# Concrete Block over limestone grade.?



## CenTexan (Aug 28, 2015)

Any of you guys have experience with laying block directly over limestone? Site location is cretaceous limestone with an estimated bearing capacity of 4,000 psf. 

Last house we built in this area was slab on grade with piers every 8-10' down into the limestone. That site required significant fill so it made sense to go slab. However, new site is relatively flat and I'm considering the effectiveness of concrete block pinned directly to the limestone (roughly 3' high) and doing a crawlspace. 

Years ago I recall seeing a northeastern rubble stone foundation being repaired and the site was similar; mason shaped the CMU to the contour of the limestone grade and laid right on top of it.

I'm talking with a local engineer about pinning #5 rebar into the limestone every 4' with epoxy, setting 12' blocks over the limestone and then grouting all blocks and finishing with a bond beam on top. But we are unsure whether a footer needs to be used.


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## JBM (Mar 31, 2011)

Engineers will take the guess work out of that. I wouldnt ask us we cant stamp anything.


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## CenTexan (Aug 28, 2015)

Ah the old "ask an engineer". I already have. Was hoping to get opinions from masons that have experience with the situation. Just because someone is an engineer and can stamp a plan doesn't mean that's the best approach. 

Engineers are competent within their typical practices; this is out of the scope of normal practice in my area.


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## concretemasonry (Dec 1, 2006)

It is very possible, but depends on the strength of the limestone and the stability. Often the upper layers of limestone can be weathered and there may be friable or loose.

How did you come up with 4000 psf? - out of a book? Your concrete block without core filling are many times stronger than the 4000 psf. - The net strength of a minimal strength CMU is 3000 psi (not psf), which is more than you need. Why do you think you need to grout all cores solidly since you can just grout where there is grout. - I have seen 22 story loadbering block buildings built out of partially grouted 6" thick block.

If the limestone is not perfectly even or level you can lay the block with a variable thickness mortar bed joint. This is done often when footings are sloppily poured and can vary a inch or more in places.


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## CenTexan (Aug 28, 2015)

Thanks, Dick.

The IRC states a presumptive load bearing value of sedimentary rock at 4,000 psf. It also states 12,000 psf for bedrock; the problem is I believe limestone is considered sedimentary rock, but this cretaceous limestone more resembles bedrock in my worthless opinion. 

I've asked a couple engineering firms if they can perform core-samples of limestone to gauge surface bearing capactiy but they acted like that was overkill and that our limestone was sufficient for pinning to. 

I'm confident that I can form up and pour concrete for a footer if necessary, but if i do that I will just go ahead and pour the entire perimiter wall out of concrete. I and the engineers are also confident that this approach will be of superior strength compared to slab on fill. 

But if I can hire a mason to lay block without a footer, I think he will get the wall built twice or three times as fast without any form-work. But I want this solution to be something that experienced masons would feel confident in, and not just get a stamp from an engineer.


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## CenTexan (Aug 28, 2015)

These are images taken directly from the area. As you can see, it's more like bedrock than sedimentary limestone. The last house we built here, the septic leach field had to be busted out with a backhoe.


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## dom-mas (Nov 26, 2011)

What JBM is saying is that it doesn't matter what we think. Your municipality will only be interested in what the engineer stamps, not what some guys on the internet have or have not done outside of that municipality. Especially people who are totally unfamiliar with that limestone formation.

Building right on bedrock (I'm fine with calling it bedrock) has a long hoistory but isn;t mentioned in the building code so it automatically defaults to an engineer. If the formation was reasonably level and flat without any fissures I would have no issue with building right on it...my municipality wouldn't allow it though. My house is from the late 1800s and built exactly like that, stone on (limestone) bedrock, the small new ('80's) addition is 10" block on a footing poured on the same bedrock. That's just how the code reads


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## Tscarborough (Feb 25, 2006)

It is not bedrock, it is limestone over bedrock, even though the limestone is thousands of feet thick. Central Texas lies over the tail end of the Appalachian mountains as bedrock.

Regardless, even in the county you will have to have a structural inspection meaning stamped plans. It will of course work fine, but that is beside the point: No stamped plan=no inspection=no building.


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