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Retaining wall physics

darrel | Posted in Construction Techniques on August 4, 2003 04:14am

We’re replacing our retaining wall this week. The wall is about 2.5′ tall. At the top, the grade goes back about 3′ flat, then inclines at about 45 degree up the top which is about 10′ above the side walk. (I’ve attached a photo…see the first pic).

The question is if we need to go with the 16″ x 6″ x 12″ blocks or can get by with the 12″ x 6″ x 8″ blocks.

The smaller blocks are about 1/3 less per square foot than the bigger ones. All of the ‘experts’ I’ve talked to this week say that the blocks are all fine up to 4′ walls. From what I’ve read, that’s pretty accurate as the only earth the walls really hold up are the disturbed backfill up to the point where earth can’t keep it’s own slope (I’m not saying that right, but I hope it makes sense ;o)

So, it seems like the smaller blocks are fine, as, at most, it only need to hold up my drainage backfill and the settled earth at a roughly 45 degree angle, which would only be the lower, flat part of the hill.

However, my neighbors, who replaced theirs last year, really think we should go with the big blocks ‘such we all have such a large hill to hold up.’

Thoughts? The wall isn’t really holding up the entirety of the hill, correct?

Since we’re on the retaining wall topic…one quick other question: The old wall we took out is mortared cement block. It’s remained relatively level, but had begun to lean forward over the years and crack and was just plain ugly (see photo). Under this wall is a cement footer…no idea how deep. As the wall never buckled/heaved during its length, is there any reason not to assume the footer is fine and should support the new wall OK? (As opposed to tearing everything out and putting in a gravel footer?)
View photo


Edited 8/3/2003 9:18:59 PM ET by DARREL510


Edited 8/4/2003 12:26:39 AM ET by DARREL510

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  1. fdampier5 | Aug 04, 2003 04:46am | #1

    where you are determines much..

      If you live in the freeze belt you need to deal with the freeze thaw cycles..

      What happens over time is the ground freezes.. and as we all learned in high school science class that when the water in the ground freezes it expands.. that expansion will try to force the wall over.  It may only move a little at a time, but it will slowly topple even a low wall if the wall is not strong enough to resist the force and there is no provision to drain the moisture from the ground around the wall..

         to deal with this freeze cycle use gravel adjacent to the wall and put a drain field in the bottom of the gravel..   If you implant pipes in the joints at the bottom you will give the wall a way to weep the moisture before it does any damage.    The gravel will shift enough to  prevent the wall from falling..

      If you put a shallow layer of topsoil over it, you can grow grass or whatever right up to the edge.. just be carefull to water the edge frequently..

     My opinion of block as a building material is rather low.. there is little strength in the joints.  I've been able to able to kick over an old joint with little more than my boot..  at best a sharp blow with a seldge hammer will topple even the best laid block wall..   I've seen buildings topple with little more than concentrated wind. 

    1. darrel | Aug 04, 2003 06:37am | #3

      Frenchy:

      Yep, we're in MN, so we certainly have the freeze/thaw problem. I will be putting in a full backfill of landscaping fabric, river rock and drainage tile.

      As for grass...nope. I'm tired of mowing this thing ;o)

      Notchman:

      The terrace thing is something we may do down the road.

      To clarify, the wall at the bottom is holding up about 3' of soil. The top of the wall goes back level about 3/4' and then the main part of the large hill rises up.

      I completely agree that the bigger the block, the more stable the wall, but I'm not really sure if the bigger block add a whole lot, as this wall isn't really holding up much. However, the neigbor disagrees...hence my posting in here. ;o)

      As for the curret footer...it's pretty rough (they just slapped mortar on as a leveler). I'll be putting down a bit of sand to level. I doubt the first row will 'hook' on it, but it is a few inched below the sidewalk, so the sidewalk actually provides a nice lip to hold the bottom course.

      1. FrankB89 | Aug 04, 2003 06:52am | #4

        I mentioned the heavier block simply because that's what I'm most familiar with. 

        However, weekend before last, I played "slave" for one of my excavators employees while he layed a wall for my Father-in-Law.  We were using the smaller block and it was pretty easy on the back and I have no doubt it will work fine. 

        The wall we did has some curves and varies in height from 6 courses (24") to 10 courses (40"), had one set of steps and where the higher courses terminated, was curled into the bank.

        It took a little over 4 hours for the two of us to lay nearly 700 blocks (six pallets), so I was a little surprised at how quickly it goes up.  The first course took the most time.

        One thing I did learn:  If your wall has much curvature, start your second and subsequent courses midwall and work both ways so you maintain staggering of the joints.

        Have fun....sounds like you pretty much know what you're doing! 

      2. darrel | Aug 04, 2003 07:26am | #6

        "The top of the wall goes back level about 3/4'"

        Ugh. Huge typo.

        Should read: ...goes back level about 3 or 4 feet.

        "Have fun....sounds like you pretty much know what you're doing!"

        Wow. I'll take that as a huge compliment coming from this crowd!

        (we'll see how it goes this week and then I'll let you know if the compliment was deserved ;o)

        "If so, you should not have a problem with the smaller blocks, unless the ground higher up is all sandy or silt and there is lot of water under causing it to want slide."

        The ground is pretty much solid clay with a foot or so of topsoil. There are days I really wish it was sandy, though. ;o)

        Not sure where my photo went...did an admin edit it? I've fixed it....

        View photo

        Edited 8/4/2003 9:13:24 AM ET by DARREL510

        1. xMikeSmith | Aug 05, 2003 12:06am | #7

          darrel..u didn't fix sh*t , your photo still doesn't post.. here.. look at this oneMike Smith   Rhode Island : Design / Build / Repair / Restore

          1. Planeman | Aug 05, 2003 12:16am | #8

            I think I was behind this guy at the airport metal detector!

            Experienced, but still dangerous!

          2. xMikeSmith | Aug 05, 2003 12:22am | #9

            dave.. it has never been determined if it IS a G-U-Y... at least not to my knowledgeMike Smith   Rhode Island : Design / Build / Repair / Restore

          3. calvin | Aug 05, 2003 12:27am | #10

            Well Mike, I've determined it's one damn goofy.Remodeling Contractor just outside the Glass City.

            Quittin' Time

          4. xMikeSmith | Aug 05, 2003 12:29am | #11

            c'mon , calvin... care to venture a guess on the gender...

            or is it just pat ?

            Mike Smith   Rhode Island : Design / Build / Repair / Restore

          5. calvin | Aug 05, 2003 01:36am | #12

            yup.  Pat.  He the no gender guy/gal from SNL?

            I was getting chills viewing that shot.  I saved it for the next heat wave.

            Remodeling Contractor just outside the Glass City.

            Quittin' Time

          6. Piffin | Aug 05, 2003 02:26am | #13

            I think it'll take a 12" block to fix that face!.

            Excellence is its own reward!

          7. Planeman | Aug 05, 2003 05:26pm | #18

            And to think I was upset because my son got a small tattoo on his arm. Some where, some parent must be so proud. I'm not judging, just observing and wondering.

            Experienced, but still dangerous!

          8. User avater
            IMERC | Aug 05, 2003 02:57am | #14

            That had to hurt falling in to a tackle box face first and all.

          9. darrel | Aug 05, 2003 04:57am | #15

            " 33497.8 in reply to 33497.7 darrel..u didn't fix sh*t "

            Nah...it's this crappy discussion board software. I had it fixed twice already and twice they've gone and changed it. 'doh.

            I'll post a photo of the new wall when we're done...

          10. Piffin | Aug 05, 2003 06:10am | #16

            Try posting it as an attachemnt the normal way instead of embedded.

            th esite is messing up tonight though. I keep geting service denied messages.

            Excellence is its own reward!

          11. BungalowJeff | Aug 05, 2003 08:03am | #17

            If the wall is 3 to 4 feet away from the slope and even less in height, then it is out of the influence area of the slope (slope failure plane). Take a poke through a basic foundations text, and this will be explained in simple diagrams in the retaining wall section....that's not a mistake, it's rustic

          12. hasbeen | Aug 05, 2003 10:04pm | #19

            Say, that's a new look for you, isn't it?   {G}Any jackass can kick down a barn, but it takes a carpenter to build one.

  2. FrankB89 | Aug 04, 2003 05:33am | #2

    My Excavation Sub does a lot of those block walls and prefers the larger 60# size.  As Frenchy points out, some attention to drainage will help. 

    The instructions that accompany the block spell out the necessity and methods of providing drainage.

    Another thing that is sometimes done that increases the wall's resistance to backfill pressures is to put a slight back slope to the bed for the first course of block so your wall will lean back a little more against the fill than if you layed it up on a level bed.

    The 4 foot height limitation is a built in safety factor for the manufacturer of the block.  Any retaining wall, at least in my area, that's over 4 feet tall requires engineering if it's to be inspected and to code.

    However, locally there are a couple of the walls that are over 12' tall and show no signs of movement after several coastal winters.

    Of course, you also have the alternative of doing a terraced wall which is attractive and is probably more effective in resisting the lateral forces.

    The systems most in play in my area (Southern Oregon Coast)  are the "Anchor Retaining Wall Systems" and their product line offers a pretty good choice of options and styles.

    I have a hunch French is referring to some other kind of block walls, 'cause these aren't easily kicked around.

    Oh, and if your concrete footing turns out to be ok, laying the block on it is fine, but you'll want to chip out any rough spots so the first course lays flush from block to block, and the back of the footing provides the edge for the lip of the block to grab, so the back line of the footing will determine how straight or wavy or curved your finished wall ends up!

     



    Edited 8/3/2003 10:38:11 PM ET by Notchman

  3. User avater
    BillHartmann | Aug 04, 2003 07:15am | #5

    Your picture did not post.

    I forgot, are you the one that posted the picture of with the wall bulging in the discussion about replacing a patio in the backyard and the problems with access?

    If so, you should not have a problem with the smaller blocks, unless the ground higher up is all sandy or silt and there is lot of water under causing it to want slide. If that was the case you would probably already see it.

    Between the setback of the blocks and following the instructions for drainage including gravel backfill and filter cloth and you should be fine.

  4. stonebm | Aug 07, 2003 07:43pm | #20

    I'd defer this to the engineers at the retaining wall manufacturer.  Although one poster mentioned that it would be OK based on the setback of the upper slope area and that the answer is apparent in any soils book, I'd issue a word of caution- slope stability analysis is never simple and the ramifications of a substandard design are obviously pretty severe.  I think most manufacturers have geotech engineers on staff and can analyze your exact situation.

    1. darrel | Aug 08, 2003 04:31am | #21

      Stone...in hindsight, you are probably correct.

      I did talk with more than one Retaining wall company, and none saw a problem with the smaller blocks, but, then again, they probably weren't engineers.

      Well, we have the whole wall out. We were doing find until we got near the stairs, where, as our neigbor informed us, the previous owners thought they'd repair the wall their themselves and fill all of the cores with solid concrete.

      Brilliant.

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