Department of Geology

Engineering Geology (CEng 2112) (Cr. Hr. 2)

For Civil Engineering 2nd Year Students
CHAPTER 4
Engineering Properties and Classification of
Rocks.
March, 2025

1
 4.1 Basic Engineering Properties of Rocks

 Engineering properties of rocks is a collective nomenclature which

includes all properties of rocks that are relevant to engineering
application after their extraction from natural beds or without
extraction i.e. insitu conditions.

 The first set include all those properties for which a rock must be
tested for selection as a material for construction such as a building
stone, road stone or aggregate for concrete making.

 The second set of properties include the qualities of a natural bed

rock as to what extent and where it exists. That would determine its
suitability or otherwise as a construction site for a proposed
engineering project.

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 Index properties in Engineering Classification of Rocks
 Since there are vast ranges in the properties of rocks, engineers rely
on a number of basic measurements to describe rocks
quantitatively. These are known as Index Properties.

 These index properties are

 Porosity,
 Density,
 Sonic velocity,
 Permeability,
 Durability and
 Strength.

3
a) Porosity is a dimensionless fraction expressing the proportion of the void
spaces to the total volume in rock . e = Vv/Vt
 In sedimentary rocks, formed by the accumulation of grains, rock fragments,
or shells, the porosity varies from 0 to 90% (In these rocks, porosity
generally decreases with age and depth below the surface).
 Some volcanic rocks (e.g. Pumice, Scoria) can have very high porosity due
to vesicles left by the escaping gas bubbles.
 In crystalline limestone and evaporites and most igneous and metamorphic
rocks, a large proportion of the pore space belong to planar cracks termed
fissures.

b) Density is defined as the mass per unit volume of the rock.

 Dry density: It is the weight per unit volume of an absolutely dried rock
specimen including the volume of the pore spaces present in the rock.

 Bulk density: It is the weight per unit volume of a rock sample with natural
moisture content where pores are only partially filled with water.

 Saturated density: It is the density of the saturated rocks or weight per unit
volume of a rock in which all the pores are completely filled with water. 4
c) Permeability is the ability of a porous material to allow a liquid to pass
through its pores.
 Since the pores are connected with each other, the flow of a liquid
takes place through the pores if there is difference in heads at the two
ends of the sample.

d) Durability: is the property of the rock to retain its strength, colour, chemical
composition and fire resistance property etc. for long period of time without
change (alteration).
 Thus an index to alteration is useful mainly in offering a relative ranking of rock
durability.
Slake durability
 Widely occurring rock materials are prone to degradation when exposed to
weathering processes such as wetting and drying and freezing and thawing
cycles.
 The main purpose of this 'slake-durability test' is to evaluate the weathering
resistance of shales, mudstones, siltstones and other clay-bearing rocks.
 This type of test is important when there are slope stability design, tunnel and for
dimension stones.
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f) Sonic velocity
 Sonic velocity can serve to index the degree of fissuring within rock
specimens.
 The sonic (sound) velocity is much slower in air than in solid rock,
the speed difference can be used to get information about open
fractures in rock masses.

Summary of Index Properties of Rocks:

 Porosity- Identifies the relative proportions of solids & voids;
 Density- a mineralogical constituents parameter;
 Sonic Velocity- evaluates the degree of fissuring;
 Permeability- the relative interconnection of pores;
 Durability- tendency for eventual breakdown of
components or structures with degradation of rock quality and
 Strength- existing competency of the rock fabric binding components.
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4.2 Engineering classification of rocks
 Rocks form part of many civil engineering projects and rock properties
are part of the exploration, design, construction and in service phases of
such projects.
 From engineering point of view, there are two broad categories of rocks:
1) Intact Rocks and
2) Rock Mass

4.2.1 Intact Rocks

 Block of rock that do not contain
mechanical discontinuities and do
have tensile strength.

 This is the smallest element of rock

block not cut by any fracture.

 There are always some micro

fractures in the rock material but these
should not be treated as fractures. 7
 Properties of Intact Rocks
The intact rocks can be described by standard geological terms such as

 Density  Hardness
 Rock name
 Porosity  Modulus of
 Mineralogy elasticity
 Strength
 Texture  Swelling and
slake durability,
 Weathering
condition and
 Degree and kind
of cementation

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Factors that affect intact rock strength

 Grain size and mineralogical composition:

 Size of intact rock,
 Shape of intact rock,
 Environment (moisture),
 Time,
 Temperature.

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 1) The effect of Grain Size and Mineralogical Composition on
Intact Rock Strength

 Grain size and mineral content of the soils have a significant

influence on strength of soils.

 Likewise, rocks are also made of grains and hence, there is a weak
zone between grains of rocks. Therefore, as grain size of rocks
increase, the size of weak zone between grains of rocks also
increase.

 Hence, as grain size of rocks increase, the abrasion resistance of

rocks (strength of rocks) decrease.

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 2) The size effect on intact rock strength
The larger the specimen, the greater the number of micro-cracks
and hence the greater the likelihood of a more cracking.

The size effect in the uniaxial complete stress-strain curve.

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 3) The effect of shape on intact rock strength
 The elastic modulus is
basically unaffected by
specimen shape.
E = Stress/Strain = σ/ ε
= (F/A)/(Δl/l0)
= Constant

 Strength and the ductility

increase as the aspect
ratio, defined as the ratio
of diameter to length, The shape effect in uniaxial compression.
increases.

 Aspect ratio, greater than

or equal to 2.5 is used in
the laboratory.
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 4) The effect of confining pressure on intact rock strength

In compression , the rock

tends to fracture normal to
the least principal stress i.e.
parallel to the major
principal stress.

Consequently, the
application of even a small
confining pressure has a
significant effect on
inhibiting the development
of these cracks.
The confining pressure effect on intact rock strength.
Thus, as confining pressure
increases, the intact rock
strength increases.
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 5) Effects of Moisture on intact rock strength

 Moisture content is known to influence the complete stress-strain

curve because of its effect, in certain rocks, on the deformability,
compressive strength and post-peak behavior.

 Some rocks and in particular those with high clay mineral

contents, may experience desiccation when exposed.

 For example, some rocks may possess high strength at their in

situ condition. But, when they are exposed to high moisture
content, their properties may change as water dries out.
 As a result, these rocks become friable and crumble
with very little applied stress.

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 6) Effects of temperature on intact rock strength

As temperature increases

 Elastic modulus and

compressive strength
decreases.
 Ductility in post-peak
behavior increases

Temperature effect on intact rock strength.

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 4.2.2 Rock Mass
An assemblage of intact rocks with discontinuities, with or without
inhomogeneity and with anisotropy.

 The overall effect of discontinuities

is that a rock mass that contains
discontinuities is weaker than the
intact rock because shear and
tensile strengths of the
discontinuities are lower than that
of the intact rock materials.
 A rock mass containing
discontinuities will be more
deformable than an intact rock.
Such deformation will normally
take place by the relative movement
along discontinuities and be plastic
rather than elastic.

Rock Mass = Intact rock (Rock material) + Discontinuities

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 Isotropic
Homogeneous Continuous

Sandstone Strength equal in

all directions

Heterogeneous Discontinuous Anisotropic

Shale Fault High
Strength
varies with
direction Low

Sandstone Joints

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 Discontinuities in Rocks

 Discontinuities are planes or surfaces that mark a change in physical or

chemical characteristics of rocks.

 They are plane of weaknesses along which intact rocks are structurally
separated.

Some Types of discontinuities

 Bedding plane
 Joints
 Fractures
 Faults
 Foliations
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According to ISRM (1978c), parameters to describe the characteristics
of discontinuities include:
a) Orientation
b) Spacing
c) Persistence (continuity)
d) Roughness
e) Wall strength
f) Aperture
g) Filling
h) Seepage
i) Number of discontinuity sets
j) Block size
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 a) Orientation

 The attitude of a discontinuity in space with respect to the

engineering work.

 It is described in terms of strike, dip amount and its dip direction.

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 Strike
 It is the line of intersection of inclined plane with horizontal
Strike Direction
 It is a direction at which discontinuity planes are oriented (i.e. it is an
angle between the north direction and strike line).
Dip angle (Dip amount)
 It is the maximum angle of inclination of a discontinuity plane from the
horizontal
Dip Direction
 It is a direction in which the discontinuity plane is dipping. It must be
perpendicular to the strike direction.

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 b) Spacing
 Spacing refers to perpendicular distance between two adjacent
discontinuities.
 It is usually expressed in terms of mean or modal spacing of a set of
discontinuities.

 Rock mass quality decreases with decrease in spacing of

discontinuities.

Classification of discontinuity spacing (ISRM, 1978c)

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 C) Persistence

 It is a trace length of
discontinuities as observed
in an exposure. It may give
a crude measure of the
areal extent or penetration
length of a discontinuity.

 The higher persistence of

the discontinuity, the lower
will be the rock mass
quality.

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 D) Roughness

 The inherent surface

roughness and waviness
relative to the mean
plane of a discontinuity.

 Both roughness and

waviness contribute to
the shear strength.

 As surface roughness of
discontinuity increases,
frictional resistance
increases and hence,
rock mass quality also
increases.
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 E) Wall strength:

 It is an equivalent compressive
strength of the adjacent rock
walls of a discontinuity.

 It may be lower than rock block

strength due to weathering or
alteration of the walls.

 The higher value of wall

strength, the higher will be rock
mass quality.

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 F) Aperture:

 The perpendicular distance between adjacent rock

walls of a discontinuity in which the intervening
space is filled with air or water.

 Rock mass quality decreases with an increase in

aperture of discontinuities.

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28
 G) Filling
 The material that separates the adjacent rock walls of a discontinuity which
is usually weaker than the parent rock.

 Typical filling materials are sand, silt, clay, breccia, gouge, mylonite,
calcite and etc.

 With the exception of discontinuities filled with strong vein materials

(calcite, quartz, pyrite), filled discontinuities generally have lower shear
strengths than clean and closed discontinuities.

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 H) Seepage
 The water flow and free moisture visible in individual
discontinuities or in the rock mass as a whole.

 Water plays a negative role by:

 Reducing the shear strength (due to uplift pressure),
 Increasing overlying weight (due to saturation),
 Causing sub-surface erosion of fine soil materials,
 Acting as a lubricant in discontinuities,
 Causing seepage and uplift force in water reservoirs
(reducing the shear strength and stability).

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 I) Number of Sets:
 The number of discontinuity
sets comprising the intersecting
discontinuity system.

 The higher number of sets of

the discontinuities, the lower
will be the rock mass quality.

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A rock mass that contains discontinuities is generally
characterized by:

 Higher deformability.

 Lower shear strength.

 Lower tensile strength, even tending to zero

perpendicular to discontinuities.

 Higher permeability (though not always the case;

depending on the infilling material).

 Higher anisotropic behavior as compared to a rock

with no discontinuities.

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 4.3 Rock Strength and Failure Criteria
4.2.1 Modes of Failures in Rocks
No single mode of rock failure exists.

Major modes of failure include the

following:
(a) Flexure: failure by bending, with
development and propagation of tensile
cracks (e.g. mine roof).
(b) Shear failure: formation of surface of
rupture where the shear stress have
become critical followed by release of
shear stress as the rock suffers a
displacement along the rupture surface.
(c) Crushing or compression failure:
occurs in intensely shortened volumes
or rock penetrated by a stiff punch.
(d) Direct tension: In rock layers on
convex upward slope surfaces and in
sedimentary rocks on the flank of an
anticline. 33
 Deformation of Rock Mass

Deformation of a rock
mass is the change in
volume or shape of the
rock mass, for a large part
due to:

 Shear displacements
along discontinuities or
openings and/or

 Closure or opening of
discontinuities.
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 4.3 Geotechnical laboratory
tests

 Unconfined compressive
strength test.
 Triaxial compression test.
 Brazilian (splitting tension)
test.
 Flexural test.
 Ring shear test.
 Point Load Test.
 Durability/Soundness Tests.

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(a) Unconfined compressive strength test:
 Uniaxial compressive strength is the ultimate
stress of a cylindrical rock specimen under axial
load.
 It is the most important mechanical properties of
rock material, used in design, analysis and
modelling.
Method:
Sample length=(2-2.5)*diameter;
 Parallel ends,
 2= 3=0
qu= unconfined compressive strength ,
A= Initial cross-sectional area of sample
P= Peak load .

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Stage I – The rock is
initially stressed, in addition
to deformation, existing
micro-cracks is closing,
causing an initial non-
linearity of the curve.

Stage II – The rock

basically has a linearly
elastic behavior with linear
stress-strain curves, both
axially and laterally.

Stage III – The rock

behaves near-linear elastic.
The axial stress-strain curve
is near-linear and is nearly
recoverable.
37
Stage IV – The rock has
undergone a rapid
acceleration of microcracking
events and volume increase.

Stage V – The rock has

passed peak stress but is still
intact, even though the
internal structure is highly
disrupt. The specimen has
undergone strain softening
(failure) deformation.

Stage VI – The rock has

essentially parted to form a
series of blocks rather than an
intact structure.
38
 (b) Point Load Strength and Point Load Strength
Index

For diametral tests:

39
 4.4 Engineering Classification of Intact Rock

For engineering uses, attempts were made to classify intact rocks

based on an individual property such as:

 UCS (Uniaxial Compressive Strength),

 Modulus of elasticity/ deformation,

 Sonic velocity,

 Point load strength index.

40
 Classification of intact rocks based on UCS according to ISRM (1979)

Classification of intact rocks based on Modulus of deformation E = Stress / Strain

41
 4.5 Classification of rock masses for engineering purpose.
 Rock mass classification is a means of evaluating the quality and expected
behaviour of rock masses based on the most important parameters that
influence its quality.
The objective of rock mass classifications are
 Identifying the most significant parameters influencing the
behaviour of a rock mass

 Divide the particular rock mass formation into groups of similar

behaviour, that is, rock mass class of varying quality

 Relate the experience of rock conditions at one site to the

conditions and experience encountered at other

 Derive quantitative data and guidelines for engineering design and

 Provide a common basis for communication between engineers and

engineering geologist.
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The main benefits of rock mass classifications

1. Improving the quality of site investigations by calling for the

minimum input data as classification parameters

2. Providing quantitative information for design purpose

3. Enabling better engineering judgment and more effective

communication on a project

Engineering Importance of Rock Mass Classifications

 To study the foundation condition
 To develop the excavation criteria
 For the tunneling work
 To maintain a stable slope 43
Rock mass Classification Systems for Engineering Purpose

 RQD, RMR & Q systems are the most commonly used rock mass
classification systems for engineering purposes

1) Rock Quality Designation index (RQD)

A) RQD for Core Sample- Deer et al. (1967)

 The Rock Quality Designation index (RQD) was developed by Deere et
al. (1967) to provide a quantitative estimate of the rock mass quality from
drill core logs.

 In general, RQD is determined from spacing of discontinuities.

 RQD is defined as the percentage of intact core pieces longer than 100
mm (4 inches) in the total length of a core.

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Total length of core run = 200cm

RQD (%) Rock quality

<25 Very poor
25-50 poor
50-75 fair
75-90 Good
90-100 excellent

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46
B. Palmström Method (Volumetric Count) for no core samples
Palmström (1982) suggested that, when no core is available but
discontinuity traces are visible in surface exposures, RQD may be
estimated from the number of discontinuities per unit volume.

• The suggested relationship for clay-free rock masses is:

RQD = 115 - 3.3 Jv

Where, Jv is the total numbers of discontinuities more than 10cm long in
1m x 1m exposed rock face.

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2. Find RQD value for the tunneling face shown below.

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 Rock Quality Designation index (RQD)

RQD (%) Rock quality

<25 Very poor
25-50 poor
50-75 fair
75-90 Good
90-100 excellent

Correlation between RQD and Rock mass quality

 The main important input that we get from the rock quality designation are fracture
frequency or density.
 Though RQD is a simple and inexpensive index, when considered alone it is not
sufficient to provide an adequate description of a rock mass because it disregards
joint orientation, joint condition, type of joint filling and stress condition.
49
 2) Rock Mass Rating System (RMR)

 Originally intended for tunnelling & mining applications, it has been

extended for the design of cut slopes and foundations.

 It was proposed by Bieniawski (1989).

 The RMR rock classification system uses six parameters to classify

rocks.
The six parameters used to determine the RMR value are:
1. Uniaxial compressive strength (qu or σu)* (R1).
Parameters for 2. Rock Quality Designation (RQD) (R2)
classification and
evaluation of its 3. Spacing of discontinuities (R3)
properties 4. Condition of discontinuities (R4)
5. Groundwater conditions (R5)
To assess stability 6. Orientation of discontinuities (R6)
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 •Thus, the RMR is determined as:

RMR= R1+R2+R3+R4+R5,

• The RMR value ranges from 0 (very poor) to 100 (excellent) for the rock
mass.

 Depending on orientation of discontinuities (i.e. strike and dip amount) with

respect to engineering construction under consideration, the sixth factor (R6)
will be added to adjust the RMR.

 Its value ranges from 0 (when orientation very favorable) to (-12 for
tunnels, -25 for foundations, and -60 for slopes) when orientation is
very unfavorable. 51
R1: Rock Mass Rating for Uniaxial compressive strength of rock

Point load index Uniaxial Compressive Strength (UCS) (MPa) Rating

(MPa)
>10 >250 15
4-10 100-250 12
2-4 50-100 7
1-2 25-50 4
For this low range 10-25 2
Uniaxial compressive 3-10 1
test is preferable
<3 0
R2: Rock Mass Rating for Rock Quality Designation, RQD
RQD Rating
90-100 20
75-90 17
50-75 13
25-50 8
52
<25 3
 R3: Rock Mass Rating for Joint spacing

Joint Spacing (m) Rating

>2 20
0.6-2 15
0.2-0.6 10
0.06-0.2 8
<0.06 5

R4: Rock Mass Rating for joint condition

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 R5: Rock Mass Rating for Ground water condition

General Condition Rating

Completely dry 15
Damp 10
Wet 7
Dripping 4
Flowing 0

R6: Rock Mass Rating (Rating adjustment for discontinuity orientation)

Strike and dip orientation Rating Increment for
of joint Tunnels Foundation Slopes
Very favorable 0 0 0
Favorable -2 -2 -5
Fair -5 -7 -25
Unfavorable -10 -15 -50
Very unfavorable -12 -25 -60
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 Assessment of Joint Orientation Effect on Tunnels
(Dips Are Apparent Dips Along Tunnel Axis) (Bieniawski, 1989)

Effect of Joint Orientation on Stability of Dam Foundation

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 The total RMR of a rock mass places the rock in one of the five classes of
quality of the rock
Rock mass class and their ratings
Class Description of Rock mass RMR
I Very good rock 81-100
II Good rock 61-80
III Fair rock 41-60
IV Poor rock 21-40
V Very poor rock 0-20

Class no. I II III IV V

Average stand 10 years for 5m 6 months for 4m 1 week for 3m span 5 hrs for 1.5m span 10 min for 0.5m
span span span
time
Cohesion of the >300 200-300 150-200 100-150 <100
rock
mass, kPa
Internal friction >450 40-450 30-400 30-350 <300
angle of the rock
mass
Cavability Very poor Will not cave Fair Will cave readily. Very good
readily. Large
fragments. Good framentation. 56
Guidelines for excavation and support of 10 m span rock tunnels in
accordance with the RMR system (After Bieniawski, 1989).

57
Exercise 2
A tunnel is to be driven through highly weathered basalt with a
dominant joint set dipping at 60o along with the strike direction
of the drive. Index testing and logging of diamond drilled core
give typical point load strength index values of 1.8MPa and
average RQD values of 18%. The slightly rough, soft infilling
characteristics with the thickness of 6mm and highly weathered
joints seen to be persistent for 9.88m and are spaced at 300 mm.
Tunneling conditions are anticipated to be wet. Determine :
A. The RMR value?
B. A Suitable Excavation Criteria?
C. The Support systems for this tunnel?

58
No Parameters Given value Rating

R1 Uniaxial compressive strength or point load 1.8 Mpa 4

strength index
R2 Rock Quality Designation (RQD) 18 % 3
R3 Spacing of discontinuities 300mm=0.3m 10
R4 Discontinuity length (persistence) 9.88m 2
Separation (aparture) 6mm 0
Condition of
Roughness Slightly rough 3
discontinuities 6
Infilling Soft infilling 0
Weathering condition Highly weathered 1
R5 Groundwater conditions Wet 7
R6 Orientation of dipping at 60o along with the Very favourable 0
discontinuities, strike direction of the drive, so
this means the strike direction is
perpendicular to tunnel axis,
A)RMR = R1+R2+R3+R4+R5+R6 =4+3+10+6+7+0=30,
Rock mass class IV and it is Poor rock
59
B) C)

60
61
62
Rock Mass Rating System (After Bieniawski, 1989).

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