Rock Mass Classification
Submitted To: Prof. Sohail Mustafa
Submitted By: Ahmed Younhais Tariq
Institute of Geology
THE UNIVERSITY OF AZAD JAMMU & KASHMIR
Introduction to Rock Mass
Rock mass classification schemes have been
developed to assist in (primarily) the collection of
rock into common or similar groups.
The first truly organized system was proposed by
Dr. Karl Terzaghi (1946) and has been followed by
a number of schemes proposed by others.
Terzaghi's system was mainly qualitative and
others are more quantitative in nature.
The following subsections explain three systems
and show how they can be used to begin to
develop and apply numerical ratings to the
selection of rock tunnel support and lining.
This section discusses various rock mass
classification systems mainly used for rock tunnel
design and construction projects.
Today rock tunnels are usually designed
considering the interaction between rock and
ground, i.e., the redistribution of stresses into
the rock by forming the rock arch.
However, the concept of loads still exists and
may be applied early in a design to "get a
handle" on the support requirement.
The concept is to provide support for a height
of rock (rock load) that tends to drop out of
the roof of the tunnel.
Rock Mass Rating (RMR):
Rock Mass Rating (RMR) system
is a geomechanical classification
system for rocks, developed by Z.T
Bieniawski between 1972 and 1973.
the feasibility and preliminary design
stages of a project, when very little detailed
information is available on the rock mass
and its stress and hydrologic characteristics.
The use of a rock mass classification
scheme can be of considerable benefit.
This may involve using the classification
scheme as a check-list to ensure that all
relevant information has been considered.
the other end of the spectrum,
one or more rock mass classification
schemes can be used to build up a
picture of the composition and
characteristics of a rock mass to
provide initial estimates of support
requirements, and to provide
estimates of the strength and
deformation properties of the rock
Parameters of RMR:
Six parameters are used to classify a
rock mass using the RMR system:
Uniaxial compressive strength of rock
Rock Quality Designation (RQD)
Spacing of discontinuities
Condition of discontinuities
Orientation of discontinuities
of six parameters is assigned a
value corresponding to characteristic
of rock. These values are derived from
field surveys. The sum of the six
parameters is the "RMR value", which
lies between 0 and 100.
RMR =Ja1 + Ja2 + Ja3 + Ja4 + Ja5
Classification table for the
0 - 20
21 - 40
41 - 60
61 - 80
81 - 100
Applications Of Rock Mass
Mass Rating has found wide
application in various types of
engineering projects such as tunnels,
slopes, foundations, and mines.
mass classification systems have
gained wide attention and are frequently
used in rock engineering and design.
However, all of these systems have
limitations, but applied appropriately and
with care they are valuable tools.
RMR system is applied to coal and
hard rock mining.
The RMR system is also applicable to
slopes and to rock foundations. This is a
useful feature which can assist with the
design of slope near the tunnel portals as
well as allow estimates of deformability of
rock foundation for bridges and dams.
Other special uses includes applications to
assess rock rippability, cuttability and
RMR may be applied for
classification of the stability and
support estimates of tunnels and
rock caverns, preferably in jointed
It may be used for planning
It is less useful for prescription of
rock support during construction.
It is not likely that RMR is suitable to
express the effects of pre-grouting.
Using Rock Mass Classification
two most widely used rock mass
classifications are Bieniawski's RMR
(1976, 1989) and Barton et al's Q
Both methods incorporate geological,
geometric and design/engineering
parameters in arriving at a quantitative
value of their rock mass quality.
similarities between RMR and
Q system from the use of
identical, or very
similar, parameters in calculating
the final rock mass quality rating.
Uniaxial compressive strength of
strength of rock can be evaluated
using a laboratory compression test
on prepare core.
for rock classification purposes it
is satisfactory to determine
compressive strength approximately
using the point load test on intact
pieces of drill core.
Uniaxial compressive strength of
of a material is verified by
applying compressive load until
failure occur due to fractures in core
stresses exceeds the bearing
limit it cracks the core sample. These
cracks are produce along the weaker
crack produced then we can
note the clock reading. That point
shows the maximum compressive
strength of rock.
Orientation of Discontinuities:
of the joints relative to the work
can have an influence on the behavior of
Bieniawski recommended adjusting the
sum of first five rating numbers to account
for favorable or unfavorable orientation.
No points are subtracted for very
favorable orientation of joints up to 12
points are deducted for unfavorable
orientation of joints in tunnels and up to 25
for unfavorable orientation in foundation.
orientation of joint sets cannot be
found from normal routine drilling of
rock masses but can be determined
from drill core with special tools or
of the borehole using a
television or camera down hole will
reveal orientation of joints and
absolute orientation will also be
obtained from logging shafts and
Adjustment in RMR for joint
orientation on the
Modifications to RMR for mining:
Mass Rating (RMR) system was
originally based upon case histories drawn
from civil engineering.
developed the Mining Rock
Mass Rating (MRMR) system by modifying
the Rock Mass Rating (RMR) system of
the MRMR system the stability and
support are determined with the following
RMR = IRS + RQD + spacing + condition
RMR = Rock Mass Rating
IRS = Intact Rock Strength
RQD = Rock Quality Designation
Spacing = expression for the spacing
Condition = condition
of discontinuities (parameter also dependent on
groundwater presence, pressure, or quantity of
Comparison of MRMR and
MRMR = RMR x adjustment factors
Adjustment factors = factors to compensate for:
the method of excavation, orientation of
discontinuities and excavation, induced stresses,
and future weathering.
adjustment factors depend on future
(susceptibility to) weathering, stress
environment and orientation.
combination of values
of RMR and MRMR determines the socalled reinforcement potential.
mass with a high RMR before the
adjustment factors are applied has a high
reinforcement potential, and can be
reinforced by, for example, rock bolts,
whatever the MRMR value might be after
Parameters of MRMR:
parameters to calculate the RMR value
are similar to those used in the RMR system.
This may be confusing, as some of the
parameters in the MRMR system are
modified, such as the condition parameter that
includes groundwater presence and pressure
in the MRMR system whereas groundwater is
a separate parameter in the RMR system.
number of classes for the parameters and
the detail of the description of the parameters
are also more extensive than in the RMR
Rock Structure Rating (RSR):
Structure Rating(RSR) is a
quantitative method for describing
quality of a rock mass and then
appropriate ground support.
Categories of RSR:
are considered two general
Rock type; joint pattern; joint
orientations; type of discontinuities;
major faults; shear sand folds; rock
material properties; weathering or
Size of tunnel; direction of drive;
method of excavation.
Parameter A : Geology
appraisal of geological
structure on the basis of:
Rock type origin (igneous, metamorphic,
Rock hardness (hard, medium, soft,
Geologic structure (massive, slightly
faulted/folded, moderately faulted/folded,
Parameter B: Geometry
of discontinuity pattern with
respect to the direction of the tunnel
drive on the basis of:
Joint orientation (strike and dip)
Direction of tunnel drive.
Parameter C:Effect of
of groundwater inflow and joint
condition on the basis of:
Overall rock mass quality on the basis
of A and B combined.
Joint condition (good, fair, poor).
Amount of water inflow (in gallons per
minute per 1000 feet of tunnel).
et al. (1974) at the Norvegian
Geotechnical Institute (NGI) proposed
the Rock Mass Quality (Q) System of
rock mass classification on the basis of
about 200 case histories of tunnels and
It is a quantitative classification system,
and it is an engineering system
enabling the design of tunnel supports.
concept upon which the Q system is
based upon three fundamental
a. Classification of the relevant rock mass
b. Choice of the optimum dimensions of
the excavation with consideration given
to its intended purpose and the required
factor of safety,
c. Estimation of the appropriate support
requirements for that excavation.
Q-system for rock mass
classification is developed by Barton,
Lien and lunde. It expresses the quality
of rock mass, on which, the design and
support recommendations are based
for the underground excavations.
The Q- value is determined by the
Q = RQD/Jn x Jr/ Ja x Jw/SRF
RQD = Rock Quality Designation
Jn = Joint Number
Jr = Joint Roughness
Ja = Joint Alteration
Jw = Joint Water Reduction Number
SRF = Stress Reduction Fraction
=Degree of Jointing(or block size)
=Joint Friction(inter-block shear strength)
Q values can be determined in different
ways, by geological mapping in underground
excavation, on the surfaces , or alternatively
by core logging. The most correct values are
obtained from geological mapping
underground. Each of Six Parameters is
determined according to description found in
The Q values varies between 0.001 and
1000. Please note that it is possible to get
higher values and slightly lower values by
extreme combinations of parameters. In such
odd cases one can use 0.001 and 1000
respectively for determination of support.
RQD (Rock Quality Designation):
is the sum of length ( between natural
joints ) of all core pieces more than 10 cm
long as a percentage of the total core
will therefore be a percentage
between 0 to 100. If 0 is used in the Q
formula it will give a Q value of 0 and
therefore all RQD values between 0 to 10
are increased to 10 when calculating the Q
RQD (Rock Quality Designation):
is used as a simple classification
system for the stability of rock masses.
Using the RQD values, 5 rock classes are
Very Poor (>27 joints per m3)
0 - 25
Poor (20 - 27 joints per m3)
25 - 50
Fair (13 - 19 joints per m3)
50 - 75
Good (8 - 12 joints per m3)
75 - 90
Excellent (0 - 7 joints per m3)
90 - 100
In the underground opening it is
usually possible to get a three
dimensional view of rock mass. A
three dimensional RQD may therefore
be used. That’s means that the RQD
values are estimated from the no of
joints per .
The following formula may be used :
RQD = 115 - 3.3 Jv
Where is the number of joints per
RQD is intended to represent the rock
mass quality in situ. When using diamond
drill core, care must be taken to ensure
that fractures, which have been caused by
handling or the drilling process, are
identified and ignored when determining
the value of RQD.
Stress Reduction Factor:
describes the relation between
stress and rock strength around an
underground opening. The effect of
stresses can usually be observed in
an underground opening as spalling,
slabbing, deformation, squeezing,
dilatancy and block release. However,
sometime may pass before the stress
phenomena are visible.
Joint Roughness Number:
depend on joint wall surfaces. If they are
undulating, planner, rough or smooth. Joint
description is based on roughness in two
The terms rough, smooth and slickenside
refer to small structures in a scale of cm and
mm. This can be evaluated by running a finger
along the joint wall; small scale roughness will
then be left.
2. Lange scale roughness is measured on a dm
to m scale and is measured by lying a one
meter long ruler on the joint surface to
determine the large scale roughness
Rock wall contact , and
Rock wll contact before 10 cm of shear movement
Rough or irregular, Undulating
Rough, Irregular, Planar
No rock wall contact when sheared
Zone containing clay minerals thick enough to prevent
rock wall contact when sheared
Joint Set Number:
Shape and size of the blocks in the rock mass
depends on the joints geometry.
There will often be 2 to 4 joint sets at a certain
Joints in it will be nearly parallel to one another
and will display a characteristic joint spacing.
Joints that do not occur systematically or that
have a spacing of several meters are called
However, the effect of spacing strongly depend
upon the span or height of the underground
If more than one joint belonging to a joint set
appears in a underground opening, it has an
effect on the stability and should be regarded
Table for determination of joint
Joint set Number:
Massive, no or few joints
One joint set
One joint set plus random joints
Two joint sets
Two joint sets plus random joints
Three joint sets
Three joint sets plus random joints
Four or more joint sets , randomly
heavily jointed “Sugar Cube “ etc
Crushed Rocks, earth like
=Degree of Jointing(or block size)
This fraction represents the relative
block size in the rock masses.
In addition to RQD and Jn. It is also
useful to make notes of the real size
and shape of the blocks, and the joint
Joint Alternation Number:
In addition to the joint roughness the joint infill
is significant for joint friction. When
considering joint infill, two factors are
important; thickness and strength. These
factors depends on the mineral composition.
In the determination of joint alternation
number, the joint infill is divided into three
categories ; (a, b and c) based on thickness
and degree of rock wall contact when sheared
along the joint planes.
Joint water reduction Factor:
Joint water may soften or washout the
mineral in fills and there by reduce the
friction on the joint planes. Water
pressure may reduce the normal stress
on the joint wall and cause the blocks to
shear more easily.
A determination of joint water reduction
factor is based on inflow and water
pressure observed in a underground
opening. The lowest Jw values(Jw < 0.2)
represent large stability problems.
=Joint Friction(inter-block shear
New Austrian Tunneling method
term New Austrian Tunneling
Method Popularly Known as NATM got
its name from Salzburg (Austria).
It was first used by Mr. Rabcewicz in
1962. It got world wise recognition
The first use of NATM in soft ground
tunnel in Frankfurt (Europe) metro in
The basic aim of NATM is for getting
Definition of NATM:
New Austrian Tunnelling Method is
a support method to stabilize the tunnel
perimeter by means of sprayed concrete
,anchors and other support and uses
monitoring too control stability”.
Principles of NATM:
of the strength of rock mass
Rock mass classification
Summary of procedure in
SHOTCRETING AT THE EXCAVATED
PLACING OF THE WIREMESH ALONG THE
FACEOF THE TUNNEL
ERECTION OF THE LATTICE GIRDER
ALONG THE FACE OF THE TUNNEL
PERTICULAR TYPE OF ROCKBOLTING
SHOTCRETING THE WHOLE
POINTS TO CARRY OUT A
of rock mechanics
Selection of a proper profile
Design of flexible support and slender lining (in
rock) Careful excavation
Maintenance of rock strength, avoidance of
loosening and over-breaks
Direct contact of rock/soil and support
Continuous control by geotechnical
Installation of support without delay and in