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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:
- Bates, R.L. and Jackson, J.A., EDS., Glossary of Geology, American Geological Institute, Falls
Church, Va, 1980.
- I.S.R.M., Suggested Methods for the Quantitative Description of Discontinuities in Rock Masses.
Summary of Terms for Describing Rock Cores
Grain Size
| Term | Grain Size |
| Fine-grained | Not visible to barely visible with naked eye |
| Medium-grained | Barely to easily visible with naked eye; up to 1/8" (3 mm) |
| Coarse-grained | > 1/8" (3 mm) |
Continuity
| Term | Length of Drill Core Stem
Pieces |
| Sound | > 8" (200 mm) |
| Slightly Fractured | 4"-8" (100-200 mm) |
| Moderately Fractured | 1"-4" (25-100 mm) |
| Extremely Fractured | < 1" (25 mm) |
Discontinuity Description
| Fracture Spacing (Joints, Faults, Other Fractures) | Bedding Spacing (May Include Foliation or Banding) |
| Description | Spacing | Description | Spacing |
| Extremely close | < 3/4 in (<19 mm) | Laminated | < 1/2 in (12 mm) |
| Very close | 3/4 in - 2-1/2 in (19 - 60 mm) | Very thin | 1/2 in - 2 in (12 - 50 mm) |
| Close | 2-1/2 in - 8 in (60 - 200 mm) | Thin | 2 in - 1 ft (50 - 300 mm) |
| Moderate | 8 in - 2 ft (200 - 600 mm) | Medium | 1 ft - 3 ft 300 - 900 mm) |
| Wide | 2 ft - 6 ft (600 mm - 2.0 m) | Thick | 3 ft - 10 ft (900 mm - 3 m) |
| Very wide | 6 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
| Term | Description | Grade |
| Unweathered | No visible sign of rock material weathering, perhaps slight discoloration on major discontinuity surfaces | I |
| Slightly weathered | Discoloration 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 weathered | Less 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 weathered | More 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 weathered | All rock material is decomposed and/or disintegrated to soil. The original mass structure is still largely intact. | V |
| Residual soil | All 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 transported | VI |
| The terms and description below help to define some of the descriptions used in the above table. |
| Fresh | No visible sign of weathering of the rock material. |
| Discolored | The 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 |
| Decomposed | The 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. |
| Disintegrated | The 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
| Grade | Description | Field Identification | Uniaxial Compressive Strength, PSI (MPa) |
| R0 | Extremely weak | Indented by thumbnail | 40-150 (0.3-1) |
| R1 | Very weak | Crumbles under firm blows with point of geological hammer, can be peeled by a pocket knife | 150-700 (1-5) |
| R2 | Weak rock | Can be peeled by a pocket knife with difficulty, shallow indentations made by firm blow with point of geological hammer | 700-4000 (5-30) |
| R3 | Medium strong | Cannot be scraped or peeled with a pocket knife, specimen can be fractured with single firm blow of geological hammer | 4000-7000 (30-50) |
| R4 | Strong rock | Specimen requires more than one blow of geological hammer to fracture it | 7000-15,000 (50-100) |
| R5 | Very strong | Specimen requires many blows of geological hammer to fracture it | 15,000-36,000 (100-250) |
| R6 | Extremely strong | Specimen 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. |
| Grade | Description | Field Identification | Uniaxial Compressive Strength, KSF (KPa) |
| S1 | Very soft clay | Easily penetrated several inches (cm) by fist | 0.5 (25) |
| S2 | Soft clay | Easily penetrated several inches (cm) by thumb | 0.5-1.0 (25-50) |
| S3 | Firm clay | Can be penetrated several inches (cm) by thumb with moderate effort | 1.0-2.0 (50-100) |
| S4 | Stiff clay | Readily indented by thumb but penetrated only with great effort | 2.0-5.0 (100-250) |
| S5 | Very stiff clay | Readily indented by thumbnail | 5.0-10.0 (250-500) |
| S6 | Hard clay | Indented 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 joints | 4 |
| B. Rough or irregular, undulating | 3 |
| C. Smooth, undulating | 2 |
| D. Slickensided, undulating | 1.5 |
| E. Rough or irregular, planar | 1.5 |
| F. Smooth, planar | 1.0 |
| G. Slickensided, planar | 0.5 |
| No Rock Wall Contact Along Discontinuity Surface |
| H. Zone containing clay minerals thick enough to prevent rock wall contact | 1.0 (nominal) |
| I. Sandy, gravelly, or crushed zone thick enough to prevent rock wall contact | 1.0 (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 epidote | 0.75 |
| B. Unaltered joint walls, surface staining only | 1.0 |
| 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 |
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Contact
Jesus Rohena
Office of Bridges and Structures
202-366-4593
E-mail Jesus
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