Steepest incline on bedrock for keyed/doweled footings?

[abarker]

I've seen several references to keying and/or doweling spread footings into bedrock when the incline of the bedrock exceeds 1V:6H. However, I have not been able to find a reference for the steepest recommended incline before consideration should be given to blasting, etc, to level it. Does anyone have a reference for this?

 

[jike]

I would think this would be dependent upon how many pins are put into bedrock and their shear capacity.

Even 1:6 seems steep to me (10 degrees). What kind of factor of safety against sliding is this?

 

[abarker]

jike,
Thanks for your comments. If we look only at geometrical affects of the incline (not how wind loads, etc affect the design), and assuming a coefficient of friction between rock and concrete of about 0.65, then the FS would be about 3 with a 1:6 incline (and no pins). This isn't too bad if you don't have much for other lateral loadings. However, at 1:4 that FS would drop to about 2 (again w/o pins).

I understand we can always design the foundation to resist sliding using the shear capacity of anchors, the question is at what point should we as engineers start to get uncomfortable about how much of the capacity is taken up by anchors? For instance, to maintain the FS=3, a 1:4 slope would require a 33% increase in resistance. Is that reasonable for rock anchors? The answer may be that "it depends" (mostly on magnitude of load and slope vs. footing orientation), but I'm just looking for general guidelines for what engineers have done in the past (i.e, the ole' "standard of care" due diligence).

 

[jdonville]

abarker,

It would likely depend on, as you say, the relative magnitude of lateral loads. Also, the depth of overburden above the rock, scour criteria, seismic considerations, etc.

Of course, depending on the size of the footprint, you could always incorporate a shear key or keys.

I saw a presentation a year or so back for a massive bridge foundation that incorporated multiple drilled shaft extensions below the sloped bedrock surface as shear keys or large dowels, depending on your perspective.

Also, how does the analysis change when you have a sloped bedrock surface and you 'stair-step' the rock surface to accomodate the incline?

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[abarker]

jdonville,
I believe the discussion of anchors would dissappear with a "stair-step" configuration because I assume each of the steps would be less than a 1:6 slope. This type of configuration is what we would recommend next if the inclination of the bedrock is too steep for comfort with anchors.