Geotechnical Engineering

4.1 Introduction

It has already been stated that certain terms such as ‘Gravel’, ‘Sand’, ‘Silt’ and ‘Clay’ are used
to designate a soil and are based on the average grain-size or particle-size. Most natural soils
are mixtures of two or more of these types, with or without organic matter. The minor compo-
nent of a soil mixture is prefixed as an adjective to the major one—for example, ‘silty sand’,
‘sandy clay’, etc. A soil consisting of approximately equal percentages of sand, silt, and clay is
referred to as ‘Loam’. The differentiation between ‘coarse-grained soils’ and ‘fine-grained soils’
has already been brought out in Chapters 1 and 3.

In this chapter, certain procedures for field identification of the nature of a soil, as well
as certain generalised procedures for classification of a soil with the help of one of the systems
to be dealt with, will be studied in some detail.

4.2 Field Identification of Soils

Basically, coarse-grained and fine-grained soils are distinguished based on whether the indi-
vidual soil grains can be seen with naked eye or not. Thus, grain-size itself may be adequate to
distinguish between gravel and sand : but silt and clay cannot be distinguished by this tech-
nique.

Field identification of soils becomes easier if one understands how to distinguish gravel
from sand, sand from silt, and silt from clay. The procedures are given briefly hereunder :

Gravel from Sand

Individual soil particles larger than 4.75 mm and smaller than 80 mm are called ‘Gravel’ ; soil
particles ranging in size from 4.75 mm down to 0.075 mm are called ‘Sand’, (Refer IS : 1498-
1970 ‘‘Classification and Identification of Soils for General Engineering Purposes’’—First Re-
vision). These limits, although arbitrary in nature, have been accepted widely. The shape of
these particles is also important and may be described as angular, sub-angular, rounded, etc.,
as given in Sec. 3.3. Field identification of sand and gravel should also include identification of
mineralogical composition, if possible.

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Chapter 4

IDENTIFICATION AND

CLASSIFICATION OF SOILS

DHARM