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Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Appendix B - Description of Rock Core Samples

Glossary of Terms Used in Rock Core Boring Logs

Arenaceous: A sedimentary rock descriptive term that signifies the rock consists in part of sand-size particles.

Argillaceous: A sedimentary rock descriptive term that signifies the rock is comprised of a large percentage (but less than 50%) of clay.

Bedding: A surface, generally planar or nearly planar, that visibly separates each successive layer of stratified rock from the preceding or following layer.

Swirly bedding: Tightly curved, wavy pattern throughout texture of rock.

Color-banding: Shades of alternating color in very thin bands parallel to the bedding. Differing lithology or grain size in the various bands is possible.

Discontinuity: A collective term for most types of joints, bedding planes, schistosity planes, shear and fault zones.

Fault: A fracture or fracture zone along which there has been recognizable displacement.

Fissile: Exhibiting the property of easily splitting into very thin layers parallel to the bedding.

Friable: Easily crumbled, as would be the case with rock that is poorly cemented.

Grain size:
Fine-grained (rock): Grain size not visible to just barely visible with naked eye.
Medium-grained (rock): Grain size barely to easily visible with the naked eye; up to 1/8 in. (3 mm).
Coarse-grained (rock): Grain size 1/8 in. (3 mm) or greater.

Joints: A break of geological origin in the continuity of a rock mass along which there has been no visible displacement.
Horizontal: Natural breaks inclined to a horizontal plane from 0° to 5°.
Low angle: Natural breaks inclined to a horizontal plane from 5° to 35°.
Moderately dipping: Natural breaks inclined to a horizontal plane from 35° to 55°.
High angle: Natural breaks inclined to a horizontal plane from 55° to 85°.
Vertical: Natural breaks inclined to a horizontal plane from 85° to 90°.

Mottling: Irregular color patches of limited extent.

Oolitic: Composed of smooth, rounded granules.

Parting: Natural break in the rock caused by change in lithology or grain size, parallel to the bedding. Unlike joints, which can be limited in extent or trend by the thickness of the formation, partings are usually persistent in every direction parallel to the bedding. Often marked by a very thin bed or seam of soft rock or mineral. Stylolitic partings are rough, irregular, and faced with argillaceous materials (see Stylolite).

Pit: Cavity up to 1/4 in. (6mm) size.

Shear: A localized expression of strain resulting from stresses that cause or tend to cause slippage along a plane at the contact of two contiguous parts of a body.

Slickensides: Smooth, highly polished argillaceous facing on a shear. Trace slickensides are not highly polished, but marked by some sign of small movement, such as very small polished areas and/or parallel grooves and striations on a joint face.

Stylolite: A surface, usually in homogeneous carbonate rocks, marked by an irregular and interlocking penetration of the two sides; in cross section it resembles a suture; the seam is characterized by a concentration of clay, carbon, or iron oxides.

Surface Planarity:
Planar - A flat surface.
Stepped - A surface with asperities or steps. The height of the asperity should be estimated or measured.
Wavy - A moderate undulating surface; curved, smoothly uneven.

Surface Roughness:
Very Rough - Near vertical steps and ridges occur on the discontinuity surface.
Rough - Some ridges and side-angle steps are evident; asperities are clearly visible; and discontinuity surface feels very abrasive.
Slightly Rough - Asperities on the discontinuity surface are distinguishable and can be felt.
Smooth - Surface appears smooth and feels so to the touch.
Slickensided - Visual evidence of polishing exists.

Trace: Amount less than 10%; not common.

Vug: Cavity larger than a pit; from 1/4 (6 mm) to 2 in. (50 mm) in size.

Many of the terms above were defined in the following two references:

  1. Bates, R.L. and Jackson, J.A., EDS., Glossary of Geology, American Geological Institute, Falls Church, Va, 1980.
  2. I.S.R.M., Suggested Methods for the Quantitative Description of Discontinuities in Rock Masses.

Summary of Terms for Describing Rock Cores

Grain Size
TermGrain Size
Fine-grainedNot visible to barely visible with naked eye
Medium-grainedBarely to easily visible with naked eye; up to 1/8" (3 mm)
Coarse-grained> 1/8" (3 mm)
Continuity
TermLength of Drill Core Stem Pieces
Sound> 8" (200 mm)
Slightly Fractured4"-8" (100-200 mm)
Moderately Fractured1"-4" (25-100 mm)
Extremely Fractured< 1" (25 mm)
Discontinuity Description
Fracture Spacing (Joints, Faults, Other Fractures)Bedding Spacing (May Include Foliation or Banding)
DescriptionSpacingDescriptionSpacing
Extremely close< 3/4 in (<19 mm)Laminated< 1/2 in (12 mm)
Very close3/4 in - 2-1/2 in (19 - 60 mm)Very thin1/2 in - 2 in (12 - 50 mm)
Close2-1/2 in - 8 in (60 - 200 mm)Thin2 in - 1 ft (50 - 300 mm)
Moderate8 in - 2 ft (200 - 600 mm)Medium1 ft - 3 ft 300 - 900 mm)
Wide2 ft - 6 ft (600 mm - 2.0 m)Thick3 ft - 10 ft (900 mm - 3 m)
Very wide6 ft - 20 ft (2.0 - 6 m)Massive> 10 ft (3 m)
Discontinuity Orientation (Angle): Measure the angle of discontinuity relative to a plane perpendicular to the longitudinal axis of the core. (For most cases, the core axis is vertical; therefore, the plane perpendicular to the core axis is horizontal.) Record orientation (angle) on log. For example, a horizontal bedding plane would have a 0 degree angle.
Weathering
TermDescriptionGrade
UnweatheredNo visible sign of rock material weathering, perhaps slight discoloration on major discontinuity surfacesI
Slightly weatheredDiscoloration indicates weathering of rock material and discontinuity surfaces. All the rock material may be discolored by weathering and may be somewhat weaker externally than in its fresh condition.II
Moderately weatheredLess than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a continuous framework or as corestones.III
Highly weatheredMore than half of the rock material is decomposed and/or disintegrated to a soil. Fresh or discolored rock is present either as a discontinuous framework or as corestones.IV
Completely weatheredAll rock material is decomposed and/or disintegrated to soil. The original mass structure is still largely intact.V
Residual soilAll rock material is converted to soil. The mass structure and material fabric are destroyed. There is a large change in volume, but the soil has not been significantly transportedVI
The terms and description below help to define some of the descriptions used in the above table.
FreshNo visible sign of weathering of the rock material.
DiscoloredThe color of the original fresh rock material is changed. The degree of change from the original color should be indicated. If the color change is confined to particular mineral constituents, this should be mentioned
DecomposedThe rock is weathered to the condition of a soil in which the original material fabric is still intact, but some or all of the mineral grains are decomposed.
DisintegratedThe rock is weathered to the condition of a soil in which the original fabric is still intact. The rock is friable, but the mineral grains are not decomposed.
Strength or Hardness
GradeDescriptionField IdentificationUniaxial Compressive Strength, PSI (MPa)
R0Extremely weakIndented by thumbnail40-150 (0.3-1)
R1Very weakCrumbles under firm blows with point of geological hammer, can be peeled by a pocket knife150-700 (1-5)
R2Weak rockCan be peeled by a pocket knife with difficulty, shallow indentations made by firm blow with point of geological hammer700-4000 (5-30)
R3Medium strongCannot be scraped or peeled with a pocket knife, specimen can be fractured with single firm blow of geological hammer4000-7000 (30-50)
R4Strong rockSpecimen requires more than one blow of geological hammer to fracture it7000-15,000 (50-100)
R5Very strongSpecimen requires many blows of geological hammer to fracture it15,000-36,000 (100-250)
R6Extremely strongSpecimen can only be chipped with geological hammer>36,000 (>250)
Assess the strength of any filling materials along discontinuity surfaces in accordance with the following descriptions and grades.
GradeDescriptionField IdentificationUniaxial Compressive Strength, KSF (KPa)
S1Very soft clayEasily penetrated several inches (cm) by fist0.5 (25)
S2Soft clayEasily penetrated several inches (cm) by thumb0.5-1.0 (25-50)
S3Firm clayCan be penetrated several inches (cm) by thumb with moderate effort1.0-2.0 (50-100)
S4Stiff clayReadily indented by thumb but penetrated only with great effort2.0-5.0 (100-250)
S5Very stiff clayReadily indented by thumbnail5.0-10.0 (250-500)
S6Hard clayIndented with difficulty by thumbnail>10.0 (>500)
  • Grades S1 to S6 apply to cohesive soils for example clays, silty clays, and combinations of silts and clays with sand, generally slow draining. If non-cohesive fillings are identified, qualitatively identify, e.g., fine sand.
  • Discontinuity wall strength will generally be characterized by grades R0-R6 (rock) while S1-S6 (clay) will generally apply to filled discontinuities.
Joint Roughness (Jr) Number
 Jr
Rock Wall Contact Along Discontinuity Surface
A. Discontinuous joints4
B. Rough or irregular, undulating3
C. Smooth, undulating2
D. Slickensided, undulating1.5
E. Rough or irregular, planar1.5
F. Smooth, planar1.0
G. Slickensided, planar0.5
No Rock Wall Contact Along Discontinuity Surface
H. Zone containing clay minerals thick enough to prevent rock wall contact1.0 (nominal)
I. Sandy, gravelly, or crushed zone thick enough to prevent rock wall contact1.0 (nominal)
Joint Alternation (Ja) Number
 Ja
Rock Wall Contact, or Coating <1/8 in. (3 mm) Thick
A. Tightly healed, hard, non-softening, impermeable filling. i.e., quartz or epidote0.75
B. Unaltered joint walls, surface staining only1.0
C. Slightly altered joint walls. Non-softening mineral coatings, sandy particles, clay-free disintegrated rock etc.2.0
D. Silty- or sandy-clay coatings, small clay-fraction (non-softening)3.0
E. Softening or low friction clay mineral coatings, i.e., kaolinite, mica. Also chlorite, talc, gypsum, graphite, etc., and small quantities of swelling clays.4.0
No Rock Wall Contact, Continuous Coatings <1/4 in (5 mm) Thick
F. Sandy particles, clay-free disintegrated rock etc.4.0
G. Strongly over-consolidation, softening, clay mineral fillings. (Continuous, <5 mm in thickness)6.0
H. Medium or low over-consolidation, softening, clay mineral fillings. (Continuous, <5 mm in thickness)8.0
J. Swelling clay fillings, i.e., montmorillonite (Continuous, <5 mm in thickness). Value of Ja depends on percent of swelling clay-size particles and access to water, etc.8.0-12.0
No Rock Wall Contact, Continuous Coatings >;1/4 in (5 mm) Thick
K., L., M. Crushed rock and clay (see G., H., J., for description of clay condition)6.0, 8.0 or 8.0-12.0
N. Zones or bands of silty- or sandy clay, small clay fraction (nonsoftening)5.0
O., P., R. Thick continuous zones or bands or clay (see G., H., J. for description of clay condition)10.0, 13.0 or 13.0-20.0

Sample project form

Sample project form

Sample project form

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Updated: 06/19/2013
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000