The Mohr-Coulomb failure theory represents which of the following equation as the line that represents the failure envelope ?
1
\({t_f} = c + {\sigma _f}\tan^2 ɸ\)
2
\({t_f} = c - \sigma _f^{}\tanɸ\)
3
\({t_f} = c + \sigma _f^2\tanɸ\)
4
\({t_f} = c + \sigma _f^{}\tanɸ\)