Environmental Geology Spring 2010

Mass Wasting/Landslides/Land Subsidence

Local example: 1 dead in 1995 slide in Hot Springs, Arkansas when slide collapsed part of the Hot Springy Dingy novelty shop.

Slope Elements:

Convex slope near top of hill.

Free Face (steep part of slope)

Concave slope at base of hill (wash slope/debris slope)

Angle of repose: steepest angle loose material can maintain

Types of movement

Flow – behaves like a fluid (may be wet or dry)
Slide – movement along a failure surface (planar slide or curved as n a slump)
Free Fall
Slump – mass moves down along a curved failure surface

Safety Factor:

SF = Resisting Forces/Driving Forces = Shear Strength/Shear Stress

Shear Strength and Resisting Forces are equivalent – This is what holds the mass in place

Shear Stress is the force that is acting to make the object move

If safety factor is less than one the slope is likely to fail.

Increasing the slope causes shear stress to increase

Reducing friction causes shear stress to increase

Adding weight to the mass causes shear stress to increase

Examples of slides:

Frank Slide Alberta Canada – covered the town of Frank, Canada – 70 dead

Gros Ventre Slide Wyoming:

Slide began in 1920,  massive failure occurred in 1925.

Slide mass blocked river creating a natural reservoir.  Over next two years reservoir filled finally overtopping the dam in the winter of 1926-1927 as a result of heavy snow and rain.  River rapidly cut down through the slide material releasing a wall of water that inundated Kelly, Wyoming, killing 5 of 65 inhabitants.

Water contributes to slides in several ways:

Adds weight
Reduces cohesion if saturated or if completely dry (if intermediate moisture content then cohesion is generally increased).
Acts as a lubricant
Facilitates frost wedging
Causes expansion and contraction of certain clay minerals
Liquefaction

Vegetation
Provides cover
Binds soil
Adds weight

Vaiont Dam (Italy) 1963

Geology of site: Limestone interbedded with shale with bedding sloping in toward river.  Limestone was karstic (caverns and fractures).

Previous evidence of large slides in area.

Thin arch dam constructed, reservoir filled began to observe creep rate gradually increase over a 3 year period from 1 to 30 cm/week to about 25 cm/day.  On the day before the slide the creep rate increased to 100 cm/day.  Massive slide filled reservoir, displacing water both upstream and downstream. The dam held but the wall of water went over the top and flooded the town of Vaiont (2,600 dead).  Air blast from the slide mass blew windows out and lifted roofs from homes on far side of reservoir.

Yungay, Peru – 1962 – debris flow with no apparent trigger kills 4,000 in town of Ranrahirica.

In 1970 in same area an earthquake triggers a slide that moves down the slope at about 300 km/hr.  The upper part of the slide is a vertical fall.  The mass then runs up and over a large glacial hill launching megaton boulders – smashing homes.  Town of Yungay was buried 30 meters deep.

Gansu Province, China – 1920

Loess Flow (dry flow of silt) – earthquake triggered this event. Historic accounts indicate the mass flowed like water with eddies and whirlpools.  Anecdotal story of survivors whose farm rafted from one hill across the valley and up the adjacent hill coming to rest with only minimal disturbance to their house and orchard.  The story goes that they began farming at their new location.

Methods to stabilize Slopes

Retaining walls – only function with proper drainage.

Pinning

Reduce gradient – add benches

Subsidence and Catastrophic collapse

Subsidence – sinking or depression of the surface

Collapse – material collapses into near surface void.

Subsidence is related to compression of fine grained material through dewatering often resulting form extraction of fluids through aquifer depletion and oil and gas production.

Examples:

Mexico City – 8.5 meters
Tucson, Arizona – subsidence resulted in large Earth fissures.
San Joaquin Valley, California – meters
Las Vegas, Nevada – 1.5 meters
Houston area – 1 to 3 meters (results from both aquifer depletion and oil production).

Other causes of subsidence include drainage of swamps and subsequent decay of organic matt as occurred in the Florida, Everglades.

Collapse:

Example shown was Winter Park sink in Florida 1981 – Limestone sink resulting from loss of roof support due to depletion of the aquifer as a result of heavy use for municipal water needs.

Example 2: Salt Solution mining near Hutchinson, Kansas resulted in 1975 collapse creating a surface crater about 700 feet diameter and 80-100 feet deep.