Properties of Soils

Weathering

Mechanical: Frost Wedging, unloading, salt crystal growth, stream abrasion, sand blasting, root wedging

Chemical: Acidification of water, hydrolysis of silicate minerals, dissolution of carbonate minerals (calcite-limestone) and some silicates, oxidation of iron cations, chelation of metal cations by organic compounds, hydration

Weathering Products:

gravel (granules, pebbles, cobble, boulders)
----------- 2 mm -----------
sand (mostly quartz)
--------- 1/16 mm ---------
mud (silt-sized quartz, clay-sized clay minerals)
------ molecular size ------
soluble ions (silica, calcium, iron, etc.)

For detailed discussion of weathering click on Weathering


Soils

the importance of soil
1) natural resource (for agriculture, for construction material)
2) soil erosion
3) as a sediment source (via erosion)
4) as a water filter (for water infiltrating the ground)
5) as a contaminant sink
6) as a bearing material (for building on)

definitions of soil
soil scientists and geologists:
     a mix of mineral weathering products and organic matter differentiated into distinct horizons
engineers:
     any unconsolidated material (clay through gravel size) that may be excavated without blasting

To soil scientists and geologists, soil is a mixture of mineral matter produced by the weathering of bedrock or other parent material (e.g., glacial till) and organic matter supplied by plant growth. These materials are modified by continued weathering, downward leaching via infiltrating rainwater, and mixing via animal burrowing.

Soil Forming Processes:

1) physical and chemical weathering of parent material
2) incorporation of organic matter2) incorporation of organic matter
3) downward leaching of soluble ions and translocation of fines (clays)

typical soil profile in temperate climate
O horizon:
100% organic (leaf litter, etc.)
A horizon: organic rich (~5-10%), depleted in fines and soluble ions
     coarse, crumbly texture (lacking clay), dark color (organic carbon)
B horizon: organic poor, enriched in fines and soluble ions
     compact, dense texture, light in color (little organic carbon,
     clays are tan/gray color)
C horizon: physically and chemically weathered parent material
parent material

additional horizons:
E or AEhorizon:
a leached zone above the B horizon
     light in color, high in silica, low in organics and fines
     common in sandy soil (good drainage) and cool climate
     typically associated with forest soil
     soils with E horizon are called podzols
K or BK horizon:
a layer of CaCO3 or enriched in CaCO3 or (Ca,Mg)CO3
      in semi-arid and arid climate, carbonates precipitate from downward percolating water
      where water becomes saturated due to
           - decrease in soil CO2 and therefore acidity (below root zone)
           - or, loss of water due to evapotranspiration (less water -> saturation)
     soils with K horizon are called pedocals

Soil Formation is affected by 6 factors
1) climate
2) vegetation
3) soil organisms
4) parent material
5) topography
6) time

Universal soil Loss Equation (USDA)

     A = f(R,K,L,S,C,P)

meaning, soil erosion is a function of the following factors:

R = erosive impact by rainfall (energy and intensity of rainfall)
K = soil erodibility factor (coarse grains need more energy to transport, clay is cohesive and takes more energy to erode)
L = slope length factor (long slopes gather more water and greater erosive flow velocity)
S = slope angle factor (water flows faster down steeper slopes)
C = cropping management factor (cover crops, no-till, etc.)
P = support practice factor (contouring, strip cropping, etc.)


Soil Classification

- Climate Classification (soil scientists)
based on differences in soil profiles developed in different climate conditions

Tropical - Equatorial: hot and wet: Pedalfers = Laterite
severe chemical weathering and leaching; even silica dissolved and leached
laterite residue = aluminim and iron hydroxides (bauxite and goethite)
soil profile is very deep but is very poor in nutrients

Desert Belts: hot and dry: Pedocals
poorly developed soil profiles because little organic matter and downward leaching
caliche (calcite rich) soil horizon develops due to evaporation of soil moisture

Temperate mid-Latitude: warm-cool, moderate rainfall: Podzol
typical A, B, C soil horizons

Polar: cold and dry
little or no soil development because little vegetation and leaching; permafrost
much frost wedging-mechanical weathering, little chemical weathering


- Textural Classification (engineers)

Soil can be classified according to the percentage of sand vs. silt vs. clay as determined by sieve analysis of soil samples and plotting on a ternary diagram (see right).

Particle-size distribution diagrams are also based on sieve analysis.
They show how well graded a soil is.
A well graded-soil comprises all particle sizes (which to the geologist would constitute a poorly-sorted sediment)

fine-grained (silt and clay rich) soils present engineering problems
clays are sheet silicates and have low shear strength
certain clays (esp. montmorillonite) swell when wet ("fat clays")

 

 

 

A more detailed textural characterization is attained using particle size distribution curves


(from Rahn, 1996)

Soil Gradation

Soil is said to be well-graded if it is composed of a wide spectrum of grain sizes (this would be considered poorly-sorted to a geologist). Well-graded soils typically have the best engineering properties.

As a measure of the gradation of a soil, the uniformity coefficient, Cu, is calculated as the following ratio:

Cu = D60 / D10

D60 = soil particle diameter at which 60% of the mass of a soil sample is finer
D10 = the diameter at which 10% is finer

Cu > 6 is well-graded


Engineering Properties of Soil

Recognize that soil = solid + water + air.

The texture (grain sizes) in a soil will determine how
it will behave in the absence and presence of water.

Soil Moisture and Moisture Potential (Porosity)
water content = (weight of water / weight of solid) * 100%
void ratio = volume of void / volume of solid
porosity = (volume of void / volume total) * 100%
Atterberg Limits
LL Liquid Limit - moisture content at which soil changes from a plastic solid to liquid
PL Plastic Limit - moisture content below which soil changes from plastic to crumbly solid
PI Plasticity Index - Difference between LL and PL (PI = LL - PL)
Other Properties
density (mass per volume)
compaction (artificially increasing the density by loading and/or vibrating)
consolidation (the slow settlement of soil as a load gradually squeezes water out; fast in coarse soils, slow in clay-rich soils)
liquefaction/sensitivity to vibration (sudden change to a liquid-like state induced by shaking)
strength (resistance to deformation)
cohesion (the more clay, the greater the cohesion)
friction
compressibility
erodibility (fine grained materials are easily transported, but clays are difficult to initially erode due to cohesion)
permeability (ease with which water flows through it; finer grained materials are less permeable)
corrosion (tendency to corrode materials and structures, esp. metals, laid in the ground)
ease of excavation

The Unified Soil Classification System (USCS), first developed by Cassagrande to classify soils for building airplane runways on in World War II, and the classification system developed by the American Association of State Highway and Transportation Officials (AASHTO) use sieve analysis and liquid limit and plasticity index to classify soils by type as a general aid in predicting their suitability as a building substrate, any problems that might be expected, and techniques to improve them and mitigate problems.

The best soils are well-graded soils containing gravel and sand (coarse materials have the greatest shear strength), but also silts to fill in the large void spaces and clay to add cohesion.

Fine soils, those rich in silt and/or clay, are problematic.

- The suitability of soil as a load-bearing substrate may be improved by compaction (artificially loading +/- vibration) to increase the soil density / reduce void space. Clay soils have a fairly consistent load-supporting ability (resistance to compaction) over a range of moisture as opposed to silts and sands which are best compacted at a specific moisture content. However, the density of silty and sandy soils may be increased by a greater amount than clay which has low permeability and does not allow water to be squeezed out easily. Although mechanical compaction typically does not affect the entire thickness of the soil column, it will typically suffice for the load requirements for roadways, walkways, etc.. It is not typically done for building foundations.

- Clay soils have high strength when dry due to clay's cohesion, and the dry strength increases with increasing PI. However, they have no shear strength when wet. Silt and fine sand are cohesionless and of low strength when dry but stronger when damp.

- Plastic clay soils are not sensitive to vibration whereas silt and, especially, fine sands may settle or liquefy as a result of vibrations from building activities or earthquakes. The fine sands and silts my initially be in a low density state and the vibration can cause the grains to tumble into a more compact state, but this reduction in void space may cause the soil to be instantly saturated and liquefy.

- Wind-deposited silts, loess, are particularly unstable and prone to large settlement under load when saturated.

- Certain clays (montmorilonite) have high very high shrink-swell potential as a result of wetting and drying. These are so-called "fat clays." As clays absorb water and expand they can exert tons of upward pressure per square foot and can easily disrupt residential foundations and parking lot and roadway pavement.

Summary of engineering properties of fine-grained soils
1 from Military Soils Engineering Manual (1992)
USCS
Soil Type
Value for Embankments1

Compaction
Characteristics1

Value for
Foundations1
AASHTO Grade
For Road Beds
(lower is better)
ML
poor stability, may be used with proper control
good to poor, needs careful moisture control
very poor, susceptible to liquefaction
A-4
CL
stable, impervious cores, and blankets
fair to good
good to poor bearing value
A-6
MH
poor stability
poor to very poor
poor bearing value
A-5
CH
fair stability with flat slopes
fair to poor
fair to poor bearing value
A-7

Problem Soils
loess (may be classified ML or MH in USCS)
fine, loose sands and silts (vibration and/or changing water content can cause large, unequal settlement)
highly compressible clays (soft texture)
highly plastic soils and expansive clays (shrink-swell; anchor foundations below depth of annual moisutre change)