Which of the following is NOT a step involved in geotechnical testing?

The planting of vegetation is not a step involved in geotechnical testing because geotechnical testing primarily focuses on assessing the physical and engineering properties of soil and rock in order to determine their suitability for construction projects. Such testing typically includes methods like soil sampling, compaction testing, and the evaluation of materials and subsurface conditions.

In contrast, the other options relate directly to actions that are part of evaluating soil stability and suitability. Compaction around a barrel, for instance, involves examining how soil behaves when compacted, which is crucial for foundation and structural integrity. Inspecting unsuitable materials helps ensure that only suitable soil or rock is used in construction, avoiding complications later in the project. The construction of a cut-off trench can be part of a geotechnical strategy to manage groundwater and improve the stability of structures. Each of these actions responds to the particular needs of geotechnical engineering, while planting vegetation typically pertains more to landscaping or ecological efforts rather than initial geotechnical testing.