Author Topic: Foundation for Surface Bedrock  (Read 13317 times)

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rock

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Foundation for Surface Bedrock
« on: February 09, 2007, 02:05:14 PM »
I am seeking advice on a foundation. Our land is at about 9000' in Colorado.  We are planning to build this summer and want to get going on the foundation.  Problem: Were we want to locate the cabin is all surface bedrock (smooth and mostly level).  I'd like to use a pier and post foundation if possible.  Any advic on how to do this?  Do I need to drill into the bedrock?  If so, with what . . . . ?  Any advise is welcome.  I would assume a concrete pad is also a good option, but I'd like the cabin elevated 2-3 feet above the ground.
Thanks in Advance-

jraabe

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Re: Foundation for Surface Bedrock
« Reply #1 on: February 09, 2007, 03:02:03 PM »
If you have a massive rock ledge that is unlikely to move due to wimpy things like frost action, then you already have a nearly endless footer. All you need to do is pin the concrete pier to that footing.

I would suggest a drilled pin or large diameter rebar set with epoxy into the rock and then tied into a rebar running down the center of the tube footing you will pour on top of it. Cut the cardboard tube footings to the same height, put in your brackets and you should be done. An 8" tube should be fine for the heights you are talking.

glenn-k

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Re: Foundation for Surface Bedrock
« Reply #2 on: February 09, 2007, 07:18:58 PM »
A decent sized Bosch Hammer drill will drill a 3/4 hole in rock 5 inches deep in about 5 minutes.  There are many models.  There are many brands.  I like the Bosch because it is very fast for the money.  Having rock you may want to buy one but you can also rent one.  Expect to pay around $500 or so for one of the larger ones to do a decent job in rock.  Check the ratings in their literature or online for the proper one to suit your needs.  It is also very handy to have one that will chisel - rotary drilling action must stop for this .  Note that all models are not equal.  Some do not stop to chisel.  You can even chisel wood with it.  

We also use it to drive wedge anchors such as Red Heads or Hilti.  Just stop rotation - put on a chisel point or use the drill at the risk of breaking it- hit the trigger and in goes the anchor.  Note that hard driving can bang up the end of the bolt but usually the nut and washer are on already anyway.

« Last Edit: February 16, 2007, 06:39:52 PM by glenn-k »

MountainDon

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Re: Foundation for Surface Bedrock
« Reply #3 on: February 09, 2007, 10:06:08 PM »
Hi! Welcome! I'm just curious about where in CO your cabin will be. I'm in northern NM at about 8700 feet or so.   :)  

rock

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Re: Foundation for Surface Bedrock
« Reply #4 on: February 16, 2007, 11:38:49 AM »
The Cabin will be East of Pikes Peak (Colorado Springs Area) 7 miles northeast of the town of Guffey.  It sits in a bit of a strange weather pattern and has never seen more than about 6" of snow.  

rock

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Re: Foundation for Surface Bedrock
« Reply #5 on: February 16, 2007, 11:52:12 AM »
Thanks for the advice John and Glen.  You seem to have confirmed my hopes!  Sonotubes it is.  Could I drill into the bedrock, epoxy some rebar that runs from the drilled hole to the middle rise of the concrete tube and crook it, or do I need to secure it better in the concrete?  I will look into the hammer drill to cut the whole.  Thanks again,  I very much appreciate your help.  I am afraid you all may be hearing from me more often as I begin this process as a novice.  The Cabin will be small 16x20 or 16x24 and done as a "storage building" rather than a home.
Thanks.

glenn-k

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Re: Foundation for Surface Bedrock
« Reply #6 on: February 16, 2007, 07:02:03 PM »
I think it will hold fine if it is long enough.  I would make it go near the top of the sonotube.  It is quite common for engineers to design a cage of rebar to set inside the sonotube to hold the concrete column together.  Maybe about 3 - 5 to 6"dia.  rings of 3/8 rebar (#3) tied around 4 vertical #4's that run from the rock to near the top of the sonotube around the anchor bar.  Use regular tie wire to tie the rings around the verticals then drop it in the sonotube.  Put your anchor in the top and fill er up with concrete.  Tap sides to make sure it all gets full.  This may be overkill but should keep things together.  Common rebar laps are 20 bar diameters. (10 inches for 1/2").  I usually go more.

Offline MikeOnBike

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Re: Foundation for Surface Bedrock
« Reply #7 on: December 19, 2009, 11:54:55 PM »
Common rebar laps are 20 bar diameters. (10 inches for 1/2").  I usually go more.

We have a similar situation.  The cabin will sit on a large piece of bedrock.  The piers will range from 6" to ~3ft high.  How high should the pin/rebar stick up out of the rock?  Seems it might as well stick out most of the height of the pier.

I have helped with a lot of foundation pours but have never designed the rebar for a pier.  Can someone point me to a resource on the rebar setup for a pinned pier?

Thanks, Mike in Idaho...

Offline glenn kangiser

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Re: Foundation for Surface Bedrock
« Reply #8 on: December 21, 2009, 06:07:49 AM »
For the most holding power, I would drill into the rock and epoxy the rebar in.  Again consider that for the most holding power deeper is better so 1/2 inch about ten inches.


As you suggested, full length would be best  on the rebar - I am assuming you are pouring the piers.  Consider adding diagonal bracing to the longer ones at least - possibly add a J bolt through the side of the form for future attachment.  I would also suggest rebar cages and stirrups every foot with a couple near the top about an inch apart.  Some will say that is overkill.  The stirrups can be just 3/8 rebar bent in a circle with a good lap.  It keeps everything together should a crack develop.

Maybe others can find a typical installation example.
Note that I am not an engineer and if you feel the need one should be consulted.  I am just going from past experience in similar situations.
"Always work from the general to the specific." J. Raabe

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Offline Redoverfarm

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Re: Foundation for Surface Bedrock
« Reply #9 on: December 21, 2009, 09:20:55 AM »
Mike I had to add to a bridge pier once and instead of epoxy I drilled my holes at a 45 deg angle from the center.  Then drilled one the opposite direction from the center.  I allowed the re-bar to cross each other in the center.  Once the concrete is poured there is no way for any uplift of the re-bar pins as they will be working against each other. I was adding 3' pour onto  of a 48" deep pour. Cold concrete joint.  I am not sure now exactly how many pins I used somewhere in the neighborhood of 20-25.  Each were about 2' long X 1-1/4" with 1/2 drilled and the  other half sticking out that made the cross.  I then attached horizontal re-bar to the X for a wall.  But you could do the same and leave enough out to make the X and then tie your caging to that.  Make sense.   ???.  


Concrete pier
     ---X---
     Bedrock
« Last Edit: December 21, 2009, 09:59:20 AM by Redoverfarm »

Offline MikeOnBike

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Re: Foundation for Surface Bedrock
« Reply #10 on: December 21, 2009, 07:56:39 PM »
Yeah, I think I get the idea.  I like the X design to prevent uplift.  Yes, we will pour the piers in place.  I'm all for putting extra rebar in.  I only want to build this thing once.  My concern is that there is no side/lateral support for the piers like you have if they are sunk into the ground and the rock/concrete surface is a cold joint.  I planned to brace any pier over about a foot high.

The current plan is a 20x28ft version of John's 20x30 single story.  I believe it only requires two rows of piers on the long side but I think we may use a middle row to add some stability given that it is kind of a house on stilts.

Here is a photo of the rock we are going to set the cabin on.  The camera is in about the middle of the cabin location.  The ledge in the lower third is as far forward as the piers will go.


Offline Redoverfarm

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Re: Foundation for Surface Bedrock
« Reply #11 on: December 21, 2009, 08:24:27 PM »
Mike just be carefull as not to shatter the rock.  Too many holes in certain rocks will cause then to crack and defeat the purpose.  I would think that three pins at an angle then one larger in the middle straight verticle.  All tied together for the concrete pour to secure them as one. I think I would find something a little larger than # 4 or 5 re-bar.  Maybe a #7 or 8 for the pins. Re-bar has little shear strength.  I was always told that it's strength is the pull against it horizontal surface thus the ribs in it's design. I can give you a good example at my house with a retaining wall that was re-bar pinned to the foundation.  The wall leaned and the re-bar pins had to snap.  The wall had moved approximately 5-6" verticle. I was howeer able to push it back and install deadmans but I learned to depend very little on re-bar's shear ability.

If you are using Sono tubes or box formed piers you might consider some "J" bolts with the treads exposed and enough length to penetrate your bracing material at a pre determined heigth. This will give you something to attach your bracing to the piers and then up to your beams or rim joist. They can be wired in place to the re-bar put inside the forms.  

Offline MikeOnBike

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Re: Foundation for Surface Bedrock
« Reply #12 on: December 21, 2009, 09:02:30 PM »
Probably sonotubes unless I have a lot of scrap lumber on hand when we get ready to pour.  Yes on the J-bolts, the bracing is a must.  One of my biggest concerns is getting enough lateral stability in the pier foundation.  I might investigate a pin with greater shear strength.

The solid rock creates some problems but the view at this location is really great.


Offline glenn kangiser

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Re: Foundation for Surface Bedrock
« Reply #13 on: December 21, 2009, 10:00:43 PM »
Nice Looks a lot like the Snake but smaller ? when I flew a plane up to Boise.

Interesting thoughts from an engineering forum:

Quote
"DaveVikingPE (Structural)     
13 Feb 02 11:42
Thanks, Ishvaag!

I've got a CMU building that's going to be partly demolished. The building is a one-storey, multi-bay garage, with three sides closed, oneside permanently open, very simple. Basically, we're cutting the building in half and putting in a new wall. My wall design calls for connecting a new wall to the rear wall and a column at the "front" of the garage. The new wall, which is a load-bearing shear wall - of course - will be attached to the front column (a 24 x 24 CMU solid column) utilizing wall ties dowelled into the front column, connected with a high-strength epoxy chemical adhesive anchor. The manufacturer's specs, which are quite detailed, give "good numbers" were I to use a 1/2" diameter threaded rod (SAE - whatever). I, however, would prefer to use #4 rebar, Grade 60, because I can bend it and hence get the appropriate lap with the new wall's bond beams, etc. Since grade 60 rebar has "better" properties than threaded rod, I *know* I will get more positive numbers than threaded rod (i.e., tensile and shear strengths). However, I was at a loss to find shear strengths and, in desperation, threw the question to the Eng-Tips crowd. I've decided, after some more research, that I can assume a shear strength for #4 Grade 60 of around 8,000 psi.

Quote
"RiBeneke (Structural)     
2 Mar 02 5:18
I think that the shear strength of a steel bar in a concrete-masonary interface will seldom be the controlling factor.

Under load the steel will tend to deform to an S-shape as the concrete crushes slightly at the interface.  Then you will not have pure shear at the interface, and the controlling factors are concrete (or masonary) strength, and steel bending and tensile strength and also pullout strength of the steel into the concrete.

Be careful that if you have changed from threaded bar to smooth bar, the pullout strength may be reduced because of less effective bond."

Grade 60 rebar is stronger than grade 40 - 60000psi tensile strength.  From the above then 8000 psi shear  onm 1/2 inch rebar.  Thinking about leverage on a big retaining wall with tons of unstable mud behind it and the leverage advantage from the top of the wall to the toe of the wall, it could be pretty easy to shear.

A few of those 1/2 inch bars or larger if you wish should not have a problem if diagonal braces are added at about a 45 degree angle from the bottom just above the rock to the framing of the house.

You will not have the leverage there to speak of because the top is stable and the load is transferred down to the rock by the diagonal brace.
"Always work from the general to the specific." J. Raabe

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Please put your area in your sig line so we can assist with location specific answers.

Offline MikeOnBike

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Re: Foundation for Surface Bedrock
« Reply #14 on: December 21, 2009, 11:21:25 PM »
A lot smaller than the Snake.  You can step across this body of water.  The creek does dump into the Owyhee river which then dumps into the Snake.  We have 80 acres and this cuts diagonally across it.

Thanks for all of the engineering help.  I think with some careful consideration the piers will be just fine.  I'll create a thread with pictures when we get started. 

Offline Redoverfarm

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Re: Foundation for Surface Bedrock
« Reply #15 on: December 22, 2009, 03:15:56 AM »
Mike I am not an Injunear   ;D  by any means just from what I have picked up through the years and used in the past that have worked.

BTW you will have a nice looking setting.  What about a well and septic? [scared]

Offline glenn kangiser

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Re: Foundation for Surface Bedrock
« Reply #16 on: December 22, 2009, 04:22:00 AM »
I'm not an engineer either but work with them quite often and understand the principles enough that I can discuss design with them reasonably well.

If I was building what you are I would not worry about it as long as I braced well from the bottom and used a decent number of pins into the rock.

Soil braced foundations can get muddy and give.  If you build it on the rock it has no place to give - no compaction - no movement, no give.

Unless of course a massive earthquake fissure just decides to open somewhere within the 20 foot wide spot you have chosen to put your house on.....  [waiting]
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Offline Whitlock

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Re: Foundation for Surface Bedrock
« Reply #17 on: December 22, 2009, 04:24:04 AM »
I have some ?'s about this idea. What type of rock are you putting this on? Is that a schist or slate?
What is the lowest temp.in your area? How are you going to run the pipes for your pluming?
Is there enough top soil for a septic?

I ask this becuse I have drilled alot of rock and also have put in piers in bedrock.
As I say IN bedrock. You have to drill a bore hole to be safe.
Maybe your well driller can do this for you?
They never work by just drilling in rebar and pouring concrete piers.( unless you have the right type of bedrock)
The concrete rebar and rock work aganst each other. The rock will swell 10% in the winter I don't remeber how much the concete swells but it won't be the same. You can't keep the bed rock from sucking up ground water.Your concrete won't last 20 years if it gets below 32 much.
You could rock bolt metal ancors in. But you still need to drill a 7/8's hole 3 ft. deep.
And this will only work if your rock is stable enough.

Lets start bye finding out what kind of rock you have.


Good Luck,W
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Offline glenn kangiser

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Re: Foundation for Surface Bedrock
« Reply #18 on: December 22, 2009, 04:44:20 AM »
Good one, W.  Freezing is a good consideration.

Whitlock mentions that the foundation that was breaking was on slate which will take on some moisture then freeze and expand but the piers on granite or basalt didn't have the problem.

Note that some things that are considered bedrock can be soft.  Keeping water away from under the house can help also.

Again, if there are concerns consult with a local professional familiar with the rock.

"Always work from the general to the specific." J. Raabe

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Offline Redoverfarm

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Re: Foundation for Surface Bedrock
« Reply #19 on: December 22, 2009, 05:17:49 AM »
Whitlock do you recall what they used to use to embed pipe and steel into concrete and rock before the advent of epoxy.  My Dad had once told me how they did it with something like phospourus/lead  (?) around the post.

You can usually tell if the rock is suitable when you first start drilling or go out to a surround area and take a sledge to it.  Easily fractures or flakes and it will not usually work.  Believe it or not a lot of rock are more pourus than you think.  If you don't think so put a river rock near a flame/fire and chances are that it will break apart.

Offline rick91351

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Re: Foundation for Surface Bedrock
« Reply #20 on: December 22, 2009, 08:21:37 AM »
John:  Where I am from the old guys used a lot of molten sulfur in those situations so I have been told.  I have seen some cable leads anchored as such and have been there for 80+ years.    Most likely a poor mans way, money was hard to come by, sulfur most likely was easy to come by and was very easy to melt and pour.  I also have heard about molten lead being used.  Would that be the phos/lead?  But with advances in epoxies I think you could bound just about anything to anything.  That is if you can get a hole bored for the anchors.  A person might have to give a mining engineer a call or poke around on the internet a little.

MikeOnBike:  Thanks for the pictures.  Wow - Great spot, do you have any springs there close by?  The rock out cropping in the foreground is that where you intend to build?  It sort of looks like what we called Owyhee County shale when I was running around rock-hounding.  But then I sure am not any type of geologist anymore.  (Hope to go back and kick over a few rocks now and then after I retire.)  But best leave that to those like Whit and Glenn who seem to crave underground houses and mines - dirt and rust.  ;D

rlr
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Offline Redoverfarm

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Re: Foundation for Surface Bedrock
« Reply #21 on: December 22, 2009, 10:46:51 AM »
John:  Where I am from the old guys used a lot of molten sulfur in those situations so I have been told.  I have seen some cable leads anchored as such and have been there for 80+ years.    Most likely a poor mans way, money was hard to come by, sulfur most likely was easy to come by and was very easy to melt and pour.  I also have heard about molten lead being used.  Would that be the phos/lead?  But with advances in epoxies I think you could bound just about anything to anything.  That is if you can get a hole bored for the anchors.  A person might have to give a mining engineer a call or poke around on the internet a little.


rlr

Rick that was it. Sulfur. It seems that phospourus was something that burned hot and that was in the back of my mind to disolve all this snow.  Anyway you are probably 100% now that I think about it.  I doubt it was a combination of the two just one or the other.  Thanks for setting me straight.  d*

Offline Whitlock

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Re: Foundation for Surface Bedrock
« Reply #22 on: December 22, 2009, 12:02:26 PM »
Whitlock do you recall what they used to use to embed pipe and steel into concrete and rock before the advent of epoxy.  My Dad had once told me how they did it with something like phospourus/lead  (?) around the post.

You can usually tell if the rock is suitable when you first start drilling or go out to a surround area and take a sledge to it.  Easily fractures or flakes and it will not usually work.  Believe it or not a lot of rock are more pourus than you think.  If you don't think so put a river rock near a flame/fire and chances are that it will break apart.

They called it Neat Cement Grout.I know they used melted lead and zinc for the cable sockets.
Never herd of sulfur you would think it would rust out the anchor ??? Good to know.

Thanks guys,W
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Offline Whitlock

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Re: Foundation for Surface Bedrock
« Reply #23 on: December 22, 2009, 04:21:00 PM »
 But best leave that to those like Whit and Glenn who seem to crave underground houses and mines - dirt and rust.  ;D


Dirt Rust and mines are your friends without them there would be no-


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Offline rick91351

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Re: Foundation for Surface Bedrock
« Reply #24 on: December 22, 2009, 05:31:21 PM »
Gee Whit up here in Idaho we use those little old things like that for fishing sinkers in Hells Canyon during Steelhead season.  Find'em by the bucket fulls all the time we do.  Yip right there on them gravel bars while your a fish'n.  Just gently tap a couple them around your line and you can really sail it out down to those deep holes.  You know some them fish eat so many of them you just can not get them up off the bottom after you hook'em.  They just sit there on the bottom ya know just like your are hung up or something.  But they is all bellied out in the rocks.  Only thing you can do is just cut the line and fix yourself another rig and it try again.   ;)       
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