Find the AASHTO soil group and group index from sieve passing percentages, liquid limit, and plasticity index, with classification steps.
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Worked examples
These are the solver's own reference problems — the answers come from published sources or independent hand computation, never from the solver itself. The solution below is the live solver output for each.
Classification per AASHTO M 145 by left-to-right elimination: the FIRST group whose limits all pass is the classification.
Result: A-1-a (0) — stone fragments, gravel and sand, general subgrade rating excellent to good.
Solution steps
Granular screen
With 35% or less passing the No. 200 sieve the soil is a granular material (A-1, A-3, A-2 families); more would make it a silt-clay material (A-4 to A-7).
F = 5 % ≤ 35 % → granular
Test A-1-a
Stone fragments/gravel: tight limits on all three sieves and PI ≤ 6.
By definition the group index of A-1, A-3, A-2-4 and A-2-5 soils is always taken as zero.
GI = 0
= 0
Classification: A-1-a (0)
Group A-1-a — stone fragments, gravel and sand; general subgrade rating excellent to good. Reported with the group index in parentheses.
Results
Group index GI (whole number)
0
Percent passing the No. 200 sieve (F)
5
Where this answer was checked
source
AASHTO M 145 classification table (as tabulated in the NCEES FE Reference Handbook) — A-1-a limits: No10 ≤ 50, No40 ≤ 30, No200 ≤ 15, PI ≤ 6; GI ≡ 0
verified by
hand-recomputed
derivation
40 ≤ 50 ✓, 20 ≤ 30 ✓, 5 ≤ 15 ✓, PI 3 ≤ 6 ✓ → first group passes → A-1-a. GI of A-1 soils is always 0.
Example 2
A-2-6 with partial GI: No200=30, LL=35, PI=15 → GI = round(0.75) = 1
Given
passing no10
80
passing no40
60
passing no200
30
liquid limit
35
plasticity index
15
Assumptions
Classification per AASHTO M 145 by left-to-right elimination: the FIRST group whose limits all pass is the classification.
Result: A-2-6 (1) — silty or clayey gravel and sand, general subgrade rating excellent to good.
Solution steps
Granular screen
With 35% or less passing the No. 200 sieve the soil is a granular material (A-1, A-3, A-2 families); more would make it a silt-clay material (A-4 to A-7).
F = 30 % ≤ 35 % → granular
Test A-1-a
Stone fragments/gravel: tight limits on all three sieves and PI ≤ 6.
fails: passing No. 10 = 80 % ≤ 50 % is violated
Test A-1-b
Coarse sand mixture: No. 40 and No. 200 limits with PI ≤ 6.
fails: passing No. 40 = 60 % ≤ 50 % is violated
Test A-3
Clean fine sand: mostly finer than No. 40, few fines, nonplastic.
fails: passing No. 200 = 30 % ≤ 10 % is violated
A-2 family (silty or clayey gravel and sand)
Granular soils that fail A-1/A-3 fall into A-2; the suffix comes from the LL split at 40 and the PI split at 10.
LL = 35 % ≤ 40 %; PI = 15 % > 10 %
Partial group index for A-2-6
A-2-6 and A-2-7 use only the PI term of the group index formula; a negative value is reported as zero and the result is rounded to the nearest whole number.
GI = 0.01·(F − 15)·(PI − 10)
GI = 0.01 × 15 × 5 = 0.75 → 1
= 1
Classification: A-2-6 (1)
Group A-2-6 — silty or clayey gravel and sand; general subgrade rating excellent to good. Reported with the group index in parentheses.
Results
Group index GI (whole number)
1
Percent passing the No. 200 sieve (F)
30
Where this answer was checked
source
AASHTO M 145 classification table and group index formula — A-2-6: F ≤ 35, LL ≤ 40, PI ≥ 11; partial GI = 0.01(F−15)(PI−10)
verified by
hand-recomputed
derivation
Elimination: A-1-a fails (F 30 > 15), A-1-b fails (F 30 > 25), A-3 fails (PI ≠ 0); F 30 ≤ 35 → A-2; LL 35 ≤ 40 and PI 15 ≥ 11 → A-2-6. Partial GI = 0.01·(30−15)·(15−10) = 0.01·15·5 = 0.75 → rounds to 1.
Example 3
A-6 clay: No200=60, LL=32, PI=15 → GI = 6.25 → 6
Given
passing no10
95
passing no40
80
passing no200
60
liquid limit
32
plasticity index
15
Assumptions
Classification per AASHTO M 145 by left-to-right elimination: the FIRST group whose limits all pass is the classification.
Result: A-6 (6) — clayey soil, general subgrade rating fair to poor.
Solution steps
Silt-clay screen
With more than 35% passing the No. 200 sieve the soil is a silt-clay material (A-4 to A-7).
F = 60 % > 35 % → silt-clay
LL and PI grid
The silt-clay groups split on the liquid limit at 40 (A-4/A-6 low vs A-5/A-7 high) and the plasticity index at 10 (silty ≤ 10 vs clayey ≥ 11).
LL = 32 % ≤ 40 %; PI = 15 % > 10 %
Group index
The group index penalizes fines content and plasticity; a negative value is reported as zero and the result is rounded to the nearest whole number.