AASHTO Soil Classification

Steve and Oksana

How to Classify a Soil Using the AASHTO Classification System:

    Created by Tarzaghi and Hogentogler in 1928, it was one of the first engineering classification systems. Intended specifically for use in highway construction, it still survives as the American Association of State Highway and Transportation Officials (AASHTO) system. It rates soils for their suitability for support of roadway pavements, and is still widely used in such projects.
    The AASHTO system uses both grain-size distribution and Atterberg limits data to assign a group classification and a group index to the soil. The group classification ranges from A-1 (best soils) to A-8 (worst soils). Group index values near 0 indicate good soils, while values of 20 or more indicate very poor soils. However, a soil that may be "good" for use as a highway subgrade might be "very poor" for other purposes, and vice versa.
    The system itself requires only that a portion of soil to pass through a 3-inch sieve. If any material does not pass the 3-inch sieve, its percentage by weight should be recorded and noted with the classification.
    The table below can be used to determine the group classification. Begin on the left side with A-1-a soils and check each of the criteria. If all have been met, then this is the group classification. If any criterion is not met, move to the right and repeat the process, continuing until all the criteria have been satisfied. Do NOT begin at the middle of the chart.

    The group index can be found by using the following equation: ( 5.1 page 140 in text):

    Group Index = (F-35) [0.2+0.005(wL - 40)] + 0.01(F-15)(IP - 10)
                F= fines content (expressed as a percentage)
                wL= liquid limit
                IP= plasticity index

    When evaluating the group index for A-2-6 or A-2-7 soils, use only the second term in equation 5.1. For all soils, express the group index as a whole number. Computed group index values of less than zero should be reported as zero.
    Finally, express the AASHTO soil classification as the group classification (A-1 through A-8), followed by the group index in parentheses. For example, a soil with a group classification of A-4 and a group index of 20 will be reported as A-4(20).

-Caduto,D. (1998), "AASHTO Soil Classification System," Soil Classification, Chapter 5 in Geotechnical Engineering, pp139-140, ALAN APT.

AASHTO Soil Classification System (from AASHTO M 145 or ASTM D3282)
General Classification Granular Materials (35% or less passing the 0.075 mm sieve) Silt-Clay Materials (>35% passing the 0.075 mm 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
0.425 (No. 40) 30 max 50 max 51 min
0.075 (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 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
eg. 5.2 (text p. 141)
A soil sample is given: LL=44,
                                  PL=21, therefore pasticity index :23    ( 44-21=23)
                                  92% passing #10 sieve,
                                  74% passing # 40 sieve
                                  54% passing #200 sieve , since more than 35% passes (A1-A2-7)
A-4: LL too high
A-5: Ip too high
A-6: LL too high
A-7: all criteria met, yay! A-7-6 because Ip is greater than LL minus 30...
Group Index # per formula above(5.1, pg 140) 
9=(54-35) [0.2+0.005(44-40)] + 0.01(54-15)(23-10)
A-7-6 (9)