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I am attempting to build a privacy fence that will sit just behind a retaining wall (as in, on the ground behind the wall and not on the wall itself).

While digging today I realized that while the ground behind the retaining wall does have ~10" of soil on top, underneath the thin layer of soil seems to be gravel.

I assume that the retaining wall looks almost exactly like this underneath https://imgur.com/cYi6Jf4.

The good news is I only need to set two posts into this gravel, but I am a bit of a loss on how to create the hole for the post. A few questions for anyone who may have some knowledgeable advice

First and foremost, will setting two posts in concrete behind this wall create any structural risk to the wall?

How should I dig 2 foots deep holes into straight gravel? Will a rented auger even work through it? Will a post hole digger and a digging bar work any better?

Thanks in advance!

  • I personally think an auger bit could get the gravel out as it spins, but I am very curious on if there are other solutions already proven. Great question. – Evil Elf Apr 28 at 11:53
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    It would be most helpful to directly embed the image in your post. Edit your post and click the "landscape" looking button above the text entry box and paste the link to the .jpg there. This makes it easier for everyone involved. – FreeMan Apr 29 at 13:45
  • What exact kind of gravel is it? If it is well-rounded 'pea' gravel then you should be able to drive a post, or a post-holder right through it, but not of it is just small hardcore. – Mike Brockington Apr 29 at 14:43
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I watched my fence installers and there where some areas that had a lot of stone. They would take a pointed metal rod and jam it into the hole to loosen up the rock and then use the post hole digger to get the loosened rock out. He alternated between the pointed rod and the post hole digger. Two men dug 9 holes by hand and finished the fence in one day.

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    The metal rod was probably a digging bar, aka a "spud bar". – llogan Apr 28 at 19:55
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First and foremost, will setting two posts in concrete behind this wall create any structural risk to the wall?

You will not be adding any loads to the retaining wall that are of much concern. The wood screen wall is relatively very light so not adding surcharge to the soil behind the retaining wall and any forces from wind are relatively minor. The gravel is there as part of the water drainage system.

Related to the gavel: if water is not allowed to drain away it would add very significant loads to the wall that would very much be a concern. However, the adding of two posts into the gravel shouldn't change the effectiveness of the overall draining system and in addition they are located at the top of the retaining wall where the drainage is least important. The drainage is meant to prevent a full wall-height of water behind the wall like a dam on a river situation.

How should I dig 2 foots deep holes into straight gravel? Will a rented auger even work through it? Will a post hole digger and a digging bar work any better?

An auger would probably tear things up too much. I think that it is probably most likely that the problem you'll have is not getting through the gravel, but actually that the gravel will be too loose and want to fall back into the hole as you dig it. A post hole digger would be the way that I would go with it.

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  • "...not adding surcharge to the soil..." Could you define "surcharge"? I've not heard it used in this context. "in addition they are located at the top of the retaining wall" OP specified that the fence would be "behind" the wall, not "on top of". – FreeMan Apr 29 at 13:48
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    surcharge are vertical loads, live loads such as a driveway (car, fire truck, cement truck) dead loads such as house foundation). The post holes are -in to- the gravel at the top of the wall, aka the top of the water draining system "the adding of two posts into the gravel shouldn't change the effectiveness of the overall draining system and in addition they are located at the top of the retaining wall where the drainage is least important" – Ack Apr 29 at 14:37
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Once it's in place most gravel is not all that mobile, but it's also not hard to dig.

I would not bother with a power auger, a clamshell posthole digger should get it done easily. I've just revisited some areas I backfilled with gravel to make some modifications, and got in just fine with a clamshell and a shovel - any large rocks were not in the area I backfilled, so unless the "gravel" had abnormally large stones in it, it should be excellent digging.

I would also skip the concrete, which tends to cause more problems than it solves, if it solves any. Just pack/tamp the fill back in very well, in 2" layers, after setting the post, for which purpose a bar or pipe may be handy.

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Having dug post-holes for a deck 2 summers ago through a former gravel driveway, I'd say your success level will vary with they type of gravel you encounter.

Our driveway had been covered with very large "crushed" gravel - many pieces were fist-sized. In this situation, the auger worked great at getting some of the rocks out, but it would hit others and bind, twisting the motor instead of the bit. We ended up using a combination of powered auger, post-hole digger, spud-bar and laying on the ground with an old screwdriver to pry some of the rocks out. Each hole was an adventure in frustration and soreness.

However, if your gravel layer is pea gravel (small, rounded rocks) usually used for drainage precisely because they don't compact and lock in place, the powered auger may be able to grab them and pull them out. You may have to allow the auger to spin into a bit of gravel then haul it up while the bit is spinning to have the bit pull the loose rock out of the hole. As you go deeper, you'll essentially be doing squats with the weight of the auger, so prepare for a lot of thigh burn!

If your gravel is compacted crushed gravel used as a footing, you may well be able to get the auger to bite into it and pull it up, though the squat method may be necessary for this, too.

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A tiny bucket sideways and a garden shovel or just a shop vac (post hole diggers are too heavy; hate 'em).

The 2' hole is going to end up being about 1.5 feet in diameter or more (the angle of repose of gravel is more than 45, but not enough...). Post hole diggers are for compacted soil, where the hole needs only be as large as the tool because cave-ins don't happen once you're passed the overburden.

You can lift a fifteen pound tool out of a hole with 1 pound of dirt in it... way too many times, or you can get down there and scoop it out hand fulls at a time.

Granted, I've seen some dudes hustle the shit out of a post hole digger no matter what it is, but that's not how I want to live my life.

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  • What a loose pile does (and actual gravel rather than pea stone isn't all that slumpy even in just a pile, unless you vibrate it) and what it does when placed and settled are quite different. As I mentioned, I've been digging into stuff I gravel backfilled (years ago) to make some changes. Without acting like a gorilla, it holds a vertical face quite nicely. – Ecnerwal Apr 29 at 3:32
  • The shop vac is a great idea. Maybe with PVC tube wider than the post to prevent cave-in? – Ben Apr 29 at 11:56

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