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rjschwar 04-29-2010 03:57 PM

Fence Post on Concrete
I have an ecology block retaining wall that I want to put a fence on. Right now, the top of the retaining wall is about a foot lower than the rest of the yard. (Dirt basically drops off to the drainage rock)

I would also like to cover the ugly top of the ecology blocks. I thought I could do all of this by attaching the 4x4 fence posts (6' spacing) somehow to the concrete, putting a 2x12 on the bottom 12" of the post (kind of a 1' retaining wall on top of the ecology blocks) fill that with dirt, so now the post is anchored to the ecology block with 1' of dirt on one side, and nothing on the other, then build the 6' fence from the new ground level which will be 1' above the post anchor.

Firstly does anyone see any issues with this? Will I be able to find a method to secure the post to the ecology block solid enough that it will be structurally sound?

Secondly, what is the best way to anchor the post? Right now I'm considering two options, either a base that accepts the 4x4 with four anchor bolts that hold it down. Like this:

My main concern is whether the wedge or sleeve anchors will split the concrete as it isn't reinforced.

The other option would be to use something like this:

and bore out a hole, put this in, and fill w/ epoxy.

With that method I'm not sure if the bracket would be strong enough to support the fence and retain the soil.

Any thoughts or ideas?



Yoyizit 04-29-2010 04:02 PM


Originally Posted by rjschwar (Post 435405)
My main concern is whether the wedge or sleeve anchors will split the concrete as it isn't reinforced.

"As a rule of thumb, the expansion anchor industry has established a minimum standard of ten (10) anchor diameters for spacing between anchors and five (5) anchor diameters from an unsupported edge. When vibration or sudden impact are part of the load conditions, the spacing should be increased."

rjschwar 04-29-2010 04:12 PM

I had read that before, which is why I was a little concerned because the retro-fit anchor has holes that appear to be about 4" apart, and says to use 1/2" anchors.

Yoyizit 04-29-2010 04:23 PM


Originally Posted by rjschwar (Post 435413)
I had read that before, which is why I was a little concerned because the retro-fit anchor has holes that appear to be about 4" apart, and says to use 1/2" anchors.

4"/10 = 0.4". That seems close enough to 1/2". It also depends on the compressive strength of your concrete.

kwikfishron 04-29-2010 05:11 PM

How big are these are these blocks you want to attach to? How much do they weigh? How are they connected to the block below other than gravity and the tongue they sit on?

Willie T 04-29-2010 05:33 PM

I don't see attachment to the top of eco-blocks to be the way to go.

Why would you not remove sections of block and sink the fence posts as normally done? Either cut block pieces to fill in up to the sides of the posts, or fill in the holes with gravel.

As for finishing off the tops of the blocks, they sell smooth caps for these blocks.

rjschwar 04-29-2010 05:52 PM

2 Attachment(s)
The blocks are 2'x2'x6' solid concrete and each weighs about 3000 lbs. This is a gravity wall w/ no tie backs or anything. Each block has a hump on top that fits in a groove of the one above. I found a couple pics to show the wall from below and above, which are attached. The picture from the top doesn't show it but there is currently about a 1' drop from the dirt to the top of the block at the end of the yard.

I know they have caps for ecology blocks, but I can't find anyone around me sells them. The places that sell ecology blocks don't sell the caps. I'm not sure what you mean by removing sections of block.

Another option I had thought of would be to anchor the fence posts to the edge of the blocks through the sides of the posts, like a railing of a deck. I could have it hang the post down 2 feet or so and sink four anchors or so in that way, but it seems like bolting to the top would be stronger. Not sure.

Willie T 04-29-2010 06:32 PM

3 Attachment(s)
Remove a top block.

Cut a 7" or 8" corner out

Put the stone back in place

Drop a fence post in

Block off the open side with plywood

Pour concrete around the post

Remove the plywood after concrete is set

Backfill again

The post will sit on the second row of blocks, allowing any water that goes down alongside the post to seep out.
Of course, this is one of those things best planned out and thought of before setting the blocks.
Digging out dirt and bolting to the backsides might be easiest.

If you had in mind to try this, you could use a core drill and drill down into the blocks to drop in posts. No moving blocks that way. Any large plumber should have a core drill.

kwikfishron 04-29-2010 06:57 PM

Since you have some substantial blocks of concrete, digging out and bolting to the yard side is the strongest way to go. Bolting to the outside is not as strong but would work if done properly and give you 2 extra feet of yard. As far as your links, maybe #1 and that’s a big maybe and definitely not #2.

rjschwar 04-29-2010 07:13 PM

I didn't think about that. I don't have the ability to move the blocks at this point, but I might be able to cut away the corner of each top block. I'm not sure I want to do that though as I don't have the tools to do it.

That seems similar to doing a deck railing kind of design. If I didn't cut away the corner, and just anchored the posts to the side of the ecology block with say 4 anchors. If I'm remembering my physics correctly I basically have a lever w/ the fulcrum at the bottom.

So say 9' 4x4 (2' below the ecology blocks, 1' for dirt/"retaining wall", and 6' for fence. The pull out strength of the 1/2" wedge anchors is said to be 2400 lbs. I will use half of that to be on the really safe side, so 1200 lbs per anchor. If I put 4 anchors, at 5", 10", 15", and 20" from the top of the ecology block I see that as 1200 lbs at each point 19", 14", 9", and 4" from the fulcrum. Thus 1200(19+14+9+4) = x*108. (108 is assuming a force at the top of the 9' post) That means x = 511 lbs of force at the very top of the post could be supported by the 4 bolts. This seems like plenty of strength, but I don't know.

I could always use 3 5/8 inch anchors instead, or use a 10 foot post and have 3' below the top of the ecology block.

Does this add up for anyone else, physics was a few years ago. I kind of like this idea as it gives me even a bit more yard, and I don't have to invest a couple hundred bucks in the brackets that bolt down to the top. The more I think about it, the more I think that this would indeed be stronger than bolting to the top where all the force from the bolts is only a few inches from the fulcrum.

Any thoughts or something I missed?

Bolting to the outside is greatly preferred because 1) 2' extra of yard, which I would really like to have, and I think would be nice when we sell our house in a few years. and 2) all the drain rock behind the wall I imagine would be a huge pain to dig out to the depth I need to bolt the posts. Plus if I ever need to replace the posts, it would be a lot easier from the exposed side. Do my equations add up do you think? I think the core drilled hole would be my second best option perhaps, if you don't think the bolting to outside of blocks would hold up.

concretemasonry 04-29-2010 07:29 PM

Keep in mind that the Ecology Blocks (Mafia Block) are a gravity wall with not connection between the units except the sloop joint/recess. Because of this, you can expect some small movement that you do not get with a rigid concrete wall. Evewn though the movement is small, it can be cumulative and the streeses can cause problems with connections thatv are expected to be firm or rigid.


Willie T 04-29-2010 07:29 PM

I'd probably bolt to the inside since core drilling might run $100 per hole. (Don't know)

One thing to keep in mind is that there COULD be considerably more force exerted on those posts than you might think when the wind comes whistling from the backfilled side of the wall in a storm. It could possible tip the upper row of blocks when the fence acts as a sail. Pretty doubtful, though..... although I once saw a small motel (one story, about 6 or 8 rooms) blown over to about 20 or 30 degrees in a hurricane, foundation still attached.

If the wind blew the other way, it might not be as much of a problem.

rjschwar 04-29-2010 07:47 PM

I'm not too worried about the wall tipping over. It is in the back yard, so the house would provide some wind block in big storms. My neighbor has a fence on top of like 8"x17" stackable retaining wall blocks, that appear to have a hole drilled through them so that one of the simpson strong-tie things (from the second link above which appear to be the weakest option) can sit in them, and then refilled w/ concrete or something. That hasn't had any issues.

In terms of the blocks moving, that is possible that they will move a bit, but it seems to me that the fence boards would also provide a bit of flex to accommodate any movement. Each post will only be bolted to a single block. It seems like if anything is going to move, the fence sections between posts would act somewhat to absorb it.

That would be pretty amazing to see w/ the motel. I live in Seattle, and we don't get any wind near hurricane force.

tpolk 04-29-2010 08:05 PM

drill holes in the block, cement in a lag bolt with hydraulic cement with the lag part sticking up 4-6" or more, drill center hole in post and thread on to lag

rjschwar 04-30-2010 02:02 PM

I think what I will do is put 2 3/4" x 10" wedge anchors per post. This seems to be the cheapest, easiest method, and will give me the most yard.

If I put anchors at 6" below the top, and 17" below the top, that would be 7" and 18" from the bottom, fulcrum point. The tension in the lowest psi concrete listed, 2000 psi at the minimum embedded depth of 3 3/8 is 8740lbs. I think this is ultimate tension so if I take 1/4 of that, I get 2185. So say 2000lbs. gives me 2000(18+7) = x * 108 (inches of 9' pole, 6' fence 1' dirt 2' below ecology block) and that gives me 462 lbs at the top of the fence, or if I assume the force from wind is middle of the fence, 72" above fulcrum, that gives 694 lbs.

I can sink them in about 4-5", giving me 5-6 inches hanging out, and once the 4x4 is on, 1-2" for the bolt. This will give even more strength.

By my calculation, if we somehow were to get even 75 mph wind, the force would be 75^2*.00256 = 14.4 lbs/ft^2, or 518.4 per 36ft^2 panel of fence, thus those two anchors should be plenty.

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