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When referring to consistence vs consistency. Which one would a soils scientist use?



What is consistence?

How must it be stated?

The degree of resistance to rupture or deformation when external pressure is applied to a soil.

Must be stated relative to initial soil wetness


What two forces are critical to consistence ?
How do they effect soil?

Adhesion and cohesion
- lead to the attraction of soil particles to pore water and the attraction of soil particles to each other.


Which 6 factors contribute to soil consistency?

-particle size distribution
-Organic matter content
-Clay minerology
-soil solution chemistry (presence of salts)
-soil structure
-sample pretreatment (how soil is handled)


What %porosity are correlated with high and low Db soils

High Db = low porosity
Low Db = high porosity


Does high porosity soil drain well?

No. Because of hydrostatic relations between much smaller pore spaces. ( pores get clogged or are too small for water to permeate )


Which soils are most affected by water?

Low Db soils ( fine textured ) - ex. Silt and Clay


How does the addition of water effect the properties of silty and clayey soils?

-Soil strength decreases as water content increases
-soils swell up when water content increases
-fine grained soils are subject to liquefaction at very high water content


How does decreasing water content affect fine textured soils?

As water content reduces from high to medium, soils become plastic and reduce in volume.

If it is further reduced soils will become semi solid at the point where the volume no longer changes


What are atterburg limits?

The water content boundaries at which soils change states.
LL - Liquid limit: moisture level at which soil begins to behave as a liquid.
SL - Shrinkage limit: The moisture level at which soils will no longer decrease in volume due to loss of moisture.
PL - Plastic limit: The moisture content at which a soil begins to behave as a plastic material.

Can be
-solid (very low moisture)
-semi solid (Low)
-plastic (Medium)
-liquid (Very high)
*Note: The atterburg limits differ depending on the soil type, therefor can sometimes be used to identify soils at a given moisture level.


How are LL and SL determined?
What about PL?

LL and SL are determined as a function of soil moisture content

PL is determined by using a mechanical process.


What is the Plasticity Index or (PI)?

The difference between the Liquid limit and the plastic limit
PI = LL - PL


What plasticity index would a cohesion-less soil have ?

A soil that did not stick together would have zero plasticity index and be termed "non-plastic"


Why is the PI important?

It helps us classify fine textured soils.


What are Phyllosilicates?
might be a bonus Q...

Flat clays that are highly plastic and have a high PI


What type of clays are highly plastic and has a high PI?

flat clays such as Phyllosilicates


What is soil consistence?

The degree and kind of cohesion and adhesion that a soil exhibits. So basically it is a soils degree of firmness.


How is consistence expressed?
how is it measured?

As one of 5 states
-very soft
-very stiff

measured by the soils resistance to a blunt object (there is a field tool called a penetrometer that the second years get to use) but us lowly first years apparently have to use your thumbs.


How do the definitions of consistence vary between soil scientists and engineers.

Consistence - level of resistance to rupture or deformation

Consistency - Resistance to penetration or the strength at which soil particles are held together.

basically same fricken thing...


Briefly define cohesion and adhesion

Cohesion: Attraction of water molecules to each other (hydrogen bonding)

Adhesion: The attraction of water molecules to non water particles such as soil.


Whats the difference between absorption and adsorption?

In adsorption water is drawn toward the substance but does not permeate it

In absorption water is drawn into a substance usually by osmosis.


Briefly describe the plasticity classes

Non plastic - will not form a 6mm diameter, 4cm long wire that will support itself
Slightly plastic - 6mm diameter wire will support itself but 4mm diameter will not
Moderately Plastic - 4mm, 4cm long wire supports itself but 4mm diameter does not
Very plastic - 2mm diameter wire supports itself


What does soil structure refer to?

How primary particles come together with respect to arrangement of soil particles and pore space.


What things is soil structure dependent on?

- Parent material type
- The environmental conditions under which the soil formed
- The amount and type of clay present
- the organic materials present
- the recent history of site and soil management.
- The development of PEDs


What are peds and how are they separated from one another?

Peds are secondary soil structures that form from the aggregation of primary soil particles that are bound together by gluing agents such as oxides and salts.

They are separated from eachother by surfaces of weakness


How is soil structure classified in respect to grade?

-Structureless: No observable ped formation
-Weak: Poorly formed indistinct peds
-Moderate: Well-formed distinct peds, moderately evident but not in undisturbed soil.
-Strong: Durable peds that are quite evident in undisturbed soil and adhere weakly to one another. Become separated when soil is disturbed.


How is structure classified?

Grade form and size of particles


Briefly describe the development of soil structure.

-Soil formation starts with a structureless condition (single grain or massive)
- Describes the formation of Peds and aggregates.
-Soil structure is formed by action that pushes soil particles together.


How does climate affect structure of soil?
Give an ex.

Environmentally driven physical processes that result in changes in the amount and distribution and state of water exhert a major influence on soil structure.
ex. (Shrinking-swelling, freezing-thawing result in volume changes in the soil, which over time produce distinct aggregations of soil particles.)


Where does climate affect soil most?

Effects of climate are most prominent in the upper soil profiles. As the depth below ground increases, the intensity of these processes decrease.