Upload
justina-hart
View
233
Download
1
Embed Size (px)
Citation preview
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