CHAPTER 4.

0 (abbreviated)

BORING LOG PREPARATION

4.1 GENERAL

The boring log is the basic record of almost every geotechnical exploration and provides a detailed record
of the work performed and the findings of the investigation. The field log should be written or printed
legibly, and should be kept as clean as is practical. All appropriate portions of the logs should be completed
in the field prior to completion of the field exploration.

A wide variety of drilling forms are used by various agencies. The specific forms to be used for a given type
of boring will depend on local practice. Typical boring log, core boring log and test pit log forms endorsed
by the ASCE Soil Mechanics & Foundations Engineering Committee are presented in Figures 4-1 through
4-3, respectively. A proposed legend for soil boring logs is given in Figure 4-4 and for core boring logs in
Figure 4-5. This chapter presents guidelines for completion of the boring log forms, preparation of soil
descriptions and classifications, and preparation of rock descriptions and classifications.

A boring log is a description of exploration procedures and subsurface conditions encountered during
drilling, sampling and coring. Following is a brief list of items which should be included in the logs. These
items are discussed in detail in subsequent sections:

C Topographic survey data including boring location and surface elevation, and bench mark location
and datum, if available.
C An accurate record of any deviation in the planned boring locations.
C Identification of the subsoils and bedrock including density, consistency, color, moisture, structure,
geologic origin.
C The depths of the various generalized soil and rock strata encountered.
C Sampler type, depth, penetration, and recovery.
C Sampling resistance in terms of hydraulic pressure or blows per depth of sampler penetration. Size
and type of hammer. Height of drop.
C Soil sampling interval and recovery.
C Rock core run numbers, depths & lengths, core recovery, and Rock Quality Designation (RQD)
C Type of drilling operation used to advance and stabilize the hole.
C Comparative resistance to drilling.
C Loss of drilling fluid.
C Water level observations with remarks on possible variations due to tides and river levels.
C The date and time that the borings are started, completed, and of water level measurements.
C Closure of borings.

Boring logs provide the basic information for the selection of test specimens. They provide background data
on the natural condition of the formation, on the ground water elevation, appearance of the samples, and
the soil or rock stratigraphy at the boring location, as well as areal extent of various deposits or formations.
Data from the boring logs are combined with laboratory test results to identify subgrade profiles showing
the extent and depth of various materials at the subject site. Soil profiles showing the depth and the location
of various types of materials and ground water elevations are plotted for inclusion in the geotechnical
engineer’s final report and in the plans and specifications. Detailed boring logs including the results of
laboratory tests are included in the text of the report.

4-1
4.2 PROJECT INFORMATION

The top of each boring log provides a space for project specific information: name or number of the project,
location of the project, drilling contractor (if drilling is contracted out), type of drilling equipment, date and
time of work, drilling methods, hammer weight and fall, name of personnel logging the boring, and weather
information. All information should be provided on the first sheet of each boring log.

4.3 BORING LOCATIONS AND ELEVATIONS

The boring location (coordinates and/or station and offset) and ground surface elevation (with datum) must
be recorded on each boring log. Procedures discussed in Section 2.5.3 should be used for determining the
location and elevation for each boring site.

4.4 STRATIGRAPHY IDENTIFICATION

The subsurface conditions observed in the soil samples and drill cuttings or perceived through the
performance of the drill rig (for example, rig chatter in gravel, or sampler rebounding on a cobble during
driving) should be described in the wide central column on the log labeled “Material Description”, or in the
remarks column, if available. The driller's comments are valuable and should be considered as the boring
log is prepared. In addition to the description of individual samples, the boring log should also describe
various strata. The record should include a description of each soil layer, with solid horizontal lines drawn
to separate adjacent layers. It is important that a detailed description of subsurface conditions be provided
on the field logs at the time of drilling. Completing descriptions in the laboratory is not an acceptable
practice. Stratification lines should be drawn where two or more items in the description change, i.e.,
change from firm to stiff and low to high plasticity. Minor variations can be described using the term
'becoming'. A stratification line should be drawn where the geological origin of the material changes and
the origin (if determined) should be designated in the material description or remarks column of the log.
Dashed lines should be avoided.

The stratigraphy observations should include identification of existing fill, topsoil, and pavement sections.
Careful observation and special sampling intervals may be needed to identify the presence and thickness
of these strata. The presence of these materials can have a significant impact on the conclusions and
recommendations of the geotechnical studies.

Individual strata should be marked midway between samples unless the boundary is encountered in a sample
or special measurements are available to better define the position of the boundary.

4.5 SAMPLE INFORMATION

Information regarding the sampler types, date & time of sampling, sample type, sample depth, and recovery
should be shown on each log form using notations and a graphical system or an abbreviation system as
designated in Figures 4-4 and 4-5. Each sample attempt should be given a sequential number marked in
the sample number column. If the sampler is driven, the driving resistance should be recorded at the
specified intervals and marked in the sampling resistance column. The percent recovery should be
designated as the length of the recovered sample referenced to the length of the sample attempt (example
550/610 mm).

4-2
4.6 SOIL DESCRIPTION / SOIL CLASSIFICATION

Soil description/identification is the systematic, precise, and complete naming of individual soils in both
written and spoken forms (AASHTO M 145, ASTM D-2488), while soil classification is the grouping of
the soil into a category based on index test results; e.g., group name and symbol (AASHTO M 145, ASTM
D-2487). The soil's description should include as a minimum:

C Apparent consistency (for fine-grained soils) or density adjective (for coarse-grained soils)
C Water content condition adjective (e.g., dry, damp, moist, wet)
C Color description
C Minor soil type name with "y" added if fine-grained minor component is less than 30 percent but
greater than 12 percent or coarse-grained minor component is 30 percent or more.
C Descriptive adjective for main soil type
C Particle-size distribution adjective for gravel and sand
C Plasticity adjective and soil texture (silty or clayey) for inorganic and organic silts or clays
C Main soil type name (all capital letters)
C Descriptive adjective “with” for the fine-grained minor soil type if 5 to 12 percent or for the
coarse-grained minor soil type if less than 30 percent but 15 percent or more (note some practices
use the descriptive adjectives “some” and “trace” for minor components).
C Descriptive term for minor type(s) of soil
C Inclusions (e.g., concretions, cementation)
C The Unified Soil Classification System (USCS) Group Name and Symbol (in parenthesis)
appropriate for the soil type in accordance with AASHTO M 145, ASTM D 3282, or ASTM D
2487. For classification of highway subgrade material, the AASHTO classification system is used.
C Geological name (e.g., Holocene, Eocene, Pleistocene, Cretaceous), if known, (in parenthesis or
in notes column)

The various elements of the soil description should generally be stated in the order given above. For
example:

Fine-grained soils: Soft, wet, gray, high plasticity CLAY, with f. Sand; Fat CLAY (CH); (Alluvium)

Coarse-grained soils: Dense, moist, brown, silty m-f SAND, with f. Gravel to c. Sand; Silty SAND
(SM); (Alluvium)

Some local practices omit the USCS group symbol (e.g., CL, ML, etc.) but include the group symbol at the
end of the description. When changes occur within the same soil layer, such as change in apparent density,
the log should indicate a description of the change, such as “same, except very dense”.

4.6.1 Consistency and Apparent Density

The consistency of fine-grained soils and apparent density of coarse-grained soils are estimated from the
blow count (N-value) obtained from Standard Penetration Tests (AASHTO T-206, ASTM D 1586). The
consistency of clays and silts varies from soft to firm to stiff to hard. The apparent density of coarse-grained
soil ranges from very loose to firm to dense and very dense Suggested guidelines in Tables 4-1 and 4-2 are
given for estimating the in-place consistency or apparent density of soils from N-values.

4-3
The apparent density or consistency of the soil formation can vary from these empirical correlations for a
variety of reasons. Judgment remains an important part of the visual identification process. Mechanical
tools such as the pocket (hand) penetrometer, and field index tests (smear test, dried strength test, thread
test) are suggested as aids in estimating the consistency of fine grained soils.

In some cases the sampler may pass from one layer into another of markedly different properties; for
example, from a dense sand into a soft clay. In attempting to identify apparent density, an assessment
should be made as to what part of the blow count corresponds to each layer; realizing that the sampler
begins to reflect the presence of the lower layer before it reaches it.

The N-values in all soil types should be corrected for energy efficiency, if possible (ASTM D 4633). An
energy efficiency of 60% is considered the reference in the U.S. In certain geotechnical evaluations of
coarse-grained soil behavior (relative density, friction angle, liquefaction potential), the blow count (N-
value) should be normalized to a reference stress of one atmosphere, as discussed in Chapters 5 and 9.

Note that N-values should not be used to determine the design strength of fine grained soils.

4.6.2 Water Content (Moisture)

The amount of water present in the soil sample or its water content adjective should be described as dry,
moist, or wet as indicated in Table 4-3.

4.6.3 Color

The color should be described when the sample is first retrieved at the soil's as-sampled water content (the
color will change with water content). Primary colors should be used (brown, gray, black, green, white,
yellow, red). Soils with different shades or tints of basic colors are described by using two basic colors;
e.g., gray-green. Note that some agencies may require Munsell color and carry no inferences of texture
designations. When the soil is marked with spots of color, the term “mottled” can be applied. Soils with
a homogeneous texture but having color patterns which change and are not considered mottled can be
described as “streaked”.

TABLE 4-1

EVALUATION OF THE APPARENT DENSITY OF COARSE-GRAINED SOILS

Measured Apparent Relative

N-value Density Behavior of 13 mm Diameter Probe Rod Density, %
0-4 Very loose Easily penetrated by hand 0 - 20
> 4 - 10 Loose Firmly penetrated when pushed by hand 20 - 40
>10 - 30 Firm Easily penetrated when driven with 2 kg. hammer 40 - 70
>30 - 50 Dense A few centimeters penetration by 2 kg. hammer 70 - 85
>50 Very Dense Only a few millimeters penetration when driven with 2 kg. 85 - 100
hammer

4-4
 TABLE 4-2

EVALUATION OF THE CONSISTENCY OF FINE-GRAINED SOILS

Unconfined
Uncorrected Compressive
N-value Consistency Strength, qu, kPa Results Of Manual Manipulation
<2 Very soft <25 Specimen (height = twice the diameter) sags under
its own weight; extrudes between fingers when
squeezed.
2-4 Soft 25 - 50 Specimen can be pinched in two between the thumb
and forefinger; remolded by light finger pressure.
4-8 Firm 50 - 100 Can be imprinted easily with fingers; remolded by
strong finger pressure.
8 - 15 Stiff 100 - 200 Can be imprinted with considerable pressure from
fingers or indented by thumbnail.
15 - 30 Very stiff 200 - 400 Can barely be imprinted by pressure from fingers or
indented by thumbnail.
>30 Hard >400 Cannot be imprinted by fingers or difficult to indent
by thumbnail.

TABLE 4-3

ADJECTIVES TO DESCRIBE WATER CONTENT OF SOILS

Description Conditions
Dry No sign of water and soil dry to touch
Moist Signs of water and soil is relatively dry to touch
Wet Signs of water and soil wet to touch; granular soil exhibits some free water when densified

4.6.4 Type of Soil

The constituent parts of a given soil type are defined on the basis of texture in accordance with particle-size
designators separating the soil into coarse-grained, fine-grained, and highly organic designations. Soil with
more than 50 percent of the particles larger than the (U.S. Standard) No. 200 sieve (0.075 mm) is designated
coarse-grained. Soil (inorganic and organic) with 50 percent or more of the particles finer than the No. 200
sieve is designated fine-grained. Soil primarily consisting of less than 50 percent by volume of organic
matter, dark in color, and with an organic odor is designated as organic soil. Soil with organic content more
than 50 percent is designated as peat. The soil type designations follow ASTM D 2487; i.e., gravel, sand,
clay, silt, organic clay, organic silt, and peat.

4-5
Coarse-Grained Soils (Gravel and Sand)

Coarse-grained soils consist of gravel, sand, and fine-grained soil, whether separately or in combination,
and in which more than 50 percent of the soil is retained on the No. 200 sieve. The gravel and sand
components are defined on the basis of particle size as indicated in Table 4-4.

The particle-size distribution is identified as well graded or poorly graded. Well graded coarse-grained soil
contains a good representation of all particle sizes from largest to smallest, with # 12 percent fines. Poorly
graded coarse-grained soil is uniformly graded with most particles about the same size or lacking one or
more intermediate sizes, with # 12 percent fines.

TABLE 4-4

PARTICLE SIZE DEFINITION FOR GRAVELS AND SANDS

Soil Component Grain Size Determination

Boulders* 300 mm + Measurable
Cobbles* 300 mm to 75 mm Measurable
Gravel
Coarse 75 mm to 19 mm Measurable
Fine 19 mm to #4 sieve (4.75 mm) Measurable
Sand
Coarse #4 to #10 sieve Measurable and visible to eye
Medium #10 to #40 sieve Measurable and visible to eye
Fine #40 to #200 sieve Measurable and barely discernible to the eye

*Boulders and cobbles are not considered soil or part of the soil's classification or description, except under
miscellaneous description; i.e., with cobbles at about 5 percent (volume).

TABLE 4-5

GROUP SYMBOLS FOR ORGANIC SOILS

Group Symbol Group Name Remarks

SO organic silty SAND > 12% fines below A-line
SO organic clayey SAND > 12% fines below A-line
SP-SO poorly graded SAND with organic silt 5 - 12% fines below A-line
NOTE: The present USCS does not allow the identification of whether the fines' liquid limit is less than or
equal to and greater than 50.

4-6
 TABLE 4-6

ADJECTIVES FOR DESCRIBING SIZE DISTRIBUTION FOR SANDS AND GRAVELS

Particle-Size Adjective Abbreviation Size Requirement

Coarse c. < 30% m-f sand or < 12% f. gravel
Coarse to medium c-m < 12% f. sand
Medium to fine m-f < 12% c. sand and > 30% m. sand
Fine f. < 30% m. sand or < 12% c. gravel
Coarse to fine c-f > 12% of each size1
1
12% and 30% criteria can be modified depending on fines content. The key is the shape of the particle-size
distribution curve. If the curve is relatively straight or dished down, and coarse sand is present, use c-f, also use
m-f sand if a moderate amount of m. sand is present. If one has any doubts, determine the above percentages
based on the amount of sand or gravel present.

Sedimentation Test: A small sample of the soil is shaken in a test tube filled with water and allowed to
settle. The time required for the particles to fall a distance of 100 mm is about 1/2 minute for particle sizes
coarser than silt. About 50 minutes would be required for particles of .005 mm or smaller (often defined
as "clay size") to settle out.

For sands and gravels containing more than 5 percent fines, the type of inorganic fines (silt or clay) can be
identified by performing a shaking/dilatancy test. See fine-grained soils section.

Visual Characteristics: Sand and gravel particles can be readily identified visually but silt particles are
generally indistinguishable to the eye. With an increasing silt component, individual sand grains become
obscured, and when silt exceeds about 12 percent, it masks almost entirely the sand component from visual
separation. Note that gray fine-grained sand visually appears siltier than the actual silt content.

Fine-Grained Soils

Fine-grained soils are those in which 50 percent or more pass the No. 200 sieve (fines) and the fines are
inorganic or organic silts and clays as defined by the plasticity chart (Figure 4-7) and decrease in liquid limit
(LL) upon oven drying (Table 4-7). Inorganic silts and clays are those which do not meet the organic
criteria as given in Table 4-7. The flow charts to determine the group symbol and group name for fine-
grained soils are given in Figure 4-8a and b. These figures are identical to Figures 1a and 1b in ASTM D
2487 except that they are modified to show the soil type capitalized; i.e., CLAY. Dual symbols are used
to indicate the organic silts and clays that are above the "A"-line. For example, CL/OL instead of OL and
CH/OH instead of OH.

To describe the fine-grained soil types, plasticity adjectives, and soil types as adjectives should be used to
further define the soil type's texture, plasticity, and location on the plasticity chart.

4-7
Figure 4-6: Flow Chart to Determine the Group Symbol and Name for Coarse-grained Soils.

Inclusions

Additional inclusions or characteristics of the sample can be described by using "with" and the descriptions
described above. Examples are given below:

C with petroleum odor

C with organic matter
C with foreign matter (roots, brick, etc.)
C with shell fragments
C with mica
C with parting(s), seam(s), etc. of (give soils complete description)

Layered Soils

Soils of different types can be found in repeating layers of various thickness. It is important that all such
formations and their thicknesses are noted. Each layer is described as if it is a nonlayered soil using the
sequence for soil descriptions discussed above. The thickness and shape of layers and the geological type
of layering are noted using the descriptive terms presented in Table 4-11. Place the thickness designation
before the type of layer, or at the end of each description and in parentheses, whichever is more appropriate.

4-8
 ENGINEERING SOIL TEST BORING RECORD November 3, 2001
Elevation Stratum Sample Sample Soil Penetration
Remarks and raw
(ft-msl) Depth Visual Soil Description Depth Recovery Sym. N 60
SPT data
+182.2 (ft) (ft) (in) K (blows/ft)
+180 0.3 Top soil, grass, and roots

Loose gray-brown clayey fine

6.0 16 7 (2+3+4)
SAND (SC)
7.0
Soft blue-tan clayey SILT
+170 12.0 16 3 (0+2+1)
(MH)
Groundwater
14.5
z w = 15.5 feet
Firm yellow-tan clean to
(Nov. 8, 2001)
slightly silty fine SAND (SP to
SP-SM)
20.5 18 32 (11+14+18)
+160
21.5

Firm yellow-tan clean fine to

medium SAND (SP) 28.0 11 28 (+13+15+13)
30.0
+150
Loose white to yellow slightly
silty medium to coarse SAND
(SP)
36.0 11 5 (+2+3+2)

39.0

+140 Very stiff green fine-medium

sandy CLAY (CL) 43.5 16 20 (+10+10+10)

45.5

+130 (+6+7+8)
Stiff green-gray silty to sandy
52.5 18 15
CLAY (CL)
60.2
Dense white medium SAND (+20+22+20)
63.5 10 42
+120 (SP) with shells
64.0
REFUSAL at 64 feet
Driller: E. Van Halen
Soil Symbols K (Unified Soil Classification System) Other Symbols Boring Number: AGB-1
Top Soil Water Date Drilled: Oct/29/2001
CL Level Job Number 32335
MH Site Location: Tampa
CH Florida
SP Test Method: ASTM D 1586
Hammer Type: Diedrich Automatic
Notes: (ER =82%)
N = Penetration in blows per foot (ASTM D-1586) Sampler: Drive (split-barrel)
N60 = (Ef/60) * N measured = Energy-Corrected N-value Drilling Method: Hollow Stem Augers
Ef = Energy Efficiency of Hammer Used Make of Drilling Rig: CME-850
ER = energy ratio per ASTM D-4633 (truck mounted)
Examples of descriptions for layered soils are:

C Medium stiff, moist to wet 5 to 20 mm interbedded seams and layers of: gray, medium plastic,
silty CLAY (CL); and lt. gray, low plasticity SILT (ML); (Alluvium).

C Soft moist to wet varved layers of: gray-brown, high plasticity CLAY (CH) (5 to 20 mm); and
nonplastic SILT, trace f. sand (ML) (10 to 15 mm); (Alluvium).

Geological Name

The soil description should include the field supervisor’s assessment of the origin of the soil unit and the
geologic name, if known, placed in parentheses or brackets at the end of the soil description or in the field
notes column of the boring log. Some examples include:

a. Washington, D.C. - Cretaceous Age Material with SPT-N values between 30 and 100 bpf:
Very hard gray-blue silty CLAY (CH), damp [Potomac Group Formation]
b. Newport News, VA - Miocene Age Marine Deposit with SPT- N values around 10 to 15 bpf:
Stiff green sandy CLAY (CL) with shell fragments, calcareous [Yorktown Formation].

4.6.5 AASHTO Soil Classification System

The AASHTO soil classification system is shown in Table 4-12. This classification system is useful in
determining the relative quality of the soil material for use in earthwork structures, particularly
embankments, subgrades, subbases and bases.

According to this system, soil is classified into seven major groups, A-1 through A-7. Soils classified under
groups A-1, A-2 and A-3 are granular materials where 35% or less of the particles pass through the No. 200
sieve. Soils where more than 35% pass the No. 200 sieve are classified under groups A-4, A-5, A-6 and
A-7. These are mostly silt and clay-type materials. The classification procedure is shown in Table 4-12.
The classification system is based on the following criteria:

I. Grain Size: The grain size terminology for this classification system is as follows:
Gravel:fraction passing the 75 mm sieve and retained on the No. 10 (2 mm) sieve.
Sand:fraction passing the No. 10 (2 mm) sieve and retained on the No. 200 (0.075 mm) sieve
Silt and clay: fraction passing the No. 200 (0.075 mm) sieve
ii Plasticity: The term silty is applied when the fine fractions of the soil have a plasticity index of
10 or less. The term clayey is applied when the fine fractions have a plasticity index of 11 or
more.
iii. If cobbles and boulders (size larger than 75 mm) are encountered they are excluded from the
portion of the soil sample on which classification is made. However, the percentage of material
is recorded.

To evaluate the quality of a soil as a highway subgrade material, a number called the group index (GI) is
also incorporated along with the groups and subgroups of the soil. This is written in parenthesis after the
group or subgroup designation. The group index is given by the equation

Group Index: GI=(F-35)[0.2+0.005(LL-40)] + 0.01(F-15) (PI-10) (4-1)

4-9
where F is the percent passing No. 200 sieve, LL is the liquid limit and PI is the plasticity index. The first
term of Eq. 4-1 is the partial group index determined from the liquid limit. The second term is the partial
group index determined from the plasticity index. Following are some rules for determining group index:

C If Eq. 4-1 yields a negative value for GI, it is taken as zero.

C The group index calculated from Eq. 4-1 is rounded off to the nearest whole number, e.g.,
GI=3.4 is rounded off to 3; GI=3.5 is rounded off to 4.
C There is no upper limit for the group index.
C The group index of soils belonging to groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 will always
be zero.
C When calculating the group index for soils belonging to groups A-2-6 and A-2-7, the partial
group index for PI should be used, or

GI=0.01(F-15) (PI-10) (4-2)

In general, the quality of performance of a soil as a subgrade material is inversely proportional to the group
index.

Figure 4-9: Range of Liquid Limit and Plasticity Indices for Soils in Soil Classification
Groups A-2, A-4, A-5, A-6 and A-7 (AASHTO Standard M 145, 1995)

4 - 10
4.7 LOGGING PROCEDURES FOR CORE DRILLING

As with soil boring logs, rock or core boring logs should be as comprehensive as possible under field
conditions, yet be terse and precise. The level of detail should be keyed to the purpose of the exploration
as well as to the intended user of the prepared logs. Although the same basic information should be
presented on all rock boring logs, the appropriate level of detail should be determined by the geotechnical
engineer and/or the geologist based on project needs. Borings for a bridge foundation may require more
detail concerning degree of weathering than rock structure features. For a proposed tunnel excavation, the
opposite might be true. Extremely detailed descriptions of rock mineralogy may mask features significant
to an engineer, but may be critical for a geologist.

4.7.1 Description of Rock

Rock descriptions should use technically correct geological terms, although local terms in common use may
be acceptable if they help describe distinctive characteristics. Rock cores should be logged when wet for
consistency of color description and greater visibility of rock features. The guidelines presented in the
"International Society for Rock Mechanics Commission on Standardization of Laboratory and Field Tests"
(1978, 1981), should be reviewed for additional information regarding logging procedures for core drilling.

The rock's lithologic description should include as a minimum the following items:

C Rock type
C Color
C Grain size and shape
C Texture (stratification/foliation)
C Mineral composition
C Weathering and alteration
C Strength
C Other relevant notes

The various elements of the rock's description should be stated in the order listed above. For example:

"Limestone, light gray, very fine-grained, thin-bedded, unweathered, strong"

The rock description should include identification of discontinuities and fractures. The description should
include a drawing of the naturally occurring fractures and mechanical breaks.

4.7.2 Rock Type

Rocks are classified according to origin into three major divisions: igneous, sedimentary, and metamorphic,
see Table 4-13. These three groups are subdivided into types according to mineral and chemical
composition, texture, and internal structure. For some projects a library of hand samples and photographs
representing lithologic rock types present in the project area should be maintained.

4 - 11
 TABLE 4-13

ROCK GROUPS AND TYPES

IGNEOUS
Intrusive Extrusive Pyroclastic
(Coarse Grained) (Fine Grained)
Granite Rhyolite Obsidian
Syenite Trachyte Pumice
Diorite Andesite Tuff
Diabase Basalt
Gabbro
Peridotite
Pegmatite

SEDIMENTARY
Clastic (Sediment) Chemically Formed Organic Remains
Shale Limestone Chalk
Mudstone Dolomite Coquina
Claystone Gypsum Lignite
Siltstone Halite Coal
Sandstone
Conglomerate
Limestone, oolitic

METAMORPHIC

Foliated Nonfoliated
Slate Quartzite
Phyllite Amphibolite
Schist Marble
Gneiss Hornfels

4 - 12
4.7.3 Color

Colors should be consistent with a Munsell Color Chart and recorded for both wet and dry conditions as
appropriate.

4.7.4 Grain Size and Shape

The grain size description should be classified using the terms presented in Table 4-14. Table 4-15 is used
to further classify the shape of the grains.

4.7.5 Stratification/Foliation

Significant nonfracture structural features should be described. The thickness should be described using
the terms in Table 4-16. The orientation of the bedding/foliation should be measured from the horizontal
with a protractor.

4.7.6 Mineral Composition

The mineral composition should be identified by a geologist based on experience and the use of appropriate
references. The most abundant mineral should be listed first, followed by minerals in decreasing order of
abundance. For some common rock types, mineral composition need not be specified (e.g. dolomite,
limestone).

4.7.7 Weathering and Alteration

Weathering as defined here is due to physical disintegration of the minerals in the rock by atmospheric
processes while alteration is defined here as due to geothermal processes. Terms and abbreviations used
to describe weathering or alteration are presented in Figure 4-5.

4.7.8 Strength

The point load test, described in Section 8.2.1, is recommended for the measurement of sample strength in
the field. The point-load index (Is) may be converted to an equivalent uniaxial compressive strength and
noted as such on the records. Various categories and terminology recommended for describing rock
strength based on the point load test are presented in Figure 4-5. Figure 4-5 also presents guidelines for
common qualitative assessment of strength while mapping or during primary logging of core at the rig site
by using a geological hammer and pocket knife. The field estimates should be confirmed where appropriate
by comparison with selected laboratory tests.

4.7.9 Hardness

Hardness is commonly assessed by the scratch test. Descriptions and abbreviations used to describe rock
hardness are presented in Table 4-17.

4 - 13
 TABLE 4-14

TERMS TO DESCRIBE GRAIN SIZE OF (TYPICALLY FOR) SEDIMENTARY ROCKS

Diameter
Description (mm) Characteristic

Very coarse grained > 4.75 Grains sizes are greater than popcorn kernels
Coarse grained 2.00 -4.75 Individual grains can be easily distinguished by eye
Medium grained 0.425 -2.00 Individual grains can be distinguished by eye
Fine grained 0.075-0.425 Individual size grains can be distinguished with difficulty
Very fine grained < 0.075 Individual grains cannot be distinguished by unaided eye

TABLE 4-15

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- Completely worn. Grain edges or corners are not present. No secondary edges or corners
rounded are present.

4 - 14
 TABLE 4-16

TERMS TO DESCRIBE STRATUM THICKNESS

Descriptive Term Stratum Thickness

Very Thickly bedded >1m

Thickly bedded 0.5 to 1.0 m
Thinly bedded 50 mm to 500 mm
Very Thinly bedded 10 mm to 50 mm
Laminated 2.5 mm to 10 mm
Thinly Laminated < 2.5 mm

TABLE 4-17

TERMS TO DESCRIBE ROCK HARDNESS

Description (Abbr) Characteristic

Soft (S) Reserved for plastic material alone.

Friable (F) Easily crumbled by hand, pulverized or reduced to powder and is too soft to be cut with a
pocket knife.
Low Hardness (LH) Can be gouged deeply or carved with a pocket knife.
Moderately Hard (MH) Can be readily scratched by a knife blade; scratch leaves a heavy trace of dust and scratch
is readily visible after the powder has been blown away.
Hard (H) Can be scratched with difficulty; scratch produces little powder and is often faintly visible;
traces of the knife steel may be visible.
Very Hard (VH) Cannot be scratched with pocket knife. Leave knife steel marks on surface.

4.7.10 Rock Discontinuity

Discontinuity is the general term for any mechanical crack or fissure in a rock mass having zero or low
tensile strength. It is the collective term for most types of joints, weak bedding planes, weak schistosity
planes, weakness zones, and faults. The symbols recommended for the type of rock mass discontinuities
are listed in Figure 4-5.

The spacing of discontinuities is the perpendicular distance between adjacent discontinuities. The spacing
should be measured in centimeters or millimeters, perpendicular to the planes in the set. Figure 4-5 presents
guidelines to describe discontinuity spacing.

The discontinuities should be described as closed, open, or filled. Aperture is used to describe the
perpendicular distance separating the adjacent rock walls of an open discontinuity in which the intervening
space is air or water filled. Width is used to describe the distance separating the adjacent rock walls of filled
discontinuities. The terms presented in Table 4-18 should be used to describe apertures.

4 - 15
Terms such as "wide", "narrow" and "tight" are used to describe the width of discontinuities such as
thickness of veins, fault gouge filling, or joints openings. Guidelines for use of such terms are presented
in Figure 4-5.

For the faults or shears that are not thick enough to be represented on the boring log, the measured thickness
is recorded numerically in millimeters.

In addition to the above characterization, discontinuities are further characterized by the surface shape of
the joint and the roughness of its surface. Refer to Figure 4-5 for guidelines to characterize these features.

Filling is the term for material separating the adjacent rock walls of discontinuities. Filling is characterized
by its type, amount, width (i.e., perpendicular distance between adjacent rock walls) and strength. Figure
4-5 presents guidelines for characterizing the amount and width of filling. The strength of any filling
material along discontinuity surfaces can be assessed by the guidelines for soil presented in the last three
columns of Table 4-2. For non-cohesive fillings, then identify the filling qualitatively (e.g., fine sand).

TABLE 4-18

TERMS TO CLASSIFY DISCONTINUITIES BASED ON APERTURE SIZE

Aperture Description

<0.1 mm Very tight

0.1 - 0.25 mm Tight "Closed Features"
0.25 - 0.5 mm Partly open
0.5 - 2.5 mm Open
2.5 - 10 mm Moderately open "Gapped Features"
> 10 mm Wide
1-10 cm Very wide
10-100 cm Extremely wide "Open Features"
>1 m Cavernous

4.7.11 Fracture Description

The location of each naturally occurring fracture and mechanical break is shown in the fracture column of
the rock core log. The naturally occurring fractures are numbered and described using the terminology
described above for discontinuities.

The naturally occurring fractures and mechanical breaks are sketched in the drawing column. Dip angles
of fractures should be measured using a protractor and marked on the log. For nonvertical borings, the
angle should be measured and marked as if the boring was vertical. If the rock is broken into many pieces
less than 25 mm long, the log may be crosshatched in that interval, or the fracture may be shown
schematically.

4 - 16
The number of naturally occurring fractures observed in each 0.5 m of core should be recorded in the
fracture frequency column. Mechanical breaks, thought to have occurred due to drilling, are not counted.
The following criteria can be used to identify natural breaks:

1. A rough brittle surface with fresh cleavage planes in individual rock minerals indicates an
artificial fracture.

2. A generally smooth or somewhat weathered surface with soft coating or infilling materials, such
as talc, gypsum, chlorite, mica, or calcite obviously indicates a natural discontinuity.

3. In rocks showing foliation, cleavage or bedding it may be difficult to distinguish between

natural discontinuities and artificial fractures when these are parallel with the incipient
weakness planes. If drilling has been carried out carefully then the questionable breaks should
be counted as natural features, to be on the conservative side.

4. Depending upon the drilling equipment, part of the length of core being drilled may
occasionally rotate with the inner barrels in such a way that grinding of the surfaces of
discontinuities and fractures occurs. In weak rock types it may be very difficult to decide if the
resulting rounded surfaces represent natural or artificial features. When in doubt, the
conservative assumption should be made; i.e., assume that they are natural.

The results of core logging (frequency and RQD) can be strongly time dependent and moisture content
dependent in the case of certain varieties of shales and mudstones having relatively weakly developed
diagenetic bonds. A not infrequent problem is "discing", in which an initially intact core separates into discs
on incipient planes, the process becoming noticeable perhaps within minutes of core recovery. The
phenomena are experienced in several different forms:

1. Stress relief cracking (and swelling) by the initially rapid release of strain energy in cores
recovered from areas of high stress, especially in the case of shaley rocks.

2. Dehydration cracking experienced in the weaker mudstones and shales which may reduce RQD
from 100 percent to 0 percent in a matter of minutes, the initial integrity possibly being due to
negative pore pressure.

3. Slaking cracking experienced by some of the weaker mudstones and shales when subjected to
wetting and drying.

All these phenomena may make core logging of fracture frequency and RQD unreliable. Whenever such
conditions are anticipated, core should be logged by an engineering geologist as it is recovered and at
subsequent intervals until the phenomenon is predictable. An added advantage is that the engineering
geologist can perform mechanical index tests, such as the point load or Schmidt hammer test (see Chapter
8), while the core is still in a saturated state.

4 - 17