I am looking for advice on how to go about digging fence post holes 42 inches down (to exceed 40 inch frost line) safely.and properly.

I was told that I must manually dig the hole within 18 inches of any marked utility lines. I have these utility boxes in the back corner of my lot (3 of them) as well as drainage sewer my sump pump drains into.

First, the town came out and marked some lines that run in and out of my property line that go to two street lights in front of my house. They weave in and out of the property line over and out a foot or so making it pretty tedious to put my fence as close as I can to the property line. So I am going to do my best and manually dig a few of the holes.

I am confused on how to dig the hole. Some people say use a post hole digger, and others say use a spade sliding along the top horizontally rather than vertically. If I use a spade and dig a trench 42 inches down this seems like a lot of work, and also how would I back fill the hole properly. I plan on using tamped crushed rock for the back fill. Would I have to make some sort of square form to back fill the earth in and fill the post hole properly? Does square vs circle surrounding make a difference? If I used a the post hole digger, how do I safely cut into the ground without worrying about cutting into the line? I feel like I can go slowly, but I don't want each hole to turn into a 4hr project a piece!

Any advice would be great as this is my first time putting up a fence. I don't mind the leg work/labor.. I just want to do the job right and maximize the real estate of my backyard without giving it away to my neighbors.

Also, how deep is the sump pump line usually? The only marking I have of it is from the plat of survey showing it running just past the utility boxes near the property line.


  • How many holes are you talking about?
    – mmathis
    Sep 7, 2016 at 13:24
  • I have to go and mark them still, but just looking at it, could be 4-5. The rest will be done with a power auger where it is safe. It will be in total around 360 linear feet of fence for the whole backyard.
    – eaglei22
    Sep 7, 2016 at 13:37
  • I have never put a fence post in that deep. The wiring should be 24" below the surface. Just about any digging tool shovel , clam shell type post hole digger or hand screw can cut the insulation on Under ground feeder wires. They are quite good at locating the wires but if pipes for the sump don't have a trace wire they cant mark them luckily pipe will hold up a little better than wire but can still be damaged when digging.
    – Ed Beal
    Sep 7, 2016 at 13:43
  • I may just do the whole side because of the wire, which would be about 100ft or so then.. so closer to 10.
    – eaglei22
    Sep 7, 2016 at 13:43
  • Ed, where is your frost line recorded at? I have to put it that deep because of the frost line is recorded at 40 inches, otherwise the posts will raise due to heaving.. I have seen them raising around here due to them not be deep enough.
    – eaglei22
    Sep 7, 2016 at 13:45

4 Answers 4


To follow up, I ended up just paying a fencing company to do it. So the headache and stress became their problem. Plus if I hit the line and hurt myself or had to pay to have it repaired it would have negated any savings of doing it myself; it didn't seem worth the risk.

The crew got nice and close to the property line, and hand dug near the proposed marked lines. They only went 36 inches down though in those areas, and never came across any lines. Although CAUTION should always be taken for anyone digging as all scenarios are different.

So I guess the answer is, as a DIYer you have to weigh the pro's and con's of the risks. In my case the con's began to outweigh the pro's and paying a company was better for me as I ended up going with a nice white cedar fencing with 5/8 thick and roughly 6 inches wide pickets. I used an oil based stain called, "Ready Seal" which I brushed on for a nice, thick, coat, and it turned out beautiful.

Had I done it myself, I was going to use the pressure treated cheaper material, which wouldn't have looked as nice. Also, I started to brainstorm workarounds trying to avoid the buried wire, and there were too many exceptions.

Anyway that is my follow up.


When digging near other connections, or utilities, consider using a pressure washer to dig! It works amazingly. Its messy though. We used a gas powered pressure washer, and even in our hard southern Arizona caliche clay ground, it was much quicker than any digging other than the ditch-witch for the trenches NOT near any critical utilities. enter image description here

  • Nice! How long did it take you to get your trench cleared with the pressure washer? Also what PSI did you use? I considered doing this for one of my backyard projects but never did for whatever reason.
    – eaglei22
    Apr 7, 2021 at 20:54
  • 1
    Maybe an 30 min to an hour to dig this hole. The hard part is moving the much from the hole with a bucket or cup. You can see to my left, i set up a "lake" with a dam to keep the much from running back into the cleared trench. The pressure washer on full pressure with the biggest nozzle tip easily sliced up the dirt into chunks (that was the easiest way to remove dirt)
    – mark f
    Apr 12, 2021 at 22:21

If it were my property I'd either offset the fence line 5' into my property and work cautiously because utility marking can still be missed. If you have your fence over top of the utilities there may be issues when they come to do work on the lines; they will destory your fence because you're within their right of way.

The other option I'd look at would be to use a very shallow base in that area. You didn't specify what type of fence but most of them will handle a bit of frost heave and survive. Take a look at the sidewalks in your neighborhood, those are shallow foundations that don't go more than about 6 inches deep. If the sidewalks are in decent shape (will you really notice if one fence post is 1 inch above the next post?) then you could go with shallow bases. A possible solution would involve digging below the topsoil with a spade shovel, flatten it and place a deck block and mount your post to the top. (similar to http://www.lowes.com/pd/Concrete-Deck-Block-Common-7-in-x-11-in-x-11-in-Actual-7-125-in-x-10-375-in-x-10-375-in/50113084)

  • You must have a huge piece of land to be okay sacrifice 5ft in my case, 5ft * 100 500sqft of backyard space.. I understand the consequences of an easement, I did the paperwork already and pulled the permit and HOA approvals. in the rare occasion if I have to take off the fencing and pull a few posts thats not a problem. That's why I am using screws, and will be tamping crushed rock not cement to set the posts in.
    – eaglei22
    Sep 7, 2016 at 13:56
  • Not really, I just have experience with people digging into buried utility lines. Sometimes the answer for the safest way to do something is to change the plan. Sep 7, 2016 at 14:02

I would call the utility company back out and have them walk with you over your proposed digging site. I'm sure they will assist with hole placement.


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