Seen in a mailing list from the USA:
>How do I get a [small, low-voltage] cable under a 4ft concrete walkway? >
>This is very easy, you go to your local hardware store and pick up nozzle
>for your hose that is designed for the job (about 3 bucks) and drill your
>way with water pressure under the path.
>
>I have done this 5 times and I am amazed how fast it works, it took me about
>7 minutes to get under a 4 foot path. Dig a 6" sump on the side of the path
>so you can cup out the excess water.
>
The consensus of this and other replies (covering a range of ground conditions) was that it's messy, but that's the way to do it. Basically use about a 1in hose to drill under the path, chop both ends off and then thread the cable through the pipe.
Anybody here tried it?