Page 5 - Structural Highway Design_Neat

a). AASHO Soil Classification

Soils are divided into two major groups, granular materials containing
35% or less material passing a No. 200 sieve, and clay and clay-silt
materials, containing more than 35% passing a No. 200 sieve. The soil
components as recognized by this system are as follows:

Gravel – materials passing a 3 in. sieve and retained on a No. 10 sieve

Coarse Sand – materials passing a No. 10 sieve and retained on a No.
40 sieve

Fine Sand – materials passing a No. 40 sieve and retained on a No.
200 sieve

Combined silt and clay – material passing the No. 200 sieve.

b). Unified Soil Classification

Letter symbols used for this soil classification system are as follows:
G – Gravel
S – Sand
M – Silt
C – Clay
W – Well Graded
P – Poorly Graded
U – Uniformly Graded
L – Low Liquid Limit
H – High Liquid Limit

Combinations of the letters designated above are used to identify the
soils.

For example, GP indicates gravel that is poorly graded, and CH
designates highly compressible clay. Soil components used in the Unified
Soil Classification System are as follows:

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