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Rock @ 3'

Rock @ 3'

Rock @ 3'

I know this is a very stupid question but I am being bugged constantly by the architect to get an idea for estimate purposes of what the footing sizes would be for a 7 story structure. He keeps telling me to assume rock at 3' depth (river bed in the area). I know what to assume for regular footings for estimate sake, 2000-3000 psf. But the heck could I assume (which I would clearly state to the arch) for a rock at 3.

RE: Rock @ 3'

if you don't have a geotech report or boring log or personal experience with seeing the rock at 3 feet, than don't assume it. besides, what kind of rock is it and what would be the strength? unless you have a degree in geology, don't even try to make that assumption. ask the architect for supporting data. By the way, "river bed in the area" does not generally indicate you are close to bedrock, at least not around here. We have riverbeds around here with over a thousand feet of alluvium before you hit bedrock.

RE: Rock @ 3'

Thanks for the response. I fully agree with you.

RE: Rock @ 3'

You don't have a stupid question.  You have a stupid architect.

RE: Rock @ 3'

Rock on!

Mike McCann
MMC Engineering


RE: Rock @ 3'

Tell the Architect that you would like him to check this out on site, bring a shovel and a sack lunch, and send you pictures of the results.     

Mike McCann
MMC Engineering


RE: Rock @ 3'

If it is rock, it might be the worst possible rock. it would be prudent to design appropriately for this and disclose it in your  report to him.

Timing has a lot to do with the outcome of a rain dance.

RE: Rock @ 3'

A post hole auger or a backhoe should quickly verify this (at least in the area you dig).


RE: Rock @ 3'

That's funny, I think the greenbook deffinition of unforseen cuircumstances is a .....three feet? Assume nothing whithout supporting boring reports.

RE: Rock @ 3'

River bed ----> loose alluvial soils(?)--> shallow groundwater(?)-->
                liquefaction potential(?) ----> unacceptable         
                settlement(?)----->excavation dewatering problems(?)

Assuming the bearing capacity to size the foundations is the least of your potential problems!


RE: Rock @ 3'

I am in the Minnesota River Valley, with granite bedrock running from exposed to 7 meters below surface in the immediate locale.  It varies sharply in depth depending on where a testbore is taken.  There is a water table directly above bedrock, with plenty of seasonal springs, with the overburden being kaolin clay/decomposed granite and a smattering of topsoil.

As others have mentioned, nothing can be assumed, and you will most definitely want an accurate geo site survey to produce an accurate estimate.  A few undesirable characteristics and a low estimate could bite you by a factor of 10 or more.  It's either that or go with a "shoot the moon" estimate.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

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