In the USCS, the most logical engineering geologic classification of a till would be which of the following options?

• A. GM
• B. GC
• C. GM-GC
• D. GP

Answer: (C)

Gravel-sized debris with a noticeable fine fraction is described in USCS by the coarse-grained base (gravel) plus a modifier for the fines. When the fine portion is a mix of silt and clay, you label it as GM-GC to show that the fines cannot be attributed to just one type. Till, as glacial debris, is typically unsorted and contains a mixture of coarse particles with a substantial amount of fines that often include both silt and clay. That combination is best communicated by describing the soil as a gravel with silt and clay fines, i.e., GM-GC. If the fines were solely silt, you’d use GM; if solely clay, GC; and GP would apply if the gravel was poorly graded with little or no fines.