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GEOLOGIC AND INDEX PROPERTIES OF ROCK TABLE 3
ROCK GROUPS AND TYPES
The most basic characterization of rock for engineering Igneous
purposes is a description of rock core based on visual and Intrusive Extrusive
physical examination. The International Society of Rock (coarse-grained) (fine-grained) Pyroclastic
Granite Rhyolite Obsidian
Mechanics (ISRM) proposed a standardized method for Syenite Trachyte Pumice
descriptions of rock masses from mapping and core logging Diorite Andesite Tuff
("Basic Geotechnical Description of Rock Masses" 1981). A Diabase Basalt
Gabbro
summary of the ISRM method as given by Wyllie (1999) is Peridotite
adopted in the FHWA manuals on subsurface investigations Pegmatite
and soil and rock properties (Mayne et al. 2001; Sabatini Sedimentary
et al. 2002) and is summarized here. Clastic (sediment) (chemically formed) (organic remains)
Shale Limestone Chalk
Mudstone Dolomite Coquina
A rock mass is described in terms of five categories of Claystone Gypsum Lignite
properties, as follows: Siltstone Halite Coal
Conglomerate
Limestone, oolitic
1. Rock Material Description--a. Rock type, b. Wall Metamorphic
strength, c. Weathering Foliated Nonfoliated
2. Discontinuity Description--d. Type, e. Orientation, f. Slate Quartzite
Roughness, g. Aperture Phyllite Amphibolite
Schist Marble
3. Infilling--h. Infilling type and width Gneiss Hornfels
4. Rock Mass Description--i. Spacing, j. Persistence, k.
Number of sets, l. Block size/shape
5. Groundwater--m. Seepage.
rock types listed in Table 3 based on lithologic characteristics
that include color, fabric (microstructural and textural fea-
Each of the 13 parameters listed (a through m) is assigned
tures), grain size and shape (Tables 4 and 5), and mineralogy.
a description using standardized terminology. Descriptive
Sedimentary rock descriptions should include bedding thick-
terms are given in Tables 3 through 6 and in Figure 11, which
ness (Table 6). The rock unit name, which may be a formal
is an example of a Key used for entering rock descriptions on
name of a formation or an informal local name, should be
a coring log and includes details of several categories.
identified; for example, Bearpaw Shale or Sherman Granite.
Rock Material Descriptors Compressive strength of rock core can be evaluated us-
ing simple field tests with equipment commonly available
Rock type is defined in terms of origin (igneous, sedimentary, (knife, rock hammer, etc.) and summarized in the Key of
or metamorphic) and then further classified into one of the Figure 11 ("Rock Strength") or evaluated from point load
TABLE 4
TERMS TO DESCRIBE GRAIN SIZE OF SEDIMENTARY ROCK
Description Diameter (mm) Characteristic
Very coarse grained >4.75 Grain sizes are greater than popcorn kernels
Coarse grained 2.004.75 Individual grains can be easily distinguished by eye
Medium grained 0.4252.00 Individual grains can be distinguished by eye
Fine grained 0.0750.425 Individual grains can be distinguished with difficulty
Very fine grained <0.075 Individual grains cannot be distinguished by unaided eye
TABLE 5
TERMS TO DESCRIBE GRAIN SHAPE (for sedimentary rocks)
Description Characteristic
Angular Showing very little evidence of wear. Grain edges and corners are sharp. Secondary
corners are numerous and sharp.
Subangular Showing definite effects of wear. Grain edges and corners are slightly rounded off.
Secondary corners are slightly less numerous and slightly less sharp than in angular
grains.
Subrounded Showing considerable wear. Grain edges and corners are rounded to smooth curves.
Secondary corners are reduced greatly in number and highly rounded.
Rounded Showing extreme wear. Grain edges and corners are smoothed off to broad curves.
Secondary corners are few in number and rounded.
Well-rounded Completely worn. Grain edges and corners are not present. No secondary edges or
corners are present.
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TABLE 6 tests or uniaxial compression tests conducted on specimens.
TERMS TO DESCRIBE STRATUM The rock strength descriptions given at the bottom of the
THICKNESS
second page of the Key correspond to the seven categories
Descriptive Term Stratum Thickness of rock strength, R0 through R6, of the ISRM ("Basic Geo-
Very thickly bedded >1 m
Thickly bedded 0.5 to 1.0 m technical Description of Rock Masses" 1981), with R0 cor-
Thinly bedded 50 mm to 500 mm responding to extremely weak rock and R6 corresponding
Very thinly bedded 10 mm to 50 mm to extremely strong rock. The degree of physical disinte-
Laminated 2.5 mm to 10 mm
Thinly laminated <2.5 mm gration or chemical alteration of rock can be described by the
terms and abbreviations given in the Key. Weathering and
alteration reduces shear strength of both intact rock and
discontinuities.
FIGURE 11 Key for rock core description (sheet 1).
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FIGURE 11 (continued ) (sheet 2).
Discontinuity Descriptors to the core. Roughness and surface shape of joint surfaces is
best measured in the field on exposed surfaces at least 2 m in
A discontinuity is defined as any surface across which any me- length and can be described using the terms in the Key or
chanical property of a rock mass is discontinuous. Discon- quantified in terms of a Joint Roughness Coefficient (Barton
tinuity descriptors are summarized in Figure 11 (Key), items a 1973). Aperture is the width of a discontinuity with no infill-
through g. Types of discontinuities include faults, joints, shear ing and can be classified according to Box c of the Key.
planes, foliation, veins, and bedding. Orientation refers to the
measured dip and dip direction of the surface (or dip and
strike). Dip is defined as the maximum angle of the plane to Infilling
the horizontal and dip direction (strike) is the direction of the
horizontal trace of the line of dip measured clockwise from Infilling is the term for material separating adjacent rock
north, in degrees. Determination of dip and dip direction from walls of discontinuities. Infilling is described in terms of its
core samples is possible using oriented coring techniques, type, amount, and width (Key). Additional laboratory testing
borehole televiewers, downhole cameras, or other devices may be conducted to determine soil classification and shear
capable of establishing orientation of the discontinuity relative strength of infilling materials. Direct shear tests provide a
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means to measure shear strength of joints with infilling, as
described by Wyllie and Norrish (1996). Infilling properties LENGTH OF SOUND
L = 250 mm
CORE > 100 mm
vary widely and can have a significant influence on rock RQD =
PIECES
TOTAL CORE RUN LENGTH
mass strength (RMS), compressibility, and permeability.
L=0 250 + 190 + 200
HIGHLY WEATHERED RQD = × 100%
1200
DOES NOT MEET
SOUNDNESS REQUIREMENT
Rock Mass Descriptors RQD = 53% (FAIR)
L=0
Spacing is the perpendicular distance between adjacent dis- CENTER LINE
CORE RUN TOTAL LENGTH = 1 2 0 0 m m
PIECES < 4"
& HIGHLY WEATHERED
continuities. Spacing has a major influence on seepage and
mechanical behavior and can be described using the terms in
Figure 11 (Key). Persistence refers to the continuous length L = 190 mm
or area of a discontinuity and requires field exposures for its
determination. L=0
< 4"
MECHANICAL
BREAK CAUSED
The number of sets of intersecting discontinuities has a BY DRILLING L = 200 mm
PROCESS
major effect on RMS and compressibility. As the number of
sets increases, the extent to which the rock mass can deform
without failure of intact rock also increases. Field mapping L=0
NO RECOVERY
or observations made in exploratory pits or large excavations
provide the best opportunity to map multiple sets of discon- FIGURE 12 RQD determination of rock core (after Deere and
tinuities. Block size and shape is determined by spacing, per- Deere 1989).
sistence, and number of intersecting sets of discontinuities.
Descriptive terms include blocky, tabular, shattered, and measured along the centerline or axis of the core, as shown
columnar, while size ranges from small (<0.0002 m3) to very in Figure 12.
large (>8 m3).
Only natural fractures such as joints or shear planes
should be considered when calculating RQD. Core breaks
Seepage
caused by drilling or handling should be fitted together and
Field observations of seepage from discontinuities should the pieces counted as intact lengths. Drilling breaks may be
be described whenever it can be observed. The presence identified by fresh surfaces. For some laminated rocks it may
and type of infilling controls joint permeability and should be difficult to distinguish natural fractures from those caused
be described wherever seepage is observed. Seepage can by drilling. For characterization of rock mass behavior rele-
range from dry to continuous flow under high pore water vant to foundation design it is conservative to not count the
pressure length near horizontal breaks. RQD should be performed as
soon as possible after the core is retrieved to avoid the effects
of deterioration, which may include slaking and separation of
Rock Quality Designation core along bedding planes, especially in moisture-sensitive
rocks like some shales. It is also desirable because RQD is a
A simple and widely used measure of rock mass quality is quantitative measure of core quality at the time of drilling
provided by the RQD (rock quality designation, ASTM when the rock core is "fresh" and most representative of in
D6032). RQD is equal to the sum of the lengths of sound situ conditions.
pieces of core recovered, greater than 100 mm (4 in.) in
length, expressed as a percentage of the length of the core Rock assigned a weathering classification of "highly weath-
run. Originally introduced by Deere (1964), the RQD was ered" or above should not be included in the determination of
evaluated by Deere and Deere (1989), who recommended RQD. RQD measurements assume that core recovery is at or
modifications to the original procedure after evaluating its near 100%. As core recovery varies from 100%, explanatory
field use. Figure 12 illustrates the recommended procedure. notes may be required to describe the reason for the variation
Several factors must be evaluated properly for RQD to pro- and the effect on RQD. In some cases, RQD will have to be
vide reliable results. determined on the basis of total length of core recovered, rather
than on the length of rock cored. One state (Florida) uses per-
RQD was originally recommended for NX size core, but cent core recovery as an index of rock quality in limestone.
can also be used with the somewhat smaller NQ wireline
sizes and with larger wire line sizes and other core sizes A general description of rock mass quality based on RQD
up to 150 mm (6 in.). RQD based on the smaller BQ and is given here. Its wide use and ease of measurement make it
BX cores or with single-tube core barrels is discouraged an important piece of information to be gathered on all core
because of core breakage. Core segment lengths should be holes. Taken alone, RQD should be considered only as an
