Can Rebar and Tiebacks Stabilize a Shifting Retaining Wall?

[CreatesMoreWork]

I saw a similar thread on this issue - so I'm cross posting this forum with a builders forum as well - as I'm very worried about this, and am seeking input ASAP... I apologize if this is off-topic - but I really need some advice ASAP...

Last week I cleared an area out next to a fence that has a slight slope on it (about 1' within 2-3' distance). I cut the ground down equal to the neighbor's fence line, and then put in a retaining wall consisting of 2x12s, staked in with 18" steel stakes.

Then, I poured a long slab with the intention of building a firewood shelter on it. The next day, I noticed that one part of the retaining wall has moved about 1/2" or so - so I staked it with 48" long, 1/2 thick rebar at several points to arrest the movement.

I'm now trying to determine the best path to stabilize the hillside and arrest further movement of this slab, or the retaining wall beneath it... I've been considering driving some rebar in at an angle against the slab, or even tunneling beneath it in places to pour smaller bench footings...but I'm not sure...

When you have a second, could you take a look at the attached and give me your thoughts on this...

Thanks -

 

[Pyrrhus]

Could you provide a sideways picture?

 

[FredGarvin]

How high up is the top part of that slab? It looks about a foot or so to me. That can be a lot of weight you're holding back.

You sure don't have any real room to do anything on the side with the fence. Did the 2x12's actually move or did they bend? Also, what orientation are the 2x12's in? Is the wide dimension of the board parallel or perpendicular to the ground?

Either way, I don't think this is going to be easy. At first glance I'd say you are looking at one of three things:

1) The movement stops and all will stay put and life is good.
2) You pull it all up and possibly use 4x4's with rebar through each piece to stabilize.
3) You tie back the existing wall somewhat like they do with collapsing basement walls
http://www.engineeredfoundations.com/images/basement_Wall_Repair/retain04.jpg

I'd be curious to see what the folks on the builder's forum come up with. I always wondered if there were other options for something like this.

 

[CreatesMoreWork]

How high up is the top part of that slab? It looks about a foot or so to me. That can be a lot of weight you're holding back.

You sure don't have any real room to do anything on the side with the fence. Did the 2x12's actually move or did they bend? Also, what orientation are the 2x12's in? Is the wide dimension of the board parallel or perpendicular to the ground?

Thanks Fred... The top of the slab is probably 19" or so in total height about the ground plane of the neighbor's yard (12" + 7" of the slab). The 2x12s moved, I think - in the worst place about 1/2" out - they're oriented vertically (see the attached drawing).

Images follow of the two places where the wall has moved - looking down on the wall from above. Also attached are two drawings - showing the current condition, and some thoughts on mitigating some of the problem.

Some friends who've looked at it think that the movement may just be from the wall squeezing out the backfill (since it's soft) - and that the slab is just exerting downward pressure - not lateral movement.

Right now the consensus is to drive a couple of 4' rebar beneath the slab at 45 degree angles, to arrest/slow any additional movement, and then wait the winter (our rainy season) to see what happens...

 

[CreatesMoreWork]

See the last attached image of the second gap...

 

[Pyrrhus]

Some friends who've looked at it think that the movement may just be from the wall squeezing out the backfill (since it's soft) - and that the slab is just exerting downward pressure - not lateral movement.

The movement of the retaining wall?, The slab is generating a surcharge on the backfill therefore it is contributing to lateral pressure.

Right now the consensus is to drive a couple of 4' rebar beneath the slab at 45 degree angles, to arrest/slow any additional movement, and then wait the winter (our rainy season) to see what happens...

Well depending on the soil, specially for some types of clay there will be swelling because of humidity thus increasing the lateral pressure on the retaining wall.

Btw, the slab is 7 inches thick? just concrete right (no reinforcement)?

 

[CreatesMoreWork]

Yeah, 7 inches thick - with 1/2" rebar reinforcement in a box frame about 1/3 of the way from the bottom of the slab...

 

[django]

Angled supports, sound good. But make sure that you have left holes in the retaining wall, for the rain water to drain through, or else it will serve as a dam and the combined weight will bring the whole thing down. django

 

[Michael_E]

This thing is only 12" tall and the only problem that you're liable to have is that the retaining wall will keep moving letting the slab tilt once the soil gets wet. Any thin items pounded into the hill under the slab won't do anything since they are too thin to provide resistance to soil moving past them. The fix is simple but will cost you some $$ in additional concrete. I've attached a sketch showing the fix.

Basically just leave your retaining wall in place, (add a few supports to keep it in place when you pour the concrete behind it). Under cut the slab by about 6" or so and fill the space behind the retaining wall with concrete. You can add some rebar for some assurance if you'd like, but at 12" tall you probably don't need it. Remember to vibrate the new concrete well to get it to "float" up under the slab since you're bound to have a hollow spot under it once you start undercutting. You really don't have to worry much about drainage with such a short wall.

 

[CreatesMoreWork]

This thing is only 12" tall and the only problem that you're liable to have is that the retaining wall will keep moving letting the slab tilt once the soil gets wet. Any thin items pounded into the hill under the slab won't do anything since they are too thin to provide resistance to soil moving past them. The fix is simple but will cost you some $$ in additional concrete. I've attached a sketch showing the fix.

Basically just leave your retaining wall in place, (add a few supports to keep it in place when you pour the concrete behind it). Under cut the slab by about 6" or so and fill the space behind the retaining wall with concrete. You can add some rebar for some assurance if you'd like, but at 12" tall you probably don't need it. Remember to vibrate the new concrete well to get it to "float" up under the slab since you're bound to have a hollow spot under it once you start undercutting. You really don't have to worry much about drainage with such a short wall.

Thanks Michael - I appreciate the input. I've had that idea suggested to me also. A friend of mine (a commercial contractor) is coming over on Saturday, and we'll probably end up doing just that - in combination with drilling two holes in each slab - one at each end - and then running vertical rebar down into them so they won't move laterally. I like you're idea also, however - and we'll probably end up doing that as well...

Thanks again -

 

[Willox Perez]

Right! Michael I agree with Your reply.

 

[Jasongreat]

Your probably already done with this project. I have built miles and miles of MSE(material stabilizing excavation) walls at work, they are a bit larger than yours but some of the procedures may help you out if you haven't finished yet. We alway backfill the wall with 3/4 inch gravel, it will be at about 94% compaction just dumping it in the hole, and it won't settle like dirt grade will. The next thing is we always use tiebacks, that is strips of galvonized metal with cleats on it, they are laid back and covered with dirt that is compacted so that the weight of the dirt holds the wall back, we use some that are as long as 30 ft but our walls are about 50 feet high, I would think with a 1 foot tall wall you could probably get away with less than 5 ft depending on your soil(sandy longer, clayish shorter). Any larger builder supply shop should have some straps and you could probably attach them to your existing concrete by drilling some holes and using an epoxy. I wouldn't trust epoxy on a wall any higher than a couple feet but it would be cheaper and easier than forming and pouring more concrete,imo. Hope this helps.