Landslides, Part 1 La Conchita slide, Jan 2005

Outline Definitions Factors important for mass movements Timescales of movement Examples

What are mass movements? Material moves downslope under force of gravity Occur throughout U.S. and the world Often triggered by earthquakes, volcanoes, rainstorms, human activities In U.S., average $1.5 billion, 25 deaths per year

Gravity: Key Force 1 lb rock Material on horizontal surface - no problem because no motion driven by gravity Gravity pulls with 1lb towards center

Gravity: Key Force Downslope component leads to slides Just need some energy or trigger to get it started 1 lb rock Gravity force split into components perpendicular to surface, and parallel to surface

Factors Influencing Mass Movement Nature of slope materials Steepness of slope Water content Slope stability

Angle of Repose Angle of Repose: the maximum angle at which a pile of unconsolidated particles can rest, increases with grain size

Classification of mass movement is based on dominant material, fluid content, and velocity of movement.

Creep Slowest motion, very widespread Downhill motion of soil and uppermost bedrock layer, rates of few mm/year

How Creep Works Surface materials expand and contract, resulting in net motion downslope

Creep - Expansion and Contraction Ways to get expansion: Water in pores freezes - expands in volume Abundance of clay minerals - absorb water easily Heating of ground surface Ways to get contraction: Thawing Drying Cooling

Evidence of Creep

Evidence of Creep

More Speed! More substantial, higher velocity movements - important for hazards Several internal and external factors important for increasing the odds of movement

External Factors: Steepen slope Remove support at base Waves, streams, people Add mass to top Sedimentation, human effects

Internal Factors Weak materials Water Decrease cohesion Geologic conditions

Weak Materials Clays most common problem; very abundant Sheetlike structure can absorb, release water Some clay minerals lead to large slope failures Quickclay: mix of clay, water, salt that can collapse from vibrations (blasts), rain Example: Canada

Water: Important for strengthening and weakening Strengthens - surface tension Weakens by Pore pressure Adding weight Interaction with clays

Importance of Water Content Saturated sand flows easily because of interstitial water Surface tension in damp sand increases cohesion Dry sand is bound only by friction

Other water weakening methods Weight - excess water adds mass on the slope Clay interaction - can attach to clays because of positive/negative charges on water molecule

Decrease in Cohesion Occurs through erosion Rocks expand as they reach surface (less pressure than at depth) Open fractures, allows water to enter

Geologic Conditions Slopes can be weak due to “pre-existing” geologic conditions Ancient slide surfaces Slope of rock layers relative to slope of hillside Structures such as joints, faults, clay layers in rocks

Rock Dip is Important Rock layers dip at angles less than hillslope - conditions ripe for failure

Triggers of Movements Immediate cause Long-term Rains Earthquakes Thawing cycles Construction Long-term Gravity

Types of Faster Movements

Falls Hard rock splits along joints, weak zones Detaches from cliff, free falls to ground Can shatter, leading to dust clouds Triggers: rain, frost, earthquakes

Rock fall - Yosemite 1996 162,000 ton mass fell off valley wall Large tree kill, thick dust blanket 1 death

Slides Movement above failure surface Can be rotational Or translation Curved slip surfaces Or translation Fail along planar surface

Rotational Slides Move down and out on curved slip surfaces (sometimes called slumps) Can trap water, lead to more instability

Rotational Slide Example Rotational landslide in France

Translational Slide Move down and outward on ~planar surfaces of weakness Can be joints, faults, clay layers, etc Can go as Block slides Debris slides Lateral spreading

Next Time More slides and flows