Lectures on Rock Mechanics Lectures on Rock Mechanics • SARVESH CHANDRA SARVESH CHANDRA Professor D Department of Civil Engineering t t f Ci il E i i Indian Institute of Technology Kanpur KANPUR, 208016 India email: sarv@iitk ac in email: sarv@iitk.ac.in
The problem in mathematics is black and white but the real world is grey –Albert Einstein ld i Alb t Ei t i
Rock Mechanics Problems 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 bedding What 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 Material Rock as a Construction Material • For laying structural foundations to support For laying structural foundations to support structures • For constructing Underground openings g g p g • For protecting slopes • For supporting railway tracks – Ballasts For 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 Quaternary 10 000 10,000 Pleistocene Geolo 2 million Pliocene 5 million Cenozoic Miocene 26 million gic gic Tertiary y Oligocene g 38 million ll Eocene 54 million Time Paleocene 65 million Cretaceous 130 million M s z ic Mesozoic Jur ssic Jurassic Scale 185 million Triassic 230 million Permian 265 million Pennsylvanian Carboniferous Carboniferous 310 million 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 Soil Some organic content, May need to Unsuitable VI no original structure save and re-use V V Completely Completely Decomposed soil, some Decomposed soil, some Scrape Scrape Assess by soil Assess by soil weathered remnant structure testing IV Highly Partly changed to soil, Scrape NB Variable and weathered soil > rock corestones unreliable III Moderately Partly changes to soil, Rip Good for most weathered rock > soil small structures II II Slightly g y Increased fractures and Blast Good for weathered mineral staining anything except large dams I Fresh rock Clean rock Blast Sound Engineering classification of weathered rock
Primary Rock Types by Geologic Origin Origin Sedimentary Types Metaphorphic Igneous Types Clastic Carbonate Foliated Massive Intrusive Extrusive Grain Aspects Conglomerate Limestone Gneiss Marble Pegmatite Volcanic Coarse Breccia Breccia Conglomerate Granite Sandstone Limestone Schist Quartzite Diorite Tuff Medium Siltsone Chalk Phyllite Diabase Shale Calcareous Slate Amphibolite Rhyotite Basalt Fine Mudstone Mudstone Obsidian
Index Properties of Intact Rock • Specific Gravity of Solids, G s • Unit Weight, γ • Porosity, n • Ultrasonic Velocities (V p and V s ) ( s ) p • Compressive Strength, q u • Tensile Strength, T 0 • Elastic Modulus, E R (at 50% of q u ) Elastic Modulus, E R (at 50% of q u )
Specific Gravity of Rock Minerals galena pyrite it barite olivine dolomite dolomite calcite chlorite feldspar quartz Common Minerals serpentine Average G s = 2.70 gypsum halite halite 0 1 2 3 4 5 6 7 8 Specific Gravity of Solids G Specific Gravity of Solids, G s Reference Value (fresh water)
Unit Weights of Rocks 28 γ sat = γ water [ G s (1-n) + n] γ 3 ) t, γ T (kN/m 3 26 26 24 Unit Weight 22 20 Saturated Dolostone Granite 18 Graywacke Limestone G s = Mudstone Siltstone 2.80 Sandstone Tuff 16 2.65 2.50 14 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Porosity n Porosity, n
Geologic Mapping of Rock Mass Features Features
INHERENT COMPLEXITIES INHERENT COMPLEXITIES 1 1. R Rock fracture k f t ─ under compressive stresses 2. Size effects ─ response of rock to loading affected by the size of th the loaded volume” (joints & fractures) l d d l ” (j i t & f t ) 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 ─ softening softening of clay seams & argillaceous rocks (e.g. shales)
COMPLEXITIES…. COMPLEXITIES…. 5. 5. Weathering 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
Cavernous limestone Cavernous limestone Coffin Bay
STRUCTURAL FEATURES or DISCONTINUITIES DISCONTINUITIES 1) Bedding planes 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 bedding planes (axial (axial plane or fracture cleavage)
Folding Folding
DISCONTINUITIES DISCONTINUITIES 3) Faults 3) Faults – shear displacement zones - sliding Faults may contain – Fault gouge (clay) – weak F lt ( l ) k – Fault breccia (re-cemented rock) – weak – Rock flour Rock flour – weak weak – Angular fragments – may be strong
Defects Defects
Defects Defects
DISCONTINUITIES DISCONTINUITIES 4) Shear zones 4) Shear zones – bands of materials - local shear failure 5) Dykes 5) Dykes – igneous intrusions (near vertical) – weathered dykes, e.g. dolerite weathers to weathered dykes, e.g. dolerite weathers to montmorillonite – unweathered dykes attract high stresses 6) Joints – breaks with no visible displacement
Joint Patterns 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
JOINTS JOINTS 1) Open ) p Filled Healed ( or closed ) 2) Stepped Undulating Pl Planar 2B) each of the above can be Rough Smooth Smooth Slickensided
JOI NT CLASSES ( AS 1 7 2 6 -1 9 9 3 ) I I Stepped St d R Rough h II Smooth II II Slickensided Slickensided IV Undulating Rough V V Smooth Smooth VI Slickensided VII VII Planar Planar Rough Rough VIII Smooth IX IX Slickensided Slickensided
Order of Description of Rocks ( AS 1 7 2 6 -1 9 9 3 ) rock name ROCK MATERIAL grain size (Table A6) g ( ) COMPOSITION texture and fabric (Table A7) colour e.g. Basalt, fine, massive, vesicular, dark grey to black
Order of Description of Rocks ( AS 1 7 2 6 -1 9 9 3 ) strength (Table A8) ROCK MATERIAL CONDITION CONDITION weathering (Table A9) e.g. VL strength, XW OR EH strength, FR
Order of Description of Rocks ( AS 1 7 2 6 -1 9 9 3 ) structure ROCK MASS defects (much information required) defects (much information required) PROPERTIES PROPERTIES weathering of joints Structure: sedimentary rocks – bedded, laminated sed e ta y oc s bedded, a ated metamorphic – foliated, banded, cleaved igneous rocks igneous rocks – massive, flow banded massive flow banded
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
DESIGN IN ROCK 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 • Rock anchoring? R k h i ? • Drilling and blasting procedures • Monitoring of stability – the observational method
Intact Rock Intact Rock • Heterogeneous H t • 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)
Recommend
More recommend
