MAC 102: Formation of Soils

Types of mechanical weathering

• Unloading due to erosion (stress relief)

• Impact loading

• Thermal loading and unloading

• Wetting and drying

• Freezing

Types of chemical weathering

• Solution (rainwater can contain some very dilute acids)

• Oxidation and reduction (soil can contain iron)

• Leaching (the migration of ions downward through soil

Factors affecting the formation of residual soils

• CLimate (hot and humid enhance reactions)

• Organisms

• Relief (topology and drainage conditions)

• Parent Material

• Time

What is laterite

A very widespread residual soil deposit found in equatorial regions and is formed from the weathering of basic igneous rocks

What is the agent for colluvium soil and what is its transportation process

Agent: Gravity

Transportation Process: Moving down hills

What is the agent for marine soil and what is its transportation process

Agent: Water

Transportation Process: Sea

What is the agent for alluvium soil and what is its transportation process

Agent: Water

Transportation Process: River flood planes

What is the agent for estrurine soil and what is its transportation process

Agent: Water

Transportation Process: River channel

What is the agent for lacustrine soil and what is its transportation process

Agent: Water

Transportation Process: Lake

What is the agent for aeolian soil and what is its transportation process

Agent: Wind

Transportation Process: Deposition in deserts

What is the agent for glacial soil and what is its transportation process

Agent: Ice

Transportation Process: Deposition by ice

What is the agent for periglacial soil and what is its transportation process

Agent: Ice

Transportation Process: Deposition by freeze/thaw effects

What are properties of colluvium soil

• Large size range

• Usually unsorted

• No bedding

• Can contain shear surfaces

• Liable to become unstable

What are engineering implications of colluvium soil

• Rockfalls

• Landslides

• Creep

What are properties of marine, alluvium, estrurine, lacustrine soils

• Soils sorted into beds

• Generally low strength and highly compressible

• Strength increases with depth

• Varying permeabilities

What are engineering implications of marine, alluvium, estrurine, lacustrine soils

• Moved by receding floods

• Deposition into sea/lake

What are properties of aeolian soil

For aeolian sands:

• Rounded particles

• Highly compressible

For aeolian loess:

• Rapidly eroded by water

• Sudden decrease in strength

What are properties of glacial and periglacial soils

For lodgement till (boulder clay) which is the base of the ice sheet:

• Wide range of particle sizes

• Over consolidated deposit

• Low compressibility, high strength

• Experiences weight of ice sheet

For ablation till (moraines) which is in or on top of ice:

• Wide range of particle sizes

• Normally consolidated deposit

• High compressibility, low strength

• Is not compressed by ice sheet