[CrossWorks]

I'm in the process of estimating a deck which will be basically set upon a sloped edge of ledge in the backyard of the homeowners. My thinking is to hammer drill some holes about 4" deep and epoxy the rebar pins in place. The pins will extend up about 8-10" and I'll tie some horizontal pieces to help keep the concrete support together.

Then I plan on forming some 8" square boxes that will conform to ledge slope and be about 8-12" high overall. The overall weight on these supports will be minimal per square ft. They'll be supporting a 4-6" x 20' platform and fro there some steps going down.

I'm heading to Cape Elizabeth this afetrnoon and take some pictures which I'll post tonight so as to give you guys a better perspective of the lay of the land. But in the meantime, is the approach above more or less the correct one when tieing into ledge? Just never had to do it before.

My only concern would be "shale ledge"

Dave...GBAR...Ron...Jaros...anyone. Your thoughts?

 

[Bob Mariani]

Good plan, but drill deeper. I would go into the shale at least 10" And use 1/2" threaded rod, epoxied to the hole. This can then go through your support boxes and connect to a post support.

 

[12penny]

crossworks....If it where my insurance, I'd call someone that knows rocks and engineering. I like to have someone sign off on something like that. Maybe I'm too cautious, but I sleep pretty good.

 

[Ron6519]

What is this ledge made out of? Heaved, sedimentry rock would have issues, especially if the layers were steeply sloped. An homogenous igneous or metamorphic rock would be very good.
If the rock were appropriate, I agree with Bob about the rebar being deeper. I would plan the holes to be as deep as they project out of the ledge. The diameter of the concrete pier would depend on the load put on it. While I don't have the qualifications to size it, I would not make it less then 12" in diameter.
Ron

 

[Scuba_Dave]

I hit ledge at 40" deep for a post
I wasn't too worried about drilling & pinning

They hit ledge on my garage - maybe 2' down in one spot ?
They dug at it pretty good to break off anything loose on top
Then they drille dit & pinned it
Not extremely critical as it was buried 2' down

The only other spot was already 4' down

It depends upon the ledge
Exposed you really don't want to run the risk of it breaking off
So far I haven't had to work on any exposed ledge

 

[CrossWorks]

Thanks guys. Well it is Maine Coast ledge. I will point out to the customer what's involved in terms of stability and the importance thereof. But I do know that the live load will be limited 90% of the time since this will be simply a few platforms and a run of maybe 8 steps at one point.

I'd say the slope is probably [from top to bottom] about 8-9' in a 15' run. Normally you would think unless they decide to have 30 people go up this stairway at once...your good with what I described above for an approach to do this. But the thing is, that's how this has to be planned (overkill). It only takes that one unplanned, unexpected situation for things to go badly very quickly.

12' round pier won't be neccessary though (I guess you need to see it Ron), but, deeper holes I agree with. I'll probably get a "rock guy" {tech term?????} and have him assess the situation.

Couldn't get pic's cause it's raining cat's-n-dog's right now. But I'll revisit this tread later.

Thanks again.

 

[Ron6519]

Rock guy= geologist, but the local building inspector might do the trick.
Ron
(Geology major, long ago and far away)

 

[CrossWorks]

Thanks. Meeting with local inspector Friday morning. Should know then.

 

[Bob Mariani]

The person that deals in this is a soils engineer, not a geologist.