Leaking sprinkler line

4-zone residential system with timer-controlled valves.
Just noticed water spurting up alongside the "supply side" of the walk when one particular zone is activated. I've found where the polyethylene pipe (one of two at that point) passes at right angles under the walk. but the water was coming up at a point a few feet away in the opposite direction from the sprinkler heads that this pipe feeds. I dug down at the opposite side of the walk and found not two pipes, or even one pipe, but some of that corrugated black plastic pipe -- presumably put there to protect the sprinkler pipes against over-enthusiastic digging in what was previously a garden bed; it's now lawn. I haven't yet dug out from the walk to see how far the corrugated pipe extends, but I'm guessing that it's at least the width of the former garden bed.
Even with a trench (dug in the search for the crossing point) alongside the walk, most of the water is flowing from under the walk some distance from where the pipes pass under the walk, rather than right alongside the pipes, but that might simply be the line of least resistance because of permeability issues.
Any bright ideas for dealing with this situation effectively?
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