Classification of soils for road-construction purposes
The AASHTO Soil Classification System was developed by the American Association of State Highway and Transportation Officials, and is used as a guide for the classification of soils and soil-aggregate mixtures for highway construction purposes. The classification system was first developed by Hogentogler and Terzaghi in 1929,[1] but has been revised several times since.
AASHTO Soil Classification System (from AASHTO M 145 or ASTM D3282)
General Classification
Granular Materials (35% or less passing the 0.075 mm (No. 200) sieve)
Silt-Clay Materials (>35% passing the 0.075 mm (No. 200) sieve)
Group Classification
A-1
A-3
A-2
A-4
A-5
A-6
A-7
A-1-a
A-1-b
A-2-4
A-2-5
A-2-6
A-2-7
A-7-5 A-7-6
Sieve Analysis, % passing
2.00 mm (No. 10)
50 max
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0.4255 mm (No. 40)
30 max
50 max
51 min
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0.0755 mm (No. 200)
15 max
25 max
10 max
35 max
35 max
35 max
35 max
36 min
36 min
36 min
36 min
Characteristics of fraction passing 0.425 mm (No. 40)
Liquid Limit
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40 max
41 min
40 max
41 min
40 max
41 min
40 max
41 min
Plasticity index
6 max
N.P.
10 max
10 max
11 min
11 min
10 max
10 max
11 min
11 min1
Usual types of significant constituent materials
stone fragments, gravel and sand
fine sand
silty or clayey gravel and sand
silty soils
clayey soils
General rating as a subgrade
excellent to good
fair to poor
Plasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30.
^Hogentogler, C.A.; Terzaghi, K. (May 1929). "Interrelationship of load, road and subgrade". Public Roads: 37–64.
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