Foundation on equivalent soil mass bedrock within a slope.

[peelwvu]

I have a project in which a bridge abutment foundation is proposed on bedrock of very poor quality and is being analyzed as equivalent soil mass. The foundation is also next to a slope. My question is do I need to evaluate the bearing resistance using AASHTO, 2010 Section 10.6.3.1.2c or will the fact that it is on bedrock, albeit very poor bedrock, negate this.

 

[FlashSet]

Bedrock can become weathered and decomposed. At one extreme, severely weathered rock no longer has any rock properties and is soil. A geotechnical engineer needs to decide if the bedrock is poor rock or if the underlying stratum is soil.

 

[Mccoy]

peelwu, it would be advisable that you briefly explain what AASHTO requires since not everyone has that code (I do not).
Like the previous poster already said, if extremely decomposed, a rock mass may behave like gravelly soil, otherwise like an isotropic poor quality rock mass, the best procedure would be to analyze it both ways, you may have a lower bound for the gravelly soil and an upper bound for the poor quality isotropic rock mass.
It stands to reason that the presence of the poor bedrock does not negate the evaluation of the bearing resistance since its behaviour approaches that of a soil mass.

 

[peelwvu]

Sorry for the AASHTO code for a footing on or near a slope the bearing capacity equation neglects the Df term and Nc and Ng become Ncq and Ngq which are determine based on how the footing is situated within the slope. The bedrock has been determined to be very poor quality shale. I assumed that I would need to evaluate it more as a soil and take in to account the slope but I wanted to make sure there wasn't something I was missing.

 

[Mccoy]

KK, I know that kind of corrective factors, the rationale is that the failure surface is interrupted by the slope so you have less frictional area=less resistance to failure. Since that is a conservative assumption anyway, you cannot be wrong if you use the AASHTO suggestions.

the above reasoning may be incorrect though if the failure surface is constrained by some joints, like it can happen in jointed anisotropic rock masses. A very poor shale will be an isotropic medium with a behaviour similar to soil, probably it is reasonable to assume that the failure surface under the foundations will follow a pattern similar to that of soils.