Under which condition should Method C be used for standard proctor testing?

Method C for standard proctor testing is specifically designed to address the density and moisture characteristics of granular soils that may contain coarser materials. This method is utilized when there is a certain proportion of material retained on a sieve, which impacts the effectiveness of the compaction process being measured.

In this context, the correct choice states that Method C should be used when 30% or less material is retained on a 3/4" sieve. This is significant because the size of the particles influences the compacted density achievable with Proctor tests. When the finer particles make up a majority of the sample, the density results obtained are more representative of compactible, cohesive soils. Excess larger particles could lead to less effective compaction since they do not interact with the finer particles in the same manner.

The threshold of 30% allows for a mix that is manageable for testing while ensuring that the results remain valid for practical applications, particularly in assessing the moisture-density relationship of the soil.

Other options either specify a higher percentage of retained material or the wrong sieve size, which would not align with the guidelines set forth for Method C usage, potentially leading to inaccurate or unreliable test results. Thus, recognizing the correct parameters under which Method C is applicable is critical for