The capstone at Lligwy on Anglesey is set very low to the ground on quite squat orthostats, a pit was dug beneath the capstone to increase the head room to 2m or more. Our visit:-

http://www.megalithics.com/wales/lligwy/lligmain.htm

This is sort of an odd halfway house between the suggested capstone dug out and supported in situ, and the standard built-on-the-old-ground-surface chamber. Although the interior space is now silted up to only around 1m tall, it does feel a little like being inside a WW2 bunker with vision slits at head height (try our VR panorama of the interior).

We would love to know if the capstone was set this way by design or expediency (me back's gone Dai, this'll have to do), at 1m thick and 5m square it would certainly have been a handful.

Maybe one to visit when we finally get that time machine we've been after.

As a design engineer, viewing it from a purely engineering point of view, yes it would be possible. Obviously, without them getting stress engineers in to do the calculations necessary, it would be a case of trial and error at first. Either that, or over-engineer it to make it failsafe, which is very possible.

However, from a practical point of view, I have to wonder how often the circumstances would arise to make anyone experienced in it, i.e. getting it right the next time after a failed attempt, when there probably wouldn't be that many opportunities to attempt it.

It's a good theory that might have worked in the past. It'd be good if archaeology could prove it had been done. Why doubt that it could be achieved, after all you just have to look at the trilithons at Stonehenge to realize what massive engineering problems they could overcome.

All the best,
TE.

A site I have yet to visit, but this one might be a candidate:

http://www.themodernantiquarian.com/site/8180/carn_llechart_burial_chamber.html