Study Notes: Geomorphology and Geotechnical Units 4–8

1 Introduction and General Overview (Learning Unit 4 – Geomorphology)

  • Geomorphology = the study of landforms, surface processes, relief (form/topography) and sediments on Earth.
  • Exogenic processes: surface-acting processes (erosion, deposition) such as water, wind, chemical, glacial, gravity-driven processes.
  • Endogenic processes: tectonic-related below-surface processes (uplift, faulting, folding).
  • Goal in engineering: combine geology, geomorphology and engineering to identify hazards, problem soils, material behavior and suitable construction materials; inform site selection, hazard mitigation, design input, etc.
  • Unit emphasizes exogenic processes affecting everyday engineering (erosion, deposition) and provides foundation for later units on hazards (Unit 5) and soils/materials (Units 6–8).
  • Module layout (Figure outlines) places Geomorphology as Unit 4 within a broader sequence of Civil Engineering Design, Basic Geology, and related topics.

2 Purpose and Expected Outcome of the Learning Unit

  • Provide a broad overview of geomorphology for engineers.
  • Objectives: identify geological hazards; identify problem soils; predict material properties/behavior; identify potential construction materials.
  • End-of-unit capabilities:
    • Argue the importance of geomorphology in engineering projects (hazards, geotechnical constraints, material properties, potential sources).
    • Illustrate the concept of “equilibrium state” in geomorphology.
    • List major geomorphological processes covered.
    • Differentiate between process, activities and products.
    • Understand the role of soils in the rock cycle; soil-forming factors; soil profiles.
    • List soil types by grain size with definitions and limits.
    • Distinguish between transported and residual soils; describe soil origins and their relation to landforms (Block diagram descriptors).
    • Differentiate disintegration vs decomposition; describe weathering processes and their significance.
    • Understand progressive reduction, exfoliation and spheroidal weathering; factors affecting weathering; relation to climate.
    • Understand the three-stage process of erosion; differentiate erosion types; explain erosion-factors.
    • Apply Mohr-Coulomb criterion to erosion/mass movement and discuss slope protection.
    • Understand nine-unit Land Surface Model; recognize erosion as a design constraint via desk study/site walkover.
    • Propose erosion protection measures for site-specific conditions; identify geotechnical constraints in depositional environments; locate potential material sources (gravel, sand, clay).

3 Definitions and Abbreviations

  • Definitions provided in sections; refer to original sources (Weinert, 1980; others).
  • Abbreviations used across learning units; consult the Glossary in each section for context.

4 The Importance of Geomorphology

  • Surface/landforms provide:
    • Subsurface geology indicators;
    • Historic/current geomorphological activity;
    • Anticipated soil profile (shallow rock vs deep compressible, gravel vs clay);
    • Potential problem soils and hazards (e.g., slope instability);
    • Expected engineering properties and construction material sources (clay, sand, gravel, pedocretes, etc.).
  • Geomorphology is essential in geotechnical/site investigations and is considered good practice in standards (SAICE/SANS). It supports planning, feasibility, hazard mitigation, and project economics.
  • Integrated geomorphological and geological input during planning helps identify fatal flaws and constraints early.

5 Concepts of Equilibrium State and Base Level

  • Equilibrium state: balance between opposing processes (uplift vs downcutting; erosion vs deposition; soil formation vs removal).
  • Base level concept: erosion cannot proceed below a threshold; deposition does not occur above it. The sea level is the ultimate erosional base level; temporary base levels can be created by dams and groundwater tables.
  • Figures illustrate: (a) temporary base levels; (b) erosion/deposition progression; (c) ultimate base level equilibrium with surface and soil levels at sea level.

6 Geomorphic Processes, Activities and Products

  • Agents/drivers: rivers, wind, ice, waves, gravity, etc.
  • Three main activities: erosion, transportation, deposition.
  • Products: erosional landforms and depositional landforms.
  • Processes by category (examples):
    • Aeolian (wind); Biological; Fluvial (water); Glacial (ice); Hillslope (mass wasting); Igneous; Tectonic; Marine.
  • Example landform descriptors (from Figure 3 in the unit): flat/steep hills, channels, terraces, floodplains, dunes, etc.
  • Key takeaway: linking processes to landforms helps predict soils and materials in engineering contexts.

7 Brief Geomorphic History of South Africa and the Concept of Scale

  • Six geomorphic cycles initiated by tectonic episodes:
    1) Gondwana planation (early Jurassic, ~190 Ma);
    2) Post-Gondwana cycle (Early Cretaceous, ~135 Ma);
    3) African cycle (Mid-Cretaceous, ~100 Ma);
    4) Post-African I cycle (Miocene, ~20 Ma);
    5) Post-African II cycle (Pliocene, ~5 Ma);
    6) Quaternary cycle (Pleistocene, ~2 Ma).
  • Emphasize: cycles reflect tectonic forcing; processes progress toward equilibrium via erosion/transport/deposition.
  • Scale of geomorphology: continental to micro-environment; focus on regional to site-specific understanding for engineering applications.

8 Formation of Soils

  • Soil definition (engineering perspective): unconsolidated material covering most of the Earth, product of rock weathering, may be transported or residual; includes all pedogenic formation (cementation) levels.
  • Soils include transported and residual varieties; regolith refers to soil/fragmented rock over bedrock.
  • Soil horizons and soil profiles form as bedrock/regolith weathering/decomposition proceeds.
  • Soil relates to rock cycle and to lithology, tectonics, and geomorphological processes.
  • Rock cycle illustration (Figure 4) helps interpret soil profiles and landforms.

9 Weathering (Disintegration and Decomposition)

  • Weathering = general term for disintegration (physical) and decomposition (chemical).
  • Disintegration: physical rock breakdown; Decomposition: chemical alteration (change to secondary minerals) in the presence of air, hydrosphere, biosphere.
  • Self-study guidance emphasizes the Weinert (1980) reference for climate, N-values, slope effects on soils, rock types, Bowen reaction series, and weathering products.
  • 9.3 Physical Weathering (Disintegration): mechanisms include joint formation (stress relief), salt crystal growth, freeze-thaw, diurnal temperature cycles, heat events, root wedging; joints/discontinuities facilitate ingress of water and weathering.
  • 9.4 Chemical Weathering (Decomposition): drivers include H+, OH−, O2−; processes include dissolution and leaching, hydrolysis/hydration, oxidation/reduction, carbonation, and combined reactions.
  • 9.5 Progressive Reduction During Weathering: larger exposed surface areas accelerate chemical weathering; jointing increases surface area and weathering rate.
  • 9.6 Exfoliation and Spheroidal Weathering: onion-like onion-layering and spheroidal weathering lead to large blocks (corestones) within soils; important for slope stability and excavation planning.
  • 9.7 Factors Affecting Weathering: mineralogy, rock type/structure, slope angle, climate, biotic activity, human impact, time.
  • 9.8 Climate-weathering relationship: higher temperature and rainfall favor chemical weathering; colder/polar climates favor physical weathering; Figures show climate vs chemical/mechanical weathering relationships.

10 Erosion (Mobilisation, Transportation and Deposition)

  • Erosion = three-stage process: mobilisation (detachment), transportation, deposition.
  • Energy and material properties govern thresholds for movement; higher energy required to mobilise; deposition occurs as energy decreases.
  • Erosion has engineering consequences: undercutting foundations, silting of reservoirs, piping, etc. (Photograph of excessive erosion).
  • 10.1 Types of Erodibility:
    • Dispersive soils: colloidal/clay deflocculation; suspension when electrical charges overcome van der Waals forces; problematic soils (covered elsewhere).
    • Slaking materials: degrade with moisture cycles; lack true cohesion; soil suction provides apparent cohesion; common example: slaking mudrock.
    • Erodible materials: non-dispersive with true cohesion; require additional forces (water impact, shear, gravity) to mobilise; in this unit, erodible and dispersive soils are treated as erodible soils for mitigation discussions.
  • 10.2 Factors Affecting Erosion: climatic conditions, topography, hydrology, geology (soil characteristics), vegetation, human activity.
  • 10.3 Mohr-Coulomb Criterion and Erodibility:
    • Equation (classical): auf=c+auanext(phi)au_f = c + au an ext{(phi)}
    • Effective stress form: au=c′+(au−u)anext(phi′)au = c' + ( au - u) an ext{(phi')}
    • Factor-of-safety concept: extFoS=extResistingForce(ShearStrength)extDrivingForce(ShearStress)ext{FoS} = \frac{ ext{Resisting Force (Shear Strength)}}{ ext{Driving Force (Shear Stress)}}
    • Terzaghi/modified Terzaghi forms connect shear strength to effective stress, cohesion, friction angle, and pore pressures.
    • The mass remains stable if resisting forces exceed driving forces; instability if FoS < 1.
  • 10.4 Transportation Agents – Water, Gravity, Wind, Ice:
    • Water: mobilisation and entrainment depend on flow velocity; there are velocity thresholds for particle entrainment and deposition; fine sands are most susceptible to erosion; deposition occurs where velocities drop below threshold.
    • Gravity: slope angle controls shear stress; mass movements include slumps, falls, slides; flows include slurry and granular flows; types such as solifluction, debris flows, mudflows, creep, earth flows, grain flows, debris avalanches.
    • Wind: modes of transport include creep, saltation, suspension; laminar near-surface flow dominates; wind erosion mitigated by vegetation, barriers, stabilisation methods.
    • Ice: glacial transport; capable of moving a wide range of grain sizes; deposition by ice creates corestones and chaotic deposits; ice-related hazards include rockfalls and avalanches.
  • 10.4 Transportation – Physics and stability: Framing in terms of FoS and Mohr-Coulomb remains central; stability depends on resisting shear strength vs driving forces (gravity, water, wind).
  • 10.4.4 Agent – Water (Details): velocity thresholds for entrainment and deposition; Figure 11 shows velocity vs particle size; deposition occurs when flow drops below critical velocity; Figure 12–13 illustrate particle movement and bed load.
  • 10.4.2 Agent – Gravity (Details): slope stability failure types; slope movement regimes; mass-flow vs flow; Figures describe slope failure types (slump, fall, slide) and flow types (slurry vs granular).
  • 10.4.3 Agent – Wind and 10.4.4 Agent – Ice provide similar driving-force vs resistance discussions; note laminar vs turbulent flow near-surface and relevance to depositional environments.
    1. Fact-of-Safety analogy: Angels vs Devils (Figure 10.7 analogy) to conceptually illustrate balance of forces; more angels than devils -> stable; more devils -> unstable.
  • 10.6 Summary: mass movement depends on geometry, loads, pore pressure, cohesion, friction angle, and vegetation among others.

11 Land Surface Models

  • Describes land-surface modeling approaches to classify slope units for erosion risk assessment.
  • Four-unit model (older), nine-unit model (more detailed):
    • Four-unit: Upper convex slope, cliff, straight segment, lower concave slope.
    • Nine-unit: Interfluve, seepage slope, convex slope, fall face, transportational mid-slope, colluvial foot-slope, alluvial toe-slope, channel wall, channel bed.
  • Key exercise: relate units to energy regime, motion direction, and deposition/redeposition zones; use to plan erosion control strategies.

12 Erosion Recognition

  • Recognize erosion constraints via visual indicators: regional erosional features, siltation patterns, murkiness of water as indicators of dispersive soils.
  • Siltation evidence indicates deposition and erodibility; murkiness implies dispersive soils and erodibility.
  • Desk-study desk practice: assess hazards from regional erosion, siltation data and water turbidity; plan mitigation accordingly.
  • The unit includes worked desk-study examples (Figures 22–27) to practice recognition and risk assessment.

13 General Erosion (Slope) Protection

  • Slope protection decisions depend on:
    • Cause of damage; climatic conditions; soil type; consequences (structural failure, safety, environmental impact).
  • Potential erosion protection measures:
    • Topsoil placement with improved grading or non-dispersive horizons; mulch/seeding; reinforced turf mats; hydro-seeding with vegetation; vetiver hedgerows to trap sediment; rock/gravel protection (pitching, gabions); compaction to increase shear resistance; chemical stabilization to increase cohesive strength; geotextiles; gabion mattresses; barriers (retaining walls); chutes and stilling basins; slope angle adjustments; drainage design.
  • Agricultural measures: proper grazing management, longer fallow, reduced deforestation, fire control.

14 Typical Geotechnical Constraints Associated with Mode of Transport and Depositional Environment

  • Table-based descriptors link transported soil origins to expected problems:
    • Littoral/mobile dune sands, estuarine deposits, aeolian sands, alluvial deposits, lacustrine deposits etc.; problems include collapsible fabric, variability in compaction, compressibility, dispersiveness, etc.
  • Special notes: collapsible fabrics in wind-deposited sands; compressibility in estuarine/deltaic deposits; slope stability constraints with coarse colluvium; aeolian soils and poor compaction; alluvial deposits and high compressibility, dispersivity, etc.
  • The origin and transport mechanism determine the material properties, deposition environment and thus engineering behavior. (Self-study: SAPEM/other references.)

15 Potential Construction Materials Associated with Landforms

  • Use landforms and depth of soil profiles to predict material types (clay, silt, sand, gravel, boulders) and pedocrete formation potential.
  • Combine geological inputs with vegetation/fauna indicators to infer material sources and suitability for construction materials (clays for clay liners, gravels for fills, pedocrete cemented horizons, etc.).
  • Self-study: Obtain South African Pavement Engineering Manual – Material Sources (Chapter 8 – Material Sources) and study landforms related to gravels and sands; relate to residual vs transported gravels; evaluate problems associated with residual vs transported gravels for engineering.

16 Self-Assessment Activities

  • Revisit self-study prompts; address self-assessment questions to test understanding.

17 Further Reading

  • Additional recommended texts and sources (Weinert, 1980; Skinner et al., 2004; Craig & Knappett, 2012; etc.).
  • Provided in unit (web resources listed in the transcript).

19 References

  • Bell, Brink, Craig, de Vallejo, Williams, Paige-Green, SAICE, SAPEM, Weinert, etc. (full bibliography in the transcript).

Connections Across Units (Key Takeaways for Exam)

  • Geomorphology informs soil formation, weathering, erosion and sedimentation; understanding landforms helps predict soil types and problem soils.
  • Equilibrium state and base level concepts underpin long-term landscape evolution and guide hazard assessment and mitigation in civil design.
  • Weathering processes link mineralogy, climate and landforms to soil properties; progressive reduction and exfoliation shape unit considerations for mass movements.
  • Erosion processes and Mohr-Coulomb-type strength criteria underpin slope stability analysis and design of erosion-control measures.
  • Land Surface Models provide a framework to segment slopes by energy and movement potential for planning slope protection and site investigations.
  • Geomorphology, geology and soils knowledge combined with site investigation standards (SAICE, SANS) guide phased investigations, parameter targeting and design input for engineering works.

Formulas and Key Equations (LaTeX)

  • Mohr-Coulomb shear strength criterion (total stress):
    auf=c+auanheta,au_f = c + au an heta, where c is cohesion,
    and heta is the angle of internal friction.
  • Mohr-Coulomb with effective stress (pore pressure u):
    au=c′+(au−u)anheta′,au = c' + \bigl( au - u\bigr) an heta' ,
    where c' is effective cohesion and
    heta' is the effective friction angle.
  • FoS against erosion (safety against movement):
    extFoS=extResisting(ShearStrength)extDriving(ShearStress).ext{FoS} = \frac{ ext{Resisting (Shear Strength)}}{ ext{Driving (Shear Stress)}}.
  • Ultimate bearing capacity for cohesionless soils (Terzaghi-like form for friction soils):
    qu = c' Nc + qd Nq + rac{1}{2}
    ho d N_
    ho,
    where $Nc$, $Nq$, and $N_
    ho$ are bearing-capacity factors dependent on
    the friction angle; $q_d$ is the net overburden pressure; $d$ is footing size/internal dimension; and $
    ho$ is unit weight.
  • Ultimate bearing capacity for cohesive soils (Skempton-like form):
    qu = c' Nc + qd Nq + rac{1}{2}
    ho d N_
    ho. (Formulation shown in unit; context-appropriate substitutions apply.)
  • Pile capacity (single pile):
    P<em>u=P</em>sextshaft+P<em>bextbase+W,P<em>u = P</em>s^{ ext{shaft}} + P<em>b^{ ext{base}} + W, where $Ps^{ ext{shaft}}$ is shaft resistance, $P_b^{ ext{base}}$ is base resistance, and $W$ is pile weight.
  • Elastic settlement (granular soils, Hooke’s law form):
    extε=auE,extorSeextexpressionsdependentonfoundationgeometryandE,ext{ε} = \frac{ au}{E}, ext{ or } S_e ext{ expressions dependent on foundation geometry and } E,
    with $E$ the modulus of elasticity; for sodal joints see specific forms in the unit.
  • Wave velocities (P- and S-waves) relationships:
    Vp = rac{1}{ ho}igl(K + frac{4}{3}Gigr)^{1/2}, ag{6.1} Vs = iggl( rac{G}{
    ho}iggr)^{1/2}, ag{6.2}
    with $K$ bulk modulus, $G$ shear modulus, and $
    ho$ density.
  • Relationship between $K$, $G$, $E$, and $
    u$ (Poisson’s ratio):
    K=E3(1−2<br/>ν),ag6.3 g=E2(1+<br/>ν).ag6.4K = \frac{E}{3(1-2<br />\nu)}, ag{6.3} \ g = \frac{E}{2(1+<br />\nu)}. ag{6.4}
  • Reflectivity impedance relation (simplified form):
    R = rac{Z2 - Z1}{Z2 + Z1}, ext{ where } Zi = hoi V_i.
  • Resistivity and conductivity relation:

    ho = rac{1}{\sigma}, ext{ with units } [
    ho] = ext{Ohm m},
    [\sigma] = ext{S/m}.