Fs= safety factor = (shear strength)/(shear stress) Shear stress: force causing movement parallel to slope; If fs is less than 1, then the slope is unstable and prone to increases with slope angle failure. On a slope, gravity has two components. As the slope angle increases, gs decreases while gs (the shear stress) increases. At a certain point, gs exceeds the shear strength, and failure of the mass. Clays which swell and shrink with amount of water present (smectite) Sensitive clays, which rearrange their structure and liquefy. In sum, the above materials all exhibit: also have little strength. Low resistance to: what causes and triggers landslides. If the planes are parallel to the slope, they may promote sliding. Loss of root systems which binds soil and promote stability. Can be caused by erosion, fire, construction, logging (all these can remove/alter amount of vegetation vegetation) Hurricane mitch (1998) wiped out 100% of the infrastructure. Cascading hazards : drought wildfire debris flows.