U.S. GEOLOGICAL SURVEY CIRCULAR 954 

Core Index, A Numerical 

Core-Logging Procedure for 

Estimating Rock Quality 

Core Index, A Numerical 

Core-Logging Procedure for 

Estimating Rock Quality 

By John R. Ege 

U.S.GEOLOGICAL SURVEY CIRCULAR 954 

Procedures to be followed for rapid 

characterization of rock quality 

1987 
Department of the Interior 

DONALD PAUL HODEL, Secretary 

U.S. Geological Survey 

Dallas L. Peck, Director 

Free on application to the Branch of Distribution, Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225 
CONTENTS 

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Engineering rock-classification systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Case histories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Comparison of the core-index and Rock Quality Designation systems . . . . . . . . . . . . . . . . 13 

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

ILLUSTRATIONS 

Page 

FIGURE 1. Sketch of the three logging categories used for calculating the core-index number . . . . . . . . . . . . . . 2 

2. Sketch outlining method for accounting for effects of discontinuities in intact core . . . . . . . . . . . . . 4 

3. Photograph of oil shale core assigned a core-index value of 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

4. Photograph of oil shale core assigned a core-index value of 94 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

5. Graph of relative rock hardness and degree of weathering ranked against joint frequency for 732 m 

of granitic core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

6-8. Graphs showing: 

6. Porosity, dry-bulk density, laboratory compressional velocity, and laboratory shear velocity from 

granitic cores ranked against core-index numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

7. Static Young's modulus, static shear modulus, static bulk modulus, and unconfined compressive 

strength from granitic cores ranked against core-index numbers . . . . . . . . . . . . . . . . . . . . . . . 9 

8. Dynamic Poisson's ratio, dynamic Young's modulus, dynamic shear modulus, and dynamic bulk 

modulus from granitic cores ranked against core-index numbers . . . . . . . . . . . . . . . . . . . . . . . 10 

9. Profile of an exploratory borehole showing intervals of the borehole classified according to core-index 

values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

10-12. Graphs showing: 

10. Core-index values from two adjacent vertical exploratory borings in oil shale, Piceance Creek Basin, 

Colorado . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

11. Comparison between a synthetic geologic plan constructed from horizontal borehole data using 

lithologic and core-index logging and a geologic plan mapped in a tunnel driven parallel to the 

exploratory boring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

12. Relation between Rock Quality Designation and discontinuity frequency . . . . . . . . . . . . . . . . . . . 14 

Ill 

Core Index, a Numerical Core-Logging Procedure 

for Estimating Rock Quality 

By John R. Ege 

ABSTRACT 

The core index, a numerical core-logging procedure used by 

the U.S. Geological Survey since 1968 in engineering-geology 

projects at the Nevada Test Site and other locales, provides 

an objective means for estimating rock quality. The logging 

concept is based on the observation that rock-mass strength 

is reduced by discontinuities cutting the rock. The technique 

utilizes a combined discontinuity (joint, fault, bedding plane, 

contact, foliation, banding) frequency and core-loss measurement to calculate the core-index number. The core-index number 

ranks the rock from continuous to discontinuous (solid to 

fragmented) on a scale of 0 to 100, the higher values indicating 

an increasing degree of fragmentation. The basic core-logging 

unit is a 10-cm (4-in.) length of core bounded by natural discontinuities. Three categories are used in logging core: lost core, 

core lost from the core barrel during drilling and assumed to 

be highly fragmented core less than 10 em (4 in.) in length; 

broken core, recovered core that is fragmented by natural 

discontinuities into pieces 10 em (4 in.) or less in length; and 

intact core, recovered core greater than 10 em (4 in.) in length. 

Intact core cut by discontinuities is related to the basic 10-cm 

(4-in.) core unit by multiplying the number of discontinuities 

in a logged interval by 0.1 m for metric units or 0.3 ft (4 in./12 

in.) for English units. 

The core-index (Cl) number is calculated from: 

Core loss (m) + Broken core (m) 

CI= + Number of discontinuities X 0.1 (m) 

Drilled hole interval (m) 

Core loss (ft) + Broken core (ft) 

CI= + Number of discontinuities X 0.3 (ft) 

Drilled hole interval (ft) 

X 100 

X 100 

Results of a study ranking physical property measurements 

of unconfined compressive strength, Young's modulus, shear 

modulus, bulk modulus, and compressional and shear velocities 

made on granitic cores against CI numbers showed decreasing 

values in the physical property measurements with increasing 

values of cr. 

INTRODUCTION 

Core index, a numerical core-logging procedure 

first developed at the Nevada Test Site (Nye 

County, Nev.) between 1965 and 1967 and since 

modified (Ege, 1968), has been in continuous use 

by the U.S. Geological Survey (USGS) on 

1 

numerous engineering-geology projects. The corelogging procedure evolved from exploratory programs conducted by the USGS at the Nevada Test 

Site (NTS) that were aimed at assessing geologic 

conditions at sites of proposed mined and bored 

underground openings. The need to establish a 

relatively simple, objective, and precise means of 

describing lithologic and structural site conditions 

that could be used by engineers for site evaluation, 

design, and construction became very apparent 

during the early phases of the exploration 

program. 

The core index, a modified discontinuityfrequency percentage, was first applied to granitic 

cores taken from the Climax Stock at the NTS and 

later applied to volcanic and sedimentary rocks at 

various NTS sites. The logging procedure is based 

on the observation that rock-mass strength is 

reduced by discontinuities cutting the rock and 

that generally the greater the number of discontinuities per volume of rock mass, the less competent the rock is as an engineering material. 

Goodman (1976, p. 40) described a single discontinuity as comprising two mating surfaces and a 

space or filling. Obert and Duvall (1967, p. 310-

312) stated that a discontinuity is a plane of 

weakness in the rock which, under applied stress, 

has the potential of failing before failure occurs in 

the intact rock. Examples of geologic discontinuities are joints, fractures, faults, foliation, 

schistosity, banding, bedding contacts, and 

cleavage. 

The CI ranks the rock from 0 "excellent" to 100 

"very poor," where 0 signifies continuous sound 

core and 100 totally fragmented core over the 

drilled interval. The CI is, therefore, a measure of 

discontinuities, increasing in value as the number 

of discontinuities increases over the drilled interval. The logging procedure has removed some of 

the subjective structural descriptions of cores 

made by individuals examining core from borings 

and has established a stm;tdard by which an estimate of rock quality can be made. The procedure 

has been applied to a variety of rock types at the 
NTS and other locales, and an extensive body of 

core information has been gathered by USGS 

personnel. 

ENGINEERING ROCK-CLASSIFICATION 

SYSTEMS 

A number of workers have established systems 

that attempt to standardize and quantify descriptions of rock material and rock masses ('Thrzaghi, 

1946; Deere, 1963; Coates, 1964; Deere and others, 

1967; Barton and others, 1974; Bieniawski, 1974). 

Information used by these systems to rank the 

rock include number, spacing and orientation of 

discontinuities,. roughness of discontinuity surfaces, type of material filling discontinuities, 

degree of alteration of rock, groundwater conditions, depth of rock mass, and strength of rock. 

These systems range from those that require many 

parameters and provide a detailed engineering 

analysis of rock conditions to those that examine 

a few core parameters and provide a rapid estimate 

of rock quality. The former are elaborate, informative design tools, requiring considerable effort 

and time. For example, Barton and others (197 4) 

and Bieniawski ( 197 4) utilize numerous geologic, 

hydrologic, and rock parameters that provide 

detailed information for design and construction 

of underground openings such as tunnels. The latter are fast, simple exploratory tools, such as the 

Rock Quality Designation (RQD) (Deere, 1963; 

Deere and others, 1967) which uses a single 

measure of discontinuity spacing in determining 

estimates of rock quality. 

PROCEDURE 

Conceptually, the CI logging technique rests on 

a combined measurement of discontinuities, core 

loss, and broken core (fig. 1 ). A core length of 10 em 

(4 in.) separated by natural discontinuities is the 

basic core unit; all measurements are based on this 

unit length of core. Intact core refers to core pieces 

whose lengths are greater than 10 em (4 in.). Core 

loss is defined as rock so fragmented by natural 

discontinuities or crushing that its pieces cannot 

be retained in the core barrel. Broken core is determined by measuring all increments of core separated by natural discontinuities into pieces less 

than 10 em ( 4 in.) in size. 

First, the recovered core is laid out in a core tray 

or in core boxes, and the length of borehole to be 

logged is determined. The selected hole length can 

be some convenient unit, for example, 10m (30ft), 

w 

a: 

0 

u 

1-

(/J 

0 

...J 

FIGURE 1.-The three logging categories used for calculating 

the core-index number. Lost core, core not recovered from drilling. Broken:core, core fragmented by natural discontinuities 

into pieces 10 em (4 in.) or less in length. Intact core, core 

recovered in lengths greater than 10 em (4 in.). 

or perhaps the length of the core barrel. Second, 

the total amount of recovered core is measured and 

recorded, subtracted from the drilled hole length, 

and noted as core loss in terms of meters (feet). In 

some instances core loss may be caused by rock 

grinding during drilling and may not be the result 

of natural processes. The core logger should be 

aware of this possibility and correct measurements 

of core loss related to drilling procedures. Third, the 

total increments of broken core are recorded as 

length of broken core in meters (feet). Again, care 
must be taken to include only core fragments cut 

by natural discontinuities and not fractures 

created by drilling or handling. Fourth, a count of 

discontinuities cutting and separating the intact 

core into discrete pieces is recorded. 

Natural discontinuities are potential planes of 

weakness and are accounted for in the CI system. 

The lengths of the intact core pieces formed by 

discontinuities are referenced to the basic 10-cm 

(4-in.) logging unit. Thus, if 1 m of core is cut 

perpendicularly by 10 equally spaced discontinuities (excluding the butt end), the core will be 

divided into ten 10-cm core lengths. This configuration would then be equivalent to 1 m of 

broken core. If 1 m of core is cut by four discontinuities, then a maximum of 0.4 m of 10-cm pieces 

could exist, assuming a 1 0-cm discontinuity spacing in sequence. This then would be comparable 

to 0.4 m of broken core. Multiplying the number 

of discontinuities counted in a specified length of 

intact core by 0.1 m is a means of including the 

effect of all discontinuities, regardless of orientation, by the logging procedure (fig. 2). For the 

unique case in which a discontinuity cuts a piece 

of core parallel to its axis, the discontinuity is 

counted as a single discontinuity in the calculation. 

Similarly, when using the English system of 

measurements, 1ft of intact core is cut into 4-in. 

pieces by three equally spaced discontinuities. 

Multiplying the number of discontinuities contained in a specified length of core by 0.3 ft (4 in./ 

12 in. X 1 ft) will again arbitrarily equate the 

intact core discontinuity count to a comparable 

length of broken core (fig. 2). 

The following examples illustrate the use of the 

core-logging procedure and calculation of the coreindex value, which is the designated value relating 

the structural properties of the core to rock quality. 

Rock type: Granite 

Total length of borehole: 40 m 

Specified borehole logging unit: 10m 

Hole 

interval Core loss Broken core Discontinuity 

(m) (m) (m) count 

0-10 2.0 1.7 

10-20 1.6 .9 

20-30 1.1 .7 

30-40 .7 .3 

Core index (CI) (in percent) 

Core loss (m) + Broken core (m) 

+ Number of discontinuities 

22 

16 

12 

4 

CI = _X_0._1---'-(m_____:_) ----,------------ X 100 

Drilled hole interval (m) 

3 

Hole 

interval 

(m) 

2.0 m + 1.7 m 

CI 

0-10 CI = __ +_2-2-X_0_.1_m __ X 100 = 59 

10m 

1.6 m + 0.9 m 

10-20 CI = __ +_1-6-X_0_.1_m_ X 100 = 41 

10m 

1.1 m + 0.7 m 

+ 12 X 0.1 m 20-30 CI =-------X 100 = 30 

10m 

0.7 m + 0.3 m 

+ 4 X 0.1 m 30-40 CI = ------- X 100 = 14 

10m 

Rock type: Ash-fall tuff 

Total length of borehole: 160 ft 

Specified borehole logging unit: 20 ft 

Hole 

interval Core loss Broken core Discontinuity 

(ft) (ft) (ft) 

0-20 3.0 1.0 

20-40 2.7 .8 

40-60 .9 .4 

60-80 .9 .3 

80-100 1.3 .7 

100-120 3.3 2.1 

120-140 2.1 1.6 

140.,-160 .8 .7 

Core index (CI) (in percent) 

Core loss (ft) + Broken core (ft) 

+ Number of discontinuities 

count 

4 

6 

3 

1 

2 

10 

8 

3 

CI = _X_0_._3_:_(f--'-t) ________ X 100 

Drilled hole interval (ft) 

Hole 

interval 

(ft) 

3.0 ft + 1.0 ft 

CI 

0-20 CI = __ +_4_X_0_.3_f_t- X 100 = 26 

20ft 

2.7 ft + 0.8 ft 

+ 6 X 0.3 ft 20-40 CI = ------- X 100 = 26 

20ft 
0 1 0 Centimeters 

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FIGURE 2.-Method for accounting for effects of discontinuities in intact core. The basic core unit, broken core, used in calculating the core-index number is a 10-cm 

(4-in.) core length bounded by natural discontinuities. To account for the effects of discontinuities separating intact core (core lengths greater than 10 em (4 

in.)) and relate the intact core to broken core, multiply the number of discontinuities cutting the intact core by 0.1 m (0.3 ft). 
Hole DISCUSSION interval 

(m) CI The examples of CI calculations presented in the 

previous section illustrate that increasing values 

0.9 ft 1- 0.4 ft of CI have a corresponding increase in discon40-60 CI = 

1- 3 X 0.3 ft X 100 = 11 

tinuites contained in the core. On a scale of 0-100, 

20ft the higher CI values designate a very fragmented 

rock and, therefore, can be related to a less com0.9 ft 1- 0.3 ft petent engineering material (figs. 3 and 4). In fact, 

60-80 CI = 

1- 1 X 0.3 ft X 100 = 8 observations based on numerous site-evaluation 

20ft projects made by the author and others have produced a rule of thumb which states that rock hav1.3 ft 1- 0.7 ft ing CI values in general greater than 80 and in 

80-100 CI = 

1- 2 X 0.3 ft X 100 = 13 many cases greater than 50 has proven to be a dif20ft ficult construction medium, and the rock so classified has been designated as poor or problem rock. 

3.3 ft 1- 2.1 ft Estimates of rock quality determined from ranges 

100-120 CI = 1- 10 X 0.3 ft X 100 = 42 of CI values are shown below. These designations 

20ft are based on field observations. 

2.1 ft 1- 1.6 ft 

120-140 CI = 1- 8 X 0.3 ft Core index Rock quality 

X 100 = 31 20ft 0-10 Excellent 

10-30 Good 

0.8 ft 1- 0.7 ft 30-50 Fair 

140-160 CI = 1- 3 X 0.3 ft 50-80 Poor 

X 100 = 12 80-100 Very poor 20ft 

FIGURE 3.-Example of oil shale core assigned a core-index value of 6 over borehole logging interval of 15 m (50 ft). 

5 
FIGURE 4.-Example of oil shale core assigned a core-index value of 94 over borehole logging interval of 15 m (50 ft). 

Examples of rock types on which the CI procedure 

has been applied by USGS investigators include 

granite, ash-fall and ash-flow tuff, rhyolite and 

basalt, argillite and shale, limestone and dolomite, 

sandstone, quartzite, granite gneiss, and amphibolite schist. 

In 1968, in a study conducted at the NTS, I 

qualitatively examined 732 m (2,400 ft) of granitic 

core for degree of weathering and relative hardness 

with respect to number of joints (the dominant 

discontinuity) contained in core recovered over 

15-m (50-ft) lengths of bored hole. I ranked the 

properties of degree of weathering and relative 

hardness against joint frequency (joint per meter 

or foot) measured in the core over the 15-m or 50-

ft intervals (fig. 5). The results of the comparison 

showed a decrease in hardness and an increase in 

weathering with concurrent increase in joint frequency. An observ~!_ble change in both the relative 

hardness and degree of weathering was apparent 

at a frequency of 10 joints per meter (three joints 

per foot) or at an approximate joint spacing of 

10 em (4 in.). It was on the basis of this information obtained from the granitic cores that the 

10-cm (4-in.) core length was selected as the basic 

core unit used for the logging procedure and, 

subsequently, applied to other rock types. 

All geologic discontinuities have the capability 

of separating or failing along their respective 

planes and, therefore, are sources of weakness. Discontinuities will vary, however, with rock type, 

and the dominant forms of discontinuities contained in a particular rock mass depend on its 

properties, history, and geometry. Granite, a brittle igneous rock, can split along joint planes; shale, 

a low modulus sedimentary rock, can separate 

along bedding planes; and basalt, a brittle volcanic 

rock, can fail along both flow banding and joint 

planes. Judgment must be used when examining 

core, and discontinuities in all their forms should 

be considered and recorded in terms of their effects 

on the strength of the rock mass. 

PHYSICAL PROPERTIES 

Granitic cores were selected from intervals of 

core taken from the Climax Stock, NTS, ranked 

according to CI values, and submitted to the 

USGS laboratory in Denver, Colo., for physical 

property tests. The three CI groupings chosen 

ranged from good to poor rock, or CI = 0-30 percent, 31-60 percent, and 61-100 percent. Tests 

were conducted on 31-42 cores for each of the 

following 12 properties: porosity, drybulk density, 

static Young's modulus, static shear modulus, 

static bulk modulus, unconfined compressive 

strength, compressional velocity, shear velocity, 

dynamic Poisson's ratio, dynamic Young's modulus, dynamic shear modulus, and dynamic bulk 

modulus. The averages and ranges of the results, 

plotted for each test (figs. 6-8), show a decrease 

in property value as CI values increase with the 

exception of porosity; porosity becomes greater 

with increasing CI. 

The physical property test results made on the 

granitic cores indicate that in addition to a reduction in structural continuity of the rock mass, 

there is an accompanying lowering of laboratory 

physical property values. For example, the means 

of unconfined strength values decreased about 50 

percent from 206.8 MPa (30,000 lbf/in2) in the 0-

49 percent group to 103.4 MPa (15,000 lbf/in2) in 

the 80-100 percent group, and the means of compressional velocity decreased about 13 percent 

(5,608-4,877 m/s or 18,400-16,000 ft/s) within the 

same CI categories for the granitic rock samples. 

Thus the CI system may have application in 
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FIGURE 5.-Relative rock hardness and degree of weathering ranked against joint frequency (joints per meter) for 732 m (2,400 ft) of granitic core. A, 

Relative rock hardness was estimated by an iron-nail scratch test. B, Degree of weathering was estimated from visual examination of the amount 

of feldspar alteration in the granite. (Modified from Ege, 1968.) 
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CORE INDEX, IN PERCENT 

FIGURE 6.-Porosity (A), dry-bulk density (B), laboratory compressional velocity (C), and laboratory shear velocity (D) from 42 samples of granitic cores ranked against core-index numbers. Minimum, maximum, and 

average of physical properties plotted for the following core-index groups: 0--30, 31-60, 61-100. 

8 
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CORE INDEX. IN PERCENT 

FIGURE 7.-Static Young's modulus (A), static shear modulus (B), static bulk modulus (C), and unconfined 

compressive strength (D) from granitic cores ranked against core-index numbers. Minimum, maximum, 

and average of physical properties plotted for the following core-index groups: 0-31, 30-60, 61-100. 
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u 

~ <( 

z 

>-

c 

0 

c 

0-30 31·60 61·1 00 

CORE INDEX, IN PERCENT 

l I 

0-30 31·60 61-100 

CORE INDEX. IN PERCENT 

1-

0 

0 

LL. 

w 

a: 

<( 

:::> 

0 

(/) 

a: 

w 

&:L 

(/) 

Cl 

z 

:::> 

0 

&:L 

0 

X 

~ 

~ 7 ....1 

:::> 

c 

0 

~ 

f/) 

CJ 5 z 

:::> 

0 

>-

0 

~ <( 

z 

>- c 2L-------~-------L------~------~ 

B 0·30 31·60 61·1 00 

:I: 

u 

~ 8.0 

a: 

w 

0.. 

(/) 

~6.0 :::> 

0 

0.. 

"' 0 

.... 5.0-

X 

~ (/). 

:::> 

54.0 

Cl 

0 

~ 

CORE INDEX, IN PERCENT 

1 ·0 L_ _____ 0_-..1_3_0 ______ 3_1 -.L6_0 _____ 6_1_1.-1_0_0 ____ _ 

D CORE INDEX. IN PERCENT 

FIGURE 8.-Dynamic Poisson's ratio (A), dynamic Young's modulus (B), dynamic shear modulus (C), and 

dynamic bulk modulus (D) from granitic cores ranked against core-index numbers. Minimum, maximum, 

and average of physical properties plotted for the following core-index groups: 0-30, 31-60, 61-100. 

10 
estimating changes in granitic rock properties (and 

possibly those of other lithologies) resulting from 

the effects of discontinuites. 

CASE HISTORIES 

The CI method was used to evaluate a construction site at the NTS in the Climax Stock, a jointed 

granodiorite-porphyritic quartz monzonite intrusive 

body. Desigri criteria for a 21-m (70-ft) diameter, 

hemispherical-shaped chamber constructed in 

granitic rock required that the structure be mined 

in competent high-strength rock at a depth of at 

least 91 m (300ft) (Ege, 1983). The limiting factor 

for siting the chamber was to find as continuous 

a rock mass as possible at the prescribed depth. The 

site-appraisal program, in addition to geologic mapping, included three exploratory boreholes inclined 

in such a way that the borings would pass through 

the proposed hemisphere at three different levels. 

The recovered cores from the three borings were 

structurally analyzed using the CI method and a 

location chosen that met design requirements. A 

profile of one of the exploratory holes (fig. 9) shows 

the plot of CI numbers along the alignment of the 

boring and the recommended location for the proposed chamber. The site was selected in a zone of 

low CI numbers (ll<CI<36) which designated a 

volume of rock containing relatively few discontinuities. Construction of the hemispherical 

chamber at the selected location was completed successfully in a region of structurally competent rock 

and confirmed the prediction of suitable rock for 

mining the uniquely shaped opening. 

The CI method was also used in determining a 

site for a mine in northwestern Colorado. Specifications developed by the U.S. Bureau of Mines for 

an experimental shaft and mine for testing mining techniques in the oil shale of the Piceance Creek 

Basin of northwestern Colorado called for a 

carefully designed geologic and hydrologic evaluation of several potential sites (Ege and others, 

1978). An acceptable site that would be typical of 

conditions expected to be encountered by commercial mining of oil shale had to include the following stratigraphic and structural sequences: (1) an 

overburden thickness of 245-365 m (800-1,000 ft) 

to the top of the Mahogany zone, an upper oilshalebearing unit; (2) a thickness of about 120 m (400 ft) 

of the leached saline zone, an evaporite unit thickening toward the center of the basin which has 

ALTITUDE ALTITUDE 

IN METERS IN FEET 

1550 .-----------------------------------------------------------------------,5100 

1500 

1450 

1400 0 

0 

20 40 

Drill 

Rig 

60 METERS 

50 1 00 1 50 200 FEET 

FIGURE 9.-Exploratory borehole showing intervals classified according to core-index values (numbered). Boring penetrates 

granitic rock of the Climax Stock, Nevada Test Site, in which location of a large hemispheric chamber was determined, 

in part, from the core analysis. Dashed lines bound possible fault zone. 

11 
undergone dissolution by ground water and represents a structurally weak zone lying below the 

Mahogany zone; (3) a continuous oil-shale zone 

below the leached zone at least 150-300 m (500-

1,000 ft) thick and averaging 70 L/t (20 gal/short 

ton) of in-place shale oil; and (4) no areas containing major fault or fracture systems. TWo 

exploratory cores, taken from boreholes (C-171 and 

C-34) drilled in one of the potential mine sites, were 

logged by the USGS using the CI procedure. The 

Surface altitude= 1,937m (6,356 ft) 

ALTITUDE HOLE DEPTH 

METERS FEET METERS FEET 

A 

results of the core logging, presented in figure 10 

using the local geophysical "Black" marker as the 

datum level, document the areally discontinuous 

and vuggy nature of the evaporite, caused by 

dissolution and resulting in high CI values 

(>50 percent). TWo other structural intervals 

appear on both core logs, a fractured zone lying 

immediately above the leached evaporites and an 

interval of core near the bottom of the hole that 

readily separates into thin disks. 

ALTITUDE 

B 

Surface altitude= 1,830m (6,003 ft) 

500 

A groove 

Q) 

c 

0 

N 

-o 

Q) 

.r; 

u 

"' Ql 

_J 

~- -~--~;.1 __ -l'F= 

Orange marker 

FIGURE 10.- Core-index values from two adjacent vertical exploratory borings (A and B) in oil shale, Piceance Creek Basin, 

Colorado. The high core-index values locate the pervasive saline "leached zone" above a lower rich oil-shale zone. The low 

core-index values below the saline zone indicate a continuous interval of intact rock. See figures 3 and 4. Disked core at 

the bottom of boring represents core separating into 1- to 10-cm-wide fragments (disks) at right angles to the core axes. 

12 
In constructing a 609-m (2,000-ft) long tunnel 

complex in zeolitized ash-fall tuff 396m (1,300 ft) 

below Rainier Mesa at NTS, the CI method was 

also used. Prior to construction, a 472-m (1,400-

ft) horizontal borehole was drilled parallel to the 

proposed tunnel alignment to forecast rock properties and construction conditions, a standard procedure for tunnel design at the NTS (Fairer and 

others, 1979). The recovered core from the 

exploratory boring was logged for lithology and 

CI numbers. A synthetic geologic plan map and 

cross section were constructed along the proposed 

tunnel axis utilizing the core data and known 

geology of Rainier Mesa. The geologic interpretations were incorporated into the overall tunnel 

planning. During mining of the tunnel complex, 

geologists mapped the geology penetrated by the 

tunnels and constructed "as-built" maps and cross 

sections. Comparison of the predicted geology as 

interpreted from exploratory cores and the 

geology mapped along the tunnel alignment is 

A BOREHOLE 

BOREHOLE~ 

0 

ENDOF ~ 

/0 """' B 

shown in plan views (fig. 11). Locations of faults 

were based on both CI numbers and careful 

examination of core. In this example, faults cutting low-strength zeolitized ash-fall tuff exhibited 

CI values greater than 25 percent. The designation of rock units was determined from lithologic 

characteristics observed in the cores. 

COMPARISON OF THE CORE-INDEX AND 

ROCK QUALITY DESIGNATION SYSTEMS 

The CI and RQD techniques are similar in that 

both are relatively uncomplicated and fast procedures that estimate rock quality by discontinuity counts or spacing. The widely used RQD 

core-logging procedure (Deere, 1963; Deere and 

others, 1967) is a modified core-recovery method 

in which all pieces of sound core greater than 10 

em (4 in.) are counted as recovery. The sum of the 

lengths of the sound core are divided by the length 

NO 

CORE 

COLlAR OF 

BORE HOLE 

100 ,--L--~----~--~----L_ __ _L __ ~----~--_l----~~=~~~L_--~=-~-----.100 

50 ~--------~----------------------------~~-=~~+- +r~ 

J_~--=4~--~~~--~~==~~==~--~~~_2~~~~~~~ 

hd+-4---++-----t 50 

1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 

0 

0 FEET 

c 400 300 

ROCK UNIT CONTACT- Y Letter designates 

stratigraphic unit 

200 

EXPlANATION 

FAULT- Arrow designates 

dip of fault, bar and 

ba ll shows direction 

of displacement 

100 0 METERS 

DRIFT OUTLINE 

FIGURE H.-Comparison between a synthetic geologic plan (B) constructed from horizontal borehole data (C) using lithologic 

and core-index logging and the geologic plan (A) mapped in a tunnel driven parallel to the exploratory boring. The· tunnel 

was mined in Tertiary ash-fall tuff below Rainier Mesa, Nevada Test Site. (Modified from Fairer and others, 1979.) 

13 
of the drilled interval and a percent of recovered 

sound core is calculated. This recovery percentage 

is designated as the RQD and ranges from 0 for 

very poor rock to 100 for excellent rock. 

Rock fragments less than 10 em in the RQD procedure are considered to have been formed by 

jointing, faulting, close shearing, and weathering 

within the rock mass, and they are not included 

in the recovered-core measurements. Deleterious 

factors affecting rock quality, for example, 

weathered and alterated incompetent zones and 

closely spaced fractures, are, therefore, accounted 

for by not recording the smaller pieces. In this 

manner, a general estimate of engineering 

behavior of the rock mass is made. 

One difference between the CI and RQD 

methods is that CI defines the form of fragmented 

rock in terms of lost core and broken pieces which 

may aid in interpreting the style of "weak" zones. 

For example, closely spaced fractured rock may 

appear as broken core, whereas soft highly weathered or altered rock may be washed from the core 

barrel and appear as lost core. 

Another difference between the CI and RQD 

techniques is that the CI procedure, by recording 

the number of discontinuities in the greater-than1 0-cm sound core, provides information on the 

general occurrence of discontinuities as sources of 

planes of weakness throughout the rock mass. 

This can be an important consideration when 

working with very large underground openings 

where discontinuity spacing greater than 10 em, 

the RQD threshold value, may have to be considered as a stability factor. 

Priest and Hudson (1976), from analysis of 

field-obtained scanline discontinuity data, indicated that a negative exponential distribution 

of discontinuity values is a good approximation 

to the true distribution patterns. The authors 

using the negative exponential distribution of 

discontinuity spacing values established a relation 

between a theoretical RQD and the average 

number of discontinuities per meter (fig. 12): 

RQD = 100e-0·

1x (O.IA + 1) 

where 0.1 is the RQD threshold value of 0.1 and 

'A is the average number of discontinuities per 

meter. 

A result shown by the above equation and by 

the upper projection of the linear approximation 

of RQD with a conventional threshold value of 

0.1 m (fig. 12) is that RQD is insensitive to a mean 

discontinuity spacing greater than 0.3 m ('A = 3). 

Kulhawy (1978) discussed similar arguments. Core 

index, as previously mentioned, takes into account 

all discontinuities intersected along a borehole or 

scanline and, therefore, has sensitivity to a wider 

range of discontinuity spacing. 

f- Linear approximation ROD*= -3.68 >.. +11 0.4 for 6< >.. <16 

z 90 w 

u 

~ 80 a.. 

~ 70 

z 

0 

i= 60 

<l: 

z 

(.!)50 

Inflection point P 

Deviation due to 

clustered discontinuities 

Deviation due 

to evenly spaced 

discontinuities 

Theoretical curve 

RQD*=1 OOe -o.ll'(0.1 >.. +1) 

8 1 0 1 2 1 4 1 6 1 8 20 22 24 26 28 30 32 34 36 38 40 

AVERAGE NUMBER OF DISCONTINU ITIES 

PER METER.(>..) 

FIGURE 12.-Relation between Rock Quality Designation (RQD) and discontinuity frequency (Priest and Hudson, 1976). 

14 
CONCLUSIONS 

Core index (CI), a core logging system based upon 

frequency of natural discontinuities segmenting the 

core, has been in use by the USGS since 1968 as 

an evaluation tool for determining structural conditions related to engineered structures built in 

rock. The CI number, a quantity calculated from 

the logging procedure, provides an objective means 

for estimating rock quality in terms of its engineering properties and projecting geologic structures 

that may be encountered at the construction site. 

Correlation of rock-engineering conditions areally 

is possible between nearby exploratory boreholes 

and has been successfully used by the USGS at the 

NTS and other locales since inception of the 

technique. 

The CI number is calculated from: 

Core loss (m) + Broken core (m) · 

+ Number of discontinuities 

CI = _x_o_.1----'('--m--'--) ________ x 100 

Drilled hole interval (m) 

Core loss (ft) + Broken core (ft) 

+ Number of discontinuities 

CI = _X_0_._3 ....:.._(ft_:_) _______ X 100 

Drilled hole interval (ft) 

A significant difference between the CI and RQD 

procedures is the inclusion of discontinuities as a 

separate parameter in calculating the CI number. 

This accounting of discontinuities makes the CI 

technique more sensitive to variations in rock 

quality, as compared to RQD, for average discontinuity spacings greater than 0.3 m based on the 

RQD threshold value of 10 em (fig. 12) (Priest and 

Hudson, 1976). 

Results of a study ranking physical property 

measurements of unconfined compressive strength, 

Young's modulus, shear modulus, bulk modulus, 

and compressional and shear velocities made on 

granitic cores against CI numbers showed decreasing values in these physical property measurements 

with increasing values of CI. Porosity 

measurements, as would be expected, showed 

increasing values with increasing values of CI. 

REFERENCES CITED 

Barton, N., Lien, F., and Lunde, J., 1974, Engineering classification of rock masses for the design of tunnel support: Rock 

Mechanics, v. 6, no. 4, p. 189-236. 

Bieniawski, Z. T., 1974, Geomechanics classification of rock 

masses and its application in tunneling: International 

Society for Rock Mechanics, 3d, Denver, Colorado, 

September 1974, Proceedings, v. 2A, p. 27-32. 

Coates, D. F., 1964, Classification of rocks for rock mechanics: 

International Journal of Rock Mechanics and Mining 

Sciences, v. 1, no. 3, p. 421-429. 

Deere, D. U., 1963, Technical description of rock cores for 

engineering purposes: Rock Mechanics and Engineering 

Geology, v. 1, no. 1, p. 16-22. 

Deere, D. U., Hendron, A. J., Jr., Patton, F. D., and Cording, 

E. J., 1967, Design of surface and near-surface construction in rock, in Fairhurst, Charles, ed., Failure and breakage 

of rock: Symposium on Rock Mechanics, 8th, Minneapolis, 

Minnesota, September 1966, Proceedings: New York, 

American Institute of Mining, Metallurgy and Petroleum 

Engineers, p. 237-302. 

Ege, J. R., 1968, Stability index for underground structures in 

granitic rock: Geological Society of America Memoir 110, 

p. 185-197. 

__ 1983, Use· of joint planes in constructing a large 

hemispheric-shaped chamber in granitic rock: Association 

of Engineering Geology Bulletin, v. 20, p. 333-338. 

Ege, J. R., Leavesely, G. H., Steele, S. G., and Weeks, J. B.,1978, 

Site evaluation for U.S. Bureau of Mines experimental oil- . 

shale mine, Piceance Creek Basin, Rio Blanco County, Colorado: U.S. Geological Survey Open-File Report 78-390, 42 p. 

Fairer, G. M., Townsend, D. R., Carroll, R. D., Cunningham, 

M. J., Muller, D. C., Healey, D. L., and Ellis, W. L., 1979, 

USGS investigations in connection with the Mighty Epic 

event, U12n.10 tunnel, Nevada Test Site: U.S. Geological 

Survey Report USGS-474-228, 189 p.; available only from 

U.S. Department of Commerce National Technical Information Service, Springfield, VA 22161. 

Goodman, R. E., 1976, Methods of geological engineering: St. 

Paul, West Publishing Company, 472 p. 

Kulhawy, F. H., 1978, Geomechanical model for rock foundation 

settlement: American Society of Civil Engineers Proceedings, Journal of the Geotechnical Engineering Division, 

v. 104, no. GT2, p. 211-227. 

Obert, Leonard, and Duvall, W. I., 1967, Rock mechanics and the 

design of structures in rock: New York, John Wiley, 650 p. 

Priest, S.D., and Hudson, J. A., 1976, Discontinuity spacings 

in rock: International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, v. 13, no. 5, p. 

135-148. 

Terzaghi, K. C., 1946, Introduction to tunnel geology; Rock 

defects and loads on tunnel supports, in Proctor, R. V., and 

White, T. L., Rock tunneling with steel supports: The Commercial Shearing & Stamping Co., Youngstown, Ohio, p. 

15-99. 

15