Chapter 5

Mass Movements and Subsidence

Causes vs. TriggersCause – situation that leads to failure Trigger – event that produces

failureNatural geologic – fractures, layers PhysicalNatural morphology Heavy rainfall/rapid snowmelt Uplift (tectonic or volcanic) Rapid drawdown Subsurface erosion Earthquake

Vegetation loss (fire, drought) Volcanic eruption Freeze-thaw action FloodingHuman Human Excavation or loading Explosions Deforestation Mining Irrigation or water leakage

Resolution of forces on a slope

Driving force – downhill componentResisting force – depends on underlying

material

Safety factor = resisting force/driving forceif > 1, slope is stableif < 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 AlaskaMarch 1964 Earthquake

Translational slide

Types of flows – notice differences

Debris flow Earthflow

Creep

Turtle Mtn/Frank Slide, Alberta1903

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 CO2

Alabama 1972130 m long - 46 m deep