Rock Classification and Field Methods (Notes)

Rock Classification and Field Methods

• Purpose: Move from understanding rocks to how to classify rock masses for engineering design; in NZ, classify to infer engineering behavior from descriptions seen in boreholes and on-site investigations.
• Context: Last lecture on rock; next: soil. We’ll cover rock classification, field methods, and then begin soils if time allows.
• Reference material: NZ Geotechnical Society field classification and description of soil and rock (2005). Post to Canvas; current but being updated by NZGHS; new version in a few years, changes expected to be moderate.
• Governing bodies and professional context:
  • New Zealand Geotechnical Society (NZGS) governs geotechnical engineers in NZ.
  • Engineering New Zealand administers accreditation for degrees and related professional paths.
  • Related societies: NZ Society of Geotechnical Engineers (NZSGS?); NZSCA (New Zealand Society for Earthquake Engineers); other specializations (water, transportation, etc.).
• Practical goal for engineers in the field:
  • If you encounter five meters of highly fractured sandstone in a borehole, you should be able to identify the situation and infer implications for design.
  • You may work with a geologist on a drill rig to classify material; the classification informs expectations of engineering behavior.

Intact rock vs rock mass

• Intact rock: idealized, continuous material with uniform properties, potentially spanning kilometers; strongest material on average; robust and resistant to breakdown over the timescale of interest.
• Real-world condition: rock mass often dominates behavior due to defects.
  • Defects include fractures, joints, weathering, seams, and other discontinuities.
  • Even strong intact rock can behave weakly if the rock mass contains pervasive discontinuities.
• Engineering implication: consider rock mass behavior (discontinuities, weathering) rather than just intact strength when designing foundations, slopes, tunnels, etc.

Four key aspects of rock mass description (order of description)

• Weathering
• Color
• Fabric
• Rock name
• Rationale: Weathering, color, and fabric often carry more engineering significance than rock name alone; descriptions aim to summarize rock mass characteristics at the site.
• Note: Weathering status is used to separate rock-like behavior from soil-like behavior on a spectrum; transitions are not binary (there is a gray zone between rock and soil).

Weathering descriptions and the rock vs soil spectrum

• NZ skyline table (weathering scale) from fresh rock down to residual soil: unweathered fresh rock → slightly weathered → moderately weathered → highly weathered → (rock essentially gone) → residual soil.
• Practical use: determine whether the mass will behave like rock or like soil; high weathering can push behavior toward soil-like, but exceptions exist (some highly weathered rocks still behave like rock due to fabric or bedding).
• Field note: borehole descriptions typically begin with weathering status (e.g., unweathered, slightly weathered, moderately weathered, highly weathered).
• Important nuance: if weathering is very high, rock may behave as soil; if weathering is moderate, behavior may still be rock-like; engineers must adjust design accordingly.

Color (qualitative indicators)

• Color reflects mineral composition and weathering effects.
• Examples and interpretations:
  • Pinkish color often indicates plagioclase presence (geologist cue; not required to memorize exact mineral naming for engineering).
  • Red/orange stains can indicate iron oxidation and weathering processes within joints.
  • Darker colors often indicate moisture or water flow through the rock; e.g., a joint with ongoing water flow may appear darker.
• Practical use: color helps geologists infer mineral content, weathering state, and moisture condition at a glance; field charts exist to match color with standard categories.
• Qualitative process: color categories include light vs dark plus hue (pink, red, yellow, brown, etc.).

Fabric and bedding

• Fabric: arrangement and size of mineral particles within the rock.
  • Fine fabric: particles < 25 mm (roughly 25 mm or smaller).
  • Coarse fabric: larger grains influence stiffness and strength.
  • Massive: no observable fabric (no preferred alignment).
• Bedding (for sedimentary rocks): alignment and thickness of beds/layers.
  • Bed thickness varies from very thin (lamination, a few millimeters) to thick (layers tens of centimeters; examples: 5–6 cm in the figure; 20–60 mm occasionally described as “thinly bedded”).
  • Very thick beds can exceed 2 m (paleo-dunes, etc.).
  • Bedding orientation matters: horizontal bedding (strike ~0°) is easier to cut; dipping bedding can create planes of weakness along which failure is more likely if cut obliquely.
• Importance: bedding orientation and fabric affect anisotropy in strength and failure planes; engineers must note bedding planes and their orientations to anticipate potential planes of weakness.

Metamorphic rocks and foliation

• Metamorphic rocks can develop foliation due to differential stress (crystal flattening under long-term vertical compression).
  • This creates planes of weakness parallel to the foliation direction.
  • Orientation and thickness of foliation are described similarly to bedding.
• Engineering takeaway: treat foliation as a potential weakness plane; orientation relative to structure and loading is critical for stability assessments.

Qualifying paragraph and rock mass description

• Typical sequence of information in a rock mass description:
  • Weathering status
  • Color
  • Foliation or bedding orientation
  • Rock name (rock type)
  • Additional characteristics: strength indicators, discontinuities, and geologic information (formation, age, etc.)
• The key point for engineers: weathering, discontinuities, bedding/fabric, and overall rock mass properties drive stiffness, strength, and shear behavior more than rock name alone.

Rock strength and testing concepts

• Intact rock strength testing (lab/controlled conditions):
  • Unconfined compression test (UCS): specimen is prepared as a cylinder; measure unconfined compressive strength.
  • Thresholds for strength categories are tabulated; a very strong rock typically has UCS > 250 MPa ($ ext{UCS} > 250 ext{ MPa}$).
  • Strength in a laboratory setting may not represent in-situ rock mass, where discontinuities dominate.
• Field or quick qualitative tests (for rapid assessment):
  • Hammer test: strike with a hammer; if rock chips off easily, strength is lower; many blows needed to fracture imply higher strength.
  • Pocket knife test: ability to scrape the rock indicates weaker material; more resistant to scraping indicates stronger rock.
  • Hammer indentation tests and surface hardness tests provide quick ordinal strength indicators.
• In-situ and rock mass testing considerations:
  • For rock mass strength, intact rock strength is a small fraction of the overall rock mass strength due to discontinuities.
  • Discontinuities (joints, faults, fractures) control deformation and failure more than intact strength.
• Realistic lab vs field tests:
  • Unconfined compression tests are straightforward on rocks but not applicable to loose soils; for soils, confined compression tests are more relevant (to be covered later by another instructor).
  • In-situ testing (e.g., point load test, Schmidt hammer) provides field estimates of strength and helps calibrate lab data.
• Notes on equipment and terminology:
  • Point load test: field method showing a small rock specimen loaded at two points to determine a compressive strength estimate.
  • Schmidt hammer: device that measures rebound to infer surface/near-surface strength.

Discontinuities: orientation, spacing, roughness, aperture, and infill

• Discontinuities describe defects in the rock mass that can govern mechanical behavior:
  • Orientation: described in 3D using strike and dip.
  • Strike: the imaginary line of intersection between a bedding/foliation plane and a horizontal plane; described with a compass bearing (e.g., 90° corresponds to east–west orientation depending on dataset).
  • Dip: angle between the plane and the horizontal, between 0° and 90°, always perpendicular to strike; examples: dips of 16°, 20°, 30°.
  • Dip direction: in map view, a tick marks the dip direction and angle; the line’s orientation on a map indicates which way the plane tilts.
  • Spacing: the distance between discontinuities measured perpendicular to the joint direction; typical values: closely spaced (~
  • Roughness: qualitative description of joint surface roughness.
  • Categories include: polished/smooth, slightly rough, rough to very rough, and qualitative “polished as beads” descriptions; nine categories used in practice.
  • Aperture (or opening): width of the opening of a joint; categories: closed, gapped, open.
  • Very small apertures: < 2 mm; moderately wide: ~10–20 mm; larger openings indicate different slipping behavior.
  • Infill: material filling the joint can be:
  • clay-like (e.g., bentonite, clay) which can drastically reduce shear strength along the plane,
  • mineral infill with cementation effects,
  • very fractured and disturbed fill (often a concern in faulted systems).
• Engineering consequences:
  • Discontinuities control shear strength across joints and potential planes of failure.
  • Open apertures with slippery infill reduce inter-block resistance and can dominate deformation behavior.
  • The presence of infill materials like clay (e.g., bentonite) can create large loss of shear strength along a plane, creating potential failure planes.

Discontinuity materials and bentonite example

• Bentonite (a clay with high plasticity) can fill joints and create a slippery, weak plane with potential for significant weakness if present.
• Example phrasing: “bentonite-like clay filling” indicates a high-risk lubrication plane that needs to be addressed in design.
• This type of description demonstrates how the presence of a specific infill material alters the predicted behavior of a joint or fracture in the rock mass.

Example classifications (how to read and interpret logs)

• Example 1: Unweathered, grey, foliated metamorphic rock (schist) described as follows:
  • Rock mass: strong (high UCS in lab tests)
  • Foliation dips 20° below the surface; well-developed; shear zones along foliation; widely jointed spacings.
  • Inference for design: strong rock with directional weakness due to foliation; consider orientation of foliation when sizing excavations or anchors.
• Example 2: Highly weathered sandstone described as yellow-brown, homogeneous with little fabric information; expected to behave like soil or very weak rock.
  • Discontinuities: joints are closely spaced, narrow; potential for slip along close planes, high weathering reduces strength.
  • Inference for design: treat as soil-like; potential for high settlement or shear failure along planes; monitor weathering progression.
• Example 3: Greywacke bedrock with very weathered sandstone layers; description highlights bedded nature and weathering progression from moderately to slightly weathered.
  • Geologic information: formation name (e.g., Mount Pleasant formation in the South Island region) provides regional context.
  • Inference: layer transitions can influence strength and deformation; account for layering and bedding orientation in design.
• Takeaways from these examples:
  • Weathering status, color, and bedding/fabric cues, combined with rock name, yield a practical picture of rock mass behavior.
  • Discontinuities (orientation, spacing, roughness, aperture, infill) provide critical information for predicting strength and stability.
  • The goal is to translate a field observation into a defensible engineering assessment of potential behavior.

Field observations and borehole logging procedures

• Borehole logging workflow:
  • Drill through rock to collect samples; bring samples to lab for testing and log the rock mass as you observe it.
  • Log depth along borehole (e.g., depth increasing from the surface to 8–9 meters below ground surface in example).
  • Describe at each interval: weathering status, color, bedding/fabric, rock type, and discontinuities.
  • Note weathering indicators: staining along fractures suggesting weathering and natural fracture networks.
  • Distinguish natural fractures from those caused by coring or drilling by looking for weathering around fractures and other indicators of in-situ origin.
• Example borehole log interpretation heuristics:
  • If there is weathering around a fracture, it likely represents a natural fracture; lack of weathering around a fracture may indicate a drilling-induced fracture (needs caution).
  • Logs often include a sequence of rock types as depth increases (e.g., silt, siltstone, sandstone), with weathering statuses and discontinuities described for each interval.
• Visualization and interpretation:
  • Logs can be plotted as depth vs description to identify layering, weathering gradients, and preferential fracture orientations with depth.
  • Borehole logs support the pairing of lab results with in-situ conditions to refine design assumptions.
• Field practice context:
  • Logs provide essential data before any excavation or foundation design.
  • In advanced courses or project work (e.g., Foundations Engineering), more detailed interpretation of borehole data is taught and practiced.

Practical takeaways for engineers

• When sampling rock on-site, document weathering, color, fabric, bedding, and rock name, then focus on discontinuities: orientation, spacing, roughness, aperture, and fill.
• Treat rock strength as a function of rock mass rather than solely intact rock strength; discontinuities typically dominate behavior.
• Use a spectrum model for rock vs soil behavior; be prepared to classify a material as rock-like or soil-like depending on the degree of weathering and the presence of discontinuities.
• Field tests (hammer scratch test, pocket knife scratch test, Schmidt hammer, point load in the field) provide rapid, qualitative or semi-quantitative estimates of strength when lab tests are not immediately available.
• Always correlate borehole logs with lab tests to get a cohesive understanding of material properties and to identify potential planes of weakness.
• Understand that NZ-specific guidelines exist to standardize descriptions; refer to NZGS field classification and description guidelines and related resources for consistency in reporting.

References and resources mentioned

• NZ Geotechnical Society field classification and description of soil and rock (2005; current guideline; available on Canvas).
• ROC Minerals Index (supplementary resource for rock types and properties).
• Borehole logs as a foundational data source for site investigation and design.
• For deeper study: Foundations Engineering course materials and lab practices for rock and soil testing.

Transition to soils

• The last portion of the lecture signals the transition from rock classification to soils discussion; soils will be explored in more depth in coming lectures.
• Expect continued emphasis on field methods, classification schemes, and the interplay between rock mass properties and soil-like behavior in engineering design.