and herein lies a major issue with unqualified advice to ground staff.
it looks like we can add aboriculture to the list of things Obe is not so proficient in.
for those playing along at home, don't put any fill or turf over the tree roots of an existing tree, the more mature the tree, the higher the chance of killing the tree.
here is the first site I came across in a search to give some more details
There's another thread on here about this and AhBen rekons you can do it so long as you dont do it all the way to the trunk, that made sense to me.