The Origin of Soils

8/3/2019 The Origin of Soils

1/23

CE 330: Soil Mechanics

Basic Geologyand the Origin of Soils

Dante Fratta

University of Wisconsin-Madison


2/23

Definitions

Geology: is the science that deals with rocks,

minerals, and subsurface rocks. This science also

studies the formation, structure and behavior of

these materials.

Engineering Geology: deals the the application of

geologic concepts into engineering practice.


3/23

Rocks and Soils

Rocks for a geologist is any natural formedaggregate or mass of mineral matter, whether ornot coherent, constituting an essential and

appreciable part of the earth crust

Rocks for an engineer is a hard durable materialthat cannot be excavated without blasting

These definitions are sometimes conflicting.


4/23

Rocks and Soils

Most rocks are cemented Most rocks have low

porosity

Weathering can greatly alterthe rocks properties.

Depending on scale, rocksare considered adiscontinuous material.

Rocks can have the largestprincipal stress either in thevertical or horizontaldirection (complex stresshistory).

Most soils are not cemented Most soils have large

porosity

Weathering barely alters thesoil properties.

Depending on scale, soils areconsidered a continuousmaterial.

Most soils have the largestprincipal stress in thevertical directions.

AM consultants KSU


5/23

Minerals

Minerals are naturally occurring solids withspecific structures and chemical composition.

There are thousand of minerals, and the vast

majority form rocks (rock-forming minerals) Properties of minerals control the properties of

rocks

Identifications is done using physical (color, Mohs

Scale), chemicals properties (evaluatingreactions), and structure (microscope, X-raydiffraction)


6/23

MineralsMohs Scale

1 Talc (scratched by

finger nails)

2 Gypsum (~ 2.2

fingernail)

3 Calcite (~ 3.2 copper

penny)

4 Fluorite5 Apatite (~ 5.1

pocketknife)

6 Orthoclase (~ 6.5 steel

needle)

7 Quartz (scratches glass

with ease)

8 Topaz (scratches

glass)

9 Corundum (cuts glass)10 Diamond


7/23

Minerals - Examples

Quartz (SiO2): is a verycommon mineral. Mohshardness 7. Chert is a type

of amorphous silicate thatmay react with Portlandcement.

Feldspar (orthoclaseKAlSi3O8 and plagioclase

NaAlSi3O8 ): is the mostabundant mineral. Mohshardness 6 (orthoclase)


8/23

Minerals - Examples

Calcite (CaCO3): is soluble inwater which can transport it. Itmay cement soils when

precipitate out of solution. Itreacts with hydrochloridic acid.Mohs hardness 3.

Dolomite: similar composition tocalcite (it has magnesium)

Mica (Muscovite is white;Biotite is black): semi-transparent flakes or sheets withvery low friction coefficient.

Pitt Univ.

Pitt Univ.


9/23

Minerals - Examples

Gypsum: it is a whitish mineral that has cementing

properties. It is used in the manufacturing of Portland

cement and dry walls. It is soluble in water.

Pyroxene, Amphibole, Hornblende, Olivine: areferromagnesian minerals.

Limonite, Magnetite: are minerals that contain iron

(Fe2O3). They have rusty color (e.g., Georgia clay)

Ice (solid H2O). Water is not considered a mineral. Diamond: is the hardest mineral of all. It is composed of

pure carbon.


10/23

The Geologic Cycle

The geologic cycle explains the process of rock

and soil formation

Molten

MAGMA

Igneous

RockMetamorphic

Rock

Sedimentary

RockSoils

(Coduto 1999)

metamorphosis

metamorphosisweatheringweathering


11/23

The Geologic Cycle

Igneous Rocks: they are formed by the cooling of lava.

There are two types: intrusive and extrusive rocks.

Intrusive rocks cool very slowly and have large grains(e.g., granite, diorite, gabbro)

Extrusive rocks cool very fast and have small grains (e.g.,

basalt, andesite, rhyolite).


12/23


13/23

The Geologic Cycle

Sedimentary Rocks: are formed

the the transformation of soils

back to rock by a process

known as induration orlithification.

There are two types of rocks:

clastic and carbonate rocks.

CalState-Los Angeles


14/23

The Geologic Cycle

Clastic rocks: are formed by the hardening of soil deposits

due to the pressure of overlying materials and cementation

of water soluble minerals, iron oxides and carbonates.

They usually show layers (strata). These layers are known

as bedding planes.

Typical examples include: sandstone, claystone,

conglomerate, shale (indurated clays)

Slacking: is the process by which fine-grained clastic rock

deteriorate due to excavation and exposure. Slacking can

be a problem for engineering structures.


15/23

The Geologic Cycle Carbonate rocks: they are formed by organic materials that

accumulate and become indurated.

Typical examples include limestone (CaCO3water and

carbonic acid soluble may trigger sinkhole big

problem in Florida, Karst topography), chalk (a very soft

rock), dolomite (mineral dolomite instead of calcite).

(from USGS 2000)


16/23

The Geologic Cycle

Metamorphic Rocks: they are formed by the action of largepressure and temperature. These actions produce changes in

mineral-forming rocks.

The metamorphic process generally improves the engineering

properties of the rock as it reduces the porosity, and increasesthe hardness and the strength.

Examples: Foliated (slatederived from shale, schistlarge

mica content, gneissderived from granite) and non-foliated

rocks (quartzite and marble).

Scienceclarified.com


17/23

Formation, Transport and

Deposition of Soils Residual Soils: these are soils that remain in the same

place of the formation (the rate of formation is faster than

the rate of transport).

These types of soils are typical of tropical regions.

Deposits can be several hundred meters deep.

Examples: decomposed granite, saprolite (rotten rock; e.g;

Piedmont formation, GA), literite (tropical regions, usually

cemented with iron oxidesrusted color)


18/23


Deposition of Soils Glacial Soils: the action of glaciers grinds down soils and

rock and transports the resultant materials over largedistances.

The formed material is very heterogeneous. There areparticles of different sizes and mechanical properties.

Examples: Till: directly deposited by the glacierE.g., ablation (morraines)

and lodgement (hardpan) tills

glacio-fluvial: are formed after the ice melted due to outwash

glacio-lacustrine: formed by the deposition of fine soils in largebodies of water, they are usually layered


19/23


Deposition of Soils Glacial Soils


20/23


Deposition of Soils Alluvial Soils (fluvial soils): are soils transported by

streams and rivers.

Very common soils, found in rivers flood plains.

They usually have large groundwater aquifers.

These soils are segregated by size along the length of the

river (large particlesgravels and sand - at the origin and

small particles at the deltasilts and clays).

In arid areas, the evaporation of water leaves cemented

agents that are deposited in the soil forming very hard

materials known as caliche.


21/23


Deposition of Soils Lacustrian and Marine Soils: lacustrian are soils deposited in

lakes, while marine soils are deposited in the sea and ocean

floor.

Grain sizes vary from silts to clays and deposited in uniformlayers or poor engineering properties

Deltas are a common example of marine soils (Mississippi,

Nile, Amazon, and Parana deltas).

Univ. of Idaho


22/23


Deposition of Soils Aolian Soils: are soils transported and deposited by the

wind. They are very poorly graded (uniform) and show

very high porosity.

The transport mechanisms include: suspension (dust

storms), saltation (dunes), and creep (dunes).

Colluvial Soils: are soils transported by gravity, either

slowly or fast.

Examples: downhill creep, landslide, mudflow.


23/23

Bibliography

Coduto, D. (1999). Geotechnical Engineering. Principles and Practice.

Prentice-Hall.

McCarthy, D. (1998). Essential of Soil Mechanics and Foundation.

Prentice-Hall.

Budhu, M. (2005). Soil Mechanics and Foundations. Wiley. Encyclopedia Britannica (2001). Web Site. http://britanica.com

Liu, C. and Evert, J. B. (2001). Soils and Foundations. Prentice-Hall.

Documents

The Origin of Soils