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Environmental Geology – Mass movements

Mass movements

Reading: This week: 8 and 9Next week: 9 and 11

Surface processes:

Environmental Geology – Mass movements

Mass wastingplate tectonics vs. gravity

Cost in U.S.• $1.5 bill/year • 25-50 deaths/year

Environmental Geology – Mass movements

Slope stabilitygravity vs. friction

Important factors:

• Sediment type (size, angularity)

• Fluid content

• Vegetation

• Triggering events

Environmental Geology – Mass movements

Angle of repose

maintained by mass wasting

Slope stability

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Environmental Geology – Mass movements

Effects of fluids – a littleSlope stability

Surface tension binds sediment particles together

Environmental Geology – Mass movements

Effects of fluids – a lotSlope stability

Too much fluid increases pore pressure and pushes sediment particles apart reducing friction

Clays: can absorb water into their crystal structure turning them into gel-like sediments

Environmental Geology – Mass movements

VegetationSlope stability

Root systems bind together sediment particles

Here erosion has removed material from beneath the layer held together by

vegetation

When fire removes that vegetation erosion proceeds rapidly

…a trigger of sorts

Environmental Geology – Mass movements

Triggering - EarthquakesSlope stability

Stress of passing seismic waves can reduce inter-particle friction below the threshold to slide

Nevados Huascaran, Peru debris flow:

• Resulting from a mag 7.7 earthquake in 1970

• Flow travelled at 600 mph

• Buried 2 towns, 18,000 dead

Southern CA:• Many slides

resulted from Northridge earthquake

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Environmental Geology – Mass movements

Triggering – Rain stormsSlope stability

Rapid influx of fluid into pore spaces increases pore pressure causing slope failure

Brazilian mudflow following heavy rainfall

Swollen rivers rapidly undercut slopes

Environmental Geology – Mass movements

Failure in rock

Freeze-thaw process

breaks up

Weak layers with high silt/clay content can fail during earthquakes

Environmental Geology – Mass movements

Types of mass movements

Characterized by • material involved (rock or debris)• speed of the movement (cm/yr to km/hr)• slide or flow (one unit or fluid-like)

Environmental Geology – Mass movements

Rock mass movementsRock fall:freeze-thawhigh velocity!talus slopes

Rock slide:freeze-thaw, undercuttinghigh velocity

debris piles

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Environmental Geology – Mass movements

Unconsolidated mass movementsdebris: soils, broken up rock, vegetation and often human construction, cars etc

Creep:• gradual movement of

surface soils• 1 to 10 mm/yr• causes structures to tilt

Environmental Geology – Mass movements

Unconsolidated mass movements

Earthflow:• Fluid mass movements• speeds up to km/hr• fine grained soils

Debris flow:• Fluid mass

movements• speeds up to

km/hr• rock fragments

supported by muddy matrix

Environmental Geology – Mass movements

Mud flowsArmero Columbia 1985

• flows of finer muds and sands

• significant volumes of water

• very high velocities (km/hr)

How do these relate to other natural hazards we have discussed?

Environmental Geology – Mass movements

Slumps and debris slidesMovement of unconsolidated units

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Environmental Geology – Mass movements

Natural causes of landslides

1982 Thistle, Utah

Rainstorms, undercutting of slopes, earthquakes

Environmental Geology – Mass movements

Human acceleration of landslide potential

• removing vegetation

• steepening slopes

• adding construction to slopes

Environmental Geology – Mass movements

The Vaiont Dam, Italy1960 slide:

• Small slide raised awareness and initiated monitoring

October 1963:

• Landslide filled reservoir causing dame to be over topped

• Flooded many towns down stream, 3000 drowned

Environmental Geology – Mass movements

Slope reductionPreventative measures

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Environmental Geology – Mass movements

Retention structuresPreventative measures

Thick low wall have been more effective

than thin high coverings

Rock bolts are very effective at stabilizing rock surfaces

Fences catch small

debris

Environmental Geology – Mass movements

Fluid removalPreventative measures

• Prevent absorption with waterproof covering

• Drain pore fluids – some soils will drain well, others will not

Environmental Geology – Mass movements

Recognizing hazard1. The historic record: landslides

are recurrent events

2. Tilted structures, cracked construction