Foundation for a big rock

[brotherson]

Just wondering if anyone has any ideas for anchoring a big rock (like a memorial stone)

It's about 1.5 tonnes and the base is about 700x700mm. It tapers smaller in the vertical direction. It's about a metre high. It's quarry rock so the surface is quite uneven.

Best I have so far is to drill and chemically anchor deformed bar into the base of the rock, and then cast a concrete base around those rods. I haven't worked out the intermediate support of the rock yet and I may have to drill into the rock which would damage the appearance.

It sounds impractical. Does anyone know how this is typically done? I know there are many many instances of this kind of thing. The idea is to have any concrete footing below the ground so that at the top all you can see is rock and grass.

Thanks

 

[msquared48]

Talk to a stome mason...

Mike McCann
MMC Engineering

 

[brotherson]

Thanks for the response.

If there are any other comments from people with any experience about this it would be good and failing that I'll see what I can do about a stone mason.

Cheers.

 

[MikeHalloran]

Do your local vandals have access to backhoes?

I'm just wondering why a rock needs a foundation to rest on the ground, and what forces (value and direction) you wish to make it resist.



Mike Halloran
Pembroke Pines, FL, USA

 

[brotherson]

Probably!

It's to be a memorial stone with a plaque to be put up in a public place so it needs to stay upright. It'll have to put up with wind really and no doubt there will be the odd reasonble effort to push it over.

I wouldn't have to account for the odd motivated vandal with a backhoe but thought if it was in concrete then the person pushing on it would be standing on the footing that he's trying to overturn. I think a rock of those dimensions simply embedded in the ground would be easy to push over.

 

[MikeHalloran]

A search on "headstone installation" without the quotes was productive.


Mike Halloran
Pembroke Pines, FL, USA

 

[msquared48]

Rock on...

Mike McCann
MMC Engineering

 

[bookowski]

How would you first install the deformed bars and then pour a concrete base around it.... doesn't that mean the stone needs to be 'floating' temporarily to pour the base below? I suppose you could prop it and then remove the shores.

I have no idea so making this up but... what if you core the rock bottom and insert and grout a structural shape in there that projects out. I have no idea of size or loads, that would determine the shape or embedment. Maybe a steel tube , or a WF etc. Stick a base plate on that projecting steel.

Pour a footing with a pocket for the future base plate and anchors. Set stone with steel projection/base plate into the pocket and grout/tighten anchors per usual base plate. Grout pocket to encase steel shape and make flush with b.o. stone.

 

[ztengguy]

bury it in the ground a foot or two?? or pour a pad around it. Surely that should do it.

 

[racookpe1978]

True, this sounds like a silly question: but it really isn't.

Has a LOT of troubling "artistic" problems concealed within the problem as well.

For example - Will the inscription be done on-site (after the foundation is poured) or will you get the rock delivered with an an inscription that needs to be leveled and set with a vertical face and lettering?

Does the memorial sponsor "want" a rock that looks "natural" and be surrounded by grass (A tombstone looks very different than the Plymouth rock for example.) ?

Or will he/her accept a curb and visible foundation?

Are you setting this into a dirt fill or into something solid that won't settle and move over 50-80 years?

Any chance of flooding or water damage scouring away the foundation or rock?

When you get the rock, can you turn it upside down to drill the proposed rebar holes for the bolts or anchor rods?

 

[brotherson]

Hey there

Thanks for the additional replies, I didn't realise they'd been made!

bookowski - The rock sits upright pretty well on its own, but wouldn't take a huge amount of effort to push over. Hence the concrete footing. Instead of drilling into the base (vertically), I'd put a dowel bar in each face (hotizontally) about 150mm above the base of the rock. Sit it in a ~350mm deep hole, put some mesh in, pour a ~250mm footing and put 100mm soil/ plant grass above. The concreteis to make a person unable to push it over, the mesh to stop it cracking up and the dowels to get the rock to engage the footing to some degree.

An easier option would be to leave out the dowels altogether but I'm not sure how to put a number to the depth of concrete that would be a stable footing in this case. My gut tells me 250mm-300mm would be fine.

recookpe - The plaque and inscriptions are all done by a stonemason already. It was done with the rock sitting at it's natural lie on the base as it is so sitting in in a (level) hole will not affect this. The fotting has to be hidden, so I plan to dig the hole 100mm deeper than the depth of the footing. The fill is pretty crap. It's just variable disturbed fill that gets saturated after rain. It'll probably rock around a bit but as long as the footing doesn't break up, it shouldn't become unstable. I doubt there'd be any scour but even if there were, someone will just come along with a dirt and shovel and get some dirt back in.

Cheers

 

[EdStainless]

And this isn't in a place that sees freeze/thaw cycles right?
If it does then you need to get below the frost line.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[jayrod12]

Our office did something similar here in Manitoba.

A large (56"x56"x56") limestone cube that had been etched by some artist needed to be mounted on a corner(fricken artists). We had a large (maybe 72" diameter by 18" or 24" thick) concrete pad that was supported on a singular 24" diameter cast-in-place concrete friction pile. (we are on about 60' of clay here subjected to freeze-thaw). We epoxied a single large diameter(3") solid stainless steel rod into the limestone cube. Then drilled a hole into our pad and then lifted the whole cube and epoxied the anchor into the hole.

the limestone cube corner had a roughly 12"x12" flattened area to help with stability.

not sure if this helps or not.

 

[jayrod12]

This is what it looked like once it was installed

 

[msquared48]

Looks like a baby Borg spaceship.

Mike McCann
MMC Engineering

 

[jayrod12]

@mike
My thoughts precisely when the artist came to us with the design. But I'm smarter than to say that out loud.