I want to know how to determine water table height and phreatic line in earthen dam with rock fill and clay core.
Okay have a look at this article it may be some use, it's the Journal of GeoEngineering, Vol. 8, No. 1 April 2013 Pgs 27-32 so you may be able to get a hard copy if the link fails, it talks about clay core dams and seepage calculations thereof, including phreatic pathway estimation.
If I remember correctly rock fill in earth dams is treated as earth for the purposes of working out seepage lines etc... as it is generally small enough (particle size), relative to the dam as a whole, to have little or no effect on total mean permeability.