Classify inorganic soil from gradation and Atterberg limits using the USCS flowchart. Follow the steps to a group symbol and soil name.
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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.
Example 1
SW-SM dual symbol: sand, 8% fines, Cu=8, Cc=1.5, ML fines
Given
gravel percent
10
sand percent
82
fines percent
8
cu
8
cc
1.5
liquid limit
30
plastic limit
26
Assumptions
Classification follows the ASTM D2487 (USCS) flowchart for inorganic soils; organic soils (OL/OH/PT) are not evaluated.
Result: SW-SM (well-graded sand with silt)
Solution steps
Gradation summary
Fines (passing the No. 200 sieve) decide the top-level split: 50% or more fines is fine-grained, less is coarse-grained.
fines = 8 %; sand = 82 %; gravel = 10 %
Coarse-grained soil: gravel or sand?
With less than 50% fines the soil is coarse-grained; the larger coarse fraction names it.
Fines between 5% and 12% take a dual symbol: gradation (W/P) first, fines type (M/C) second.
5 % ≤ fines = 8 % ≤ 12 %
Gradation shape: well or poorly graded?
Well-graded sand requires Cu ≥ 6 with Cc between 1 and 3.
Cu = D60/D10 and Cc = D30² / (D10·D60)
Cu = 8 and Cc = 1.5; limits: Cu ≥ 6 and 1 ≤ Cc ≤ 3 → well graded
Plasticity chart (A-line) check
The A-line PI = 0.73(LL − 20) separates clays (on or above) from silts (below); the hatched CL-ML zone is PI between 4 and 7 on or above the A-line.
PI_A-line = 0.73·(LL − 20)
A-line at LL = 30: PI_A = 0.73 × 10 = 7.3; soil PI = 4 (below the A-line)
Classification: SW-SM
Group symbol SW-SM — well-graded sand with silt (ASTM D2487 group name).
Results
Fines content (passing No. 200)
8
Gravel fraction
10
Sand fraction
82
Coefficient of uniformity Cu = D60/D10
8
Coefficient of curvature Cc = D30²/(D10·D60)
1.5
Plasticity index PI = LL − PL
4
Where this answer was checked
source
ASTM D2487 flowchart criteria (as tabulated in the NCEES FE Reference Handbook) — 5–12% fines → dual symbol; Cu ≥ 6 and 1 ≤ Cc ≤ 3 → SW; PI below A-line → M
verified by
hand-recomputed
derivation
Fines 8% < 50 → coarse; sand 82 > gravel 10 → S. 5 ≤ 8 ≤ 12 → dual. Gradation: Cu = 8 ≥ 6 and Cc = 1.5 in [1,3] → W. Fines: PI = 30 − 26 = 4; A-line = 0.73(30−20) = 7.3; 4 < 7.3 → below A-line → ML → M. Symbol SW-SM, name 'well-graded sand with silt' (gravel 10% < 15% adds no modifier).
Example 2
sandy lean clay CL: 65% fines, LL=40, PL=18
Given
fines percent
65
sand percent
30
gravel percent
5
liquid limit
40
plastic limit
18
Assumptions
Classification follows the ASTM D2487 (USCS) flowchart for inorganic soils; organic soils (OL/OH/PT) are not evaluated.
Result: CL (sandy lean clay)
Solution steps
Gradation summary
Fines (passing the No. 200 sieve) decide the top-level split: 50% or more fines is fine-grained, less is coarse-grained.
fines = 65 %; sand = 30 %; gravel = 5 %
Fine-grained soil
Half or more of the soil passes the No. 200 sieve, so the group is decided entirely on the plasticity chart.
fines = 65 % ≥ 50 %
Plasticity index
PI is the plastic range of the fines.
PI = LL − PL
PI = 40 − 18 = 22
= 22
Plasticity chart (A-line) check
The A-line PI = 0.73(LL − 20) separates clays (on or above) from silts (below); the hatched CL-ML zone is PI between 4 and 7 on or above the A-line.
PI_A-line = 0.73·(LL − 20)
A-line at LL = 40: PI_A = 0.73 × 20 = 14.6; soil PI = 22 (on or above the A-line)
Classification: CL
Group symbol CL — sandy lean clay (ASTM D2487 group name).
Results
Fines content (passing No. 200)
65
Gravel fraction
5
Sand fraction
30
Plasticity index PI = LL − PL
22
Where this answer was checked
source
ASTM D2487 flowchart criteria (as tabulated in the NCEES FE Reference Handbook) — ≥50% fines → plasticity chart; PI > 7 and above A-line, LL < 50 → CL