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I need to build a mailbox post (for two mailboxes) and have confirmed underground power and data cables exist.

Unfortunately the plans don't specify the exact position or depth and it's likely the data cables are 300mm down. I don't have access to a metal detector.

Can I get away with 200mm concrete footings? With 50mm of gravel under the concrete?

I'm thinking of making wide shallow concrete footings, with steel bolts anchoring the posts. There will be two timber posts 300mm apart connected above ground with railings (planning to make it look like our fence).

Does this sound reasonable?

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A post set in the normal way but with only 200 mm depth won't be very secure.

Call the underground utility locating/marking service, if one exists in your area, and then dig your post holes carefully. It may turn out that the buried lines do not conflict with your plan or that your plan can be adjusted to work around them.

You can definitely trade depth of the post for mass of the footing. A concrete slab sitting on the surface (no ground penetration at all) will support a very large mailbox. For example, cluster mailboxes like those below are commonly bolted to a section of flatwork. (image credit: https://kentonbrothers.com/generalinfo/cluster-mailboxes/)

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Your "wide shallow concrete footings" sounds similar to this idea. If a large concrete surface at ground level doesn't suit you then you could put it below ground. Excavate a large but shallow hole nearly to the buried cables, pour a concrete slab (or place a precast concrete pad), attach posts, and then replace soil over it so that only the posts rise above ground.

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  • Google Call Before You Dig for your locality's phone number, +1.
    – Mazura
    Dec 5, 2020 at 4:32
  • @Mazura I have called and received plans - there are several services directly under where the mailbox must be situated (corner of driveway and footpath). The plans mostly warn to allow for services being a few metres away from where they are on the plans... and fines payable if damaged not to mention i could be killed by the high voltage power. Dec 5, 2020 at 5:37
  • I've never had to call it. Aren't they supposed to come out and put little flags in the ground?
    – Mazura
    Dec 5, 2020 at 5:42
  • @Mazura they emailed me a series of PDF plans from relevant companies and government departments. Dec 5, 2020 at 9:47
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    @AbhiBeckert I get the idea you're in a part of the world that isn't the USA. Most states here have a utility locating service. Somebody comes out and paints a stripe on the ground or places flags to mark buried lines. They don't say how deep the line is, and in my state they require hand digging within 60 cm to either side of the mark. Anyway, you should be safe to hand dig. The power cables are likely armored and/or in conduit and the data cables will surely be in conduit. Conduit resists a human-powered shovel pretty well.
    – Greg Hill
    Dec 6, 2020 at 5:12

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