‌‍‍‍Soil

For engineering purposes, soil is defined as the uncemented aggregate of mineral grains and decayed organic matter (solid particles) with liquid and gas in the empty spaces between the solid particles. soil is used as a construction material in the various civil engineering projects, and it supports structural foundation.

soils are formed by either (i) physical disintegration or (ii) chemical decomposition of rocks.

Physical disintegration or mechanical weathering of rocks to smaller particles is due to the action of agents such as the expansive forces of freezing water in fissures, due to sudden changes of temperature or due to the abrasion of rock by moving water or glaciers. Temperature changes which cause volume change set up tensile and shear stresses in the rock alternately leading to the fracture of even large rocks. This types of rock weathering takes place in very significant manner in variation in temperature at sunrise and sunset.

Erosion by wind and rain is very important factor and a continuing event.cracking forces by growing plants and roots in voids and crevasses of rock can force fragments apart. Coarse grained soils, such as gravel and sand are formed by the process ophysical disintegration.

when chemical decompostion or chemical weathering of rocks takes place, original rock minerals are transformed into new minerals by chemical reactions. Chemical decomposition can transform hard rock minerals into soft, easily carbonation, desilication and leaching. Chemical decomposition of rock results in the formation of clayey soils.

on the basis of origin of their constituent (formation of soil), soils can be divided into two large groups:

1. Residual soils
   • Residual soils are those that remain at the place of their formation as a result of the weathering of parent rocks. The depth of residual soils depends primarily on climatic conditions and the time of exposure. An important characteristics of residual soil is that the sizes of grains are indefinite
2. Transported soils
   • Transported soils are soils that are foundat locations far removed fran their place of formation. The Transporting agencies of such soils are glaciers, wind and water. Transported soils are further classified according to the transporting agency and method of deposition.
     1. Alluvial deposit: Soils that have been deposited from suspension in running water.
     2. Lacustrine deposit: Soils that have been deposited from suspension in still, freshwaters of lakes.
     3. Marine deposit: Soils that have been deposited from suspension in sea water.
     4. Aeolian deposit: Soils that have been transported by wind.
     5. Glacial deposit: Soils that have been transported by ice.

Many of these transported salsare loose and soft to a depth of several hundred feet. Therefore, difficulties with foundations and other types of construction are generally associated with transported soils.

Names of soils that are generally used in practice:

1. Bentonite: It is the clay formed by the decomposition of volcanic ash with a high content of montmorillonite. It exhibits the properties of clay to an extreme degree.
2. Black cotton soil: It is a residual sail containing high percentage of the clay minerals montmorillonite. It has very low bearing capacity and high swelling and shrinkage properties.
3. Kaoline, China clay: It is very pure form of white clay used in ceramic industry.
4. Calcarious soil: It is the soil containing calcium carbonate.
5. Marl: It consists of calcareous sands, clays a loam.
6. Caliche: It is an admixture of clay, sand and gravel cemented by calcium carbonate deposite from the ground water.
7. Peat: It is fibrous aggregate of finer fragments of decayed vegetable matter. It is very compressible.
8. Loam: It is a mixture of sand, silt and clay.
9. Shale: It is a material in the state of transition from clayto slate.
10. Varved Clays: These are secondary deposits consisting of alternate thin layers of silt and day.
11. Muck: It denotes the mixture of fine soil particles and highly decomposed organic matter.
12. Loess: It is windblown deposits of silt. It is generally of uniform gradation, with the particle size between 0.01 to 0.05 mm.

Reference:

1. a reference book Soil Mechanics for Bachelor of Engineering by Er. B. Pandit and et.al.