SOILS AND ENVIRONMENT

We can save 700 lira by not doing soil testing!

Introduction to Soils

Definition varies with viewer• Soil scientists--altered earth materials that can support

rooted plant life

• Engineer--earth material that can be removed without blasting

• Both perspectives are important in environmental geology

Introduction to Soils

• Land Use– Capability– Waste Disposal – site suitability– Natural hazards

• Soil Profiles– Weathering

• Physical– Frost wedging– Thermal changes– Exfoliation/unloading– Organic activity

Talus

Frost Wedging

Mechanical Weathering

Increase in surface area by mechanical weathering

Exfoliation dome

Weathering

• Chemical Weathering– Process by which rocks are

decomposed by chemical alteration (acids, water, gases)

• dissolution: dissociation of solids in water

• Oxidation: reactions with O2 which form oxides or hydroxides

• Hydrolysis: Incorporation of water into the mineral structure. Reaction between the H+ ions and OH- ions of H20

• Carbonation-reaction of CO2 with cations

Weathering• Biological weathering

– Roots– Lichen/moss– Organisms

• Weathering of a granite– Quartz: no alteration; residual mineral– Feldspar: forms clays– Mica: forms clays

Chemical Weathering

Rates of weathering

• Factors:– Particle size/surface area

– Rock characteristics

– Climate (temperature and moisture)

• Stability of individual minerals determined by the pressure and temperature conditions under which they formed

• Goldich stability series

Goldich & Bowen Series

Bowen Goldich

Pyrite Oxidation and Hydrolysis--Results in Aggregate failure in PortlandCement Concrete

Stadium

Sidewalk

Weathering

FeS2 + H2O Fe(O,OH)n + SO2

SO2 + CaCO3 CaSO4 X 2H20

Soils

• Residual vs. Transported • Soil Horizons• Variables

– Climate– Topography– Parent material– Maturity/time– Organic activity/vegetation

Weathering

Weathering

Soil Horizons

• Result of horizontal and vertical movement of materials

• Layering parallel to the surface

• Usually restricted to the upper 2 meter

• May not be a good system for some parts of Florida

Soil Profiles – Horizon Descriptions

• O or A Horizons– Highly conc. organic material– Differences are in % organics – O is the organic litter horizon– A contains more mineral matter

• E Horizon

– Occurs below the A or O (if present)

– Has had its iron-bearing components leached;

• Zone of leaching = E-horizon plus A-Horizon

• B Horizon— Zone of accumulation: enriched in clays, Fe-oxide, silica, carbonate or other materials leached from overlying horizons

• may be several types– Bt-argillic (enriched in translocated

clay minerals)– Bk-contains carbonate layer– Bh-organic matter

• K Horizon – Dominated by calcium carbonate– Soils of this type are called caliche – Typical of dry areas

Soil Profiles – Horizon Descriptions

• C horizon--partially altered parent material• R horizon--unaltered rock horizon

Soil Color--may indicate how well a soil is drained

• O and A may be dark due to organic matter; may be white if leached

• E if present, may be white• Bk may be white, if present• Note: color is also a factor of original parent

material, and Fe-rich materials may produce red soils w/ little soil profile development

Soil Profiles – Horizon Descriptions

B horizon usually shows most color variation

well-drained = well-aerated = oxidizing = reddish color of B horizon

Poorly drained = wet = more reducing conditions for Fe = more of a yellowish color

Soil Profiles – Horizon Descriptions

Soil Texture

• Varies with:– Sand (0.05-2 mm) – Silt (0.05 to 0.002 mm)– Clay (below 0.002 mm)

Tips: – Sand can see ind. Grains w/ naked

eye. Feels gritty; crunches between teeth

– Silt: can see ind. Grains w/ 10X hand lens, feels like flour

– Clay, cannot see ind. Grains w/o microscope, cohesive, mix w/ water and rub on back of hand. When dry won’t dust off

Soil Structure

Descriptions of peds (soil aggregates)

Ind

icat

or

of

Inc

rea

sin

g a

ge

as

b

-ho

rizo

n c

on

ten

t in

cre

ase

s

Relative Profile Development

– Degree of development indicates the age of the soil

– May range from hundreds to hundreds of thousands years

Soil Chronosequences– Arrangement from youngest to oldest based on

profile development– Good indicator of the relative stability of an area :

Important for hazards assessment. (See fig 3.4 in book)

Soil Fertility

• Fertility– Capacity of soils to supply nutrients (N, P, K) for

plant growth

– Some are naturally fertile (soil developed on some glacial deposits and floodplain deposits)

– Can be manipulated using fertilizers and irrigation and adversely affected by interrupting natural processes like flooding

• Water in Soils: can greatly affect strength, and shrink/swell potential– Saturated-- voids are filled with water,

– Unsaturated--void not filled with water

Soil Classification - Taxonomy

• Soil taxonomy (U.S.D.A.)– Basis for classification– Chemical and physical – Genetic scheme (origin implied)

• Six fold hierarchy (see Table 3.1)– Order--11 orders based on:

• Morphology (# and types of horizons present)• Nutrient status• Organic content• Color• General climatic conditions

Soil Classification

– Suborder (each step down from order gets more specific)• Great Group

– Subgroup

» Family

» Series

• Problems:– Useful for agricultural purposes– May be too complex for most applications– Lacks engineering data

• Benefit: Individual county reports. County soil surveys are v. useful info sources

Florida SoilsMap

Distribution of Soils

– FL is State with greatest number of orders• Diverse topography • Diverse climates

Engineering Properties of Soils

• Unified Soil Classification System– Used by engineers and the military– Classification Parameters

• Coarse textures– Size

– Gradation

• Fine textures– Clay content (generally minus-200 mesh, 73 m)

– Organic matter

Unified Soil Classification System

FIN

E-G

RA

INE

D

CO

AR

SE

-GR

AIN

ED

>50 % largerthan 0.074 mm

>50 % smallerthan 0.074 mm

Cla

y

Silt

s

Sa

nd

s

Gra

ve

ls GW = well-graded gravelGP = poorly graded gravelGM = silty gravelGC = clayey gravelSW = well-graded sandSP = poorly graded sandSM = silty sandSC = clayey sandML = siltMH = clayey siltOL = organic silt

CL = silty clayCH = high plastic clayOH = organic clay

Mostly Organics

Clean(<5 % fines) Dirty(>12 % fines)

Clean(<5 % fines) Dirty(>12 % fines)

Non-plastic

Plastic

PT = peat and muck

•Plasticity Index= (liquid limit - plastic limit)

•Values less than 5•may change from a solid to a liquid easily

•Values greater than 35•may expand/contract on wetting

Atterberg Limits

Plasticity (related to water content)

•Proctor Density --this is a laboratory test used to determine the "ultimate" dry density and "optimum" moisture content for a soil

sample. It varies with moisture content

Strength of Soil

• Cohesion in fine grained a consequence of electrostatic

forces (clay minerals). In unsaturated coarser–grained soils grain boundary wetting and surface tension provides cohesion (sand-castle analogy)– Can be destroyed if completely dry (low PI)– Can be destroyed if completely saturated (increased

pore pressure; high PI)

• Frictional forces – Grain-to-grain contact– Function of density, size and shape (roundness)

Soil Strength is a combination of two factors

Engineering Properties of Soils

• Sensitivity--changes in soil strength due to vibration or excavation; clays more sensitive than sand or gravel. Can lead to liquifaction.

• Compressibility--tendency to consolidate or decrease volume; can cause settling. Coarse-grained ( gravels, sand) tend to be less compressible than fine

• Erodibility--ease of removal by wind or water. Function of

cohesiveness and degree of consolidation • Permeability--measures the ease with which a liquid moves through

a material (related to porosity, but not always). Clean sand and gravels are v. permeable. Decreases w/ increasing fines. Clay v. low permeability