Rock slope? What would you correlate SPT values of 100 blows/15 mm as? Look at this, based on what you have provided, as a rock slope analysis. What are the joint patterns, dipping planes? See Wyllie's Rock Slope Engineering book as a starter.

I agree totally with BigH. Unfortunately, very few projects ever have any information on the rock's joints, dips, etc. Oriented rock cores are also rare. If you don't know the orientation of the rock bedding and fractures, it is hard to check for sliding of the rock mass.

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What is your slope height and gradient? Is seismic Kh a concern? Is this in a remote location where failure is not catastrophic or is it in urban area? What is the performance of this slope in the past few decades? Yo will need to work with engineering geologist on it as well.

If it is a rock slope then you need to classify the rock mass using RMR or Q-system. This is engineering geology stuff (I defer to the geology people on this).

There are some free software (rocscience.com) to compute the c and phi from RMR. From that you can do your stability analysis. Check out Practical Rock Engineering by Hoek. That may fill in some of the knowledge gap. For smaller projects you may be able to make some conservative estimates of joint spacing from RQD. May be able to get strike and dip from outcrops or geo maps. For larger projects this information is obtained from televiewer logs.

I would advise you to do the rock mass strength analysis using RMR or Q system as described by molerat2210 secondarily to a kinematic analysis of the jointing. The most important failure mode is going to be sliding along joints. Your primary design consideration is the potential for daylighting (joints are flatter than the slope angle) either joints in the plane shear (single joint striking parallel and dipping out of the slope) or wedge (2 joints, intersection dips out of slope) orientations. This can be done by a stereonet analysis. Go to the project site, measure strikes and dips of joints exposed at the surface (hopefully you have rock outcrops to work with). Plot these orientations on a stereonet and compare these to the strike of the wall to evaluate the potential for daylighting. Also consider the friction angle in your kinematic analysis. If the joints are daylighted and are steeper than the friction angle, sliding will occur. You will need to do a few direct shear tests on rock joint samples. Ideally, the samples would come from diamond drill core samples.

If you have a daylighted condition with major joint sets (ie bedding in sedimentary rocks), then either flatten the slope angle to avoid the daylighted condition or change the strike of the slope (if possible). If you can't do either, you will have to install rockbolts.

I find SPT blowcounts more convenient to use. But it depends.
I agree with what molerat2210 said. A Q system or an RMR will be needed in order to classify the rock mass.
You should perform a rock mass strength analysis and in order to do that, you need to use a Q system or an RMR, just like what molerat2210 said. Having only RQD is not enough. You need more than that. Try performing a rock mass strength analysis, too.
When it comes to performing a design, i am comfortable with RQD. However, we need to perform a rock mass strength to be more accurate.

Rock mass analysis is an important analysis consideration jackkingsford32, but it's always second in terms of importance to kinematic analysis of jointing when it comes to rock slope engineering.

I agree with what molerat2210 said. A Q system or an RMR will be needed in order to classify the rock mass.

What's your end use? Keep in mind that Q system is specifically designed to determine ground support requirements. If you don't intend to design ground support (ie split sets, cable bolts, lagging, etc.), then don't waste your time with Q.

If you want to determine shear strength for stability analysis of the potential for rock mass failure, record RMR or GSI then use the Hoek-Brown method of calculating rock mass shear strength. However, be forewarned that the most important mode of failure in rock slope engineering is sliding along joints and not rock mass failure. As I've posted before, I'd encourage you to measure the joints and focus your efforts on the impact of joints on stability. Do a kinematic analysis of the potential for sliding along joints.

Cheers!

Nobody has mentioned the isotropy versus anisotropy condition which is clearly explained in the hoek's free e-book.
IF the rock mass is very fractured (4 or more joint sets randomly oriented and the scale of the slope is not small compared to the joints spacings) then it makes up an isotropic medium, which will not exhibit as a rule a preferential sliding direction. Other factors will govern and the phi-c pair can be defined by the simple hoek & Brown Roclab freeware.

If the rock mass is anisotropic then directionality will govern and c-phi pairs must be defined for the the critical joint surfaces, according to the criteria exposed by the other posters.

To get back to the original question, if you have pictures of cores, their RQD's and some data on the intact rock you migth be able to define phi and c with some practice, I agree with BigH that SPT has no use in rocks, unless it's weak or very weak rocks but then we should have some reliable correlations for these.