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Dry Pack in underpinning

bluegoat | Posted in Construction Techniques on August 21, 2009 01:31am

Hi,

I am underpinning a foundation and my engineer has spec’d a way I haven’t done it before.

4′ sections leaving a 3″ gap between the old and the new and using “Dry Pack” after the new section has cured.

His drypack is speced as “mixture of 1 part sand to 1 part cement with just enough water to make it hold it’s shape. Throughly tempered in place with a short length of 2×4 and pounded with an 8 pound hammer, resulting in complete filling of irregularites to assure full bearing.”

I gave this method a brief trial and it seemed quite hard to temper the drypack in place as it kept flaking away when pounded. If the planes of the old foundation was perfectly level with the new one and just enough to put a 2×4 horizontally into the 3″ gap and pound then I could see the method working quite well but the irregularities I had made this difficult. Perhaps I am making my dry pack too dry? I made it so that it will just form a ball but if touched will crumble in that ball shape.

Any idea if “sterilized play sand” is adequate for this? That’s the only grade my lumbar yard seems to stock. Also should I figure 7 days for 90% strength before I excavate the surrounding 4′ sections of foundation?

I’ve currently got a 6×6 column supporting a 6×6 beam and I am thinking of replacing it with steel as I am dropping the slab. Any idea what the equivalent screw jack would be? It seems odd that the steel tables list HSS of 3″x3″ at 66kips but the screws jacks are rated at 6.6kips. I suppose this is due to the load concentration in the screw section of the jack. Are screw jacks ever left in as permanent columns given the 1/10th load rating?

Thanks for reading!

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Replies

  1. WayneL5 | Aug 21, 2009 02:24am | #1

    Usually play sand is not good for concrete.  The reason is that proper play sand is beach sand, with the grains worn rounded, so it is comfortable for play.  Concrete sand has to be sharp so it locks.

    1. bluegoat | Aug 21, 2009 05:43pm | #10

      Good point, I'll have to call around for better sand.

  2. User avater
    SamT | Aug 21, 2009 02:28am | #2

    Add just a tad more water.

    Make your golf ball and drop it 1"  to 2" onto a table, board, sidewalk, etc. If it breaks into a zillion pieces, add a drop more water. If it breaks in two or three pieces, you're ok.

    If it squashes down to 3/4 original or smaller size, too much water.

    You're filling a 3" by 4' by X' gap, right? If so add some fine broken gravel, 1/4" to 3/8".

    Is the 4' dimension horizontal?

    SamT
    A Pragmatic Classical Liberal, aka Libertarian.

    I'm always right!
    Except when I'm not.

    1. bluegoat | Aug 21, 2009 05:30pm | #7

      Yeah 4' horizontal, 3" tall, 8" deep to match existing foundation wall thickness. Thanks for the water instructions I'll give those a try.

      1. User avater
        SamT | Aug 21, 2009 08:15pm | #13

        Is the gap open on the back side, or is it closed so you have something to pack against?

        If it's closed off, I would mix some 0" slump mud with 1/2 aggregate, trowel it in to make a ramp about  2" thick at the open side to as near the old foundation as possible at the back, come back tomorrow and DP the gap left.

        A lot less manhours and much easier, since DP is really not the best for filling a 3"x8"x4' space. Actually, it's about the worst.

        DP is usually used to fill small spaces like beeholes in pours and under thread mounted flanges, (0" to 1",) and maybe some large cracks.SamTA Pragmatic Classical Liberal, aka Libertarian.

        I'm always right! Except when I'm not.

        1. bluegoat | Aug 22, 2009 07:58am | #16

          The backside is closed with dirt. I am essentially underpinning the footing of the existing wall. The footing just happens to be the same thickness as the rest of the wall or you could consider this no footing. The house is from about 1925 so seems like it has held up pretty well so the bearing capacity of the soil must be pretty good. I'll run your suggestion by the engineer it did seem like an extra ordinary amount of work on my test section.Any idea on wait time between the dry pack and excavation around the surrounding sections.

  3. clinkard | Aug 21, 2009 05:58am | #3

    What about non shrink grout? I have uncovered walls discovering temp screw-jacks, (the 6'-8' kind) wondering why on earth would you leave them in. I have also broken a few due to the aluminum collars shearing.

    1. User avater
      Jeff_Clarke | Aug 21, 2009 03:05pm | #4

      Nice idea (we've used non-shrink grout in bluestone joints because it doesn't shrink/crack) but I'd say too expensive for the much-larger extent of this application.

      Jeff

      1. wane | Aug 21, 2009 03:16pm | #5

        why leave the gap in the first place?, why not build your form couple inches above and out from the footing, then pour and vibrate?

        1. User avater
          SamT | Aug 21, 2009 04:14pm | #6

          why leave the gap in the first place?, why not build your form couple inches above and out from the footing, then pour and vibrate?

          Shrinkage. As each 4' section shrinks at different times, it makes it impossible for the new to properly support the old.SamTA Pragmatic Classical Liberal, aka Libertarian.

          I'm always right! Except when I'm not.

          1. bluegoat | Aug 21, 2009 05:41pm | #9

            The previous engineered underpinning I did was done without leaving a gap. It overlapped the original foundation on the inside of the house. I am not sure why the shrinkage doesn't matter in that case. The reason why I am using the different method this time is that the house is limited to 34' width and if I use the overlap method then I loose 3" on each side or at least 6" from the 34' and the additional width of any covering materials.The overlap in my previous job was also required for lateral soil loading whereas in this case the foundation is really just in a bearing capacity with most of the basement above ground.

      2. bluegoat | Aug 21, 2009 05:32pm | #8

        I asked the engineer about non-shrink grout as well and he gave me the same answer. I could do it, it would just be quite a bit more expensive material wise.

  4. YesMaam27577 | Aug 21, 2009 06:20pm | #11

    Sound to me like the mix he is specifying, is somewhat similar to deck mud that tilers use in shower decks. Although his blend is different, the concept is the same.

    And the advice that SamT gave you about how to tell when it's too dry/too wet is good.

    Politics is the antithesis of problem solving.
    1. chairmon | Aug 24, 2009 06:46pm | #24

      No we deck mud at 4:1 or there aboutCraig

  5. mike_maines | Aug 21, 2009 07:24pm | #12

    Dry packed mortar is more common in commercial work than in residential, from what I understand.  It's easier to place than wet mortar, has sufficient bearing strength, and doesn't cost much. 

    You should be able to get mason's sand wherever you get your portland cement.  Or just get a mortar mix (after checking with your engineer of course).

    Screw jacks should not be used for permanent support.  Order round steel tube, or use a concrete-filled lally column.  No way to tell what size you'd need, but a 3 1/2" lally will support a lot of weight.  Your engineer will be able to help you with sizing. 

  6. john_carroll | Aug 21, 2009 09:09pm | #14

    Bluegoat,

    I've done the same basic thing on many occasions. I posted some photos of my own crawlspace to basement job in this discussion:   http://forums.taunton.com/tp-breaktime/messages?msg=73774.8. Note the joint at the top of the wall, too.

    I'm not sure why your engineer would want such a wide mud-pack joint. My guess is that he thinks it would be easier. I made mine a minimum of 3/4-in.--just enough space to get a tuck pointer in. The bottom of the footing was very uneven and the joint got fairly thick--up to 2-in.--in places. I wouldn't make it 3-in. because that makes it harder to keep the mud from slumping.

    Filling the joint is strenuous and tedious. Put the mud in in layers. Keep it dry and press it in place in order to fill every nook and cranny under the footing.  



    Edited 8/21/2009 2:18 pm ET by Mudslinger

    1. Henley | Aug 21, 2009 11:36pm | #15

      3" does seem like a lot of packing. Perhaps a bit more then 3/4 so that you could tamp it with a broom
      handle would be less work then a tuckpointer tho.

    2. bluegoat | Aug 22, 2009 08:15am | #17

      I saw your discussion quite interesting.I am going to run keeping the joint to a minimum like you recommend by the engineer.When you say putting the mud in layers is there no concern with cold joints when doing layers of mud?Thank you for the response.

      1. User avater
        SamT | Aug 22, 2009 02:28pm | #18

        The reason why I am using the different method this time is that the house is limited to 34' width and if I use the overlap method then I loose 3" on each side or at least 6" from the 34' and the additional width of any covering materials.

        You're doing this from inside the basement?

        Any idea on wait time between the dry pack and excavation around the surrounding sections.

        Next morning.SamTA Pragmatic Classical Liberal, aka Libertarian.

        I'm always right! Except when I'm not.

        1. User avater
          SamT | Aug 22, 2009 03:11pm | #19

          bg,

          Youknow, when I said shrinkage was the reason for drypaking, it's 'cuz that's the only reason I could come up with.

          Now, I think it may be because the work is being done from the inside and will be finished.

          Make a form like in the figure, using screws to attach the cleat (behind the ripped 4x4) to the form. Pour and vibrate throught the opening made by the ripped 4x4.

          As soon as the mud has firmed up, maybe 10 minutes, remove the cleat and 4x4, and cut the concrete wedge off flush. Use a coarse, 12" Sawzall blade like a pull saw, or even a trowel. Lightly mist where you cut it right away and 10-20 mins later. If you're still around, repeat after an hour or more.

          Be sure to create a void at least 1/2" thick by a couple of inches tall behind the existing wall to allow air to escape the incoming 'crete. 

          View ImageSamTA Pragmatic Classical Liberal, aka Libertarian.

          I'm always right! Except when I'm not.

          1. bluegoat | Aug 24, 2009 06:56am | #21

            That would be the ideal option. Close to zero extra labor vs the labor intensive dry pack. I'll see what the engineer has to say about it as it would save me a lot of time and money. Thanks for the image !!

          2. bluegoat | Aug 24, 2009 06:17pm | #22

            Good news. The engineer is ok with your method. Thank you for the suggestion!

          3. User avater
            SamT | Aug 24, 2009 06:36pm | #23

             SamTA Pragmatic Classical Liberal, aka Libertarian.

            I'm always right! Except when I'm not.

          4. bluegoat | Oct 28, 2009 03:40am | #25

            Attached are some progress pictures. First with the form work and the 2nd stripped where I used a hammer to break off the protrusion.Thanks again for the diagram and suggestion.Hardest part was drilling a hole into the hardpan dirt 15" horizontally to dowel tie the pieces of underpinning together. I just used my rotary hammer and a carbide bit on hammer drill mode... not sure what might have been a less painful method.

      2. john_carroll | Aug 22, 2009 05:22pm | #20

        When you say putting the mud in layers is there no concern with cold joints when doing layers of mud?

        No. The layers are from back to front. I don't let it dry; I let it stiffen up slightly and keep adding to it until I work it out to the front surface.

        Just have at it. You'll get the feel for the consistency of the mud and develop methods for applying it as you work. One technique that I use to get the mud in the groove is to make standard mud, spread it on a piece of plywood and let it set up slightly. Then I cut off pieces (about 4-in. by 4-in.) and slide them in the slot. I pack the mud with either a 1 x 4 or a tuck pointer.

        I also put on a rubber glove and make a mortar ball in my left hand and cut off and push mud in with a tuck pointer held in my right hand.

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