It varies tremendously... the type of soils, whether cohesive or non-cohesive and the degree of plasticity for cohesive soils. It also varies substantially depending on the type of 'rock' they came from... too many variables... can range from about 3% to a 'whole bunch'.
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik