In what situation is the sand cone method applicable according to D1556?

The sand cone method, as outlined in ASTM D1556, is specifically suited for determining the in-place density of soils. This method is primarily applicable to soils that do not contain significant amounts of rock or coarse material larger than 1.5 inches in diameter. The rationale behind this is that the sand cone method relies on a uniform flow of sand to fill the voids left by an excavated soil sample, allowing for accurate measurement of density.

When coarse materials or large rocks are present, they can disrupt the uniformity of the sand filling process. This limits the effectiveness of the method, as the sand may not adequately fill the voids, leading to inaccurate density results. Therefore, the sand cone method is not suitable when substantial amounts of larger particles are present in the soil matrix.

In contrast, the method is ideal for fine and medium-textured soils, such as sands, silts, and certain types of clay, where the particle sizes allow for proper sand compaction and filling of voids. This characteristic makes option C the appropriate choice.