physico mechanical properties of rock materials and details of different laboratory as well as field tests for determining behaviour of different rock materials in the field of mining and civil engineering

1. Physico-Mechanical
Properties of Rock Materials
Siva Sankar Ulimella M.Tech
Under Manager
Project Planning, SCCL
Email: uss_7@yahoo.com
PHYSICO-MECHANICAL PROPERTIES OF ROCKS
A Rock material is an aggregate of mineral particles
The performance of the rock, under a particular condition
depends upon physical and mechanical properties of rock
materials
The Physical properties may be known as Index properties,
which describes the rock material and helps in classifying them
The Mechanical properties may be known as Strength properties
and they will give an information about the performance of rock
materials, when subjected to a particular loading system.
When we talk of Rock Strength we generally understand that:
Rock material is generally strong in compression.
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2. PHYSICO-MECHANICAL PROPERTIES OF ROCKS
Rocks exhibit a brittle type behaviour when unconfined, but become
more plastic as the level of confinement increases.
Conditions in the field are primarily compressive and vary from
unconfined near the opening walls to confined at some distance
from the opening.
The strength of a rock is affected not only by factors that relate to
its physical and chemical composition such as its mineralogy,
porosity cementation, degree of alteration or weathering, and water
content, but also by the methods of testing, including such factors as
sample size, geometry, test procedure, and loading rate.
Physical Properties of Rock Material
The physical properties of rocks affecting design and
construction in rocks are:
• Mineralogical composition , structure, and texture;
• Specific gravity G
• Unit weight
• Density
• Void ratio e
• Porosity n
• Moisture content w
• Degree of saturation, S
• Coefficient of Permeability k
• Electrical and Thermal properties
• Swelling
• Anisotropy
• Durability
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3. Physical Properties of Rock Material
Mineralogical composition is the intrinsic property controlling the
strength of the rock Although there exist more than 2000 kinds of
known minerals, only about nine of them par take decisively in
forming the composition of rocks. They are:
• Quartz
• Feldspar
• Mica
• Hornblende (Amphiboles)
• Pyroxenes
• Olivine
• Calcite
• Kaolin, and
• Dolomite
Physical Properties of Rock Material
Specific gravity is the ratio of the density of solids to the density of water.
M 1
G= S ⋅
VS ρW
(where M S = mass of solids and VS -volume of solids)
Unit weight ( γ )
W
γ=
V
( W is the total weight of the sample and V the total volume of the sample)
Density is a measure of mass per unit of volume. Density of rock material various, and
often related to the porosity of the rock. It is sometimes defined by unit weight and
specific gravity. Most rocks have density between 2,500nd 2,800 kg/m3.
Dry Density, Bulk Density, and Saturated Density
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4. Physical Properties of Rock Material
Void ratio (e) is the ratio of the volume of voids (VV) to the volume of solids (VS)
VV
e=
VS
Wd G γ
γ Dry = = ⋅γW =
V 1+ e 1+ w
Porosity (n) describes how densely the material is packed. It is the ratio of the non-solid
volume (VV) to the total volume (V) of material. Porosity therefore is a fraction between 0
and 1.
V e V − (WS / Gγ W )
n= V = =
V 1+ e V
1 VS
=
1+ e V
(The unit weight of water = 1 g/cm3 = 1 t/m3 = 9.81 kN/m3 = 62.4 lb/ft3)
Physical Properties of Rock Material
Porosity decreases with increasing age of the rock and depth of the rock
Porosity is a measure of water – holding capacity of a rock material
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5. Physical Properties of Rock Material
Moisture Content (M): it is the ratio of weight of water in the voids to the weight
of dry solids in the rock sample
M = W w / W s, where M = Moisture Content, Ww = Weight of water, and
Ws = Weight of Solids
Degree of saturation (S): it is defined as the volume of water in the void to the
total volume of voids in the rock sample
S = Vw / Vv, where Vw = volume of water, and Vv = volume of voids
The rockmass having higher porosity has higher degree of saturation
Permeability (k): the ability of porous material to allow a liquid to pass through
its pores, units: cm/sec, or m/sec
Q=kiA
Q= discharge through area, i= hydraulic gradient
Electrical properties:
Most of the rocks are dielectric in nature and measurement of Dielectric
constants used for data interpretation
Electric resistivity method used in geophysical prospecting
Physical Properties of Rock Material
Thermal Properties: Increase in temperature makes rock weaker due to the
formation of cracks in the rockmass
Coefficient of thermal expansion of the rocks: increase in length due to a
change in temperature
Swelling: it is an increase in volume of the mass due to suction of water or due
to contact of water for a longtime
Swelling is more in weaker type rocks
Anisotropy: properties of the elements of the rock mass are not similar in
every direction, due to sequence of rock formation, i.e., due to existence of
bedding planes, etc.
Anisotropic material has some weakness in a particular direction
Sedimentary rocks have high degree of anisotropy
Durability : it is the resistance to destruction.
If rock is more durable means it will last for a longer period when put into use.
It depends upon the nature of environment against which the rock is going to be
used. Swelling index or slake durability test is used to describe nature of
weathering
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6. EXAMPLES 1.
A cylindrical specimen of moist clay has a diameter of 38 mm, height of 76 mm
and mass of 174.2 grams. After drying in the oven at 105 0 C for about 24 hours,
the mass is reduced to 148.4 grams. Find the dry density, bulk density and water
content of the clay. Assuming the specific gravity of the sample grains as 2.71,
find the degree of saturation.
Solution
Strength and Deformation Properties
of Rocks
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7. Idealized diagram showing the transition from intact rock to a heavily jointed
rock mass with increasing sample size.
• Figure above illustrates the difficulty in finding a
realistic failure criterion for rock masses, it shows the
transition from intact rock material to a heavily
jointed rock mass.
• The underground excavation designer is concerned
with all stages in this transition.
• The stability of the entire system of u/g openings
which make up a mine depends upon the behaviour of
the entire rock mass surrounding these openings.
• The rockmass may be heavily jointed that it will tend
to behave like an assemblage of tightly interlocking
angular particles with no significant strength under
confined conditions.
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8. In considering the behaviour of rock as an
engineering material in transition from intact rock to
heavily jointed rock mass, the quantity and quality
of experimental data decrease rapidly as one
moves from the intact rock sample to the rock mass.
Because small samples are easy to collect and to
test under a variety of laboratory conditions.
Experimental difficulties increases significantly
in tests on samples with a single set of a joint to
multiple sets. Further the full scale testing on
jointed rock mass is a real challenge both in
terms of testing as well as expense.
Taking all these factors into consideration, it is seen that the
failure criteria which will be of significant use to the
underground excavation designer should satisfy the following
requirements:
•It should adequately describe the response of an intact rock
sample to the full range of stress conditions likely to be
encountered underground. These conditions range from
uniaxial compression, tension, to triaxial compression
•It should be capable of predicting the influence of one or
more sets of discontinuities upon the behaviour of a rock
sample.
•It should provide some form of projection, even if appropriate,
for the behaviour of a full scale rock mass containing several
sets of discontinuities.
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9. Mechanical or Strength Properties of Rocks
Strength : Ability of a material to resist an externally applied load, but
In Rock mechanics, strength is the Force per unit Area required to bring about
rupture in a rock mass at a given environmental conditions.
Classification of strength: depending upon type of loading and the stresses, the
strength in general may be classified as
Compressive Strength
Tensile strength, and
Shear Strength
For determining the above strength values the tests are conducted either on intact
rock specimens in the laboratory tests or on rockmass in the field, i.e., insitu
strength tests
In the laboratory there are direct Methods for the determination of above strength
values in the laboratory and also indirect methods for the determination of above
strength values roughly in the laboratory or at the field site
Mechanical or Strength Properties of Rocks
Compressive Strength
The compressive strength of a material is a measure of its ability to resist
uniaxial compressive loads without yielding or fracture.
The most common measure of compressive strength is the Uniaxial
compressive strength or unconfined compressive strength. It is one of the
most important properties used in design, analysis and modeling.
Direct Methods:
1.Uni axial Compression Test
2.Tri axial Compression Test
Indirect Method :
1.Point Load Test
2.Schmidt hammer Test
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10. Mechanical or Strength Properties of Rocks
Direct Method: It requires a preparation of sample as accordance to ISRM
(International Society of RockMechanics).
Uniaxial compressive strength (UCS) of rock material and deformation
behavior under loading is verified by applying compressive load until failure
occurs in the core by a fracture in the middle using high capacity Compressive
testing machines
ISRM Standards for Testing of Rock Specimens in Laboratory
(a) Test specimens shall be right circular cylinders having a height to diameter ratio of 2.0-
3.0 and a diameter preferably of not less than NX core size, approximately 54 mm. The
diameter of the specimen should be related to the size of the largest grain in the rock by
the ratio of at least 10:1.
(b) The ends of the specimen shall be flat to 0.02 mm and shall not depart from
perpendicularity to the of the specimen by more than 0.001 radian (at 3.5 mm) or
0.05 mm in 50 mm.
(c) The sides of the specimen shall be smooth free of abrupt irregularities and straight to
within 0.3 mm over the lull length of the specimen.
(d) The use of capping materials or end surface treatments other than machining is not
permitted.
(e) The diameter of the test specimen shall be measured to the nearest 0.1 mm by averaging
two diameters measured at right angles to each other at about the upper-height, the mid-
height and the lower height of the specimen. The average diameter shall be used for
calculating the cross-sectional area. The height the specimen shall be determined to the
nearest 1.0 mm.
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11. ISRM Standards for Testing of Rock Specimens in Laboratory
(f) Samples shall be stored, for no longer than 30 days. in such a way as o
preserve the natural content, as far as possible, and tested in that condition. This
moisture condition shall be reported in accordance with “Suggested method for
determination of the water content of a rock sample”.
(g) Load on the specimen shall be applied continuously at a constant Stress rate
such that failure occur within 5 -10 min. of loading, alternatively the stress rate
shall be within the limits of 0.5—1.0 MPa/s.
(h) The maximum load on the specimen shall be recorded in newtons (or
kilonewtons and mega-newtons where appropriate) to within 1%.
(i) The number of specimens tested should be determined from practical
considerations but at least five are preferred.
Uniaxial compressive strength (UCS)
UCS is given by the ratio of load at failure or rupture to cross-sectional area of the
specimen
If the length-to-diameter ratio of the rock specimen is less than 2, the measured
compressive strength, Ca should be corrected to give the standardized compressive
strength, C0, by means of the following equation:
Where:
D = diameter of specimen, in.
L = length of specimen, in.
C a= measured compressive strength, lb/in.
C0 = corrected (computed) compressive strength of an equivalent L/D = 2
specimen.
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12. Compressive Testing Machines
Universal Testing Machine
Manual
INSTRAN Testing Machine
With UTM, the axial displacement w.r.t. load is to be recorded manually with the
help of proving ring, while lateral deformation recorded using dial gauges or
strain gauges coupled to LVDT,If provided.
With Instran machine Displacement between Loading Platens will give axial
displacement of the specimen under loading and directly get recorded in
connected computer
Lateral displacement will be recorded in computer using the special attachment
shown below or manually recorded using strain gauges coupled to LVDT
Lateral Displacement Measurement
12

13. Uniaxial stress-strain curves for different rock
types
Peak stress
Post peak characteristics
are different in different
rock types
Uniaxial compression
Class I: is a stable fracture propagation
which means that when the max load
bearing capacity is exceeded, still some
external work has got to be done for
further destruction of the specimen
Class II: unstable fracture
propagation takes place such that
the amount of energy stored in
the specimen at the moment
when its max load carrying
capacity is just exceeded is
sufficient to maintain the crack
growth
13

14. Post failure behaviour of rock in compression
Cyclic loading
The behaviour of the rock under compression until the rock
has lost its strength is as shown in the following figure.
Load deformation
Curve if loading-unloading
is not followed
Compressive Strength
The load-deformation characteristics in UCS for loading and
unloading cycles follow the following behaviour:
1. On loading , the curve eventually joins that for a specimen
in which the axial displacement increases with time
2. As displacement continues in the post-peak region, the
portion of the total displacement that is irrecoverable
increases
3. The loading-unloading-loading loop shows some hysteresis
4. The apparent modulus of the rock which can be calculated
from the slope of the reloading curve, decreases with post-
peak deformation and progressive fragmentation of the
specimen
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15. Rock Failure characteristics in UCS
Spalling phase
Shear
Fracture
The fracture pattern of specimen is divided into 8 distinct regions.
I-III are marked with closure of pre-existing cracks as well as
coalescence of random crack formation, crack growth and sliding
on existing crack surface
IV extension of the small fractures parallel to the line of loading. The
cracks appear at the center of the specimen height and dilation is
prominently seen. The peak strength is also reached
V Spalling of the dilated specimen starts at the beginning of the
region V of stress-strain curve, continues in the region VI followed
by a steeply inclined shear fracture plane and it grows into the
region VII
VIII Loose mass of the broken material is held together due to friction
15

16. Failure modes in compression
Rock Specimens before & After failure in Uni-Axial compression
16

17. Triaxial compression of rock samples – Direct Method
When the rock specimen is subjected to
confining pressure in addition to vertical
pressure, the strength exhibited by rock
specimen is known as Triaxial compressive
strength
Axial loading by Compressive testing machine
and Confining pressure usually oil pressure from
external source
Usually tests on atleast five specimens, each at a
different confining pressure needed to define
peak strength envelope Sigma 1 Vs Sigma 3
This test also helps in determining shear
Stress Strain Curve in Triaxial strength parameters of rock material from the
Compression Mohr’s envelope drawn from test results
Axi Symmetric Triaxial compression
Mohr’s Envelope
Triaxial Cell
17

18. Tri-axial compression
Figure Complete axial stress-axial strain curves obtained in tri-axial compression
tests on Tennessee Marble at various confining pressures (Wawersik & Fairhurst
1970).
Effects of Confining Pressure
A number of important features of the behaviour of rock in tri-axial
compression can be seen, such as with increasing confining
pressure,
(a) the peak strength increases;
(b) there is a transition from typically brittle to fully ductile
behaviour with the introduction of plastic mechanism of
deformation;
(c) the region incorporating the peak of the axial stress-axial
strain curve flattens and widens;
(d) the post-peak drop in stress to the residual strength reduces
and disappears at high confining stress.
The confining pressure that causes the post-peak reduction in
strength disappears and the behaviour becomes fully ductile
(48.3 MPa in the figure), is known as the brittle-ductile
transition pressure. This brittle-ductile transition pressure
varies with rock type.
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19. CONFINED
LOAD
UNCONFINED
DISPLACEMENT
Effects of Confining Pressure
Effects of Pore water Pressure
A series of triaxial compression tests was
carried out on a limestone with a
constant confining pressure of 69 MPa,
but with various level of pore pressure
(0-69MPa). There is a transition from
ductile to brittle behaviour as pore
pressure is increased from 0 to 69 MPa.
In this case, mechanical response is
controlled by the effective confining
stress (σ3' = σ3 – u).
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20. Rock Specimens before & After failure in Triaxial compression
Compressive Strength - Indirect Test:
A.Point Load Strength Index Test
Point load test of rock cores can be conducted diametrically and
axially. In diametrical test, rock core specimen of diameter D is
loaded between the point load apparatus across its diameter.
The length/diameter ratio for the diametrical test should be
greater than1.0.
Uncorrected point load strength, Is, is calculated as:
Is = P 2
De
Where:
P = Load at failure in (kN)
De= equivalent diameter for a circular core (m)
20

21. Compressive Strength - Indirect Test:
UCS = 14 x Is for Indian Coal measure rocks
UCS = 21 x Is in other cases
UCS = Uniaxial Compressive Strength
Is = point Load strength
Compressive Strength - Indirect Test:
Schmidt or rebound Hammer Test:
It normally tests on surface hardness of rock sample as it is also easy to use and
handle. The sample can be in core or in block shape and it is non-destructive type
of test. The best part of the test is that the sample used for the previous test can be
used again.
Schmidt or rebound Hammer
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22. Tensile strength Tests
Tensile strength of a material is defines as the maximum tensile stress which a
material is capable of developing
In nature rockmass is rarely subjected to direct tension, but it is subjected to
tensile stresses
Rocks are weak in tension
Direct Tests:
In this Rock specimen is subjected to uni-axial tensile loading along its axis.
The principal difficulties associated with tensile tests on rock the prevention of
failure within the grips and the elimination of bending in the specimen.
Indirect Method:
Brazilian Test : (Mellor & Hawkes,1971)
Where
T is the tensile strength, P is the maximum compressive load recorded during the test,
D is the diameter, and t is the thickness of the test specimen.
Rock Specimens before & After failure in Brazillian Test
22

23. Tensile strength Tests
Brazilian test in which tensile failure is induced in a disc by compressing it across a
diameter.
Point load Test :
Point load is approximately 0.8 times the uni-axial Tensile strength
UTS = 0.8 x Is
UTS = Uniaxial Tensile Strength
Is = point Load strength
Shear strength Tests
Shear strength of may be defined as the maximum resistance to deformation due to
shear displacement caused by shear stress
Shear strength in a rockmass is derived from the surface frictional resistance along the
sliding plane, interlocking between individual rock grains and cohesion in sliding
surface of the rock.
Shear strength Tests
It mostly deals with the shear strength and shear behavior of the shearing and
weakness planes of the rock which hold together a rock specimen.
This is the most expensive laboratory strength tests, as it requires special kind
of methodology for acquiring the samples from the site as fracture planes to be
tested and utmost relatively complex testing procedures
In general there are two methods for evaluation of Shear Strength of rocks;
1. Direct Shear Test
a. Shear Box Test
b. Shear Test on Rock Cubes
2. Indirect Shear Test – Punch shear Test
23

24. Shear Box Test:
Constant Normal Load
(CNL)
Arrangement for shear
Testing
Complete setup of Shear Testing Portable shear Testing apparatus
apparatus with online acquisition system
Constant Normal Load Condition
σ n = Constant Free to move
τ
Rock slope stability
Constant normal
load (CNL) （non-reinforced）
(After Barton)
24

25. Direct shear test apparatus
(1) The Constant Normal Load (CNL) is applied on single rock joint through a
loading yoke connected to a loading lever.
(2) The shear displacement is applied through the advancement of a lead screw which
is pushing the shear box assembly. A high sensitivity proving ring (5 MPa) is used
for measuring the shear load.
Direct shear test apparatus
Vertical dial gauge Loading Yoke
Shear Box
Specimen
Proving Ring
Lead Screw
Turret Gear Box
Horizontal dial gauge
25

26. Two halves of the joint ready for molding One half placed in concrete mold
Leveling the Sample Samples after mold is set
Sample Preparation
Fig: Surface profiler Fig: Brush profiler
Joint roughness Coefficient Measurement
26

27. Shear behaviours of rock joint (i = 150)
Peak shear stress region
1.5 Mpa
Residual stress region
1.Mpa
0.5 Mpa
0.25 Mpa
Larger shear stresses are obtained
under higher normal stress levels
Positive dilation is shown in the
residual region and negative dilation
is generated when a shear starts.
Fig. Shear stress-horizontal displacement and dilation curves at
0.502 mm/min shear rate (asperity angle i = 150)
1. Mobilization of friction with beginning
of stress. This usually occurs with in
the first 1mm of shear displacement.
2. Mobilization of roughness with the
beginning of dilation.
3. Peak shear strength at which
contribution from JRC is maximum.
4. Beyond peak stress roughness is
gradually destroyed with the
declining of dilation.
Fig; Ideal shear Stress Vs
Displacement Curve
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28. Relation between strength Properties
Uni-axial Compressive Strength = 7.5 times of Shear strength
= 10.5 times of Tensile Strength
= 14 to 21 times of Point Load Strength
Index
Elastic Properties of Rocks :
Elastic constants are evaluated by Uniaxial compression, Uniaxial Tension or
Flexural Strength tests and choice depends up on the type of loading expected in
field
28

29. Elastic properties of rocks
Fig. Stress-strain curve with yield point, peak strength, post-peak
ductile and brittle behaviour.
Elastic properties of rocks
Tangent Modulus
of elasticity
Secant Modulus
of elasticity
Fig Stress strain relationship for determination of Young’s modulus (E) and
Poisson’s ratio
29

30. Elastic properties deformation in rocks
Modulus of Elasticity
Rate of change of strain as a function of stress. The slope of the
straight line portion of a stress-strain diagram. Tangent modulus
of elasticity is the slope of the stress-strain diagram at any point.
Secant modulus of elasticity is stress divided by strain at any
given value of stress or strain. It also is called stress-strain ratio.
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31. Rock Material Classification
Compressive Strength (MPa)
Range Description
0.25 – 1.00 Extremely weak
1–5 Weak
5 – 25 Medium strong
25 – 50 Strong
50 – 100 Very strong
100 - 250 Very Very Strong
>250 Extremely strong
Point Load Strength Index
Range Description
1–2 Average
2–4 Strong
4 -8 Very Strong
>8 Exceptionally strong
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32. Angle of Internal Friction (Degrees)
Range Description
< 15 Very Poor
15 – 25 Poor
25 - 35 Fair
35 - 45 Good
45 Very Good
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