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Attaching footing to retaining wall?

Posted by drdna801 10a (My Page) on
Fri, Dec 18, 09 at 23:14

Due to a miscommunication, a concrete retaining wall was poured without any footing. The wall is 4'H x 65'L x 8"W with #4 rebar. The site is pure sand, in a area that was previously sand dunes by the ocean. The proposed fix is to add a footer that will be 18"W x 12"H along the entire width of the inner wall, drilling 6" holes into the wall 3" from the bottom spaced 12" apart and using epoxy to secure #4 right angle rebar to connect the wall to the footing.

Will this be sufficient? My primary concern is mechanical failure due to the rebar not being tied in. Is there another solution you can think of, short of a total re-do?

Thanks!
Adrian


Follow-Up Postings:

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RE: Attaching footing to retaining wall?

  • Posted by laag z6CapeCod (My Page) on
    Sat, Dec 19, 09 at 7:16

A footing is beneath a wall, not next to it. A wide footing on a poured retaining wall is usually the mechanism to give it strength in what is called a "cantelever wall". The idea is that the structure becomes an "L" shape where the backfill sits on the horizontal portion of the "L" and acts much like when you step on a rake - it is much more difficult to push it over.

Rebar pinned between a slab and a wall is basically equal to a set of hinges rather than a solid "L".

Pease don't tell me that they want to put the slab in front of the retaining wall instead of behind it. If they do, they are either idiots or they are not serious and just trying to get you off their backs.

A 4' slab of concrete falling over would be deadly if someone were to be next to it when it happens. This is nothing to shortcut.


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RE: Attaching footing to retaining wall?

"Miscommunication"? A miscommunication is no pepperoni on your meat-lovers pizza. This... wow. The only way I could see someone pouring a wall sans footer is this was their first job ever, or they were trying to pull a fast one.

If the above solution was proposed by a professional engineer, bears his/her stamp, and the local permit office and your insurance carrier agree with it, I still think it's a bizarre and unlikely solution but the engineer is usually right. If this is the contractor's solution, I'm exceptionally dubious.


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RE: Attaching footing to retaining wall?

Well, the plans were all to code, but the two principal contractors on the job were out that day and their workers did not follow the plans. A total demo and re-do would be a huge project in terms of money and time. I suggested we demo small sections and put in intermittent inverted cross footing scaled larger to accomodate the torque force of the entire wall, but the contractor suggested this solution. Just don't want the wall to fall over and trying to repair a bad situation.

Any ideas for how you would approach the situation to try and come up with a fix?


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RE: Attaching footing to retaining wall?

If the plans were "all to code" as you say but the built wall is not then it should be demolished and rebuilt as it was supposed to be. You say there is time and money involved, this should be covered by whoever was contracted to build according to the plan.


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RE: Attaching footing to retaining wall?

  • Posted by laag z6CapeCod (My Page) on
    Sat, Dec 19, 09 at 20:57

Ink is exactly right. They did not follow the contract. It is their expense to fix it.

Are you the property owner or project manager?


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RE: Attaching footing to retaining wall?

  • Posted by bahia SF Bay Area (My Page) on
    Sun, Dec 20, 09 at 15:32

I find it incredible that this "miscommunication" could happen in just one day. Doing form work for a wall of this size would have taken many days to do, and the main contractors must have been gone all week, and never once checked up on construction progress while the form work was being done. Negligence of this magnitude should not be tolerated, and in my view these contractors should have to pay for it, as required by contract law.

If the client is willing to consider alternatives to complete demolition, with additional work obviously to be done at the contractor's expense, there probably are better solutions that what is being proposed. I'd suggest getting a consulting structural engineer involved, and investigate whether driving steel I-beams at regular spacing in front of the wall might be a solution. However, if there was no oversight on form work, who even knows if sufficient rebar was actually placed in the vertical wall section that was installed.


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RE: Attaching footing to retaining wall?

  • Posted by laag z6CapeCod (My Page) on
    Sun, Dec 20, 09 at 17:09

No, there is no problem setting up 65' of 8' forms and pouring it in a day with a good set of forms. We used to do a house a day with no problem back in the eighties when I lumped forms during the winter. The toughest most time consuming part of that job would be bracing a straight wall of forms so that it did not fall over or blow out while pouring it.

Dunes are shifting sands. Does anyone realize how unstable a straight 8' retaining wall is, never mind the sand.Another problem is that dunes grow (or shrink) and you could wind up with a much bigger surcharge on that wall on the top side. Or the wall can shift the flow of the wind and scour out an undermining on the low side.

Splat! Not good.


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RE: Attaching footing to retaining wall?

Well, I am the owner and came home to see the workers pouring concrete into open, unframed, untamped soil while other workers were PUSHING pieces of rebar into already poured concrete. I told them to stop and the owner of the concrete company, who didn't believe me until he came out ans saw it for himself. He said he would do whatever it takes to make it right, but frankly that would be a major demolition work for that much reinforced concrete. The neighbors are already unhappy and this will put the whole project behind schedule.

Now, I am no structural engineer, but I did a few simple calculations:

Proposed retrofit involves drilling 5" holes every 12" OC into existing wall and using epoxy to connect to a new poured footing.

For the pond, proposed retrofit is to connect the lower 4 subterranean portion of the wall to the three sided enclosure with rebar and pour concrete to connect the two parts.

A. BACK WALL. Dimensions of wall are 25L x 3H x 8"W = 2.08 cu.yd.
4000 lbs./cu.yd. (concrete) x 2.08 = 8320 lbs.
The wall retains a volume of 25 x 20 x 4 of soil = 2000 cu.ft.
130 lbs./cu.ft. (wet sand) x 2000 = 260,000 lbs.
Estimated foliage on this land is considered negligible.
Because of excellent drainage of sand, hydrostatic pressure is negligible.
Total 268,320 lbs.
#6 Rebar at a depth of 5" yields a surface area of 11.8 sq. in. (5*(0.75)*pi)
Spacing 12" OC yields 23 dowels x 11.8 = 271 sq.in. total
Thus 268,320 lbs./ 271 sq.in. = 990 psi
The shear strength of epoxy is 2000-8000 psi, so it should hold.

B. SIDE WALL. Dimensions are 40L x 2H x 8"W = 2.22 cu.yd.
4000 lbs./cu.yd. (concrete) x2.22 = 8,888 lbs.
The wall retains a volume of 20x 40 x 2 of soil = 1600 cu.ft.
130 lbs./cu.ft. (wet sand) x 1600 = 208,000 lbs.
Estimated foliage on this land is considered negligible.
Because of excellent drainage of sand, hydrostatic pressure is negligible.
Total = 216,888 lbs.
Total surface area for 38 #6 rebar pieces = 449 sq.in.
Thus 216,888 lbs./ 449 sq.in. = 480 psi.
The shear strength of epoxy is 2000-8000 psi, so it should hold.

C. POND WALL. Dimensions are 4L x 8H x 1W = 1.19 cu.yd.
4000 lbs./cu.yd. (concrete) x 1.19 = 4760 lbs.
The wall retains a volume of 10 x 4 x 8 of soil = 320 cu.ft.
130 lbs./cu.ft. (wet sand) x 320 = 41,600 lbs.
Estimated foliage on this land is < 8,000 lbs.
Because of side drainage, hydrostatic pressure is negligible.
Total = 54,360 lbs.
In a worst case scenario, the new concrete will fail to adhere to the old, placing
the full weight upon the cross section of the rebar. The shear strength of rebar is much less that its tensile strength. Calculating from SAE shear strength ratings for standard steel rod:
54,360/3310 = 16 pieces of #6 rebar, 6" OC or
54,360/5890 = 10 pieces of #8 rebar, 10" OC.
Thus the repair should be effective.

I appreciate the feedback on this issue.


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RE: Attaching footing to retaining wall?

There's a pond in this scenario? (picture Macauly Caukin with hands upside his face from 'Home Alone')


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RE: Attaching footing to retaining wall?

  • Posted by laag z6CapeCod (My Page) on
    Sun, Dec 20, 09 at 19:55

You need unbiased professional help. Don't try to get an answer on the internet. You just laid out huge evidence for a neglegence suit if anyone suffers damages due to a failure of any of this. Cover your butt and get the proper help with this.

I don't know where you are getting your calculations from (websites, I'd guess), but a free standing concrete wall with nothing behind it can still fall over. Backfilling it makes it more likely to fall over. There are several ways in which a wall can fail. It is not simply mass against mass.

This story keeps changing. What happened to the 8' high 65' long wall?


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RE: Attaching footing to retaining wall?

Well you did the right then upon discovering the quality control failure (no footing and improperly installed re-bar) by ordering a work stoppage. I am perplexed about why there is no continued QC follow through by demanding re-work from the contractor.


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RE: Attaching footing to retaining wall?

  • Posted by bahia SF Bay Area (My Page) on
    Sun, Dec 20, 09 at 21:48

From what the owner has additionally said, it sounds like this project was not done with a building permit,(form work should have a required inspection by city prior to concrete pour), and as it appears that no form work was installed(?), and I don't know where Laag gets the 8 foot wall height, but a 4 foot wall requires engineer's plans as part of the submittal and mandates a building permit if done to code. I won't dispute that it may be possible to do a concrete job with premanufactured form work in one day, but most projects that I have designed with concrete form work, (admittedly more complex than a single length of straight wall), require multiple days of digging and form work installation and additional day(s) for installing rebar, all prior to the scheduled pour. If this is for a private residential project, it is unlikely that there was a large enough crew to do all this work in a single day.

It sounds like the contractor(s) must have been the low bidders on the job, or were simply negligent or ignorant of proper construction techniques to do the work in a professional, to code manner. Given the information supplied, I would recommend that delaying the schedule is the least of the owner's worries, and that the entire wall should be demolished and rebuilt to code appropriate plans, with the proper inspections performed. The performance of the contractor(s) would lead me to believe that the entire job is being built in a substandard manner, and diligence on the owner's part to protect his own interest and minimize lawsuits from a potential wall failure is paramount.

The whole scenario seemed at first so unlikely, I thought it was a joke posting...


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RE: Attaching footing to retaining wall?

One of the architects I deal with won't use epoxy anchors for ledger attachments on decks, because it's too difficult to ensure perfect application of the epoxy in each hole. Without proper application of the epoxy, it won't have near the strength required. You're asking a lot of this jerry-rigged fix, as a wall carries a pretty significant load. And, you're expecting a contractor who can't properly build a wall to engineer a corrective action and install it?

Here's what I'm getting out of all this: it sounds like you have a pretty complex project, with a lot of moving parts. You state that you had plans that specified a properly constructed concrete wall, and hired a contractor to execute the project to those specifications. For whatever reason, be it honest mistake (how?), incompetence, or shadiness, the specifications were completely ignored. Unless I'm missing something, it sounds like the contractor is now peddling a band-aid solution, and because it's expedient and doesn't require much pushing back on your part, you're anxious to get confirmation that this will work. Personally, I think your best option is to have the contractor demo his mistake and redo it the right way. Alternately, you should consult with a structural engineer and follow his/her advice. Even though the wall is now shorter than in your first post, there is still significant risk and liability in bodging together a solution.


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RE: Attaching footing to retaining wall?

  • Posted by laag z6CapeCod (My Page) on
    Mon, Dec 21, 09 at 6:56

I see that the 8'wall was on his other thread.

This is why saving money by being your own general contractor is not always going to save you money.

There is a little saying in my neck of the woods regarding homeowner's acting as contractors: "do it yourself and save = &^%$ yourself, its cheaper".


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RE: Attaching footing to retaining wall?

It's been a long time since any of my structural engineering classes so the technology could very well have changed, but it would seem to me that retrofitting a footing and connecting it with epoxied, angled rebar would not come close to having the structural integrity that a correctly designed and constructed footed retaining wall would possess.

I'm pretty sure I would have been flunked if I were to have offered this as a viable alternative :-)

Again, I agree with others that this is not an appropriate venue for the technicalities of this discussion. You need the intervention of a professional, licensed structural engineer.


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RE: Attaching footing to retaining wall?

  • Posted by catkim San Diego 10/24 (My Page) on
    Mon, Dec 21, 09 at 15:39

If the neighbors are unhappy now, imagine how unhappy they will be when this retrofitted wall begins to deteriorate and the contractor is long gone (with your money). Imagine how this will sound in a courtroom in front of judge and jury: "Now, I am no structural engineer, but I did a few simple calculations." You have a chance to save your butt now; as bahia recommended, demolish and rebuild to code.


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RE: Attaching footing to retaining wall?

The calculations of A, B, & C above do not reflect the forces at play and are useless. There is no way I would try to pin a footer to the back of that wall.

There is a fix for most wall problems. But in your case, having the forms from the first pour to re-use, will probably make it cheaper to demo and rebuild.

Also, I get the feeling that you have failed to communicate exactly what the situation is. You said: "Well, I am the owner and came home to see the workers pouring concrete into open, unframed, untamped soil while other workers were PUSHING pieces of rebar into already poured concrete." What function was this concrete pour if not a footing, with vertical rebar being set for the wall tie?

Photos and original plan drawings would be useful to understand your project.


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RE: Attaching footing to retaining wall?

Using my imagination would I be wrong in guessing that our OP is more invested in this project than we have been lead to believe? Some of us have been lead down this path before and have expressed our view based on this experience.


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RE: Attaching footing to retaining wall?

  • Posted by bahia SF Bay Area (My Page) on
    Mon, Dec 21, 09 at 23:59

pls8xx was observant in picking up the reference to no forms and pushing rebar into the concrete could very well have been a footing base with vertical tie bars being placed, rather than the wall itself. There is no way that a 4 foot exposed concrete wall could be poured in place without form work to hold it together, against SAND as the subsoil base. A lot of this scenario just doesn't add up.


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RE: Attaching footing to retaining wall?

Well I am seeing a structural engineer tomorrow. I will update with the recommendations, which will most likely be demo and re-do.

To clarify the primary concrete contractors were away and the workers demonstrated pure incompetence by failing to follow the plans and pouring before inspection occurred.

Concrete was poured in untamped sand as a weird attempt at a footing I guess but it is loose, unformed, without rebar crossties, and extends only about three inches horizontally.

The 65' wall is 2' tall at one end and 4' tall at the other end of the yard. This is the "no footer issue."

The 8' wall is only 4' long and is designed to house the reservoir and backsplash for the spill on a water feature. There is a 4' x 4' platform that should have had rebar connected to the wall.

Hope this clears up any confusion as I originally posted this in two threads.

Thanks!


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RE: Attaching footing to retaining wall?

There is absolutely no question to anyone of professional calibre about the status of this wall.
It should be demolished and properly constructed with the code compliant permited inspections.

Your neighbors might be unhappy about your construction work but could you imagine if this wall was inappropriately amended with substandard building practices and failed thus killing one of your neighbor's children ?
How pissed would they be then ?

Think smart.


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RE: Attaching footing to retaining wall?

The structural engineer said demolition would not be required. He agreed with my proposed fix and is drawing up plans with the specifications. Good news, sort of.


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RE: Attaching footing to retaining wall?

"The neighbors are already unhappy and this will put the whole project behind schedule."

If this is the most important thing that worries you, you should not be allowed with any decision making. You put people's feelings over savety?


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Attaching footing to retaining wall?

This is more like mate in less than 10.

Is there any kind of rule of thumb

Here is a link that might be useful: Pedway


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