1. Soil Classification
N. Sivakugan
Duration: 7 min. 20 s

2. Objectives To develop a systematic way to describe and classify soils;
To group soils of similar geotechnical characteristics; and
To assign symbols.

3. Major Soil Groups Grain size (mm) Cohesive soils
Granular soils or Cohesionless soils
Clay
Silt
Sand
Gravel
Cobble
Boulder
0.002
0.075
2.36 63
200
Grain size (mm)
Fine grain soils
Coarse grain soils

4. 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.

5. Grain Size Distribution
Determination of GSD:
In coarse grain soils …... By sieve analysis
In fine grain soils …... By hydrometer analysis
hydrometer
Sieve Analysis
stack of sieves
sieve shaker
soil/water suspension
Hydrometer Analysis

6. Grain Size Distribution Curve
can find % of gravels, sands, fines
define D10, D30, D60.. as above.

7. Well or Poorly Graded Soils
Well Graded Soils
Wide range of grain sizes present
Others, including two special cases:
(a) Uniform soils – grains of same size
Gravels: Cc = 1-3 & Cu >4
(b) Gap graded soils – no grains in a specific size range
Sands: Cc = 1-3 & Cu >6

8. Relative Density (Dr)
Measure of how densely the grains are packed in a coarse grain soil in %.
100
Loosest
Densest
Also known as density index (ID).

9. Granular Soil Consistencies
As per AS
Relative Density (%)
Consistency Term
0-15
Very loose
15-35
Loose
35-65
Medium dense
65-85
Dense
85-100
Very dense

10. Atterberg Limits
Border line water contents, separating the different states of a fine grained soil
water content
Liquid limit
Shrinkage limit
Plastic limit
brittle-solid
semi-solid
plastic
liquid

11. 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

12. Plasticity Index (PI)
Range of water content over which the soil remains plastic
Plasticity Index = Liquid Limit – Plastic Limit
Liquid limit
Shrinkage limit
Plastic limit
water content
plastic

13. Classifying Fines Purely based on LL and PI Clays Silts
High plasticity
Low plasticity 35
Intermediate plasticity 20
100 50 40 60
Liquid Limit
A-Line PI=0.73(LL-20)
Clays
Silts

14. AS 1726 – 1993 Classification coarse grain soils fine grain soils
% of fines 5 12 50
100 YB
e.g., CH XY
e.g., SM XA
e.g., GP
XA-XY
e.g., GP-GC
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

15. 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) .