Doweling Footing to Sloping Rock

[MDavidsonPE]

I'm looking for other engineers input here on doweling into bedrock for continuous strip footings. The project is located on a significantly sloping site. Overall 20 ft drop over 100 ft length. The bedrock surface varies from 0 ft to 4 ft below the existing sloping ground surface. Loads along the proposed continuous 4 ft high foundation walls are in the order of 4 klf, with multiple point loads of 10 to 25 kips every 5 to 10 ft along their length.

I designed the footings to bear directly on bedrock, and required that the bedrock be leveled to no steeper than 6H:1V both longitud. and transverse to the footing with steps as required to achieve that. {Note that IRC requires 10H:1V max, and it doesn't mention soil or rock.] I used #5 dowels embedded 18 in. into a 1" dia. grouted filled hole the rock. The 18" depth was because since the top 6 in. of the rock surface was weathered at many places. Dowel spacing was 18" o.c., staggered along the length of wall, with a 2 ft wide footing. I thought is was completely reasonable-- but alas...

The architect came back to me saying that all the local contractors who bid the job complained bitterly that my design requirements were over-the-top-excessive and that the design was driving the foundation cost way up. [to over 200K]. They typically only go 6" into the rock with pins every 4 ft or so in similar situations. The architect sympathized with them since he is also the developer and would be paying the bill. I promised to take a second look at it.

Question 1: If you are designing footings on sloping rock with dowels, what is the maximum slope that you will accept. i.e. ?H:1V
Question 2: Assuming you analyze sliding of the footing as an inclined block, and you do it without shear friction dowels, do you include full dead load or 0.60D. Under allowable load design, using 0.6D, would you still have a FS against sliding of 1.5, or is that overkill.
Question 3: What coefficient of friction do you use for the concrete on variably fractured weathered granite bedrock? [I used 0.60 but considered pushing it to 0.75.]
Question 4: If you conclude that you need dowels to create the clamping force perpendicular to the rock surface to give you the shear friction that you need to resist sliding, do you also include the dead load in the calculation, or do you conservatively ignore it, and just use the dowel contribution.
Lots of questions. I'm very interested to hear what others do in this situation. Thanks.

 

[KootK]

My take:

#1) Anything above 40% would make me pretty nervous.
#2) 0.6D. I like the 1.5 FOS here.
#3) I'd be wanting geotechnical help with mu.
#4) I'd probably ignore dead load here.

Additional thoughts.

1) Barf.

2) I've never heard of shear friction being used in this way. I'd think that, just as in a concrete to concrete connection, you'd have to develop the dowels for fy on either side of the joint. Something to keep in mind when considering your 6" embedment.

3) In addition to geotechnical help, it would be very comforting to have some kind of in situ load testing program in place for the dowels to verify capacity.

4) I would install the dowels such that they were either plumb or slanted uphill somewhat. That way, your connection into the bedrock would be in compression rather than tension which sounds great to me.






I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MDavidsonPE]

Hi KootK:
Thanks for your clear responses.
Under your additional thoughts,
1. Barf ?- I assume you mean how unpleasant dealing with the arch/contractor's complaints.
2. How would you design the dowels to take the sliding if not in shear friction? and yes, We do need to develop the dowels- that's why I want to stick with 18" embedment, and rejected their suggestion of using 6".
3. Good idea. I had the same thought. Getting some real in-situ failure values
4. Dowels are plumb. Not sure I agree with desired compression vs tension.
Again, thanks for your helpful comments.

 

[msquared48]

I would make flat bearing at the top and bottom of the slope and design the sloped footing as a grade beam, still pinning the rock with #5 bars at 4 foot max.

Mike McCann, PE, SE (WA)

 

[KootK]

You're most welcome MDavidsonPE. The numbers below are in reference to your latest post:

1) Barf = sloped foundations make my stomach turn.

2) As you know, simple friction and shear friction are different animals. I'm hesitant to consider this shear friction in the ACI 318 sense for two reasons. Firstly, the provisions in ACI have been validated by testing and your condition has not to my knowledge. Secondly, with a layer of weathered bedrock between your two competent sliding surfaces, I'm not sure which model of shear friction would apply here. It's not the rough surface model where two competent, keyed surfaces force the joint apart under applied shear and stress the dowels in tension. I guess it bears some similarity to the smooth surface model where shear transfer is essentially true dowel action but, again, you've got that layer of weathered material in between. My instinct would be to treat the dowels as simple axial members having a component of their axial force parallel to and resisting the applied sliding shear. This concept partially informs my preference for "uphill" dowels.

4) I like compression because of the design method that I mentioned in point #2 above. With the dowel behaving as an axial loaded member (strut or tie), its development capacity in compression ought to be much higher than its development capacity in tension where you would have to deal with something akin to ACI appendix D breakout cones etc, both in the concrete and the bedrock.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I'm going to change my "what would I do" recommendation a bit:

I would use the dowels as shear dowels but not shear friction dowels. In my mind, this would eliminate the need to develop the dowels for fy into the bedrock. I would also try to get design recommendations for the dowels from a geotechnical or specialist foundations consultant if possible. Obviously, not all projects are large enough to warrant that kind of specialist attention. I would then consider any benefit gained from tilting the dowels uphill to be icing on the capacity cake.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MDavidsonPE]

KootK: Your points are well taken. I've thought about the dowel in shear vs dowel in tension that you bring up, and conclude that the tension model is cleaner, less moving parts, i.e. no dealing with lateral loads on thin steel dowels, worries of rock fracturing under high lateral point loads near rock surface, i.e., a clear vertical failure cone mechanism. Nevertheless, I can see either model would be a mechanism to resist sliding.

Related to your 40% max sloping surface level, is it based on anything, or just your gut and experience? What slope would you use and not call for use of using dowels at all. i.e, if you could make 0.6D work with FOS of 1.5, would you just forget doweling. Why dowel if the sliding can be taken care of through regular stability statics, like in a retaining wall on soil? IS it just a feel good, just in case type of thing. My gut tells me, either you need it or you don't. If you need it, do it right, not dowels embedded only 6" into rock, acting like scratch pins, but fully developed steel capable of generating a clamping force. And yes, I agree that it would be necessary to test the actual pullout capacity at representative locations in the field before going to crazy with a million dowels.

Regarding the geotech specialist suggestion, obviously that makes sense. I am originally a geotechnical engineer who morphed twenty yrs ago into a structural engineer with specialty in foundations. So why don't I have all the answers? As I indicated earlier, I thought I did, but I am being questioned so I'm looking to see how other competent engineers see it. Either to bolster my own opinions, or to open my eyes to something that I might be missing. Anybody else have any thoughts? Thanks.

 

[KootK]

Related to your 40% max sloping surface level, is it based on anything, or just your gut and experience?

Just gut feel. No real basis for it technically or experientially.

What slope would you use and not call for use of using dowels at all. i.e, if you could make 0.6D work with FOS of 1.5

Vertical.

I am originally a geotechnical engineer who morphed twenty yrs ago into a structural engineer with specialty in foundations.

Interesting. How far into your career were you before you made the switch? I've considered doing that in reverse.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.