Author Topic: Fabric Formed Foundation for Building On Boulders  (Read 11321 times)

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Offline flyingvan

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Fabric Formed Foundation for Building On Boulders
« on: January 30, 2012, 06:01:09 PM »
For both my owner/builder projects there was an issue with massive boulders right where the cabins were to go.  This was my solution---

First, dig your trenches/ posts where you can.  When you come to a boulder, drill into it and epoxy rebar in, sticking up where your stemwall will be.

Next, build these forms OVER the trenches, supported by yolks that hold them over the trench.  Each yolk will have a hole drilled in it holding up an anchor bolt.

Next, the fabric--I've used both geotextile fabric and plain old visqueen.  Both work.   You have to do some un-doing of the yolk screws to pul the fabric through, then roll it up in a 1x2 and screw it to the outside of the form.  The fabric goes over the form, lines the trench, then back out the other side.  Too loose is OK but too tight and the concrete will 'hammock' and collapse your forms.

The rebar all hangs from the yolks.  Tie it in with whatever you epoxied in to the stones.  I'm big on doing everything myself but this is the one day I needed help--- a few months to prep and about an hour to pour.  The concrete inflates the fabric and the fabric brings it to the top.

Here it is all poured.  I gave it a few days to cure so I could really wrench on the forms and not hurt the concrete

Here it is all finished.  I like how the wrinkles add character to the concrete.  I threw some color in, too.  The concrete never actually touches the lumber in the forms so you can use it in your framing without worrying the lime weakened it at all.

  Here's a picture from the first cabin, same sort of foundation, bigger boulders

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Offline MountainDon

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #1 on: January 30, 2012, 06:32:07 PM »
Clever. One thing I'd like to caution about is aimed at viewers who may see the boulder solution and decide to copy it. It won't work as well in ND as it might in San Diego. Frost is the difference.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline flyingvan

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #2 on: January 30, 2012, 06:59:06 PM »
Actually, these cabins are built at a mile high elevation and we get some pretty good snowfalls (not by ND standards, I bet) Our frost season is 7 months out of the year.  I DID have to hire a geologist to write a letter to the county saying the boulders were suitable to support a house (platonic granitic schist) but I can't imagine why frost would matter in a monolithic pour with no cold joints.  The trenches go down a minimum of 36" except where there's stone....I used the 7' to daylight rule
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Offline MountainDon

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #3 on: January 30, 2012, 07:25:16 PM »
IF the frost penetrated to a depth greater than the depth of the boulder/rock/stone/obstruction, would there not be a danger of frost lift potentially causing trouble? I'm just playing the devils advocate as many diy builders do not have the sense to do sufficient preparation with as much research or detail as it seems you have.

You at least have footings far deeper than your likely frost depth, and the boulders go deeper than that. I have seen diy builders stop at the first good sized rock and say good enough. That's not good enough if the rock only goes to 24 inches or is fractured at 24 inches when the typical frost depth may be 48 inches. Like I said initially, I've seen copycats copy something unsuited for their location. You have done your homework; it doesn't always transition well to other locations. I like to see folks show off their work. I also like to teach and prevent future issues if at all possible. No doubts meant to be cast on your project.

I have to plead ignorance of the meaning of the 7 foot to daylight rule.  ???  Might be just me.  ;D
« Last Edit: January 30, 2012, 07:53:24 PM by MountainDon »
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Offline fishing_guy

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #4 on: January 30, 2012, 08:55:48 PM »
MD stumped? ???

Not that much of an issue here in relatively flat Minnesota, but a great explanation and illustration is found here:
http://www.nachi.org/deck-inspections.htm
third picture down from the top.

Google is a great thing ;D
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Offline MountainDon

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #5 on: January 30, 2012, 09:01:53 PM »
Thanks fishing_guy.  Google IS a great thing   :)                  d*

"The 7-Foot Rule states that there should be a least 7 feet between the bottom of a footing and daylight"

Good rule. That's one of the reason we graded the slope to near flat...  :D

I guess that should specify horizontal distance as vertically it makes no sense...  ::)
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Offline flyingvan

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #6 on: January 30, 2012, 11:49:41 PM »
 According to the geologic report (it was only $175) the boulders go down at least 12'.  There is an issue with expansive soils here---so the problem isn't so much the rock moving, but the soil.  That's why I went BIG.  Concrete's relatively cheap anyway.  I'm going to tack on to MountainDon's warning---you have to build these forms really strong, pin the yokes (not yolks as I previously wrote.  Eggs make poor building materials) into the ground with rebar so they don't move, too.
  Grading flat wasn't an option.  These lots were purchased very cheap because they were 'unbuildable'---too small to have a septic field with the required 100% reserve.  Once the county was satisfied, the foot print was as big as it could be---any grading would have changed the setbacks for the leach fields.
   Here's the description of 'Seven Feet To Daylight', Don----if you're building on a slope, you dig down so a horizontal line facing the downhill from any point in the footing would just reach the natural surface.   The steeper the slope, the deeper the trench.  I kept a 7' board and level handy.  I'd rest one end on the lip where I was digging, hold the far end of the board off the ground on the downhill side, make sure it was level, and measure the distance from the high end to the ground.  So, no, you don't have to go 7' deep---unless it's a 1:1 slope (100% grade, 45 degree angle, whatever) Still confused?  Scribble a slope.  Now start with your pencil at some point on your slope and draw a straight, level line 7' into the hillside.  Now measure straight up until you reach daylight.


  Here's a picture that gives an idea of the slope---it's 4:1 overall, but very variable.  In addition to frost and slope, we sit on the second largest fault line in California--the Elsinore Fault.  This cottage is taller than it is wide and we get heavy winds (70 knots isn't unusual) and have to plan for a 4' snow load.
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Offline Squirl

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #7 on: January 31, 2012, 05:08:04 AM »
I like the fabric idea to protect the lumber for reuse.  Thank you, I will try it sometime if I can. 

If the building is heated in the winter you could use the FPSF charts to combine the methods if you couldn't test the boulder depth. Fabric is cheaper.

How did you overcome the septic issue?

Offline flyingvan

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #8 on: January 31, 2012, 07:58:00 AM »




     Someone had an easement for a driveway then didn't build in the easement.  That cut into potential leach field, so I moved the driveway.  I dug all the perk test holes myself in spots I knew were sandy, so the perk rate would be good.  I moved the required 2 9x19 parking spaces to the extreme rear.  I worked with the water department and moved a 4" water main that ran diaganolly across the lot.  Also went with infiltrators, although the county of san diego doesn't decrease the required linear feet vs. clay pipe.  It still has lots more absorption, and you don't need gravel.
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Offline mlnk

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #9 on: January 26, 2015, 08:49:15 AM »
A concrete footing is supposed to grab on to the soil. This creates strength especially for uplift. It also creates a good UFER ground. Fabric or plastic is a good idea but it should drape only to the bottom of the forms.  This also eliminates the "hammock" problem.

Offline flyingvan

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #10 on: January 26, 2015, 07:48:40 PM »
When the fabric IS the form, it has to go from one side or the other or it will float and blow out.  I tried it.  The woven geotextile fabric I use is a loose weave and not watertight, so I imagine no resistance is created between the concrete and the ground it's sitting on, but I haven't tested it.
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Offline Txvineman

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #11 on: February 20, 2015, 02:08:01 PM »
hey Flyingvan, thanks for this post.  timely for me.  Have you come across any problems since this foundation was completed?  Anything you would change?  I'm on solid limestone, and keep tossing a coin on solid pour vs block wall over footer.  i toss the coin about ten times a day and it lands on heads 5 times and tails the other 5... 

Thanks!
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Offline flyingvan

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Re: Fabric Formed Foundation for Building On Boulders
« Reply #12 on: February 21, 2015, 05:16:51 PM »
    Not a thing.  The first project has been completed 12 years now and survived the Cedar Fire

and two flash floods with no problems whatsoever.  I think it's the best solution for building on a stone/soil mix.  Just be sure the stone is suitable to support a house and follow the 7' to daylight rule for soil. 
   
    Here's a few pointers, though---try to get all the folds out of the fabric while pouring.  Fabric embedded in the concrete might weaken things and looks crappy.  Also it's probably a good idea to run a length of pipe through the lowest point (look in the background at the ABS pipe) just in case some pipe breaks or something, so you don't form a swimming pool under the house.   I played with color too.  Fail.  Just painted it instead.
     For fabric I used Mirafi geotextile fabric for all of the first build and most of the second http://www.tencate.com/amer/Images/PDS_Xseries0113_tcm29-21722.pdf  It was strong, easy to cut, and porous so excess moisture could squeeze out and pre-pour rain wouldn't ruin everything.  Also as someone else mentioned it probably allows for good earth contact for grounding.   All that said, I experimented with visqueen for a section. There are some advantages---you can see any voids and just sort of punch it to get the concrete to settle in, and you get an awesome smooth finish.  I liked the finish so much i used visqueen to form the hearth.
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