties.
21 August 2017 57
s=1
Hoek, 2000
30. 30
Nature of Rocks
• Continuum/Discontinuum
• Isotropic/Anisotropic
• Interconnected Pores: Water Pressure
• Bedding planes, Sets of joints, Faults, Fissures
• Brittles/Ductile
Brittles/Ductile
• Elastic or Elasto-Plastic
• Perfectly plastic
• Strain hardening
St i ft i
• Strain softening
• Creep: Viscoelastic/Viscoplastic, Time dependent
behaviour
21 August 2017 59
S: Single occurring Discontinuities
M: Multiple occurring Discontinuities
31. 31
S: Single occurring Discontinuities
M: Multiple occurring Discontinuities
Influence of Joints/Discontinuities
on Foundations & Excavations
32. 32
Nature of Rock
Idealization: Homogeneous, Continuous, Isotropic, Linear
g
Simplest Idealization
But rocks are non-ideal
• Seldom truly continuous as pores and fissures are
usual.
• Interconnected pores
• Isolated vugs in volcanic rocks and soluble carbonate
rocks.
• Capacity to store & transmit fluids is largely dependent
p y g y p
upon behaviour of these voids.
21 August 2017 64
33. 33
Nature of Rock
1. Microfissures: Small planar cracks, 1μm or less in width
and about a length of a crystal or two. Common in hard rocks
and occur as intra-crystalline and crystal boundary cracks.
y y y
Not seen by naked eyes. All rocks have it.
2. Microfractures: Planar cracks about 0.1 mm or less wide,
barely visible to the naked eyes. Depend on schistosity of
rock and have well defined direction in space.
3 Macrofractures: Wider than 0 1 mm may be up to several
3. Macrofractures: Wider than 0.1 mm, may be up to several
metres or more in length. Closed/Tight or Open, Gouge/Filled
material
Joints: Discontinuities along which little/no displacement has
occured, Closed/Open joints
4 F lt L f t ith l ti di l t f
4. Faults: Large macrofractures with relative displacement of
more than 0.3 m. Fault zone, Shear zone
21 August 2017 65
Nature of Rock
Large geologic fractures and faults are important for the
design of tunnels, foundations.
Microfiss res and microfract res determine the real
Microfissures and microfractures determine the real
crushing strength of rock material and mass.
• Rock Material: is the smallest element of rock not cut
by any fractures. There are always some
microfissures in the rock material
microfissures in the rock material.
• Rock Mass: refers to any insitu rock with all inherent
geomechanical anisotropies.
• Homogeneous Zone: refers to rock mass with
comparable geological and mechanical properties
comparable geological and mechanical properties
such as type of rock, degree of weathering and
decomposition, and rock structure.
21 August 2017 66
34. 34
Nature of Rock
• Laboratory Tests: carried out on rock material.
y
• Insitu Tests: carried out on rock mass.
Collectively fissures and pores do the following:
1 Non linear load deformation or stress strain response
1. Non-linear load-deformation or stress-strain response
2. Reduced tensile strength
3. Stress dependency in material properties
4. Variability and scatter in test results
5. Scale effect into prediction of behavior
6. Anisotropy
21 August 2017 67
Rock Masses
• Discontinuous and variable in space
• It is important to choose the right domain, representative
of the rock mass, affected by the structure analyzed
• Scale Effect
• Intact Rock
• Intact rock with one set of discontinuities
• Intact rock with two sets of discontinuities
• Intact rock with many discontinuities
• When the structure being analyzed is much larger than
• When the structure being analyzed is much larger than
the blocks of rock formed by the discontinuities, the rock
mass may be simply treated as an equivalent continuum
21 August 2017 68
35. 35
DETERMINATION OF ENGINEERING
PROPERTIES OF ROCKS
• Rock mass is complex and it is difficult to
p
determine rock properties.
• Direct Methods & Indirect Methods
21 August 2017 69
Direct Methods
• Laboratory and Field (Insitu) Tests
y ( )
• BIS, ISRM, ASTM
• Volume of Rock Mass affected during the tests
• Insitu tests time consuming & expensive
21 August 2017 70
36. 36
Indirect Methods
• Empirical or theoretical correlations
• Combination of intact rock and discontinuity properties
using analytical or numerical methods, Back-analysis
using field observations of prototype observations
• Current practice relies heavily on the indirect methods
• The indirect methods can also be used for checking the
test results.
• Data resulting from laboratory and insitu tests are often
not completely consistent.
• Empirical correlations can be used to check the data from
tests for the inconsistency
21 August 2017 71
Categories of Test Methods Suggested by ISRM
(after Brown, 1981)
1. LABORATORY TESTS
(a) Characterization
(i) Porosity, density, water content
(ii) Absorption
(iii) Hardness - Schmidt rebound, Shore scleroscope
(iv) Resistance to abrasion
(v) Point load strength index
(vi) Uniaxial compressive strength and deformability
(vii) Swelling and slake-durability
(viii) Sound velocity
(ix) Petrographic description
21 August 2017 72
37. 37
Categories of Test Methods Suggested by ISRM
(after Brown, 1981) Contd...
1. LABORATORY TESTS
(b) Engineering design
(b) Engineering design
(i) Triaxial strength and deformability test
(ii) Direct shear test
(iii) Tensile strength test
(i ) P bilit
(iv) Permeability
(v) Time dependent and plastic properties
21 August 2017 73
Categories of Test Methods Suggested by ISRM
(after Brown, 1981) Contd...
2 IN SITU TESTS
2. IN SITU TESTS
(a) Characterization
(i) Discontinuity orientation, spacing, persistence,
roughness, wall strength, aperture, filling, seepage,
number of sets, and block size
(ii) Drill core recovery, RQD
(iii) Geophysical borehole logging
(iv) In situ sound velocity
21 August 2017 74
38. 38
Categories of Test Methods Suggested by
ISRM (after Brown, 1981) Contd...
(b) Engineering design
(i) Plate and borehole deformability tests
(ii) In situ uniaxial and triaxial strength and deformability
test
(iii) Sh t th di t h t i l h
(iii) Shear strength - direct shear, torsional shear
(iv) Field permeability measurement
(v) In situ stress determination
