Unit 3 – Gradational Processes

 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

 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.

 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

 Two factors: 1. Steepness of slope, and 2. 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

 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

1. Vegetation provides a protective cover that cushions the impact of falling rain 2. Plant roots add strength and cohesion to slope materials 3. Vegetation also adds weight to a slope

1. Many landslides, such as shallow soil slips and debris flows, develop during rainstorm when slopes become saturated 2. Other landslides, such as slumps, develop months or even years following the deep infiltration of water in to a slope 3. 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

 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