Ah yes, the Robinia, false acacia, (otherwise it is mimosa) regrows, even cut flush with the ground, with lots of stems stuffed with very large thorns to defend itself, following this past aggression (near my home 91 , there is a corner of the forest stuffed with these stems with enormous thorns, suckers repelled, after the storm of late 1999 having killed acacias) !!!
In addition it rots much more slowly than other woods (tens of years instead of years), ideal for posts.
informative wikipedia:
http://fr.wikipedia.org/wiki/Robinia_pseudoacacia
http://fr.wikipedia.org/wiki/Mimosas
http://fr.wikipedia.org/wiki/Cat%C3%A9gorie:Acacia
The locust tree does not tolerate severe pruning and pruning. If it is absolutely necessary to do so, prefer the period just after flowering.
which explains
the one is half dead.
but it will grow back full of prickly suckers all around, unless you hunt the slightest root !!!
And its strain is hard and almost rot-proof !!!
you have to work it quickly enough after cutting before it gets too hard
Good luck !!!