ABSTRACT
LANDSLIDE TRIGGER PROCESSES Each landslide event can be ascribed to a process which triggered the failure of a potentially unstable rock mass. Cause of failure is therefore a combination of unstable structure and a trigger event. Water: rise in groundwater pressure is by far the most important single trigger factor behind landslides - see section 33. Toe removal: removing toe of a slope reduces resistance to movement. Natural toe removal: erosion by river undercutting (Gros Ventre, above); erosion by wave action causing numerous coastal slides (Folkestone Warren, section 36); glacial erosion leaving oversteepened hillsides (Mam Tor, section 35). Artificial toe removal: by quarrying or mining (Frank), excavation for building site (Hong Kong), or road widening (Catak) (all in section 33). Head Loading: adding material above neutral line of a slide increases its driving force. Portugese Bend slide, Los Angeles, 1956, activated by fill placed for a new road which added 3% to slide mass in zone above slip surface dipping 22° in weak clays. Folkestone slide, 1915, activated by rock falls from head scar (section 36). Natural head loading causes slope instability on many active volcanoes. Strength reduction: weathering ultimately weakens all slope materials; slow creep causes restructuring of c lays s t ressed within s lopes (section 34); slow processes eventually reach critical points. Vibration: cyclic and temporarily increased stresses may cause soil restructuring or rock fracturing. Artificial vibration, as from heavy road traffic (contributory at many small road failures) or from pile driving (which caused a clay slide destroying Swedish village of Surte in 1950). Earthquake vibration has caused numerous slides. 1970 earthquake in Peru started slide on Mt Huascaran which developed into debris flow moving fast enough to rise 150 m over ridge and bury 20 000 people in the town ofYungay. Many slides have complex origins, where and when a number of contributory factors coincide.
