GEOL 446Environmental Geology

PART IFOUNDATIONS OF ENVIRONMENTAL

GROLOGY

Chapter 1: philosophy and Fundamental Concepts

Chapter 2: Earth Material and Processes

Chapter 3Soils and Environment

3. Soils and the Environment

Soil is one of the most important earth materials we encounter each day, but the definition of soil is difficult.

Soil Scientists (and most ordinary people):• fine-grained, well-weathered earth material that is able to

support plant growth• focus on the physical and chemical properties

Engineers:• any earth material that can be removed without blasting• focus on particle size and the amount of organic material• engineering applications

Environmental Geologists:• must understand soil from many perspectives• characteristics affect agriculture, engineering,

hydrology, natural hazards and other aspects of land use• soil development and soil character is crucial to good

land use planning.

Read Table 3.1 (Soil Taxonomy)•

Understand the meaning of soil types, but do not memorize all of them.

Read Table 3.2 (Unified Soil Classification)• Learn the definition of each constituent that makes up soil.

3. Soils and the Environment

Soil Development

Soil is an important part of the geologic cycle and soil characteristics are influenced by parent material, climate, topography, weathering, and the amount of time a particular soil has had to develop.

As soil develops, weathering creates distinct layers in soil. We call these layers soil horizons, and each soil horizon has distinctive characteristics. Every soil has a soil profile, a list of the horizons that describe a

particular soil .

Soil HorizonsMaterials in a Soil System:

Vertical and horizontal movements create a soil profile made up of distinct layers parallel to the surface, which are called soil horizons.

Organic top layer (O)Zone of leaching (A and E)

Zone of accumulation (B)

Rock (weathered & unweathered) (C and R)

Soil

Rock

Soil Horizons

O Mostly organic materials, decomposing leaves, and twigs. Often dark brown color.

A Mineral and organic materials, light black to brown. Leaching of clay, Fe and Ca.

E Light colored materials due to leaching of clay, Ca, Mg, and Fe to lower horizons. Horizons

A and E make up the Zone of Leaching.

B Enriched in clay, Fe oxides, Silica, carbonate and other material leached from above. This is

the Zone of Accumulation.

C Partially altered (weathered) parent material, which is either rock or loose sediment.

R Unweathered (unaltered) parent material = rock.

~3m

Soil Development• A soil’s profile depends on its age and its conditions of

formation. Soil profile is the primary criteria for soil classification.

• Soils can be compared in terms of their relative development. Weakly developed soil profiles are generally younger and may have fewer horizons; well-developed soils are generally older and have more horizons.

Chronosequences

• Relative development of a series of soils allows their arrangement in a soil chronosequence. A soil chronosequence gives information about the history of the landscape. The relative development of the soils in a chronosequence tells the investigator about the climate and depositional history of the area.

Soil Texture

Texture = relative proportion of sand, silt and clay.

Texture classes:

Coarsesands, loamy sand and sandy loams with less than 18 % clay, and

more than 65 % sand.

Mediumsandy loams, loams, sandy clay loams, silt loams with less than 35

% clay and less than 65 % sand; the sand fractions may be as high as 82 % if a minimum of 18 % clay is present.

Fineclays, silty clays, sandy clays, clay loams and silty clay loams with

more than 35 % clay.

Clay(%) Silt(%)

Sand(%)

100 % CLAY

100 % SILT

100 % SAND

Clay

Sand Silt

Clay loam

LoamSandy loam Silt loam

See Figure 3.2in textbook

Soil ClassificationSoils are often referred to as being sandy or clayey, or silty.

Different countries use different standards to define sand particle and silt particle sizes.

Particle sizes

Gravel, Cobbles, and Boulders particles greater than 2 mm diameter

Coarse and medium sand particles from 2 mm to 0.2 mm diameter

Fine and very fine sand particles from 0.2 mm to 0.074 mm diameter

Silt particles from 0.074 mm to 0.004 mm diameter

Clay particles less than 0.004 mm diameter

Soil Classification

WELL SORTED WELL GRADED

Unified Soil Classification SystemF

INE

-GR

AIN

ED

CO

AR

SE

-GR

AIN

ED

>50 % largerthan 0.074 mm

>50 % smallerthan 0.074 mm

Cla

ys

S

ilts

S

and

s

G

rave

ls GW = well-graded gravelGP = poorly graded gravelGM = silty gravelGC = clayey gravelSW = well-graded sandSP = poorly graded sandSM = silty sandSC = clayey sandML = siltMH = micaceous siltOL = organic siltCL = silty clayCH = high plastic clayOH = organic clayPT = peat and muckMostly Organics

Clean<)5 % fines(

Dirty>)12 % fines(

Clean<)5 % fines(

Dirty>)12 % fines(

Non-plastic

Plastic

Water in Soils

Types of water:

Water on Earth is known by different terms, depending on where it is and where it came from.

• Meteoric water = water in circulation.

• Connate water = "fossil" water, often saline.

• Juvenile water = water from the interior of the earth.

• Surface water = water in rivers, lakes, oceans, etc

• Subsurface water = groundwater, connate water, soil, capillary water.

• Groundwater in the zone of saturation, may be fresh or saline.

Water in Soils

Moisture Content of soil is calculated as follows:

W = weight, so that:

[(Wwet - Wdry)/Wdry] x 100 = H2O content (%)

Moisture content affects the engineering properties and stability of soils. A soil that is stable in dry conditions may become unstable to support the structures built on it when saturated with water.

Engineering Properties of Soils

Plasticity• related to the water content

Liquid Limit (LL)

• water content above which the soil behaves like a liquid

Plastic Limit (PL)

• water content below which the soil is no longer plastic

Plasticity Index (PI), PI = LL - PL

• range of water contents that make the soil behave as a

plastic material

• Low PI (5 %): small change in water content, soil changes from solid to liquid

• High PI (< 35%): potential to expand and contract on wetting and drying

Engineering Properties of Soils

Expansive Soils• high content of swelling clay (montmorillonite)• soils swell when water is incorporated between clay plates• shrinking occurs when soil is dried• damage to building and road foundations

Study Table 3.3 in textbook to understand more about soil descriptions and their significant properties.

• Study the Universal Soil Loss Equation (erosion) :

A = RKLSCP

Universal Soil Loss Equation

• A = RKLSCP

A = long-term average annual soil loss for the site

R = long-term rainfall runoff erosion factor

K = soil erodibility index

L = hillslope/length factor

S = hillslope/gradient factor

C = soil cover factor (crop/vegetation and mgmt. factor)

P = erosion-control practice factor

• Used to predict the impact of sediment loss on local streams and other resources and to develop management strategies for minimizing impact.

Water in Soils

FOREST Precipitation

Interception

Evapotranspiration

Soil

Rock

Water infiltrates andruns through soil

Water in Soils

Farming Precipitation

Less Interceptionand EvapotranspirationSoil

Rock

Increased surface runoffand soil erosion fromfreshly plowed land

Increased sedimentin channel

Water in Soils

CLEARCUT Precipitation

Little Interceptionand Evapotranspiration

Soil compaction

Rock

Increased surface runoffand sediment; weaker soil

More sedimentin channel

Water in Soils

URBANIZATION Precipitation

Soil

Rock

Large increase in runofffrom urban surfaces andstorm sewers

Key Terms to Review:

• weathering• soil horizons• soil profile development• soil chronosequence• soil fertility• unified soil classification• soil strength• soil sensitivity• liquefaction

• compressibility• erodibility• permeability• corrosion potential• shrink-swell potential• expansive soils• soil pollution• desertification• water table• soil plasticity index