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Soil Survey Manual - Table 3-17

Toughness classes

Classes Criteria
Low Can reduce the specimen diameter at or near the plastic limit to 3 mm by exertion of < 8N
Medium Requires 8-20 N to reduce the specimen diameter at or near the plastic limit to 3 mm.
High Requires > 20 N to reduce the specimen diameter at or near the plastic limit to 3 mm.