1.2. Basic Terms and Their Definitions

1. SOIL: (to an engineer) Soil is the unaggregated or uncemented deposits of minerals and/or organic particles or fragments covering a large portion of the earth’s crust.It includes widely different materials like boulders, sands, gravels, clays and silts, and the range in the particle sizes in soil may vary from grains of micron size in diameter up to the large boulders.As per the geologist, Soil is the material in the relatively thin surface zone within which roots occur, and all the rest of the crust is grouped under the term rock irrespective of its hardness.

2. ROCK: A rock forms a portion of the earth’s crust having no definite shape or chemical composition.It is usually a mixture of two or more minerals and is not homogeneous.

3. ORIGIN OF SOIL: Soil is considered by the engineer as a complex material produced by the weathering of the solid rock.The formation of soil is the result of the geologic cycle continually taking place on the face of the earth. The cycle consists of weathering or denudation, transportation, deposition and upheaval, again followed by weathering and so on.

4. FORMATION OF SOILS: The soils are formed either by a. Physical/Mechanical Weathering & b. Chemical Weathering.

Physical/Mechanical Weathering: Mechanical weathering or physical disintegration of the parent rocks can occur due to the following processes: 

1. Temperature changes

2. Abrasion

3. Wedging action of ice

4. Spreading of roots of plants.

 b. Chemical Weathering; Easy transformation of hard rocks to soft and erodible materials is conducted through the process of chemical weathering or chemical decomposition. The principal types of chemical decomposition happening in rocks are 

• Hydration 

• Carbonation

• Oxidation                                                                                                 

• Solution                                                                                                

• Hydrolysis.