Attach retaining wall footing to rock
Hi there: I’m a new poster, faced with a challenge I would like some advice on. An old (and ugly) part of a retaining wall failed this spring in our back yard. Unfortunately, not just the wall failed: its large slab granite foundation also cracked as the wall pushed out. Since the replacement wall should be mortared stone to match the non-ugly part, I need to construct a decent foundation before the replacement wall. (The wall will be replaced with either mortared stone, or cinder block faced with cultured stone.) My plan is to put forms out about 6″ from the rock face, and fill to the top of the current stone foundation with concrete.
The question is how I can attach the foundation securely to the existing rock foundation for the adjacent wall. My first thought was to use a diamond core drill to drill anchor points for rebar, fixed into the hole with grout. (I was thinking as deep as feasible with the bit & hand-held drill, 6″ deep at least) Are there any other recommended ways to tie the new concrete to old rock?
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Note that the remaining cinder block will be removed; it is there to slow down erosion until the new foundation is built.
As an aside, I attributed the failure of the old wall to lack of steel reinforcement between the blocks (no rebar, no block-lock), and lack of drainage (no gravel backfill, no perforated drain pipe). With those in place, the wall should be much stronger and under less hydraulic pressure.
Any comments or suggestions would be welcome. Thanks!
Alan
Replies
Great detail on the question.
I know in Northern new york where there is only vertical load on the rock, most guys just form and pour to clean rock.
You will have lateral loads so I like the rebar option. I am guessing that a good drilling and doweling should be sufficient (and clean rock) but never did it so only stating from common sense, which i might not have.
Jeff, You read it right that the rebar doweling was meant to hold against lateral loads, to keep the footing from moving away from the adjacent rock footing for the mortared stone wall. It's good to know that the "pour and go" on clean rock holds up in New York (I'm in Kingston, Ontario, just a little north of there, and so have the same frost heave & motion concerns).
here (Colorado) we drill the rock and ankor the rebar beded with epoxy ankor cement...
min #5 bar....
a hammer drill and a 7/8-1" masonry bit is all ya need to drill the rock... (12")
and BTW... welcome to BT... great post too...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
Thank you all very much for your kind comments; I've responded to the detailed posts below.
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IMERC, Thanks; I'd heard that both epoxy and grout could be used to anchor the rebar to the existing rock. Do anyone else have any preference for one or the other (a later post suggest epoxy instead of grout, which is why I ask).Unfortunately, I'll have to use something heavier than the hammer drill + masonry bit to cut the holes. You can see in the last close-up photo a small hole in the far left-hand footing rock. That's the end product of 10 minutes of hammer drilling, and it's about a half-inch deep... =] (With that same drill + bit, I can cut a 2 inch deep hole in masonry in about 30 seconds, so I think it's just hard rock.)
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dovetail,
The movement of the footing is why I was planning the rebar; it would (ideally) tie the new footing into the rock in the hillside, so the two stay together under the lateral load from the hill. Unfortunately, I can't cut too deep into the soil there, as it's just a shallow topsoil layer (3-5") covering a deep layer of shale-type rock (weak, easily fragmented).
Thanks for pointing out the depth of the rebar as an issue; I'll go as deep as practical with the diamond bit (I'm guessing my patience with the boring process will be the limiting factor, rather than the technical limitations on the drill.)
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wrudiger,
I hadn't really considered the cantilevered idea for the wall (which is what I think you recommended; tie the vertical wall into the buried footing, so to be pushed over, the footing would have to rise against the hill soil load). The reason I was planning on expanding the footing out instead of keeping it, though, was because the planting area _above_ the wall is already quite tight, and there's lots more room on the footing's level to expand out without running short on visible gardening area.
I do hate the idea of losing sight of that nice granite, though. Unfortunately, the fact that the main slab is already fractured means I'd have to bring the wall in tighter than is practical; the pieces you see in the pics supporting the existing wall are no longer attached to the main interior block, and can't resist any lateral push from the hill.
Drystone would be lovely, too (my father has a farm a few hours away and does all his walls that way), but the expense of stone in Kingston is just too high; concrete blocks + cultured stone will end up being about half the price. (On my dad's farm, the stones "jest grow like weeds", so he has the "luxury" of free materials for his walls... =])Thanks again,Alan
RE the cantilever. That is often called the "toe" of the wall and can extend to the front or non soil side of the wall as well. Which is what you originally drew. It works by resisting overturning by using the compressive strength of the soil beneath the "Toe" to resist sinking into the ground and allowing the wall to tip. Stand up and keeping you ankles locked and not bending them try to lean forward and you will easily understand how it works and how it got it's name.
They can't get your Goat if you don't tell them where it is hidden.
epoxy if you never want that rebar to move or pull out...
we use the Hilti or Simpson 2 part ankor cement....
rarely use the grout...
also we us at least a 1-9/16 or 2" hammer drills...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
Two things. to get the most out of the new foundation it has to be buried so it is pushing against the surrounding soils. Otherwise it will slide along the surface . You may intend that I just couldn't read it in the drawing.
Second item is to make sure you get good drainage behind that wall. Dirt itself will rarely cause the wall to fail, almost always it is the combination of excess water and the mass and weight of the retained slope that causes the problem.
The wall should and needs to be pinned or connected to the footing with rebar that is bent up out of the footing. The footing acts to not only prevent the wall from sliding but to prevent overturning.
6-10" may not be a large enough "toe" for the footing though. That depends on soils, slope above the wall and extent of mass retained.
They can't get your Goat if you don't tell them where it is hidden.
Rather than extending the footing out away from the wall - and lose some planting area - you can run one back into the hillside. That will act as a foot or lever to keep the wall from tipping (weight of soil, increasing as the wall tries to tip).
The rebar epoxied (not grouted) into the stone is also a good idea. Good news is it's not a very tall wall. Dry stack, if thick enough with good enough drainage would even work.
I'd see if I could salvage those granite blocks if I could rather than burying them in concrete. Getting them out of the way will allow you to build a better base for the new wall than trying to fit around someting that may be shifting or settling. Kinda hard to tell from the pics just what's going on.
We are experiencing in my part of the country (Cincinnati Oh) a lot of what you have. Here it was caused by the drought last summer creating large cracks in the soil then lots of rain causing slippage once the cracks filled with water and acting like grease.
You need the Footer (Toe of the wall) to be a minimum below the frost line but probably deeper to provide area that it can push on so it does not kick out. In the footer you need j bars sticking up. You can lay the Bloch over them and then fill with mortar or concrete. I would use dura wire in the wall when it is laid and maybe a bond beam as a top course. Near the top but below the frost line at the top something needs to be laid back into the hill to keep the top from moving. Some refer to this as dead men. There are a number of products on the market for this. Make sure there is gravel behind the wall so water gets out. If not in the winter the water will freeze and expand and push the wall over.
A number of people have made good suggestions
Al, liked your pictures and your ideas. Here is a suggestion. Before you pour the footing go to a construction supply yard and have them precut 3/4" rebar, the depth of the footing plus the height of either the cinder block or concrete wall. Then you gain integrity and strength with out the work. This way your hydraulic lateral pressure would be transferred properly to the footing.
Just my two cents;
The fact that not only your wall but your foundation kicked out leads me to believe this is a frost issue.
Fill out your profile and we will know better.
Drainage, would be my main concerne moving forward.
It's only three courses high, so the lateral pressure isn't all that high. Plus it didn't tip, but rather pushed out. Weep holes and perforated pipe were needed.
Your plan seams good to me + drainage.
I would toe it out (the way you drew it) rather then in because it is only three courses high and there isn't all that much mass to hold it down.