Critique of CANDE/AASHTO Soil Groups

CANDE and AASHTO pipe design documents inappropriately group soil types that have greatly varying strength and compressibility properties leading to unnecessary design uncertainty. Most notably, fine-grained elastic silt (MH) is grouped with coarse-grained clayey gravel (GC). There are other groupings that also do not seem appropriate. A defensible design of buried pipe requires acceptable limits on uncertainty. The CANDE and AASHTO groups exhibit an unjustifiably broad range of soil behavior and therefore may often not result in designs of acceptable uncertainty. Additionally, the CANDE and AASHTO naming convention causes problems in referring to the soil groups by using redundant symbols. The inappropriate grouping of some of the soil classifications will be illustrated by comparing their engineering properties. An alternate grouping is proposed.