Chunil,
Rubble/boulder mounds and piles tend to settle under their own weight even before something is constructed on them more than most people would think, sometimes from 5% to 10% of the original height. The settlement under its self-weight has generally been attributed to the high localized contact stresses between the rocks and pre-existing imperfections in the boulders. The NAVFAC foundation guide suggested by ayfarm may cover this as a settlement estimate consideration.
MRM
