Lectures on Rock MechanicsLectures on Rock Mechanicssarv/New Folder/Presentation-1.pdf · 2010. 8....
Transcript of Lectures on Rock MechanicsLectures on Rock Mechanicssarv/New Folder/Presentation-1.pdf · 2010. 8....
Lectures on Rock MechanicsLectures on Rock Mechanics
• SARVESH CHANDRASARVESH CHANDRAProfessorD t t f Ci il E i iDepartment of Civil EngineeringIndian Institute of Technology KanpurKANPUR, 208016 Indiaemail: sarv@iitk ac inemail: [email protected]
The problem in mathematics is black and white but the real ld i Alb t Ei t iworld is grey –Albert Einstein
Rock Mechanics ProblemsRock 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?
Rock as a Construction MaterialRock 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.
Era Period Epoch Time Boundaries (Years Ago) Holocene - Recent Quaternary 10 000
Geologic
Quaternary 10,000 Pleistocene 2 million Pliocene 5 million Cenozoic Miocene 26 million Tertiary Oligocene ll gic
Timey 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 Greenland
What are we calling a rock?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 soilV 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 yweathered mineral staining anything except
large dams
I Fresh rock Clean rock Blast Sound
Engineering classification of weathered rock
Primary Rock Types by Geologic OriginOrigin
Sedimentary Types Metaphorphic Igneous Types
GrainAspects
Clastic Carbonate Foliated Massive Intrusive Extrusive
Coarse ConglomerateBreccia
LimestoneConglomerate
Gneiss Marble PegmatiteGranite
Volcanic Breccia
Medium SandstoneSiltsone
LimestoneChalk
SchistPhyllite
Quartzite DioriteDiabase
Tuff
Fine ShaleMudstone
Calcareous Mudstone
Slate Amphibolite Rhyotite BasaltObsidian
Index Properties of Intact Rock
• Specific Gravity of Solids, Gs
• Unit Weight, γ• Porosity, n• Ultrasonic Velocities (Vp and Vs)( p s)• Compressive Strength, qu
• Tensile Strength, T0
• Elastic Modulus, ER (at 50% of qu)Elastic Modulus, ER (at 50% of qu)
Specific Gravity of Rock Minerals
itgalena
dolomiteolivinebaritepyrite
feldsparchloritecalcite
dolomite
halitegypsum
serpentinequartz Common Minerals
Average Gs = 2.70
0 1 2 3 4 5 6 7 8
Specific Gravity of Solids G
halite
Reference Value Specific Gravity of Solids, Gs(fresh water)
Unit Weights of Rocks
26
28
3 ) γsat = γwater [ Gs(1-n) + n]
24
26
t, γ
T (kN
/m3 γ
20
22
Unit
Weigh
t
16
18
Satu
rate
d Dolostone GraniteGraywacke LimestoneMudstone SiltstoneSandstone Tuff
Gs = 2.80 2.65
140.0 0.1 0.2 0.3 0.4 0.5 0.6
Porosity n
2.50
Porosity, n
Geologic Mapping of Rock Mass FeaturesFeatures
INHERENT COMPLEXITIESINHERENT COMPLEXITIES
1 R k f t1. 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
COMPLEXITIES….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. shales)
COMPLEXITIES….COMPLEXITIES….5. Weathering5. Weathering
─ chemical/physical alteration, reduction of engineering propertiesp p
─ limestone caverns, sinkholes: ”Karst”
─ basic rocks with olivine (e.g. basalt) and pyroxene ( g ) pyminerals are reduced to montmorillonite by hydrolysis
Cavernous limestoneCavernous limestone
Coffin Bay
STRUCTURAL FEATURES or DISCONTINUITIESDISCONTINUITIES
1) Bedding planes1) 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 (axialbedding planes (axial plane or fracture cleavage)
FoldingFolding
DISCONTINUITIESDISCONTINUITIES
3) Faults3) Faults– shear displacement zones - sliding
Faults may containF lt ( l ) k– Fault gouge (clay) – weak
– Fault breccia (re-cemented rock) – weakRock flour weak– Rock flour – weak
– Angular fragments – may be strong
DefectsDefects
DefectsDefects
DISCONTINUITIESDISCONTINUITIES
4) Shear zones4) Shear zones– bands of materials - local shear failure
5) Dykes5) Dykes – igneous intrusions (near vertical)– weathered dykes, e.g. dolerite weathers toweathered dykes, e.g. dolerite weathers to
montmorillonite– unweathered dykes attract high stresses
6) Joints – breaks with no visible displacement
Joint PatternsJoint Patterns
sedimentary rocks usually contain 2 sets of joints orthogonal to each other and thejoints, orthogonal to each other and the bedding plane
JOINTSJOINTS1) Open) p
FilledHealed (or closed)
2) SteppedUndulating
PlPlanar2B) each of the above can be Rough
SmoothSmoothSlickensided
JOINT CLASSES (AS 1726-1993)
I St d R hIIIII
Stepped RoughSmoothSlickensidedII Slickensided
IVV
Undulating RoughSmoothV
VISmoothSlickensided
VII Planar RoughVIIVIIIIX
Planar RoughSmoothSlickensidedIX Slickensided
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
Order of Description of Rocks (AS 1726-1993)
ROCK MATERIAL CONDITION
strength (Table A8)
CONDITIONweathering (Table A9)
e.g. VL strength, XW
OR EH strength, FR
Order of Description of Rocks (AS 1726-1993)
ROCK MASSPROPERTIES
structuredefects (much information required)PROPERTIES defects (much information required)
weathering of joints
Structure:
sedimentary rocks – bedded, laminatedsed e ta y oc s bedded, a ated
metamorphic – foliated, banded, cleaved
igneous rocks massive flow bandedigneous rocks – massive, flow banded
DEFECTS – information needed
ti httightness
cementation or infill
smoothness or irregularity of surfaces
class of jointclass of joint
water in joints
joint orientation
joint spacingjoint spacing
DESIGN IN ROCKDESIGN IN ROCKTake 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
– the observational method
Intact RockIntact 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)
Rock MassesRock 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
Rock MassesRock Masses
DEFINITIONSDEFINITIONS
• Dip angle, βw:
the acute angle measured in a vertical plane between the line of maximum dip in a non horizontal plane and the horizontalof maximum dip in a non-horizontal plane and the horizontal plane
i e 0° ≤ β ≤ 90°i.e. 0 ≤ βw ≤ 90
• Dip direction, αw: the geographical azimuth measured in a clockwise direction from north (0°) of the vertical plane in which ( ) pthe dip angle is measured i.e. 0° ≤ αw ≤ 360°
Dip AngleDip Angle
North
Horizontal
ββw
Line of maximum dip
Dip directionDip direction
Azimuth is the direction of an object, measured clockwise around the observer's horizon from North, i.e. an object due north has an azimuth of 0°
Dip DirectionDip Direction
N thNorth
Horizontal
αw
Line of maximum dip
Quantitative Classification of Rock MMass
• Description of Joints:Description of Joints:Orientation, Persistence, Roughness, Wall Strength Aperture Filling SeepageStrength, Aperture, Filling, Seepage, Number of sets, Block size, spacing.ISRM i i ’ tISRM commission’s reportClassification of Rock MaterialBased on Uniaxial Compressive Strength
Uniaxial Compressive Ranges for someUniaxial Compressive Strength
Ranges for some Common Rock Material
Term Kg/cm2 Schist, Silt stone VW-W, Sand Very Weak- VW < 70Stone, Lime stone –VW-M,Granite, Basalt Gneiss
yWeak- WMedium Strong-MS
70-200200-700 Basalt, Gneiss,
Quartzite, Marble –MS-VS
Medium Strong MSStrong- SVery Strong- VS
200 700700-1400> 1400 MS VS
|
Very Strong VS > 1400
|
Classification for Rock Material StrengthStrength
Intact Rock ClassificationIntact Rock Classification• Rock TypeRock Type• Geologic Formation and Age
I di• Indices:– Specific Gravity, Porosity, Unit Weight,
W V l itiWave Velocities– Strength (compressive, tensile, shear)– Elastic Modulus
• What is Rock Mechanics?
R k h i iRock mechanics is a discipline that uses the
principles of mechanics top c p es o ec a cs todescribe the behaviour ofrock of engineering scale.
• 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-psitu conditions and properties of rock?
• What design parameters are to be used for rock g pslope design?
• How to stabilize slopes and underground openings?