Dual symbols in USCS: the 5 to 12 percent fines band

Updated: 2026-08-04 · ISKRIT

Between 5 and 12 % fines a coarse soil is genuinely between two classes: clean enough that gradation still controls its behaviour, dirty enough that the fines start to matter. ASTM D2487 refuses to pick and issues both symbols.

How the pair is built

The first symbol comes from gradation, exactly as for a clean soil: Cu and Cc decide W or P against the thresholds for sand or gravel. The second symbol comes from the plasticity of the fines: M if they are non-plastic silt, C if they are plastic clay. The two are joined with a hyphen, giving SW-SM, SP-SC, GW-GC and so on.

So a sand at 8 % fines with Cu = 9 and Cc = 1.4 and non-plastic fines is SW-SM. The same sand with Cc = 0.7 becomes SP-SM.

Why it matters on site

The band is where permeability changes by orders of magnitude. A clean SW drains freely. The same skeleton with 12 % silt filling its voids can drop two or three orders in hydraulic conductivity, because the fines occupy the connected pore throats rather than adding to the load-carrying frame. Frost susceptibility switches on across the same band, and so does the sensitivity of compaction to water content.

A dual symbol on a drawing is therefore a warning that filter design, drain design and frost protection all need the actual fines number, not the class.

The limit that stops the classification

The M-versus-C part of a dual symbol cannot be assigned from sieving. It requires Atterberg limits on the fraction passing 0.425 mm. Without them the honest output is the gradation symbol plus an explicit note that the second letter is undetermined, which is what a correct report shows. Above 12 % fines the same constraint applies to the single suffix, and below 5 % it does not arise at all.

Questions

Do dual symbols exist for fine-grained soils? Yes, but for a different reason: CL-ML covers the low-plasticity zone of the plasticity chart between the A-line and the ML region.

Is the 5 to 12 band the same in other systems? No. AASHTO and EN ISO 14688 draw their fines boundaries elsewhere, so a soil can be borderline in one system and firmly classified in another.

The classification tool emits the dual symbol together with a note stating precisely which input is missing.