mm

92084999
œ
r\
TECHNICAL REPORT GL-85-3

z)
iy Corps GEOTECHNICAL DESCRIPTIONS OF
neers
ROCK AND ROCK MASSES
by
William L. Murphy

Geotechnical Laboratory

DEPARTMENT OF THE ARMY

Waterways Experiment Station, Corps of Engineers
PO Box 631
Vicksburg, Mississippi 39180-0631

April 1985
Final Report
Approved For Public Release; Distribution Unlimited

DEPARTMENT OF THE ARMY

Prepared for
US Army Corps of Engineers
Washington, DC 20314-1000
Under CWIS Work Unit 31754
 PROPERTY OP
BUREAU OF RECLAMATION

C /

It °

M '16? & % / P&- ^

Q *
The findings in this report are not to be construed as an official
Department of the Army position unless so designated
by other authorized documents.

The contents of this report are not to be used for

advertising, publication, or promotional purposes.
Citation of trade names does not constitute an
official endorsement or approval of the use of
such commercial products.
 rsJ
0
LA
OO
'Cx ________Unclassified_____________
S E C U R IT Y C LA S S IFIC A TIO N o f T H I S P A G E (W hen D a te E n te r e d )

R E A D IN ST R U C T IO N S
R E P O R T D O C U M E NTA TIO N PAGE B E F O R E C O M PL ET IN G FORM
1. REPORT NUMBER 2. G O V T A C C E S S I O N NO . 3. R E C IP IE N T ’S C A TALO G NUMBER
IÀ Technical Report GL-85-3
OO
ôl-

4. T I T L E (a n d S u b title ) 5. T Y P E O F R E P O R T & P E R IO D C O V E R E D

GEOTECHNICAL DESCRIPTIONS OF ROCK AND ROCK MASSES Final report

VVS4-+-

6. P E R F O R M IN G ORG. R E P O R T NUM BER

7. A U T H O R f«; 8. C O N T R A C T OR G R A N T N U M B E R fs)

William L. Murphy

9. P E R FO R M IN G O R G A N IZ A T IO N NAM E AND ADDRESS 10. PRO GR AM E L E M E N T , P R O J E C T , TASK

A R E A & WORK U N IT N U M B E R S
US Army Engineer Waterways Experiment Station
Geotechnical Laboratory CWIS Work Unit 31754
PO Box 631, Vicksburg, Mississippi 39180-0631
11. C O N T R O L L IN G O F F IC E NAME AND ADDRESS 12. REPORT DATE
DEPARTMENT OF THE ARMY April 1985
US Army Corps of Engineers 13. NUM BER OF PAGES
Washington, DC 20314-1000 49
14. M O N I T O R I N G A G E N C Y N A M E & A D D R E S S ( i f d iffe r e n t from C o n t r o llin g O f f ic e ) 15. S E C U R I T Y C L A S S , ( o f th is re p o rt)

Unclassified
15a. D E C L A S S I FI C A T I O N / D O W N G R A D I N G
SCHEDULE

16. D I S T R I B U T I O N S T A T E M E N T ( o f th is R e p o r t)

Approved for public release; distribution unlimited.

17. D I S T R I B U T I O N S T A T E M E N T ( o f th e a b s t r a c t e n te r e d in B lo c k 2 0 , i f d iffe r e n t from R e p o r t)

18. SU PPLE M E N TA R Y NOTES

Available from National Technical Information Service, 5285 Port Royal Road,
Springfield, Virginia 22161.

19. K E Y W O RD S (C o n tin u e on r e v e r s e a id e i f n e c e s s a r y a n d i d e n t if y by b lo c k n u m b e r)

Rock
Rock core
Rock descriptors

20. A B S T R A C T (XTontBoue am r e v e r s e s id e i f r r e c e e a a ry a n d i d e n t i f y by b lo c k n u m b e r)

Geotechnical descriptors for rock and rock mass are suggested for use in
the field that can be readily understood by geotechnical engineers and con­
tractors. Several rock and rock mass properties and descriptors were deter­
mined to be important in geotechnical applications, including tunnel support,
slope and foundation stability, and rock excavation. The descriptors are rock
Ö- type, strength, discontinuity characterization, weathering, rock quality
Cn designation (RQD), ground-water conditions, and rock density. The rock name
>
(Continued)
DD ,FORM
JAM 73 1473 E D I T I O N O F I M O V 6 5 IS O B S O L E T E

o Unclassified
S E C U R I T Y C L A S S I F I C A T I O N O F T H I S P A G E (W hen D a ta E n te r e d )

N
 _________ Unclassified______________
S E C U R IT Y C L A S S IF IC A T IO N O F T H I S P A G E f H T i f i Data Entered)

20. ABSTRACT (Continued).

or type should be retained in field description, but uncommon rock names
should be accompanied by a brief definition to enable the user to relate to
more common rock types. Rock strength should be described quantitatively by
the point load index test. Descriptions of discontinuities should include
measurement and classification of aperture and a determination of whether the
discontinuity is open or tight; filling thickness and composition; wall
asperity or roughness; and orientation of individual discontinuities, sets
and systems. The use of stereographic projection and unambiguous azimuthal
notation to describe discontinuity orientation is recommended. Bieniawski’s
classification of rock weathering, which classifies degree of weathering and
describes the appearance in the field, is recommended. The use of RQD in field
descriptions as developed by Deere is suggested for certain applications.
Field recognition and description of seepage and groundwater conditions along
discontinuities based on simple observations of the amount of water present
and estimates of discharge are recommended to precede and augment the design
of more elaborate pore pressure and seepage analysis investigations. Wet
density of rock samples can be determined in the field on core specimens by
the standard Rock Testing Handbook Methods.

Unclassified
SE C U R IT Y C L A SSIF IC A T IO N O F T H IS P AG E(W hen Data Entered)
 PREFACE

» This report represents a part of the work at the US Army Engineer Water­
ways Experiment Station (WES) under Civil Works Investigational Studies (CWIS)
Work Unit 31754, !,Rock Mass Classification Systems,” sponsored by the Office,
Chief of Engineers (OCE), US Army. The OCE technical monitors were Messrs.
Paul R. Fisher and Ben Kelly.
The report was written by Mr. William L. Murphy, Engineering Geology
Applications Group (EGAG), Engineering Geology and Rock Mechanics Division
(EGRMD), Geotechnical Laboratory (GL). The principal investigator was
Mr. Hardy J. Smith, Rock Mechanics Applications Group (RMAG), EGRMD. The work
was under the direct supervision of Mr. Jerry S. Huie, Chief, RMAG, and under
the general supervision of Dr. Don C. Banks, Chief, EGRMD, and Dr. William F.
Marcuson III, Chief, GL.
Commanders and Directors of WES during the conduct of this study and
the preparation of this report were COL Tilford C. Creel, CE, and COL Robert C.
Lee, CE. Technical Director was Mr. Fred R. Brown.

i
1
 CONTENTS

Page
PREFACE ................................................................ 1
CONVERSION FACTORS, US CUSTOMARY TO METRIC (SI)
UNITS OF M E A S U R E M E N T ................................................ 3
PART I: INTRODUCTION................................................. 4
Background and Purpose .......................................... 4
Approach ........................................................ 4
PART II: DESCRIPTORS................................................. 5
Rock T y p e .......................... . ........................... 5
Strength .......................................... 7
Discontinuity................................. 13
PART III: ROCK W E A T H E R I N G .............................................. 29
D e f i n i t i o n ..................................... 29
Previous and Existing Classifications ............................ 30
Recommended Descriptors .......................................... 30
PART IV: ROCK QUALITY D E S I G N A T I O N ...................................... 32
D e f i n i t i o n ........................................................ 32
Importance and Previous U s e ........................................ 33
Recommended U s e ............................... 33
PART V:GROUND-WATER CONDITIONS ........................................ 34
Definition and Importance............. 34
Previous and Existing Classifications ............................ 35
Recommended Descriptors ............................................. 35
PART VI: ROCK D E N S I T Y .................................................. 36
Definitions and Importance ...................... . . . . . . . . 36
Recommended U s e ........................... 36
PART VII: SUMMARY AND RECOMMENDATIONS.................................. 37
R E F E R E N C E S .............................................................. 38
TABLES 1-16

2
 CONVERSION FACTORS, US CUSTOMARY TO METRIC (SI)
UNITS OF MEASUREMENT

US customary units of measurement used in this report can be converted to

metric (SI) units as follows:

Multiply By To Obtain
inches 2.54 centimetres
feet 0.3048 metres
pounds (force) 4.4482 newtons
pounds (force) per 6.8948 kilopascals
square inch
pounds (mass) per 16.01846 kilograms per cubic metre
cubic foot

3
 GEOTECHNICAL DESCRIPTIONS OF ROCK AND ROCK MASSES

PART I: INTRODUCTION

Background and Purpose

1. Rock mass classification systems currently in use produce divergent

results when used by different engineers and geologists. Geologic descrip­
tions of rock are often misunderstood by engineers and contractors or are of
insufficient engineering value. Consequently, the US Army Engineer Waterways
Experiment Station (WES) is developing rock mass classification systems for
engineering applications. The purpose of the study reported herein is to rec­
ommend geotechnical descriptors for rock mass properties and characteristics
that can be determined in the field and understood and used by geotechnical
engineers and contractors. Development of the rock and rock mass descriptors
is necessary for consistent application of rock classification systems.

Approach

2. A working list of rock mass properties and conditions important to

various engineering applications was formulated. A systematic study of exist­
ing descriptive terminology and classifications was then made to develop the
recommended geotechnical descriptors presented in this report (Table 1). The
selection of properties and conditions was strongly influenced by existing
rock mass classification systems developed by others for specific applications,
such as tunnel support, slope and foundation stability, and rock excavation.
The rock descriptors discussed in this report are rock type, strength, dis­
continuity spacing, condition of discontinuity, discontinuity orientation,
weathering, rock quality designator (RQD), ground-water conditions, and rock
density. The discussion of each rock mass property descriptor includes the
definition of the term, the importance in rock mass classification for geo­
technical use, the previous and existing classifications and measurement tech­
niques, and the recommended description and/or measurement for US Army Corps
of Engineers (USACE) usage.

4
 PART II: DESCRIPTORS

Rock Type

Definition
3. Rock type is the identification given a rock by the geologist.
Examples of rock type are limestone, dolomite, sandstone, granite, banded
gneiss, and mudstone. Rock types are defined individually in publications
such as the Glossary of Geology (American Geological Institute (AGI) 1972).
Local or colloquial rock names are sometimes used in the literature. Authors
should consult publications such as the AGI glossary to achieve uniformity in
rock identification. Formation name is another identifier of assemblages of
one or more rock types occurring in a particular location or region. Forma­
tion names may be inconsistent from one location to another, are constantly
subject to change in the literature, and are less uniform and more difficult
to define than are rock types.
Importance and existing classifications
4. The rock type is the product of a classification procedure that the
geologist performs to categorize the mode of formation and certain physical
characteristics of the rock. In addition, however, rock name implies quali­
tative information on many properties to be considered as a general guide in a
geotechnical project. Such information includes strength, predicted joint
systems, the probability of the presence of bedding planes and possible weak
zones, permeability, hardness, or resistance to abrasion, and perhaps cohesion
and angle of internal friction. For example, the rock types "granite,11
"slate," and "dense basalt" would likely represent rocks of high compressive
strength; "sandstone" would lead an investigator to suspect relatively high
permeability; "clay shale" would imply bedded sediments with low shear
strength. Most field data supplied for engineering evaluation of rock and
rock mass behavior include the rock name even though the name may not be
entered in the formal rock classification scheme, indicating that the geolo­
gist and geotechnical engineer use information implicit in the rock name in
their evaluation of a problem.
5. Attewell and Farmer (1976) discuss the relevance of geologic classi­
fication of rocks to geotechnical applications. They suggest that the stand­
ard classification based on texture, mineralogy, and origin does not

5
adequately distinguish rock types for engineering purposes. Unweathered ig­
neous rocks, for example, tend to be sound engineering materials relative to
many sedimentary rocks, although the geologic distinctions within the igneous
class are complex and many. Igneous rocks are relatively sound because they
are agglomerates of strongly bonded minerals and have low primary porosity
and high competency. Franklin (1970) realized that abundant geologic rock
names are often applied to materials that differed insignificantly in their
engineering importance. He evaluated previous attempts to simplify the geo­
logic nomenclature for geotechnical use, but concluded that no improvement in
usage could be made by a mere reduction in the number of rock classes.
6. Stagg and Zienkiewicz (1968) support the retention of geologic
naming of rocks by citing several examples of mechanical properties that can
be inferred from the rock name. They point out that texture, fabric, and an­
isotropy in the rock are usually a product of the rock’s origin (mode of form­
ation) and are related to mechanical properties of rock behavior. Most igneous
rocks, for example, are generally isotropic in mechanical properties; whereas,
many sedimentary rocks are laminated or bedded and are thus considerably an­
isotropic. The metamorphic rocks, which are often banded, foliated, and com­
posed of platy minerals, can also be very anisotropic. Geologists commonly
classify rocks on the basis of origin and mineral composition.
7. Table 2 presents a classification of common rock types based essen­
tially on physical appearance represented by color, texture, grain (or crystal)
size, and types of minerals present. The table lists the more common rock
names that may occur in a geologic report as well as other terms that are
sometimes applied synonymously to the rock or rock group. Igneous rocks can
be described as coarse (phaneritic) or fine grained (aphanitic) depending on
whether the mineral crystals can be seen by the naked eye. The many rock
types within the igneous groups differ basically in the relative percentages
and kinds of the feldspar minerals and the amount of quartz they contain.
The general color of the rock also reflects its mineralogy. Volcanic rocks
are further classified by their grain size and mode of emplacement (i.e., flow,
intrusion, pyroclastic fall, etc.). The sedimentary rocks are described as
clastic (particulate or mechanically deposited) and nonclastic (primarily
chemical precipitates), and as coarse or fine grained. Nonclastic sedimentary
rocks are subdivided into organic and inorganic types. The grain-size bound­
ary between coarse and fine (Table 2) is ambiguous because of the recognition

6
of siltstone as an identifiable rock type. Some sedimentary rock types (marl-
stone, for example) exhibit characteristics of both divisions. The metamor-
phic rocks can show pronounced anisotropy and have been subdivided into iso­
tropic and anisotropic. Table 2 is not a complete list of rock names; it is
a reference guide to help the user relate less common terms and rock names
that may be encountered in the literature to more commonly used or accepted
terms.
Recommended descriptors
8. The geologic rock name should be included in geotechnical reports.
The standard and common names that are in wide geographic use or acceptance
should be used rather than the colloquial or locally popular terms. Use of
the three basic classes of rocks— igneous, sedimentary, and metamorphic— should
be continued because they are widely understood and are part of the rock—naming
process. If an uncommon rock type must be described, the name should be accom­
panied in parentheses by a brief definition so that the user of the field log
can relate to the general class of rock being described. For example, the rock
name "syenite" might be qualified by adding "the quartz-deficient equivalent of
granite," because granite is a common rock name. It should be understood, how­
ever, that the field geologist must usually make determinations of more than
just engineering properties in order to accurately correlate between borings
or exposures to determine continuity of the rock type and rock mass and to
detect the possible presence of faults or structure not sampled. Correlation
requires stratigraphic, paleontologic, and mineralogic detail that may be of
little interest to the engineer, but must appear in the field logs for use by
the geologist in his geotechnical evaluation of site. Formation and other
stratigraphical names may be used in the geologic report, but it should be
remembered that these names apply only to specific geographic locations or
regions.

Strength

Definition
9. The term "strength" as applied to a rock specimen or to a rock mass
has been defined in field investigations in many ways; for example, by quali­
tative descriptors referring to the relative density or crushing resistance of
the rock under a hammer blow, by quasi-quantitative descriptors using the hard­
ness or rebound of the specimen as determined by a simple apparatus, and by

LIBRARY
7
MAR - 6 2012

Bureau of Declamation
measurements of compressive strength derived indirectly from point-load index
and directly from uniaxial compression tests. Compressive strength is a
measurement of the compressional load required to cause an unconfined or con­
fined specimen to fail, as in a uniaxial (unconfined) or triaxial (confined)
compression tests. Shear strength is determined in the triaxial test chamber
by applying axial loads to specimens under several confining pressures or by
subjecting samples to direct shear stresses. The following discussion deals
only with field estimates of compressive strength. Field estimates of shear
strength are not common.
Importance
10. An evaluation of the strength of rock provides an upper limit of the
strength of the rock mass, represents an estimate of the true rock mass strength
in massive unjointed rock masses, and is a simple and useful means of classi­
fying the rock. Rock strength also implies the effectiveness to be expected
of tunneling and other rock excavation machines.
Previous descriptors
11. The Core Logging Committee of the South Africa Section of the Asso­
ciation of Engineering Geologists (AEG 1978) suggests a rock strength classi­
fication as shown in Table 3* which is based on hardness as defined by simple
field examination. The classification relates the strength (hardness) de­
scriptors to simple field tests based on abrasion resistance or point-load
tests, and to ranges of compressive strength derived from a relationship of
Jennings and Robertson (1969). The AEG classification is designed to offer
subdivisions of strengths, particularly in the lower ranges of stresses for
design of foundations or slope stability and in the higher ranges of stress as
in tunneling considerations. The ranges of compressive strengths used in
Table 3 are similar to those of the geomechanics classification of Bieniawski
(1979) which is based on the scheme of Deere and Miller (1966). Table 4 com­
pares the schemes of Bieniawski and Deere and Miller. The slight difference
in values in units of pounds per square inch between the two classifications in
Table 4 is for convenience in converting to metric (SI) units in Bieniawski!s
classification. Bieniawski added the 150 psi** lower limit to distinguish rock

* Tables 3, 4, and 5 are presented for discussion only, and should not be
inferred as having been accepted for use.
** A table of factors for converting US customary units of measurement to
metric (SI) units is given on page 3.

8
from soil. Bieniawski’s geomechanics classification has been recommended for
USACE consideration in tunnel support design (Bieniawski 1979). The qualita­
tive description of compressive strengths expressed by AEG’s six hardness
classes (Table 3, field test) does not correlate with similar terminology de­
scribing strength in other classifications, such as those suggested by Coates
(1970) (Table 5) and Deere and Miller (1966) (Figure 1). Medium hard rock is
characterized in Table 3 by a strength range of 1430 to 3625 psi, which would
correspond to a "very weak" rock by Coates (Table 5) and a "very low strength"
rock by Deere and Miller (Figure 1). The apparent lack of consistency between
the classifications and in the meaning of the terms "hardness" and "strength"
confuses the user. Therefore, the use of a strength classification based on
hardness is not recommended. Instead, rock strength preferably should be de­
scribed in the field investigation by quantitative strength values determined

1 2 3 4 5 6 8 20 30 40 50 60
U N IA X IA L COMPRESSIVE STRENG TH, (7a(ULT)

*E t = T A N G E N T M O DULUS A T 50% U L T IM A T E STRENGTH

Figure 1. Rock strength classification (modified from

Deere and Miller 1966)

9
by accepted standard strength tests such as uniaxial compressive tests or
point-load indices (paragraph 12). A qualitative descriptor, using the range
of terms from "very low" to "very high" strength, can be added if desired,
preferably with the strength ratings of Bieniawski (Table 4).
Index tests for strength
12. Rebound tests. Good correlation between field rebound tests and
compressive strength was reported by Deere and Miller (1966) and between point­
load and compressive strength by Deere and Miller (1966), D fAndrea, Fischer,
and Fogelson (1965), and Franklin, Broch, and Walton (1971). Deere and Miller
(1966) compared Shore scleroscope and Schmidt hammer rebound indices with uni­
axial compressive strengths of intact NX core specimens of 13 geologically dis­
tinct rock types. From the somewhat curvilinear relationships, Deere and
Miller developed logarithmic rock strength charts with which uniaxial compres­
sive strength can be estimated if the Shore or Schmidt (Figure 2) values and
the unit weight of the rock are known. The scleroscope and Schmidt hammer are
similar instruments that impart a definite amount of energy to a rock specimen
by a free-falling hammer in the scleroscope and by a spring-loaded hammer in
the Schmidt hammer. The rebound of the hammer from the rock surface is mea­
sured and recorded. The devices are relatively inexpensive and rapid and
simple to employ on NX core at a project site. However, the rebound tests
DRY U NIT WEIGHT, 7 PCF DRY U NIT WEIGHT, 7_, PCF

0 5 '10 15 20 25 30 35 40 45 50 55 60
SHORE HARDNESS. Sh SCHMIDT HARDNESS, R (L-HAMMER)

NOTE: DISPERSION LIMITS DEFINED NOTE: 1) HAMMER VERTICAL DOWNWARD

FOR 75% CONFIDENCE 2) DISPERSION LIMITS DEFINED FOR
75% CONFIDENCE

Figure 2. Rock strength charts based on^Shore (left) and Schmidt

(right) hardness tests (from Deere and Miller 1966)

10
are insensitive to strength changes, and the results are strongly influenced
by variation in testing techniques (WES 1982). Attewell and Farmer (1976)
question the usefulness of rebound indices, but concede that they may have
value as spot checks on rock strength. The Geological Society Engineering
Group Working Party (1977) of Great Britain states that there is only a 75 per­
cent probability that the laboratory-determined uniaxial compression strength
will fall within 50 percent of the strength determined by the rebound hammer
test in the correlation chart (Figure 2) of Deere and Miller (1966).
13. Point-load test. The point-load test index is conducted by apply­
ing compressive point loads diametrically to a specimen and measuring the load
at failure. A point-load strength index is derived from the failure load and
the distance between loading platens. The specimen actually breaks in tension,
but a linear relationship between point-load index and compressive strength has
been shown by several authors. D TAndrea, Fischer, and Fogelson (1965) demon­
strated a linear relationship for data for rock core specimens from 49 loca­
tions, Bieniawski (1975) for data for rock from four locations, and Deere and
Miller for rock core samples from 27 locations. Their combined data are shown
in Figure 3, a graph of point-load tensile strength index versus uniaxial com­
pressive strength. The graph (Figure 3) relates data for various rock core
diameters. Bieniawski (1975) developed a chart for converting point-load in­
dex of cores of other than NX standard size. Specimens of irregular shape can
also be tested with the point-load apparatus (Attewell and Farmer 1976, and
Franklin 1970). An informative dissertation on the history of development,
effects of specimen shape factors, and testing procedures for the point-load
strength test is available in a paper by Broch and Franklin (1972). Use of
the point-load apparatus as an index for the quick field evaluation of com­
pressive strength is recommended as a proposed standard (No. 325-82) in the
Rock Testing Handbook (WES 1982).
Recommended rock strength descriptors
14. Strength is an important property required to describe adequately a
rock type when index classification is used in engineering applications.
Simple hammer and penknife tests have been used but seldom give objective,
quantitative, or reproducible results. The uniaxial (unconfined) compression
test has been widely used for rock strength classification but requires ma­
chined specimens and is therefore a slow technique, essentially confined to
the laboratory. This report recommends that the point-load test index be used

11
 MN
OR MPa
m2
3.5 7 10.5 14.0 17.5

to provide rock strength descriptions from the field. The point-load test has
proven to be a reliable method of determining rock strength properties, and
portable equipment that lends itself well to field use is commercially avail­
able. The advantages of the point-load test are:
cu Smaller forces are needed so that a small and portable testing
machine may be used.
b^. Specimens in the form of core or irregular shapes are used and
require no machining.

12
 jc. More tests may be made for the same cost of uniaxial compression
tests which allows adequate sampling even when rock conditions
are variable.
d.. Fragile or broken materials can be tested, so there is less
chance of results being biased in favor of more competent
strata.
e^. Results show less scatter than those for uniaxial testing as
reported by Broch and Franklin (1972).
j£. Measurement of strength anisotropy is simplified.

If a strength classification is desired in addition to point-load values, the

scheme of Bieniawski (1979) (Table 4) should be used. The report of field
point-load test results should include the point load index corrected to a
reference diameter of 50 mm by use of a correction chart and the uniaxial
compressive strength derived from an index-to-strength conversion graph. The
Rock Testing Handbook (No. 325-82) should be consulted for procedures in con­
ducting and reporting point-load index tests.

Discontinuity

Definition
15. The term discontinuity" encompasses all perceivable breaks or divi­
sions in a rock mass. Discontinuities include structural features such as
faults and joints and depositional features such as bedding planes, erosional
surfaces, and other contacts. Some engineers define "joint" as any break in
the continuity of the rock mass, including structural (stress) breaks and
bedding features. Most usage distinguishes joints from bedding. However,
"bedding" implies that the discontinuities are parallel or subparallel;
whereas, a joint system usually consists of several sets of joints at differ­
ent orientations. "Bedding" also implies that adjoining rock types may be
different in character (for example, in grain size or strength); whereas,
joints often separate a rock mass of unchanging rock type. Joints, faults,
and bedding planes sometimes occur congruently, for example, in the case of
joints occurring along bedding planes. Most often, however, a distinction
between structural and bedding features can be made in the field. The dis­
tinction between bedding and jointing should be retained.
Importance
16. The importance of discontinuity analysis is expressed in the

13
following quotation from the International Society for Rock Mechanics (ISRM)
(1978):

The majority of rock masses, in particular those

within a few hundred meters from the surface, behave as
discontinua, with the discontinuities largely determining
the mechanical behavior. It is therefore essential that
both the structure of a rock mass and the nature of its
discontinuities are carefully described in addition to
the lithological description of the rock type. Those pa­
rameters that can be used in some type of stability
analysis should be quantified whenever possible.
For example, in the case of rock slope stability cer­
tain quantitative descriptions can be used directly in a
preliminary limit equilibrium analysis. The orientation,
location, persistence, joint water pressure and shear
strength of critical discontinuities will be direct data
for use in analysis. For purposes of preliminary investi­
gation the last two parameters can probably be estimated
with acceptable accuracy from a careful description of the
nature of the discontinuities. Features such as roughness,
wall strength, degree of weathering, type of infilling
material, and signs of water seepage will therefore be im­
portant indirect data for this engineering problem.
For the case of tunnel stability and estimation of
support requirements, all the descriptions will tend to be
indirect data since a direct analysis of stability has yet
to be developed. However, a careful description of the
structure of a rock mass and the nature of its disconti­
nuities can be of inestimable value for extrapolating
experience of support performance to new rock mass environ­
ments. Descriptions should be sufficiently detailed that
they can form the basis for a functional classification of
the rock mass.
In time, as descriptions of rock masses and discon­
tinuities become more complete and unified, it may be pos­
sible to design engineering structures in rock with a min­
imum of expensive in situ testing. In any case careful
field description will enhance the value of in situ tests
that are performed, since the interpretation and extrapo­
lation of results will be made more reliable.

Previous classifications and descriptors

17. For engineering purposes descriptions of discontinuities should be
quantitative when possible, pertinent to engineering usage, and should include
characteristics readily measurable or determinable in the field. Character­
istics of discontinuities that meet the above restrictions are spacing (or
bed thickness), true orientation or attitude within the rock mass and rela­
tive orientation with respect to excavation surfaces, and condition (surface

14
roughness, width of opening, degree of weathering, and filling material prop­
erties) . Some or all of the above characteristics have been applied in rock
classification for engineering purposes by Bieniawski (1979); Deere, Merritt,
and Coon (1969); Coates (1970); John (1962); Underwood (1967); Barton, Lien,
and Lunde (1974); Franklin (1970); AEG (1978); and others. Other descriptions
of discontinuity geometry such as surface area and area intensity have been
suggested by Fookes and Denness (1969) (in Attewell and Farmer 1976). The
following discussion develops descriptive terminology for discontinuity spac­
ing, condition, and orientation.
Discontinuity spacing
18. Determination of spacing. Spacing is the distance separating planes
of discontinuity in a rock mass. The term "joint spacing" is analogous to the
term "bedding thickness." Ideally, the spacing applies to the three-dimensional
rock mass, but realistically measurements of spacing are usually made in the
field in one or two dimensions. Borehole core and photolog spacing measure­
ments are one-dimensional (along a line) and most rock exposure measurements
are two-dimensional (in a plane). Borehole measurements are biased in favor
of discontinuities lying at nearly right angles to the borehole axis and
against those lying parallel to the borehole axis because more of the former
intersect the borehole. Similar but less severe bias occurs in two-dimensional
rock exposure measurements. The geologic report should qualify the reported
spacing values by stating the methods used to determine spacing. Preferably
the report should make the determination of three-dimensional spacing by
analyzing all complementary data from boreholes, trenches, cuts, and other
exposures.
19. Previous classification/description schemes. Bedding thickness was
classified by McKee and Weir (1953), and their classification was adopted by
Pettijohn (1957) for geologic usage. Their terms (Table 6) were based on
field examinations of sedimentary rock units. Rock strata* less than 1 cm
thick were termed "laminations," and strata greater than 1 cm thick were
termed "beds." Subdivisions of the two major groups (beds and laminations)
were added for classification (Table 6). The splitting properties listed in
Table 6 are vaguely defined and have been used by geologists in the field in

* "Strata" (stratum) is the general term for layers of rock; "bed" and "lami­
nation" are terms of magnitude.

15
lieu of quantitative bed thickness values. The splitting terms are widely
used and are included for completeness only. Deere (1964) developed a similar
quantitative scheme for engineering purposes and applied it to joint spacing
as well as strata thickness (Table 7). Figure 4 compares discontinuity
spacing schemes of several authors. The scheme of Deere (1964) is similar to
an earlier rock mechanics scheme devised by Klaus John (1962). The Core Log­
ging Committee of the South Africa Section of AEG (1978) proposed a purely
logarithmic division of discontinuity spacing in a range from 0.3 to 100 cm.
Coates’ (1970) rock mechanics classification used the terms "broken," "blocky,"
and "massive" to describe the spacing of rock mass discontinuities with a nar­
rower range of values than the above schemes. No rationale for the choice of
numerical values of the various divisions of spacing categories was given by
the authors, but all of the schemes approximate a log-scale division because
the widths of the categories on a logarithmic scale are roughly the same within
a given scheme. Log-plots allow subdivisions of equal widths at both extremes
of a scale. The difference between the four schemes is basically the number of
categories (or classes) and the position of division points between classes.
Deere’s engineering usage scheme allows good resolution in the middle of the
scale (close to moderately close to wide), is widely published, and agrees

THICK V. THIN THINLY

V. THICK B E D D E D BDD | THIN B E D D E D ^ BEDDED L A M IN A T E D L A M IN A T E D __
* M rKFF A N n W F IR
T
120 CM 60 CM 5 CM 1 CM 0.2 CM
(1953)

VERY C L O S E -
O C C A S IO N A L I W IDE C LO SE I C R U SH ED K. JOHN
(1962)
200 CM 20 CM 2 CM 0.5 CM
MOD.
VERY W IDE | W IDE | C LO SE | C LO SE V ERY CLO SE D E ER E (1964)
(BEDDIN G T ER M S
(V. THICK) I ? THICK) IfM E D IU M ) (THIN) (VERY THIN)
IN PAREN.)
120 IN. 36 IN. 12 IN. 2 IN.
M A SSIV E | BLO C K Y | B R O K E N \ VERY B R O K E N COATES
(1965)
72 IN. 12 I n . 3 IN.
VERY CLO SE
| CORE LOG. COMM.
VERY WIDE ( I WIDE^ I M E D IU M CLO SE (V. THIN) ^ | V. IN T EN SELY OF A.E.G. (1978)
(V. THICK) * I 1t h ICK) 1"
T H IN r * ~r \ ~ V L A M IN A T E D (BEDDIN G T ER M S IN PAREN.)
100 CM 30 CM 10 CM 3 CM 1 CM 0.3 CM
-IN T E N SE L Y LAM INAT ED ,
FOLIAT ED, C LV D
1000 200 100 20 10 1.0 0.2
D I S C O N T I N U I T Y S P A C IN G , C M

Figure 4. Discontinuity spacing divisions of several investigators

16
well with subdivisions of the spacing scale widely used by geologists (McKee
and Weir 1953; and adopted by Pettijohn 1957). Deere’s system has also been
adopted by Bieniawski (1979), whose geomechanics classification has been rec­
ommended for USACE usage.
20. Recommended descriptor for spacing. This report recommends a
slightly modified discontinuity spacing terminology and class division of
Deere (1964). The actual spacing measured in the field should also be stated
because further subdivisions may sometimes be needed for special cases.
Deere’s system does not subdivide bedding thickness less than "very fine” (less
than 2 in.); whereas, McKee and Weir (1953) subdivided further by defining
strata less than 1 cm as ’’laminated” (Table 6 and Figure 4). Laminations are
commonly ascribed to shales, and because shales are an important rock type in
engineering problems, the terms ’’laminated” and "laminations" should be re­
tained. The system of Deere (1964) can then be modified by assigning "very
thin bedding" to the range 1/2 in. (in place of 1 cm) to 2 in., and "laminated"
to strata less than 1/2 in. thick. Table 8 shows the system adopted in this
report. A term analogous to "laminated" for use with joints is not considered
necessary. The system omits "massive," which is often used to describe rock
units that display no visible bedding planes and often behave isotropically
and appear homogeneous, such as a massive sandstone. Although the term "mass­
ive" is in widespread use, it is believed unnecessary in the system because the
terms "very wide" and "very thick" can be applied. Bedding or joint geome­
tries, such as cross-bedding, that are not adequately described by the terms
in Table 8 should be described separately.
Condition of discontinuity
21. Definition and importance. Condition describes the roughness, the
degree of weathering, the width of opening (or aperture), and the character
and presence of filling material of rock mass discontinuities. Condition is
an important consideration in classifying rock mass quality for engineering
purposes. For example, joint condition accounts for as much as 25 percent
of the rock quality rating of Bieniawski’s (1979) geomechanics classification
scheme. Joint roughness and joint alteration are essential factors in comput­
ing the rock mass quality of Barton, Lien, and Lunde’s (1974) system. Franklin
(1970) included openness (width), roughness, and infilling material for his
fissure descriptions for a mechanical classification of rock properties. The
Core Logging Committee for AEG, South Africa Section (1978), recognized as

17
significant to engineering behavior of rock masses the following discontinuity
features:
a.. Separation of fracture walls (aperture) .
Filling.
c_. Roughness (asperity).
d_. Orientation.
The shear strength along discontinuities and the stability of the rock mass
are affected by the height and strength of surface irregularities (roughness)
and the strength and thickness of the filling material, which is often clayey
and considerably weaker than the host rock. Aperture determines the secondary
permeability or effective porosity of a rock mass. Orientation (discussed
under a separate heading) of discontinuities and sets* of discontinuities in­
fluence the stability of excavations in rock. Weathering of discontinuities
is discussed in Part III.
22. Aperture. Discontinuity wall separation has been described quali­
tatively and quantitatively. Franklin (1970) suggested using only the terms
"tight11 and "open" to describe discontinuity aperture. Similarly, Deere (1964),
referring primarily to discriptions of discontinuities in rock cores, preferred
the terms "tight" for discontinuities the surfaces of which could be tightly
fitted together and "open" for those the surfaces of which could not be inti­
mately mated. Deere also recommended that the ranges in aperture be recorded
for open discontinuities. Other writers have suggested quantitative ranges and
divisions for classifying aperture, but with considerable disagreement, as shown
in Table 9. The usefulness of aperture description is in the determination of
secondary permeabilities (or effective porosity) and water inflows and in eval­
uating the shear strength of the rock mass as controlled by discontinuities.
23. The effect of aperture on shear strength should be evaluated with
respect to the filling within the discontinuity and the roughness (asperity
amplitude and waviness) of the surfaces. For example, the combined effects of
aperture, roughness, and filling were summarized in AEG, South Africa Section
(1978) after a discussion by D. R. Piteau:
aL. With tight discontinuities (no separation, no filling), the
shear strength depends on properties of the wall rock.
b_. With open discontinuities with measurably thick filling but
with some interlocking of asperities, the shear strength depends

* Discontinuities having the same orientation comprise a "set."

18
 on both filling properties and on wall rock strength.
c.. With open discontinuities with thick filling and no interlocking
of asperities, the shear strength is controlled by the properties
of the filling material.

Figure 5 illustrates the influence of aperture and filling thickness on discon­

tinuity shear strength.

Figure 5. Relationship between shear strength and normal

stress for discontinuities with different thickness of
gouge infilling (Hoek and Bray 1974)

24. Effective porosity* of a rock can be estimated from analysis of the

volume of open discontinuities determined from borehole photographic or tele­
vision logging. Effective porosity was defined for the borehole photography
analysis of jointing in the foundation of Teton Dam** as the total open dis­
continuity (joint) volume divided by the volume of the boring. For the Teton
Dam analysis, a determination of joint condition and aperture was made for
every visible joint in the boring walls. Joint condition was described as
tight if no aperture was present, open if separation of the walls was
consistent, and partially open if the joint walls did not remain separated

* The term "effective porosity" as used herein denotes the fracture porosity
of rocks that have little or no primary (grain) porosity.
** D. C. Banks. 1977. "Borehole Photography Analysis, Teton Dam," Letter
Report, US Army Engineer Waterways Experiment Station, Vicksburg, Miss.

19
throughout the film record. The volume of partially open joints was halved
for effective porosity calculations; the volume of open joints was taken at
100 percent. The primary considerations in the analysis, however, were
whether a joint was tight or open and the actual aperture of the open joint.
25. Discontinuity apertures can also be measured in excavation made in
rock if the excavation surfaces are fresh, for example, in machine-bored tun­
nels. Borehole determinations can be made using impression-type packers, which
expand against the borehole walls and take imprint of wall irregularities such
as open discontinuities. The ISRM (1978) emphasizes, however, that measure­
ments of the exposed surfaces of open discontinuities may not be representative
of water-conducting potential because wall roughness may reduce flow veloci­
ties, and flow in joints may be tubelike rather than sheetlike. Also, open
discontinuities may be filled or closed at some distance from the measured ex­
posure. In situ permeability testing (pump testing, bailing, falling head,
etc.) is a more reliable indicator of flow through apertures.
26. Recommended descriptors for aperture. For the above reasons the
classification or division of ranges of aperture is considered unnecessary in
the general description of discontinuities in the field. Instead, the simple
determination of tight or open should be made and the actual aperture measured
normal to the plane of the discontinuity recorded along with and qualified by
other joint conditions including filling and roughness. Special applications
requiring a subdivision of classes of aperture, such as the "partially healed"
subdivision for porosity estimation mentioned above, can be approached for
specific cases requiring more detailed investigative procedures.
27. Filling. The material within the walls of a discontinuity should
be described in terms of its thickness, relative grain size and, if possible,
its composition. Fillings such as calcite and gypsum that are subject to
removal under construction stresses or by solution may produce greater aper­
tures than those initially measured. If the thickness of such a filling is
recorded, the effect of subsequent widening of the aperture can be predicted
or expected. Fillings of cohesionless materials such as wall rock alteration
products or infiltrating clastic materials may flow out when the rock mass
is excavated. Fillings of clays with a high activity number* can undergo

* Activity of a clay is defined as the plasticity index divided by the weight

percent of particles smaller than 0.002 mm.

20
considerable volume change in the presence of varying moisture conditions.
Brekke and Howard (1972) suggest that swelling clays can cause a loss of
strength through swelling and may produce considerable swelling pressure when
confined. Low activity or inactive clays are relatively weak materials with
correspondingly low resistance to shear along discontinuities. Fillings of
metamorphic minerals such as chlorite, talc, and graphite impart low coeffi­
cients of friction to discontinuity walls even when present in thin coatings
(Brekke and Howard 1972). Thick fillings of materials of low seismic velocity
attenuate shock waves and can influence blasting results in rock excavation.
Table 10 describes materials often filling discontinuities and the potential
problems associated with the fillings (Brekke and Howard 1972).
28. Recommended descriptors for filling. The thickness of the filling
(width of the filled discontinuity) limits the degree to which discontinuity
wall roughness increases shear strength along the discontinuity (paragraph 29).
The minimum and maximum thickness of the filling should be measured and deter­
mination made of the mineralogy and approximate grain size and gradation of
the filling. If the mineralogy cannot be determined by field observation, a
sufficient sample of the material should be obtained for laboratory determina­
tion, especially where the presence of active clays is suspected. The ISRM
(1978) suggests the thickness be measured to 10 percent and an estimate made
of the average (modal) width. Description of important complex filled discon­
tinuity zones such as shear zones should be accompanied by a scaled sketch of
the zone (ISRM 1978). Water conditions of filled discontinuities should be
described as suggested in paragraphs 45-48.
29. Roughness. Roughness (asperity) of discontinuity walls is described
by the presence or absence of surface irregularities and their magnitudes.
Site investigation should include sufficient description of surface roughness
to aid in the design of laboratory and in situ shear strength testing programs.
For example, Goodman (1968) evaluated the effects of roughness, filling thick­
ness, and water content on load deformation curves of a large number of labora­
tory direct shear and several in-situ block shear tests on discontinuities.
Figure 6 summarizes Goodman's evaluation in which he defined four types of
stress-strain responses. Figure 6 also illustrates the effects of condition,
especially roughness, on peak strengths, residual (ultimate) strengths, and
stiffnesses. The several peaks displayed on the stress-strain curves for

21
 TYPE 1 - HEALED AND TYPE 2 -CLEAN,
INCIPIENT JOINTS SMOOTH FRACTURES

DISPLACEMENT, CM DISPLACEMENT, CM

TYPE 4 - FILLED JOINTS,

TYPE 3 -CLEAN, SHEAR ZONES,
ROUGH FRACTURES AND SHALE PARTINGS

DISPLACEMENT, CM DISPLACEMENT, CM

Figure 6. Typical shear stress-deformation relationships for

various discontinuity surface conditions (after Goodman 1968)

type 3 discontinuities (clean, rough fractures) are reportedly caused by over­

riding of asperities during shear displacement. The responses of type 4 dis­
continuities (filled joints, shear zones, and shale partings) were sensitive to
water content and filling thickness. The clean surfaces (types 1-3) report­
edly were unaffected by water contents.
30. Bieniawski's (1979) Geomechanics System of rock mass classification
and the AEG (1978) Core Logging Committee's terminology suggest a roughness
description scheme based on visual examination in the field. Barton, Lien, and
Lunde's (1974) Q System for rock mass classification also evaluates roughness
(an important factor in the Q System) from simple field observations. The
asperities described by the above authors are small enough (amplitudes of

22
millimetres or tenths of an inch) to be readily apparent on core-sized samples.
Surface irregularities of larger amplitude are described collectively as wavi­
ness. Waviness refers to large-scale undulations which affect the strength of
the in situ discontinuity. Characteristics of waviness are not apparent on
small specimens obtained from drilled core. Unevenness refers to small-scale
roughness which affects the strength normally sampled in laboratory or medium­
sized in situ shear tests. Characteristics of unevenness are apparent on
drilled core specimens.
31. Recommended descriptors for roughness. The International Society
for Rock Mechanics (ISRM 1978) suggests means for measuring roughness by pro­
filing and other methods. However, actual measurement of asperity amplitude
and wavelength on discontinuity surfaces is tedious, nonstandardized, and only
practical on large exposed surfaces. It is reasonable to suggest a simple
qualitative roughness description terminology for field use. Four previously
published roughness classification schemes are shown in Table 11. Bieniawski’s
(1979) and AEGfs (1978) schemes are similar: (a) their terms are defined,
(b) their terms are simple, and (c) their schemes have fewer categories than
the other schemes. Bieniawski’s scheme is recommended primarily because it
distinguishes the condition of slickensides, the polished and striated sur­
faces characteristic of shear planes. Bieniawski’s descriptions and defini­
tions are given in Table 12.
Orientation
32. The orientation of discontinuities can be described in absolute
terms (orientation in space) and in relative terms (orientation with respect
to excavation surfaces, tunnel axes, stress .fields, etc.). Orientation can
apply to individual discontinuities, and to sets of discontinuities making
up a system. Bieniawski (1979), following Wickham, Tiedemann, and Skinner
(1972), preferred a qualitative assessment of orientation relative to the
alignment of the axis of a driven tunnel, and developed descriptive termi­
nology for various relative alignments and dips (Table 13). Bieniawski ex­
tended the use of the orientation classification to foundations and slopes
in his geomechanics classification, but did not explain the extension.
Hoek and Bray (1974) recognized the importance of discontinuity orienta­
tion to slope stability. Figure 7 illustrates several simple types of
slope-problems produced by adverse orientation of planes of weakness. The
use of stereoplots to describe the interactive geometry of the slope and

23
 N

a. CIRCULAR FAILURE IN OVER­

BURDEN SOI L, WASTE ROCK OR
HEAVILY FRACTURED ROCK WITH
NO IDENTIFIABLE STRUCTURAL
PATTERN.

b. PLANE FAILURE IN HIGHLY

ORDERED STRUCTURE SUCH AS
SLATE.

c. WEDGE FAILURE ON TWO INTER­

SECTING DISCONTINUITIES.

d. TOPPLING FAILURE IN HARD

ROCK WHICH CAN FORM COLUMNAR
STRUCTURES SEPARATED BY
STEEPLY DIPPING DISCONTINUITIES.

Figure 7. Main types of slope failures and stereoplots of

related structural conditions (after Hoek and Bray 1974)

failure planes in Figure 8 is discussed below.

33. The importance of discontinuity orientation to blasting efficiency
and to excavation stability were discussed in the Corps manual on systematic
drilling and blasting for surface excavation (Department of the Army, Office,
Chief of Engineers (OCE) 1972). Orientation may adversely or favorably control
alignment of the cut face, produce overbreakage of the rock mass by transmittal
of blast energy beyond the design grade or surface, cause ravelling of the
excavated surface, or result in postexcavation failure of the finished excava­
tion walls. Similar effects apply to underground excavation blasting.
34. Orientation notation. The orientation of planar discontinuities
can be determined at a single point on the plane by recording the direction
of a horizontal line on the plane (the strike) and the maximum angle of

24
»

TRACE OF PLANE

reference
(HEMI)SPHERE

ft
Figure 8. Representation of plane in stereographic projection from
reference hemisphere (top) to stereonet (bottom)
inclination of the plane from the horizontal (the dip). Strike is commonly
recorded in the field as the number of degrees between 0 and 90 west or east
of north (for example, N50°W). Dip is recorded as the number of degrees be­
tween 0 and 90 below the horizontal in a vertical section perpendicular to the
strike of the plane. Numerical methods for analyzing orientation and frequency
data require that the usual geologic description be converted to a system in
which the strike is recorded unambiguously by the use of a single number be­
tween 0 and 180 or 360 degrees, rotated clockwise or counterclockwise from
north or south depending on the convention used and the dip by a single num­
ber between 0 and 90 or 180 degrees rotated according to a convention. Nu­
merical conversion of strike and dip data from the field has been used by
Hendron, Cording, and Aiyer (1980) for vector analysis of stability of slopes
cut by discontinuities. Numerical conversion has also been used to permit
computer analysis and processing of borehole photography data on discontinui­
ties, especially in inclined borings for which the recorded apparent orienta­
tion data must be converted to true orientation.*
35. Data analysis. Discontinuity orientation data are commonly analyzed
by plotting the orientations on the two-dimensional projection of a reference
sphere by the technique of stereographic projection. Figure 8 shows a plane
represented on the reference sphere projection, or stereonet, by a single
point, the pole, which represents the intersection with the lower hemisphere
of a line normal to the plane and passing through the center (0) of the sphere.
The pole is unique to the plane of that orientation and all discontinuity
planes recorded can be represented on a single stereonet. The stereonets of
discontinuities associated with slope failures on the right side of Figure 7
illustrate the clustering or grouping of poles. The preferred orientations
of groups or sets of planes can be readily seen by contouring the clusters.
Stereonets are constructed by equal-area projection (a Lambert or Schmidt net)
or by equal-angle projection (a Wulff net), and may be constructed to equa­
torial or polar projections. The equal-area projection, or Schmidt net, re­
tains correct areal distribution of projected poles and is used for statistical
analysis and contouring of groups of discontinuities. The equal-angle, or
Wulff net, is most commonly used for analysis and graphical solutions of
structural problems due to the ease of plotting the traces of planes in the

* For example, see Banks (1977), op. cit.

26
projection. Discontinuity orientation can be simply displayed on a joint
rosette, which shows the number of joints (discontinuities) occurring in each
sector of a 360° compass face. Discussions of the construction and use of
stereographic projections can be found in Billings (1954); Ragan (1973); Hoek
and Bray (1974); Coates (1970); Goodman (1976); and Hendron, Cording, and
Aiyer (1980). John (1968) explains in detail the use of stereographic pro­
jection of discontinuity data in stability analyses of slopes in jointed rock,
and includes the application of factors of safety and active and passive
forces. Another useful discussion can be found in Attewell and Farmer (1976),
but it should be recognized that the upper hemisphere projection is used.
Figure 9 illustrates the simplified analysis of the stability of a wedge
formed by the intersection of two planar discontinuities with a slope (example
of Coates (1970)). In the example, two joint planes, one striking N10°E and
dipping 60°NW (Plane 1 in Figure 9) and the other striking N30°E dipping
40°SE (Plane 2), intersect to form a potential wedge in a slope face striking
N75°E and dipping 35°SE. The joint poles are also shown. The intersection of
the joint planes, line OJ (Figure 9), plunges 11 deg and extends beyond the
trace of the slope plane on the stereonet. Therefore, sliding of the wedge
is possible. If Point J were inside the slope projection, the intersection
would have a steeper plunge than the slope and sliding of the wedge could not
occur.
36. The attitude (orientation) of the joint planes (Figure 9) could
have been recorded numerically as, for example, with joint plane number 1,
190°, 60° if a convention were used whereby strike is recorded as an azimuth
clockwise from north, dip direction is understood to be strike plus 90°, and
dip is between 0° and 180°. Or the attitude could be recorded as 280°,
60°, whereby 280° is the dip direction (in lieu of "strike plus 90°") using
azimuth.
37. Recommended descriptors for orientation. Other conventions are
sometimes used, however, but until a standard convention is established for
recording attitudes numerically for automatic data processing and analysis,
the practice of recording strike, dip, and direction of dip should be contin­
ued. The geologist should be aware, however, that specific projects may re­
quire the recording of discontinuity orientations in numerical (azimuthal)
notation. Pole diagrams (stereonet plotting) of discontinuity distribution
and frequency are an efficient and well known method of displaying absolute

27
 I

Figure 9. Stereonet analysis of joint wedge/slope stability problem

(after Coates 1970)

orientation and orientation relative to surface and subsurface excavations,

and their use in geotechnical reports is encouraged. Field methodology for
determining the orientation of discontinuities in rock slopes is presented in
ETL 1110-2-300 (OCE 1983).

28
 PART III: ROCK WEATHERING

Definition

38. Weathering is the disintegration and decomposition of rock in place

by mechanical and chemical processes. Rock is attacked by weathering agents
on exposed surfaces, such as excavation walls and natural outcrops and along
joints and other discontinuities that extend into the rock mass. The zone of
weathering is most pronounced near the discontinuities and exposed surfaces.
The degree of weathering in a rock mass depends on (a) the area of exposed
surface, (b) the age of the exposed surface, (c) the extent of access to the
rock mass along discontinuities and through pores, (d) the chemical composi­
tion (mineral content) and texture of the rock, (e) the environment or climate,
and (f) the position and chemistry of the ground water. A useful discussion
of the relative susceptibilities of the common rocks and minerals to break­
down by weathering is presented by Dornbusch (1982).
39. Mechanical, or physical, weathering of rock occurs primarily by
(a) freeze expansion (or frost wedging) of water that seeps into pores and
open discontinuities, particularly in temperate climates; (b) thermal expansion
and contraction from severe daily temperature variations, especially in arid
regions; and (c) cycles of wetting-drying, particularly in the clay-rich rocks.
Chemical weathering occurs by the reaction of water, acids and bases, oxygen,
and carbon dioxide with mineral constituents of the rock. Iron sulfides
combine with oxygen to form the commonly occurring red oxides of iron by the
process of oxidation. Carbon dioxide dissolved in water readily dissolves
soluble carbonates such as limestones and dolomites to produce the networks of
caves and solution-enlarged discontinuities of karst regions. Many clay min­
erals, which are significant in stability of geotechnical structures, are
formed from silicates of the igneous rocks by the addition of water (hydroly­
sis) under certain conditions to form hydrous compounds. For example, feld­
spars, common igneous minerals, alter in the presence of water to illite or
kaolinite, common clay minerals. The absorption of free water into the mineral
structure (hydration) also produces a kind of mechanical weathering by expan­
sion of the structure when a mineral undergoes growth by recrystallization.
For example, the hydration of anhydrite to reform gypsum produces a volume
change of as much as 30 to 60 percent (Robinson 1982). Clay minerals such as

29
montmorillonite filling the space between discontinuity walls may absorb water
and contribute to expansion and mechanical breaking of the rock mass.

Previous and Existing Classifications

40. The weathering of rock is recognized by a decrease in the luster of

the rock’s minerals, discoloration of the rock, separation of rock crystals or
grains along their boundaries, increased friability, and a general decrease in
competency or compressive strength. Infiltrating water may stain discontinuity
surfaces or bring in material to fill open discontinuities. The degree of
weathering present in a rock mass can be classified on the basis of simple
qualitative visual and physical inspection. Saunders and Fookes (1970) re­
viewed weathering processes and earlier (pre-1970) classification schemes of
several workers. The system widely published more recently is based on work
by the Task Committee for Foundation Design Manual of the Committee on Shallow
Foundations of the Soil Mechanics and Foundations Division of the American
Society of Civil Engineers (ASCE) (1972). Table 14 is the classification
used in Bieniawski (1979) and is an abbreviated version of the ASCE Task Com­
mittee (1972) similar to the versions of the Geological Society Engineering
Group Working Party (1977) and AEG (1978). It describes weathered rock on the
basis of appearance and feel. The terminology for degree of decomposition
(weathering) used by Little (1969) in his scheme (Saunders and Fookes 1970) is
similar to that of Bieniawski (1979), but Little based his field recognition
on relative strength of hand samples and the degree of difficulty in excava­
tion. Little also evaluated the weathering classes by their effects on engi­
neering works.

Recommended Descriptors

41. Although the descriptive terms of Table 14 are subjective in nature,

they can be rendered locally objective if the user will observe fresh (un­
weathered) cores or other samples of the rock mass and use the fresh rock as a
standard of comparison for weathered rock. This report recommends the descrip­
tive terminology of Table 14 for field description of the weathering condition
of rock samples and rock masses. The field inspector should record the depth
of weathering from exposed surfaces where possible and the thickness of the

30
weathered zone around discontinuities as well as the degree of weathering.
The determination of the decrease in strength of the weathered rock should be
made using the methods presented in paragraphs 9- 14 .

31
 PART IV: ROCK QUALITY DESIGNATION

Definition

42. Rock Quality Designation (RQD), or Modified Core Recovery, was

developed by Deere, Merritt, and Coon (1969) as a means of describing the
condition of the rock mass from core borings. The RQD is obtained by mea­
suring the cumulative (total) length of intact NX core pieces 4 in. long or
longer and dividing by the sampling depth (Figure 10). The USACE stipulates
that RQD be applied to NX core only. Other investigators (Bieniawski 1979,
and Franklin, Broch, and Walton 1971) apply RQD to NX or larger cores. The
quotient is expressed as a percentage and is used to classify the rock
quality as very poor to excellent (Table 15).

MODIFIED
CORE RECOVERY, IN. CORE RECOVERY, IN

10

0
0
0
4

0 NX (2-1/8 IN. or 54 MM) CORE

0
0

50 60 34
CORE RUN
CORE RECOVERY = 50/60 RQD = 34/60

= 83% =57% , ROCK QUALITY = FAIR (TABLE 17)

Figure 10. Rock quality determination from modified core recovery

(Deere, Merritt, and Coon 1966)

32
 Importance and Previous Use

43. The RQD has been accepted widely as a means of estimating rock
quality from cored rock. Bieniawski suggests that the RQD is a quick, inex­
pensive index for rock core quality but is insufficient to adequately describe
the rock mass quality alone because it disregards discontinuity orientation,
aperture or tightness, and condition (filling, roughness, etc.). However,
Bieniawski uses the RQD as a factor in his Geomechanics rock mass classifica­
tion. The Geological Society of Great Britain Engineering Group Working
Party (1970) elected to retain RQD as a rock quality index. Barton, Lien, and
Lunde (1974) incorporate RQD in their Rock Mass Quality rating (Q) of rock
mass classification. Most tunnel support design and analysis, for example,
require RQD as input, but RQD may or may not be used in foundation or slope
stability investigations.

Recommended Use

44. The determination of RQD is somewhat more time consuming than stand­
ard core recovery measurements but is a specified procedure for most WACE Dis­
tricts and Divisions. The RQD as defined by Deere, Merritt, and Coon (1969)
(Table 15), or a slightly modified version for metric use in which 0.1-m
(10 cm) core pieces instead of 4-in. pieces are counted, should be used when
RQD is required. Following current USACE practices, RQD should be applied to
NX core only.
 PART V: GROUND-WATER CONDITIONS

Definition and Importance

45. The effects of ground water in surface and subsurface excavations

are manifested as (a) seepage or inflow through pores and along discontinui­
ties, (b) strength-reducing pore pressure in excavation slopes and along po­
tential planes of weakness, and (c) softening or weakening of saturated rocks
and filling materials. Ground-water inflow in underground excavations inhibits
excavation activities and may wash out loose, saturated materials in pores and
filled discontinuities. Water trapped in the rock mass against impermeable or
low permeability barriers can create a buildup in pore pressures which can
lead to instability in excavation slopes, tunnel walls, and along potential
failure planes in foundations. Pore pressure reduces the effective stress on
potential planes of failure and thereby lowers the shear strength of the rock
mass. The relationship is shown mathematically in the familiar expression:

t = c + (a - u) tan (f)

where
t = the shear strength (or shear stress required to cause sliding along
a plane)
c = the cohesion of the rock or soil particles
a = the normal stress component of load on the plane
u = the pore (uplift) pressure produced by the head of ground water
(j) = the angle of internal friction along the potential failure plane

The term (a - u) is the effective stress on the plane resulting from the
reduction in normal stress by the pore pressure. As stated in Hoek and Bray
(1974), cj) and c are not much affected by the mere presence of water (water
content) in hard rock, sands, and gravels. Instead, the shear strength charac­
teristics of those materials are defined more by water pressure, u , than by
water content. Slope stability, for example, is influenced more by a small
volume of water trapped under high pressure within the rock mass than by a
large volume of water discharging freely from the excavation face.

34
 Previous and Existing Classifications

46. Bieniawski (1979) included ground-water condition in his Geomechan­

ics Classification for rock mass rating. Ground-water condition, accounting
for as much as 10 percent of the rock mass rating in his system, is assessed
by estimating the rate of inflow (discharge) and by a subjective description
such as dry, damp, wet, dripping, or flowing. Barton, Lien, and Lunde (1974)
used a Joint Water Reduction Factor, Jw , in their Q-System of rock mass
rating. Jw is a measure of the water (pore) pressure, which is responsible
for reducing the effective stress along planes of potential failure. Barton
et al. also recognized the ability of water under pressure to soften and wash
out clay-filled discontinuities.
47. Tests on rock by Colback and Wiid (1965) and by Broch (1974) showed
a general reduction in compressive strength and point-load index with increas­
ing water contents of the rocks tested. Broch (1979) suggested in further
tests that a reduction in <J> also occurred. The deleterious effects of water
content on strength have been shown only in laboratory tests of intact rock
specimens. For practical considerations of stability of rock masses the
effects of pore pressure and not water content should be given priority. Pore
pressures within the rock mass can be monitored by the installation of piezom­
eters if the piezometers are properly located. Nevertheless, the preliminary
analysis of seepage and pore pressure conditions benefits from early field
recognition and description of water conditions along discontinuities encoun­
tered in the exploration phase.

Recommended Descriptors

48. The ISRM (1978) suggests a system of seepage ratings for describing
the water conditions of filled and unfilled discontinuities in tunneling or
surface exposures (Table 16). The ratings are based on simple observations of
the amount of water present and on field estimates of discharge and relative
water pressure. This report recommends the use of the field descriptors pre­
sented in Table 16 for preliminary assessment of water conditions. The systems
of Bieniawski (1979) and Barton, Lien, and Lunde (1974) for rock mass rating
are accepted as USACE guidance.

35
 I
PART VI: ROCK DENSITY

Definitions and Importance

49. The rock density, or unit weight, is commonly used to determine the
load that the rock mass exerts on a structure and the stress that exists at a
point within the rock mass. Rock density is defined in several ways:
a.. Grain density, the ratio of the weight of dry solids to the
volume of solids (converts to specific gravity, Gs , by divid­
ing by the unit weight of water).
b^. Dry density, y^ , the ratio of the weight of the dry solids to
the total specimen volume (includes pores).
cL. Saturated density, ygat > the ratio of the weight of the satu­
rated specimen to the total specimen volume (pores filled with
water).
cL Bulk, or wet density, Ywet > the ratio of the weight of the
specimen at its natural or sampled water content to the total
specimen volume (also wet unit weight).

Wet density is the quantity most often used to estimate the rock load and is
also the most readily obtained because it requires only the weight and volume
of the intact specimen at its natural (as-sampled) water content.

Recommended Use

50. Unit weight is usually determined in the laboratory but it should

be calculated in the field on rock cores if a diamond saw to square the core
ends and a good scale are available (horizontally bedded sedimentary rocks may
not require sawing). The maximum range of variation of unit weight in nature
is only a little more than a factor of 2. The Rock Testing Handbook, Method
No. 109-80 (WES 1980) describes the procedure for determining the effective
(as-received) unit weight of a rock specimen.

36
 PART VII: SUMMARY AND RECOMMENDATIONS

51. The significant rock characteristics discussed and the classifica­

tion and descriptive systems recommended in this report are summarized below:

Significant Rock Characteristics Recommended Descriptive System

Rock type Common geologic rock name with
qualifying phrase, if neces­
sary (Table 2 and paragraph 8)
Strength Unconfined compressive strength
from point-load test (Table 4
and paragraph 14)
Discontinuity spacing Table 8 and paragraph 20
Discontinuity aperture Use "tight" or "open." Include
actual aperture measurements
(paragraph 22)
Discontinuity filling Minimum and maximum filling
thickness; mineralogy of fill­
ing; grain size (paragraph 28)
Discontinuity roughness Table 12 and paragraph 31
Discontinuity orientation Strike, dip, direction of dip;
use of pole diagrams (stereo­
graphic projection) is encour­
aged; (Figures 9 and 10 and
paragraph 37)
Weathering Table 14 and paragraph 41
RQD Figure 10, Table 15, and para­
graph 44
Ground-water conditions Table 16 and paragraph 48
Rock density Calculate wet unit weight
(paragraph 50)

37
 REFERENCES

American Geological Institute. 1972. Glossary of Geology, Washington, DC.

Association of Engineering Geologists, South Africa Section. 1978. MA Guide
to Core Logging for Rock Engineering," Core Logging Committee, Bulletin, Asso­
ciation of Engineering Geologists, Vol XV, No. 3, pp 295-328.
Attewell, P. B., and Farmer, I. W. 1976. Principles of Engineering Geology,
Wiley, New York.
Barton, N., Lien, R., and Lunde, J. 1974. "Engineering Classification of
Rock Masses for the Design of Tunnel Support," Rock Mechanics, Vol 6, No. 4,
pp 183-236.
Bieniawski, Z. T. 1975. "The Point-Load Test in Geotechnical Practice,"
Engineering Geology, Vol 9, pp 1-11.
____________ . 1979 (Sep). "Tunnel Design by Rock Mass Classifications,"
Technical Report GL-79-19, US Army Engineer Waterways Experiment Station,
Vicksburg, Miss.
Billings, M. P. 1954. Structural Geology, Prentice Hall, Englewood Cliffs,
N. J.
Brekke, T. L., and Howard, T. R. 1972. "Stability Problems Caused by Seams
and Faults," Proceedings, North American Rapid Excavation and Tunneling Con­
ference, American Institute of Mining, Metallurgical, and Petroleum Engineers,
Chicago, 111., Vol 1, pp 25-64.
Broch, E. 1974. "The Influence of Water on Some Rock Properties," Proceed­
ings, Third Congress of International Society of Rock Mechanics, Vol II,
Part A, pp 33-38.
________ . 1979. "Changes in Rock Strength Caused by Water," Proceedings,
International Society of Rock Mechanics Congress, Vol 1, pp 71-75.
Broch, E. and Franklin, J. A. 1972. "The Point-Load Strength Test," Inter­
national Journal for Rock Mechanics and Mining Science, Vol 9, pp 669-697.
Coates, D. F. 1970. "Rock Mechanics Principles," Mines Branch Monograph 874,
Mining Research Centre, Department of Energy, Mines and Resources, Ottawa,
Canada.
Colback, P. S. B., and Wiid, B. L. 1965. "The Influence of Moisture Content
on the Compressive Strengths of Rocks," Proceedings of the Rock Mechanics
Symposium, Toronto, Canada, pp 65-71.
D ’Andrea, D. V., Fischer, R. L., and Fogelson, D. E. 1965. "Prediction of
Compressive Strength of Rock from Other Rock Properties," Report of Investi­
gations 6702, US Bureau of Mines.
Deere, D. U. 1964. "Technical Description of Rock Cores for Engineering
Purposes," Rock Mechanics and Engineering Geology, Vol 1, No. 1, pp 17-22.
Deere, D. U., Hendron, A. J., Patton, F. D., and Cording, E. J. 1966. "Design
of Surface and Near-Surface Construction in Rock," Proceedings, Eighth Rock
Mechanics Symposium, Minneapolis, Minn.

38
Deere, D. U., and Miller, R. P. 1966. "Engineering Classification and Index
Properties for Intact Rock," Technical Report No. AFWL-TR-65-116, Air Force
Weapons Laboratory, Kirtland Air Force Base, N. Mex.
Deere, D. U., Merritt, A. H., and Coon, R. F. 1969. "Engineering Classifica­
tion of In-Situ Rock," Technical Report No. AFWL-TR-67-144, Air Force Weapons
Laboratory, Kirtland Air Force Base, N. Mex.
Dornbusch, W. K., Jr. 1982 (Jun). "Natural Processes Influencing Terrain
Attributes; Prediction of Residual Soil Texture in Humid Temperate Climates
of the Federal Republic of Germany and Selected Analogous Portions of the
United States - Pilot Study," Technical Report GL-82-2, Report 1, US Army
Engineer Waterways Experiment Station, Vicksburg, Miss.
Fookes, P. G., and Denness, B. 1969. "Observational Studies on Fissure Pat­
terns in Cretaceous Sediments of South-East England," Geotechnique, Vol 19,
pp 453-477.
Franklin, J. A. 1970. Classification of Rock According to Its Mechanical
Properties, Ph. D. dissertation, University of London Imperial College.
Franklin, J. A., Broch, E., and Walton, G. 1971. "Logging the Mechanical
Character of Rock," Transactions, Institution of Mining and Metallurgy, Vol 80,
pp A1-A9.
Geological Society Engineering Group Working Party. 1970. "The Logging of
Rock Cores for Engineering Purposes," Quarterly Journal of Engineering Geology,
Vol 3, pp 1-24.
__________ . 1977. "The Description of Rock Masses for Engineering Purposes,"
Quarterly Journal of Engineering Geology, Vol 10, pp 355-388.
Goodman, R. E. 1968. "Effects of Joints on the Strength of Tunnels," Techni­
cal Report No. 5, US Army Engineer District, Omaha, Omaha, Neb.
__________ . 1976. Methods of Geological Engineering in Discontinuous Rock,
West Publishing Co., St. Paul, Minn.
Hendron, A. J., Jr., Cording, E. V., and Aiyer, A. K. 1980 (Mar). "Analytical
and Graphical Methods for the Analysis of Slopes in Rock Masses," Technical
Report GL-80-2, US Army Engineer Waterways Experiment Station, Vicksburg, Miss.
Hoek, E., and Bray, J. 1974. Rock Slope Engineering, The Institution of
Mining and Metallurgy, London, England.
International Society for Rock Mechanics. 1978. "Suggested Methods for the
Quantitative Description of Discontinuities in Rock Masses," International
Journal of Rock Mechanics, Mining Science & Geomechanical Abstracts, Vol 15,
pp 319-368.
Jennings, J. E., and Robertson, A. M. 1969. "Stability of Slopes Cut in
Natural Rock," Proceedings, Seventh International Conference on Soil Mechanics
and Foundation Engineering, Mexico City.
John, Klaus. 1962. "An Approach to Rock Mechanics," Journal of the Soil
Mechanics and Foundations Division, American Society of Civil Engineering,
No. SM4.
John, K. W. 1968. "Graphical Stability Analysis of Slopes in Jointed Rock,"
Journal of the Soil Mechanics and Foundations Division, American Society of
Civil Engineering, Vol 94, No. SM2, pp 497-526.

39
Little, A. L. 1969. MThe Engineering Classification of Residual Tropical
Soils,11 Proceedings, Seventh International Conference on Soil Mechanics and
Foundations Engineering, Vol 1, Mexico City, pp 1-10.
McKee, E. D., and Weir, G. W. 1953. "Terminology for Stratification and
Cross-Stratification in Sedimentary Rocks," Bulletin, Geological Society of
America, No. 4, Vol 64, pp 381-390.
Office, Chief of Engineers, Department of the Army. 1972. "Systematic Drill­
ing and Blasting for Surface Excavations," EM 1110-2-3800, Washington, DC.
__________ . 1983 (Sep). "Characterization and Measurement of Discontinuities
in Rock Slopes," ETL 1110-2-300, Washington, DC.
Pettijohn, F. J. 1957. Sedimentary Rocks, Harper and Row, New York.
Ragan, D. M. 1973. Structural Geology, An Introduction to Geometrical
Techniques, Wiley, New York.
Robinson, E. S. 1982. Basic Physical Geology, Wiley, New York.
Saunders, M. K., and Fookes, P. G. 1970. "A Review of the Relationship of
Rock Weathering and Climate and Its Significance to Foundation Engineering,"
Engineering Geology, An International Journal, Vol 4, pp 289-325.
Stagg, K. G., and Zienkiewicz, 0. C. 1968. Rock Mechanics in Engineering
Practice, Wiley, London, England.
Task Committee for Foundation Design Manual. 1972. "Subsurface Investigation
for Design and Construction of Foundations of Buildings; Part II," Journal of
the Soil Mechanics and Foundations Division, American Society of Civil Engi­
neers, Vol 98, No. SM6, pp 557-578.
Underwood, L. B. 1967. "Classification and Identification of Shales," Journal
of the Soil Mechanics and Foundation Division, American Society of Civil Engi­
neers, No. SM6, pp 97-116.
US Army Engineer Waterways Experiment Station. 1980. Rock Testing Handbook,
No. 109-80, Vicksburg, Miss.
__________ . 1982 (Revised). Rock Testing Handbook, No. RTH 325-82, Vicksburg,
Miss.
Wickham, G. E., Tiedemann, H. R., and Skinner, E. H. 1972. "Support Deter­
mination Based on Geologic Predictions," Proceedings, Rapid Excavation Tunnel­
ing Conference, American Institution of Mining Engineers, K. S. Lane and L. A.
Garfield, eds., American Institute of Mining, Metallurgical, and Petroleum
Engineers, New York.

40
 Table 1
Working List of Properties for Rock Mass Classification

Discontinuities Compres­ Abrasion Density Joint/ Slake Fric­ Seis­ State

Engineering Rock Condi­ Orien­ Weather­ sive Resist/ or Unit Pore Dura­ Permea­ tion V mic of
Application Type RQD tion* tation Spacing ing Strength Hardness Wt Water bility bility Angle <f> Ratio** Data Stress

Slope
Stability
A A A A A A A A A
Rippability
A A A A A A A
Drillability
A A A A A
Tunnel
Boring A A A A A A A
Tunnel
Support A A A A A A A A A A
Foundations
A A A A A A A A
Subsurface
Blasting A A A A A A A A A
Surface
Blasting A A A A A A A A A
Quarry
Excavation A A A A A A
Dynamic Re­
sponse (Quake,
Blast Resist.)
A A A A A A
NOTE: Properties or conditions used in rock mass classification systems. A indicates consideration in given application.
* Roughness, aperture, filling.

— = velocity index of Deere (1969).

Vlab
 Table 2

A Classification of Common Rock Types

Common Igneous Rocks

Light-Colored(Acidic) Intermediate Dark-Colored(Basic) Ultra-Basic

Qtz Rich Qtz Deficient Qtz Rich Qtz Deficient (Composed wholly
of dark minerals,
often only one
mineral)

----- GRADATIONAL CHANGE DEPENDING ON KIND AND AMOUNT OF FELDSPAR PRESENT------ >

Coarse Texture Granite Syenite Qtz Monzonite Monzonite Gabbro Peridotite

(Plutonic or to Qtz Diorite to Diorite Pyroxenite
intrusive, (Tonalité) Dunite
phaneritic) Others

Contrasting Texture Granite Monzonite

(Porphyritic; coarse Porphyry Porphyry
crystals in fine
matrix)
Rhyolite Latite "Diabase"
Porphyry Porphyry or
"Dolerite"
Fine Texture or Rhyolite Trachyte Qtz Latite Latite Basalt ^
Glassy (Obsidian is a to Dacite to Andesite
(Volcanic or glassy form of
extrusive; Rhyolite)
lava formers;
aphanitic)

Pegmatites are igneous rocks with large crystals. Pegmatites are usually granitic and tabular.
(1) Dark fine grained igneous rocks are often called "traprock"

common bedimentary ROCKS

Clastic Rocks (Mechanical) Non-Clastic (Chemical)

Coarse Grained Fine Grained Organic Inorganic
(>1/16 or 0.06mm, Wentworth (<1/256 or 0.0004mm, Wentworth
>0.075mm, USCS, Soils) <0.075mm, USCS, Soils)

Conglomerate ) Shales Most Limestone(CaCO3) Gypsum

> (Rudites)
Claystones (Argillites or Coal(Lignite,Bituminous) Anhydrite
Lutites)

Sandstones (Arenites) Mudstones Dolomite(Ca-MgCO3) Rock Salt

Siltstone
(1/lb to l/256mm,
Wentworth)

Volcanic Breccia (volcani-clastic) Tuff (volcani-clastic) Diatomaceous Earth Some Limestones

(Silicate)
Glacial Till (Tillite) Chert

MARLSTONE
(35-65% CaC03
65-35% Clay)

Common Metamorphic Rocks

Isotropic
Anisotropic
(Non-Foliated, Massive)
(Foliated or Banded)
Quartzite
Schist
Marble
Gneiss
Hornfels
Slate

Phyllite
 Table 3
Rock Hardness Classification of the Core Logging Committee, South Africa

» Section (from Association of Engineering Geologists 1978)

Range of minimum
uniaxial compres­
Classification Field test sive strength (MPa)

Very soft rock Can be peeled with a knife, material 1 to 3

crumbles under firm blows with the (145 to 435 psi)
sharp end of a geological pick

Soft rock Can just be scraped with a knife, 3 to 10

indentations of 2 to 4 mm with firm (435 to 1450 psi)
blows of the pick point

Medium hard rock Cannot be scraped or peeled with a 10 to 25

knife, hand-held specimen breaks (1450 to 3625 psi)
with firm blows of the pick

Hard rock Point load tests must be carried out 25 to 70

in order to distinguish between (3625 to 10,150 psi)
these classifications. These re­
Very hard rock sults may be verified by uniaxial 70 to 200 (10,150
compressive strength tests on to 29,000 psi)
selected samples
Extremely hard rock 200 (29,000 psi)

Table 4
Rock Compressive Strength Classifications (from Deere and
Miller (1966) and Bieniawski (1979)

Uniaxial Compressive Strength

Description Bieniawski (1979)* Deere and Miller (1966)

Very high strength >30,000 psi (>200 MPa) >32,000 psi

High strength 15,000-30,000 psi (100-200 MPa) 16,000-32,000 psi
Medium strength 7.500- 15,000 psi (50-100 MPa) 8.000- 16,000 psi
Low strength 3.500- 7,500 psi (25-50 MPa) 4.000- 8,000 psi
Very low strength 150-3,400 psi (1-25 MPa) <4,000 psi

i * Bieniawski gives the metric (SI) measurements listed here in parentheses.

 Table 5
Rock Strength Classification (from Coates 1970)

Description Uniaxial Compressive Strength, psi

Very weak <5,000

Weak 5,000-10,000
Strong 10,000-25,000
Very strong >25 ,000

Table 6
Stratification Thickness Classification (from McKee and Weir 1953)

Thickness Term Thickness Splitting Property Terms

Thinly laminated (lamination) 2 mm Papery

Laminated (lamination) <2 mm - 1 cm Platy or shaly
Very thin-bedded (bed) 1 cm - 5 cm Flaggy
Thin-bedded (bed) 5 cm - 60 cm Slabby
Thick-bedded (bed) 60 cm - 120 cm Blocky
Very thick-bedded >120 cm Massive

Table 7
Discontinuity Spacing Scheme (from ;
Deere 1964)

Joint Spacing Term Spacing/Thickness Bed Thickness Term

Very close <2 in. Very thin

Close 2 in. - 1 ft Thin
Moderately close 1 ft - 3 ft Medium
Wide 3 ft - 10 ft Thick
Very wide >10 ft Very thick

Table 8
Discontinuity Spacing Recommended for USACE Use (modified from Deere 1964)

Joints Spacing/Thickness Bedding

1/2 in. Laminated

Very close 1/2 in. - 2“in. Very thin
Close 2 in. - 1 ft Thin
Moderately close 1 ft - 3 ft Medium
Wide 3 ft - 10 ft Thick
Very wide >10 ft Very thick
 Table 9
Some Proposed Discontinuity Aperture Classifications

Geological Society (Great Core Logging Com­

Britain) Engineering Group mittee South Africa
Working Party (1977) Section, AEG (1978) Bieniawski (1979)
Aperture Aperture Aperture
Description mm Description mm Description mm

Wide >200

Moderately Wide 60-200

Moderately Narrow 20-60

Narrow 6-20 Very Wide 5-25+ Very Wide 10-25

Very Narrow 2-6 Wide 1-5 Open 2.5-10

Extremely Narrow >0-2 Narrow 0.1-1 Moderately Open 0.5-2.5

Tight 0 Very Narrow 0-0.1 Tight 0.1-0.5

Closed 0 Very Tight < 0.1

 Table 10
Materials Filling Discontinuities and Associated Problems
(modified from Brekke and Howard 1972)
i
Material Filling Discontinuity Potential Problems

Swelling clay (montmorillonite, Subject to volume change in varying

illite, attapulgite) moisture conditions. May produce
swelling conditions when confined. May
cause lifting of excavation surfaces and
foundations

Inactive clay Represents weak material between discon­

tinuity walls, with low shear resistance
if thick enough. Can be washed out, re­
sulting in open discontinuity

Low-friction metamorphic Low resistance to sliding, especially

minerals (chlorite, talc, when wet
graphite, serpentine)

Crushed rock fragments or May ravel or run out of exposed disconti­

breccia; sandlike gouge nuity. Permeability may be high

Calcite, gypsum Soluble, may later produce larger

apertures than initially measured. May
be relatively weaker than wall rock i

i
 Table 11
Discontinuity Roughness Classification Schemes

Barton, Lien, and Geol. Soc. Working

Bieniawski (1979) AEG (1978) Lunde (1974) Group (1977)
Very rough Very rough Rough or irregular, Very rough
undulating
Rough Rough Smooth, undulating Small steps
Slightly rough Medium rough Slickensided, Defined ridges
undulating
Smooth Slightly rough Rough or irregular, Rough
planar
Slickensided Smooth Smooth, planar Smooth
Slickensided, planar Slickensided
Polished

Table 12
Bieniawskifs Discontinuity Roughness Classification
(from Bieniawski 1979)

Description _______________Definition_______________
Very rough Near vertical steps and ridges occur on
the discontinuity surface
Rough Some ridge and side-angle steps are evi­
dent; asperities clearly visible; dis­
continuity surface feels very abrasive
Slightly rough Asperities are distinguishable and can
be felt
Smooth Surface appears smooth, feels smooth
Slickensided Visual evidence of polishing

Table 13
Effect of Joint Strike and Dip Orientations in Tunneling
(from Bieniawski 1979)

Strike Perpendicular to Tunnel Axis Strike Parallel

Drive with Dip Drive Against Dip to Tunnel Axis Dip 0°-20°
Dip Dip Dip Dip Dip Dip Irrespective
45°-90° 20°-45° 45°-90° 20°-45° 45°-90° 20°-45° of Strike
Very -favor- Favor­ Fair Unfavor­ Very un­ Fair Unfavorable
able able able favorable
 Table 14
Classification of Degree of Weathering of Rocks
(after Bieniawski 1979)
i
Unweathered : No visible signs of weathering; rock fresh;
crystals bright.
Slightly weathered rock: Discontinuities are stained or discolored and
may contain a thin filling of altered material.
The discoloration may extend into the rock from
the discontinuity surfaces to a distance of up
to 20 percent of the discontinuity spacing.
Moderately weathered rock: Slight discoloration extends from discontinuity
planes for greater than 20 percent of the dis­
continuity spacing. Discontinuities may contain
filling of altered material. Partial opening of
grain boundaries may be observed.
Highly weathered rock: Discoloration extends throughout the rock and
the rock material is friable. The original tex­
ture of the rock generally has been preserved,
but separation of the grains or crystals has
occurred.
Completely weathered rock: The rock is totally discolored and decomposed
and friable. The external appearance of the
rock sample is that of soil. Internally, the
rock structure is partially preserved but grains
and crystals have completely separated.
i

Table 15
Rock Quality Designation (RQD) as an Index of Rock
Quality (from Deere, Merritt, and Coon 1969)

), percent Description of Rock Quality

0-25 Very poor

25-50 Poor

50-75 Fair

75-90 Good

90-100 Excellent I
 Table 16
Field Observations of Seepage Conditions for Filled and Unfilled Discontinuities
(after International Society for Rock Mechanics 1978)

Seepage
Rating Description, Unfilled Discontinuities Description, Filled Discontinuities

I Discontinuity very tight and dry, water Filling materials heavily consolidated* and dry;
flow along it does not appear possible significant flow appears unlikely due to very low
permeability

II Discontinuity is dry with no evidence Filling materials damp, but no free water present
of water flow

III Discontinuity is dry but shows evidence Filling materials are wet, occasional drops of
of water flow, e.g., staining water

IV Discontinuity is damp but no free water Filling materials show signs of washout; continuous
is present flow pf water (estimate discharge)

V Discontinuity shows seepage; occasional The filling materials are washed out locally; con­
drops of water, but no continuous flow siderable water flow along washout channels (es­
timate discharge and describe pressure, i.e.,
low, medium, high)

VI Discontinuity shows continuous flow of Filling materials washed out completely; high water
water (estimate discharge and describe pressures (estimate discharge and describe pres­
pressure, i.e., low, medium, high) sure, i.e., low, medium, high)

* Presumably, "consolidated" implies that low void ratio has been achieved.