Soil Origins, Transported Soils, and Reclaimed Lands: Lecture Notes on Soil Formation and Types

Induction and Lab Safety

• Induction for laboratory access will be posted on Canvas once set up.
• Labs start in week four; continued in week five with the lab in the middle of the term.
• Induction is done via an online Canvas tool for the labs in the main building (Room 405).
• Process steps:
  • Read the documents provided.
  • Complete a five-minute Canvas quiz with ten questions.
  • If you answer correctly, you send a screenshot to the instructor.
  • The instructor will point you to the correct Canvas page if there are multiple versions.
  • Post the screenshot with the statement, “Yes, I’ve done this quiz,” in the assignments area that will be created on the course Canvas page.
• Important health and safety rule: you cannot participate in laboratory sessions in the test labs until you complete the induction.
• The induction itself is not worth points, but it is mandatory.
• Compulsory nature of labs: if you don’t do the induction, you can’t attend the lab; if you don’t do the lab, you can’t finish the course.
• Next week: complete the induction and inform the instructor once finished.
• The instructor notes that the process changes annually and hopes this year goes smoothly.

Transition to Lecture Topic: Soil Origins

• The current topic is soil origins; prior discussion covered rock as the source material to understand soil behavior.
• Why origin matters: knowing where the soil comes from helps predict its behavior (strength, permeability, compressibility).
• Soil has unique time-dependent reactions to loading; not all materials react instantly.
• A historical example (Holten Kovács, 1981) illustrates time-dependent settlement:
  • A foundation applies vertical load to the ground surface, causing settlement, denoted as \Delta (settlement).
  • Settlement occurs over time; in the example, the time scale is on the order of 100 to 1400 minutes (roughly a fraction of a day).
  • In many soils, settlement can continue for years; engineers must consider time because
    time equals money and delays can upset clients.
• This time-dependent behavior is more critical for soils than for concrete or steel in structural engineering contexts; covered in greater detail in Civil 300.

Key Concepts: Soil Formation and the Rock–Soil Continuum

• Soils form from parent material, usually rock, but can be already-existing soil that is transported or eroded.
• Weathering can be physical or chemical; weathering plus erosion leads to soil formation.
• Residual soils: soils that weather in place, remaining in the same general location as the parent rock.
• Transported soils: soils moved from their original location by natural processes and deposited elsewhere.
• Sedimentary soils are deposited soils that may later be buried and transformed into sedimentary rock in the rock cycle.
• The parent material and the weathering processes influence soil mineralogy and grain size.
• Pumice is highlighted as an important volcanic material that can break down into sandy or pumice grains with unique crushable properties.
• The formation of soil is driven by multiple interacting factors including climate, topography, time, organisms, and even human activity.
• Humans can create new soil deposits through activities such as land reclamation and fill, which has implications for soil behavior and stability.

Factors Driving Soil Formation

• Climate: hot and humid climates accelerate chemical weathering; dry climates favor physical weathering processes.
• Topography: slope steepness and location influence erosion and sediment deposition; landslide susceptibility is a key concern on hilly terrain.
• Time: longer exposure to weathering and erosive forces leads to more complete breakdown.
• Biota: plants, animals, bacteria influence weathering and soil development.
• Humans: construction, mining, and land-use changes can alter soil formation and distribution.

Transported Soils: Mechanisms and Examples

• Transported soils originate from a source region and are moved to a deposition site by one of several processes:
  • Alluvium: river-transported material (also called fluvial). Deposits vary along the river course depending on energy.
  • Glacial: material moved by ice; deposits can include large boulders and a wide range of grain sizes.
  • Aeolian: wind-driven transport, capable of moving very fine particles like silt; loess is a common wind-deposited soil.
  • Volcanic alluvium: volcanic processes can transport ash and pumice; lahars are volcanically driven landslides that create layered deposits.
• Alluvium and river dynamics:
  • Near the source (upstream), high-energy conditions carry larger grains (gravel and sand).
  • Downstream and toward flatter areas, energy decreases, causing coarser particles to drop out first (gravel), then sand, then fine silt and clay near the river mouth.
  • River energy and sediment transport can vary with weather events (e.g., heavy rain in the Southern Alps causing floods, shifting sediment transport).
  • Rivers can be braided, meander, and form oxbow lakes when channels change course; deposition and erosion create complex stratigraphy.
• Terminology used by geologists:
  • Alluvium: material transported and deposited by rivers.
  • Mass movements and slope instability can produce alluvial deposits at the base of slopes.
• The geologic vocabulary and deposition history help engineers interpret site conditions and predict behavior.

Dynamic River Processes: Meanders, Braided Rivers, and Oxbow Lakes

• Meandering: a river channel gradually erodes on the outer banks and deposits on the inner banks, creating bends.
• Braided rivers: multiple channels with interwoven paths in regions with high sediment supply and variable flow; the river seeks paths of least resistance.
• Channel migration results in large floodplains and complex stratigraphy; bridges may cross wide alluvial fans or floodplains.
• Oxbow lakes form when a cut-off occurs, leaving a crescent-shaped lake from an abandoned river channel.
• These processes produce interlayered deposits of gravel, sand, silt, and clay, with properties varying by layer and depth.

Sedimentary Rock Formation from Deposits

• Sedimentary rocks form when sediments become cemented or compacted over long timescales.
• Layering arises from deposition during different periods of high and low energy (e.g., sustained gravel deposition during glacial periods, followed by sand and silt deposition as climate changes).
• The Canterbury Plains example illustrates long-term interlayering: thick gravels above, then sands and silts, followed by renewed gravel deposition from renewed glaciation.
• Over tens to hundreds of thousands of years, such layering continues, and later seismic events can interact with these layers (e.g., liquefaction concerns in sandy layers during earthquakes).
• In New Zealand, Christchurch and canterbury region show thick, alternating layers of gravel, sand, and silt due to repeated glaciation and deglaciation cycles.

Volcanic and Volcanic-Influenced Soils

• Volcanic soils arise from volcanic eruptions that deposit ash and pumice; pumice grains can be crushable and have distinctive properties.
• Lahars (volcanically driven landslides) create layered deposits similar to other transported soils.
• Volcanic ash deposits can create highly variable soils depending on deposition and subsequent weathering processes.

Residual Soils: In-Place Weathering and Auckland Examples

• Residual soils form in place from the weathering of parent rock, typically through chemical weathering, though mechanical weathering also contributes.
• Auckland-specific context: soils like Auckland clays result from in-place weathering of local rocks (sandstone, mudstone/siltstone) brought near the ground surface by uplift and exposure.
• These soils are chemically and physically distinct from transported soils, often showing unique clay minerals and behaviors.
• Residual soils can themselves be eroded and transported, becoming transported soils in new locations.
• Summary for residual soils:
  • Common in New Zealand.
  • Can be problematic for foundations (e.g., Auckland clays).
  • Behavior differs from transported soils due to chemical interactions and mineralogy.
  • Most geotechnical practice is based on transported soils; residual soils remain an active area of research and engineering practice.

Organic Soils and Peats

• Organic soils form from decomposition of plant material in swampy or water-saturated environments.
• Identifying organic soils: notable odor and sulfurous smell; typically low strength and highly compressible.
• In places like Auckland, extensive peat- or organic-rich soils exist due to historical land use and drainage conditions.
• Engineering implications: organic soils are challenging; traditional construction practices may avoid building directly on them or require special treatment.

Man-made Soils and Land Reclamation

• Human activities create artificial or highly variable soils through fill and reclamation (e.g., in urban waterfronts and port areas).
• Auckland CBD example: shoreline originally formed differently; large-scale reclamation created flat ground by dumping soil in place to support infrastructure like ports and transport hubs.
• Depth to original formation can be substantial: up to ~22\text{ m} below current ground surface in some locations.
• Properties of hydraulic fill (uncompacted or lightly compacted fill) can be extremely variable and problematic for construction.
• Lessons and hazards:
  • Hydraulic fill is prone to liquefaction under seismic shaking if not properly managed.
  • Auckland’s experience shows reduced risk due to regional seismicity differences, but remediation practices remain essential in other seismic regions.
• Global context: other cities rebuild land by dumping material in harbors (e.g., Singapore, Hong Kong) to create new land area; this is a common urban strategy in land-constrained regions.

Practical Takeaways: How Origin Affects Geotechnical Behavior

• Soil behavior is highly dependent on origin (transported vs residual vs organic vs volcanic):
  • Transported soils tend to be analyzed with standard geotech practices, with a wide suite of empirical correlations.
  • Residual soils require careful consideration of mineralogy and chemical interactions; standard transported-soil assumptions may not always apply.
• Layered soils from deposition histories can create complex, interlayered properties that affect strength, compressibility, permeability, and response to loads.
• Time and loading history matter: settlement can continue long after construction begins; time-dependent deformations influence design and long-term performance.
• Human activities can drastically alter soil properties and pose unique risks (e.g., hydraulic fill liquefaction, ground settlement from reclamation).

Quick References and Concepts to Remember

• Key soil types:
  • Alluvium (fluvial transport): energy-dependent deposition along rivers; coarse near sources, finer downstream.
  • Glacial soils: transported by ice; varied grain sizes; glacial till can be poorly sorted.
  • Aeolian soils: wind-transported; loess deposits are fine-grained.
  • Volcanic soils: ash and pumice deposition; may have layered, variable properties.
  • Sedimentary soils: deposited and later buried to form sedimentary rock; layering reflects deposition history.
  • Residual soils: weathered in place from parent rock; often chemically driven.
  • Organic soils: peat and related materials; high compressibility and low strength.
  • Man-made soils: reclaimed or hydraulic fill; potential for liquefaction if poorly compacted.
• Important concepts:
  • The soil–rock cycle, weathering, erosion, deposition, and reworking across time.
  • The impact of climate, topography, time, biota, and human activity on soil formation.
  • Time-dependent settlement and its practical implications for construction and risk management.

Final Thoughts and Real-World Relevance

• Understanding soil origins provides insight into expected soil behavior and helps engineers anticipate how soils will respond to loads, time, and environmental changes.
• Real-world examples (Auckland, Canterbury Plains, Singapore, Hong Kong) illustrate the global relevance of soil origin, transport, and modification practices.
• Ethical and practical implications include ensuring safe designs in reclaimed or fill areas, accounting for time-dependent settlements, and balancing urban development with geotechnical risk management.