PU

Rock Classification and Field Methods – Civil 200 Notes

  • Context and scope

    • Civil 200: last lecture on rock, with soil to follow for the rest of the semester.

    • Focus: rock classification, field methods, and how classification informs engineering design.

    • Three PowerPoints will cover: rock classification system (NZ-specific), field methods for classification, and demonstrations. If time allows, introduction to soils.

    • Reference material: New Zealand Geotechnical Society (NZGS) field classification and description of soil and rock (2005). Document still current; NZGS updating for a new version in a few years.

    • NZGS is part of Engineering New Zealand (governs engineers in NZ). Members include NZ Society of Geotechnical Engineers, Structural Engineering Society, NZ Society for Earthquake Engineers, etc.

  • Key concepts: intact rock vs rock mass

    • Intact rock: a single, continuous material, potentially kilometers in size, with consistent properties; strongest/most robust form on average.

    • Rock mass: rock broken by defects (fractures, joints, weathering, faults, etc.) and often behaves differently from intact rock.

    • Strength of rock mass is often controlled more by discontinuities than by the intact rock itself; analogous testing (e.g., unconfined compression) may yield high values for intact rock, but real behavior is governed by defects.

  • Weathering and the rock-vs-soil spectrum

    • Weathering is the first descriptor used to characterize a rock mass; bands from unweathered fresh rock to slightly weathered, moderately weathered, highly weathered, to residual soil (and then to nothing/soil-like behavior).

    • There is a gray zone between rock and soil; highly weathered rock may behave like rock in some engineering contexts and like soil in others. Design decisions depend on application and may shift along this spectrum.

    • Borehole logs begin with weathering descriptions (e.g., unweathered, slightly weathered, moderately weathered, highly weathered).

  • Field versus lab descriptions

    • Field notes synthesize weathering, color, fabric, and rock name; then sample and test in the lab when possible.

    • Lab tests commonly used for intact rock: unconfined compression test; results reported as unconfined compressive strength, typically in MPa.

    • When lab access is limited or for field decisions, qualitative field tests are used (hammer test, pocket-knife scratch test, indentation with hammer).

    • For soils, unconfined compression testing is not feasible due to particle dispersion; confined compression tests are typically used later in the course.

  • Rock strength testing overview

    • Unconfined compression test: requires a intact rock specimen that can be cored into a cylinder; yields a strength value which helps classify rock (

    • Example threshold:
      ext{If } ext{UCS} > 250 ext{ MPa} o ext{extremely strong}.

      Field tests:

    • Hammer test (qualitative): if rock chips on impact, suggests weaker or fractured; if no chips, suggests stronger.

    • Pocket-knife scratch test: scraping ability indicates weakness progression (easier to scratch = weaker).

    • Lab test vs field test trade-offs discussed, including the practicality of rock mass vs intact rock behavior.

  • Rock mass discontinuities: what to describe and why it matters

    • Discontinuities describe defects: orientation, spacing, roughness, aperture, and infill.

    • Orientation (3D): how a fracture is oriented in space; described using strike and dip.

    • Strike and dip concepts

    • Strike: orientation of a fracture’s line of intersection with a horizontal plane; represented as a compass bearing (0°/360° = north).

    • Dip: angle between the fracture plane and the horizontal; ranges from 0° to 90° and is always perpendicular to strike.

    • Practical description: angle/direction relative to north is shown on maps; dip direction and angle plotted with a tick mark on the map.

    • Visualization: a dipping plane intersects the horizontal plane; strike line is drawn along this intersection; dip is the angle the plane makes with horizontal, perpendicular to strike.

    • Map view vs cross-section view

    • In maps, strikes and dips are depicted with a strike line and a dip tick; north arrow is included; dip direction indicated.

    • Cross-sections show orientation in 3D, useful for understanding subsurface geometry (e.g., folds, inclined layers).

    • Bedrock layering, bedding, foliation, and planes of weakness

    • Bedding: layering in sedimentary rocks; bedding thickness ranges from millimeters to meters; thin beds imply potential planes of weakness.

    • Lamination and bedding orientation: crucial for assessing potential failure planes when cutting or excavating.

    • Metamorphic rocks: foliation (layering due to metamorphism) acts as planes of weakness; orientation and thickness of foliation matter for design.

  • Discontinuity spacing and roughness

    • Spacing: distance between discontinuities, measured perpendicular to joint direction.

    • Examples from a table: 50 mm spacing = closely spaced; >2 m spacing = widely spaced.

    • Roughness categories (nine total; qualitative):

    • Smooth/ polished (low friction) vs rough/high friction; roughness affects shear strength along discontinuities.

    • Terms used include stacked, undulating, rough; and smooth as a baby’s bottom (colloquial for polished surfaces).

    • Aperture and infill

    • Aperture describes opening width: closed, open, or gapped (incl. qualitative sizing).

    • Example widths: less than 2 ext{ mm} (very tight) and on the order of 10 ext{-}20 ext{ mm} (moderately wide).

    • The material inside the aperture (infill) matters greatly for strength; possibilities include:

      • Soil-like clay (e.g., bentonite, a clay with high plasticity that can create slip planes).

      • Newly formed mineral deposits that can cement a seam and increase strength.

      • Extremely fractured/disrupted material due to faulting.

    • Bentonite example cited as a clay with a consistency reminiscent of “baby poop,” highlighting potential slip planes.

  • Example rock classifications (field practice)

    • Example 1: metamorphic rock (unweathered, grey, foliated) with foliation dipping 20°; strong rock with shear zones along foliation and widely spaced joints; interpretation: from the south coast of the South Island.

    • Example 2: highly weathered sandstone (yellow-brown), homogeneous fabric; weak strength; closely spaced and narrow discontinuities; geologic information: Greywacke formation (bedrock of southern ranges); in Auckland each borehole log would flag the potential for soil-like behavior at surface with underlying Greywacke.

  • Borehole logs and site investigation practice

    • Boreholes are used to identify material with depth, from weathered surface to deeper, more intact rock.

    • Logs describe color, weathering, fabric, and rock mass properties; they may include terms like:

    • Moderately weathered dark reddish lava breccia; Mount Pleasant Formation.

    • Highly to moderately weathered basalt with strong porphyry and olivine; again Mount Pleasant formation (likely from South Island).

    • Stained orange and rough fractures; incipient fractures at 30°; undulating, very rough, and infilled with clay; interpreted as natural fractures.

    • Older borehole in meters: silt, siltstone, sandstone; description: moderately weathered grey laminated siltstone; demonstrates soil transitioning into sedimentary rock (weathering progression from soil to rock and back toward soil-like behavior).

    • Key practice: distinguish natural fractures from those induced by coring by inspecting weathering around fractures; weathering around a fracture suggests a natural origin.

    • Borehole logs help identify layering and discontinuities to inform engineering design and testing strategies.

    • The field logs shown illustrate detailed observations, including bedding orientation, fracture angles, roughness, and infill, to guide assessment of potential weakening planes.

  • Practical implications for engineers

    • Early and accurate rock classification informs design decisions (whether to treat as rock or soil).

    • Discontinuities and their properties (orientation, spacing, roughness, aperture, and infill) often dominate rock mass behavior and must be incorporated into analyses.

    • Bedding and foliation orientation influence potential failure planes; orientation data should be captured for 3D interpretation and design planning.

    • Field classification is a collaboration between geologists and engineers; engineers must be able to interpret field logs and understand their implications for stability, excavation, and foundational design.

  • References and further reading

    • NZGS field classification and description of soil and rock (2005): primary reference for field descriptions and classification framework.

    • ROC Minerals Index and related resources for broader mineralogical context.

  • Transition to soils

    • With this foundation on rock classification and field methods, the course will transition to soils and their engineering implications in the next lectures.

  • Summary of key takeaways

    • Distinguish between intact rock and rock mass; real-world strength is controlled by discontinuities.

    • Weathering, color, fabric, and rock name constitute the core field description order; weathering dictates whether the rock mass behaves more like rock or soil.

    • Bedding and foliation create planes of weakness that influence stability and failure mechanisms.

    • Discontinuity properties (orientation/strike-dip, spacing, roughness, aperture, and infill) are critical for engineering design and methods for evaluating rock mass strength.

    • Field and lab testing complement each other: unconfined compression tests provide UCS values for intact rock; field tests (hammer, knife scratch) offer quick qualitative assessments; soils require different testing approaches (e.g., confined compression).

    • Borehole logs are essential for mapping stratigraphy, weathering, and discontinuities to guide site-specific design decisions.

  • Key definitions and formulas

    • Strike: orientation of the line of intersection between a plane and a horizontal plane; used to describe joint orientation on maps; bearing is usually given in degrees with 0° = north.

    • Dip: angle between the plane and the horizontal, ranging from 0° to 90°; always perpendicular to strike.

    • Discontinuity spacing: distance between joints, measured perpendicular to joint direction; examples: ≈50 mm (closely spaced) vs >2 m (widely spaced).

    • Aperture: opening width of a discontinuity; described as closed (<2 mm), narrow, open (larger openings); the material inside (infill) may be clay, cemented minerals, or very fractured material affecting shear strength.

    • Unconfined compressive strength (UCS): the axial compressive stress at which a rock specimen fails in unconfined conditions; used to classify rock strength; example threshold: ext{UCS} > 250 ext{ MPa}
      ightarrow ext{extremely strong}.