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  1. #1
    Join Date
    Dec 2006
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    11

    Default Building a retaining wall with pressure treated lumber

    I'm planning on building a 20' to 23" retaining wall using green pressure treated wood. I've seen other designs but wondered if there were any concerns with my design. My plan is to put 4x4's into the ground about 18-24" deep on 4 foot centers. Then...using 3 rows of 2x8 (7.5 really) totalling 22.5" high.....but putting the 2x8's behind the 4x4's, thus exposing the 4x4's totally. To me this seems better for two reasons.....1) 4x4 is not exposed to the dirt, 2) 2x8's are supported by the 4x4's and not relying on any hardware to hoild them in place. Plus I think it would look better with the posts showing every 4 feet. Other plans I've seen seem to hid the posts. Thoughts? This is in SoCal where it never rains....and I plan on getting 10-20 years out of the wall.

  2. #2
    Join Date
    Jun 2004
    Location
    Houston & Washington Texas
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    12,104

    Default Re: Building a retaining wall with pressure treated lumber

    You are probably OK, some kind of drainage rock behind the wall with an exit for the water never hurts.

    My landscaper has done retaining walls up to 6' high using bags of redi-mix concrete. The kind you buy anywhere for $2.99. He pours a very nominal concrete beam, then stacks the bags where each row sits back toward the dirt that is being retained 1". Drives rebar down thru the bags every 3'-4' with drainage rock behind the wall. The paper eventually disintegrates, the concrete hardens. Makes for a very strong wall, cheaper than poured in place concrete. Surprisingly, we call these “bag walls”.

    The only issue is when we have to engineer it, seems it is difficult for engineers to calculate this kind of system. But I promise they work very, very well, and are cost effective.

  3. #3
    Join Date
    Jun 2004
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    Holly Springs, GA
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    3,287

    Default Re: Building a retaining wall with pressure treated lumber

    Hard to say without knowing what kind of loads the wall is holding back, what kind of soil you have, etc., but at first blush, I'd say you want the 4x4's to go much deeper than 18-24". In typical shoring designs using driven H-beams or sheeting, the members are usually driven to 2x the unsupported height, so in your case, you'd be looking to be around 4' deep.

    Also, I'm assuming you plan to pour concrete around the posts to fill in the holes you're digging? Backfilling with the native material will never compact enough to keep them from moving- as soon as you load that wall up, the posts will push forward.

  4. #4
    Join Date
    Jun 2004
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    Holly Springs, GA
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    3,287

    Default Re: Building a retaining wall with pressure treated lumber

    Quote Originally Posted by Allan Edwards View Post

    My landscaper has done retaining walls up to 6' high using bags of redi-mix concrete. The kind you buy anywhere for $2.99. He pours a very nominal concrete beam, then stacks the bags where each row sits back toward the dirt that is being retained 1". Drives rebar down thru the bags every 3'-4' with drainage rock behind the wall. The paper eventually disintegrates, the concrete hardens. Makes for a very strong wall, cheaper than poured in place concrete. Surprisingly, we call these “bag walls”.
    What do you face these walls with? I'm assuming a stack of concrete bags doesn't look attractive enough to be left exposed on the back yard of one of your mansions?

  5. #5
    Join Date
    Dec 2006
    Posts
    11

    Default Re: Building a retaining wall with pressure treated lumber

    Quote Originally Posted by Bob Kovacs View Post
    Hard to say without knowing what kind of loads the wall is holding back, what kind of soil you have, etc., but at first blush, I'd say you want the 4x4's to go much deeper than 18-24". In typical shoring designs using driven H-beams or sheeting, the members are usually driven to 2x the unsupported height, so in your case, you'd be looking to be around 4' deep.

    Also, I'm assuming you plan to pour concrete around the posts to fill in the holes you're digging? Backfilling with the native material will never compact enough to keep them from moving- as soon as you load that wall up, the posts will push forward.
    I this particular case, the hill is only 6ft high and 9 ft deep and about 35 deg.
    The soil is sandy and some small rock. That is one of the reasons I need to retain it. If you walk up it, it just gives away. You are right ...... what is the load? I think it's not bad. I have another area I'm working on and I;ve cut into the hillside 3 ft high. this area has been left unsupported for 3 years and had not deteriated one bit. So you could argue that there is no load to speak of. Loads would come from mega saturation from rainwater. ....and depending on the ht. of the hillside. A mudslide is a huge load. Being is SoCal, we do not get alot of rain, but we do have wet years and I have seen many mud slides. Getting back to the 6 ft hill, yes I am concreting in the posts.....and do not expect load problems, In fact ther alread is a retaing wall made of 4x6's horizontal on the other side of the fence (backyard) that I will be replacing, as they are pretty much shot. They are supported from the inside with galv fence posts with brackets.

  6. #6
    Join Date
    Mar 2007
    Location
    Northern Virginia
    Posts
    198

    Default Re: Building a retaining wall with pressure treated lumber

    I have built retaining walls out of 6x6's with a dead man every 8' on every other coarse of 6x6's it has worked well.
    Justin Thomas

  7. #7
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    Jun 2004
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    Holly Springs, GA
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    3,287

    Default Re: Building a retaining wall with pressure treated lumber

    Quote Originally Posted by pmaru77 View Post
    I this particular case, the hill is only 6ft high and 9 ft deep and about 35 deg.
    That's even worse than I expected, since it's not just a 2' high load you're supporting, but also the sloped load up to 6' behind the wall. While the 3' cut might not have moved yet, that doesn't mean it won't eventually- I think you're pushing your luck with the system you've proposed, but "I'm not an engineer, and I don't play one on TV"- so it's your call.

  8. #8
    Join Date
    Dec 2006
    Posts
    11

    Default Re: Building a retaining wall with pressure treated lumber

    As far as building codes or permits, one only need a permit for retaining walls above 3 ft tall. It's reall all about water saturation, and of course earthquakes. Seeing that if the dirt jumps a wall during an earthquake, I have much more to worry about :(. As far as water saturation, I see no problem. the land above this is concrete so there will not be any water soaking from up top. really, 2 ft is nuthin. many people use the concrete blocks from Home depot to go up 3 ft and even higher.....but I know those clowns have no permit for the above 3 ft walls.

  9. #9
    Join Date
    Jul 2004
    Location
    NOLA
    Posts
    4,087

    Default Re: Building a retaining wall with pressure treated lumber

    Humor me and fill in your profile agent 77.

    Wait a minute.
    This is a pro forum and I question if you meet the qualis agent 77.

    And I have some experience in Cali, you are running with scissors with hills, slopes and guessing.

    Get some help...Professional Help.
    Any you guys...stop it.
    Don't make me come back there.
    MBR

    Jeesh, you say there is no problem with the slope above because because it is concreted.
    Wonder what happens with the water that falls from the sky when it does.
    Think La Conchita
    Last edited by Bill Robinson; 07-24-2009 at 05:41 PM.

  10. #10
    Join Date
    Dec 2006
    Posts
    11

    Default Re: Building a retaining wall with pressure treated lumber

    What slope above? Flat concrete with drainage to road. Thanks for the help, as usual. I do have an ME degree but work on aircraft equiptment. I guess it's not good enuf.
    Do me a favor and delete my existence. I was looking for comments on the cosmetics but did not get any. I should learn better communication.
    Last edited by pmaru77; 07-27-2009 at 12:52 PM.

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