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forming foundation walls over bedrock

yup | Posted in Construction Techniques on April 7, 2009 05:39am

I’m in the process of forming up a small foundation and could use some advice. Approximately 30% of the foundation will run over undulating bedrock and won’t be formed on footings. I have gotten the go ahead to skip the footings on the sections of foundation running on bedrock and just build the walls right to bedrock, tying it together with rebar drilled into the rock.

If anyone has advice to offer up as to an efficient way to form these walls so they contour the irregular bedrock i’m all ears. I have heard good things about the foam forms for situations like this but they are unfortunately not an option in this case. The current plan is 5/8″ plywood with cone style snap ties, scribbed (for lack of a better word) to the bedrock.

Thanks.

DB

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Replies

  1. Piffin | Apr 07, 2009 06:52pm | #1

    I do a lot over uneven ledge and still step the footings. One main reason for using footings is to provide a level surface for setting up the wall forms. it is far easier and cheaper to scribe and whack at junk boards or to use soil at small gaps for a 6" to 12" footing form than to hack up wall forms. You also need a way to hold the wall forms DOWN so they don't float up over the pour. Doing that and keeping them straight is 300% easier when you have a footing.

    If your walls are all shorter than 4', this can be done, if you don't care much how they look, but you are really adding to the work and cost if you want to go with tall walls formed straight to the ledge.

     

     

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    1. yup | Apr 07, 2009 09:18pm | #3

      Thanks for the comments, i'm all for 300% easier. There is one section where i have already scribed the footings to contour over a less tall section of rock so i'll just keep that going over the more irregular sections as you suggest.How about a sharp projection of about 2ft in height that is too narrow to have a footing on top and too big to break out without wasting time? would i run the footing up as close as possible to either edge then scribe the plywood forming the wall to wrap from the footing on one side, up and over to the other footing? Sorry if my description is vague...DB

      1. Catskinner | Apr 07, 2009 09:42pm | #4

        The greatest consideration for a sharp projection into the cast wall is that you are including a stress riser in the wall. This is an opportunity for a crack to start that could be a problem.I'd give some thought to a little extra rebar over that projection.

        1. yup | Apr 09, 2009 05:32pm | #10

          thanks again, lots of helpful stuff being offerred. For the projection into the form, i'll go with a combo of taking some of the height off and adding additional rebar. I was planning on drilling rebar into the rock vertically to support the rebar running horizontally and will likely use the metal strap idea as well to keep the forms from lifting. While we're on the topic of pouring foundations, a portion of the wall tapers from 8" wide to 5 1/2" at the top where it meets the framing. I was planning on running a band of foam around the top then breaking it out when the forms are stripped - is that the best way? Forms won't be re-used and i won't be doing another foundation for quite a while so there may not be a need to build anything more permanent.I'm already picturing spots that can use the loving attention only a can of great stuff can provide...DB

          1. Catskinner | Apr 09, 2009 07:09pm | #11

            <<I was planning on drilling rebar into the rock vertically to support the rebar running horizontally and will likely use the metal strap idea as well to keep the forms from lifting.>>Good idea. It would also help to clean the rock as well as possible and either hit it with some bonding adhesive or a dash coat of Portland cement and Anti-Hydro right before you place the concrete.The more you can get the concrete and rock to act as a continuous component the better off you will be. I don't remember if your foundation is entirely on rock or if some is on rock and some is on dirt. If the latter, then it becomes critically important to think about what sort of vertical deflection might take place under load. If you get the sense that you could exceed about 1/600th of the run, say so and we'll see what we can do to help. There are a lot of really knowledgeable builders here.<<While we're on the topic of pouring foundations, a portion of the wall tapers from 8" wide to 5 1/2" at the top where it meets the framing.>>Taper over what distance?<<I'm already picturing spots that can use the loving attention only a can of great stuff can provide...>>Yep, Great Stuff is, well, great stuff. <G>

          2. yup | Apr 14, 2009 06:12pm | #16

            the foundation is not entirely on rock, i would say it's about 50/50 rock versus compacted fill. i've got the footings formed, including re-bar and have begun with the wall forms on top. i went with 2x10's for the walls of the footings which i scribbed to contour the rock. I am very happy i went with the suggestions here of going with footings instead of trying to scribe the plywood wall forms to fit the rock.i pressure washed the rock prior to forming and will give it another blast before pouring just to get out the debris from drilling/chipping to fine tune things and set the re-bar. thanks for the suggestion of the bonding agent, i will go with that.i'll clarify my taper question - all walls are 7 1/2" wide and in some areas they decrease in size to 5 1/2" to accomodate framing. it is only the top 4 inches of wall that needs to be narrower to account for the slab. thanks again.db

          3. Catskinner | Apr 14, 2009 07:31pm | #17

            You're welcome, glad to be of help.Because of the difference in bearing I'd give serious thought to controlling deflection. A few pieces of diagonal at the bottommost and topmost horizontals gets you a big return on the investment.Rather than taper the wall, could you step it? That's whole lot easier to do. Stop the wide forms at the step, install metal form clips, and form the narrower wall on top, pour continuous.Sounds like you've got this one roped.

          4. yup | Apr 15, 2009 12:10am | #18

            ideally by end of the weekend i'll have this one roped and ready to pour. becuase this place is a decent drive/ferry ride from where i live, it's weekends for now to get the foundation ready to build on then i'll move over for a few months to do the rest of the place.the way you describe the "taper" is more accurate, i should have said step. the top 4 inches of the wall step from 7 1/2" to 5 1/2". i have my top row of snap ties at 4" down from the top with the idea of laying a 4" tall by 2" wide piece of pink foam on it to form the step. i would then break away the foam and cut the ties with a grinder where they project from the wall so as not be be banging on the narrow portion of the wall to break the ties...db

          5. Catskinner | Apr 15, 2009 03:21am | #19

            Yep, you've got it.Give a little thought to those shear reinforcements, as the depth of the beam is where you save your money and gain your strength.Other than that, sounds like you've got it.

      2. Piffin | Apr 08, 2009 12:05am | #5

        Sorry, not picturing it. Sharp projections usually get to tangle with my jackhammer - You can rent a Brute for a day. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        1. brownbagg | Apr 08, 2009 01:12am | #6

          can of great stuff

        2. Piffin | Apr 08, 2009 01:34am | #7

          Here is a tip for some form locates on ledge. best when running the wall or footer perpendicular to the slope where both sides are aprox the same cut. Run a piece of metal banding strap or plumbers strap under the form on the ledge and bend up to each form 8-12" nail it to the form. Repeat every 12-24" Saves a lot of ties or trying to drill in the ledge or brace back as much. When the crete flows in, it pushes both forms in opposition to one another so things pretty much stay in place. I have the dowels int he ledge to tie internal rebar onto also.BB is right, the spray foam can be a quick way to seal gaps, as loing as you do it the night before, not as the crete is flowing 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

  2. ronbudgell | Apr 07, 2009 07:46pm | #2

    I agree with Piffin that a footing scribed to the rock will save you a lot of trouble. And you can form the footing with ICF.

    Lite Form ICF is among the best for this sort of application. It's good for just about nothing else, but is easy to scribe to a rough surface mainly because it does not have any incuded webs. You can strip the foam afterward, too.

    Lit Form is 8" planks of 2" extruded Dow polystyrene. You insert polyethylene ties every 8" into sawn slits. Ties of different lengths are available.

    Ron

  3. Marson | Apr 08, 2009 03:00am | #8

    I've used ICF's straight on ledge and wasn't really impressed. Had a lot of trouble keeping them straight and level--if you are going to scribe to sloped rock, you need to be spot on knowing where the forms will land on the rock--and that is no easy job. Plus, you cut the sh** out of the forms so that they take a lot of shoring.

    I agree with the others that you are better off with footings. I built a house on a really nasty site with lots of cliffs and jags and jogs. (our BI's require continuous footings with continuous rebar BTW) We held our form boards initially with rebar pegs drilled into the rock on either end of each board. Then we filled in below the form boards with crudely scribed plywood. Would have used cases of foam to fill all our gaps, but with as stiff a mud as could be pumped, we got through it with plenty of leaks but no blow outs.

    We went up from there with ICF's. Originally, we tried to keep the steps matching the height of the ICF's, but we tossed that idea shortly after fighting the site. It worked out fine, though we had to do a lot of cutting until we got one continuous course up.

  4. oldhouseguy | Apr 08, 2009 04:04am | #9

    Look at this product. It appears interesting. There is also a short Blurb in the May issue (Old Fashioned Paper on newstands now)   http://www.fab-form.com/products/fastfoot/fastfoot.html  

    As  I have a similiar potential problem;  at least a 140 YO home on a combo of pier /rubble foundation and in parts actual basement stuccoed applied as  parging adn or concrete similiar to teh surface of a gunnite /plaster pool I have been looking into options to avoid lost of excavation. We are looking to elevate an adjacent structure and tie it to main house, as 140 years ago they built on what they had , did not have a lot of power equipment, so we are looking at this as an alternative.

  5. dovetail97128 | Apr 09, 2009 07:44pm | #12

    I built one home IN the site of an old quarry pit. Solid bedrock.

    Used footings, stapled 12" wide strip of 6 mil plastic to the inside of the forms on each side and pulled it out to the outside of the form and shoveled dirt or weighted it down with sand to close the irregular gaps.
    Poured stiff mix, worked just fine.

    They can't get your Goat if you don't tell them where it is hidden.
  6. JoshRountree | Apr 09, 2009 11:23pm | #13

    I'm not usually much help when I respond, but if it were me, I might try making a HUGE contour gauge and make footings that way. Or, using something like this to make the footing with epoxied rebar tied into the footing.

    1. Piffin | Apr 12, 2009 11:43pm | #14

      That fast foot fabric method would not be good over ledge. You don't want the membrane separating crete from the rock.And installing it would require a LOT of drilling into the ledge - a lot more than needed with normal form methods.Most times there is little need to actually scribe fit a form to the rock either - set up and then mark the level on the form. No need to have the tops opf form leveled. where you have gaps at bottoms, you can fill them with scrap plywood tacked in or metal or the spray foam 

       

      Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      1. ronbudgell | Apr 13, 2009 02:44am | #15

        Piffin,

        Since it was mentioned above I have been trying (not full time or anything, my life isn't quite that lame) to imagine if Fast foot fabric form would work over rough bedrock.

        I think I agree with you that it would be a pain, but not because of drilling or separation between rock and concrete. I believe you could support the fabric form without any drilling in the rock, but making the fabric conform to the surface could be more trouble than it's worth, depending on how rough it really is. If you leave hollow spots, maybe the fabric will burst. Whether that would be a problem is another issue - one I wouldn't want to be experimenting with. 

        I don't think the separation is an issue at all. A footing will key perfectly well to the rock through a sheet of plastic.

        The easiest way I have ever found to do a job like this, and I've done a few of them,  is simply to scribe ICF blocks to the rock and form them to the required height. Forget the footing. The OP rejected ICF right at the start, so I'd have to say the next best way is an ICF footing form with foam that can be stripped and re-used elsewhere. I think only Lite Form is easy to strip off a pour. It's the only one I know of anyway.

         

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