rock mechanics rock mechanics

ROCK MECHANICSROCK MECHANICS

PROPERTIES OF INTACT ROCKIn the beginning of rock mechanics (in the early

1960s), more attention has been paid to the ), pintact rock than to the other features of rock mass.

The reason of it:– First, the subject of it related heavily to

the general mechanics of solid materials.– Second, intact rock samples are obtained

il f d illeasily from drill cores.

TESTING OF INTACT ROCKi h

• Uniaxial compressive strength testDestructive strength tests:

– Uniaxial compressive strength– Young modulus– Poisson’s ratio

• Triaxial test– Young modulus– Poisson’s ratioPoisson s ratio– Shear strength (cohesion, angle of friction)

• Point load test• Point load test• Indirect tensile strength test (brazilian test, beam

t t)test)• Direct shear strength test

TESTING OF INTACT ROCKNondestructive strength tests:

S h id h• Schmidt hammer– surface strength, estimation of strength

• US wave propagation– detetecting of microcracks inside the specimen– estimation of strength

Other tests:• Density properties, porosity

Other tests:

• Water content, water absorption• Leaking testg

– Water

COMPRESIVE STRENGTH TEST

The compressive strength is probably the most widely d d d k i iused and quoted rock engineering parameter.

Under uniaxial load conditions the maximum stress that the rock sample can sustain referred as uniaxial compressive strength (σucs or σc) .

The most useful description of the mechanical behavior of intact rock is the complete stress – strain curve of pthe compressive strength test.

From this curve can be determined the Young modulusFrom this curve can be determined the Young modulus and the post-peak behavior of the rock material.

COMPRESSIVE STRENGTH TESTCOMPRESSIVE STRENGTH TEST

FORCE – DISPLACEMENT CURVE

(Hudson&Harrison 2007)

STRESS – STRAIN CURVEThere are three zone of the curve:

I compaction zoneI. compaction zoneII. linear zoneIII. failure zone

a) Stress-strain curveVertical and horizontal displacements of thea) Stress strain curve

b) Idealized stress-strain curve (elastic-perfectly plastic)

displacements of the specimen and the stresses (shear failure)

COMPLETE STRESS – STRAIN CURVE


STRESS – STRAIN CURVE

Stress strainStress – strain curve for brittle and forbrittle and for ductile rock material:Rock samples after failureRock samples after failure


DIFFERENT STRESS – STRAIN CURVESHigh stiffnes, strength

Very brittle (basalt)

Medium stiffnesMedium strengthMedium strengthMedium brittleness(limestone)( es o e)

Low stiffnesLow strength

Low stiffnesLow strengthow st e gt

Brittle(chalk)

Low strengthDuctile(rock salt)( )


Characteristic stress-Characteristic stressstrain (s-e) curve of well knownof well known Hungarian rocks

COMPRESIVE STRENGTH

The compressive strength is not an intrinsic property. I i i i l i d d d i lIntrinsic material properties do not depend on material geometry or the loading conditions used during the test.

Because of it the height and diameter ratio of the specimen is 2:1.

• Size effect– Larger specimen has reduced compressive strength and

brittleness.

• Shape effect– When the ratio of diameter to length increases both theWhen the ratio of diameter to length increases both the

compressive strength and the ductility increases.

COMPRESIVE STRENGTH

Size effect Shape effect


COMPRESIVE STRENGTHThe strength is the maximum stress that the rock can

sustain, after it is exceeded the rock may still havesustain, after it is exceeded the rock may still have some load-carrying capacity which called residual strength.strength.


DETERMINATION OF YOUNG’S MODULUS

The Yo ng’s mod l s (E) is defined as the ratio of stressThe Young’s modulus (E) is defined as the ratio of stress to strain.

b d i d iIt can be determined in two ways:• Tangent modulus: by taking the slope of the stress –

strain curve at a given point. – The given point is conventionally at a stress level

corresponding to 50% of the peak stress.

• Secant modulus: by taking the slope of a line y g pconnecting two points on the linear portion of the curve.– This line can be anywhere of the linear portion of the curve.

YOUNG’S MODULUS


COMPLETE STRESS – STRAIN CURVE


POINT LOAD TEST

The point load test is one of the most common test in k i irock engineering.

Benefits of it:• The size and the shape of the specimen could be

varied in wide range, therefore it can preformed when g , pcylindrical specimen is not available.

• This test can easily be preformed on field as well so itThis test can easily be preformed on field as well, so it gives result very quickly.

• The value uniaxial compressive strength can be• The value uniaxial compressive strength can be estimated by the point load strength.

POINT LOAD TEST

The point load test is able to preform on field and in l b lllaboratory as well.

(Marinos&Hoek 2001)

POINT LOAD TEST

The point load test options: a) sample from surface explosureThe point load test options: a) sample from surface explosure, b) sample from core drilling (Marinos&Hoek 2001)

Determination of point load strength: F [N] is the collapsing force, De [mm] is the equivalent diameter of the sample.

WDF

DFIs 42

e [ ] s t e equ va e t d a ete o t e sa p e.

WDDes 42

LOADING CONDITIONS


TENSILE STRENGTH TESTS

Uniaxial tensile strength test is not used in engineering ipractice.

There are two reasons for that:• First, it is very difficoult to preform• Second the rock does not fail in direct tension in situSecond, the rock does not fail in direct tension in situ

conditionsThe tensile strength is normally measured by indirectThe tensile strength is normally measured by indirect

tests, in which the tensile stress is generated by compressive loadingcompressive loading.

• Brazilian test (splitting test)• Beam test (bending test)

INDIRECT TENSILE STRENGTH (BRAZILIAN TEST)

The height and diameter ratio of the specimen is 1x1.Th i i f h i i h i l

(BRAZILIAN TEST)The position of the specimen is horizontal.

i i f h il h

F 2

Determination of the tensile strength [MPa]:F: collapsing force [N]d di t f th i [ ]

hdt d: diameter of the specimen [mm]

h: height of the specimen [mm]

INDIRECT TENSILE STRENGTH (BRASIL)(BRASIL)

rock mechanics rock mechanics

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