ERTH2404-L8: Soils and Erosion

Earth’s external processes

• Weathering

• Erosion

• Formation of soil

Earth materials can be subdivided into…

• Rocks

• Soils

• Fluids

Geotechnical engineering

Geotechnical engineering: branch of civil/geological engineering that focuses on the behavior of soil, rock, and groundwater and how these materials interact with human-made structures.

Roles of geotechnical engineers

• Study the properties of soils and rocks

• Evaluate site conditions before construction

• Design foundations, slopes, retaining walls, embankments, and tunnels

• Assess natural hazards such as landslides, soil liquefaction, and ground settlement

Soil

Soil is a material composed of fragments from pre-existing rocks but lacking strong cement between particles

How do detrital sedimentary rocks form?

Detrital sedimentary rocks form after the lithification of fragments

Soil: soil science perspective

The soil science perspective (pedology, agriculture):

• Organic-rich weathered zone that supports the growth of

plants

• Focus: interrelation between physical, chemical and

biological properties of soils

Soil: engineering perspective

The engineering perspective (soil engineers):

• A soil is any material that can be excavated by a shovel

• Focus: behavior of soils under load

Soil: geological perspective

The geological perspective:

• Portion of unconsolidated material lying above bedrock*

• Focus for geologists: links between soils and the formation of

various geological material like clay, metallic ores, etc. Also,

indicator of past climates.

• Geologists rarely refer to soils as civil engineers do.

• The soils of civil engineers is for geologists:

• Unconsolidated deposits

• Surficial (superficial) deposits

• Drift if of glacial origin.

Bedrock

Bedrock: relatively hard, solid rock that commonly underlies altered rock, sediment, or soil. No unconsolidated deposits is found beneath bedrock.

Outcrop

Outcrop: A portion of bedrock visible at the surface.

Typical composition by volume of a soil which yields good plant growth

45% Mineral matter

5% Organic matter

50% Pore space:

25% Air

25% Water

Soil Composition

Total volume of soil

Volume of void space

Volume of air

Va

Volume of water

Vw

Volume of solids

Vs

Mass of solids

Ms

Porosity [%]

Void ratio [%]

Dry density [kg/m3]

Parent materials

Parent material: rock from which the soil developed

Residual soils (sedentary)

Residual soils (sedentary): The soils that hold its position of their formation, without transporting; parent material is the underlying bedrock

Transported soil

Transported soil: material has been carried from the location of origin to the location of deposition, by natural processes:

• Water

• Glaciers

• Wind

Important aspect of canadian soils

*** In Canada, most soils are transported soils (glaciers covered most of the territory, removed old loose material and left behind till after melting). Opposed to residual soils

Types of transported soils

1. Alluvial soils

2. Lacustrine soils

3. Marine soils

4. Glacial soils

5. Eolian soils

6. Colluvial soils (colluvium)

Alluvial soils

Alluvial soils: soils transported by rivers and streams

• Gravel, sand, silt with coarse/fine horizons commonly alternating

Lacustrine soils

Lacustrine soils: soils transported and deposited beneath a lake surface

Marine soils

Marine soils: soils transported and deposited in deltas, seas or oceans

Glacial soils

Glacial soils: soils transported by glaciers

Glacial till

Glacial till

Till: in geology, unsorted material deposited directly by glacial ice and showing no stratification, can be debris left behind by melting glaciers

• If deposited directly by the ice sheet, till is composed of particles of different sizes

Glaciofluval

Glaciofluvial (water circulation around glaciers)

• Sand and gravel

Glaciolacustrine/glaciomarine

Glaciolacustrine/glaciomarine (near-glacier deposits in lakes, seas)

• Clay and silt

Eolian soils

Eolian soils: soils transported by wind

• Homogeneous particle size (silt and clay-size

particles)

• High porosity

• In arid regions, fine-grained soils made of silt and

clay

• Also called Loess

Colluvial soils (colluvium)

Colluvium (deposits at base of slopes)

Colluvial soils (colluvium): soils transported downhill by gravity, either slowly (creep) or catastrophically (mass movement)

• Material moved downhill by gravity

• Mix of rock fragments, soil, and debris

• Typically found at the foot of hills or steep slopes

• Usually angular and unsorted

Colluvium (Vietnam)

• Material moved downhill by gravity

• Mix of rock fragments, soil, and debris

Soil profile

The soil profile is a vertical section of the soil that depicts all of its horizons.

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• Soil forming processes operate from the surface downward

• Soil scientists have developed a classification scheme based on horizons

Horizon

Horizon: individual layer with distinctive properties

O-horizon

O-horizon: organic matter (ex: dead leaves)

A-horizon

A-horizon:

• Mineral matter mixed with humus (partially decomposed organic matter)

• Dark color

• High biological activity

• Also called topsoil

E-horizon

E-horizon: (in Canada: called Ae)

• Mostly mineral particles with

little organic matter

• Light color, grey

• Zone where soluble substances are leached away by rain water

B-horizon

B-horizon (syn. subsoil)

• Zone where substances leached out from above layers accumulate (enriched in metals leached from the A horizons)

Leaching

Leaching: process by which soluble material in the soil (e.g. soluble minerals, nutrients, contaminants) are washed into a lower layer of soil – or – are dissolved and carried away by water

C-horizon

C-horizon:

• Partially altered parent material

• Residual soils: weathered bedrock

• In transported soils: unconsolidated material (e.g. glacial till, stream sediments)

R-horizon

R-horizon:

• Unweathered bedrock

Layers of soil

Factors controlling soil development

• Climate: the most critical factor

• Organisms

• Relief (Topography)

• Parent material

• Time

CLORPT, for short

Factors controlling soil development

Climate

Affected by temperature and precipitation

Climate: temperature

• High T: more evaporation, less infiltration

• Cold T: less evaporation, more infiltration

Climate: precipitation

• Concentrated in one season or spread over the year

Climate: Tropical forest

Climate: Coniferous forest (boreal forest)

Climate: Arctic and alpine tundra

Climate (Ottawa, Victoria, Regina)

Ottawa: 6 C and 106 cm of precipitation/year

Victoria BC: 10 C and 91 cm

Regina, SK: 2 C and 53 cm

Resolute Bay: -15 C and 9 cm (Arctic is a cold desert)

Highest in Canada: 6325 mm (6 metres!) on Haida Gwaii Islands (it rains 275 days/year)

Relationship between precipitation, amount of vegetation, temperature, and parent material depending on the biome

Soil in semi-arid and arid climates

Pedocal

• Pedo (soil) + cal (calcium)

• Soil enriched in calcium carbonate content and low organic matter. white-ish color

• Soil associated with dry grasslands

• Covers southern Saskatchewan and most of the western U.S.

Soils in temperate and humid regions

Pedalfer

Name origin: Ped (soil) + al (aluminum) + fer (iron)

Topsoil (A horizon): Sandy

• Light-colored

• Acidic because rainwater leaches many minerals

Subsoil (B horizon):

• Accumulates aluminum, iron, oxides, and clay

• Brown in color

Where they form: Mostly under forest vegetation, especially common under conifer forests, Generally fertile soils.

Found in most of southern Canada, The eastern United States

Soils in hot and wet tropical climates

Intense leaching

• Soluble elements (Na, K, Ca, Mg, Si) carried away by water

• Insoluble elements (Fe, Al) are left in place

• High relative concentration of (Fe, Al)

• Formation of laterites

• Rusty red, historically used as building material, easily cut into bricks

• Example: bauxite (aluminium ore)

Bauxite

The main ore of aluminum (Mineral Gibbsite Al(OH)3, not found in Canada).

Mines in Australia, Guinea, China, Brazil, Jamaica...

Canadian aluminum production depends on imported bauxite and abundant hydroelectric power.

It contains about 40-60 per cent alumina (aluminum oxide, Al2O3)

Soil types depending on climate

Organism Factor

Plants and animals

• mix and aerate soil

Relief Factor

• Thin soil on steep slopes

• Thick soil on flat or undulating surfaces

Topography Factor

Topography: controls position of soil in landscape

Parent material

• Glacial deposits: the majority of parent materials in Canada

• Silica-rich igneous rock (e.g. Eastern North America): -------- sandy, acidic soil

• Limestone (e.g. Western North America): ------ clay, slightly alkaline soil

Time Factor

• In fact, one centimeter of topsoil can take several hundred years or more to develop.

• Soil formation rates vary across the planet:

• The slowest rates occur in cold, dry regions (it can take more than 1,000 years to produce 1 cm)

• The fastest rates are in hot, wet regions (still very slow: several hundred years for 1 cm).

Objectives of engineering properties

• to make quantitative measurement of standard properties

• To assess suitability of soil to withstand load from structures

Cohesion

Cohesion: inherent shear strength of soils mostly due to the attracting forces between individual clay particles

• Cohesion means "stickiness"

Controlling factor is particle size

• Silt and clay: cohesive

• Sand and gravel: cohesionless

Different properties applicable to:

• Cohesionless soils (free running)

• Cohesive soils (clay, silt, with cohesive strength )

Grain size distribution

• Measured by a sieve analysis

• Laboratory test in which the soil is passed through a series of sieves with successively smaller openings

• Results plotted as a grain-size distribution curve

Sieve Analysis

Grain-size distribution curves

Variety of sizes

Engineering: Well-graded

Geology: Poorly sorted

Uniform distribution

Engineering: Poorly-graded

Geology: Well-sorted

Properties of Cohesionless soils

Engineering impact of well graded versus poorly graded soils

• Water drainage: poorly graded (well sorted) has better drainage

(because there are more void spaces in a poorly graded soil)

• These properties impact compressibility, shear strength

Properties of Cohesive Soils

Consistency: Very soft → Exudes between fingers

Consistency: Soft → Easily mouldable

Consistency: Firm → Strong finger pressure required

Consistency: Stiff → Can be indented with fingers, but not moulded

Consistency: Very stiff → Indented by sharp object

Consistency: Hard → Difficult to indent

Properties of Cohesive Soils

Consistency is determined by clay fabric

Properties of Cohesive soils

Remolding: disturbance of the soil by natural processes or during lab tests

• Clay can change from flocculated to dispersed fabric during remolding with a significant loss of strength

Sensitivity: strength in undisturbed conditions / strength in remolded conditions

Classification Systems

The purpose of a soil classification system is to group together soils with similar properties or attributes.

• Unified Soil Classification System

• British Soil Classification System

• ASTM - American Society for Testing and Materials

• AASHTO - American Association for State Highway and Transport Officials

Unified Soil Classification System: Major divisions

Major divisions:

Coarse-grained soils:

Classified according to sieve analysis

• Gravel

Coarse 75 mm to 19 mm sieve

Fine 19 mm to No. 4 sieve

• Sand

Coarse No. 4 to No. 10 sieve

Medium No. 10 to No. 40 sieve

Fine No. 40 to No. 200 sieve

Fine-grained soils:

Classified according to Atterberg limits (Liquid Limit)

• Silt

• Clay

Unified Soil Classification System: Grain size, grain-size distribution, liquid limit symbols

Grain size symbols:

Gravel G

Sand S

Silt M

Clay C

Organic soil O

Peat Pt

Grain-size distribution symbols:

Well-graded W

Poorly-graded P

Liquid limit Symbols:

High LL (>50) H

Low LL (<50) L

Soils Hazards

1. Settlement

2. Expansive clay

3. Liquefaction

4. Subsidence

Soil Hazard: Settlement

Settlement: Soil deforms under load, leads to volume decrease under a structure

• Tendency of soil to decrease in volume termed compressibility

• Consolidation test

• Test for Void Ratio (e = Vvoid / Vsolid)

Soil Hazard: Liquefaction

Liquefaction: phenomenon in which the strength of soil is reduced by vibrations, shaking, and loading

• Occurs in saturated sandy soils in which the space between particles is filled with water

• Under normal circumstances, sand responds to increased stress by the expulsion of pore water

• Under rapid stress, particles become suspended within pore water

Soil Hazard: Expansive Soil

Expansive soil: a soil that tends to change volume in response to changes in moisture content

• High moisture content = Swelling

• Low moisture content = Shrinking

• Expansive soils are a significant and costly natural hazard

• Volume increase of ≥ 3% is potentially hazardous

• Climate, vegetation,topography arer important factors

Soil Hazard: Expansive soil (Vertisols)

• Expansive soil is often related to the presence of swelling clays

• Due to the presence of smectite clay group

• Montmorillonite, Smectite, Bentonite

• Heaving and cracking of:

• Floor slabs, walls, sidewalks, driveways, and patios

• Ruptured pipelines and underground infrastructure

• Acute problem is central plains of Canada

Engineering solutions to expansive soils

1. Deep foundations that reach below the active swelling zone

2. Soil stabilization with lime or cement

3. Proper drainage around buildings

4. Flexible pavement designs for roads

Soil Hazards: Subsidence

Subsidence: sinking of the land surface

The principal causes are:

• aquifer-system compaction,

• drainage and decomposition of organic soils,

• underground mining,

• oil and gas extraction,

• hydrocompaction,

• natural compaction, sinkholes,

• and thawing permafrost

• U.S. National Research Council, 1991

• + postglacial readjustment in Canada

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Effects of land subsidence include:

• damage to buildings and infrastructure such as roads and canals

• increased flood risk in low-lying areas,

• lasting damage to groundwater aquifers and aquatic ecosystems

Soil Hazard: Rapid subsidence - sinkhole

Sinkhole: circular area of subsidence caused by

collapse into subterranean void

• Occur typically in soluble sedimentary rocks

• Example: limestone

• Diameter ≈ tens of metres

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Cover subsidence (as seen in diagram)

Cover subsidence

Sinkholes near us

Thawing permafrost

Soil Hazard: subsidence - compaction

Compaction: reduction in volume of a soil under load caused by realignment of soil particles into a denser packing

• Densification by expelling air→ occurs mainly in unsaturated soils→ water content stays essentially the same

• All types of soils can be compacted by moving grains closer together

Soil Hazard: subsidence - consolidation

Consolidation: (geotechnical sense) reduction in volume of a water-saturated soil as water flows out under load

• Consolidation by removal of subsurface fluids is usually

initiated by human activity (e.g. pumping, drainage)

• Irreversible

• It takes time for water to flow through fine-grained soil

• Slower than compaction

Difference between Compaction and Consolidation

Erosion

Erosion: detachment and transport of rock fragments by the action of agents such as...

Agents of Erosion

• Liquid water → Surface water and wave erosion

• Ice → Glaciers / Avalanches

• Wind → Eolian erosion

• Gravity → Mass movement

Factors controlling erosional rate

• Rainfall

• Intensity , duration

• Soil characteristics

• Porosity, permeability, moisture content, grain size and shape

• Topography

• Orientation of slope, slope angle, length of slope

• Vegetation

• Type and distribution

Black Canyon of the Gunnison, Colorado

The South rim (to the right) is facing the North: it weathers faster than the North face...

South rim does not receive heat from the Sun keeping moisture, accelerating weathering by freeze-thaw and chemical action

The Gunnison River erodes the material

What is the most effective erosion agent?

Running surface water is the most effective erosion agent

Grand Canyon Highlights

• ~70 Million years ago plate tectonic caused Colorado plateau to rise

• ~5M years ago, Gulf of California opened up & lowered the river’s base

• Delta in elevation formed