Soil Formation

Geotechnical Engineering

The application of the principle of geomaterials (soil and rocks)

Geotechnical Engineering Application

Foundations, Slopes, Pipelines, Roadways, Retaining walls, Dams, Landfills, Tunnels, underground constructions, geothermal energy, geological carbon storage

What do Geotechnical Engineers Do?

Cycle

Construction Site → Soil Samples → Geo-Laboratory → Soil properties → Design Office (design and analysis) → Design details

Soils starts from ______ parent material or ______

rock, magma

Percent of Oxygen by weight

47% of earth’s crust

Percent of Silicon by weight

28% of earth’s crust

Percent of Aluminum by weight

8% of earth’s crust

Percent of Iron by weight

5% of earth’s crust

Percent of Calcium by weight

4% of earth’s crust

Percent of Sodium by weight

3% of earth’s crust

Percent of Potassium by weight

3% of earth’s crust

Percent of Magnesium by weight

2% of earth’s crust

Percent of Hydrogen by weight

0.15% of earth’s crust

Igneous Rocks

Formed from magma (molten rock materials) emitted from volcanoes that has cooled and solidified.

Sedimentary Rocks

formed from sediments and animal and plant materials that are deposited in water or on land on the earth’s surface and then subjected to pressures and heat

Metamorphic Rocks

Formed deep within the earth’s crust from the transformation of igneous and sedimentary rocks into denser rocks

Rock Cycle

Bowen’s Reaction Series

Bowen’s Reaction Series Basic Rocks (Discontinuous Series) Characteristics

• Dark in color

• low silica content by may contain iron, magnesium

• form fine-grained soils (clay and silt) due to weathering)

• As temperature decreases, the composition of the minerals changes significantly

• If the melt has enough silica, mineral will change to next mineral in the lower series as temperature reduces

• If silica is not enough or temperature drops too fast, lower mineral will not form

Bowen’s Reaction Series Acidic Rocks (Continuous series)

• light in color

• high silica content (greater than 60%), with minerals like quartz and feldspar

• Typically rich in sodium and potassium, with low levels of iron and magnesium

• Form coarse-grained soils (sand and gravel) due to weathering

• As temperature/pressure decreases, minerals crystallize gradually, with plagioclase, feldspar changing from calcium-rich to sodium-rich

• If the melt has enough silica, it will form quartz and potassium feldspar at lower temperatures

• If silica is not enough or temperature drops too, fast these minerals won’t form

Physical Weathering is caused by and reactions to

Reactions

• Reduce particle size

• No change in chemical composition

• Make the coarse fraction of soil (gravel, sand, and silt)

Caused by

• Thermal Stress

• Mechanical stress (Wedging, rolling, scraping and abrasion, and human activates)

Length and specific surface area of Gravel/Sand

Length: 4.75 mm

Specific Surface Area: 1.3 1/mm

Length and specific surface area of Sand/Silt

0.075 and 80

Length and specific surface area of Silt/Clay

0.002 and 3000

Specific Surface Area Formula

Surface Area/ Weight

Surface Area Formula of a Cube

6L² / L³ =6 / L

Surface Area Formula of a Sphere

pi ( D² ) / 1/6 (pi) (D³) = 6/D

Chemical Weathering Reduction in Form

• Chemically Breaks Down Parent Material

• Changes Parent into Something Else

• Usually reduces unit weight (increasing bulk)

• New form is more stable in some ways than old material

Chemical Weathering form the _____ fraction of the soil.

fine (clay)

Hydration

Chemical Weathering process where water enters structure

Hydrolysis

Chemical Weathering process that breaks its chemical bond

Oxidation

iron-rich minerals

Carbonization

removal of limestone by solution

Common Clay Minerals

Silicon Tetrahedron: Si₁ O₄

Aluminum Octahedrons: Al₁ OH₆

Common Clay Types

Kaolinite, Illite, and Montmorillonite

Kaolinite Characteristics

• Stable physically and electrically

• Not affected much by water

• Stable so it can be used for ceramic, medicines, cosmetics, food

Kaolinite Mineral Structure Thickness and Composition

0.72 nm and silicon tetrahedron (upper layer) and aluminum octahedron (lower layer) (Hydrogen bonds)

Montmorillonite Characteristics

• Not stable physically or electrically

• very affected by water

• Crystal attract water and cations

• used for clay liners, seals for wells

Montmorillonite Composition

Silicon Tetrahedron (upper layer) and Aluminum Octahedron (middles layer) and Silicon Tetrahedron (lower layer) , Potassium bonds

Clay absorbs ______ and ______ to form ________.

water, ionic cations (salts), micelle

What is the structure of clay deposits?