Course4_RockMechanics_EngineringGeology

Geological Engineering DepartmentFaculty of Engineering

Introduction to Rock Mechanics

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ROCK STRENGTHIn a very general sense, rock strengths are related to origin; e.g.,

dense igneous rocks are stronger than dense sedimentary rocks

Rock strength is also related to:Texture (interlocking textures are stronger) Anisotropy (the existence of preferred failure planes)Mineralogy (e.g., quartz is stronger than calcite)Moisture Content (dry rocks tend to be stronger than saturated rocks)Degree and Type of Cement (e.g., quartz is stronger than calcite and complete cementation makes rock stronger than just partial cementation)

Rocks are much stronger in compression than tensionRocks deform elastically, plasticaly, brittlelyDeformation style in part a function of heat, pressure, and time

(strength is inversely proportional to all three)

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ROCK CLASSIFICATIONRock Type

Sedimentary, Metamorphic, or IgneousGrain or Particle Size

Very Coarse = >60 mmCoarse = 2 - 60 mmMedium = 60µm - 2 mmFine = 2µm - 60µmVery Fine = <2µm

WeatheringFresh -- No visible signs of weatheringSlightly Weathered -- DiscolorationModerately Weathered -- Less than half the rock is decomposed or disintegratedHighly Weathered -- More than half the rock is decomposed or disintegratedCompletely Weathered -- All rock is decomposed or disintegrated; original structure still intact (Saprolite)Residual Soil -- All rock material is converted to soil and the original structure has been destroyed

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Rockmass Strength

StrongMassive

FairDiscrete Joints

WeakLow Integrity

Relative Properties

Stope

Hutchinson and Diederichs, 1996

Hutchinson, 2000

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ROCK CLASSIFICATION (cont.)

Strength of Rock Pieces (extremely weak to extremely strong)

Discontinuity Spacing (extremely close to extremely wide)

Aperture Width (very tight to cavernous)

Rock Quality Designation (Sum of the lengths of pieces greater than 4 inches long as a percentage of the total length cored

Roughness (joint roughness coefficient)

Infilling (use Uniform Soil Classification System)

Moisture (dry, damp, moist, wet)

Staining and AlterationRock Mass Rating (a system to describe in-place rock characteristics)

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Classification of rock strength

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Soil and Rock Strength

• Unconfined compressive strength (MPa)

ConcreteConcrete1001002020

1.01.0RockRock

2502501001001010SoilSoil

0.10.10.010.010.0010.001

SteelSteel750750250250

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Intact rock strength

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Grade *

Term UCS (MPa)

Point Load Index (MPa)

Field estimate of strength Examples

R6 Extremely strong

> 250 > 10 Specimen can only be chipped with a geological hammer

Fresh basalt, chert, diabase, gneiss, granite, quartzite

R5 Very strong 100 to 250 4 to 10 Specimen requires many blows of a geological hammer to fracture it

Amphibolite, sandstone, basalt, gabbro, gneiss, granodiorite, limestone, marble, rhyolite, tuff

R4 Strong 50 to 100 2 to 4 Specimen requires more than one blow of a geological hammer to fracture it

Limestone, marble, phyllite, sandstone, schist, shale

R3 Medium strong

25 to 50 1 to 2 Cannot be scraped with a pocket knife, specimen can be fractured with a single blow from a geological hammer

Claystone, coal, concrete, schist, shale, siltstone

R2 Weak 5 to 25 ** Can be peeled with a pocket knife with difficulty, shallow indentation made by firm blow with point of a geological hammer

Chalk, rocksalt, potash

R1 Very weak 1 to 5 ** Crumbles under firm blows with point of a geological hammer, can be peeled by a pocket knife

Highly weathered or altered rock

R0 Extremely weak

0.25 to 1 ** Indented by thumbnail Stiff fault gouge

** Point load tests on rocks with a uniaxial compressive strength < 25 MPa are likely to yield highly ambiguous results.

Table 11.2: Field estimates of uniaxial compressive strength

* Grade according to Brown (1981).

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Discontinuity

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Geological Strength Index: GSI

Hoek, 2000Strength of jointed rockmass

depends on:• properties of intact rock

pieces, and• upon the freedom of these

pieces to slide and rotate under different stress conditions,

• controlled by the geometrical shape of the intact rock pieces as well as the condition of the discontinuities separating the pieces

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Post-failure Behaviour: Very Good Quality Hard Rockmass

Hoek, 2000

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Post-failure Behaviour: Average Quality Rockmass

Hoek, 2000

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Post-failure Behaviour: Very Poor Quality RockmassHoek, 2000

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Practical Examples of Rockmass Property Estimates: Massive Weak Rock, Braden Breccia, El Teniente Mine

Hoek, 2000

Hoek, 2000

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Geological Engineering DepartmentFaculty of EngineeringMassive Strong Rockmasses,

Rio Grande Pumped Storage Scheme

Hoek, 2000

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Hoek, 2000

Average Quality Rockmass, Nathpa Jhakri Hydroelectric

Partially completed 20 mspan, 42.5 m high underground powerhouse cavern of the Nathpa Jhakri Hydroelectric Project inHimachel Pradesh, India. The cavern is approximately 300 m below the surface.

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Average Quality Rockmass, Nathpa Jhakri Hydroelectric

Hoek, 2000

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Poor Quality Rockmass at Shallow Depth: Athens Metro

Hoek, 2000

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Geological Engineering DepartmentFaculty of EngineeringPoor Quality Rockmass at Shallow Depth: Athens Metro

Hoek, 2000

Hoek, 2000

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Poor Quality Rockmass under High Stress

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Course4_RockMechanics_EngineringGeology