Chapter 4

CLASSIFICATION OF ROCKS AND DESCRIPTION OF PHYSICAL

PROPERTIES OF ROCK

Introduction

Uniformity of definitions, descriptors, and identificationof rock units is important to maintain continuity ingeologic logs, drawings, and reports from a project withmultiple drilling sessions, different loggers and mappers.Also important is the recording of all significant observ-able parameters when logging or mapping. This chapterpresents a system for the identification and classificationof rocks and includes standard terminology anddescriptive criteria for physical properties of engineeringsignificance. The standards presented in this chaptermay be expanded or modified to fit project requirements.

Rock Classification

Numerous systems are in use for field and petrographicclassification of rocks. Many classifications requiredetailed petrographic laboratory tests and thin sections,while others require limited petrographic examinationand field tests. The Bureau of Reclamation (Reclamation)has adopted a classification system which is modifiedfrom R.B. Travis [1]. While not based entirely on fieldtests or field identification of minerals, many of theclassification categories are sufficiently broad that fieldidentification is possible. Even where differences in themineral constituents cannot be determined precisely inthe field, differences usually are not significant enough toaffect the engineering properties of the rock if classifiedsomewhat incorrectly by lithologic name. Detailedmineralogical identification and petrographicclassification can be performed on hand samples or coresamples submitted to a petrographic laboratory.

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If samples are submitted to a petrographic laboratory, thepetrographic classification generally will coincide with theclassification according to Travis. The petrographicigneous rock classifications are somewhat more preciseand include specialty rock types based upon mineral com-position, texture, and occurrence. For example, alamprophyric dike composed of green hornblendephenocrysts or clinopyroxene in the groundmass could beclassified as spessartite, whereas a lamprophyrecontaining biotite with or without clinopyroxene could beclassified as a kersantite. Sedimentary rockclassifications generally include grain size, type of cementor matrix, mineral composition in order of increasingamounts greater than 15 percent, and the rock type, suchas medium-grained, calcite-cemented, feldspathic-quartzose sandstone, and coarse- to fine-grained, lithic-feldspathic-quartzose gray-wacke with an argillaceous-ferruginous-calcareous matrix. Metamorphic rockclassifications include specific rock types based uponcrystal size, diagnostic accessory minerals, mineralogicalcomposition in increasing amounts greater than15 percent, and structure. Two examples of metamorphicrock descriptions are medium-grained, hornblende-biotiteschist, or fine- to medium-grained, garnetiferous,muscovite-chlorite-feldspar-quartz gneiss. The aboveclassification can be abbreviated by the deletion ofmineral names from the left to right as desired. Themineral type immediately preceding the rock name is themost diagnostic.

The term "quartzite” is restricted to a metamorphic rockonly. The sedimentary sandstone equivalent is termed a"quartz cemented quartzose sandstone."

Samples submitted to a petrographic laboratory should berepresentative of the in-place rock unit. For example, ifa granitic gneiss is sampled but only the granite portionsubmitted, the rock will be petrographically classified as

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a granite since the gneissic portion cannot be observed orsubstantiated by the thin section and hand specimen.Petrographic classifications can be related to theengineering properties of rock units and are important.

Geologic rock unit names should be simple, and generalrock names should be based on either field identification,existing literature, or detailed petrographic examination,as well as engineering properties. Overclassification isdistracting and unnecessary. For example, use "horn-blende schist" or "amphibolite" instead of "sericite-chlorite-calcite-hornblende schist." The term "granite"may be used as the rock name and conveys more to thedesigner than the petrographically correct term"nepheline-syenite porphyry." Detailed mineralogicaldescriptions may be provided in reports when describingthe various rock units and may be required to correlatebetween observations, but mineralogical classificationsare not desirable as a rock unit name unless the mineralconstituents or fabric are significant to engineeringproperties.

The classification for igneous, sedimentary, metamorphic,and pyroclastic rocks is shown on figures 4-1, 4-2, 4-3, and4-4, respectively. These figures are condensed andmodified slightly from Travis' classifications, but themore detailed original classifications of Travis areacceptable. Figure 4-5 or appropriate AmericanGeological Institute (AGI) data sheets are suggested foruse when estimating composition percentages inclassification.

Description of Rock

Adequate descriptors, a uniform format, and standardterminology must be used for all geologic investigationsto properly describe rock foundation conditions. These

FIE

LD

MA

NU

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Figure 4-1.—Field classification of igneous rocks (modified after R.B. Travis [1955]).

RO

CK

S

61 Figure 4-2.—Field classification of sedimentary rocks (modified after R.B. Travis [1955]).

FIE

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MA

NU

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62

Figure 4-3.—Field classification of metamorphic rocks (modified after R.B. Travis [1955]).

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Figure 4-4.—Field classification of pyroclastic rocks.Blocks are angular to subangular clasts > 64 millimeters(mm); bombs are rounded to subrounded clasts > 64 mm.Determine percent of each size present (ash, lapilli,blocks, and bombs) and list in decreasing order after rockname. Preceed rock name with the term "welded" forpyroclastic rocks which retained enough heat to fuseafter deposition. Rock names for such deposits areusually selected from the lower right portion of theclassification diagram above. (Modified from Fisher, 1966[2] and Williams and McBirney, 1979 [3]).

paragraphs provide descriptors for those physical charac-teristics of rock that are used in logs of exploration, innarratives of reports, and on preconstruction geologicmaps and cross sections, as well as construction or "as-built" drawings. The alphanumeric descriptors providedmay be used in data-field entries of computer generated

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Figure 4-5.—Charts for estimating percentage of composition of rocks and sediments.[4]

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logs. Chapter 5 establishes descriptors for the physicalcharacteristics of discontinuities in rock required forengineering geologic studies.

All descriptors should be defined and included in alegend when submitting data for design and/or records ofconstruction. An example of a legend and explanation isfigure 4-6, Reclamation standard drawing 40-D-6493,may be used for geologic reports and specifications wherethe standard descriptors and terminology established forrock are used during data collection.

Format for Descriptions of Rock

Engineering geology rock descriptions should include gen-eralized lithologic and physical characteristics usingqualitative and quantitative descriptors. A generalformat for describing rock in exploration logs and legendson general note drawings is:

• Rock unit (member or formation) name• Lithology with lithologic descriptors

composition (mineralogy) grain/particle size

texture color

• Bedding/foliation/flow texture• Weathering• Hardness/strength• Contacts• Discontinuities (includes fracture indexes)• Permeability data (as available from testing)• Moisture conditions

Example descriptions are presented in a later section.

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Rock Unit Names and Identification

Rock unit names not only are required for identificationpurposes but may also provide indicators of depositionalenvironment and geologic history, geotechnical char-acteristics, and correlations with other areas. A simpledescriptive name and map symbol should be assigned toprovide other users with possible engineeringcharacteristics of the rock type. The rock unit names maybe stratigraphic, lithologic, genetic, or a combination ofthese, such as Navajo Sandstone (Jn), Tertiary shale(Tsh), Jurassic chlorite schist (Jcs), Precambrian granite(Pcgr), or metasediments (ms). Bedrock units of similarphysical properties should be delineated and identified asto their engineering significance as early as possibleduring each geologic study. Planning study maps andother large-scale drawings may require geologicformations or groups of engineering geologic units withdescriptions of their engineering significance inaccompanying discussions.

Units should be differentiated by engineering propertiesand not necessarily formal stratigraphic units wheredifferences are significant. Although stratigraphicnames are not required, units should be correlated tostratigraphic names in the data report or by anillustration, such as a stratigraphic column.Stratigraphic names and ages (formation, member) maybe used as the rock unit name.

For engineering studies, each particular stratigraphicunit may require further subdivisions to identifyengineering parameters. Examples of importantengineering properties are:

• Susceptibility to weathering or presence ofalteration

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Figure 4-6.—Descriptor legend and explanation example.

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• Dominant discontinuity characteristics

• Hardness and/or strength

• Deformability

• Deletereous minerals or beds (such as swellingsusceptibility, sulfates, or clays)

For example, a Tertiary shale unit, Tsh, may bedifferentiated as Tsh1 or Tsh2 if unit 2 contains bentoniteinterbeds and unit 1 does not, and Tshc could be used asa unit name for the bentonite beds. A chlorite schist unit,Cs, may be differentiated as CsA or CsB where unit Acontains higher percentages of chlorite or talc and issignificantly softer (different deformation properties) thanunit B. A metasediment unit, ms, may be furtherdifferentiated on more detailed maps and logs as mssh

(shale) or msls (limestone). All differentiated units shouldbe assigned distinctive map symbols and should bedescribed on the General Geologic Legend, Explanation,and Notes drawings.

Descriptors and Descriptive Criteria for PhysicalCharacteristics of Rock

Lithologic Descriptors (Composition, Grain Size,and Texture).—Provide a brief lithologic description ofthe rock unit. This includes a general description ofmineralogy, induration, cementation, crystal and grainsizes and shapes, textural adjectives, and color.Lithologic descriptors are especially important for thedescription of engineering geology subunits when rockunit names are not specific. Examples of rock unit namesthat are not specific are metasediments, Tertiaryintrusives, or Quaternary volcanics.

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1. Composition.—Use standard adjectives such assandy, silty, calcareous, etc. Detailed mineral composition generally is not necessary or desirable unlessuseful in correlating units or indicating pertinentengineering physical properties. Note uniquefeatures such as fossils, large crystals, inclusions,concretions, and nodules which may be used asmarkers for correlations and interpretations.

2. Crystal or particle sizes and shapes.—Describe the typical crystal or grain shapes andprovide a description of sizes present in the rock unitbased on the following standards:

• Igneous and metamorphic rocks.—Table 4-1 isrecommended for descriptions of crystal sizes inigneous and metamorphic rocks. Crystal sizesgiven in millimeters (mm) are preferred ratherthan fractional inch (in) equivalents.

Table 4-1.—Igneous and metamorphic rock grain size descriptors

DescriptorAverage crystal

diameter

Very coarse-grained or pegmatiticCoarse-grainedMedium-grainedFine-grainedAphanitic (cannot be seen with the unaided eye

> 10 mm (3/8 in)5-10 mm (3/16 - 3/8 in)1-5 mm (1/32 - 3/16 in)0.1-1 mm (0.04 - 1/32 in)<0.1 mm (<0.04 in)

• S e d i m e n t a r y a n d p y r o c l a s t i crocks.—Terminology for particle sizes and theirlithified products which form sedimentary andpyroclastic rocks is provided in table 4-2. The size

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Table 4-2.—Sedimentary and pyroclastic rock particle-size descriptors (AGI Glossary)

USGS(soils only)

Particle size

Sizein

mm (inches)

Sedimentary (epiclastic)Rounded, subrounded,

subangular Volcanic (pyroclastic)

Particle orfragment

Lithifiedproduct

Frag-ment

Lithifiedproduct*

Boulder

Cobble

Coarse gravel

Fine gravel

Coarse sand

Medium sand

Fine sand

Fines Non- plastic Silt

Plastic Clay

300 (12)

256 (10)

75 (3) 64 (2.5)

32 (1.3)

20 (0.8)

4.75 (0.19) 4 (0.16)

2 (0.08)

1 (0.04) 0.5 (0.02)

0.42 0.25 0.125 0.074 0.0625

0.00391

Boulder

Cobble

Pebble

Granule

Very coarse sand Coarse sand

Medium sand Fine sand Very fine sand

Silt

Clay

Boulder conglomerate

Cobble conglomerate

Pebble conglomerate

Granule conglomerate

Sandstone (Very coarse, coarse, medium fine, or very fine)

Siltstone Shale

Claystone Shale

Block+

Bomb

Lapilli

Coarse ash

Fine ash

Volcanic breccia+

Agglo- merate

Lapilli tuff

Coarse tuff

Fine tuff

+ Broken from previous igneous rock block shaped (angular to subangular). Solidified from plastic material while in flight, rounded clasts. * Refer to figure 4-4.

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limits do not correspond to the Unified SoilClassification System for soil particle size but areused for the field description and petrographicclassification of rocks. These limits are theaccepted sizes in geologic literature and are usedby petrographic laboratories.

3. Textural adjectives.—Texture describes the ar-rangements of minerals, grains, or voids. Thesemicrostructural features can affect the engineeringproperties of the rock mass. Use simple, standard,textural adjectives or phrases such as porphyritic,vesicular, scoriaceous, pegmatitic, granular, welldeveloped grains, dense, fissile, slaty, or amorphous.Use of terms such as holohyaline, hypidiomorphicgranular, and crystaloblastic is inappropriate.

Textural terms which identify solutioning, leaching,or voids in bedrock are useful for describing primarytexture, weathering, alteration, permeability, anddensity.

The terminology which follows defines sizes of voids, orholes in bedrock. However, when describing pits, vugs,cavities, or vesicles, a complete description which includesthe typical diameter, or the "mostly" range, and the max-imum size observed is required. For example:

". . . randomly oriented, elliptically shaped vugs rangemostly from 0.03 to 0.06 foot (ft) in diameter,maximum size 0.2 foot; decreases in size away fromquartz-calcite vein; about 15 percent contain calcitecrystals. . .”

or". . . cavity, 3.3 ft wide by 16.4 ft long by 2.3 ft high,striking N 45EW, dipping 85ESW was. . ..”

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• Pit (pitted).—Pinhole to 0.03 ft [d in] (<1 to10 mm) openings.

• Vug (vuggy).—Small opening (usually lined withcrystals) ranging in diameter from 0.03 ft [d in] to0.33 ft [4 in] (10 to 100 mm).

• Cavity.—An opening larger than 0.33 ft [4 in](100 mm), size descriptions are required, and adjec-tives such as small, or large, may be used, if defined.

• Honeycombed.—Individual pits or vugs are sonumerous that they are separated only by thin walls;this term is used to describe a cell-like form.

• Vesicle (vesicular).—Small openings in volcanicrocks of variable shape formed by entrapped gasbubbles during solidification.

4. Color.—As a minimum, provide the color of wetaltered and unaltered or fresh rock. Reporting colorfor both wet and dry material is recommended sincethe colors may differ significantly and causeconfusion. The Munsell Color System, as used in theGeologic Society of America Rock Color Chart [5], isused to provide standard color names and assist incorrelation. The chart also provides uniform andidentifiable colors to others. Color designators areoptional unless necessary for clarity, e.g., light brown(5YR 5/6). Terms such as banded, streaked, mottled,speckled, and stained may be used to further describecolor. Also describe colors of bands, etc.

Bedding, Foliation, and Flow Texture.—Thesefeatures give the rock anisotropic properties or representpotential failure surfaces. Continuity and thickness ofthese features influence rock mass properties and cannot

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always be tested in the laboratory. Descriptors intable 4-3 are used to identify these thicknesses.

Table 4-3.—Bedding, foliation, or flow texture descriptors

Descriptors Thickness/spacing

Massive Greater than 10 ft (3 meters [m])

Very thickly, bedded, foliated, or banded

3 to 10 ft (1 to 3 m)

Thickly 1 to 3 ft (300 mm to 1 m)

Moderately 0.3 to 1 ft (100 to 300 mm)

Thinly 0.1 to 0.3 ft (30 to 100 mm)

Very thinly 0.03 [3/8 in] to 0.1 ft (10 to 30 mm)

Laminated (intensely foliated or banded)

Less than 0.03 ft [3/8 in] (<10 mm)

Weathering and Alteration.—

1. Weathering.—Weathering, the process ofchemical or mechanical degradation of rock, cansignificantly affect the engineering properties of therock and rock mass. For engineering geologydescriptions, the term "weathering" includes bothchemical disintegration (decomposition) andmechanical disaggregation as agents of alteration.

Weathering effects generally decrease with depth, al-though zones of differential weathering can occur andmay modify a simple layered sequence of weathering.

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Examples are: (1) differential weathering within asingle rock unit, apparently due to relatively higherpermeability along fractures; (2) differentialweathering due to compositional or texturaldifferences; (3) differential weathering of contactzones associated with thermal effects such asinterflow zones within volcanics; (4) directionalweathering along permeable joints, faults, shears, orcontacts which act as conduits along whichweathering agents penetrate more deeply into therock mass; and (5) topographic effects.

Weathering does not correlate directly with specificgeotechnical properties used for many rock massclassifications. However, weathering is importantbecause it may be the primary criterion fordetermining depth of excavation, cut slope design,method and ease of excavation, and use of excavatedmaterials. Porosity, absorption, compressibility,shear and compressive strengths, density, andresistance to erosion are the major engineeringparameters influenced by weathering. Weatheringgenerally is indicated by changes in the color andtexture of the body of the rock, color, and condition offracture fillings and surfaces, grain boundaryconditions, and physical properties such as hardness.

Weathering is reported using descriptors presentedin table 4-4, which divides weathering into categoriesthat reflect definable physical changes due tochemical and mechanical processes. This tablesummarizes general descriptions which are intendedto cover ranges in bedrock conditions. Weatheringtables are generally applicable to all rock types;however, they are easier to apply to crystalline rocksand rocks that contain ferromagnesian minerals.Weathering in many sedimentary rocks will notalways conform to the criteria established in

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table 4-4, and weathering categories may have to bemodified for particular site conditions. However, thebasic horizontal categories and descriptors presentedcan be used. Site-specific conditions, such as fractureopenness, filling, and degree and depth ofpenetration of oxidation from fracture surfaces,should be identified and described.

2. Alteration.—Chemical alteration effects aredistinct from chemical and mechanical degradation(weathering), such as hydrothermal alteration, maynot fit into the horizontal suite of weatheringcategories portrayed in table 4-4. Oxides may or maynot be present. Alteration is site-specific, may beeither deleterious or beneficial, and may affect somerock units and not others at a particular site. Forthose situations where the alteration does not relatewell to the weathering categories, adjusting thedescription within the framework of table 4-4 may benecessary. Many of the general characteristics maynot change, but the degree of discoloration andoxidation in the body of the rock and on fracturesurfaces could be very different. Appropriatedescriptors, such as moderately altered or intenselyaltered, may be assigned for each alteration category.Alteration products, depths of alteration, andminerals should be described.

3. Slaking.—Slaking is another type ofdisintegration which affects engineering properties ofrock. Terminology and descriptive criteria to identifythis deleterious property are difficult to standardizebecause some materials air slake, many water slake,and some only slake after one or more wet-dry cycles.The Durability Index (DI) is a simplified method fordescribing slaking. Criteria for the index are based

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Table 4-4.—Weathering descriptors

Descriptors

Diagnostic features

General characteristics(strength, excavation, etc.)§

Chemical weathering—Discoloration and/or oxidation

Mechanical weathering- Grain boundaryconditions(disaggregation)primarily for graniticsand some coarse-grained sediments

Texture and solutioning

Alpha-numeric

descriptor Descriptive term Body of rockFracture surfaces† Texture Solutioning

W1 Fresh. No discoloration, not oxidized. No discoloration oroxidation.

No separation, intact(tight).

No change. No solutioning. Hammer rings when crystalline rocksare struck. Almost always rock exca-vation except for naturally weak orweakly cemented rocks such assiltstones or shales.

W2 Slightly weathered tofresh.*

W3 Sllightly weathered. Discoloration or oxidation is limitedto surface of, or short distancefrom, fractures; some feldsparcrystals are dull.

Minor to completediscoloration oroxidation of mostsurfaces.

No visible separation,intact (tight).

Preserved. Minor leaching ofsome solubleminerals may benoted.

Hammer rings when crystalline rocksare struck. Body of rock notweakened. With few exceptions, suchas siltstones or shales, classified asrock excavation.

W4 Moderately to slightlyweathered.*

W5 Moderatelyweathered.

Discoloration or oxidation extendsfrom fractures, usually throughout;Fe-Mg minerals are “rusty,”feldspar crystals are “cloudy.”

All fracture surfacesare discolored oroxidized.

Partial separation ofboundaries visible.

Generallypreserved.

Soluble mineralsmay be mostlyleached.

Hammer does not ring when rock isstruck. Body of rock is slightly weak-ened. Depending on fracturing,usually is rock excavation except innaturally weak rocks such assiltstone or shales.

W6 Intensely tomoderatelyweathered.*

W7 Intensely weathered. Discoloration or oxidationthroughout; all feldspars and Fe-Mg minerals are altered to clay tosome extent; or chemical alterationproduces in situ disaggregation, seegrain boundary conditions.

All fracture surfacesare discolored oroxidized, surfacesfriable.

Partial separation, rockis friable; in semiaridconditions granitics aredisaggregated.

Texture al-tered bychemicaldisintegration(hydration,argillation).

Leaching of solubleminerals may becomplete.

Dull sound when struck withhammer; usually can be broken withmoderate to heavy manual pressureor by light hammer blow withoutreference to planes of weakness suchas incipient or hairline fractures, orveinlets. Rock is significantlyweakened. Usually commonexcavation.

W8 Very intenselyweathered.

W9 Decomposed. Discolored or oxidized throughout,but resistant minerals such asquartz may be unaltered; allfeldspars and Fe-Mg minerals arecompletely altered to clay.

Complete separation ofgrain boundaries(disaggregated).

Resembles a soil, partial or completeremnant rock structure may bepreserved; leaching of solubleminerals usually complete.

Can be granulated by hand. Alwayscommon excavation. Resistantminerals such as quartz may bepresent as “stringers” or “dikes.”

Note: This chart and its horizontal categories are more readily applied to rocks with feldspars and mafic minerals. Weathering in various sedimentary rocks, particularly limestones and poorly induratedsediments, will not always fit the categories established. This chart and weathering categories may have to be modified for particular site conditions or alteration such as hydrothermal effects; however, thebasic framework and similar descriptors are to be used. * Combination descriptors are permissible where equal distribution of both weathering characteristics are present over significant intervals or where characteristics present are “in between” thediagnostic feature. However, dual descriptors should not be used where significant, identifiable zones can be delineated. When given as a range, only two adjacent terms may be combined (i.e., decomposedto lightly weathered or moderately weathered to fresh) are not acceptable. † Does not include directional weathering along shears or faults and their associated features. For example, a shear zone that carried weathering to great depths into a fresh rock mass would not requirethe rock mass to be classified as weathered. § These are generalizations and should not be used as diagnostic features for weathering or excavation classification. These characteristics vary to a large extent based on naturally weak materials orcementation and type of excavation.

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on time exposed and effects noted in the field(see table 4-5). These simplified criteria do notspecify whether the specimen or exposure iswetted, dried, or subjected to cyclic wetting anddrying, and/or freeze-thaw. When reportingslaking or durability, a complete descriptionincludes the test exposure conditions. Forexample, the material could be classified ashaving "characteristics of DI 3 upon drying."Slaking is not the same as the effects of beddingseparation or disaggregation produced by stressrelief.

Table 4-5.—Durability index descriptors

Alpha-numeric

descriptor Criteria

DI0 Rock specimen or exposure remains intact withno deleterious cracking after exposure longerthan 1 year.

DI1 Rock specimen or exposure develops hairlinecracking on surfaces within 1 month, but nodisaggregation within 1 year of exposure.

DI2 Rock specimen or exposure develops hairlinecracking on surfaces within 1 week and/ordisaggregation within 1 month of exposure.

DI3 Specimen or exposure may develop hairlinecracks in 1 day and displays pronouncedseparation of bedding and/or disaggregationwithin 1 week of exposure.

DI4 Specimen or exposure displays pronouncedcracking and disaggregation within 1 day(24 hours) of exposure. Generally ravels anddegrades to small fragments.

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A field test suitable for evaluating the degreeand rate of slaking for materials, primarilyclayey materials and altered volcanics, isdescribed below. The slaking test evaluates thedisaggregation of an intact specimen in waterand reflects the fabric of the material, internalstresses, and character of the interparticlebonds.

To evaluate slaking behavior, immerse twointact specimens (pieces of core or rockfragments consisting of a few cubic inches orcentimeters) in water. One piece should be atnatural water content (wrapped jar sample) andone piece from an air-dried sample. Test resultsshould be photographed with labels to identifyspecimens and exposure times.

Results of the evaluation should be reported foreach specimen. Describe the behavior of thespecimens as follows:

a. Volume changes.—The volume of the materialreduced to individual particles should be estimatedand compared to the initial volume of material.The degree of disaggregation is described using thefollowing descriptive criteria:

None No discernable disaggregationSlight Less than 5 percent of the volume

disaggregatedModerate Between 5 and 25 percent of the

volume disaggregatedIntense More than 25 percent but less

than the total volume disaggregated

Complete No intact piece of the material remains

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b. Rate of slaking.—The following descriptors areused to identify the time to slake:

Slow slaking Action continues for several hours

Moderate Action completed within 1 hour slaking

Rapid Action completed within slaking 2 minutesSudden Complete reaction, action com- slaking pleted instantaneously

[Generally, slaking applies to rock; disaggregation appliesto soils.]

4. Character of material.—The character of theremaining pieces of material after the test iscompleted is described as follows:

No change Material remains intactPlates remain Remaining material present as

platy fragments of generally uniform thickness

Flakes remain Remaining material present as flaky or wedge-shaped fragments

Blocks remain Blocky fragments remainGrains remain Remaining material chiefly present

as sand-size grains

No fragments Remaining material entirely disag- gregated to clay-size particles

Hardness—Strength.—Hardness can be related tointact rock strength as a qualitative indication of densityand/or resistance to breaking or crushing. Strength is anecessary engineering parameter for design that

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frequently is not assessed, but plays a role in engineeringdesign and construction, such as tunnel supportrequirements, bit wear for drilling or tunnel boringmachine (TBM) operations, allowable bearing pressures,excavation methods, and support.

The hardness and strength of intact rock is a function ofthe individual rock type but may be modified byweathering or alteration. Hardness and strength aredescribed for each geologic unit when they are functionsof the rock type and also for zones of alteration orweathering when there are various degrees of hardnessand/or strength due to different degrees of weathering orchemical alteration. When evaluating strengths, it isimportant to note whether the core or rock fragmentsbreak around, along, or through grains; or along or acrossincipient fractures, bedding, or foliation.

Hardness and especially strength are difficultcharacteristics to assess with field tests. Two field testscan be used; one is a measure of the ability to scratch thesurface of a specimen with a knife, and the other is theresistance to fracturing by a hammer blow. Results fromboth tests should be reported. The diameter and lengthof core or the fragment size will influence the estimationof strength and should be kept in mind when correlatingstrengths. A 5- to 8-inch (130- to 200-mm) length ofN-size core or rock fragment, if available, should be usedfor hardness determinations to preclude erroneouslyreporting point, rather than average hardness, and toevaluate the tendency to break along incipient fracturesand textural or structural features when struck with arock pick. Standards (heavy, moderate, and lighthammer blow) should be calibrated with other geologistsmapping or logging core for a particular project.Descriptors used for rock hardness/strength are shown ontable 4-6.

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Table 4-6.—Rock hardness/strength descriptors

Alpha-numeric

descriptor Descriptor Criteria

H1 Extremely hard

Core, fragment, or exposure cannot bescratched with knife or sharp pick; can onlybe chipped with repeated heavy hammerblows.

H2 Very hard Cannot be scratched with knife or sharppick. Core or fragment breaks withrepeated heavy hammer blows.

H3 Hard Can be scratched with knife or sharp pickwith difficulty (heavy pressure). Heavyhammer blow required to break specimen.

H4 Moderately hard

Can be scratched with knife or sharp pickwith light or moderate pressure. Core orfragment breaks with moderate hammerblow

H5 Moderately soft

Can be grooved 1/16 inch (2 mm) deep byknife or sharp pick with moderate or heavypressure. Core or fragment breaks with lighthammer blow or heavy manual pressure.

H6 Soft Can be grooved or gouged easily by knife orsharp pick with light pressure, can bescratched with fingernail. Breaks with lightto moderate manual pressure.

H7 Very soft Can be readily indented, grooved or gougedwith fingernail, or carved with a knife.Breaks with light manual pressure.

Any bedrock unit softer than H7, very soft, is to be described usingUSBR 5000 consistency descriptors.

Note: Although "sharp pick" is included in these definitions, descrip-tions of ability to be scratched, grooved, or gouged by a knife is thepreferred criteria

A few empirical and quantitative field techniques whichare quick, easy, and inexpensive are available to providestrength estimates. Quantitative strength estimates can

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be obtained from the point load test. A lightweight andportable testing device is used to break a piece of core(with a minimum length at least 1.5 times the diameter)between two loading points. If a new fracture does notrun from one loading point to the other upon completionof the test, or if the points sink into the rock surfacecausing excessive deformation or crushing, the testshould not be recorded. Raw data are given with thereduced data (equations to empirically convert load datato compressive strengths are usually supplied with theequipment). The Schmidt (L) hammer may also be usedfor estimating rock strengths; refer to Field Index Testsin chapter 5. Each of these tests can be used to calibratethe manual index (empirical) properties described intable 4-6, and a range of compressive strengths can beassigned. Depending on the scope of the study andstructure being considered, laboratory testing may berequired and used to confirm the field test data.

Discontinuities.—Describe all discontinuities such asjoints, fractures, shear/faults, and shear/fault zones, andsignificant contacts. These descriptions should include allobservable characteristics such as orientation, spacing,continuity, openness, surface conditions, and fillings.Appropriate terminology, descriptive criteria anddescriptors, and examples pertaining to discontinuitiesare presented in chapter 5.

Contacts.—Contacts between various rock units or rock/soil units must be described. In addition to providing ageologic classification, describe the engineeringcharacteristics such as the planarity or irregularity andother descriptors used for discontinuities.

Descriptors applicable to the geologic classification of con-tacts are:

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• Conformable• Unconformable• Welded—contact between two lithologic units, one

of which is igneous, that has not been disruptedtectonically

• Concordant (intrusive rocks)• Discordant (intrusive rocks)

Descriptors pertinent to engineering classification of con-tacts are:

• Jointed—contact not welded, cemented, orhealed—a fracture

• Intact• Healed (by secondary process)• Sharp• Gradational• Sheared• Altered (baked or mineralized)• Solutioned

If jointed or sheared, additional discontinuity descriptorssuch as thickness of fillings, openness, moisture, androughness, should be provided also (see discontinuitydescriptors in chapter 5).

Permeability Data.—Permeability (hydraulicconductivity) is an important physical characteristic thatmust be described. Suggested methods for testing,terminology, and descriptors are available in the EarthManual and Ground Water Manual. Numerical valuesfor hydraulic conductivity (K) can be determined usingany of several methods. These values may be shown ondrill hole logs. For narrative discussions or summarydescriptions, the numerical value and descriptors may beused. Descriptors to be used—such as low,

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moderate—are those shown on figure 4-7. Whetherpermeability is primary (through intact rock) orsecondary (through fractures) should be indicated.

Example Descriptions

The examples which follow are in representative formatsfor describing bedrock in the physical conditions columnof drill hole logs and on a legend, explanation, and notesdrawing.

Core Log Narrative

Several examples of descriptions for core logs arepresented. These examples illustrate format and the useof the lithologic descriptors but do not include adescription of discontinuities.

Log with Alphanumeric Descriptors and EnglishUnits.—

. . . 12.6 to 103.6: Amphibolite Schist (JKam). Fine-grained (0.5 to 1 mm); subschistose to massive; greenish-black (5G 2/1) with numerous blebs andstringers of white calcite to 0.02 ft thick withsolution pits and vugs to 0.03 ft, mostly 0.01 ft,aligned subparallel to foliation; very thinly foliated,foliation dips 65E to 85E, steepening with depth.Moderately to slightly weathered (W4), iron oxidestaining on all discontinuities. Hard (H3), can bescratched with knife with heavy pressure, corebreaks parallel to foliation with heavy hammerblow. Slightly fractured (FD3),. . ..

RO

CK

S

87 Figure 4-7.—Permeability conversion chart.

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Log with Alphanumeric Descriptors Using MetricUnits.—

103.60 to 183.22: Sandstone (TUsa). Ferruginousquartzose sandstone. Medium-grained (0.25 to0.5 mm), well sorted, subrounded to rounded quartzgrains are well cemented by silica; hematite occursas minor cement agent and as thin coating ongrains. Moderate reddish-brown (10R 6/6).Moderately bedded, beds 250 to 310 mm thick,bedding dips 15E to 29E, averages 18E. Slightlyweathered. Hard, cannot be scratched with knife,core breaks with heavy hammer blow across beddingand through grains. Moderately fractured. Corerecovered. . ..

172.41-176.30: Claystone (TUc2). Calcareousmontmorillonitic clay with 20 percent subangular,fine sand-size quartz fragments. Strong reactionwith hydrochloric acid (HCl), grayish pink (5R 8/2).Moderately to rapidly slaking when dropped inwater. Very thinly bedded to laminated with bedthickness from 8 to 20 mm. Very intenselyweathered. Very soft, can be gouged with fingernail,friable, core breaks with manual pressure, smallerfragments can be crushed with fingers. . . Uppercontact is parallel to bedding, conformable,gradational, and intact; lower contact isunconformable, sharp and jointed but tight; dips 35E. . ..

Legend

The example which follows could be typical of a rock unitdescription on a general legend, explanation, and notedrawing. The object is to describe as many physicalproperties as possible which apply to the entire rock unitat the site. If individual subunits can be differentiated,they could be assigned corresponding symbols and

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described below the undifferentiated description. Thosecharacteristics in the subunits which are similar or areincluded in the undifferentiated unit do not need to berepeated for each subunit.

Amphibolite Schist - Undifferentiated.—Mineralogyvariable but generally consists of greater than 30 percentamphibole. Contains varying percentages of feldspar,quartz, and epidote in numerous, thin, white and lightgreen (5G 7/4), discontinuous stringers and blebs.Texture ranges from fine grained and schistose tomedium grained and subschistose. Overall, color rangesfrom greenish black (5G 2/1) to olive black (5Y 2/1).Thinly foliated; foliation dips steeply 75E to 85E NE.Weathering is variable but generally moderately wea-thered to depths of 75 ft, slightly weathered to 120 ft, andfresh below. Where oxidized, moderate reddish-brown(10R 4/6), frequently with dendritic patterns of oxides ondiscontinuities. Hard, fresh rock can be scratched slightlywith heavy knife pressure; fresh N-size core breaks alongfoliation with moderate to heavy hammer blow. Foliationjoints are variably spaced and discontinuous, spaced moreclosely where weathered. Joint sets are prominent butdiscontinuous. (Joint sets are identified in thespecifications paragraphs). Commonly altered 0.1 to 6 ftalong contacts of dikes and larger shears with epidote andquartz ("altered amphibolite" on logs of exploration).When altered, harder than amphibolite. Based on drillhole permeability testing, hydraulic conductivity is verylow to low, with values ranging from 0.09 to 130 feet peryear (ft/yr) averages 1.5 ft/yr in slightly weathered andfresh rock.

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BIBLIOGRAPHY

[1] Travis, Russell B., "Classification of Rocks," v. 50,No. 1, Quarterly of the Colorado School of Mines,Golden, CO, January 1955.

[2] Fisher, R.V., "Rocks Composed of Volcanic Frag-ments: Earth Science Review, v. I, pp. 287-298, 1966.

[3] Williams, H., and McBirney, A., Volcanology,published by Freeman, Cooper and Company,San Francisco, CA. 391 pp., 1979.

[4] Compton, Robert R., Geology in the Field, publishedby John Wiley & Sons, Inc., New York, NY, 1985.

[5] Geological Society of America Rock Color Chart,8th printing, 1995.