Download - SOILS AND ENVIRONMENT We can save 700 lira by not doing soil testing!

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

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


• 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


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 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