Geotechnical Engineering

DHARM

96 GEOTECHNICAL ENGINEERING

fibrous, nature, very low specific gravity (0.5 to 0.8), and very high compressibility and ranks
lowest as a foundation material.

‘China clay’, also called ‘Kaolin’ is a pure white clay, used in the ceramic industry.
All man-made deposits ranging from rock dumps to sand and gravel fills are termed
‘Fill’; thus, fills may consist of every imaginable material.

4.5.2 Geological Classification or Classification by Origin

Soils may be classified on the basis of their geological origin. The origin of a soil may refer
either to its constituents or to the agencies responsible for its present state.

Based on constituents, soils may be classified as :

1. Inorganic soils


2. Organic soils RSPlant lifeAnimal life
   T
   Based on the agencies responsible for their present state, soils may be classified as :
   
   
   1. Residual soils
   
   
   2. Transported soils
      (a) Alluvial or sedimentary soils (transported by water)
      (b) Aeolian soils (transported by wind)
      (c) Glacial soils (transported by glaciers)
      (d) Lacustrine soils (deposited in lakes)
      (e) Marine soils (deposited in seas)
      These have been dealt with in Chapter 1.
      Over the geologic cycle, soils are formed by disintegration and weathering of rocks.
      These are again reformed by compaction and cementation by heat and pressure.
   
   
   
   

4.5.3 Classification by Structure

Depending upon the average grain-size and the conditions under which soils are formed and
deposited in their natural state, they may be categorised on the basis of their structure, as :

1. Soils of single-grained strcture


2. Soils of honey-comb structure


3. Soils of flocculent structure
   These have also been treated in detail in Chapter 1.

4.5.4 Grain-size or Textural Classifications

In the grain-size classifications, soils are designated according to the grain-size or particle-
size. Terms such as gravel, sand, silt and clay are used to indicate certain ranges of grain-
sizes. Since natural soils are mixtures of all particle-sizes, it is preferable to call these frac-
tions as ‘Sand size’, ‘Silt size’, etc. A number of grain-size classifications have been evolved, but
the commonly used ones are :

1. U.S. Bureau of Soils and Public Roads Administration (PRA) System of U.S.A.


2. International classification, proposed at the International Soil Congress at Washing-
   ton, D.C., in 1927.