Landslides

GEOL 4093 Risk Assessment

Resources

• landslides.usgs.gov• Murck, Skinner, and Porter, 1997.

Dangerous Earth: An Introduction to Geologic Hazards, John Wiley & Sons, 300 p.

• Bloom, 1998. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms, Prentice Hall,482 p.

Mass Wasting

• Collective term for all gravitational or downslope movements of weathered rock debris

• The term “landslide” is not a real geologic term

• Flowing water not involved, but water is very important in mass wasting

Role of Water in Mass Wasting

• Adds weight to rock mass

• Generates seepage pressure by groundwater flow

The empty can on the left slides on a dry surface when the angle reaches about 17 degrees. With a little water dripping thgough a small hole in the bottom of the can on the right, sliding will occur at a much lower angle

A slope is needed to give a horizontal component of force

Angle of repose is the natural steepnees of a pile of unconsolidated material. It varies depending on grain size, grain shape, and water content.

Types of Mass Movements

• Slides—blocks

• Flows—internal shear

• Heaves—vertical

• Subsidence—vertical but no free surface

Modifiers

• Composition– rock, soil, mud, mixture

• Speed of movement– Fast, moderate, slow

Basic Types of Mass Movements

• Slides

• Flows

• Creep

• Subsidence

• There is no single perfect way to classify all of the types of mass movements

Slides

• Cohesive blocks of material

• Moven on well-defined surface

• No internal shearing

• Velocity profile of pure slide:

surface

slide plane

Flows

• Differential shearing within mass

• No clear plane of movement at base

• Velocity of flow decreases with depth in flow

• Velocity profile of pure flow:

surface

base of flow

Heave

• Forces act perpendicular to ground surface by expansion and contractin of material

• Slow, downslope movement by gravity

• Velocity profile of pure heave:

surface

base of heave

Subsidence

• No free surface

• Downward settling of material

• Little or no horizontal motion

• Main cause is slow removal of material below subsiding mass

Speed of Movement

• Creep—very slow

• Slides—fast

• Avalanche—really fast

One way to classify mass movements is by speed of movement and by composition—slurry flows versus granular flows.

Geologic Controls on Mass Movements

• Passive Controls– Nature of material

• Degree of lithification

• Degree of weathering

• Interstitial water

– Stratigraphy

– Structure

– Topogoraphy

– Climate

– Organic (vegetation/critters)

• Active Controls– Support removal

– Slope oversteepening

– Saturation with water

– Earthquakes

Examples of Passive Controls

• Nature of material– Degree of lithification (sed rocks)– Degree of weathering– Insterstitial water– All pretty self explanatory

Passive Control—Stratigraphy

– Alternating resistant and nonresistant rocks

Perched water tableSS

SH

Passive Control—Stratigraphy

– Alternating resistant and nonresistant rocks

SS

SH

collapse

Erosion removes support

Passive Control—Structure

Dipping beds, jointed rock

Creep

Effects of creep

An idealized earthflow

Mudflows from 1980 eruption of Mount St. Helens reached speeds as high as 40 m/s and traveled as far as the Columbia river, ovee 90 km away.

A 1963 landslide into the Vaiont Reservoir, Italy, caused a giant wave to overflow the dam

The 1920 Grand Banks earthquake caused a submarine landslide that broke underwater telegraph cables and disrupted communications between North America and Eurpoe. It also proved the existence of turbidity currents.

There are significant landslide hazards in the eastern U.S.

La Conchita, CA, landslide, Spring 1995. USGS photo.

Mameyes, Puerto Rico, landslide, October 1985. USGS photo.

Landslide susceptibility map and recommended land use for Congress Springs area near San Francisco

Human Intervention

• Slope modification for construction of roads, bridges, homesites can lead to destabilization of hillsides, and increased hazard of mass wasting.

Natural angle of repose exceeded

Oversteepened slope fails, equilibrium restored.

Roadcuts may expose natural planes of weakness in rocks

Mitigation

• Engineering techniques are used to stabilize slopes

• Awareness and avoidance may be the best approach in some cases

Integrated system designed to reduce landslide hazards