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Retaining wall: wood or blocks?

sideshowbob | Posted in Construction Techniques on August 3, 2004 07:09am

Howdy:

Hope someone can help as I’ve been researching with no definitive answers.

I need to build two retaining walls (26′ & 28′ long) to keep my hillside from sliding into the new fence by neighbor is going to put up (the old one has about 10″ of hill side pushing it over. I’m trying to determine if I should go with interlocking landscape blocks on a footing of gravel or 6 x 6 treated lumber.

I’ve got a good back, three years of framing experience, no budget to hire a pro, and I need to tame the hillside soon. Each wall will need to be about 3′ high, distance from one wall to the other is 3′ to 3.5′. In other words, I need to go from 6′ high to level ground across a distance of about 7′. The walls will have short sides to keep the soil from migrating out. I live in southern California, dry, sandy soil with lots of fist-sized rocks. A small garden will reside at the top close to the edge of the top wall, some moisture will always be present.

Specific questions:
(1) Do interlocking blocks give the same strength as 6 x 6 lumber, or are they about the same?
(2) Is regular pressure treated wood OK and how long will it hold?
(3) Guys at Home Depot did not know of any differences in types of treated wood available (if there are grades with more or less chemical in the wood). I have read that I should ask for a wood with a rating of .40 or better (in a HD book, ha). Is there another way to frame this question to the HD guys so they understand?
(4) If I use wood, I will use tie backs on the third and fifth rows up, spaced 4-7″ apart (counting from the bottom and estimating six rows high of 6X6). Does that sound right, overkill, not enough?
(5) If a wall built with 6X6 lumber needs tiebacks and landscape spikes, why does a same size wall made from unmortered interlocking landscape blocks not need similar reinforcement (none of the articles I’ve read recommend such for blocks)?

Any tips, info, or answers would be greatly appreciated. Thanks.

Bob

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Replies

  1. User avater
    BillHartmann | Aug 03, 2004 07:30am | #1

    First of all FORGET about LANDSCAPE TIMBERS. Those that are sold as landscpae timbers don't have a standard level of treatment.

    Teated lumber will have tags on each board that gives the level of treatment. Even the stuff at HD even if the employees don't know what it means.

    You need to go to a real lumber yard. Maybe even some one that serves ag customers.

    One concern I would have is that I understand the woods used on the west coast don't take treatment like the SYP used in the east and thus the center is not treated and any cuts are need painting with a protectant, but that is not as affect as factory treatment.

    My personal recommendation is to go with the modular blocks. I just did a 2ft high one that I did with small blocks.

    On the other side I have a 4ft high one made from railroad ties that is 25 years old and need replacement. Of course the ties where used when I started.

    I did have a piece of 2x8 CCA that I pulled out fo the ground after about 15 that while the surface was rough there was no sign of rot or termites.

    Most modular walls will go up to 4ft without any tie backs. They don't need them becaue of the banter The modular units have either a lip or a pin that causes each row to be set back (banter) from the one below so that they lean into the dirt. This is the way that loose stone walls have been build for years.

    Most of the modular wall systems are designed for a few companies and them produced under license by different manufactures. So the brands that are available with vary by area.

    In addition to the home horror stores look in the yellow pages for landscape suppliers, brick and concrete materials.

    Get some brandnames and look up the manufactures website. They usually have full set of details on how high they can go and what all details are needed to go the job correctly.

  2. Froed | Aug 03, 2004 07:38pm | #2

    I don't know anything about timber walls, but do have a bit of experience with modular block walls.  As Bill said, the blocks typically have a lip or some sort of registration feature.  This isn't really meant to be structural, but with a short wall it may not be an issue.  The keys to a modular wall, ironically enough, is not the blocks, it's what's behind them - drainage, reinforcement, and compaction.  Proper drainage is critical to this type of wall, since any water trapped behind the wall may freeze,  expand, and push the wall out.  Think frost heave.  Also, water may undermine the structural integrity of the soil.  Take clay.  When it gets saturated it loses virtually all it's strength, which could cause surface slippage and push the wall over.  So your drainage scheme is designed to take water away from the wall quickly, while keeping it away from the wall itself (so it can't push on it).

    Along with drainage is compaction.  The gravel behind the wall, and the fill behind that, must be compacted very well, otherwise you get settling, which will undermine the integrity of the wall, and you get more places for water to collect.  The compaction goes for the footing as well, which isn't really a footing, but a levelling pad.  This pad is where probably 3/4 of the work will be with a short wall like yours.

    From your description, it sounds as though you're planning a two-tier wall, with a slope above the upper wall.  From a structural perspective, this is not two independent walls, since the lower wall will see some or much of the load from the upper wall as well, depending on how tall they are, and how much horizontal space is between them.  It should be planned accordingly.  It may or may not be necessary in your case, but reinforcing grid (geotextile fabric) is what ties the wall into the hill.  This is a reinforced wall, as opposed to a gravity wall with no reinforcement.  It resists any pulling force on the wall by getting locked into the compacted fill, and serarates the fill into "lifts" for lack of a better term, so that instead of a six foot block of dirt, you have 2 three foot tall blocks, reducing potential for catastrophic failure.  As you can see, the fill behind the wall isn't just fill.  Oh yeah, the grid can be put in the wrong way, since it is directional.

    Here around Cincinnati, any wall over 2-1/2 feet (I think) is required to be designed by a PE, with checks for compaction and method during construction.  You may want to check code where you are.  Having said that, I hope I haven't scared you, since a block wall can be in the realm of diy, as long as you understand what's behind it (pun intended), sweat the details, and bear in mind what a wall failure can mean (an engineer looks cheap in comparison).

    I believe Fine Homebuilding had an article on block walls, and another on timber walls in the last few years.  Also, check out the following links for examples:

    http://www.pavestone.com/commercial/wall_anchor_diamond.html (this one has a good cross section drawing)

    http://www.anchorwall.com/ (one of several block makers)

    Good luck.  I did get a comment about timbers one time from a landscaper when I asked about wooden steps: "We don't do wood."

    1. Scooter1 | Aug 04, 2004 12:29am | #3

      I you are in California, pay no attention to anyone here unless they know Code at your City.

      In Malibu, by way of example, your 3 foot wall would have to have footings (3 foot down or if double wide only 1.5 feet) and would have to be solid CMU, grouted, and pinned with rebar.

      Call Building & Safety

      Regards,

      Boris

      "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

  3. User avater
    RichBeckman | Aug 04, 2004 05:31am | #4

    "Is there another way to frame this question to the HD guys so they understand?"

    ROFLMAO!!!!

    Thanks for the laugh!

    Rich Beckman

    Another day, another tool.

  4. User avater
    SamT | Aug 04, 2004 04:23pm | #5

    Here's some more links for you from My Favorites.

    NCMA

    In San Diego

    U of Minn, Step by Step, with lots of pictures.

    SamT

    Arguing with a Breaktimer is like mud-wrestling a pig -- Sooner or later you find out the pig loves it. Andy Engel

    1. DanH | Aug 04, 2004 05:52pm | #6

      Re wood vs block, wood is generally a lot cheaper and goes in quicker, but wood will last maybe 10 years in normal climates, and will start looking crappy after 5, while block will last 50 years if laid so it doesn't heave. If you're after cheap/quick then go with wood, but expect to have to redo it in the future.

      The biggest single mistake I see in amateur (and some pro) retaining walls is not having enough slant to the wall -- very often folks try to make them vertical or with just a little slant. On a short wall the slant is what keeps the wall from heaving outward. (On a taller wall you need deadmen or some other sort of anchoring system, but the slant is still required.)

      The second big mistake is lack of drainage. (You see this problem especially with brick or mortared block walls.) On a short wall probably all you need is gaps between the block, but for taller walls a drain tile system is probably going to be required. If wood is used, drainage is more important, because the wood tends to be tighter, with fewer "natural" gaps for drainage, and because you want to keep it as dry as possible.

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