Slope Stability and Mass Wasting

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What primarily determines slope stability in mass wasting?

Slope angle and the strength of underlying material (correct)

Soil moisture levels and temperature variations

Human activities and excavation methods

Surface vegetation and weathering processes

What occurs when the shear force exceeds the normal force on a slope?

Frictional resistance becomes negligible

Shear strength increases significantly

Downslope movement occurs (correct)

The block remains stationary and stable

Which type of mass wasting would involve a rapid movement of soil and rock?

Debris flow (correct)

Slump

Soil liquefaction

Creep

What effect does increasing the slope angle have on shear strength?

It may compromise shear strength and initiate movement Signup and view all the answers

Which of the following areas would be most susceptible to hazardous mass-wasting events?

Areas with steep topography and high rainfall Signup and view all the answers

What is the primary mechanism that leads to the movement of material in rotational landslides?

Curved rupture surface with slow movement Signup and view all the answers

Which characteristic is NOT commonly associated with landslide displacement?

Presence of vast flat areas in the middle of the move Signup and view all the answers

What type of mass wasting event are sturzstroms specifically known for?

Long runout landslides with high velocities Signup and view all the answers

What distinguishes debris flows from earthflows?

Debris flows contain coarse materials mixed with abundant water Signup and view all the answers

Which of the following is an effect of soil creep in an area?

Curved tree trunks indicating gradual movement Signup and view all the answers

What happens to the shear force and normal force when a slope becomes slightly steeper than the angle of repose?

Shear force increases and normal force decreases. Signup and view all the answers

What primary factor can significantly alter the angle of repose for sandy materials?

The presence of water content. Signup and view all the answers

Which type of bedding plane poses the greatest risk for instability on a slope?

Bedding planes parallel to the slope. Signup and view all the answers

What is the most common trigger for mass-wasting events?

Increased water content within the slope. Signup and view all the answers

Which of the following methods is NOT typically used to mitigate landslides?

Regrading slopes excessively steep. Signup and view all the answers

Study Notes

Slope Strength

Mass wasting occurs when a slope fails due to instability
Slope stability relies on slope angle and material strength
Gravity's force on a slope can be divided into shear and normal force
Shear force pushes material downslope; normal force provides resistance - friction
Shear strength is the relationship between shear and normal force
Stable slopes have higher normal force (friction) than shear force
Steeper slopes increase shear force and decrease normal force, compromising shear strength
Angle of repose is the steepest angle at which material remains stable
Angle of repose varies based on factors like grain size, composition, and water content
Water in pore spaces can increase or decrease shear strength
Water added to dry sand increases the angle of repose but too much water reduces shear strength
Planes of weakness in sedimentary rocks, like bedding planes, can affect stability
Bedding planes parallel to the slope are unstable; perpendicular to the slope are stable
Clay minerals along bedding planes can absorb water, making the surface slick and impacting shear strength

Mass-Wasting Triggers & Mitigation

Triggers can be natural, like snowmelt, rainfall, earthquakes, or human activities, like grading
Increased water content is the most common trigger
Intense rainfall, especially during El Niño, can increase water content and likelihood of landslides
Changes in surface water due to earthquakes or human structures can contribute to landslide triggers
Oversteepened slopes can also cause landslides, due to natural erosion or human development
Mitigation efforts often involve diverting and draining water from prone areas
Tarps and plastic sheeting are used to prevent water infiltration into slopes
Drains and shallow wells monitor water content in active landslides
Slope reinforcement techniques include bolting, shockcrete, buttressing, and terracing
Bolting secures loose rock material in prone areas
Shockcrete strengthens slopes by applying reinforced concrete
Buttressing adds weight at the toe of the slope and removes weight from the head to stabilize it
Terracing creates stairstep topography for slope stabilization, but needs proper scaling
Shielding, catching, and diverting runout material can also mitigate landslides
Stretchable fencing protects pedestrians and vehicles in rockfall areas
Runout channels, diversion structures, and check dams slow debris flows and divert them
Some highways have tunnels to divert landslides over roadways

Landslide Classification & Identification

Landslides are classified by movement type (falls, slides, flows, creep) and material type (rock, debris, earth)
Common morphological features include: head scarp, hummocky surfaces, talus accumulations, toe of slope
Falls are abrupt rock movements, influenced by gravity, weathering, and water
Rotational slides exhibit slow movement along a curved rupture surface
Translational slides involve rapid movement along a plane of weakness
Flows are rapid movements with abundant water, categorized as debris flow (coarse material) or earthflow (fine material)
Creep is a very slow, downward movement caused by freeze/thaw cycles in unconsolidated material
Solifluction is a specific type of soil creep in high-latitude regions due to seasonal freezing and thawing

Landslide Parts

Landslides show displacement of material, causing a gap uphill and deposition downhill
Tilted or offset structures or natural features can indicate landslide activity
Scarps or escarpments are steep terrain formed due to movement exposing subsurface material
The main scarp marks the uphill limit of the landslide and is the visible part of the slide rupture surface
The slide rupture surface is the boundary between moving and stationary material
The flanks of the landslide mark the sides of the rupture surface
The toe of the landslide is at the end of the moving material and indicates the runout distance
Rotational landslides often create a large, disturbed mound of material at the toe
Extensional cracks, sag ponds, hummocky terrain, and pressure ridges can be features of landslides

Landslide Examples

Gros Ventre, Wyoming (1925): 38 million cubic meters of sandstone slid, damming the Gros Ventre River, creating Slide Lake.
- Factors: Heavy rains and snowmelt saturated the sandstone, river undercut the slope, poorly drained soil
- Parallel bedding planes between sandstone and shale contributed to instability due to low friction
Madison Canyon, Montana (1959): Magnitude 7.5 earthquake triggered a rock avalanche damming the Madison River, forming Quake Lake
- Foliation planes of weakness in metamorphic rock parallel to the surface reduced shear strength
Mount Saint Helens, Washington (1980): Magnitude 5.1 earthquake triggered the largest landslide in recorded history followed by a volcanic eruption and lahars
La Conchita, California (1995 and 2005): Earthflows and debris flows in the same location caused damage and fatalities
- Triggers: Heavy rainfall, rising groundwater levels, saturated soil
Oso Landslide, Washington (2014): 18 million tons of material moved, killing 43 people in the Steelhead Haven community
- Unusually wet winter conditions contributed to the landslide in an area previously prone to smaller events
Yosemite National Park Rock Falls: Steep cliffs cause frequent rock falls due to tectonic fractures, exfoliation, and frost wedging
- Runout modeling helps assess the risk these falls pose to park visitors

Landslides in Utah

Markagunt Gravity Slide (21-22 million years ago): One of the largest landslides in geologic history, displacing 1,700 cubic km of material
- Evidence: Breccia conglomerates, pseudotachylytes, slip surfaces, dikes
- Likely triggered by an eruption-related earthquake releasing material from a growing laccolith
Thistle Slide (1983): Slow-moving landslide blocked Spanish Fork Canyon, causing flooding and submerging the town of Thistle
- Required expensive flood control measures and infrastructure relocation
Rockville Rock Fall (2013): 2,700 ton block of conglomerate fell from a cliff, destroying a house and killing two people
- Demonstrates the potential hazards of rock falls in steep areas### Mass Wasting
Downhill movement of rock and soil due to gravity
Occurs when a slope's angle is too steep for stability
Regolith (loose rock and soil) is usually what moves

Slope Stability

Angle of slope and shear strength of materials are key factors
Steep slopes have a higher risk of mass wasting

Triggers

Changes that might destabilize slopes
Common triggers include:
- Rapid snow melt
- Intense rainfall
- Earthquakes
- Volcanic eruptions
- Storm waves
- Stream erosion
- Human activities
Excessive precipitation is the most frequent trigger

Mass Wasting Events

Classified by their type of movement and the material involved
Share common surface features
Common types:
- Rockfalls
- Slides
- Flows
- Creep
Movement can range from slow to very rapid

Susceptibility to Mass Wasting

Areas with steep topography and frequent rainfall are at higher risk:
- California Coast
- Rocky Mountain Region
- Pacific Northwest

Examples of Mass Wasting Events

North Salt Lake Slide (2014)
- Rotational landslide that destroyed a home and damaged tennis courts
- Likely caused by a combination of:
  - Sands and gravels overlying clay
  - Recent regrading of the slope
- Heavy rains triggered the event
Bingham Canyon Copper Mine Landslide (2013)
- 65 million cubic meters of mine wall slid into the pit
- One of the largest non-volcanic landslides in history
- Radar systems warned of movement, preventing major loss of life
Other Notable Events
- 1996 - Residents in a subdivision near North Salt Lake started noticing damage to their homes. By 2012, 18 homes had become uninhabitable and were removed
- Gros Ventre Landslide (1925) - Large rockfall in Wyoming that created a lake
- Hebgen Lake Earthquake (1959) - Landslides were triggered by a magnitude 7.5 earthquake
- La Conchita Landslides (1995 and 2005) - Landslides in California that killed people
- Oso Landslide (2014) - Landslide in Washington that killed 43 people
- Markagunt Gravity Slide - Ancient landslide in Utah
- Thistle Landslide (1983) - Landslide in Utah that dammed the Spanish Fork River
- Rockville Rockfall (2013) - Rockfall in Utah that destroyed a house

Lessons Learned

Studying historical events helps scientists predict future events
Researchers gain knowledge about locations, potential impact, and ways to protect people.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the concepts of slope stability and mass wasting in this quiz. Understand the role of shear and normal forces, and learn about the factors affecting shear strength and the angle of repose. Test your knowledge on how various materials and water content impact slope stability.