Chapter 5 Mass Movements and Subsidence

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

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 Rock Debris Slides and slumps Rotational Translational Flows Mudflow or earthflow Creep

Free fall

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

Toe Crown Basal surface Head scarp Rotational slide

Anchorage Alaska March 1964 Earthquake

Translational slide

Types of flows – notice differences Debris flow Earthflow

Creep

Turtle Mtn/Frank Slide, Alberta 1903

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

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

Alabama m long - 46 m deep