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Soil Classification
N. Sivakugan
Duration: 7 min. 20 s
2SIVA
Copyright ©2001
Objectives
To develop a systematic way to describe and classify soils;
To assign symbols.
To group soils of similar geotechnical characteristics; and
3SIVA
Copyright ©2001
Major Soil Groups
0.002 200632.360.075
Grain size (mm)
BoulderClay Silt Sand Gravel Cobble
Fine grain soils
Coarse grain soils
Granular soils or Cohesionless soils
Cohesive soils
4SIVA
Copyright ©2001
Grain Size Distribution
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.
Significance of GSD:
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Copyright ©2001
Grain Size Distribution
In coarse grain soils …... By sieve analysis
Determination of GSD:
In fine grain soils …... By hydrometer analysis
Sieve Analysis Hydrometer Analysis
soil/water suspension
hydrometer
stack of sieves
sieve shaker
Grain Size Distribution Curve
can find % of gravels, sands, fines
define D10, D30, D60.. as above.
0
20
40
60
80
100
0.001 0.01 0.1 1 10 100
Grain size (mm)
D30
sievehydrometer
D10 = 0.013 mm
D30 = 0.47 mm
D60 = 7.4 mm
sands gravelsfines
% P
as
sin
g
7SIVA
Copyright ©2001
Well or Poorly Graded Soils
Well Graded Soils Poorly Graded Soils
Wide range of grain sizes present
Gravels: Cc = 1-3 & Cu >4
Sands: Cc = 1-3 & Cu >6
Others, including two special cases:
(a) Uniform soils – grains of same size
(b) Gap graded soils – no grains in a specific size range
8SIVA
Copyright ©2001
Relative Density (Dr)
Measure of how densely the grains are packed in a coarse grain soil in %.
0 100
Loosest Densest
minmax
max
ee
eeDr
Also known as density index (ID).
9SIVA
Copyright ©2001
Granular Soil Consistencies
As per AS1726 - 1993
Relative Density (%) Consistency Term
0-15
15-35
35-65
65-85
85-100
Very loose
Loose
Medium dense
Dense
Very dense
10SIVA
Copyright ©2001
Atterberg Limits
Border line water contents, separating the different states of a fine grained soil
Liquidlimit
Shrinkagelimit
Plasticlimit
0water content
liquidsemi-solid
brittle-solid
plastic
11SIVA
Copyright ©2001
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
12SIVA
Copyright ©2001
Plasticity Index (PI)
Range of water content over which the soil remains plastic
Liquidlimit
Shrinkagelimit
Plasticlimit
0water content
plastic
Plasticity Index = Liquid Limit – Plastic LimitPlasticity Index = Liquid Limit – Plastic Limit
13SIVA
Copyright ©2001
Classifying Fines
Purely based on LL and PI
20 100500
20
0
40
60
Liquid Limit
Liqu
id L
imit
A-Line
PI=0.73(LL-20)
Silts
Clays
High plasticity
Low plasticity
35
Intermediate plasticity
14SIVA
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AS 1726 – 1993 Classification
0 5 12 50 100
% of fines
fine grain soilscoarse grain soils
X: Coarse
G = Gravel
S = Sands
Y: Fines
M = Silts
C = Clays
A: Gradation
W = well graded
P = poorly graded
B: Plasticity
H = LL > 50
I = 35 < LL < 50
L = LL < 35
XA
e.g., GP
YB
e.g., CH
XY
e.g., SM
XA-XY
e.g., GP-GC
15SIVA
Copyright ©2001
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)
.
