I'd just cut off each end below the ground and leave it alone. It's much
easier to just dig a new trench and lay a new line.
I don't know how far you have to go under the asphalt, but you may be able
to dig a hole on each end, then rig up a nozzle on one end of the conduit
and use water pressure to tunnel under the slab. Naturally, this would be
much easier for a short distance. It also depends on what type of soil you
have; easy for sand, impossible for rock. :)
Otherwise, you'll probably have to cut the asphalt and dig your trench.
I've never cut asphalt, so I don't know what is needed for that. Check with
a tool rental place, I'm sure they would have something that could do it
(water cooled saw probably?).
In any case, the conduit is a wise choice. You won't have to dig as deep,
it offers protection for the wire, and you can easily replace the wire if
you need to repair or upgrade later. I'd recommend installing a larger
conduit than you need in case you need to upgrade in the future.
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