Chapter 5 Mass Movements and Subsidence Causes vs

Chapter 5 Mass Movements and Subsidence

Causes vs. Triggers Cause – situation that leads to failure Natural geologic – fractures, layers Natural morphology Uplift (tectonic or volcanic) Subsurface erosion Vegetation loss (fire, drought) Freeze-thaw action Human Excavation or loading Deforestation Mining Irrigation or water leakage Trigger – event that produces failure Physical Heavy rainfall/rapid snowmelt Rapid drawdown Earthquake Volcanic eruption Flooding Human Explosions

Resolution of forces on a slope Driving force – downhill component Resisting force – depends on underlying material Safety factor = resisting force/driving force if > 1, slope is stable if < 1, failure can occur

Angle of repose

Bedding planes on a slope

How stable is this house?

Classification of Mass Movements • Falls – free fall • • • Slides and slumps • • • Rotational Translational Flows • • Rock Debris Mudflow or earthflow Creep

Free fall

Cause of slope failure n n n 1 - steepening of slope 2 - removal of toe support 3 - addition of mass at top

Rotational slide Crown Head scarp Toe Basal surface

Anchorage Alaska March 1964 Earthquake

Translational slide

Types of flows – notice differences n Debris flow n Earthflow

Creep

Turtle Mtn/Frank Slide, Alberta 1903

Mass movements of other material n n Snow avalanches Submarine slumps – largest mass movements on Earth, but not seen

Subsidence n n n Downward movement of ground surface Not reversible once it occurs Causes n n n Removal of water or oil Compaction of clay materials (deltas) Solutioning of limestone by CO 2

Alabama 1972 130 m long - 46 m deep