For an even smoother trench job, rent one of these ....
Rocksaw International Inc. :: THE LEADER IN ROCK TRENCHING TECHNOLOGY
When I was training as a lineman, we saw pictures of a guy who was trenching alone and got a basketball size rock stuck in a big Vermeer trencher. He did not disengage the hydraulics as he pried it loose with a digging bar. When his boss showed up a couple hours later, he saw the trencher moving happily along but could not find the driver. 500' behind the trencher, he found catchers mitt sized chunks of the driver mixed in with the spoil. They were graphic enough to make us respect these things. With the right chains, you should see even smaller trenchers rip through asphalt and concrete.