04 - soil origin-1

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Civil & Environmental Engineering Department

Origin of Soil and Grain Size

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The Earth’s Crust is composed of soil and rock

Rock is a natural aggregate of minerals which are

connected by strong bonding or attractive forces.

Soil may be defined as unconsolidated material sediments

and deposits of solid particles that have resulted from thedisintegration of rock.

Origin of Soil


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Rock Cycle

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formed by one of these three different processes

I gneous Sedimentary Metamorphic

formed by cooling of

molten magma (lava)

e.g., granite

formed by gradual

deposition, and in layers

formed by alteration of

igneous & sedimentary

rocks by pressure andtemperature

e.g., limestone, shale e.g., marble

Paren t Rocks


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Intrusive Igneous rocks Deep within the earth; Large Crystals

Diorite Granite

(Light Color)

Gabbro (Dark Color)


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Lava Flow; Volcanoes


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Basalt

Rhyolite

Extrusive Igneous rocks


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Igneous Rock


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Rock Occurrence Texture Major Minerals Minor Minerals

Granite Intrusive CoarseQuartz

Na Feldspar

K Feldspar

Biotite

Muscovite

HornblendeRhyolite Extrusive Fine

Gabbro Intrusive Coarse Plagioclase

Pyroxines

Olivine

Hornblende

Biotite

MagnetiteBasalt Extrusive Fine

Diorite Intrusive Coarse Plagioclase

Hornblende

Biotite

Pyroxenes Andesite Extrusive Fine

Syenite Intrusive Coarse K Feldspar Na Feldspar

Biotite

HornblendeTrachyte Extrusive Fine

Peridotite Intrusive Coarse Olivine

Pyroxenes

Oxides of Iron

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• Rocks--weather --soils--induration--rock

• Clastic rocks

- Hardened under pressure- Cemented by CaCO3 and Iron Oxides

- Generally layered/stratified

- May be hard and durable or weak


Sedimentary Rocks

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Sedimentary Rocks

o Physically Formed Sedimentary Rocks

Conglomerate larger sized particles

Sandstone sand

Siltstone, mudstone, and shale silt and clay

o Chemically Formed Sedimentary Rocks

Limestone calcite

Rock Salt Halite

Dolomite dolomite

Gypsum gypsum

- “If Limestone is present, there is a problem!!!!”

- Engineering characteristics of Sedimentary Rocks

o Sandstone and conglomerate good for foundation

o Mudstone and Shales Problematic


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Cavities in Limestone


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• Rocks --- metamorphism ----metamorphic

rock

• Metamorphism : transformations that occur

under high temperature and pressure

• Foliated Rocks – prone to slippage along

foliation planes

• Nonfoliated Rocks – Sound Rock

Metamorphic Rocks


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o

Gneiss made from granite, gabbro, and dioriteo Slate made from shale and mudstone

o Phyllite made from slate @ the temperature of 250-3000C

o

Schist made from igneous, sedimentary, and metamorphic rocks

o Marble made from calcite and dolomite

Quartzite made from quartz rich sandstone


Metamorphic Rocks

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Metamorphic Rocks

Gneiss

Marble


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• Physical weathering- disintegration of the

rock mass

– Aids chemical weathering

• Chemical weathering - decomposition

/chemical alteration of the parent

minerals

Weathering


Weathering – the physical breakdown(disintegration) and chemical alteration

(decomposition) of rock at Earth’s surface

h

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Weathering

Mechanical or Physical Weathering


Mechanical weathering – breakingof rocks into smaller piecesTypes of mechanical weathering

•Frost wedging•Unloading•Thermal expansion•Biological activity•Slaking

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Exfoliation, or sheet joints, are common in massive

plutonic rocks. These are likely produced by a combination

of mechanisms, not simply load removal.

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Slaking is a physical process that describes spontaneous

breakdown of rock due to a number of factors, such as

exposure to air or intense heat, or submersion under water.

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Chemical weathering• Breaks down rock components and internal

structures of minerals

• Most important agent is water – Responsible for transport of ions and molecules

involved in chemical processes

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• Major processes of chemical weathering

• Dissolution – Aided by small amounts of acid in the water

• Oxidation

– Any reaction when electrons are lost from one element

• Hydrolysis – The reaction of any substance with water

– Hydrogen ion attacks and replaces other ions

o

H2O + CO2 H2CO3

o H2CO3 H+ + (HCO3)-

o 2K(AlSi3O8) + 2H+ + H2O 2K+ + 4SiO2 + Al2Si2O5 (OH)4 Orthoclase kaolinite

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~ in situ weathering (by

physical & chemical

agents) of parent rock

Parent Rock

Residual soil Transported soil

~ weathered and

transported far away

by wind, water and ice.


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Transportation of Weathered Products


(1) Glacial soils: formed by transportation anddeposition of glaciers.

(2) Alluvial soils: transported by running water anddeposited along streams.

(3) Lacustrine soils: formed by deposition in quiet

lakes (e.g. soils in Taipei basin).(4) Marine soils: formed by deposition in the seas

(Hong Kong).

(5) Aeolian soils: transported and deposited by the

wind (e.g. soils in the loess plateau,China).

(6) Colluvial soils: formed by movement of soil from itsoriginal place by gravity, such asduring landslide (Hong Kong ). (from

Das, 2007)

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Particle sizes (USCS Classification)

Gravel: > 4.75 mmSand: 4.75 mm - 0.075 mmSilt: 0.075 mm - 0.002 mmClay: < 0.002 mm

Constituents of Soil


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Clay minerals are made of two distinct structural

units.

Silicon tetrahedron

0.29 nm

aluminium or

magnesium

hydroxyl or

oxygen

0.26 nm

oxygen

silicon

Aluminium Octahedron


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tetrahedron

Several tetrahedrons joined together form atetrahedral sheet.

Tetrahedral Sheet

hexagonal

hole


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For simplicity, let’s represent silica tetrahedral sheet by:

Si

and alumina octahedral sheet by:

Al

Tetrahedral & Octahedral

Sheets


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Different Clay Minerals

Different combinations of tetrahedral andoctahedral sheets form different clay minerals:

1:1 Clay Mineral (e.g., kaolinite, halloysite):


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Kaolinite

Si

Al

Si

AlSi

Al

Si

Al

oined by strong H-bond

no easy separation joined by oxygen

sharing


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Different Clay Minerals

Different combinations of tetrahedral andoctahedral sheets form different clay minerals:

2:1 Clay Mineral (e.g., montmorillonite, illite):


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Montmorillonite

Si

Al

Si

Si

Al

Si

Si

Al

Si

0.96 nm

joined by weak

van der Waal’s bond

also called smectite; expands on contact with water


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A Clay Particle

Plate-like or Flaky Shape


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Clay Fabric

face-to-face contact

edge-to-face contact

Flocculated DispersedCivil & Environmental Engineering Department

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smectite


Kaolinite

Identifying clay minerals

Electronic Scanning Microscope

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>50% highly expansive soil

<50% highly expansive soil>50% moderately expansive soil<50% moderately expansive soil>50% less expansive soil<50% less expansive soil

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04 - soil origin-1

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