Tom Dunlap
There are a couple of tools that use a trailer mounted high PSI/CFM compressor to run a sprayer.

Google:

Air Knife
Air Spade

A lot of tree care companies have them and would be tickled to spray a trench for you. It is amazing how quickly it works. If the soil is bone dry you should water it. If its clay dampen it...you know how clay is...brick to pottery with only three drops of water.

I made two of them using black iron pipe and a spring loaded valve. I know many arborists who just use a quarter turn ball valve though. For about $20 in brand new parts, less if you're a scrounge like me, and about $120/day rental on the compressor...you have a water line set.

The link is to a thread on my forum showing what I made.

You can underbore the sidewalk too.