Perched Spread Footing on Bedrock

[JEmH]

For a private road underpass below a public road in Pennsylvania, I am considering an arch structure with footings located 12' to 15' above the elevation of the lower road. These footings would have very little lateral load, would be poured against the neat excavated rock, and would be about 5 feet below the top of rock. The rock is calcareous sandstone (or limestone, depending on who you ask.) Unconfined compression is in the 20ksi range, split tensile tests are in the 2 ksi range, and phi fm is close to 0 degrees. So here's my problem. I would know how to calculate the ultimate bearing capacity on this rock if the cut were not below it. I've seen a method in a state manual for calculating a reduction in the bearing capacity for proximity to a slope, but this seems a little shaky, although my geologist says I'm just being paranoid. Anyone have any good references for footings on rock in proximity to a cut?

 

[jdonville]

JEmH,

See NAVFAC DM7.2 (p.135) Available for download from

http://www.vulcanhammer.net

Bearing capacity is just one piece of the puzzle. You will need to perform global stability analyses on the slopes, at a minimum (I would). I would also ensure that any fractures or other discontinuities in the rock are accounted for in your analyses.

Hire a professional rock mechanics/tunnel engineer. I would imagine that the local DOT/responsible agency for the public road will require your structure to meet their design and engineering criteria.

Hope this is helpful.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[DRC1]

How does the footing of the arch not have any lateral load? Is it a tied arch?

 

[JEmH]

Thanks, Jeff. That's a snazzy reference to have around. Kind of collects up stuff that might be lying about in several other books.
Your advice to have others with experience and knowledge more suited to the project is good. I have such an engineer here, but I just wanted to be able to get my own warm and fuzzy feeling.

DRC1: this is a corrugated metal arch. It is shaped such that the "inward" force of the soil surrounding the arch tends to offset the thrust that one normally expects from an arch.