CVEN3203 Applied Geotechnics and Engineering Geology

Overview of Soils

• Soils are formed from weathering, alteration, and transportation of rocks.

• Engineering properties of soils are influenced by their formation processes.

Weathering

• Definition: Weathering is the response of rock to environmental changes.

• Types of Weathering:

  • Mechanical Weathering: Involves physical processes that break down rocks without changing their chemical composition.

  • Key processes include:

    • Destressing (main cause of new defects)

    • Gravitational creep (slabs, wedges, etc.)

    • Growth of tree roots in joints

    • Expansion of ice or clays in joints

    • Extreme temperature variations causing rock expansion and contraction.

  • Chemical Weathering: Involves chemical processes that alter minerals in rocks, making them weaker until they become soil.

  • Caused by groundwater interacting with minerals via defects in the rock or through intact fabric.

Mechanical Weathering

• Destressing Effects:

  • Rock with few defects forms new defects.

  • Rock with many defects experiences opening and extending of existing defects.

• Examples:

  • Sheet joints formed by tensile failure, commonly seen in granite and metamorphic rocks.

  • Formation of high-stress concentrations leading to rock failure in eroded valley floors.

Chemical Weathering

• Processes:

  1. Chemical reactions between minerals, water, oxygen, and organic compounds.

  2. Removal of soluble materials through leaching.

  3. Development of microcracks that increase surface area.

  4. Deposition of new products in pores or microcracks.

• Influencing Factors:

  • Climate and vegetation are dominant factors.

  • Rock types, defect patterns, time, topography, and groundwater conditions.

Weathering Profiles

• Development of profiles affected by the rate of weathering versus erosion.

• Fresh and variably weathered interfaces occur due to different rock substances and surface areas.

• Idealized profiles show weathering starting at joints and surfaces, progressing inward.

Soils from Transport and Deposition

• Soil can be transported by wind, water, and gravity once weathered from rock.

• Deposition takes place at energy change points:

  • High energy leads to heavy particles (gravel).

  • Lower energy accommodates smaller particles (sands to clays).

Alluvial Soils

• Types:

  • Deposited in river channels, flood plains, lakes, estuaries, and deltas.

• Characterized by vertical and lateral variability in soil properties.

• Alluvial deposit types:

  • Braided streams: carry large sediments exhibiting a wide size range.

  • Meandering streams: significant variability and complex deposition patterns.

Colluvial Soils

• Formed from gravity-driven deposits like slopewash, scree, and landslide debris.

• Features include:

  • Variable composition (clays to boulders).

  • Presence of timber and potential for instability.

Marine Soils

• Deposited in coastal environments, influenced by wave action and sediment transport.

• Beach formation results from greater wave upwash than backwash.

Glacial Soils

• Formed during Pleistocene glaciations, characterized by deposits of eroded materials.

• Various glacial environments:

  • Subglacial: landforms beneath large ice sheets.

  • Periglacial: frost action impacting glacial structures.

Aeolian Soils

• Formed by wind action, characterized by:

  • The movement of dry sand by rolling/bouncing.

  • Development of dunes. Different types:

  • Transverse, barchan, longitudinal, and stellate dunes.

Lateritic Soils

• Highly weathered, iron-rich soils found in tropical areas.

• Characterized by variability and cementation into rock-like masses.

Questions for Review

• Consider aspects of alluvial, colluvial, marine, and glacial soil types and their properties.

• Reflect on the implications of soil formation processes for engineering challenges and geological assessments.