dear friends from the rock mechanics field

Good day. I have what may seem a minor problem, but I cannot crack it somehow.

I have attached a page from Hoek & Brown (1997) paper, which gives elaborate spreadsheet formulas for simulation of triaxial data and conversion to Mohr-Coulomb parameters, in absence of actual triaxial tests. All the formulas are explained and mutually connected except one.

What is the formula for signt??

It is the normal stress which must be specified in order to calculate the tangent to the Mohr envelope. There is a myriad of formulas involved with the problem and I just don't see the answer. I would greatly appreciate assistance.

Cheers, Hrvoje Vučemilovič