Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 31

impart black colour to the soil. Green and blue colours may be imparted by famous compounds

such as pyrite. Absence of coloured compounds will lead to grey and white colours of the soil.

Quartz, kaolinite and a few other clay minerals may induce these colours. Light grey colour

may be imparted by small amounts of organic matter as well. Soil colour gets darkened by an

increase in organic content.

Change in moisture content leads to lightening of soil colour. A dark coloured soil turns

lighter on oven-drying. For identification and descriptive purposes, the colour should be that

of moist state and, preferably, of the undisturbed state. In general, clays are darker in colour

than sands and silts because of the capacity of the former for retention of water.

3.3 Particle Shape

Shape of individual soil grains is an important qualitative property. In the case of coarse-
grained soils, including silts, the grains are bulky in nature, indicating that the three principal
dimensions are approximately of the same order.
Individual particles are frequently very irregular in shape, depending on the parent
rock, the stage of weathering and the agents of weathering. The particle shape of bulky grains
may be described by terms such as ‘angular’, ‘sub-angular’, ‘sub-rounded’, ‘rounded’ and ‘well-
rounded’ (Fig. 3.1). Silt particles rarely break down to less that 2μ size (on μ = one micron
= 0.001 mm), because of their mineralogical composition.

Angular Subangular Subbrounded Rounded Well-rounded

Fig. 3.1 Shapes of granular soil particles

The mineralogical composition of true clay is distinctly different from the mineral com-

ponents of other soil types, thus necessitating the distinction between clay minerals and non-

clay minerals. Clay particles are invariably less than 2μ size. Microscopic studies of such soils

reveal that the particle shape is flake-like or needle-like ; clay minerals are invariably crystal-

line in nature, having an orderly, sheet-like molecular structure. Clay particles, in fact, may

consist of several such sheets on top of one another. The clay minerals, kaolinite, illite, and

montmorillonite, show such sheet structure and flaky particle shape.

3.4 Specific Gravity of Soil Solids

Specific gravity of the soil solids is useful in the determination of void-ratio, degree of satura-

tion, etc., besides the ‘Critical Hydraulic gradient’, and ‘Zero-air-voids’ in compaction. It is

useful in computing the unit weight of the soil under different conditions and also in the deter-

mination of particle size by wet analysis. Hence, the specific gravity of soil solids should be

determined with great precision.