The guidance in my state is that it's recommended to hand dig within 18" of an underground utility for everything other than gas. If it's a gas line it's mandatory to hand dig when within 18".

For digging greater than 18" from the lines you can use power without restriction.

Depending on who buried your lines, there might be marking tape (a warning that you're about to do something very bad) buried about 12" or so above the line. You never know though.

Thanks. Luckily it will only be a couple posts out of the 35, so the hand-digging won't be too terrible. We're using the concrete for a couple reasons. We already bought 30 bags of concrete :) , from previous experiences with gravel vs concrete set posts, and what all the local fence builders do. The pro's vs cons are all over the internet, so we just decided to go with what everyone around here does. We got a few quotes from local companies and all used concrete.

If I set posts in concrete, I'll dig the hole, tamp the bottom, set the post, then add a few inches of the gravel that came out of the hole back in to the hole, surrounding the base of the post. I'll then pour the concrete to fill the hole.

The concrete will form a concrete collar or ring around the post instead of concrete bucket.

Last year when I was expanding my pool equipment pump/filter area, I had to pull and relocate a few 5x5 cedar posts that had been in the ground for about 18 years. I thought I was going to have to replace them, but they were in surprisingly good shape, good enough to clean up and reuse.

MongoCT: That is exactly what we are doing as well. Gravel then concrete.

There's a reason rail road tracks built on gravel don't heave.

We never set post IN concrete; the lime interacts with the preservative chemicals, and can cause rot. This was of course, a decade ago, the newer chemicals they use in pressure treating (alas...I miss my CCA), may not.

RR tracks don't heave because the ties are set in a deep bed of crushed stone that is raised above the surrounding grade.

JDS: Correct. That deep bed of crushed stone drains spectacularly which is why the bed doesn't heave. Same with a post below grade.

I've built many fences over the years and I've never used gravel. Concrete is the only way to go. Use pressure treated posts and one to two bags of ready mix per hole. You'll have a fence that will last for decades.

I've built many fences over the years and I've never used gravel.
Concrete is the only way to go. Use pressure treated posts and one to
two bags of ready mix per hole. You'll have a fence that will last for
decades.

So how do you keep the posts from shrinking? The concrete won't shrink with the posts, hence the wobbly fences once the posts shrink.

Andrew atKERR-- So you're the guy! ;-) Boy I hated digging those concrete plugs out when I had to replace the rotten posts. Please stop doing it! You will come to regret it yourself if you stay someplace long enough to do the replacement.