Mass wasting - landslides

1. Mass wasting - landslides Unit 3 – Gradational Processes

2. Forces on Slopes • Stability is determined by the relationship between the driving and resisting forces. • Driving forces – move earth materials down • Resisting forces – oppose such movement • Slope stability is evaluated by computing a Safety Factor (SF) • The ratio of the resisting forces to the driving forces • If SF is greater than 1 a failure is likely

3. Driving a Resisting Forces • Type of earth materials • Slope angle and topography • Climate • Vegetation • Water • Time • With an elbow partner, brainstorm and make notes on how each of these variables contributes to slope stability.

4. The Role of Earth Material Type • Mineral composition, degree of cementation or consolidation, and the presence of zones of weakness • Weakness can be natural breaks in the consistency of earth materials • Shape of the slip surface is strongly controlled by the type of earth material • If very resistant rock overlies weaker rock, rapid erosion of the underlying rock may cause a slab failure and subsequent rock fall

5. Role of Slope and Topography • Two factors: • Steepness of slope, and • Amount of topographic relief • Slope = the slant or incline of the land surface • The steeper the slope, the greater the driving force • Relief = the height of the hill or mountain above the land below • High relief areas are hilly or mountainous, have tens or hundreds of meters of relief, and generally prone to mass wasting

6. Role of Climate • Climate is the weather that is typical in a place or region over a period of years to decades • Also factors in seasonal variations • Influences the amount and timing of water that infiltrates or erodes a hillslope and the type and abundance of hillslope vegetation • Arid and semi-arid climates, vegetation tends to be sparse, soils are thin, and bare rock is exposed in many areas • Free-face and talus slopes are more common • Common wasting is rock falls, debris flows, and shallow soil slips

7. Role of Vegetation • Vegetation provides a protective cover that cushions the impact of falling rain • Plant roots add strength and cohesion to slope materials • Vegetation also adds weight to a slope

8. Role of Water • Many landslides, such as shallow soil slips and debris flows, develop during rainstorm when slopes become saturated • Other landslides, such as slumps, develop months or even years following the deep infiltration of water in to a slope • Water erosion of the base of toe of a slope decreases its stability • Stream or wave erosion can reduce the safety factor by removing material at the base of a slope, thus steepening it • Spontaneous liquifaction of quick clay – clays lose their shear strength and flow as a liquid

9. Role of Time • Driving and resisting forces may change seasonally with fluctuations in moisture or level of the water table • Chemical erosion can occur when water comes into contact with rocks – carbonic acid • Changes in weathering are greater in especially wet years • Resisting forces may continuously diminish through time from weathering, which reduces the cohesion in slope materials, or from an increase in underground water pressure