A vertical cut is to be made in a soil mass having cohesion $c$, angle of internal friction $\phi$, and unit weight $\gamma$. Considering $K_a$ and $K_p$ as the cofficients of active and passive earth pressures, respectively, the maximum depth of unsupported excavation is

1. $\dfrac{4c}{\gamma \sqrt{K_p}} \\$
2. $\dfrac{2c \sqrt{K_p}}{\gamma } \\$
3. $\dfrac{4c \sqrt{K_a}}{\gamma } \\$
4. $\dfrac{4c}{\gamma \sqrt{K_a}}$