1

INDEX PROPERTIESEasily determined properties useful in the quantitative description of rocks :-

Index properties determined in the laboratory relate most closely to the behaviour of INTACT ROCK. They are of lesser importance and require caution when used in the prediction of ROCK MASS behaviour

1. POROSITY2. DENSITY3. ACOUSTIC VELOCITY4. PERMEABILITY5. STRENGTH6. DURABILITY

RockMass

Block of Intact Rock Material

Joints

LaboratorySpecimen

ROCK MASSINTACT ROCK AND DISCONTINUITIES

POROSITY

Porosity shows some useful empirical correlations :-

NEGATIVE : Unconfined Compressive StrengthDeformation modulusDensity

particularly in unweathered fresh rock types.

For similar rock types porosity also shows correlations with :-

NEGATIVE : Geological ageDepth of burial

Porosity decreases with depth

Porosity increases due to weathering

TYPICAL POROSITY VALUES

2

Dep

th b

elow

surf

ace

(km

)

Log Porosity 0.1

0

20.05 300.3

Increasing overburden

Porosity, % 20100U

niax

ial C

ompr

essi

ve S

tren

gth,

MPa 250

200

150

100

50

UCS reduction

POROSITY v DEPTH AND UCS

DENSITY OR UNIT WEIGHTRock density is often a function of porosity since most rocks have relatively similar average grain densities.

ρ and γNEGATIVE : Porosity

Weathering

POSITIVE : Geological ageBurial depthCompressive strengthDeformation modulus

DENSITIES OF MINERALS

MINERALS DENSITY gm/cm3

Halite 2.1-2.6Gypsum 2.3-2.5

Serpentine 2.3-2.6Orthoclase 2.5-2.6

Quartz 2.65Plagioclase 2.6-2.8

Chlorite/Illite 2.6-3.0Calcite 2.7

Muscovite 2.7-3.0Biotite 2.8-3.1

Dolomite 2.8-3.1Anhydrite 2.9-3.0Pyroxene 3.2-3.6Olivine 3.2-3.6Barite 4.3-4.6

Magnetite 4.4-5.2Pyrite 4.9-5.2Galena 7.4-7.6

ROCKS Density g/cm3 Unit Weight kN/m3

Granite 2.65 26

Diorite 2.85 27.9

Gabbro 3.0 29.4

Gypsum 2.3 22.5

Rock Salt 2.1 20.6

Coal 0.7-2.0

Dense Limestone 2.7 26.9

Marble 2.75 27

Shale f(depth) 2.25-2.62 22.1-25.7

Rhyolite 2.37 23.2

Basalt 2.77 27.1

Amphibolite 2.99 29.3

TYPICAL DENSITIES AND UNIT WEIGHTS OF ROCK TYPES

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ACOUSTIC VELOCITY• Acoustic velocity in rock cores is easily determined by measuring the

travel time of vibrational waves introduced by piezoelectric crystals. • Compression (P) and shear (S) waves can be measured but the morestandard index tests use only P-wave velocity

• The transmitted wave velocity depends on rock elastic properties anddensity. Fractures cause marked changes in elastic properties andtransmission velocities are sensitive to fracturing.

Oscilloscope

Transducers

Geophones

P-waveS-wave

TransmitterReceiver

Pulse Generator

Power Amplifier

Transmitter

Receiver

SpecimenPreamplifier

Oscilloscope

LABORATORY ACOUSTIC VELOCITY TESTING

ACOUSTIC VELOCITY

Transducers can be made for use under:• Uniaxial conditions or• Triaxial conditions

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Olivine 8400 Gabbro 6800-7200

Pyrite 8000 Basalt 6500-7200

Dolomite 7500 Dolomite 6500-7000

Epidote 7450 Limestone 5500-6500

Magnetite 7400 Sandstone 5000-6000

Pyroxene 7200 Granite 5500-6000

Amphibole 7200

Calcite 6600

Plagioclase 6250

Quartz 6050

Muscovite 5800

Orthoclase 5800

ROCKSMINERALS Vp(m/s) Vp (m/s)

TYPICALACOUSTIC VELOCITYVALUES Rip

Dig

Blast to loosen

Blast to fractureEH

VH

H

M

L

VLVL L M H VH EH

6

2

0.6

0.2

0.06

0.02

0.03 0.1 0.3 1 3 10 30

ROCK QUALITY CLASSIFICATION IN RELATION TO EXCAVATION

Point Load StrengthMN/m2

Frac

ture

Spa

cing

(m)

GLACIAL TILLIGNEOUS

GraniteBasaltTrap rock

ShaleSandstoneSiltstoneClaystoneConglomerateBrecciaCalicheLimestone

SEDIMENTARY

METAMORPHICSchistSlate

MINERAL & ORESCoalIron ore

D10 RIPPABLE MARGINAL NON-RIPPABLE

Velocity in metres/sec X 10000 1 2 3 4

DIG OR BLAST ???

PERMEABILITY• Permeability conveys information about the interconnection ofpore space and fissures within a rock.

• Changes in permeability with stress can be used to assess theinfluence of pores and fissures

• Flat microfissures or cracks are much more susceptible toclosure under normal pressure than spherical pores.

• Discontinuities have a major influence on permeability which isthus extremely Scale Dependent.

Microfissures or cracksSpherical pores

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Striking with geological hammer

Crumbles Breaks easily

Easy Difficult

Indenting with geological pickPeeling with knife

Extremely weak Very weak WeakMedium strong Very strong Extremely strong

Soft rock Hard rock

Requires several hard blows to break

Can only be chipped

2 6 20 60 200

ROCK STRENGTH DESIGNATION AND CRUDE TEST CRITERIA FOR ESTIMATING INTACT ROCK STRENGTH IN FIELD (ISRM)

MPa

STRENGTHPOINT LOAD TEST

An index test for rock strength is provided by the POINT LOAD TEST.

The rock is hydraulically loaded between spherically truncated, hardened, conical steel platens and fails by the development of tensile cracks parallel to the axis of loading.

Is = P/D2 P = Load at failureD = Distance between platens.

• Is is normally reported for D = 50mm called Is50

• The Uniaxial compressive strength, Co, is approx 24Is50

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Point load tester

Hardened conicalplatens

DIAMETRAL TEST

POINT LOAD SPECIMENS can be tested as :-

Diametral test :- loaded across the diameter of the core

Axial test :- loaded along the core axis

Irregular lump:- should be spheroidal.

The ISRM suggested method includes a size correction procedure to determine Is50 from diametral, axial and irregular lump tests.

L>0.5D

L>0.5D

D

L

D

W

L

D

0.3W>D>W

P (k

N)

De2 (mm2)

100 200 500 1000 2500

50

10

2

6

P50 = 18kN

Is50=18x10002500

F si

ze c

orre

ctio

n fa

ctor

De (equivalent ) core diameter (mm)

10040 80 140

1.6

1.0

0.6

0.2

Size Correction Factor

Is50= F. PD

Graphical Procedure

POINT LOAD TEST ANALYSIS

VALID TESTS

INVALID TESTS

UC

S, M

Pa

250

50Co = 22Is(50)

Is(50)

0 10 146

POINT LOAD TEST INTERPRETATION

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POINT LOAD INDEX VALUESMaterial Point Load Index (MPa)

Tertiary Sandstone and claystone 0.05-1.0

Coal 0.2-2.0

Limestone 0.25-8.0

Mudstone, shale 0.2-8.0

Volcanic Flow rocks 3.0-15.0

Dolomite 6.0-11.0

RANGE OF UCS VALUES FOR INTACT ROCK

P

P

Co=P/A

REBOUND TESTSSCHMIDT HAMMER

Originally used for non destructive testing of concrete. Four types of hammer available with varying impact energy. The rebound of the hammer mass after impact on the rock surface can be read directly from the instrument casing (Schmidt hardness number.)

Type L hammer is recommended by ISRM owing to its low impact energy.The Schmidt hardness number has been correlated with the uniaxial compressive strength of rock types

A large amount of scatter is often obtained in the field; problems include:

• influence of discontinuities:• Weathering• Alteration• Jointing• Calibration

THE SCHMIDT HAMMER and ACCESSORIES

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HARDNESSCONE INDENTORSSeveral tests are available for measuring the hardness of rocks, most adapted from metallurgy and called Indentation testsThe hardness values obtained from the indentation tests have been correlated with unconfined compressive strength, Co.

ABRASIVENESS

CERCHAR ABRASIVENESS TEST

A sharp hardened steel stylus is passed across the rock surface for a distance of 10mm under a load of 7kgf .. the unit of abrasiveness is related to the dimension of the scratch..measured using optical techniques.

SLAKE DURABILITYChanges in rock properties due to processes of chemical and mechanical breakdown (eg. exfoliation, hydration, solution, oxidation, abrasion etc.) can be very important in engineering applications. A good index test of rock degradability is the Slake Durability Index.Apparatus :-• 140mm diameter drum with 100mm long 2mm mesh walls.• trough to contain drum + water• motor drive (20rpm)

Mesh CylinderDrum lid

Trough

Drum base

100mm

SLAKE DURABILITY APPARATUSSLAKE DURABILITY

Method1. Sample consisting of 10 spheroidal lumps each approx 50g

is placed in the drum and weighed.2. Drum is placed in trough filled with water at 20oC to a

level just below drum axis and rotated at 20rpm for 10 minutes.

3. Drum is removed and material retained dried at 105oC4. Cycle is repeated and the dried material retained after 2

cycles weighed.

SLAKE DURABILITY = Weight retained 2 cyclesINDEX Initial weight

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SLAKE DURABILITY CLASSIFICATION

DURABILITY Cycle 1 (% retained) Cycle 2. (% retained Very High >99 >98 High 98-99 95-98 Medium-High 95-98 85-95 Medium 85-95 60-85 Low 60-85 30-60 Very Low <60 <30

SWELLING PRESSURE AND STRAIN

The Swelling pressure index is a measure of the pressure necessary to constrain at constant volume, an undisturbed rock specimen immersed in water. The apparatus used is similar to a soil consolidometer.

Swelling Pressure = F/A F = Max axial swelling ForceIndex, Isp A = Cross sectional area of

specimen.

The Swelling strain index is a measure of the axial swelling straindeveloped when a radially confined undisturbed rock specimen is immersed in water.

Swelling Strain = d/L *100% d = Max swelling displacement

Index, Iss L = Initial height of specimen

Rigid Plate

Porous plate

Porous plateSample

Ring

CONFINED SWELL TEST

Standpipe