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?
Best Answer
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/)
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.