I've used a trackhoe/Gradall for years on
archaeological projects. What your post
appears to impart is a situation where the
machinery went too deep. That's a fault of
the operator or more properly of the person
who was guiding the machinery rather than the
technique.

The gradall has such good use here in VA that
it is called The State Trowel. It is used
properly to remove deposits such as plowzone
from the top of those deposits one wants to
excavate. The proper use includes not
removing more than is warranted by over
cleaning.

A pan is also useful but because it is not a
precision tool, dirt removal should never be
done down to the bottom of the layer to be
removed. Those things leave giant tracks
which can disturb the deposits one wants to
excavate. I'd never go nearer than 4 inches
above the deposit I wanted to clean down to
with a pan or other blunt instrument. After
that, using a more precision guided
instrument is needed, i.e., a gradall. The
scale of the operation will dictate the
machinery used. A gradall is limited to
swath's of a certain width depending upon the
length of the boom. One can have two adjacent
swaths but if the area is more than those two
wide, the spoil will need to be removed or
moved. In that case, one might realistically
investigate using a pan.

A properly operated toothless bucketed
trackhoe/gradall can take off half inch
increments and leave a smooth surface. It all
depends upon two factors: First, the skill of
the operator. Second, the skill of the
archaeologist guiding the operator. When I
use one, each bucket swipe is watched for
appropriate angle and depth and I micromanage
the bejabbers out of their use. If in doubt,
I stop above where I want to be and do the
rest by hand. The great temptation is to
clean that extra swipe which can cause the
removal of complete layers if not noted.

>From your post, it appears that the operators
went deeper than they should have. I don't
think blaming the technique is appropriate,
rather, blame the monitoring archaeologists.
By definition, plow zone is a uniform soil
which contains vertically and horizontally
compromised materials, not intact graves.
Clearly, there was intrusion into the soils
beneath the plowzone. That's an error by the
operator and/or the archaeologist, not the
technique.

Heavy equipment has its place on
archaeological sites, but like anything else,
including the trowel, it can be mis-used. The
old saying that the "nut behind the wheel
caused the accident" appears to be
appropriate. Consider carefully before
throwing babies out with the bath-water, etc.

Lyle