What is a critical characteristic for the sand used in the sand cone method concerning its grading?

In the sand cone method, the critical characteristic of the sand used is that it should be uniformly graded. Uniformly graded sands have a consistent distribution of particle sizes, which is important for achieving accuracy in the test. The uniformity ensures that the sand has predictable behavior when it flows into the voids of the soil being tested. This predictability allows for a more reliable measurement of the soil's density, which is key for determining the compaction and suitability of the soil for construction purposes.

Using a uniformly graded sand helps to ensure that the cone of sand will fill the excavated hole in a consistent manner without leaving any voids, leading to more accurate results. Other types of grading, such as coarsely graded or finely graded sands, might not fill the hole as effectively or could lead to variations in the test results due to different flow characteristics or packing densities. A wide variety of particle sizes could also introduce inconsistencies, resulting in an inaccurate measurement of the soil’s density. Therefore, uniformly graded sand is essential for the integrity of the sand cone method.