We've got Phyllostachys Heteroclada and Nigra in our backyard,
surrounded by plastic barriers that are 30" deep. The ends of the
barriers overlap a few inches and are joined together by a metal strip
and bolts. This was installed by a contractor.

I'm finding that the seal isn't very effective - small roots have
worked their way through the overlapping area where the strip is, and
this opened up enough space to lets some rhizomes escape. The strip has
bolts every 3-4" but apparently this isn't close enough.

I can't see any easy fix to this, other than just digging inside the
barrer near the seams and removing the rhizomes and roots. What I
thought might be useful would be some sort of long, narrow saw that I
could run along the inside of the barrier to cut the rhizomes.

Are there any tools or methods that might help make this easier? I
think one mistake the contractor made was not making sure the barrier
angled outward. Not much I can do about it now...at least, not easily.
The barriers are about 1 1/2 feet from a fence line, so it's tough to
work on.