Do you want a cohesion, c-value for designing rock dowels/bolts/anchors? If you want a value to determine the pullout capacity of the dowels, then you want the bond strength, not cohesion. This is a great reference paper:
Littlejohn, G.S., "Design Estimation and Load Holding Capacity of Ground Anchors"
Sorry, but I don't know where it was printed. You might be able to find it by searching technical journals, including international journals.
In that paper there is a table of values for design. In the table the closest rock type to fractured basalt is just plain basalt with a working bond of 1.21-1.38 N/mm^2 and an ultimate bond of 3.86 N/mm^2. You might use a working bond of 1.2 N/mm^2 to determine the allowable capacity of a dowel grouted into the rock.
For example, you have #6 bars so you might want a 1" drilled hole with a perimeter of about 80mm (sorry for mixing units). A one foot deep (300mm) hole gives 24,000 mm^2. At 1.2 N/mm^2, the allowable bond between grout and rock is 28.8 kN, or about 6.5 kips. That is less than the tensile capacity of the rebar (est'd 0.6 fy, or about 36ksi), so you might want to make the dowels deeper. The reason is that it is good practice to make steel yield be the controlling factor in the design. Remember, this is a tension calculation that helps with uplift of the footing, for overturning or otherwise.
If you are putting the dowels in the footing to resist shear, then that calculation doesn't do much for you. In the case of shear, you have #6 bars with 0.44 in^2 cross section. Multiply that area by the steel strength with appropriate shear reduction factor (maybe check ACI 318 building code).
Is your wall in a rock cut? If so, then you can check earth pressure using the following paper:
Frydman, Sam and Israel Keissar, "Earth Pressure on Retaining Walls Near Rock Faces," Journal of Geotechnical Engineering, Vol. 113, No. 6, June 1987, ASCE.
If the retained soil is not rock, earth pressure will be based on some anticipated movement of the wall. If the wall structure is stiff and prevented from rotating at the top (cantilevered concrete will bend even if the foundation doesn't move a bit), then you have at-rest (Ko) conditions. If you have some movement, the earth pressure will be less, down to a lower bound of active (Ka) conditions which requires about an inch of tilt per 20 feet of wall height.
Good luck.
