Notes of soil classifications

1. Soil Classification
Lecture 3 (Week 1)

2. Description vs classification
• It is necessary to adopt a formal system of soil
description and classification in order to describe the
various materials found in ground investigation. Such
a system must be meaningful and concise in an
engineering context, so that engineers will be able to
understand and interpret.

3. Soil description
Description of soil is a statement that
describes the physical nature and state of
the soil. It can be a description of a
sample, or a soil in situ. It is arrived at by
using visual examination, simple tests,
observation of site conditions, geological
history, etc.

4. Soil classification
Classification of soil is the separation of soil into
classes or groups each having similar characteristics
and potentially similar behavior.
A classification for engineering purposes should be
based mainly on mechanical properties: permeability,
stiffness, strength.
The aim of a classification system is to establish a set
of conditions which will allow useful comparisons to
be made between different soils. The system must be
simple. The relevant criteria for classifying soils are
the size-distribution of particles and the plasticity of
the soil.

5. Specific Gravity (Gs)
Specific gravity is defined as the ratio of the unit
weight of a given material to the unit weight of
water. The specific gravity of soil solids is often
needed for various calculations in soil mechanics.
It can be determined accurately in the laboratory.
Table in the next slide shows the specific gravity of
some common minerals found in soils. Most of the
values fall within a range of 2.6 to 2.9. The specific
gravity of solids of light-colored sand, which is
mostly made of quartz, may be estimated to be
about 2.65; for clayey and silty soils, it may vary
from 2.6 to 2.9.

7. Mechanical Analysis of Soil
Mechanical analysis is the determination of
the size range of particles present in a soil,
expressed as a percentage of the total dry
weight. Two methods generally are used to
find the particle-size distribution of soil:
(1) sieve analysis—for particle sizes larger than
0.075 mm in diameter, and
(2) hydrometer analysis—for particle sizes
smaller than 0.075 mm in diameter.

8. Hydrometer Analysis
Hydrometer analysis is based on the principle
of sedimentation of soil grains in water. When
a soil specimen is dispersed in water, the
particles settle at different velocities,
depending on their shape, size, weight, and
the viscosity of the water.
For simplicity, it is assumed that all the soil
particles are spheres and that the velocity of
soil particles can be expressed by Stokes’ law,
according to which:

12. Sieve analysis
Sieve analysis consists of shaking the soil
sample through a set of sieves that have
progressively smaller openings. U.S. standard
sieve numbers and the sizes of openings are
given in Table below.

13. Steps
• The sieves used for soil analysis are generally 203 mm in diameter.
To conduct a sieve analysis, one must first oven-dry the soil and then
break all lumps into small particles.
• The soil then is shaken through a stack of sieves with openings of
decreasing size from top to bottom (a pan is placed below the stack).
• Figure in the next slide shows a set of sieves in a shaker used for
conducting the test in the laboratory. The smallest-sized sieve that
should be used for this type of test is the U.S. No. 200 sieve.
• After the soil is shaken, the mass of soil retained on each sieve is
determined.
• When cohesive soils are analyzed, breaking the lumps into individual
particles may be difficult. In this case, the soil may be mixed with
water to make a slurry and then washed through the sieves.
• Portions retained on each sieve are collected separately and oven-
dried before the mass retained on each sieve is measured.

15. Calculations
Once the percent finer for each sieve is calculated (step 5), the
calculations are plotted on semi-logarithmic graph paper with percent
finer as the ordinate (arithmetic scale) and sieve opening size as the
abscissa (logarithmic scale). This plot is referred to as the particle-size
distribution curve.

17. Particle-Size Distribution Curve – how to
use it??
A particle-size distribution curve can be used
to determine the following four parameters for
a given soil:
1. Effective size (D10): This parameter is the
diameter in the particle-size distribution
curve corresponding to 10% finer. The
effective size of a granular soil is a good
measure to estimate the hydraulic
conductivity and drainage through soil.

21. • The particle-size distribution curve shows not only the range of
particle sizes present in a soil, but also the type of distribution of
various-size particles. Such types of distributions are
demonstrated in Figure on the next slide.
• Curve I represents a type of soil in which most of the
soil grains are the same size. This is called poorly
graded soil.
• Curve II represents a soil in which the particle sizes are
distributed over a wide range, termed well graded. A
well-graded soil has a uniformity coefficient greater
than about 4 for gravels and 6 for sands, and a
coefficient of gradation between 1 and 3 (for gravels
and sands). A soil might have a combination of two or
more uniformly graded fractions.
• Curve III represents such a soil. This type of soil is
termed gap graded.

23. Example

24. Particle Shape
The shape of particles present in a soil mass is
equally as important as the particle-size
distribution because it has significant influence on
the physical properties of a given soil.
However, not much attention is paid to particle
shape because it is more difficult to measure. The
particle shape generally can be divided into three
major categories:
1. Bulky
2. Flaky
3. Needle shaped

25. Bulky particles
Bulky particles are formed mostly by
mechanical weathering of rock and minerals.
Geologists use such terms as angular,
subangular, subrounded, and rounded to
describe the shapes of bulky particles.

28. Flaky particles
Flaky particles have very low sphericity—
usually 0.01 or less. These particles are
predominantly clay minerals.
Needle-shaped particles
Needle-shaped particles are much less
common than the other two particle types.
Examples of soils containing needle-shaped
particles are some coral deposits and
attapulgite clays.