Soil Classification
Prof Neelam Belani
Assistant Professor, Civil Engineering Department
IT,NU.
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Objectives
To develop a systematic way to
describe and classify soils;
To group soils of similar geotechnical
characteristics; and
To assign symbols.
2
Major Soil Groups
Cohesive
soils
Clay
Granular soils or
Cohesionless soils
Silt
0.002
Sand
0.075
Gravel
2.36
Cobble
63
Boulder
200
Grain size (mm)
Fine grain
soils
Coarse grain
soils
3
Grain Size Distribution
Significance of GSD:
To know the relative proportions of different
grain sizes.
An important factor influencing the
geotechnical characteristics of a coarse grain
soil.
Not important in fine grain soils.
4
Grain Size Distribution
Determination of GSD:
In coarse grain soils …... By sieve analysis
In fine grain soils
…... By hydrometer analysis
hydrometer
stack of sieves
sieve shaker
soil/water suspension
Sieve Analysis
Hydrometer Analysis
5
100
80
% Passing
hydrometer
sieve
60
fines
sands
gravels
40
20
D
30
D10 = 0.013 mm
D30 = 0.47 mm
D60 = 7.4 mm
0
0.001
0.01
0.1
1
10
Grain size (mm)
Grain Size Distribution Curve
can find % of gravels, sands, fines
define D10, D30, D60.. as above.
6
Well or Poorly Graded Soils
Well Graded Soils
Wide range of grain sizes present
Gravels: Cc = 1-3 & Cu >4
Sands: Cc = 1-3 & Cu >6
Poorly Graded Soils
Others, including two special cases:
(a) Uniform soils – grains of same size
(b) Gap graded soils – no grains in a
specific size range
7
Relative Density (Dr)
Measure of how densely the grains are
packed in a coarse grain soil in %.
0
Loosest
100
emax  e
Dr 
emax  emin
Densest
Also known as density index (ID).
8
Granular Soil Consistencies
As per AS1726 - 1993
Relative Density (%)
0-15
15-35
Consistency Term
Very loose
Loose
35-65
Medium dense
65-85
Dense
85-100
Very dense
9
Atterberg Limits
Border line water contents, separating the
different states of a fine grained soil
0
Shrinkage
limit
brittlesolid
Plastic
limit
semisolid
Liquid
limit
plastic
water content
liquid
10
Atterberg Limits
Liquid Limit (wL or LL):
Clay flows like liquid when w > LL
Plastic Limit (wP or PL):
Lowest water content where the clay is still plastic
Shrinkage Limit (wS or SL):
At w<SL, no volume reduction on drying
11
Plasticity Index (PI)
Range of water content over which the soil
remains plastic
Plasticity Index = Liquid Limit – Plastic Limit
0
Shrinkage
limit
Plastic
limit
Liquid
limit
plastic
water content
12
Classifying Fines
Purely based on LL and PI
Intermediate plasticity
Low
plasticity
Liquid Limit
60
40
High
plasticity
Clays
20
0
Silts
0
20
35
50
Liquid Limit
100
13
IS 1726 – 1993 Classification
coarse grain soils
fine grain soils
% of fines
0
5
12
XY
50
e.g., CH
e.g., SM
XA
e.g., GP
100
YB
XA-XY
e.g., GP-GC
X: Coarse
Y: Fines
A: Gradation
B: Plasticity
G = Gravel
M = Silts
W = well graded
H = LL > 50
S = Sands
C = Clays
P = poorly graded
I = 35 < LL < 50
L = LL < 35
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Summary
Sieve analysis – for coarse grain soils
Hydrometer analysis - for fine grain soils
Classify coarse by GSD and
fines by Atterberg limits (PI-LL chart)
.
15
Classification of
Soils
16
U.S. Army Corps of Engineers (Triangular Chart Classification, 1943)
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Arthur Casagrande
Born: August 28, 1902
Migrated to US in US, as research assistant
at Bureau of Public Roads, with Karl
Terzaghi at M. I. T.
Worked on soil classification, shear test and
frost action on soil
Initiated soil mechanics
University, 1932
at
Harvard
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