Remember that you might get a sprinkler installation man to do this one task for you. Also, cable installers do this all the time. These two industries have a special machine that would do this job in 15-20 minutes. If you find a small (as in one man) company, they might to it for a very small fee, especially if they frequently come to your neighborhood for other work.
Try to see who does sprinkers/ or cable installations in your neighborhood. Many cable companies use independent contractors to install cable tv wires, and I get the impression most of them are one man ventures.
Hope this helps.
The water pressure idea is also good, and is good advice if you wish to try it yourself. It is a common method.
--James--