Q$3($a$)$ Considering slope stability, list and discuss briefly, with the aid of sketches if necessary, the three major types of slides.

$[7$ marks$]$

$($b$)$ Using the Limit Equilibrium approach estimate the critical height, $H_c,$ of a vertical cut made in a deep deposit of clay $($see Figure Q$3)$ which has a bulk unit weight, ${}_s,$ and an undrained shear strength, $S_u,$ constant with depth. Assume undrained conditions. The resulting $H_c$ should only be a function of $S_u$ and ${}_s.$

$[13$ marks$]$