The blog of Alvaro Boiero

"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi

The blog of Alvaro Boiero


"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi

The AASHTO soil classification system

The AASHTO soil classification system is based on the same parameters as the Unified Soil Classification System (USCS). However, its use is restricted to highway projects and, to certain applications involving the characterization of backfill materials. Do you want to know more about the AASHTO system? Continue reading...

Origins of the AASTHO Soil Classification System

In the 1920s, the U.S. Bureau of Public Roads (now the Federal Highway Administration, FHWA) developed an extensive research program on soils as materials for construction of local or secondary roads (farm-to-market-roads), basically county or state roads.


Figure 1 Farm-to-market roads (Source: illustration of Zohar Lazar, available at

Such research program led to the development of the Public Roads Classification System (PR), which was proposed by Hogentogler and Terzaghi (1929). This system was based on the stability characteristics of soils used directly as a road surface or covered by a thin tar or asphalt pavement.


Over the years, the PR system was modified several times to consider the stability of soils as pavement subgrade. The last version dates back to 1942, which is essentially the present day standard AASHTO M-145 and ASTM D-3282 (Holtz et al, 2011).

Fundamentals of the AASHTO system

In the AASHTO system, soils can be classified according to eight (8) main groups A-1 to A-8, based on their grain size distribution, liquid limit and plasticity index. Soils in groups A-1, A-2 and A-3 are coarse-grained materials, while those included in groups A-4, A-5, A-6 and A-7 are fine-grained. Group A-8, on the other hand, includes soils such as peat, organic composts and other soils with high organic matter content, which are identified by visual inspection. Table 1 shows the form used to apply the AASTHO system.

Table 1 Classification of soils and soil-aggregate mixtures (Source: ASTM D-3282).

The qualitative evaluation of a soil as a road subgrade is carried out by considering the group index (GI), which is a number that depends on the percentage passing through the #200 sieve (F200), the liquid limit (LL), and the plasticity index (PI). The expression to calculate the group index is as follows:

GI = (F_200 – 35) x [0,2 + 0,005 x (LL – 40)] + 0,01 x (F_200 – 15) x (IP – 10)

The higher GI value of a soil, the lower its quality as a subgrade. Thus, a GI of 20 or higher is indicative of a very poor material to be used for this purpose. As shown in Table 1, the higher fine particle content of the soil, the lower its quality as a subgrade, and the higher its GI.


Comparison between the AASTHO and USCS systems

In this blog, we have discussed the Unified Soil Classification System (USCS), and now, when reviewing the AASHTO method, the question arises: are the soil samples classification equivalents using both systems?

In relation to this question, Liu (1967) analyzed different aspects associated with both classification systems. The result of his work is summarized in Table 2, which shows the comparison between both systems, highlighting the most probable, possible, and possible but improbable classifications based on the USCS, for the different groups contemplated in the AASHTO system.

Table 2 Comparison between USCS and AASHTO method for classifying soils (Source: Liu, 1967. Available at

The observed differences between the two systems, as shown in Table 2, are evident. However, Liu (1967) concludes that these significant differences between USCS and the AASHTO method should not be surprising, especially considering their origins and purposes.


Therefore, it may be appropriate to pay attention to Holtz et al (2011), who mention that, although the AASHTO system can be useful to determine the quality of materials to be used in backfills, sub-bases and bases of pavements, and subgrade, it is important to consider the original purpose for which it was developed.


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