1 Mass Movements GLY 2010 – Summer 2012 - Lecture 17.

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Mass Movements

GLY 2010 – Summer 2012 - Lecture 17

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Mass Movement - Definition

• Transporting of earth materials downslope due to gravity, without the aid of a transporting medium such as water or ice

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Mass Movement Triggers

• Gravity

• Vibration

• Water

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Gravity

• Gravity constantly pulls materials downslope, opposed by friction

• If gravity overcomes friction, the material moves

• The steeper the slope, the greater the proportion of full gravity works on the material

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Vibration• Earthquakes, explosions, or other seismic

energy sources energize particles on the slope, and they begin to move around - some of them may slide downhill

• If enough slides at once, an “avalanching effect” is generated, pushing the material downhill ahead and triggering mass movement

• A blast of air, approaching hurricane force, may precede the front of a large mass movement

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Earthquake Triggered Slide

• Madison Canyon Earthquake of August 1959• An estimated 19 people are still beneath the quake rubble

Water

• The weight of water soaking into soil, increases the force from gravity

• This may be enough to overcome friction and start mass movement

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Water’s Roles

• Water acts as “glue”• Water acts as a “lubricant”

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Water As Glue - Surface Tension

• Water has a very high surface tension

• Small amounts of water in sediment, which is essentially all surface, act to hold the sediment together

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Water As Lubricant

• As more water is added, it forms layers with greater thickness between particles

• Surface tension is no longer important, because the water layer is thicker

• Water acts as a lubricant, cutting frictional forces, and making mass movement easier

Angle of Repose

• The maximum angle at which loose sediment can form a stable slope

• Different materials will have different angles of repose

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Angle of Repose - Examples

• Typical angles: Fine sediment -

30-35° Coarse

sediment - to >40° (talus slopes)

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Angle of Repose, Snow

Snow Type Temperature Angle of Repose

Fresh, Dry -35ºC 63º

Fresh, wet -4ºC Close to 90º

Wet, 24% water

>0ºC 2º

Avalanches form when the angle of repose is exceeded

Avalanche Video

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Artificial Oversteepening

• Man activities may create unstable slopes

• Examples: Road building Mining

Blasting narrow roads through mountains often leads to slope failure

In mountainous states, particularly those that get a lot of rain, road failure is a fact of life

Natural Oversteepening

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Types of Slow Mass Movements

• Creep

• Solifluction

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Creep• Creep is the imperceptibly slow down-slope

movement of soil and near-surface rock materials• Process is generally not directly observed -

instead, creep is best discerned through the movement or response of objects affected by the process

• Typical movement = mm’s to cm’s per year• Extremely common on sloping terrain

Creep Mechanism

• Expansion and contraction leads to creep

• May be due to freeze/thaw or rain/evaporation cycles

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Effect of Creep

• The tilted headstone is the result of creep

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Bent Curbs Due To Creep

• Hollister, California – Left, 1966; Right, 1992

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Leaning Fence Posts – Barnes County, N.D.

• Fenceposts, being shallowly-seated objects, are particularly prone to creep, and have tilted down-slope under creep almost to a horizontal position

• Small trees on this slope likewise show the influence of creep: as their trunks are progressively tilted over by creep, their new growth responds phototropically back upwards towards the sun, leading to a curved appearance of the trunkPhoto by D.P. Schwert, North Dakota State

University

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CREEP Cass County, N.D.

• Photo under an I-94 overpass at Fargo

• Concrete pads have shifted down-slope on soils affected by creep

• Upward squeezing of the pads is induced by differential down-slope flow velocities of soil and "rock" materials.Photo by D.P. Schwert, North Dakota

State University

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Solifluction

• A type of “fast” creep

• Occurs in cold regions where the soil remains frozen most of the year (permafrost)

• May occur in other boggy soils, especially after vegetation removal

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Solifluction Terraces

• Solifluction near Fairbanks, Alaska

Effects of Permafrost

• Thawing of Permafrost leads to slow subsidence of structures built on it

• Becoming a very common problem at high northern latitudes

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Effects of Permafrost

• When a rail line was built across this permafrost landscape in Alaska, the ground subsided.

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Rapid Mass Movements

• Slides

• Falls

• Flows

• Debris avalanche

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Rapid Mass Movement Rates

• Rapid mass movements are much faster than creep or solifluction

• They may move within seconds to a minute or so, at rates of meters per second, or more slowly, or days and weeks, at rates of meters per day

Slides

• A single intact mass (rock, soil, or unconsolidated material) moves downslope along a slip plane

• Slip planes are planes of weakness 30

Slip Plane Geometry

• Slip planes are usually flat

• If they are curved, the slide moving along a concave slip plane is known as a slump

• Some rotation of the material is involved in slumping

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As the photo illustrates, even though the Gros Ventre rockslide occurred in 1925, the scar left on the side of Sheep Mountain is still a prominent feature.

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Slump Animation

• Computer simulation of a deep-seated "slump“ type landslide in San Mateo County, California

• Over 250,000 tons of rock and soil moved in this landslide

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Failure Along Metamorphic Foliation Surface

• Slaty cleavage seen here is at a high angle• Failure has occurred along the foliation surface

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Slump Failure, McClure Pass, Colorado

• Note half buried automobile

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Falls

• Fastest type of mass movement

• Occurs when material on a near vertical cliff breaks free and falls freely to the surface below

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Picture of Fall

• A fall has left an obvious scar, and a talus pile• Failure was caused by artificial oversteepening of the surface to make a road

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Recent Rock Fall

• Recent rock fall, Zion National Park, Utah

Fall Video

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Ocoee River Valley – US 64

• On November 10, 2009, a series of rockfalls preceded a landslide, which would have killed people had geologist Vanessa Bateman not cleared the area prior to the slide

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Flows

• Mass of mostly unconsolidated material moving downslope as a viscous liquid May be dry or wet Move rapidly, especially when wet Considerable mixing may occur during the

movement

Mass Movement Animation

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Types of Flows

• Flows are broken into categories: Earthflow Mudflow Debris flow

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Earthflow

• Dry masses of clay or silt regolith• They have high viscosity, and typically move

relatively slowly (meters/hour to meters/minute)• The slow movement usually precludes loss of life,

but they do cause substantial property damage

Earthflow Effect

• This small, tongue-shaped earthflow occurred on a newly formed slope along a recently constructed highway

• It formed in clay-rich material following a period of heavy rain

• Notice the small slump at the head of the earthflow

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Mudflow

• Composed of wet mixtures of mud and water, they move swiftly

• Mudflows often develop after heavy rains (cloudbursts) in semi-arid regions, where sparsely vegetated slopes have masses of loose regolith

• Canyons in semi-arid deserts are prone to mudflows

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Lahars

• Lahars are a special type of mudflow produced on the slopes of a volcano

• Volcanic ash and hot gases melt accumulated snow and glacial ice, producing large quantities of mud

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Debris Flow

• Similar to mudflows, but consisting of particles larger then sand-sized, often with some boulders of a meter or more

• Because of the larger particles size, they require steep slopes

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Debris Flow Animation

• Computer simulation depicting the Sourgrass debris flow (Sierra Nevada, North Fork of the Stanislaus River), of January 1, 1997

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Debris Avalanche• Very steep,

unvegetated slopes may produce an avalanche

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Wildfires and Debris Flows

• Wildfires can lead to destructive debris-flow activity• In July 1994, a severe wildfire swept Storm King

Mountain west of Glenwood Springs, Colorado, denuding the slopes of vegetation

• Heavy rains on the mountain in September resulted in numerous debris flows, one of which blocked Interstate 70 and threatened to dam the Colorado River

• A 3-mile length of the highway was inundated with tons of rock, mud, and burned trees

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Glenwood Springs, Colorado

• Personal injuries and damage to 30 vehicles engulfed by these flows

• Transportation along the Interstate 70 corridor was brought to a standstill for a day

• Business and emergency operations in the Glenwood Springs area were seriously impeded  

Photo: Jim Scheidt, U.S. Bureau of Land Management

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Soils and Mass Movements

• Quick clays - Waterlogged clay sediments, when energized by seismic waves from earthquakes or explosions, can lose adhesion, and become a viscous liquid

• Resulting flows can be very large Ex. One in Quebec in 1971, in which almost

seven million cubic meters of quick clay moved and killed 31 people, doing hundreds of millions of dollars in damage

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Colluvium

• Any material deposited as the result of mass movement

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Indications of Potential Mass Movement

• Stream undercutting

• Springs on a cliff face

• Cracks near cliff top

• Hillside creep

• Talus

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Avoidance of Mass Movements• Certain activities, such as farming or

ranching, may take place if the potential for rapid, unpredictable failure is low

• Avoidance is facilitated by the development of risk maps, which inform people of where dangerous areas are

• However, many people already work or live in danger zones, and are understandably reluctant to abandon property or homes

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Hazard Maps

• Example of an experimental map released to emergency planning and response teams in the San Francisco Bay Area

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Prevention of Mass Movement• Control of nature is a very difficult, and

usually a very risky, business• If control is attempted, we must understand

the subsurface geology, revealing for example, whether their are dangerous boundary layers along which slippage might occur

• Then strategies can be planned to reduce the danger, which may be either structural or non-structural

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Non-structural Solutions

• Non-structural methods often involve planting vegetation, including grasses, shrubs, and fast-growing trees

• Cement is sometimes injected into a moving slide to try and strengthen unconsolidated material

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Non-structural Solutions Cont.• Dewatering solutions are often the

most useful

• Water can be prevented from entering a slide by the construction of barriers or diversions to keep water away from the potential mass movement site

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Non-structural Solutions Cont.

• Active site can also be dewatered by drilling into the toe of the slide and inserting perforated pipes which help water to flow out quickly, or by drilling wells and pumping water out

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Structural Solutions

• Retaining walls, which attempt to contain the slide may be built

• Site may be regraded, to reduce slope angles

• Wooden or steel pilings can be driven through the slide, to try and strengthen it

• Terraces can be built to catch material which starts to slide