AASHTO Soil Terminology
AASHTO Soil Terminology
AASHTO Soil Terminology
AASHTO soil terminology comes from AASHTO M 145, “Classification of Soils and
Soil-Aggregate Mixtures for Highway Construction Purposes”. Aggregate terminology
comes from AASHTO M 147, “Materials for Aggregate and Soil-Aggregate Subbase,
Base and Surface Courses”. Basic terms include:
Gravel Material passing a 75-mm (3-inch) sieve and retained on a 2.00-mm (No. 10) sieve.
Coarse Sand Material passing a 2.00-mm sieve (No. 10) and retained on a 0.475-mm (No. 40) sieve.
Fine Sand Material passing a 0.475-mm (No. 40) sieve and retained on a 0.075-mm (No. 200)
sieve.
Silt-Clay Material passing a 0.075-mm (No. 200) sieve.
Silt Fraction Material passing the 0.075 mm and larger than 0.002 mm.
Clay Fraction Material smaller than 0.002 mm.
Note that these definitions are AASHTO definitions and are slightly different than those
of the Unified Soil Classification system (ASTM). The table below shows the AASHTO
soil classification system (from AASHTO M 145).
AASHTO Soil Classification System (from ASTM M 145)
Sieve Analysis, %
passing
2.00 mm (No. 10) 50 max --- --- --- --- --- --- --- --- --- ---
51
0.425 (No. 40) 30 max 50 max --- --- --- --- --- --- --- ---
max
10 35 35 35 35
0.075 (No. 200) 15 max 25 max 36 min 36 min 36 min 36 min
max max max max max
Characteristics of
fraction passing 0.425
mm (No. 40)
40 40 40 40
Liquid limit --- --- 41 min 41 min 41 min 41 min
max max max max
10 10 10 10
Plasticity index 6 max N.P. 11 min 11 min 11 min 11 mina
max max max max
stone
Usual types of significant fragments, fine
silty or clayey gravel and sand silty soils clayey soils
constituent materials gravel and sand
sand
General rating as a
excellent to good fair to poor
subgrade
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
a