Dowel into rock for sliding resistance

[JStructsteel]

If placing a dowel into rock for sliding resistance, (basically shear) do you have them just grout in, or epoxy, or just hammer the heck of of them into a tight hole?

 

[azcats]

We did a couple large spread foundations sitting on top of rock and ended up requiring a shear key be cut into the rock. These footings were formed above 'grade' so there was no passive resistance from surrounding soils. Contractor didn't complain and this was easier to quantify & install. We were asking the questions that you were and couldn't get solid answers quickly.

 

[BAretired]

I have never used rock anchors, but check out some of these sites:

https://www.google.com/search?q=rock+anchors&ie=utf-8&oe=utf-8

BA

 

[KootK]

There was a recent, related discussion here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JStructsteel]

My issue is not getting the bars to take tension, or the shear capacity of the bar, its just how to install it. Footing is not outside kern, its just sliding, due to EOR being a asshat and requiring at rest pressures behind a tall wall.

 

[BAretired]

An asshat?

BA

 

[Gumpmaster]

Are you placing tiebacks into rock to prevent sliding or just vertical rock anchors down from the wall and into the rock?

Talk with a good geotechnical engineer. There's an actual methodology to designing the rock anchors. You have to take into account both local and global failure modes. Tiebacks to resist sliding are typically angled back so that they primarily act in tension. I guess you'd get some dowel action from a vertical bar, but it would probably be less reliable than a bar acting in tension. I think the geotechs would frown on it. If that's your concept, why not just use a shear key like azcats said?

 

[JStructsteel]

Asshat is someone being totally unreasonable.

They are not tiebacks in my mind, just a dowel to resist the balance of sliding resistance I need to make it work. No tension. A #5 bar, drilled and grouted about 12" into the rock.

I cant get them to give me a capacity of the the rock, and they dont want to chip away at the rock to provide a keyway. Its a contractor trying to do it as cheap as possible.

 

[cvg]

good luck with obtaining rock characteristics. Rock is a highly variable thing, joints, faults, weathering and varying degrees of hardness and strength. Most geotechs will not give you this number and guarantee the contractor will not. If you want to anchor the structure to the rock and tension the anchors during construction, you can then get adequate data to support your design. I don't expect your contractor will like to install anchors either, they can be difficult to do and expensive.

you might find more information here:

http://www.williamsform.com/Ground_Anchors/ground_anchors.html

suggest spin lock or resin anchors

 

[JStructsteel]

Its a moot point for me, as stated above, the DOT required At rest pressures, for the walls. Its fine with active (most likely what will happen) pressures. its a feel good for someone at the DOT.