- Soil classification is generally done to organize the different types of soil in different groups on the basis of their engineering properties.
- There are so many popular system of soil classification system. AASHTO soil classification system is one of them.
- AASHTO soil classification system is given by the American Association of State Highway and Transportation officials.
- The AASHTO soil classification system was first developed by Terzaghi and Hogentogler in 1929 and has been revised many times. The AASHTO soil classification system was initiated by Highway Research Board in 1943.
- The AASHTO soil classification system is generally used for soil classification for highway construction projects and is very much used worldwide.
- AASHTO soil classification system is mainly based on sieve analysis and consistency limits.
- In AASHTO soil classification system, soils are classified in group and subgroups. There are 7 major groups present in AASHTO soil classification system which are further divided into subgroups.
A1 ➡ A-1-a , A-1-b
A2 ➡ A-2-4 , A-2-5 , A-2-6 , A-2-7
A3
A4
A5
A6
A7 ➡ A-7-5 , A-7-6
- In AASHTO soil classification system, Group index is also included along with soil groups and subgroups. Group index is a number used to refer the quality of soil used as subgrade material in highway. Its value generally ranges from 0 to 20. For two soils falling under same group and subgroup , the soil having lower GI value is better highway subgrade material in comparison to that of soil having lower GI value.
Step by Step Procedure for AASHTO soil classification system
1. Distinguishing coarse grained / fine grained
First of all, find the percentage of soil passing US # 200 sieve (0.075 mm opening)
- If % passing ≤ 35% ➡ coarse grained soil
- If % passing＞35% ➡ fine grained soil
2. Determination of group and subgroups
- For coarse grained soil, % of soil passing through US sieve # 10, #40 and #200 is also required in addition to liquid limit and plasticity index.
Now, after finding all of the mentioned values, compare the data from the AASHTO table of coarse grained. To compare go through the top to bottom until a line is found matching all of the properties calculated here.
- For fine grained soil, only liquid limit and plasticity index is required.
Now, just compare the data from the AASHTO table of fine grained. To compare go through the top to bottom until a line is found matching all of the properties calculated here.
3. Determination of Group Index
for A-1-a, A-1-b, A-3, A-2-4, A-2-5, GI = 0
for A-2-6 and A-2-7, GI = 0.01 (F₂₀₀ -15) (PI -10)
GI = (F₂₀₀ - 35) [ 0.2+0.005(LL-40) ] + 0.01 (F₂₀₀ -15)(PI - 10)
if GI ＜ 0 then take GI =0 and if GI ＞0 , the round off to nearest whole number.
4. Naming of soil
Finally as per AASHTO soil classification system, soil➡ soil group/subgroup followed by GI in parenthesis. e.g. A-7-5(10) , A-5(9).
AASHTO Table for coarse/fine grained soil
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AASHTO table for coarse grained
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| AASHTO table for fine grained |
AASHTO soil classification example
Q. The result of sieve analysis of three soil is given next. It is required to classify these soils according to the AASHTO Classification system. Use the attached AASHTO Classification of Highway Subgrade Materials Table.
What is the classification of soil A,B and C?
sol:-
% passing # 200 sieve = 38 ➡ fine grained soil
plasticity index= (LL- PL) = 42-23 = 19
on comparing from AASHTO table for fine grained
soil group = either A-7-5 or A-7-6
as PI= 19 > (LL-30) ⇒ soil group = A-7-6
Group Index, GI = (F₂₀₀ - 35) [ 0.2+0.005(LL-40) ] + 0.01 (F₂₀₀ -15)(PI - 10)
= (38-35) ( 0.2+0.005(42-40)) + 0.01 (38-15)(19-10)
= 2.7 ≈ 3
so, soil A = A-7-6(3)
% passing # 200 sieve = 33 ➡ coarse grained soil
% passing # 10 = 77 , #40 = 50
and LL = 46 , PI= 46-29= 17
on comparing from AASHTO table for coarse grained
soil group = A-2-7
Group Index, GI = 0.01 (F₂₀₀ -15)(PI - 10)
= 0.01 (33-15) (17-10) = 1.26 ≈ 1
so, soil B = A-2-7(1)
% passing # 200 sieve = 63 ➡ fine grained soil
plasticity index= (LL- PL) = 47-24 = 23
on comparing from AASHTO table for fine grained
soil group = either A-7-5 or A-7-6
as PI= 23 > (LL-30) ⇒ soil group = A-7-6
Group Index, GI = (F₂₀₀ - 35) [ 0.2+0.005(LL-40) ] + 0.01 (F₂₀₀ -15)(PI - 10)
= (63-35) ( 0.2+0.005(47-40)) + 0.01 (63-15)(23-10)
= 12.82 ≈ 13
so, soil C = A-7-6(13)
