What are the requirements for soil when using sand cones according to D1556?

The requirement that the soil must be stable and firm enough to withstand minor pressures is essential when using sand cones for density tests as outlined in D1556. This stability ensures that when the sand is poured into the hole created by the sand cone, the sides do not collapse and the integrity of the test is maintained. A stable and firm surface allows for accurate measurements of the volume of the hole created, which is crucial in determining the density of the soil.

In contrast, overly loose or unstable soils could lead to inaccurate density readings as they may crumble and change shape under pressure. This makes it vital for soils being tested to maintain a sufficient level of firmness to ensure reliable results, reinforcing the importance of this requirement in the testing process.