Rock mechanics focuses on studying the properties and behavior of intact rock and rock masses. Testing of intact rock samples involves destructive strength tests like uniaxial compression and triaxial tests as well as nondestructive tests like Schmidt hammer and sonic wave propagation. The compressive strength test is widely used in rock engineering to determine parameters like the Young's modulus. The complete stress-strain curve obtained from compression testing provides information on the rock's strength, stiffness and failure behavior. Other tests like point load and Brazilian tests are also used to indirectly measure the tensile strength of rock samples.

1. ROCK MECHANICSROCK MECHANICS

2. PROPERTIES OF INTACT ROCK
In the beginning of rock mechanics (in the early
1960s), more attention has been paid to the), p
intact 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.

3. TESTING OF INTACT ROCK
i h
• Uniaxial compressive strength test
Destructive 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

4. TESTING OF INTACT ROCK
Nondestructive 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

5. 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 ofp
the 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.

6. COMPRESSIVE STRENGTH TESTCOMPRESSIVE STRENGTH TEST

7. COMPRESSIVE STRENGTH TESTCOMPRESSIVE STRENGTH TEST

8. FORCE – DISPLACEMENT CURVE
(Hudson&Harrison 2007)

9. STRESS – STRAIN CURVE
There are three zone of the curve:
I compaction zoneI. compaction zone
II. linear zone
III. failure zone
a) Stress-strain curve
Vertical 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)

10. COMPLETE STRESS – STRAIN
CURVE
(Hudson&Harrison 2007)

11. COMPLETE STRESS – STRAIN
CURVE
(Hudson&Harrison 2007)

12. STRESS – STRAIN CURVE
Stress strainStress – strain
curve for
brittle and forbrittle and for
ductile rock
material:
Rock samples after failureRock samples after failure
(Hudson&Harrison 2007)

13. DIFFERENT STRESS – STRAIN
CURVESHigh stiffnes, strength
Very brittle (basalt)
Medium stiffnes
Medium strengthMedium strength
Medium brittleness
(limestone)( es o e)
Low stiffnes
Low strength
Low stiffnes
Low strengthow st e gt
Brittle
(chalk)
Low strength
Ductile
(rock salt)( )
(Hudson&Harrison 2007)

14. Characteristic stress-Characteristic stress
strain (s-e) curve
of well knownof well known
Hungarian rocks

15. 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.

16. COMPRESIVE STRENGTH
Size effect Shape effect
(Hudson&Harrison 2007)

17. COMPRESIVE STRENGTH
The 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.
(Hudson&Harrison 2007)

18. 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 liney g p
connecting two points on the linear portion of the
curve.
– This line can be anywhere of the linear portion of the curve.

19. YOUNG’S MODULUS
(Hudson&Harrison 2007)

20. COMPLETE STRESS – STRAIN
CURVE
(Hudson&Harrison 2007)

21. 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 wheng , p
cylindrical 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.

22. POINT LOAD TEST
The point load test is able to preform on field and in
l b lllaboratory as well.
(Marinos&Hoek 2001)

23. 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.
WD
F
D
F
Is
42


e [ ] s t e equ va e t d a ete o t e sa p e.
WDDe
s
42

24. LOADING CONDITIONS
(Hudson&Harrison 2007)

25. 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
conditions
The 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)

26. 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 [ ]
hd
t

 d: diameter of the specimen [mm]
h: height of the specimen [mm]

27. INDIRECT TENSILE STRENGTH
(BRASIL)(BRASIL)