Just posted part of this on Moe333's thread on a related topic.
See Chuck Ladd's Terzaghi Lecture:
Ladd, C.C. (1991), "Stability Evaluation During Staged Construction," the 22nd Terzaghi lecture, ASCE Journal of Geotechnical Engineering, vol.117, no. 8, pp. 1540-615.
He includes a lot on c/p ratios, and gives typical values. It's a good summary of undrained strength issues in general (in addition to SHANSEP), including the importance of triaxial compression vs simple shear vs triaxial extension in the undrained strength, i.e., c/p is not a constant for a given clay - it varies considerably with stress path
Better yet, get a vane or cone, and remember it's just a bunch of dirt and we can't get terribly precise in trying to determine its engineering properties.