Hi Usmi
I have read a paper written by Hoek & Bray regarding your query, but I could not recover the title.
My understanding there is a curve for each rock, then the c & θ is depending on its overburden pressure.
Thank you for your reply to this thread.
I have came to know to determine C = 0.145*Cu ( cu is UCS value ) but i was unable to get the right reference. and for θ as well.