1. According to AASHTO Standard Specs: 1.1. Axial Capacity on Rock for piles driven to competent rock, the structural capacity of the piles in article 4.5.7 will generally govern the design axial capacity. For piles driven to weak rock such as Shale and mudstone or poor quality weathered rock, a static load test is recommended.
1.2. For drilled shafts, it is advisable to ignore side resistance for shafts socketed into weak tock. and for tip resistance in accordance with Table 22.214.171.124.2A on rock w/ RQD< 25%, use qult for an equivalent soil mass.
1.3. For Spread Footings on rock w/ RQD<25%, use qult for an equivalent soil mass.
P.S. Even if your foundation is not for a bridge or if it is for a bridge where AASHTO doesn't govern, it is worth considering these recommendations in determining your ultimate geotechnical capacities.