Replacing Retaining Wall - Learned a new trick

HeatsTwice Posted By HeatsTwice, Jul 4, 2011 at 6:55 PM

  1. HeatsTwice

    HeatsTwice
    Minister of Fire 2.
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    Jan 7, 2008
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    Loc:
    Santa Rosa, California
    I've got a 100' retaining wall made up of 2"x12"x12' horizontal members and 6"x6" vertical. The vertical members of a 12' section rotted away and failed this winter under all of the rains.

    Digging new cement holes for off set vertical members would have been a real pain so I decided to use the old cement cores and somehow take out the old rotted wood plug in the hole of each.

    First I started with a 1" auger drill bit. Too small. I couldn't chip away any of the wood around the hole with my breaker bar since it was wet and spongy and the hole too small. Spent 3 hours trying.

    Then I decided to try to burn it out with harbor freight propane tourch wand. 4 hours of burning took me down 3 inches. Still no luck.

    Then I tried to sink an 12"x3/4" lag screw into the wood and pull it out with a 8 ton hydraulic jack pushing up on 4 feet of a 6"x6" chained to the end of the screw embedded in the wood plug. It stripped the screw right out.

    By this time, I was already out $80 buying tools which didn't do the job.

    After some research I bought a 2 9/16" wood auger and put it on an 18" chuck extension (seen below). 10 minutes later I had cored the wood plug with a hole big enough for me to break out even the wet wood with my bar. Job done in 30 minutes.

    I post this solution only because I spent so much time figuring it out that others may want to know about it. I feel like a dunce for not knowing about this solution in the first place.

    Once the holes where clear of wood, new posts just dropped in. The rest of the retaining wall was up on 30 minutes and back fill done in 20.

    If I had to dig new holes for new posts it would have taken many hours and I would have ended up with a lot of extra dirt to dispose of. Now that I have the technology to reuse the old cement shafts, I save time (I can drill out and remove the old wood in about 30 minutes) and money (not having to buy 200 lb of post mix for every new hole) and I don't have to dispose of many yards of dirt/clay.

    BTW, after drilling the hole in the wooden plug, I use a shop vac to suck out the wood dust. It also does a good job of getting out any rocks which may have fallen into the hole. If you hit a rock with that bit - game over and its time to swap in a new blade. That was an important feature of that bit - the fact that new blades and center tap screws can be added if they wear out.
     

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  2. semipro

    semipro
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    Thanks for posting. Good info. Sounds like something I would do.
     
  3. ROBERT F

    ROBERT F
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    Sep 2, 2009
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    Only issue might be from cooking the post hole mix, and weakining it with heat. once concrete gets heated past a piont it tends to get very brittle/weak. probably just fine though. not a strucual wall or anything!!
     
  4. HeatsTwice

    HeatsTwice
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    Jan 7, 2008
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    You are correct. In fact I "tried" burning it out but it didn't work. From now on I will be boreing the wood out with the auger.
     
  5. heat seeker

    heat seeker
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    Good thread! I will file this away for future reference. This idea has other applications, too.
     
  6. HeatsTwice

    HeatsTwice
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    Jan 7, 2008
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    Thanks. I should mention that a "proper" retaining wall has 2/3 under ground and 1/3 above ground. This one had 1/2 and 1/2.

    The only reason I mention this is that if you drill too far, you'll hit dirt or the bottom of the cement cup. Very bad for the bit. So beware.

    I don't know the best way to keep from bottoming out the bit on an unknown wall, other than to drill 12 inches first, then 8 inches and with the point of the breaker bar, slam the bottom of the wood core in the center and listen to the bar "ring" as it would if it hits cement.

    If you don't hear a ring with the bar and its dirt down there instead of a cement cup, you will only know it when the drill jerks suddenly (not a smooth pass throught the wood anymore). This is the only clue you will have to imediatly stop drilling for the sake of the bit.
     
  7. HeatsTwice

    HeatsTwice
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    Hi, I forgot to ask what other applications you had in mind? Very interested.
     
  8. heat seeker

    heat seeker
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    Feb 25, 2011
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    Well, nothing specific at this time, but the method is generally useful, IMO. The general principle of boring out the center to weaken and provide room to work with is an excellent one. I have an older pole barn that has some rotted rafters and poles, so I will be thinking about how to replace them. It was obviously not built to any real code, so some of the posts sit on the concrete pillars, and some are set in the pillars. It appears to have been built in three stages, each one different from the other. I'm debating on letting it just fall apart and build a new one, or repair this one.

    What I was really getting at is that I'll file this in my brainbox for future use - if I can remember it by then…!

    (Nope, I don't burn pine….)
     
  9. HeatsTwice

    HeatsTwice
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    I know this thread is old. But now I burn out the wood by dropping lit chunks of charcoal in the hole and just letting it smolder all night. I also drop in a 5 foot section of electrical EMT conduit (metal tube) so the bottom of the whole has air sucked to it through it. If the coals go out or get cold, I drop in a lit one and, with a shop vac in reverse blow air in the conduit. It turns the charcoal at the bottom of the hole into a blast furnace. This very quickly burns out even the wettest wood at the bottom of each cement plug.

    This solution is by far the most cost and labor effective method of removing the remaining wood from the inside the cement footing of a retaining wall. The whole processes of burning out the plug takes about an hour and does not require drill/drill bits.

    I do not worry about weakening the cement since the only purpose it serves is as a sabot between the wood and dirt.
     
  10. Dobish

    Dobish
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    Oct 26, 2015
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    When I have been putting my posts in, I have been using a bag around the post, filled with around 1/2" of sand, then Seal up the top of the bag, and bury it. This way, if you need to remove them, you can without having to deal with all of the pain in the ass.



    it works pretty well!
     
  11. fbelec

    fbelec
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    thanks dobish. i now have to adapt this fence post thing to a lamp post with wire and not wet the sand in the bag. i don't use cement now. cement rots the metal post.
     
  12. Dobish

    Dobish
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    Oct 26, 2015
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    I don't use wet sand, i do it dry and compact it. Metal fence post should be easy, just bury some cable, drill a hole in the pipe, run the wire up to the top where you can connect it and use a small junction box...
     

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