The two major soil classification systems in the world are: American Association of State Highway and Transportation Officials (AASHTO) and Unified Soil Classification System (USCS). The similarities between the two systems lies in the fact that they originate from the same basis of classification: particle size analysis and plasticity characteristics. They are also approximately similar in the soil types obtainable from both groups as shown in Table 1
Table 1; Similarities of soil types between AASHTO and USCS
| AASHTO system | USCS |
| Symbols | Symbols |
| A-1-a | GW, GP |
| A-1-b | SW, SM, GM, SP |
| A-2-4 | GM, SM |
| A-2-5 | GM, SM |
| A-2-6 | GC, SC |
| A-2-7 | GM, GC, SM, SC |
| A-3 | SP |
| A-4 | ML, OL, MH, OH |
| A-5 | MH, OH, ML, OH |
| A-6 | CL |
| A-7-5 | OH, MH, CL, OL |
| A-7-6 | CH, CL, OH |
Where,
GM = well graded gravels
GP = poorly graded gravels
GM = silty gravels
GC = clayey gravels
SW = well-graded sands
SP = poorly graded sands
SM = silty sands
SC = clayey sands
ML = inorganic silt of low plasticity
CL = inorganic clays of low to medium plasticity
OL = organic silt of low plasticity
MH = inorganic silt of high plasticity
CH = inorganic clays of high plasticity
OH = organic clays of medium to high plasticity
Pt = peat, muck and other highly organic soils
The differences between the two systems is as tabulated below
Table 2; Differences between AASHTO and USCS
| S/No | AASHTO | USCS |
| 1 | Used to find suitability of soils as subgrade material for highways | Used for determining suitability of soil for general use |
| 2 | Soil termed fine-grained if more than 35% passes No. 200 (0.075mm) sieve | Soil termed as fine-grained if more than 50% passes No. 200 sieve |
| 3 | Sieve No 10 (2.0 mm) is used to divide the soil into gravel and sand | Sieve No 4 (4.75 mm) is used to divide the soil into gravel and sand |
| 4 | Gravelly and sandy soils not clearly defined | Gravelly and sandy soils clearly defined |
| 5 | There is no place for organics soils | Organic soils are classified as OL and OH and as Peat (Pt) |
| 6 | It is time-consuming to use due to the process of elimination | It is more convenient to use |
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Reference
Arora, K.R. (2008). Soil Mechanics and Foundation Engineering. 7th edition. Standard Publishers Distributors, New Delhi
