1. Soil Classification Prof. Basuony El-Garhy
Geotechnical Engineering and Foundations
Civil Engineering Department
Faculty of Engineering
Copyright March 10, 2011

2. Objectives
The classification of soils is mainly based on one or two index properties of soil which are described in detail in previous lectures.
The major objectives of soil classification are:
To develop a systematic way to describe and classify soils.
To group soils of similar geotechnical characteristics.
To assign symbols.

3. Major Soil Groups Grain size (mm) Sieve No 200
Fine grained soils Cohesive soils
Coarse grained soils Cohesionless soils
Clay
Silt
Sand
Gravel
Cobble
Boulder
0.002
0.074 2 6
200
Grain size (mm)
Sieve No 200

4. Soil classification systems
Particle size classification (MIT)
Textural or triangular classification
Unified soil classification

5. Particle size classification or MIT Classification
0.002 mm
0.006
2 mm
0.02
0.06
0.2
0.6
Fine
Med
Coarse
Clay
Gravel
Silt
Sand
0.002 mm
2 mm
0.06

6. Triangular classification
Soils in nature rarely exist separately as gravel, sand, silt, clay or organic matter, but are usually found as mixtures with varying proportions of these components.
Using the grain size limits in the table below for sand, silt and clay, a triangular classification chart was developed for classifying mixed soils.

7. Triangular classification
Silt
Sand
Clay
Gravel
0.002 mm
0.05 mm
2 mm
The first step in the classification of soil is to determine the percentages of sand, silt and clay in a given sample by mechanical analysis.

8. Triangular classification

9. Example 1: sand = 30%, silt = 40%, clay = 30%

10. Example 2: sand = 10%, silt = 40%, clay = 50%

11. Example 3: sand = 40%, silt = 48%, clay = 12%

12. Disadvantage of Triangular classification
Disadvantage of this method is that the textural name as derived from the chart does not always correctly express the physical characteristics of the soil. For example, since some clay size particles are much less active than others, a soil described as clay on the basis of this system may have physical properties more typical of silt.

13. Unified Soil Classification
The unified soil classification system is considered the most popular system used in the world to classify the soils. The system is taken into consideration the grain size and the consistency of the soil in the classification.
In the unified classification each soil is given a 2 letter classification (e.g. SW). The following procedure is used.

14. Symbols used in the Unified Soil Classification
G = Gravel
S = Sand
M = Silt
C = Clay
W = Well graded
L = Low plasticity
P = Poorly graded
H = High plasticity
O = organic material

15. Procedures of Unified Soil Classification
Coarse grained (if < 50% Passing No. 200 sieve )
Prefix S (sand ) if > 50% Passing No. 4 sieve
Prefix G (Gravel) if < 50% Passing No. 4 sieve
Suffix depends on %fines
if %fines < 5% suffix is either W or P
if %fines > 12% suffix is either M or C
if 5% < %fines < 12% Dual symbols are used

16. Procedures of Unified Soil Classification
To determine W or P, calculate Cu and Cc
% Passing
D10
D30
D60

17. Procedures of Unified Soil Classification
To determine W or P, use Cu and Cc as follows:
If prefix is G then suffix is W if Cu > 4 and 1 < Cc < 3 otherwise use P .
(GW = well graded gravel; GP = Poorly graded gravel)
If prefix is S then suffix is W if Cu > 6 and and 1 < Cc < 3 otherwise use P.
(SW = well graded sand; SP = Poorly graded sand)

18. Procedures of Unified Soil Classification
Fine grained (if > 50% Passing No. 200 sieve )
prefix M and C determined from plasticity chart

21. classify the soil of the grading curve shown below
Example
classify the soil of the grading curve shown below
% Passing
Example

22. Determination the % of fines (silt and clay)
% Passing
11%
Example
0.074 mm
%fines (% Passing No. 200 sieve) = 11% - Dual symbols required

23. Determination of Cu and Cc
% Passing
Example
D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm
Cu = 12.5, Cc = 1.38

24. Determination the constitutes of the soil
97%
% Passing
11%
Example
4.75 mm
0.074 mm
Gravel = 3%: Sand = 86%; Silt and Clay = 11%

25. Sample classification
The percentage of sand in the soil sample is 86% , hence Prefix is S
For the values of Cu = 12.5 and Cc = 1.38, then the Suffix1 = W
The symbol of the soil is SW (well graded sand)
From Atterberg Tests
LL = 32; PL = then PI = = 6
Using the plasticity chart to classify the fine grained soil

26. Plasticity chart 6 32

27. Sample classification
From the Plasticity Chart for the values of LL = 32 and PI = 6 the Suffix2 = M,
Then, the symbol of the soil is SM (silty sand)
Dual Symbols of the soil are SW-SM
To complete the classification the Symbols should be accompanied by a description