In small projects with light loads and
"known" soil materials, foundation
decisions are made by the architect,
engineer or contractor at the beginning
of every project (and hopefully well
before the first bit of earth is turned).
In most cases, the foundation systems
they select perform as anticipated. In
other cases, those occasional pockets of
subterranean mystery that deviate
from the norm—the local dump that
has long been abandoned and filled in so
it looks innocent from the surface, the
perched high-water table, or the
hydraulic, "quick," condition—see to it
that they don't work.
Because in the study of earth
materials we often are dealing with
the unknown, foundation "engineering"
is more of an art than a science.