Chp 7.2 (Rock Testing)_3

8/10/2019 Chp 7.2 (Rock Testing)_3

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Testing of rockTesting of rockTesting of rockTesting of rock

The most important scope in Rock Mechanics - is

measuring & determination of rock properties &deformational behaviour under specific loading,using recommended testing methods & procedures.

These engineering characteristics of rock includestrengths (compressive, tensile & shear), mode of

failure (bentuk kegagalan) & rock constants(modulus of elasticity E, modulus of rigidity G &Poissons ratio )

Testing of rockTesting of rockTesting of rockTesting of rock

Specific testing methods can be used to evaluate & tomeasure strengths & characteristics of rocksnumerically (in addition to the subjective description

provided by geologists).

Standard test mehods & procedures, laboratory & field

are usually according to International Society of RockMechanics(ISRM) (Ref. 5).Other standards include S. African SRM, ASTM, NGI & BS.


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Laboratory tests for testing of rock samplesLaboratory tests for testing of rock samplesLaboratory tests for testing of rock samplesLaboratory tests for testing of rock samples::::

Rocks are naturally occurring material, they areinhomogeneous & anisotropic. Even rock of the same

class, collected from the same location exhibit variation(kepelbagaian).

Testings are to measure & to evaluate change in rock

properties & behaviour when acted upon by loading. Theproperties include physical, index & strength of rockmaterials. Behaviour include mode of deformation &failure.

Laboratory tests for testing of rock samplesLaboratory tests for testing of rock samplesLaboratory tests for testing of rock samplesLaboratory tests for testing of rock samples::::

Are generally divided into 2 types:

[a] Index test & indirect strength test

(Ujikaji indeks & ujikaji secara tak lansung).

[b] Direct strength test(Ujikaji kekuatan secara lansung)

The types are generally based on methods of testing &nature/type of data obtained.


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[a] Index test & indirect strength test :[a] Index test & indirect strength test :[a] Index test & indirect strength test :[a] Index test & indirect strength test :

Index tests are relatively simple & rapid, but do notprovide fundamental property. Data is just an

indicator (petunjuk) on property being tested.Apparatus is generally simple and portable (allow testto be conducted on site).

Tests may not require detailed sample preparation.Certain tests are non-destructive type & do notinvolve failure of samples (cost saving for sample canbe reused).

[a] Index test & indirect strength test :[a] Index test & indirect strength test :[a] Index test & indirect strength test :[a] Index test & indirect strength test :

Data is not suitable for detailed design purpose but, itis valuable for preliminary / pre-feasibility

assessments. Common tests include:

PointPointPointPoint----load index test,load index test,load index test,load index test,Schmidt / Rebound Hammer (surface hardness) test,Schmidt / Rebound Hammer (surface hardness) test,Schmidt / Rebound Hammer (surface hardness) test,Schmidt / Rebound Hammer (surface hardness) test,Slakes durability index test,Slakes durability index test,Slakes durability index test,Slakes durability index test,Sonic wave velocity test (PUNDIT),Sonic wave velocity test (PUNDIT),Sonic wave velocity test (PUNDIT),Sonic wave velocity test (PUNDIT),Uniaxial compressive strength test (portable type).Uniaxial compressive strength test (portable type).Uniaxial compressive strength test (portable type).Uniaxial compressive strength test (portable type).

Brazilian / Indirect tensile strength test.Brazilian / Indirect tensile strength test.Brazilian / Indirect tensile strength test.Brazilian / Indirect tensile strength test.[Types of equipment are shown in Figure 7.2]


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Fig. 7.2: PointFig. 7.2: PointFig. 7.2: PointFig. 7.2: Point----load index test & Schmidt (rebound)load index test & Schmidt (rebound)load index test & Schmidt (rebound)load index test & Schmidt (rebound)hammer testhammer testhammer testhammer test

PointPointPointPoint----load test on irregular block sampleload test on irregular block sampleload test on irregular block sampleload test on irregular block sample


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Irregular block sample placed in between pointedIrregular block sample placed in between pointedIrregular block sample placed in between pointedIrregular block sample placed in between pointed

loading platensloading platensloading platensloading platens

Schmidt hammer test on irregular block sampleSchmidt hammer test on irregular block sampleSchmidt hammer test on irregular block sampleSchmidt hammer test on irregular block sample


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Slake durability index test apparatusSlake durability index test apparatusSlake durability index test apparatusSlake durability index test apparatus

Slake durability index test apparatusSlake durability index test apparatusSlake durability index test apparatusSlake durability index test apparatus


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Aggregate sample for Slake durability testAggregate sample for Slake durability testAggregate sample for Slake durability testAggregate sample for Slake durability test

Aggregate sample for Slake durability testAggregate sample for Slake durability testAggregate sample for Slake durability testAggregate sample for Slake durability test


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Slaking (disintegration) of rock sample after testSlaking (disintegration) of rock sample after testSlaking (disintegration) of rock sample after testSlaking (disintegration) of rock sample after test

Ultrasonic velocity test apparatusUltrasonic velocity test apparatusUltrasonic velocity test apparatusUltrasonic velocity test apparatus


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Mode of failure of sampleMode of failure of sample diametrical fracturediametrical fractureindicating failure is due to tensile stressindicating failure is due to tensile stress

CompressionCompressionCompressionCompressionCompressionCompressionCompressionCompression

Tensile stressTensile stressTensile stressTensile stressTensile stressTensile stressTensile stressTensile stress

Disc-shaped sample for Brazilian Test.


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Brazilian (indirect tensile strength) test to obtaintensile strength of rock sample.

Mode of failure of sample diametrical fractureindicating the association of tensile stress


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IfIf rock massrock mass is discontinuous (with fractures likeis discontinuous (with fractures like

joints) .. what sort ofjoints) .. what sort of tensile strengthtensile strength would awould a

rock mass has? Is tensile strength a criticalrock mass has? Is tensile strength a critical

property to be determined extensively?property to be determined extensively?

UCS test using portable compression machine : dataobtained is stress at failure.


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PointPointPointPoint----load Index testload Index testload Index testload Index test::::

Is a quick & simple test to undertake. Sample can becore (teras) or irregular block. Equipment is easy tohandle and portable. Test can be undertaken on site.Data obtained is an index (indicator) for strength of

sample tested.A simple test & therefore, no constraint on number oftest.

Index value obtained (Is) can be converted to UCS:UCS 24 Is (Broch & Franklin, 1972).

Irregular block sampleIrregular block sampleIrregular block sampleIrregular block sample


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Irregular block sampleIrregular block sampleIrregular block sampleIrregular block sample

Core sample obtained rock drillingCore sample obtained rock drillingCore sample obtained rock drillingCore sample obtained rock drilling


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Core sample obtained rock drillingCore sample obtained rock drillingCore sample obtained rock drillingCore sample obtained rock drilling

Schmidt / Rebound Hammer testSchmidt / Rebound Hammer testSchmidt / Rebound Hammer testSchmidt / Rebound Hammer testTest on surface hardness of rock sample using Schmidt hammer

(L-type), a portable & simple equipment to handle. Sample can be

core or block. Test is non-destructive & sample can be re-used.

Index data obtained is rebound number (R). The stronger is the

surface the higher is the R value.

R is related to the surface strength (JCS) of rock sample tested:

Log10 JCS = 0.00088 () (R) + 1.01 (Broch & Franklin, 1972).


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Weathered sampleWeathered sampleWeathered sampleWeathered sample(Grade II to III):(Grade II to III):(Grade II to III):(Grade II to III):R = JCSR = JCSR = JCSR = JCS UCSUCSUCSUCS

Fresh sample (Grade I):Fresh sample (Grade I):Fresh sample (Grade I):Fresh sample (Grade I):R = JCSR = JCSR = JCSR = JCS UCSUCSUCSUCS

Effect of rebound no. R on weathered rock surfaceEffect of rebound no. R on weathered rock surfaceEffect of rebound no. R on weathered rock surfaceEffect of rebound no. R on weathered rock surface

Fresh rockFresh rockFresh rockFresh rock(Grade I):(Grade I):(Grade I):(Grade I):R = JCSR = JCSR = JCSR = JCS UCSUCSUCSUCS

WeatheredWeatheredWeatheredWeatheredrock (Graderock (Graderock (Graderock (Grade

II to III):II to III):II to III):II to III):R = JCSR = JCSR = JCSR = JCS UCSUCSUCSUCS


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Slake durability testSlake durability testSlake durability testSlake durability test

Test is to assess the resistance of rock sample toweakening & disintegration when subjected to drying& wetting (weathering process). The stronger the rockthe higher is SDI.

Sample is soaked in water and cycled for a given time.Cycling process can be repeated up to 2 sets. Slake

durability index Isd for hard rock can be as high as100% and soils usually exhibit very low index of lessthan 30%.

Ultrasonic velocity testUltrasonic velocity testUltrasonic velocity testUltrasonic velocity test

Test is non-destructive & equipment is portable &simple.

Test involves transmitting Primary-wave through coresample, and data obtained is wave propagation

velocity (Vp).

The denser the specimen (less voids) the higher is theVp. Rocks display a higher Vp compared to soils.

(note: denser means stronger)


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Type Of Rocks P-wave velocitym/s

Dry, loose topsoils and silts. 180-370

Dry sands, loams; slightly sandy or gravely soft clays. 300-490

Dry gravels, moist sandy and gravely soils; dry heavysilts and clays; moist silty and clayey soils.

460-910

Dry, heavy, gravely clay; moist, heavy clays; cobblymaterials with considerable sands and fines; soft shales;soft or weak sandstones

910-1460

Water, saturated silts or clays, wet gravels. 1460-1520

Compacted, moist clays; saturated sands and gravels;soils below water table; dry medium shales, moderatelysoft sandstones, weathered, moist shales and schists.

1460-1830

Hardpan; cemented gravels; hard clay; boulder till;compact, cobbly and bouldery materials; medium to

moderately hard shales and sandstones, partiallydecomposed granites, jointed and fractured hard rocks.

1680-2440

Hard shales and sandstones, interbedded shales andsandstones, slightly fractured hardrocks.

2440-3660

Unweathered limestones, granites, gneiss, other denserocks.

3660-6100

Brazilian or indirect tensile testBrazilian or indirect tensile testBrazilian or indirect tensile testBrazilian or indirect tensile test

Test is to measure the uniaxial tensile strength of rocksample indirectly using Brazilian test. Equipment is

similar to portable UCS test.

Sample is disc-shaped (thickness, t = 0.5 diameter,

d). Load apply is compression. Sample fails withdiametrical fracture.

Tensile strength, T = [0.636P / d t]

where P is compressive load at failure.


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Index test, data obtained & indicationIndex test, data obtained & indicationIndex test, data obtained & indicationIndex test, data obtained & indication....

Name of test Data obtained and indicationUniaxial compressive

test (UCT).

Uniaxial compressive strength (UCS). Indication on

strength and resistance against loading andfracturing.

Point-load index

strength.

Is(can be converted to UCS). Indication is similar

to UCS

Slake durability index

test.

Isd(Slake durability Index). Indication on

resistance against slaking (pemeroian) and degreeof bonding between mineral grains. Resistanceagainst weathering.

Schmidt / Reboundhammer test.

R (Rebound Number). Indication on surfacehardness (strength) and resistance against impact

and abrasion. R is related to UCS

Tensile strength Tensile strength (T). Indication on resistance

against fracturing and degree of bonding mineralgrains.

Ultrasonic VeolcityTest

Vp (P-wave velocity). Indication on denseness andcompactness.

[b] Direct strength test[b] Direct strength test[b] Direct strength test[b] Direct strength test::::

Test procedure requires detailed preparation of

sample (standard shape & finishing). Samplepreparation process is equipment related & it iscostly.

The testing itself involving sophisticated & largeequipment, thus detailed testing procedures. Mayrequire complex analysis, again it is costly.

However data obtained is fundamental property &direct presentation of property being evaluated.


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[b] Direct strength test[b] Direct strength test[b] Direct strength test[b] Direct strength test::::

Expensive tests to conduct therefore, limited number

of tests.Since data is direct fundamental property, it can beused for detailed design. Tests include:

Permeability of rock.Modulus of deformation elastic modulus (E) &Poisson ratio ().

Uniaxial & Triaxial compressive strength test (Kekuatanmampatan satu-paksi (UCS) & tiga-paksi).

Shear strength test on weakness planes (joint &foliation).

Universal compression machine for uniaxial & triaxialstrength test (500 kN capacity)


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Universal compression machine, equipped with closed-circuit servo-controlled unit (3000 kN capacity)

Universal compression machine, equipped with closed-circuit servo-controlled unit (3000 kN capacity)


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Digital controller for input of test parameters intotesting program e.g. strain-rate & maximum load etc.

Servo-controlled (power pack) unit - for complex testingprogram e.g. compression under strain-controlled

(complete stress-strain curve) & cyclic loading.Essential for study on deformation behaviour of rock.


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Preparation of rock core sample using laboratorycoring machine (tungsten carbide coring bit)

Coring of rock block in laboratory to obtain cylindrical

sample 54 mm diameter & 108 mm height


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Coring of rock block in laboratory to obtain cylindricalsample 54 mm diameter & 108 mm height

Trimming of core sample to the required height orlength using diamond disc cutter.


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Trimming of core sample to the required height orlength using diamond disc cutter.

Lapping of core sample to ensure end surfaces aresmooth and perpendicular to core axis.


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Prepared core samples 54mm dia. & 108mm height.

Prepared core samples 54mm dia. & 108mm ht.


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Prepared core samples 54mm dia. & 108mm ht.

Uniaxial compression testUniaxial compression testUniaxial compression testUniaxial compression test::::Requires preparation of sample, as per ISRM.Cylindrical shape (H : D = 2) & specific finishing ofsample surface.

UCS of rock material & deformation behaviour underloading is verify by applying compressive load until

failure using high capacity universal testing machine.If UCT is conducted with measurement on vertical &horizontal strain, a number of rock properties can bedetermined UCS, strain at failure, E & .

Plotted Curve Stress (MPa) versus Strain (%) in

Figure 7.5.Value of E & for various rock type is as Table 7.1.


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Uniaxial compression testUniaxial compression testUniaxial compression testUniaxial compression test::::

From Fig 7.2, elastic modulus of rock (E) is the gradientof the graph at 50% ultimate stress (UCS):

E = stress / strain = () / ()unit is MPa; is % & E is GPa

Poisson ratio is the ratio of radial (lateral) strain to

axial (vertical) strain taken at 50% UCS:

= r / a

Axial strain at failure is a at ultimate UCS. Strain atfailure indicate the brittleness & ductility of sample.

Core samples for UCS test with axial (vertical) andradial (horizontal) strain measurements


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Electric foil strain gauges (20 - 30 mm gauge length)

to measure strains

2 sets of gauges placed at vertical position (axialstrain) and horizontal position (radial strain)


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Typical plot of stress versus strains curve

Uniaxial and latera l strain vs. normal stress

0

20

40

60

80

100

120

140

160

-0.15 -0.05 0.05 0.15 0.25 0.35

Strain (%)

Uniaxialstress(MPa)

Figure 7.5: Stress versus strain (Figure 7.5: Stress versus strain (aa && rr) curve) curve

a (%)r (%)

Slope = Et, Eav

Slope = Es

c= 104 MPa

c/2

120

80

40

-0.2 -0.1 0 0.1 0.2 0.3

a


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Typical values of E &Typical values of E & for various types of fresh rockfor various types of fresh rock

Rock type Elastic modulus,E (GPa)

Poissonsratio,

Andesite, Basalt 60 0.20

Gabbro, Dolerite 90 0.20

Coal 3 0.42

Dolomite 70 0.15

Gneiss 60 0.24

Granite 60 0.22

Limestone 70 0.30

Quartzite 80 0.17

Sandstone 20 0.15

Shale 12 0.10

Failure under uniaxial compression testFailure under uniaxial compression testFailure under uniaxial compression testFailure under uniaxial compression test::::

Types and modes of failure of sample during testingindicate the degree of hardness & brittleness

(kerapuhan) of rock. A number of characteristics canbe interpreted about the sample tested.

Under uniaxial compression, rock sample fails in asudden manner & fracture planes (satah kegagalan)are distinctive. For strong rock like gabbro (igneous),crushed material (dust) and fragmentations is very

minimal.


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Failure under uniaxial compression testFailure under uniaxial compression testFailure under uniaxial compression testFailure under uniaxial compression test::::

For softer rock like shale (sedimentary rock), failure isgradual and fragmentation & powdered rock is moreobvious. Fracture planes is less significant.

Brittle (rapuh) & hard materials like rocks the strainat failure is relatively smaller & the stress at failure is

higher.

Mode and shape of failure are shown in Figure 7.6

Stress-strain curve for rock samples of differenthardness: ductile and brittle

Stress (MPa)

Strain (%)

Sample A

Sample B


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Modes of failure of rock sample under loading

Fracture planes in failed rock sampleFracture planes in failed rock sampleFracture planes in failed rock sampleFracture planes in failed rock sample


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Fracture planes in failed rock sample (UCT)Fracture planes in failed rock sample (UCT)Fracture planes in failed rock sample (UCT)Fracture planes in failed rock sample (UCT)

Fracture planes in failed rock (basalt) sampleFracture planes in failed rock (basalt) sampleFracture planes in failed rock (basalt) sampleFracture planes in failed rock (basalt) sample


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Fracture planes in failed rock sampleFracture planes in failed rock sampleFracture planes in failed rock sampleFracture planes in failed rock sample

Triaxial Compression testTriaxial Compression testTriaxial Compression testTriaxial Compression test::::

Triaxial compression test (mampatan 3-paksi) is toevaluate the strength of rock under confinement(terkurung), e.g. rock samples obtained from deep

seated rock mass (effect of hydrostatic pressure p =gh, due to weight of overburden).

Rock displays a higher strength when confined anddeformation behaviour becomes ductile / strain-hardening).


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Triaxial Compression testTriaxial Compression testTriaxial Compression testTriaxial Compression test::::

The test is used to assess load bearing capability ofrock at depth (20 m from surface). Example thebedrock (batu dasar) where a large structure is to be

founded & strength of rock surrounding a deep tunnel.

Test requires high capacity compression machine,Hoeks cell & confining pressure generator.

Typical engineering properties of rockTypical engineering properties of rockTypical engineering properties of rockTypical engineering properties of rock

Rock class Rock type Unconfined

compressive

strength

c[MPa]

Tensile

Strength

t[MPa]

Modulus

of

elasticityE [GPa]

Point

load

IndexIs(50)

[MPa]

Angle of

Friction,

0

Limestone 50 - 200 5 - 20 20 - 70 0.5 - 7 33 - 40

Mudstone 5 - 15 0.1 - 6

Sandstone 50 - 150 5 -15 15 - 50 0.2 - 7 25 - 35

Siltstone 5 - 200 2 - 20 20 - 50 6 - 10 27 - 31

Sedimentary

rockShale 50 - 100 2 - 10 5 - 30 27

Gneiss 100 - 200 5 - 20 30 - 70 2 - 11 23 - 29

Marble 100 - 200 5 - 20 30 - 70 2 - 12 25 - 35Metamorphicrock Quartz 200 - 400 25 - 30 50 - 90 5 - 15 48

Basalt 100 - 300 10 - 15 40 - 80 9 - 14 31 - 38

Gabbro 100 - 300 10 - 15 40 - 100 6 - 15 Igneous rock

Granite 100 - 200 5 - 20 30 - 70 5 - 10 29 - 35


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Hoeks Cell for use in triaxial compression test

Hoeks Cell for use in triaxial compression test


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Test set-up for triaxial compression test

Test set-up for triaxial compression test


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Direct shear testDirect shear testDirect shear testDirect shear test::::Shear test is to evaluate shear strength & shearbehaviour of weakness plane in rock (not shearing of

the intact rock material)

This is the most expensive laboratory strength tests,

for it requires special method for acquiring ofsamples from site (fracture plane to be tested) &relatively complex testing procedures.

Shear strength of the weakness planes (fractures &joint) in rock mass is important for project involvingexcavation in rock e.g. slope & tunnel.

Joint is the weakest point in this core sample, sliding orJoint is the weakest point in this core sample, sliding orJoint is the weakest point in this core sample, sliding orJoint is the weakest point in this core sample, sliding or

shearing is likely to occur along this fractureshearing is likely to occur along this fractureshearing is likely to occur along this fractureshearing is likely to occur along this fracture


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Joint is the weakest point in this core sample, sliding orJoint is the weakest point in this core sample, sliding orJoint is the weakest point in this core sample, sliding orJoint is the weakest point in this core sample, sliding orshearing is likely to occur along this fractureshearing is likely to occur along this fractureshearing is likely to occur along this fractureshearing is likely to occur along this fracture

Direct shear testDirect shear testDirect shear testDirect shear test::::

Shear test is to obtain shear strength parameters :cohesion (c), basic friction angle (), & shear strength

().

These parameters are greatly depending on thetexture, roughness & degree of weathering of fracture

surface being tested.

Value of (dry) is between 300 (siltstone & slate) & 400

(limestone & basalt).


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Typical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surface

Estimation of joint shear strength:Estimation of joint shear strength:Estimation of joint shear strength:Estimation of joint shear strength:

The shear joints will affect the stability of excavation face

created in jointed rock mass.

If sufficient information is available to allow realistic

analysis of stability, then some quantitative assessment of

joint strength is possible.

The shear strength can be estimated perfectly adequately

from the following formula, provided that exposed joint

surfaces are available in situ, or at least in core samples.

== nn tan { JRC logtan { JRC log1010 ( JCS /( JCS / nn)) ++ bb }}


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== nn tan { JRC logtan { JRC log1010 ( JCS /( JCS / nn)) ++ bb }}

:: peak shear strengthpeak shear strength

nn : effective normal stress: effective normal stress

JRC: joint roughness coefficientJRC: joint roughness coefficient

JCS: joint wall compressive strength (obtained ReboundJCS: joint wall compressive strength (obtained Rebound

Hammer test on joint surface).Hammer test on joint surface).

bb : is the basic friction angle (obtained from residual shear: is the basic friction angle (obtained from residual sheartest on flattest on flat unweathered surfacesunweathered surfaces

The shear strength estimated using the above formulaThe shear strength estimated using the above formula

includes apparent cohesion of the jointincludes apparent cohesion of the joint


In practiceIn practice bb may vary between 25may vary between 2500 and 35and 3500, and the, and the

assumption of 30assumption of 3000will be adequate.will be adequate.

JCS is obtained from Rebound Hammer test on joint surfaceJCS is obtained from Rebound Hammer test on joint surface

and R is converted to JCS using formula:and R is converted to JCS using formula:

LogLog1010

JCS = 0.00088 (JCS = 0.00088 () (R) + 1.01 (Broch & Franklin, 1972).) (R) + 1.01 (Broch & Franklin, 1972).

JRC can be interpolated from roughness profile.JRC can be interpolated from roughness profile.

A simpler, but more approximate means of estimatingA simpler, but more approximate means of estimating

frictional shear strength (excluding apparent cohesion) isfrictional shear strength (excluding apparent cohesion) is

to take into account of the roughness using descriptionto take into account of the roughness using description

given in the following table and add the basic friction angle.given in the following table and add the basic friction angle.


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Typical roughness profiles and JRC numberTypical roughness profiles and JRC number

Roughness and frictional strength of jointRoughness and frictional strength of jointRoughness and frictional strength of jointRoughness and frictional strength of jointRoughness and frictional strength of jointRoughness and frictional strength of jointRoughness and frictional strength of jointRoughness and frictional strength of joint

DDeessccrriippttiioonnooffrroouugghhnneessss FFrriiccttiioonnaannggllee

SSmmooootthh

DDeeffiinneeddrriiddggeess

SSmmaallllsstteeppss

VVeerryyrroouugghh

BBaassiicc++2200

BBaassiicc++6600

BBaassiicc++110000

BBaassiicc++114400

Note: Slickensiding on joint surfaces will reduce the angle ofNote: Slickensiding on joint surfaces will reduce the angle offriction very considerably, and the presence of gouge or otherfriction very considerably, and the presence of gouge or otherinfilling in the joint aperture may totally control the jointinfilling in the joint aperture may totally control the jointstrength. In such cases, take the angle of friction to be 15strength. In such cases, take the angle of friction to be 1500..


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Why joint is susceptible to weathering?

If the surface of joint A is weathered to grade III, andjoint B is grade I, which joint exhibits higher shearstrength?

If the surface roughness of joint A and B are of similarroughness, how weathering affects the shear

strength of joint A ?

What test is used to measure the compressivestrength of joint surface ?

Frequently asked questionsFrequently asked questionsFrequently asked questionsFrequently asked questions

Portable shear box apparatus suitable for test onPortable shear box apparatus suitable for test onPortable shear box apparatus suitable for test onPortable shear box apparatus suitable for test oncore sample (Roctest Phicore sample (Roctest Phicore sample (Roctest Phicore sample (Roctest Phi 10)10)10)10)


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Portable shear box apparatus suitable for test onPortable shear box apparatus suitable for test onPortable shear box apparatus suitable for test onPortable shear box apparatus suitable for test on

core sample (Roctest Phicore sample (Roctest Phicore sample (Roctest Phicore sample (Roctest Phi 10)10)10)10)

Large shear box apparatusLarge shear box apparatusLarge shear box apparatusLarge shear box apparatus


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Large shear box apparatusLarge shear box apparatusLarge shear box apparatusLarge shear box apparatus

Shear box section for rectangular sampleShear box section for rectangular sampleShear box section for rectangular sampleShear box section for rectangular samplesize 150size 150size 150size 150 220220220220 220 mm (H220 mm (H220 mm (H220 mm (H LLLL B)B)B)B)


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Shear box section for rectangular sampleShear box section for rectangular sampleShear box section for rectangular sampleShear box section for rectangular samplesize 150size 150size 150size 150 220220220220 220 mm (H220 mm (H220 mm (H220 mm (H LLLL B)B)B)B)

Sample of rectangular joint size 150Sample of rectangular joint size 150Sample of rectangular joint size 150Sample of rectangular joint size 150 220220220220 220220220220mm (Hmm (Hmm (Hmm (H LLLL B)B)B)B)


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In situ or field testIn situ or field testIn situ or field testIn situ or field test::::

Field or in situtests are to assess properties of rock at

location where they are found.

Simple laboratory index tests (e.g. point-load &Rebound hammer) can also be undertaken on site as

to save time & minimise disturbance of samples.

Field test include large-scale direct strength testsundertaken on site. Tests require elaboratepreparation & equipments & hence very expensive,complex & time consuming.

In situ or field testIn situ or field testIn situ or field testIn situ or field test::::

In situstrength tests are undertaken when propertiesof rock is very critical to the design and detailedassessment under actual environment is essential.

Most importantly, the cost justifies the importance ofthe data.

The main advantages of field full-scale test are:it involves a larger size of sample (inclusive of large-scale discontinuities; in situsample is thereforeundisturbed & more representative of field conditions.


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Roughness of joint surface may induce dilation during

shearing of joint blocks. Dilation is the verticaldisplacement that leads to joint opening.

Texture of joint surface (roughness)


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Texture of joint surfaceTexture of joint surfaceTexture of joint surfaceTexture of joint surface roughness (kekasaran)roughness (kekasaran)roughness (kekasaran)roughness (kekasaran)

Joint orientaion that may contribute to rock sliding,Joint orientaion that may contribute to rock sliding,Joint orientaion that may contribute to rock sliding,Joint orientaion that may contribute to rock sliding,note the typical profile of the jointnote the typical profile of the jointnote the typical profile of the jointnote the typical profile of the joint


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Properties of rock determined in laboratory & fieldProperties of rock determined in laboratory & fieldProperties of rock determined in laboratory & fieldProperties of rock determined in laboratory & field::::

Table 7.2 shows the commonly tested rock properties

that include:Laboratory moisture content, hardness, slaking,permeability, elasticity modulus & strength.

Field moisture content, ultrasonic velocity,permeability & shear strength.

When interpreting field & lab data, the conditions ofthe in situ rock mass must be carefully considered,such as degree of weathering & prevailingdiscontinuities.

Properties of rock determined in laboratory & fieldProperties of rock determined in laboratory & fieldProperties of rock determined in laboratory & fieldProperties of rock determined in laboratory & field::::

Take for example the effect of weathering, a difficult

parameter to be accommodated and considered indesigning a structure although its effect on rockproperties is paramount.

Sample use in laboratory strength test is usually fresh(grade I). On site, the rock body may be weathered.Therefore, a reduction factor must be imposed on to

the laboratory data (Figure 7.6). This is similar to theuse of F.O.S. in design, to cater for the uncertaintyaspects & parameters that are difficult to measure.


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Fresh rock (weathering grade I)Fresh rock (weathering grade I)Fresh rock (weathering grade I)Fresh rock (weathering grade I)

Weathered rock (Gred V)Weathered rock (Gred V)Weathered rock (Gred V)Weathered rock (Gred V)


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Strength Reduction Factor (SRF): Effect of differentweathering grades ( I to V) on strength of rock.

Grade VI is soils!

Core samples are collected by drilling & coring in different locations and depthin rock mass on site. Depending on the proposed structure and conditions of the

samples obtained, different type of test may be undertaken on each sample

g

g'

f

f'

S3S2

S1

J1

J2


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Strength classification of rocks based on point

load index strength (Broch & Franklin, 1972)

Strength Classification Is (MN/m2) Equivalent UCS (MN/m2)

Very strong >6.7 >100

Strong 3.35-6.7 50-100

Moderately strong 0.85-3.35 12.5-50

Moderately weak 0.4-0.85 5-12.5

Weak 0.12-0.4 1.25-5

Very weak rock or hardsoil

0.05-0.12 0.6-1.25

Is = 24 UCS

Durability Classification (Gamble, 1971)

Group name anddescription

% retained after one 10min. cycle, Id1(dry

weight basis)

% retained after two 10min. cycle, Id2(dry

weight basis)

Very high durability >99 >98

High durability 98-99 95-98

Medium Highdurability

95-98 85-95

Medium durability 85-95 60-85

Low durability 60-85 30-60

Very Low durability


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Uniaxial compressive and tensile strengths ofrocks (Pitts, 1984)

Rock Type Uniaxial Compress.Strength (MPa)

Uniaxial TensileStrength (MPa)

Granite 100-250 7-25

Dolerite 200-350 15-35

Basalt 150-300 10-30

Sandstones 20-170 4-25

Mudrocks 10-100 2-10

Limestones 30-250 5-25

Gneisses 50-200 5-20

Typical static mechanical properties of some

common rock types (Bengt Stillborg, 1985)

Rock class Rock type Unconfinedcompress.strength

c[MPa]

TensileStrength

t[MPa]

Point loadIndex

Is(50) [MPa]

Limestone 50 - 200 5 - 20 0.5 - 7

Mudstone 5 - 15 0.1 - 6

Sandstone 50 - 150 5 -15 0.2 - 7Siltstone 5 - 200 2 - 20 6 - 10

Sedimentaryrock

Shale 50 - 100 2 - 10

Gneiss 100 - 200 5 - 20 2 - 11

Marble 100 - 200 5 - 20 2 - 12Metamorphicrock Quartz 200 - 400 25 - 30 5 - 15

Basalt 100 - 300 10 - 15 9 - 14

Gabbro 100 - 300 10 - 15 6 - 15Igneous rock

Granite 100 - 200 5 - 20 5 - 10


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Classification of rock types based on unconfined

compressive strength (McLean & Gribble, 1980)Descriptive terms UCS (MPa) Rock types

Very weak rock.Weak rock.Moderately weakrock

Moderatelystrong rock

Strong rock.

Very strong rock.

Extremely strongrock.

< 1.251.25 5.0.5.0 12.5

12.5 50.0

50 100

100 200

> 200

Some weakly compacted sedimentaryrocks, some very highly weatheredigneous or metamorphic rocks, boulder-clays.

Some sedimentary rocks, some foliatedmetamorphic rocks, highly weatheredigneous and metamorphic rocks.

Some low-grade metamorphic rocks,marbles, some strongly cementedsandstones (silica cement), someweathered and metamorphic igneousrocks.

Mainly plutonic, hypabyssal andextrusive igneous rocks (medium tocoarse grained), sedimentary quartzites,strong slate, gneisses.

Fine-grained igneous rock, metamorphicquartzites, some hornfelses.

Chp 7.2 (Rock Testing)_3

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