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How to pour a slab on ledge?

Started by rmallaire, August 13, 2017, 10:45:05 AM

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rmallaire

Hello everyone,

I have been doing some tree clearing on my land in southern New Hampshire. Eventually I am planning on building a 18x24 four bent timber frame.     There is a small high spot that I particularly want to build it on due to the low risk of worrying about drainage.  As you can see in the pictures, I have a couple of large rocks that have been exposed which I was hoping to dig up and remove, however, they might be much larger than expected. Once I got all of the trees and stumps removed, I started scraping away the top soil and tried moving the rocks but they might be enormous or at least too big for my 6-ton dozer. Even if the two rocks remain I can still build to the side of them. Aside from these two large exposed rocks, once I scraped away the topsoil, I started getting into an area that has rock after rock and eventually hit ledge. 

I originally wanted to pour a foundation 4' below grade and pour a slab.  Now given that I am in ledge, I will have to build on top of it if even possible.

Some of my initial thoughts / questions include:

I am looking for what my options are to build on ledge.

I am looking for advise on how it would be to perhaps pour an Alaskan slab / monolithic slab on top of ledge. And if I do this slab I am concerned about the point loads from each post if they would be too much on the shallow outter wall since it will now be 4' deep.

Any thoughts and comments will be appreciated. Thank you

snowstorm

If they are rocks do not pour Crete on it. Ledge is ok. I have done the ground work on several houses that part of the foundation was poured on ledge. A rock even a big one will move up with the frost

Bruno of NH

I have built a lot on ledge
Make sure it's solid clean it good and pin it.
My own house is on pined ledge and some is under my floor slab
Lt 40 wide with 38hp gas and command controls , F350 4x4 dump and lot of contracting tools

Banjo picker

The two replys you have gotten come form your neck of the woods.  I and a lot of others probably don't know what you mean by ledge.  ???  And there are no pictures.  Banjo
Never explain, your friends don't need it, and your enemies won't believe you any way.

Don P

Banjo, I believe he's hit the Canadian Shield, the oldest rock on the planet. If memory serves it's about a half a billion years older than the next oldest, the rock under me that runs out down in Mobile Bay, the Appalachians. I assume they've found graffiti from Alley Oop on the bottom side  :D

VTwoodworker

There will be no issues from loading of the slab if it is placed on solid rock.  The slab will be prone to shrinkage cracks when poured on rock.  The concrete will shrink as it cures and because it will be restrained by the rock it will cause tension and shrinkage cracks.  Limit the water content of the concrete and use plenty of steel to control the size of the cracks.  The other issue could be radon gas from the rock.  Radon gas can be a problem with foundations on rock.  I suggest testing for it before placing the slab and if it is detected vent systems can be designed to mitigate it but it would be under the slab.

snowstorm

I would not pour on a big rock. I have dug a lot of rocks out of town roads. Some as big as a small car. The frost will push them up. I put in a new drain a while back. One rock I dug out was 15 ft long by 4 ft by 3 ft . It was about all my little cat wanted

rmallaire

Thank you for the replies so far.  I was unable to upload photos to this post but I added the pictures to my personal gallery. Thank you

rmallaire


Bruno of NH

X2 on the rocks I had to blast and move 2 rocks the size of a 20ft containers
Lt 40 wide with 38hp gas and command controls , F350 4x4 dump and lot of contracting tools

rmallaire

I don't think blasting is an option because my well is about 75' away and my house is about 125' away.   I scraped away dirt in about a 30' diameter area and kept digging up one boulder after another (as you can see in the pictures). Near the large exposed rock, I believe there is a strip of ledge or it could very well be large boulders that my dozer just doesn't have enough power to move. 

That said if it is a rocky area, I simply should not build there because rocks will continue being lifted from the frost?  If it is indeed ledge I can pin it though?

Bruno of NH

You need to get a bigger machine in there to poke around . Just my opinion
I thought my home site was just one big rock and I was wrong.
Lt 40 wide with 38hp gas and command controls , F350 4x4 dump and lot of contracting tools

rmallaire

I agree, that is a good idea. I need to rent an excavator before drawing the conclusion that it's ledge. Thank you

thecfarm

They dug some big ones when they was digging here. We did have to blast for one corner of the house. But you said you did not want to blast because of the well. The well guys hit ledge here too. I have no idea how far down they hit ledge.
Yours don't look bad at all. The contractor hauled alot of rocks off for his retaining wall.
Model 6020-20hp Manual Thomas bandsaw,TC40A 4wd 40 hp New Holland tractor, 450 Norse Winch, Heatmor 400 OWB,YCC 1978-79

snowstorm

how much well casing do you have? that will tell you how deep the ledge is. at the well anyway

rmallaire

My well has 40' of casing.  Further back in my woods, there is a linear strip of ledge, which is in line with the rock \ area I want to build. 

Yesterday, before I read everyone's replies I scraped away the top soil into a separate pile and then scraped some of the base material from the high side of hill to the low side, making a flat buildable pad.  After reading the replies, it makes sense that if there are more large boulders beneath that pad I created, the frost will lift those boulders up, hence lifting a monolithic slab up with them. 

I think my next step should be to get an excavator in there and see if I can dig out the rocks, then dig down 4' to where I could pour footings and properly pour a floor. 

As always, any suggestions are appreciated as I do not have prior experience with this. 


snowstorm

Frost walls are nice but expensive . Most of the time a slab on 1 to 2 ft of compacted gravel works just fine. If pouring a 6" slab dig down another 6" next to your form boards so the edges are 12" thick. For a ft or so wide. It's had to tell from the picture but looks like it is quite a bit out of level. If you don't have a laser or transit buy a pop level. They are not as good but they are cheap

rmallaire

Thanks for the reply. 

Yes it's certainly out of level, no where near ready for building.  I ran the dozer for a little while yesterday morning moving material from the higher spot to the lower spots making rough passes. I'll be using a transit when it comes time to finish grade. 

With the couple feet of crushed gravel, are the larger boulders beneath them that are above the frost line not an issue?




snowstorm

Get what you can for rocks out. The bigger ones.bank run is fine then finish with 6" of 1" minus.  You can put 2" blue board under the concrete. Some say it will not freeze under. Crushed stone in overkill.  Costs more and if you end up having to fill and low spots with a rake and shovel  you won't enjoy it

Bruno of NH

Lt 40 wide with 38hp gas and command controls , F350 4x4 dump and lot of contracting tools

thecfarm

Well casing? Forgot how much. All I know is they used a couple lengths and hit ledge and started boring. Went down 140 feet,40 gallons a minute. I never got to see it being drilled. By the time I got here,they was gone. Only took a few hours. According to the contractor,the well guy said,this will be good water. He saw all the rocks coming up the road.
Model 6020-20hp Manual Thomas bandsaw,TC40A 4wd 40 hp New Holland tractor, 450 Norse Winch, Heatmor 400 OWB,YCC 1978-79

repmma

To follow up what Snowstorm has suggested research "frost protected monolithic slab" or "alaskan slab" and you can get plenty of info on insulating a slab for Northern climates. The wing insulation out around the edge of the slab is very important otherwise the frost will work in from the perimeter of the slab.  If you put say 4' of insulation out past the slab perimeter in our area ito not suppose to reach underneath the slab (horizontal wing insulation is suggested to extend out same distance as frost depth).  If I'm remembering it all correctly.  It's apparently quite common in Scandinavian regions and Alaska.
Thomas 8020, Timberjack 225C, Ford 5030 with Norse 450 winch, stihl saws and 142 acres to manage.

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