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?