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wimvb

breaking ground, but will this do as pier foundation?

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Today I started breaking ground for my obsy build. Although it feels more like breaking back atm. The obsy is planned to have room for two piers and will only be used for imaging.

The area where I'm building is old forrest ground on top of moraine. In short: roots and rocks. I managed to lift a few rocks out of the way, but then hit upon this monolith. Question is: will it do for a pier foundation? The rock is roughly 1.2 x 0.8 x 0.5 m and too heavy to move even with a bobcat excavator. Sweden is mostly bedrock with boulders of various sizes. Once you hit bedrock, it can't get more solid and stable, but even boulders of this size can move due to frost, which can go down as far as 0.5 m. The boulder definitely is not lying on top of bedrock, and if it can't be used, I will probably need to pour a foundation right next to it.

Btw, my temporary roll off shed obsy is in the background. The rough outline of the new obsy is also drawn in this image.

obsy1and2.thumb.jpg.0c3d0e19a01958d09c8af24001bfbdc1.jpg

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I'd clean away the debris and surround and cover it with concrete. It's going nowhere! Often when concreting foundations, rocks and rubble can be added to fill out and reinforce the foundation. I'd leave it be!

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Posted (edited)
On 07/04/2019 at 15:40, wimvb said:

The area where I'm building is old forrest ground on top of moraine. In short: roots and rocks. I managed to lift a few rocks out of the way, but then hit upon this monolith. Question is: will it do for a pier foundation? The rock is roughly 1.2 x 0.8 x 0.5 m and too heavy to move even with a bobcat excavator. Sweden is mostly bedrock with boulders of various sizes. Once you hit bedrock, it can't get more solid and stable, but even boulders of this size can move due to frost, which can go down as far as 0.5 m. The boulder definitely is not lying on top of bedrock, and if it can't be used, I will probably need to pour a foundation right next to it.

 

You have my full sympathy. I hit bedrock within inches of the soil surface- but I still had several feet to go down, along a 12m long stretch of slope. It took months to get through!

47528907502_48dd8c34a5_k.jpg

47528904642_5d0e192de3_k.jpg

Fortunately my biggest rock was only wheelbarrow size- at least I know the pier is concreted to the bedrock!!

33704877388_ad5db5a5ec_k.jpg

 

Edited by laser_jock99
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1 minute ago, laser_jock99 said:

Fortunately my biggest rock was only wheelbarrow size- at least I know the pier is concreted to the bedrock!!

Wouldn't that have been an ideal anchor for your pier?

Holes drilled into it, bolts fixed in with resin, pier on top.

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I though all one needed to do was connect to the bedrock so as the pier is stable! No need to go any deeper.

My pier was 18" deep before I hit concrete from a building that was there some years earlier. So, I poured concrete into the hole with the pier inserted. Never had any issues, even with all the weight in top :) Solid !!

 

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Plenty of Y'Tube videos on how to split big rocks into manageable bits.

Dave

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6 minutes ago, pete_l said:

Wouldn't that have been an ideal anchor for your pier?

Holes drilled into it, bolts fixed in with resin, pier on top.

All the resulting rubble found a use as backfill for the retaining wall on the downward side of the slope.

46666407605_9275c75662_k.jpg

 

 

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4 minutes ago, Davey-T said:

Plenty of Y'Tube videos on how to split big rocks into manageable bits.

Dave

 

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If the frost can get down to 0.5m then I think that boulder is probably too close to the surface to assume it will not move.

But perhaps you could break it up into more manageable pieces.  There are chemicals you can buy to pour into holes drilled in rocks that expand and crack the rock, or you could try the really old-fashioned way and hammer dry wooden pegs into drilled holes and then soak them with water.

James

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7 minutes ago, laser_jock99 said:

 

Wow, that's one nice pier base shattered to pieces.

We actually did some minor blasting when we built our house. Bedrock that was about 30 cm too high up.

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1 minute ago, wimvb said:

Wow, that's one nice pier base shattered to pieces.

The guy probably wants a swiming pool though!

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7 minutes ago, JamesF said:

If the frost can get down to 0.5m then I think that boulder is probably too close to the surface to assume it will not move.

... 

James

Yes, my thoughts too. One alternative solution is to cover the pier base with thick styrofoam insulation, at least 1 m to each side. That way frost may not reach the rock.

Or I could try to move the rock out of the way without having to lift it.

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2 minutes ago, laser_jock99 said:

The guy probably wants a swiming pool though!

Oh right, not everyone is into star gazing. ?

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36 minutes ago, laser_jock99 said:

 

I think there are methods that just use hand tools, just think Egyptians building stuff.

Dave

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14 minutes ago, Davey-T said:

I think there are methods that just use hand tools, just think Egyptians building stuff.

Dave

Plugs & feathers, possibly?

James

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2 minutes ago, Davey-T said:

I think there are methods that just use hand tools, just think Egyptians building stuff.

Dave

Yes, but they used a lot of cheap manpower. I don't think slavery is legal anymore. ?

In Sweden they used to drill holes in a rock and just filled those with water. The frost did the rest. But I'd like to have my obsy ready for next season.

When I built an extension to our summer cottage, I destroyed the ball bearings of my power drill while trying to drill holes in granite. I had to rent a Hilti chisel hammer to do the job properly. A Hilti is about the smallest hand tool I'd want to use on a rock like this.

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Plugs and feathers at work:

Personally I'd have used a bigger hammer and let the weight do the work for me.

James

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That doesn't look too dufficult. But I wonder, how it will work on granite?

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5 hours ago, laser_jock99 said:

I  supect granite will be a bit tougher. Amazing to think the Ancients made these huge granite obeliske with just copper & bronze tools......

https://en.wikipedia.org/wiki/Unfinished_obelisk

 

Hmm, maybe I could chisel a pier out of that rock. A mini obelisk.

On earnest though, granite is a lot harder than sandstone, the plug & feather technique won't work.

 

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It's not about hardness, though.  I'd guess it's more about tensile strength, though I'm no materials scientist.  Here's an example that claims to be done with granite:

James

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Yes, that sounds and looks like granite. So, it is possible. I can borrow a chisel hammer or strong power drill from a colleague. Maybe I'll have a go at it.

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Posted (edited)

Just an idea...

Maybe some local masonry or memorial headstone manufacturers would collect it free of charge? :)

Edited by RolandKol

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I guess there's no shortage of rock in Sweden, a bit that size would cost a small fortune in the UK.

Dave

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The rock will have been there for centuries if not thousands of years and will have settled into the ground to be as solid as the rock of Gibraltar.  Leave the rock alone, scrub it so it is clean and concrete around it.  The concrete will adhere to the rock before pouring.  

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