3.1 Introduction
As an aid for the soil and foundation engineer, soils have been divide into basic categories
based upon certain physical characteristics and properties. The categories have been rela-
tively broad in scope because of the wide range of characteristics of the various soils that exist
in nature. For a proper evaluation of the suitability of soil for use as foundation or construction
material, information about its properties, in addition to classification, is frequently neces-
sary. Those properties which help to assess the engineering behaviour of a soil and which
assist in determining its classification accurately are termed ‘Index Properties’. The tests re-
quired to determine index properties are in fact ‘classification tests’. Index properties include
indices that can be determined relatively quickly and easily, and which will have a bearing on
important aspects of engineering behaviour such as strength or load-bearing capacity, swell-
ing and shrinkage, and settlement. These properties may be relating to individual soil grains
or to the aggregate soil mass. The former are usually studied from disturbed or remoulded soil
samples and the latter from relatively undisturbed samples, i.e., from soil in-situ.
Some of the important physical properties, which may relate to the state of the soil or
the type of the soil include soil colour, soil structure, texture, particle shape, grain specific
gravity, water content, in-situ unit weight, density index, particle size distribution, and con-
sistency limits and related indices. The last two are classification tests, strictly speaking. These,
and a few properties peculiar to clay soils, will be studied in the following sections, except soil
structure and texture, which have already been dealt with in Chapter 1.
3.2 Soil Colour
Colour of soil is one of the most obvious of its features. Soil colour may vary widely, ranging
from white through red to black ; it mainly depends upon the mineral matter, quantity and
nature of organic matter and the amount of colouring oxides of iron and manganese, besides
the degree of oxidation.
Iron compounds of some minerals get oxidised and hydrated, imparting red, brown or
yellow colour of different shades to the soil. Manganese compounds and decayed organic matter30Chapter 3