footings sliding on rock

[releky]

I only designed foundations for soil and never on rock (and don't intend to as there is no rock in my vicinity) but for foundations on rock... won't the foundation just slide on the rock during seismic activity since there is no friction between them?

 

[msquared48]

Yes. That's why you drill into the rock, and grout in rebar dowels. Standard practice here...

Mike McCann
MMC Engineering

 

[releky]

Do you also do it to soil? How stiff should the soil be before you consider doweling them?

 

[hokie66]

I doubt that the coefficient of friction on most forms of rock is less than for soil. However, footings on rock would typically be smaller, so that is why sliding would be more of an issue.

 

[releky]

For a mat foundation, do you put dowels on the rock underneath too?

For a spread footing.. We don't usually put dowels on the soils below.. just gravels before pouring the foundation concrete. Do you put dowels on all soils for all sizes of footings?

what formula do you use for the coefficient of friction?

 

[hokie66]

It is about 0.3 for concrete on either rock or soil. You have to reduce the resulting friction force based on your factor of safety.

For a mat foundation, which is usually defined as a large footing under a whole building or major portion thereof, I haven't heard of dowels being provided. Perhaps keys into rock if they are required. I don't see how dowels into soil would help.

 

[releky]

mat foundations don't have dowels because they are resistant to sliding or because the dowels can't stop the sliding?

 

[hokie66]

Mat foundations support a lot of gravity load, so a lot of frictional resistance. I am not saying that there are no cases where additional horizontal resistance is required, but such cases would be rare.

 

[dhengr]

Releky:
You could always saw cut, drill, chip,.... etc. slots or key ways into the rock, in various directions. Then you have a shear key in conc. problem/detail, and a much more effective means of transmitting lateral loads, than a few grouted dowels. Although, the grouted dowels might be needed for other reasons, in some instances.

 

[releky]

dhengr, would you agree stiff soil is sometimes better than rock because stiff soil can serve as cushion for the building and foundation so the upper structure won't attract more forces or load path that can't be cushioned in the soil? Rock is so hard the load path would end up in the foundation and can't transfer this to rock.