Short note on formation of soils in Geotechnical Engineering

Soils are formed by two factors:

a) Physical disintegration

b) Chemical decomposition of rocks

i) Physical Disintegration: Physical disintegration of mechanical weathering of rocks.

1. Abrasion: Since water, wind and glaciers move over the surface of rock, abrasion and scouring takes place. Due to abrasion, formation of soil starts.
2. Wedging Action of Ice: In cold climates, the wedging action of ice can have a significant impact on rocks and other surfaces. When water enters cracks and pores in rocks and freezes, it undergoes expansion. This expansion exerts immense pressure on the surrounding rock, causing it to crack and break apart. Over time, repeated freeze-thaw cycles can lead to the gradual disintegration of rocks. This natural process, known as the wedging action of ice, is a powerful force that shapes landscapes and contributes to the formation of features such as cliffs, gorges, and talus slopes.
3. Temperature changes: Different minerals of a rock have different coefficient of thermal expansion. Unequal expansion and contractions of these minerals occur due to temperature changes. When the stresses induced due to such changes are repeated many times, the particles get detached from the rocks and the soils are formed.
4. Spreading of roots of plants: When plants, such as trees and shrubs, take root in the cracks and fissures of rocks, they exert forces on the surrounding rock. As the roots grow and expand, these forces act upon the rock, causing the segments of the rock to be forced apart. Over time, this continuous pressure can lead to the disintegration and fragmentation of the rocks themselves. The spreading of roots of plants plays a significant role in the process of weathering and erosion. By penetrating into the rock crevices, the roots not only help to anchor the plants securely but also contribute to the physical breakdown of the rocks.

ii) Chemical Decomposition:

1. Hydrolysis: In this process, water reacts with minerals in the rock, causing them to break down and form new minerals. For example, feldspar can be hydrolyzed to form clay minerals like kaolinite.
2. Oxidation: Oxygen in the air reacts with certain minerals, such as iron, to form oxides or hydroxides. This can give the soil a reddish or yellowish color.
3. Carbonation: Carbon dioxide from the atmosphere dissolves in water to form carbonic acid. This acid can then react with calcium carbonate in rocks, such as limestone, to form soluble calcium bicarbonate. This process is important in the formation of caves.
4. Solution: Some minerals are soluble in water and can be dissolved and carried away by water. For example, salts like sodium chloride can be dissolved and transported in water, leaving behind a residue of other minerals.
5. Chelation: Certain organic compounds, such as acids produced by plant roots, can bond with metal ions in rocks and make them more soluble. This can result in the release of nutrients like iron and aluminum.
6. Ion exchange: In this process, certain minerals in the soil can exchange their ions with surrounding solutions. For example, clay minerals can adsorb and release nutrient ions, making them available to plants.
7. Hydrolysis: Hydrolysis is a key chemical process in which water molecules are dissociated into H and OH ions. This reaction plays a crucial role in various natural phenomena. One fascinating aspect of hydrolysis is its impact on rock minerals. Through this process, hydrogen cations effectively displace metallic ions, such as calcium, sodium, and potassium, resulting in the formation of new soils. This chemical decomposition contributes to the rich diversity and fertility of our planet’s land surfaces. Whether it’s in the erosion of rocks or the formation of mineral-rich soil, hydrolysis is an essential force shaping the Earth’s geology.

These chemical processes play a crucial role in the formation of different types of soil and the availability of nutrients for plant growth.