Classification of Discontinuities
Two bases for classification of
discontinuities in a rock mass :
Genetic geological classification
Classification based on mechanical
characteristics

Geological Genetic Classification
 Specifies the nature of the discontinuity in
describing a rock mass.
It is essential to distinguish between a joint
and a fault, and between a joint and a bedding
plane
only partly satisfactory for engineering
purposes

Table 10 Genetic classification of discontinuities
Rock group Syngenetic Epigenetic
Igneous Cooling joints
Flow surfaces
Plutonic contacts
Dykes, Sills and veins
Faults
Tectonic joints
Sheeting
Sedimentary Bedding planes
Internal laminations
Cross bedding
Shrinkage joints
Unconformities
Faults
Tectonic joints
Cleavages
Bedding plane
shears
Sheeting
Metamorphic Foliations, Metamorphic
layering, joints
Faults, Tectonic
joints, Sheeting

Mechanical Classification of Discontinuities

Analysis of Fracture Systems
Mode of Analysis
Geometrically in the first instance
This depends on:
Firstly, certain recognition of origin of fractures
Secondly, the system being homogeneous
 i.e. that the pattern, at least statistically, is
uniform for the mass under investigation.
If this condition is not satisfied, the mass is
divided into units or sub-areas each having a
uniform fracture pattern

Geometric Analysis of Fracture
Systems
Use statistical analysis to:
a) establish the average orientation of a
set
b) determine the sets which constitute a
system

Applicable Methods
a) The fracture rosettes
 Only strikes are considered.
 Method does not take account of the dip
of the fracture, hence, does not include
horizontal fractures.
(Horizontal fractures equally important as
dipping fractures when determining the
mechanical characteristics of a rock mass)

An Example of Fracture Rose

b) Equal Area Analysis
Poles of fractures are plotted on an
equal area net bringing out the
concentrations by counting and
contouring (Fig 12).
(This method takes account of both
strike and dip and also includes
horizontal fractures in the analysis).

Example of equal area diagram of poles of fractures

Description of Persistence
Description Modal trace
length
Very low persistence <1 m
Low persistence 1 – 3 m
Medium persistence 3 – 10 m
High persistence 10 – 20 m
Very high persistence > 20m

Stress Analysis
• Qualitative analysis of stresses in rock mass is
performed using the equal area method
• Fig. 13 shows the ideal fracture system
resulting from a system of stresses.
• The fractures intersect in σ2, the
intermediate principal stress; σ1, the
maximum principal stress is the acute
bisector of the shear fractures, normal to σ2
and lying in the σ1 σ2 plane; and σ3 is normal
to the σ1 σ2 plane

σ2
σ1
σ3
σ1
σ2
σ3

Permeability of the rock mass
 Flow of water through the intact rock
material under the influence of
hydraulic pressure referred to as
Primary permeability .
 Many rock materials are impervious.
 Presence of fractures, other
discontinuities and solution cavities
impart secondary permeability to the
rock mass.

Secondary permeability is determined
by the Lugeon or packer test.
The test is conducted in a bore hole in
which a packer or a double packer is
used to seal off a section of the rock
mass
Water is pumped under pressure into
the test section and allowed to saturate
the rock
The loss of water over a specified
period is then determined.

Packer Test

Test usually carried out over a test section
1m long under a pressure of 1 MPa.
A loss of 1lmin-1 represents 1 Lugeon (~ a
coefficient of permeability of 10-4 cm s-2).
Secondary permeability depends on:
a) Spacing of discontinuities
b) Continuity
c) Infilling
d) Openness
e) degree of interconnection of the
discontinuities in the rock mass.

Seismic Velocity in the Rock Mass
Seismic Velocity is the velocity of
propagation of pressure waves through a
rock mass.
Seismic velocity is a function of a multitude
of rock properties including:
a) Density
b) Porosity
c) Composition
d) Cementation
e) Degree of fracturing.

E. G:
Variations in the degree of fracturing or
degree of weathering can be correlated
with seismic velocity.
1) The effects of discontinuities in a rock
mass can be established by comparing
the velocity (VF) of compressional wave
in the rock mass with the sonic velocity
(VL) of intact rock measured on a core
in the laboratory.

PUNDIT (Portable Ultrasonic Non-Destructive Tester)

• For a fresh rock mass with only a
few, tight discontinuities,
VF / VL is close to unity.
• With an increase in the degree of
fracturing,
VF / VL is less than 1.

Rock Quality Designation
Rock Quality Designation (RQD) is a fracture
index used in many rock classification systems

Determination of RQD

Standard terms to describe the
quality of rock mass based on RQD
RQD (%) Description of rock
quality
0-25
25-50
50-75
75-90
90-100
Very poor
Poor
Fair
Good
Excellent

Mechanical classification of
rock mass

Objectives of rock mass
classifications
• Identify the most significant parameters
influencing the behaviour of a rock mass.
• Divide a particular rock mass formulation
into groups of similar behaviour – rock
mass classes of varying quality.
• Provide a basis of understanding the
characteristics of each rock mass class

Objectives cont’d
• Relate the experience of rock conditions
at one site to the conditions and
experience encountered at others
• Derive quantitative data and guidelines
for engineering design
• Provide common basis for
communication between engineers and
geologists

Benefits
Improving the quality of site
investigations by calling for the minimum
input data as classification parameters.
Providing quantitative information for
design purposes.
Enabling better engineering judgement
and more effective communication on a
project.

NOTE:
No all-purpose mechanical or
engineering classification of rock mass
has been developed due to:
a) the number of parameters involved
b) the diversity of interests of the
engineer.

The South African Geomechanics
Classification (SAGC)
Rock mass is divided into a number of
units each having a characteristic set of
properties
(e.g. uniform fracture pattern and
fracture spacing)

Parameters determined for each
unit
• Uniaxial compressive strength
• RQD
• Weathering characteristics
• Joint and other fracture spacing
• Fracture openness, continuity and
infilling
• Orientation of fractures
• Groundwater conditions

• Based on these criteria, five classes are
established.
• Relative indices are awarded the various
parameters (see Table 12).
• The parameters are considered to be of
unequal importance hence, each is
assigned a weighted minimal value
called a rating.
• The rock quality is defined by the sum of
all of the values of the various
parameters

APPLICABILTY
Applicable to rock slopes and foundations
even though it was primarily designed for
underground mining.
DEFICIENCIES
 Does not account for anisotropy or elasticity
which are important considerations in
determining the stress distribution around
openings.
 Does not take account of the tensile strength
of the rock material –an important parameter
in mining operations.

Geomechanical classification of
jointed rock masses

Rock Structure Rating
Developed for use in mining.
Rock Structure Rating considers three basic
parameters namely:
1) Rock type, folding and discontinuities
2) Joint spacing and orientation
3) Water inflow and joint openings.
Each parameter is assigned a value and the rock
structure rating RSR is defined by the sum of
(1), (2) and (3).
•

Rock Mass Quality
• The Rock Mass Quality system due to Barton
et al (1974) was designed for underground
excavation. It considers six parameters as
follows:
i) RQD
ii) joint set number Jn
iii) joint roughness number Jr
iv) joint alteration number Ja
v) joint water reduction factor Jw
vi) stress reduction factor, SRF.

Concluding Remarks
The SAGC system is of more general
applicability than any of the above. It is
simple and partly quantitative.
All the above systems are applicable to
hard, brittle, jointed rock masses but
would certainly give conservative results
if applied to other rocks