Geotechnical Rock Deformation: Mass, Fractures, Folds, and Faults

Overview: Rock Deformation and Engineering Relevance

• Rocks, in engineering contexts, range from beach pebbles to kilometer-sized masses; intact rock can be extremely strong, but real rocks exist as masses with fractures and defects.
• Engineering properties of rock are controlled not just by the intact rock, but by interfaces between pieces and the frictional capacity along those interfaces.
• Weathering and erosion are accelerated by fractures in rock masses; this is a key reason why rock masses, not just single blocks, matter for geotechnical design.
• The course focuses on deformation of rock deposits, including folds and defects (faults), and how these features influence engineering performance.
• Geological hazards, especially faulting, are major geotechnical concerns that shape design and assessment.
• Reference material mentioned: Open Geology textbook (read up to section 99.5).
• Real-world relevance: understanding rock types and deformation helps in planning structures, slopes, reservoirs, tunnels, and other infrastructure; not accounting for defects can lead to failure.
• The instructor emphasizes a practical, site-specific approach: geotechnical engineers must understand rock and soil at the site and how they will behave under loading and environmental conditions.

Rock Mass vs Intact Rock; Fractures and Friction

• Intact rock (e.g., granite, basalt) is extremely strong in isolation, but natural rock masses are fractured and broken by tectonic and other stresses.
• The strength of rock is governed mainly by friction at interfaces between rock blocks, not the axial strength of an intact crystal.
• In regions with active tectonics (e.g., New Zealand), plate interactions (Pacific Plate vs Australian Plate) induce earthquakes, volcanism, stress accumulation, and resultant rock deformation.
• Rock mass properties are influenced by geometric processes of the landscape and weathering processes that convert rock to soil.
• Engineering significance: rock mass behavior is governed by fracture networks and their frictional properties; this controls stability, slope performance, and failure modes.

Deformation Processes Driving Rock Mates

• Deposits deform due to Earth processes, mainly tectonic movement:
  • Compression from converging plates -> potential shortening and buckling.
  • Shearing from sliding plates -> shear deformation.
  • Tension from plates moving apart -> extensional deformation.
• Tectonic movement can be gradual (e.g., ~1 cm/year) or rapid (earthquakes with displacements of meters).
• Other drivers include magmatic intrusion (plutons) causing localized compression around intrusions, and changes in gravity loading (e.g., glaciation/loss of ice).
• Large-scale processes lead to both brittle (faulting) and ductile (folding) deformation depending on depth, temperature, and rock type.
• Weathering accelerates due to fractures, aiding soil formation.

Ductile vs Brittle Deformation

• Ductile behavior occurs under higher temperature and/or slower strain rates and at greater depths:
  • Folding is a classic ductile response with rocks behaving like a viscous solid under sustained stress (Mars bar analogy).
  • If compression continues, layers may deform into wavelike folds rather than fracturing.
• Brittle behavior dominates near surface or under rapid loading:
  • Faulting occurs when rocks are pushed past each other, creating cracks and displacements.
  • Classic example in NZ: Alpine Fault, where the Pacific Plate moves past the Australian Plate along a long, nearly horizontal fault.
• Metaphor and examples used in lecture:
  • Mars bar analogy for ductile deformation (temperature and stress influence folding and thinning in the middle).
  • A biscuit-like analogy for brittle, shear/rupture behavior when rocks fail along planes.
• Key factors controlling ductile folding:
  • Temperature: warmer rocks deform more ductilely.
  • Strain rate: slower deformation promotes ductility; rapid loading favors brittleness.
  • Rock type: granites tend to fold slowly and may deform more ductilely at depth; sedimentary rocks can be more brittle and fracture more easily.
  • Depth: deeper rocks experience higher confining pressures that favor ductile behavior; near surface, brittle failure dominates.
• Folding typically develops at depth and is later exposed by erosion/removal of overburden.

Fold Structures

• Basic features of folds:
  • Hinge: where the fold curves most sharply; the highest curvature point along the fold.
  • Axial plane: the plane that contains the hinge lines of successive layers; represents the main plane of folding.
  • Limbs: the two sides of the fold away from the hinge.
• Simple fold types:
  • Syncline: a trough-like fold open downward; the youngest rocks often at the outside (top in typical cross-section).
  • Anticline: an arch-shaped fold with the oldest rocks at the core; the top resembles a capital A (mnemonic).
  • Monocline: a simple step-like fold with a single bend and no distinct two limbs.
• Complex folds occur when multiple stresses act along different planes, and most folds in the crust are not exposed at the surface due to erosion and glaciation.
• Engineering relevance of folds:
  • The orientation of bedding relative to potential loads affects stability of slopes, foundations, and excavations.
  • Water can migrate along bedding planes; impermeable vs permeable layers influence hydraulic behavior.
  • Anticlines can trap hydrocarbons; synclines can host accumulations because of capillary and permeability contrasts.
• Field examples:
  • Otago region, classic anticlines; Southern Alps show large-scale synclines.
  • Folding alters the perceived bedding orientation at the surface; apparent horizontal layers may actually be tilted due to folding.
• Engineering considerations for folds:
  • Slope stability: if layers are tilted, the stability of slopes changes and learning about weak planes is critical.
  • Water movement: RIDERS along bedding planes controls seepage channels and groundwater flow.
  • Structural planning: dams, tunnels, and other structures must account for the orientation and strength of folds.
• Oil industry relevance: anticlines with impermeable layers beneath can trap oil in adjacent sandstone; exploration targets rely on fold geometry.

Practical Engineering Implications of Folds

• Bedding orientation changes stability and slope geometry; a bed that is originally horizontal may become tilted, altering failure planes and design assumptions.
• Water movement tends to follow weaker or more permeable bedding planes, affecting reservoir behavior and seepage control.
• Reservoirs and dams:
  • The orientation of rock beds relative to the hydrostatic force matters for stability.
  • If beds are oriented perpendicular to the resultant force, the rock may have higher strength in resisting the load.
  • If beds are oriented such that a strong plane aligns with the direction of resultant forces, there can be weakness and potential failure.
• Tunnels and underground excavations:
  • Fractured or folded zones may collapse into excavations if not reinforced.
  • Permeable layers can allow water inflows into tunnels, requiring drainage and waterproofing measures.
  • Orientation of the tunnel intersecting a fold can lead to uneven stress distributions and possible failures.
• Practical design principles (conceptual, not a full method):
  • Prefer locations where bedding and rock mass properties align to resist expected loads.
  • Avoid placing critical structures where the expected resultant force aligns with weak planes or folded zones.
  • Coordinate with geologists to map folds, bedding, and potential water pathways before dam/tunnel construction.

Defects: Planar Defects; Planar Fractures, Joints, and Bedding

• Planar defects include fractures and joints, and can include cleavage in metamorphic rocks.
• Bedding planes in sedimentary rocks create planar weaknesses along which shear or sliding can occur.
• Lethargic boundaries (interfaces between different rock deposits) are often zones of weakness where different lithologies meet (e.g., East Coast Phase Formation vs volcanic deposits in Auckland).
• Joints are fractures with no displacement; they occur as planar cracks and can be filled with air, water, or infill material.
• Joints characteristics:
  • Surfaces range from smooth to very rough, depending on stress history and rock type.
  • Multiple joint sets exist, often reflecting different stress histories.
  • Joints can form due to tectonic stresses, cooling, unloading, changes in groundwater pressure, or human activities (e.g., fracking).
• Fracking and fluid-induced joint opening:
  • Injecting water (or other fluids) can create or expand joints, increasing permeability and potentially influencing rock stability.
• Unloading (release of overburden) and unloading-related joint opening can occur when overlying material is removed, causing the rock to pop or crack.
• Cooling joints form in igneous rocks as they crystallize and contract; classic examples include columnar basalts (e.g., Giant's Causeway; Devil's Causeway; Waitomo caves in NZ).
• Joints occur in sets reflecting historical stress directions; later stresses can generate new joint sets that cross-cut older ones.
• Engineering implications of joints:
  • Joints can compromise stability during excavation or tunneling because rock blocks can detach along joint planes.
  • Excavation planning should account for joint orientation and potential block movements.
• Concrete analogy: similar jointing may occur in rock due to thermal contraction and tension, highlighting the universality of fracture concepts across materials.

Bedding (Sedimentary Planes) and Zonal Defects

• Bedding represents layered deposition histories: sand, silt, shale, etc., laid down in layers with varying properties.
• Bedding planes are planes of weakness in sedimentary rocks; they often exhibit low strength parallel to bedding and higher strength perpendicular to bedding.
• Permeability differences across bedding planes influence groundwater flow and reservoir behavior.
• Engineering concerns with bedding:
  • Stability of slopes cutting into layered rocks, where layers are weak in the potential slip direction.
  • Channeling of water and petroleum through more permeable layers.
• Zonal defects (fragmented zones) occur when fracturing propagates through surrounding material, creating:
  • Angular fragments ranging in size from fists to large boulders.
  • Shear zones with closely spaced joints forming wedges and smeared clay infill in shear interfaces.
  • Varying strength across the damaged zone due to infill clays and weathering products.
• Voids and cavities along bedding/contact zones and in lava tubes or volcanic deposits can undermine foundations if unknown prior to construction.
• Practical implications:
  • Voids can collapse under load if not identified and filled or supported.
  • The presence of a void reduces bearing capacity and can necessitate remediation (grouting, underpinning, or rerouting structures).
• Examples of voids: lava tubes (e.g., NZ volcanic fields; Rangitoto lava caves) and karst-like cavities in volcanic and sedimentary rocks.

Faults: Major Displacements and Their Engineering Consequences

• Faults are fractures with significant displacement due to tectonic movement; they can dramatically change rock properties across the fault plane.
• Fault types (based on movement and dip direction):
  • Strike-slip faults: movement is mainly horizontal, parallel to the fault trace (e.g., Alpine Fault in NZ; long linear feature with minimal vertical offset).
  • Dip-slip faults: movement along the dip direction and include:
  • Normal faults: extensional displacement where one side moves down relative to the other.
  • Reverse faults: compressional displacement where one side moves up relative to the other.
• Hanging wall and foot wall terminology:
  • Hanging wall: the block that lies above the fault plane relative to a miners' perspective.
  • Foot wall: the block that lies below the fault plane.
• The Alpine Fault (NZ): a prominent strike-slip fault with ~400 km length, representing major horizontal shear of tectonic plates.
• Fault movement and water; faults can either impede or facilitate water flow:
  • Some faults act as barriers (holding water) while others act as conduits (permeability along the fault plane).
  • Abrupt changes in deposits across a fault can lead to unexpected material properties and failure modes.
• Faults generally result from earthquake shaking or slow creep (creep zones such as Hecate subduction zone) and can be accompanied by water-flow changes and permeability alterations.
• Engineering implications of faults:
  • Tunneling through faulted zones can cause sudden changes in rock properties and potential collapse if not properly reinforced.
  • Water inflow along faults requires drainage and stabilization strategies.
  • Unexpected faults can derail projects; thorough geological mapping and cautious planning are essential.
• Voids and cavities associated with faults can also appear (e.g., post-event voids or fault-block dissolution), posing hazards for foundations.

Practical Guidelines for Engineering Projects

• Always account for deformation and defects when designing; rock mass behavior is controlled by defects and their orientations.
• Site investigations should identify the orientation of folds, bedding, cleavage, fractures, and faults to assess stability, drainage, and load paths.
• Acknowledge multidisciplinary collaboration:
  • Geologists map and characterize features; engineers design reinforcement and support.
  • Structural engineers collaborate on reinforcement strategies for tunnels and excavations.
  • Geotechnical engineers model stability, seepage, and settlement considering defect networks.
• Design implications for dams, tunnels, and slopes:
  • Select siting where rock mass orientation provides the best resistance to resultant forces and reduces potential failure planes.
  • Avoid intersecting highly fractured or folded zones with peak loads or high pore pressures.
  • Plan for seepage management in permeable layers; implement drainage, grouting, or cut-off walls as needed.
  • For tunnels, anticipate preferential stress directions and reinforce accordingly to prevent block falls or inflows.
• The course emphasizes connecting high-level geological concepts to on-site engineering design and risk mitigation, with real-world examples from New Zealand geology (Alpine Fault, Southern Alps, Rangitoto lava caves).

Summary: Key Takeaways

• Deformation of rock deposits is governed by defects and their interfaces, not just the intact rock strength.
• Rocks are rarely homogeneous; fractures and fault networks dominate mechanical behavior and stability.
• Tectonic processes drive both gradual and catastrophic deformations, including folding (ductile) and faulting (brittle).
• Fold structures (anticlines, synclines, monoclines) and bedding planes strongly influence slope stability, water movement, and structural planning.
• Planar defects (joints, cleavage, bedding) and zonal defects (blocky fragmentation, wedges, clays) shape the risk profile of excavation, tunneling, and dam construction.
• Faults (strike-slip, normal, reverse) restructure rock masses and alter permeability; major NZ example is the Alpine Fault; understanding fault geometry is crucial for safe engineering practice.
• Practical design must account for the interaction of stress with rock structure, layering, and groundwater; location, reinforcement, and drainage strategies are driven by defect orientations and properties.
• Acknowledgement of the need to consult geologists and use field measurements (compasses, levels) to inform engineering decisions.

Equations and Numbers (LaTeX)

• Hydrostatic force on a vertical plane (per unit width) of height H in a dam:
  F_h = rac{1}{2}
  ho g H^2
• Location of the hydrostatic resultant from the base of the dam:
  y_h = rac{H}{3}
• If combining forces from weight W (acting through its center) and hydrostatic force Fh (acting at yh), the resultant R location along the base can be expressed as:
  xR = rac{W xW + Fh xh}{W + F_h}
• Notation: typical geologic timescales and movements discussed include:
  • Slow tectonic creep on the order of ~1 cm per year: ext{rate} \sim 1\,\text{cm yr}^{-1}
  • Large earthquakes with displacements on the order of meters (rapid, brittle deformation).
• Approximate dimensions mentioned:
  • Alpine Fault length: ext{length} \approx 400\,\text{km}
• Depth-dependent behavior: deeper rocks tend to show ductile behavior due to higher temperatures and confining pressures; near-surface rocks tend to fail brittily.

Connections to Real-World Relevance

• Understanding rock mass behavior is essential for designing dams, tunnels, slopes, and foundations; ignoring defects can lead to catastrophic failures.
• Oil and gas exploration relies on folding geometry to identify traps (anticlines) and permeability contrasts across bedding and mineral boundaries.
• Groundwater and reservoir management depend on layering and fault zones controlling flow paths.
• Monitoring and mitigating seismic hazards require knowledge of fault orientations, slip tendencies, and potential water flow along faults.

Ethical and Practical Implications

• Engineers have a responsibility to anticipate geologic hazards and minimize risk to public safety and the environment.
• Multidisciplinary collaboration (geologists, civil/structural engineers, hydrologists) is essential for safe designs and risk reduction.
• Historical mistakes (e.g., placing structures across unknown faults or permeable fault zones) highlight the need for thorough subsurface characterization and adaptive design.

Closing Notes

• Expect to integrate field observations (compass, level) with geologic maps and borehole data to inform design decisions.
• The course emphasizes that a robust understanding of rock deformation, faults, and bedding is foundational to effective geotechnical engineering practice.
• Review the open geology textbook and Section 99.5 for deeper context and examples to reinforce these concepts.