01 week 12 - lecture note - a slide per page - gray.pdf

8/11/2019 01 Week 12 - Lecture note - a slide per page - gray.pdf

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Introduction to Rock Mechanics

ENB371: Geotechnical Engineering 2ENB371: Geotechnical Engineering 2

Chaminda GallageChaminda Gallage


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Outline of the lecture Introduction

Rock formation and rock types

Rock Classification Engineering Classification more focused on applications

Shear strength of discontinuities


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Introduction -1Rock mechanics adopts many of the techniques developed in soilmechanics such as: Mohr Coulomb law

But, the behaviour of rocks is far more complex than the behaviour of

soils, in many cases rock mechanics uses techniques unknown to soilmechanics.

Rock mechanics deals with properties of rock and special

methodology required for design for rock related components ofengineering schemes

The rock considered in rock mechanics is in fact the rock mass, which

composes intact rock materials and rock discontinuities.Rock discontinuities dominate the mechanical and engineering

behaviours of rock. The existence of discontinuity depends on the

scale. The discontinuous nature and scale dependence feature is notcommon in other man-made materials.


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Introduction -2

Rock mechanics is applied to various engineering disciplines:

Civil

Mining

Hydropower

In civil engineering, it involves:

FoundationSlope

Tunnel.


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Intact rock and Rock mass

Intact rock: This isthe smallest elementof rock block not cut

Introduction -3

y any rac ure.

Rock mass: In-situ

rock together with itsdiscontinuities andweathering profile.


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Intact rock

Intact rock

Introduction -4

texture etc.)

Mechanical properties (eg. UCS, PointLoad Index etc.)


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Rocks are divided into three groups according totheir origin

Igneous Rocks Metamorphic Rocks

Rock types and their formation -1

ROCK CYCLEa scheme that represents the processes of

continuous changes that connect the three major groups of

rocks:


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Rock Cycle


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IGNEOUS ROCKS are formed when hot molten rock

material, called magma, solidifies

These samples represent igneousThese samples represent igneous

rocks, although each exhibitrocks, although each exhibit

different textures.different textures.


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Sedimentary Rocks

Sediments are formed by weathering (mechanical,chemical) of rocks and by organic matters (animal, trees)

Sediments are transported and deposited in layers

Sediments can harden into sedimentary rock by Pressure(weight of above layers) and cementation (mineral dissolved

in water).

Sedimentary rock covers about three-quarters of continentalareas and most of the sea floor


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Sedimentary rocks are typically layered,(although layering is not diagnostic of only

sedimentary rocks)


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Metamorphic rocks form deep in the earth where high temperature,

great pressure, and chemical reactions cause one type of rock to change

into another type of rock

Metamorphic rocks begin to form at 12-16 km beneath the earth'ssurface.

They begin changing at temperatures of 100- 800 0C for a few million

ears it can turn into a new kind of rock

Metamorphic Rocks


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Metamorphic Rocks


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Rock mass classification

During preliminary design stages of a project, when very little detailed

information on the rock mass and its stress and hydrological characteristics

is available, the rock mass classification schemes can be used to provide

initial estimate of support requirements, and to provide estimates of strengthand deformation properties of the rock mass

Why do we require rock classification?

Stability of rock slopes- civilengineering

Tunnel design

Petroleum engineering- extractionof crude oil and natural gas

Underground and open cut mining


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Rock mass classifications are based

on the following parameters:Quality of rock

Intact rock strengthFracture spacing, orientation

Fracture roughness

Fracture condition (eg. tight or loose)

Wet or dry conditionGeology


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Site investigationWe carry out site investigations

to determine the quality of rockbeneath the soil.

How to measure parameters required for rock classification?


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Rock Quality Designation Index (RQD)


(After Deere, 1989)


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Correlation between RQD and RockMass Quality(after Deere, 1968)

RQD (%) Rock Quality

< 2 V r r

25 - 50 Poor

50 - 75 Fair

75 - 90 Good

90 - 100 Excellent


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Example for RQD

From a core run of 1.50 m, the total core

recovery was 1.25 m as shown in the

figure. Calculate RQD for the rock core

sample and identify the quality of therock mass.

50

250

75

100

125

50

10015

0100

50

125

100runcoretheoflengthTotal

cm10piecescoreofLength

>

=

RQD

1001500

)125150100125100250(

+++++=RQD

%57=RQD Rock quality: Fair


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Measurement of intact strength

Unconfined compressive strength (UCS or qu) Point load index test - UCS = 24 Is(50)

Correlation factor may vary for certain rock types.


comparative study.

Is = P/D2

Triaxial strength tests - high pressure steel cells.


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UCS apparatus Rock specimens normally tested

under compression. No lateral confining pressure

M ximum m r iv tr t


rock

failure is known as UCS


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Point Load Tester

According to Brook (1985), the formula to

convert the force reading to Is(50) value is asfollows:

I = FP / D 2

UCS = 24 Is(50)

s e

Where

F = size correction factor = (De/50)0.45

P = applied load (MN)

De = (4A/)0.5

A= minimum cross sectional area of the specimen (m2)

The units of the point load index are MPa and whereas the test is

considered to cause tensile failure


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Triaxial Test High pressure


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Rock strength Laboratory estimationStrength class

Extremely High (EH)

Very High (VH)

Unconfined CompressiveStrength (UCS), MPa

>250

100-250

High (H)Medium High (MH)

Weak (W)

Very Weak (VW)

Extremely Weak(EW)

50-10025-50

5-25

1-5

0.6*-1


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Direct shear apparatus for rock


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Rock mass classification systems

The two most widely used rock mass classifications are:

Rock Mass Rating (RMR) Bieniawski (1984), ISRM1978

Rock mass Quality (Q system) NGI (1974)


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RMR classification

RMR classification is based on the following 6

parameters: UCS of intact rock

RQD

Joint spacing Joint condition (eg. tight, rough?)

Ground water condition, and

Joint orientation (eg. dip angle)

RMR = relevant ratings


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G id li f i d f 10 k l RMR


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Guidelines for excavation and support of 10 m span rock tunnels- RMR

system


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E l f RMR


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Example for RMRA tunnel is to be driven through weathered granite rock. It has a joint set of 59degrees against

the tunnel direction. The point load index of 8MPa and RQD value of 73% were estimated.The joints are spaced at 390mm. They are slightly rough and slightly weathered with a

separation of


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Rock mass quality (Q) system


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E i l t Di i (D )


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Equivalent Dimension (De)

Oth U f l E ti


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Other Useful Equations

Permanent roof supportpressure


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Shear Strength of RockDiscontinuities

Introduction


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All rock masses contain discontinuities such as bedding planes, joints,

shear zones and faults

Rock mass failure is controlled by sliding on the discontinuities

To analyses the stability of individual rock blocks, it is necessary to

understand the shear strength of discontinuities of rock which separate

Shear Strength of Planar Surfaces


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gBedding surface is planar having no surface irregularities or undulations.

Filling material presents between two surfaces.

anglefrictionResidualstrength,shearResidual

firctionofanglesurface,ceentedofCohesioncstress,Normalstrength,shearPeak

rr

n

==

====

p

The Basic Friction Angle b


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The Basic Friction Angle b Shear strength of smooth rock surfaces without any filling materials

between them

r = n tan b

The roughness component (i) is then added to basic friction angle(b) to get effective friction angle of the discontinuities

Typical Values


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Typical b Values

Shear Strength of Rough Surfaces


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Shear Strength of Rough Surfaces

After Patton (1966)

r = n tan (b + i),

where i = slope of the asperity = tan-1

(v/h) atmaximum dilation

Bartons Equation for i


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q

i = JRC log (JCS/n)where n = mean effective normal stress acting on the joint surface

JRC = Joint roughness coefficient

JCS = Joint wall compressive strength (JCS UCS)UCS = Unconfined (uniaxial) compressive strength

Field Estimate of JRC -1


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The joint roughness coefficient (JRC) can be estimated by comparing the appearance of

discontinuity surface of rock with standard profile published by Barton & Choubey

(1977).

Field Estimate of JRC -2


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As an alternative method, the joint roughness coefficient (JRC) can be estimated by

measuring amplitude of asperity Barton (1982).

Field Estimate of JCS ( UCS)


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The joint compressive strength (JRC) can be estimated in the field by performing

Schmidt rebound hammer test ( Deere and Miller ,1966).

log JCS = 0.00088 r R + 1.01

R = Schmidt rebound hammer number

r = Unit weight of rock

Scale Effect on JRC and JCS


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J J

Barton and Bandis (1982)

JRCo, JCSo andLo (length) refer to 100 mmlaboratory scale samples andJRCn, andLn refer to in

situ block sizes.

Instantaneous Shear Strength (i, ci ) -1


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g (i , i )

Because of non-linear strength envelope, c an need to be defined fordesign purposes

** When pore water (u) is present in a rock mass, n (normal stress) will be replaced by

effective normal stress (n= n u) in all the equations presented in this topic

01 week 12 - lecture note - a slide per page - gray.pdf

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