Rock Mechanics

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Lectures on Rock Mechanics

• SARVESH CHANDRA

SARVESH CHANDRA

Professor

D

t

t f Ci il E i

i

Department of Civil Engineering

Indian Institute of Technology Kanpur

KANPUR, 208016 India

email: sarv@iitk ac in

email: [email protected]

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The problem in mathematics is black and white but the real ld i Alb t Ei t i

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Rock Mechanics Problems

• How will rock react when put to men’s use?p

• What is the bearing capacity of rock on surface an at depths?

• What is the shear strength of rocks? • What is the shear strength of rocks?

• What is the response of rocks under dynamic / earthquake type loading?

• What is the modulus of elasticity of rock and how to get it?

• What are the effects of rock defects (jointing beddingWhat are the effects of rock defects (jointing bedding planes, schistocity, fissures, cavities and other

discontinuities) on its strength?

• What are the mechanisms of failure of rocks? • What are the mechanisms of failure of rocks?

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Rock as a Construction Material

• For laying structural foundations to supportFor laying structural foundations to support structures

• For constructing Underground openingsg g p g • For protecting slopes

• For supporting railway tracks – BallastsFor supporting railway tracks Ballasts

• As base and sub-base for roads and runways • As aggregate in concrete

• As aggregate in concrete • Making facia for buildings.

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Era Period Epoch Time Boundaries (Years Ago) Holocene - Recent Quaternary 10 000

Geolo

gic

Quaternary 10,000 Pleistocene 2 million Pliocene 5 million Cenozoic Miocene 26 million Tertiary Oligocene ll

gic

Time

y g 38 million Eocene 54 million Paleocene 65 million Cretaceous 130 million M s z ic Jur ssic

Scale

Mesozoic Jurassic 185 million Triassic 230 million Permian 265 million Pennsylvanian Carboniferous 310 million Carboniferous 310 million Mississippian 355 million Paleozoic Devonian 413 million Silurian 425 million Ordovician 475 million Cambrian 570 million Precambrian 3.9 billion

Earth Beginning 4.7 billion

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What are we calling a rock?

Grade Description Lithology Excavation Foundations

VI Soil Some organic content, no original structure

May need to save and re-use

Unsuitable

V Completely Decomposed soil, some Scrape Assess by soil

V Completely weathered

Decomposed soil, some remnant structure

Scrape Assess by soil testing

IV Highly weathered

Partly changed to soil, soil > rock Scrape NB corestones Variable and unreliable III Moderately weathered

Partly changes to soil, rock > soil

Rip Good for most small structures

II Slightly Increased fractures and Blast Good for

II g y

weathered mineral staining anything except large dams

I Fresh rock Clean rock Blast Sound

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Primary Rock Types by Geologic

Origin

Sedimentary Types Metaphorphic Igneous Types

Grain Aspects

Clastic Carbonate Foliated Massive Intrusive Extrusive

Coarse Conglomerate Breccia

Limestone Conglomerate

Gneiss Marble Pegmatite Granite Volcanic Breccia Medium Sandstone Siltsone Limestone Chalk Schist Phyllite Quartzite Diorite Diabase Tuff Fine Shale Mudstone Calcareous Mudstone

Slate Amphibolite _Rhyotite _Basalt

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Index Properties of Intact Rock

• Specific Gravity of Solids, G

_s

• Unit Weight,

γ

• Porosity, n

• Ultrasonic Velocities (V

(

_p_p

and V

_s_s

)

• Compressive Strength, q

_u

• Tensile Strength, T

₀

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Specific Gravity of Rock Minerals

it galena dolomite olivine barite pyrite feldspar chlorite calcite dolomite halite gypsum serpentine

quartz _{Common Minerals}

Average G_s = 2.70

0 1 2 3 4 5 6 7 8

Specific Gravity of Solids G

halite

Reference Value Specific Gravity of Solids, G

s

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Unit Weights of Rocks

26 28 3 ) γsat = γwater [ Gs(1-n) + n] 24 26 t, γT (kN/m 3 γ 20 22 Unit Weigh t 16 18 Saturated Dolostone Granite Graywacke Limestone Mudstone Siltstone Sandstone Tuff G_s = 2.80 2.65 14 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Porosity n 2.50 Porosity, n

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Geologic Mapping of Rock Mass

Features

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INHERENT COMPLEXITIES

1 R k f t

1. Rock fracture

─ under compressive stresses 2. Size effects

─ response of rock to loading affected by the size of th l d d l ” (j i t & f t )

the loaded volume” (joints & fractures) 3. Tensile strength

─ is low (similar to concrete); HOWEVER a rock mass can have even less tensile strength

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COMPLEXITIES….

4. Groundwater effects

─ water in joints: if under pressure, reduces

normal stress (less resistance along joints)

─ water in permeable rocks (e.g. sandstone) → soil

like response

─

─ softeningsoftening of clay seams & argillaceous rocks (e.g.

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COMPLEXITIES….

5. Weathering 5. Weathering

─ chemical/physical alteration, reduction of engineering

properties p p

─ limestone caverns, sinkholes: ”Karst”

─ basic rocks with olivine (e.g. basalt) and pyroxene ( g ) py minerals are reduced to montmorillonite by hydrolysis

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Cavernous limestone

Coffin Bay

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STRUCTURAL FEATURES or

DISCONTINUITIES

1) Bedding planes

2) Folds

– tension joints at the crest of a fold (strike, dip & shear joints)

& s ea jo s)

– folding may cause shear failure along

bedding planes (axial

bedding planes (axial plane or fracture cleavage)

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Folding

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DISCONTINUITIES

3) Faults

– shear displacement zones - sliding

Faults may contain

F lt ( l ) k

– Fault gouge (clay) – weak

– Fault breccia (re-cemented rock) – weak

Rock flour weak

– Rock flour – weak

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Defects

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Defects

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DISCONTINUITIES

4) Shear zones 4) Shear zones

– bands of materials - local shear failure

5) Dykes 5) Dykes

– igneous intrusions (near vertical)

– weathered dykes,weathered dykes, e.g. dolerite weathers toe.g. dolerite weathers to

montmorillonite

– unweathered dykes attract high stresses

6) Joints

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Joint Patterns

sedimentary rocks usually contain 2 sets of joints orthogonal to each other and the

joints, orthogonal to each other and the bedding plane

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JOINTS

1) Open) p

Filled

Healed (or closed)

2) Stepped

Undulating

Pl

Planar

2B) each of the above can be Rough

Smooth Smooth

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JOINT CLASSES

(AS 1726-1993)

I St d R h I II II Stepped Rough Smooth Slickensided II Slickensided IV V Undulating Rough Smooth V VI Smooth Slickensided

VII Planar Rough

VII VIII IX Planar Rough Smooth Slickensided IX Slickensided

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Order of Description of Rocks

(AS 1726-1993)

ROCK MATERIAL rock name

grain size (Table A6)

COMPOSITION g ( )

texture and fabric (Table A7) colour

e.g. Basalt, fine, massive, vesicular, dark grey to black

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Order of Description of Rocks

(AS 1726-1993)

ROCK MATERIAL CONDITION

strength (Table A8) CONDITION

weathering (Table A9)

e.g. VL strength, XW OR EH strength, FR

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Order of Description of Rocks

(AS 1726-1993)

ROCK MASS PROPERTIES

structure

defects (much information required) PROPERTIES defects (much information required)

weathering of joints

Structure:

sedimentary rocks – bedded, laminated sed e ta y oc s bedded, a ated

metamorphic – foliated, banded, cleaved igneous rocks massive flow banded

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DEFECTS – information needed

ti ht

tightness

cementation or infill

smoothness or irregularity of surfaces

class of joint class of joint water in joints joint orientation joint spacing joint spacing

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DESIGN IN ROCK

Take into account: Take into account:

• Local geological structure

• Shear strength of the rock mass • Shear strength of the rock mass • Impact of water on stability

R k h i ? • Rock anchoring?

• Drilling and blasting procedures • Monitoring of stability

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Intact Rock

H t

• Heterogeneous

• Anisotropic (soils less so) • Spatial variability (soils the same)

• Yield mechanisms are non-linear & depend on stress level and rock type

• Failures are often brittle (soils strain soften or harden past the peak strength)

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Rock Masses

C t i di ti iti ith littl t il

• Contain discontinuities with little tensile strength

• Scale effect

─ response is dependent on stressed volumeresponse is dependent on stressed volume

• Affected by groundwater & weathering • In-situ stresses difficult to estimate

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Rock Masses

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DEFINITIONS

• Dip angle, β_w:

the acute angle measured in a vertical plane between the line of maximum dip in a non horizontal plane and the horizontal of maximum dip in a non-horizontal plane and the horizontal plane

i e 0° ≤ β ≤ 90°

i.e. 0 ≤ β_w ≤ 90

• Dip direction, α_w: the geographical azimuth measured in a

clockwise direction from north (0°) of the vertical plane in which ( ) p

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Dip Angle

North

Horizontal

ββ_w

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Dip direction

Azimuth is the direction of an object, measured clockwise around the

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Dip Direction

N th North Horizontal α_w

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Quantitative Classification of Rock

M

Mass

• Description of Joints:

Description of Joints:

Orientation, Persistence, Roughness, Wall

Strength Aperture Filling Seepage

Strength, Aperture, Filling, Seepage,

Number of sets, Block size, spacing.

ISRM

i i ’

t

ISRM commission’s report

Classification of Rock Material

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Uniaxial Compressive Ranges for some Uniaxial Compressive

Strength

Ranges for some Common Rock Material

Term Kg/cm2 _{Schist, Silt stone}

VW-W, Sand Very Weak- VW < 70

Stone, Lime stone –VW-M,Granite, Basalt Gneiss y Weak- W Medium Strong-MS 70-200 200-700 _{Basalt, Gneiss,} Quartzite, Marble –MS-VS Medium Strong MS Strong- S Very Strong- VS 200 700 700-1400 > 1400 MS VS | Very Strong VS > 1400 |

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Classification for Rock Material

Strength

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Intact Rock Classification

• Rock Type

Rock Type

• Geologic Formation and Age

I di

• Indices:

– Specific Gravity, Porosity, Unit Weight,

W V l iti

Wave Velocities

– Strength (compressive, tensile, shear) – Elastic Modulus

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• What is Rock

Mechanics?

R k h i i

Rock mechanics is a

discipline that uses the principles of mechanics to

p c p es o ec a cs to

describe the behaviour of

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• How to correlate the properties of rock studied inHow to correlate the properties of rock studied in the laboratory with in-situ properties?

• What in-situ test methods will provide actual in-p situ conditions and properties of rock?

• What design parameters are to be used for rock g p slope design?

• How to stabilize slopes and underground openings?