The strength of the intact pieces of rock will be imaterial to the shear strength of the slope.  The strength will be determined by the material between the pieces of intact rock.  You need to sample that material to determine strenght parameters for analysis.

longisland,
I would reccomend to you the software "Roclab", freely downloadable from Hoek's website.
It overcomes some drawbacks of the Bieniawski method related to soft and very fractured rocks. Has a specific section on flysch formations. Yields conventional phi and c and Hoek & Bray parameters for slopes as well.
Pretty good when there is lack of one or few well-defined joints or joints systems.
Developed in the highway project in Greece, together with professor Marinos of Athens. I tried it out with good results, particularly in sandstone-shale sequences.
Good preliminary evaluation prior to eventual more detailed study such as the one suggested by geopavetraffic (although I fear the latter is hardly applicable in the weathered & fractured conditions remarked by longisland).

Hi,
I'm adopting phi = 41 as stated in FHWA soilnail document. The value is derived from SPT N value > 50. Since we will be excavating the slope eventually, I'll assume the cohesion to be 0 on the safe side. A sheared zone was found on the 1st berm from the bottom. The rock layers are deformed & the tocks are heavily jointed.

Thanks for the reply

That value would entail an extremely fractured rock equivalent to an isotropic, dense and possibly weakly cemented breccia-like deposit.
That's a possible conservative approach.

I have also used Hoek-Bray method using Roclab (also downloadable from the "Rocscience" site)to good effect, giving what I judged to be realistic mohr-coulomb parameters in weak fragmented rock.