Geotechnical Engineering

DHARM

IDENTIFICATION AND CLASSIFICATION OF SOILS 105

Boundary Classification for Fine-grained Soils
The fine-grained soils whose plot on the plasticity chart falls on, or practically on A-line, wL =
35-lines, wL = 50 and lines shall be assigned the proper boundary classification. Soils which
plot above the A-line, or practically on it, and which have a plasticity index between 4 and 7
are classified ML—CL.

Black Cotton Soils

These are inorganic clays of medium to high compressibility. These are characterised by high
shrinkage and swelling properties. When plotted on the plasticity chart, these lie mostly along
a band above A-line. Some may lie below the A-line also. ‘Kaolin’ behaves as inorganic silt and
usually lies below A-line; thus, it shall be classified as such (ML, MI, MH), although it is clay
from the mineralogical standpoint.

The classification procedures for coarse-grained soils and for fine-grained soils, using
this system, may be set out in the form of flow diagrams as shown in Figs. 4.5 and 4.6.

Relative Suitability for General Engineering Purposes

The characteristics of the various soil groups pertinent to roads and airfields value as subgrade,
sub-base and base material, compressibility and expansion, drainage characteristics, and
compaction equipment (in qualitative terms), ranges of unit dry weight. CBR value percent,
and sub-grade modulus—are also tabulated.

Characteristics pertinent to embankments and foundations—value as embankment
material, compaction characteristics, value as foundation material, requirements for seepage
control (in qualitative terms), ranges of permeability and unit dry weight—are also tabulated.

Characteristics pertinent to suitability for canal sections—compressibility, workability
as a construction material and shearing strength when compacted and saturated are also
given in relative or qualitative terms.

This information is supposed to serve the purpose of a guideline or an indication of the
suitability of a soil based on the I.S. Classification System. Important and large projects will
need detailed investigation of the soil behaviour—first-hand. A comparison between IS Classi-
fication and Unified Classification shows many points of similarity but only a few points of
difference, especially in classifying fine-grained soils.

4.6 Illustrative Examples

Example 4.1: Two soils S 1 and S 2 are tested in the laboratory for the consistency limits. The

data available is as follows :

Soil S 1 Soil S 2

Plastic limit, wp 18% 20%

Liquid limit, wL 38% 60%

Flow index, If 10 5

Natural moisture content, w 40% 50%

(a) Which soil is more plastic?

(b) Which soil is better foundation material when remoulded?