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Concrete Retaining Wall

| Posted in Construction Techniques on July 18, 2006 08:47am

I am reconstructing a retaining wall originally constructed with bridge planks that have rotted away permitting a large cement patio to settle substantially to the point of cracking on its lengh about half way down the middle. I intend to have a contractor raise it by pumping in cement. But first I want to resecure the patio with a concrete retaining wall.’

The wall is 40 feet in lengh and about 3 feet in height (above grade). I have dug footings 42 inches deep by about 12 inches wide. I want part of the wall to support the cement slab and part to extend beyond the slab/patio for finishing with a small block edge slightly higher than the slab. The property is in central Iowa and code requires the footing at 36 inches. Due to inaccessibility to the area, I intend to pour the footings and level them along the 40 feet and then finish up to the slab/patio with block.

My questions involve the support that I need in the concrete footings and the block wall. I intend to install rerod in the footings. Is there a standard for the amount of rerod that should be in the 12x 36 footings for 40 linear feet in order to stop the frost from breaking up the footings? Also, do I need the rods to extend up into the cement blocks to tie everything together. Also, is there a product to tie the block back into the ground. The patio and fill area was originally constructed 26 years ago so I don’t think there should be much settling but again I am worried about the frost pushing out against the cement blocks that will be above the grade of the ground and footings. thanks for any suggestions anyone might have.

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Replies

  1. john7g | Jul 18, 2006 09:10pm | #1

    What kind of block are you planning to use? 

    1. PRSVRNC | Jul 18, 2006 09:15pm | #2

      The Block is a cement block that same as one would use for a basement wall.

  2. User avater
    BillHartmann | Jul 18, 2006 11:42pm | #3

    I would have used the modular retaining wall blocks, And they don't need concrete footings and they don't have to be as near deep.

    They "work" by by stepping each block back, bantering them, so that they "lean" against the overburden.

    Bantering is hard to do with blocks. If you don't banter the walls then you need use rebar to tie them together and to the footing to prevent overturning. And the flooting might need to be wider.

    I have seen some details on this, but don't remember where. Try the library for books on concrete work.

    As far as frost heaving it is the same for any type of wall. You need to keep the space behind the wall DRY.

    Install drain pipe at the bottom of the wall. Backfill with a drainable rock, 3/4" clean cushed rock for example. Then use filter cloth before backfilling.

    1. PRSVRNC | Jul 19, 2006 12:59am | #5

      I have used the type of block that has a step notch at the rear of each block if that is what you mean by bantering. I have not found these to be very effective in holding a wall of dirt. Perhaps these are not the blocks you are referring to .

      I hate to hear about easier alternatives after digging the footings by hand  in a area full of tree roots; however, I want this wall to hold up and last. So I am interested in whatever you think will make a strong wall.

      1. brownbagg | Jul 19, 2006 01:15am | #6

        I,m glad I dont live up north, that 53 yard just for footing, over $5000. I think as a wall, who cares about frost. go with a shallow footing. I know it migh heaves but its onle a wall. maybe pier footing with a grade beam.Construction up north is so different from here.

      2. calvin | Jul 19, 2006 01:36am | #7

        To hold a good amount of dirt, you might be surprised and educated to look at the published industry details.  I cannot recommend the book but have looked at one from a masonry/concrete association 15 yrs ago.  I was amazed at the engineering that went into their designs for retaining walls.  I remember a goodly amount of substantial rebar set in an L shaped pour, footing and wall, all in one.  The L shape as you want the wall to stand straight against the weight and pressure, not tipping over.  The detail listed both the L under the area to be retained and on the opposite side (more your situation).  I think with you're wanting to rest a slab on top of the footing you might get away with less width in the footing than was listed in the manual (almost as much width as height in the L).  If you want it to last do the studying now, and best of luck.A great place for Information, Comraderie, and a sucker punch.

        Remodeling Contractor just outside the Glass City.

        Quittin' Time

         

        1. VaTom | Jul 19, 2006 05:22am | #8

          Here's one.  I prefer the footing under the fill, but stay with the rebar recommendations.  No structural problem with a cold joint between footing and wall.PAHS Designer/Builder- Bury it!

          1. calvin | Jul 19, 2006 06:33am | #9

            I like the leg down, further using the surrounding earth to hold it up.  What's the book?A great place for Information, Comraderie, and a sucker punch.

            Remodeling Contractor just outside the Glass City.

            Quittin' Time

             

          2. VaTom | Jul 19, 2006 03:26pm | #11

            What's the book?

            Don't know.  These are pages photocopied by an archy for me maybe 15 yrs ago.  When I bought engineering for our place, I took the opportunity to ask a lot of retaining wall questions and got a good feel for footing design.  Which is mostly ignored around here.  Lots of problem walls. 

            Wasn't very long after I did 180' lin ft of 4' wall for a client.  He asked why I wanted so much footing but agreed when I showed him drawings.  Wasn't a lot more expensive and the wall's been fine for a decade.

            Here's another:PAHS Designer/Builder- Bury it!

          3. calvin | Jul 19, 2006 03:39pm | #12

            I know when I looked at wall details I was amazed at what "should" be done.  Having witnessed many problem walls over the years, I can understand the thinking.  Thanks for refreshing my memory.

            You coming up this way again this summer?A great place for Information, Comraderie, and a sucker punch.

            Remodeling Contractor just outside the Glass City.

            Quittin' Time

             

          4. VaTom | Jul 20, 2006 06:40am | #19

            Having witnessed many problem walls over the years, I can understand the thinking. 

            The engineer was great in explaining the many failures I'd seen in house foundations, particularly with a walk-out basement, in which the buried wall is essentially a retaining wall.  Around here, rarely done adequately, resulting in sidewall cracks.

            Retaining walls are generally much worse.  Gets expensive replacing them.

            Not Michigan this year, but Erie calls.  I've got a zoning issue that's going to very much tie me up for awhile protecting (I hope) most of my net worth.  Signed on for a sail to Buffalo a little later, probably Sept.  New Riegel ribs?  Guess I could deliver...  LOL

            Chased a bear off the patio last night after it stuck its nose into the door glass.  Our guests were sure surprised.  3-4 yr old.PAHS Designer/Builder- Bury it!

          5. calvin | Jul 20, 2006 01:00pm | #20

            If you pass by here on the way to the dock, give me a call.A great place for Information, Comraderie, and a sucker punch.

            Remodeling Contractor just outside the Glass City.

            Quittin' Time

             

      3. john7g | Jul 19, 2006 04:22pm | #13

        I think the 'bantering' term refers to leaning the wall into/towards the dirt you're trying to retain.  With a vertical wall you're kind of going against gravity  Ever see a dirt cliff?  If you have, weather and gravity reshape it to some kind of slope eventually.  With the plan you started this thread with, the block would soon tip out or the mortar joints would fail due to the force pushing on them.  Using the block with the lip that creates the angled-back wall as they are placed creates the slope that gravity tolerates more and since they don't use mortar drainage from behind the wall is better.  Cutting back the slope and providing the best drainage that you can with the (as previously recommended) stone and drain tile with the silt filters is the only way to lengthen the life of any type of retaining wall that you decide to use. 

  3. peteshlagor | Jul 19, 2006 12:23am | #4

    What Bill says.

    too bad you did all that digging...

     

  4. User avater
    SamT | Jul 19, 2006 08:33am | #10

    Oooh! You got a mess.

    I hate to be an anal orifice, but first I gotta tear up some of your ideas, (and work.)

    I want part of the wall to support the cement slab. . .I intend to have a contractor raise it by pumping in cement. [After the new wall.]

    So ya plan to do two mutually exclusive things first?

    The wall is 40 feet in lengh and about 3 feet in height (above grade).

    I'm assuming you mean 3' above Lower Grade, that is the grade the patio sits on 3' higher than the rest of the place.

    however, I want this wall to hold up and last. So I am interested in whatever you think will make a strong wall.

    OK, here goes; Excavate the 2' of the upper grade next to the lower grade down to the level of the lower grade. . . EXCEPT UNDER THE PATIO.

    At the edges of the patio, perpendicular to the retaining wall, carefully dig two 8" wide trenches as down to the lower grade and extending 3' down the patio edges. You will pour these wings before the mudjacker arrives, possibly after you've laid the blocks and at the same time you grout them.

    Here's the way I see your concrete and rebar;

    View Image

    The rebar (pink) that seems to be under the patio slab is actually in the stabilizer portion of the retaining wall footing to either side of the patio. The stabilizer will be covered with the soil of the upper grade. Plus some drainage tile and gravel.

    Here's a face on view that shows what spacing I'ld use.

    View Image

    The verts that run from the bottom of the footing to the top of the blocks are spaced 24" OC. This will put one in every third block void. 8" away from them are Ell bars that run from the bottom of the footing to its' top, then turn and run into the stabilizer. 8" from them, are Ell bars that run from the back of the stabilizer to it's front, then turn and run to the top of the blocks. The blocks will have rebar in 2 out of 3 voids. Even the footing and the stabilizer will have bar-bar-space. All this is crossed by horizontal rebars spaced about 1' OC in the footing and stabilizer and between each course of blocks. Go ahead and put verts at 8"OC or 16" OC from bottom of footing to top of block in front of the patio. All rebar would be #4 except the 2 at the bottom of the footing would be #5.

    Patio wings; You can't mudjack the patio until you've made sure the jacking mud can't run out from under the edges. After you've laid the block wall, place a temp form 8" from the edges of the patio and pour the wings with block grout. The wings finish elevation must be at least 1/64" above the bottom of the slab WHEN IT'S AT FINISH ELEVATION.

    Drainage; two choices: 1) Don't grout between the blocks on the bottom course and let it drain out the full length of the wall. 2) Lay 3" or 4" tile to open air at one, or both, end/s of the wall. In either case you will need a gravel and fabric burrito around the tile. 

    Backfilling; FOR SEVEN DAYS after you grout the wall and wings, KEEP THEM VERY WET. Then back fill and compact.

    I probably left something out. . .so ask questions iffen you don't *See* *the* *light*

    ps, Not resposible for typos after 2400 hours

    SamT
    1. PRSVRNC | Jul 19, 2006 05:31pm | #14

      Wow, what a great source of information. Thanks to everyone that responded. For SamT, I have a couple of questions I understand that  the rebar from the footings are place 24" OC and the rebar from the stabilizer are set over 8" and are spaced on 24"OC. But I don't understand what you mean in the second paragraph when you say; " put verts at 8"OC or 16"OC from the bottom of footings to top of block in front of the patio". Did you mean horizontals?

      On the wings, are you suggesting that I form these up and  pour them without  building footings? Also you suggest pouring Block grout in the wings. Is that something different than the concrete I will pour for the footings and stabilizer?

      Thanks for the excellent analysis and drawings. They are very helpful. I have never laid block with rebar in it so I assume that the dimensions of 24"OC will space the rebar so it will go thru all the blocks without any difficulty. Would it be best to wait to fill the block with concrete after the last course is laid to allow the ability to move the rebat somewhat if necessary or should I fill the block as I build the wall?

      Thanks again for all the help!--Bill

      1. User avater
        SamT | Jul 19, 2006 06:26pm | #15

        But I don't understand what you mean in the second paragraph when you say; " put verts at 8"OC or 16"OC from the bottom of footings to top of block in front of the patio".

        The stabilizer (or Outrigger) portion of the footing will not be under the patio slab so there is no use for Ell rebar in the patio area.

        On the wings, are you suggesting that I form these up and  pour them without  building footings?

        Yes. The Wings are just a dam to hold back the mudjacking grout. They must be strong enough to resist the lateral forces generated when you compact the backfill of the retaining wall.

         Also you suggest pouring Block grout in the wings. Is that something different than the concrete I will pour for the footings and stabilizer?

        Yes. The footings and stabilizers should use concrete with larger aggregate than the pea gravel mix you fill the blocks with.

        Would it be best to wait to fill the block with concrete after the last course is laid to allow the ability to move the rebar somewhat if necessary

        Absolutely.

        Remember, in construction, God is in the layout.

         SamT

        1. PRSVRNC | Jul 19, 2006 09:03pm | #18

          Thanks, I hope to start pouring concrete by Saturday. I'll let you know how it works out.

  5. woodway | Jul 19, 2006 06:49pm | #16

    Your wall is 3 feet above grade and sits on 42 inch deep footing...don't know about your area but you probably can't get a permit to build it unless it's engineered. If you get it engineered, then all these questions you are having will disappear. Of course there are those who would build it anyway but then you could encounter problems down the road that you don't see at the moment. From your description, this wall needs to be able to withstand the test of time and it's holding back some important features of your home. If this was just holding back landscaping etc. then it probably doesn't matter but this is your home and you should get it done correctly. Again, maybe your area is different but code here reads "anything over 4 feet from bottom of footing to top of wall needs to be engineered." There's a reason that's in the code and I suspect it's because of past retaining wall failures and potential significant loses of either property of life/health issues.

    The Code raises it's ugly head once again!

    1. PRSVRNC | Jul 19, 2006 08:52pm | #17

      Woodway--According the city I reside in they do not require a permit unless it is an integral part of a dwelling. this dwelling was constructed in 1978 and a year later the then homeowner decided to add a 40x12 ft patio behind the house. The retaining wall constructed out of bridge planks was done soley to permit the patio to be level with the back of the house,due to a sloping lot,  not to give support to the rear of the house. So I believe the only engineering necessary is to hold the load of the fill dirt, that has been there for over 25 yrs and to support the edge of the patio. But I appreciate your suggestion of getting professional engineering advice. From the quality of the suggestions in this thread, I feel comfortable that the way it is "engineered" with the footings, rerod and stablizer will hold the dirt and patio for many years to come. Only time will tell, I guess.

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