When you mentioned "Clay", I automatically start thinking about undrained soil shear strength.
A less accurate, but cheaper method of investigating the underlying soil strengths would involve the use of a SPT. Bear in mind that shear strengths from SPT testing can easily have +/- 30% source of error.
A better method of assessing the soil strengths would involve the use an electronic cone testing unit called a CPT. If you also did a few soil shear vane tests then you could really get a grip on the soil properties. The remolded soil strength values from the vane might become very important if the clay is sensitive.
Simple CPT data interpretation programs will give you soil strength, soil type, over consolidation ratio, density, soil sensitivty, equivalent SPT N(60) values and a host of other soil parameters. Some CPT contractors even give all these interpeted values to the Engineer as part of the service.
I would also be looking at the CPT logs in great detail for any sort of weak layer which might act as a sliding plane.
A full and complete understanding of the soil strengths would enable you to perform your slope stability analysis with greater confidence.
A review of the seismic record and related seismic standards for your area will start you on the correct path with respect to the underlying fault.
Best of luck with your project.
Coneboy
