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How to Repair Rodent Damage to a Drystack retaining Wall

 
pollinator
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Location: Zone 3-4 (usually 4) Western South Dakota, central Black Hills
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I’m trying to figure out what to do about my mom’s retaining wall. (We live with her as caregivers in her house on our land.) As you can see, it’s suffering rodent damage.  When it was built, there was a steep slope which was backfilled as the rocks were stacked. Now mice and chipmunks have mined around behind it to the point that it’s standing primarily by virtue of balance. Other than gravity, there’s not a lot supporting it.

I guess I could just pull it down and try to restack it. I could probably lift most of the higher up rocks. Only I’m afraid of the dirt behind them tumbling down uncontrollably and just making a big mess. I’m thinking maybe this should never have been dry stacked at all and that perhaps I ought to mortar it. I’ve never done anything like this but I suppose I could manage it. I’d hire someone but that’s a lost cause. It’s all but impossible to get anyone out where we are, no matter how profitable the job. We have three roofing jobs paid for by insurance... one of them very significant... can’t even get a bid... so that’s not an option. I’m surprised we even managed to get anyone out here to do the original construction. Everything here is diy, so we’re on our own with this.

Tools we have that seem significant to me: tractor with backhoe and loader, wheelbarrow, shovels, etc. I’m building a garden cart soon. 1/2” drill, mortar mixing “bit”, masonry tools... I guess we could buy/rent a small cement mixer if necessary. Obviously fixing the rockwork is a spring/summer job, but I’m just trying to figure out a plan. It’s bothering me. DH is recovering from a bone marrow transplant and suffering from arthritis in his hands, so it’s mostly gonna be me doing the heavy work. He’s good with the big machines, though. I can operate them but I’m not fond of doing it.🤨

So... I’m just hoping to get some advice on books, videos, what would be the best way to rehab this wall... anything you think I ought to know/consider/etc. I’ve ordered spring chickens (Buckeyes) to hopefully take charge of the mice, btw, so hopefully that will work. Many of the chippies have died of... well... copper b-b poisoning? It’s hard to get mice that way though, and they seem to have figured out all the traps...

Anyway, a big thanks for any advice you can offer me. 😊
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Retaining Wall
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Detail 1
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Detail 2
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Detail 3
 
gardener
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Location: Upstate NY, zone 5
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Obviously the rodents didn't help, if they have made piles of dirt in front of the wall, but a drylaid retaining wall will not be durable for the long term if it is not built with stone in depth as well as height. I can't tell from the pictures - is the front face vertical, or sloped back, and by how much? It appears to be bulging out around midheight, which is a sign of insufficient depth of masonry. In zone 3/4, I expect your frost depth would make a mortared wall heave and crack without significant foundation below grade, while a well-built dry wall can shift a bit and settle back without damage.

A tractor with loader is a very good tool if you have heavy stones; you can bring them to wall-top height, then just lever them out into position. Do you have more stone you could use to deepen the wall? It does not all need to be pretty.
 
Cindy Skillman
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Thanks, Glenn! It’s just as you see, and it ought to have a vertical profile but has been bulging out little by little. No thickness behind the facade... that makes sense. I wondered about that at the time, but I figured they were the masons and didn’t want to be rude... I used to be much less bold. My dad was a builder; I foolishly thought they were all mostly like him. (Sigh) Yes, we have many, many rocks. I guess it needs digging out, fortifying behind by maybe some percentage of the wall’s height and then sloped inward (thinned gradually) as it gets taller? Would it be better to slope the vertical face back toward the higher ground?

I really appreciate your help. 🙂 It sounds fairly simple... hard work, but maybe not so hard to understand as I had feared.
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Detail 4
 
Glenn Herbert
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Okay, it is as I feared. Yes, it will need to be taken down and rebuilt. Fundamental rules of drylaid stonework are to batter (slope) the face(s) about 1:6, and to make the base somewhere around 1/4 to 1/2 of the height (this would vary depending on the character of the stone available). Your stone looks reasonably squared and very textured, so would be quite durable when properly built. The base needs to be thickest, and can taper toward the top, and the face needs to slope back into the hillside. The stones should be bedded not perfectly flat, but tilted back into the hill a bit. This way, they will always want to fall back into the hill. You want some of the biggest flat stones in the base, and many or most of the stones need to have their length going back into the wall rather than stretched along the face. A combination alternating between headers and stretchers can be good. Some of the heavier stones should be on the top course, so they hold the ones below them in place and will be hardest to dislodge.
 
pollinator
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That wall will be beautiful when it's done; I'm already admiring your future garden, Cindy

I have a question for you, Glenn, if you don't mind:

In block stone walls, it is recommended to backfill with coarse gravel against the interior of the wall before backfilling with the dirt - is this the same in drystack stone walls, or would a different course of action for backfill be recommended?
 
Cindy Skillman
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That’s what I figured too, Glenn, from what you said before. I don’t mind so much, as long as I have some idea what to do. Thanks for your wisdom. 🙂 The wall stones were brought in. The stone we have on site (for back fill) is more irregular; mostly quartz chunks with some granite, smaller mostly, though you can always find big ones by trying to dig a post hole...😥 Does that sound like something that would work?

One thing you said that I didn’t understand:

Fundamental rules of drylaid stonework are to batter (slope) the face(s) about 1:6.


I’m not sure what the ratio 1:6 means?

This is what I’m roughly invisioning from what I understand you to say:

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Cross section of drystack retaining wall
 
pollinator
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It may not be ideal aesthetically, but would driving some stakes into the ground at the base and pulling heavy fencing like cattle panels along the stone be an option? It would function like a gabbion fence, holding the slope for the life of the wire. Not a permanent or ideal solution, but would keep it from collapsing for awhile with fairly low cost. You could grow vines a climate zone or a more above your location on the fence to make it look better and stack functions.
 
Glenn Herbert
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That sketch is exactly right. 1:6 means sloping 1" for every 6" of height.

As long as your on-site stone is irregular and somewhat toothy, it will work fine for backup. Round, smooth stones are not so useful, being mainly good for drainage. Putting things like that or gravel behind the stacked stones will help water to escape, minimizing frost heave.
 
Cindy Skillman
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Not a bad idea, Ben, but the wall isn’t really even necessary... it’s just that my mom paid so much for it, and it *is* nice not to have that steep drop-off from the garage down to what’s now become the livestock area... especially since it’s in the shade and currently icy as all getout. I’d probably just let it crumble though, if not for the esthetic considerations. It would probably improve drainage. (Now I think of it, we could likely do something about the drainage while repairing it.) I’ve seen the curtains of chain link cascading down around the railway tunnels/turned trails. At first I thought they were really ugly... but now I think of them as steampunk chic. 😛 DH still thinks they’re ugly, but I daresay he’d think more fondly of them if he’d ever had a rock land on his bike. 🙃

Glenn, cool! And yes our rock is nothing if not irregular and toothy. Since you mention gravel, we have a pile of 1/4” clear crushed limestone for road maintenance. Would it be good to use some of that in with the bigger fill stones?
 
Glenn Herbert
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Yes, poured in to fill gaps after placing larger stones, that crushed limestone would do very well. I might concentrate on using it nearest the dirt bank, as it will inhibit dirt from sifting in between the larger stones and maintain drainage.
 
Cindy Skillman
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This forum is the best and I really appreciate all the great advice from everyone! 😀 We will do just that when the ground thaws and dries. I feel so much better about the whole situation now that there’s a plan. I’ll definitely be back with pics and probably questions when we get going next spring/summer. Thanks so much. Glenn, I’m very grateful for your help.
 
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These folks obviously have much more rock wall expertise than me but I had a crazy permi-ish anti-trash thought while reading the post. You have a backhoe. Good chance of having too many old tires. Why not remove some of the dirt behind the wall, add in tires like building an earthship, then adjust/restack the wall & add some backfill to smooth the ground & hide the tires???

Agreed. Permies rocks. The forums rock. The people rock. Would probably require ten lifetimes to absorb all the great info available here. Thanks for doing your part to make it that much better.
 
Cindy Skillman
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Surprisingly we haven’t amassed much in the way of old tires... sounds interesting, though. I’ll be interested in what the experts have to say, too. 🙂
 
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