What is the method for determining unconfined compressive strength?

The method for determining unconfined compressive strength involves using estimation techniques such as a pocket penetrometer or thumb penetration tests. These tools provide quick, field-based assessments of the strength of cohesive soils like clay. A pocket penetrometer measures the resistance of the soil to penetration, giving a direct reading of compressive strength. Thumb penetration tests rely on the operator's tactile feedback to gauge soil strength, which can offer a preliminary understanding of the material’s stability.

The other methods listed would not suffice for practical determination. Laboratory testing is typically more controlled and yields precise values, but it is not the only method available. Relying solely on visual inspection lacks the quantitative measurements required to assess unconfined compressive strength accurately. Meanwhile, standard soil classification books provide valuable reference material but do not offer direct measurements for unconfined compressive strength. They might help classify soils and understand their characteristics but do not measure strength directly.