Introduction

• What is Mass Wasting?
  • The downslope movement of rock, regolith, and soil under the direct influence of gravity.
  • The step that follows weathering.
  • The combined effects of mass wasting and erosion by running water produce stream valleys.
• Location, Location, Location
  • Occur in every state and U.S. territory.
  • Nearly all occur in mountainous regions
  • Weak or fractured materials + steep slope = landslides.
• Flatland Problems
  
  • Flat areas may not be as stable as they appear because
    
    • Expansive soils
      
    • Peat bog areas
      
    • Loose sediment
      
    • Uncompacted fill
      
    • Quick clays
      
    • Limestone caverns
      
    • Active faults
      
    • Subsiding areas
      
  • 2. Flat land problems are amplified on hillsides
    

The Origins of Landslides

• The Origins of Landslides:
  • Gravity
  • Other Factors:
    • Water
    • Steep Slopes
    • Vegetation
    • Geology
    • Earthquakes
• Water
• Triggered when water builds up in the slope
  • heavy rains
  • snowmelt
  • poor drainage.
• When water builds up in a slope several things can occur.
  • First, water > the weight and < the strength of the material in the slope.
  • Pore pressure in the slope >, and clay minerals become hydrated and expand.
  • Minerals holding individual grains together may dissolve.
• Weather comes in cycles with dry periods of 15 years and wet of 12 years
  
  • Deluges of 0.5" in 5 or 10 minutes or 3 - 6"/day
    
  • The greater the storm frequency, intensity, and duration the greater the hillside damage
    
  • Hills become pocked with soil failures
    
• Construction / new developments
• Drainage systems needed for some slopes
• Impermeable surfaces can cause excess runoff.
• Steep Slopes
  • Slopes fail occur when gravitational forces exceed the strength of the rock or soil which comprise the slope.
• Occurs when
  • Slope is over steepened
  • A heavy load is placed at the top of the slope
  • Removing material at the base of the slope.
• Vegetation
• The type of vegetation present (or not) on a slope can affect slope stability.
• Plants with deep roots hold bedrock and loose materials together.
• Shallow-rooted plants provide stability for soils and other surficial deposits.
• Slope stability can be affected if vegetation is removed.
• Reseeding or replanting slopes of questionable stability is often recommended.
• Geology
• The geology of a region greatly affects slope stability.
• Adverse Geological Structure
  
  • Bedding surfaces of weak rocks dip downslope and are unsupported at the lower end
    
  • Similar problems with faults, joints, foliation etc
    
• Weak Rocks
  • Many Southern California rocks are rich in clay or silt
    
    • Montmorillonite swells in water & disintegrates
      
    • Clay may cause instability on a 5º slope
      
  • Interbedding of sandstone and clay is a common problem
    
  • Serpentine, the state rock, causes many landslides
    
• Earthquakes
• Earthquakes, or any other type of strong vibrations, can trigger landsides on slopes which are already unstable.
• In general, only earthquakes that are a magnitude 4.0 or greater will be strong enough to cause a slide.
• Human caused Landslides
  
  • Cut slopes produce removal of hill support and failure occurs
    
    • Failure is inevitable if structure is right
      
    • Open cracks may be an early sign that failure is coming
      
  • Uncontrolled fills may settle, erode, slough away or slip in mass
    
    • Uncontrolled fills are indicated by old soil and a lack of benches
      

Introduction | The Origins of Landslides | Classification | Measuring and Monitoring | Prevention And Mitigation Of Landslides | Notable Slides |Links | top | Geol 357 Lecture Home

Classification

• Classified by
  • Type
  • Material
  • Rate of movement
• Type
  • Slide: movement parallel to planes of weakness and slope.
    • Rotational landslides
      • Occur in homogeneous material
      • Curved slide plane
    • Translational landslide
      • Typically occurs along a planar surface
      • Common in S. California in Miocene-aged Formations

      Development of a rotational landslide in homogeneous material.

      The material fails, creating a curved slideplane. The gap at the top of the slide is called the scarp, and the excess material at the base of the slide is called the toe.

      Translational landslides begin in bedrock, such as siltstone, which is dipping in the same direction as the slope topography.

      Erosion occurs at the baseof the hillside, either by a stream or even by a roadcut. The bed no longer has anything to support it at its base, and begins to slide downslope.

• Creep: gradual movement of slope materials
• Slump: complex movement of materials on a slope
• Topple: the end-over-end motion of rock down a slope.
• Fall: material free falls
• Flow: viscous to fluid-like motion of debris.
• Torrent: a sporadic and sudden channelized discharge of water and debris
• Material
• Debris, mud, earth, rock
  
  Table 1 below (modified from Varnes, 1978) summarizes one way of naming slope-failure deposits and their associated slope failure type
  

  TYPE OF MATERIAL			TYPE OF MOVEMENT
  BEDROCK	ENGINEERING SOILS
  	Predominantly coarse	Predominantly fine
  Rock fall	Debris fall	Earth fall	Falls
  Rock topple	Debris topple	Earth topple	Topples
  Rock slump	Debris slump	Earth slump	Slides	rotational	few units
  Rock block slide Rock slide	Debris block slide Debris slide	Earth block slide Earth slide		translational	many units
  Rock spread	Debris spread	Earth spread	Lateral Spreads
  Rock flow (deep creep)	Debris flow (soil creep)	Earth flow	Flows
  Combination of two or more principal types of movement			Complex

• Rate of Movement
  • Fast
    • Slumps
    • Rockslides
    • Debris flows
    • Earthflows
  • Slow
    • Creep
    • Solifluction
      • the gradual flow of a saturated layer that is underlain by an impermeable zone

Introduction | The Origins of Landslides | Classification | Measuring and Monitoring | Prevention And Mitigation Of Landslides | Notable Slides |Links | top | Geol 357 Lecture Home

Measuring and Monitoring

• Important to determine if the slide is moving, and the rate of movement.
• Geologists use a variety of instruments
  
  • Pressure sensors – rainfall amounts and groundwater
    
  • Displacement sensors – movement
    
  • Geophones – ground vibration
    
  • GPS units – movement and dimensions of the slide.
    

from: http://vulcan.wr.usgs.gov/Projects/CalifLandslide/Maps/landslide_monitor.html

Testing the solar-powered radio telemetry system used for real-time monitoring. -- USGS Photo by Mark Reid, USGS/Menlo Park (http://vulcan.wr.usgs.gov/Projects/CalifLandslide/Publications/ReidLaHusen/report.html)

Measuring landslide movement using a surface extensometer. Extensometer crosses several scarps (breaks that expose the reddish soils) at the head of the landslide. -- USGS Photo by Richard LaHusen, USGS/CVO (http://vulcan.wr.usgs.gov/Projects/CalifLandslide/Publications/ReidLaHusen/report.html)

Caltrans drill rig installing subsurface extensometers (to measure landslide movement) and pore-water pressure sensors (to measure ground-water conditions) in the Mill Creek landslide. -- USGS Photo by Mark Reid, USGS/Menlo Park (http://vulcan.wr.usgs.gov/Projects/CalifLandslide/Publications/ReidLaHusen/report.html)

Introduction | The Origins of Landslides | Classification | Measuring and Monitoring | Prevention And Mitigation Of Landslides | Notable Slides |Links | top | Geol 357 Lecture Home

V. Prevention And Mitigation Of Landslides

• Reasons
  
  • Worldwide, landslides cause death and millions of $ in damage
    
  • Very little effort to mitigating this natural hazard in the U.S.
    
  • Only one region - Los Angeles, California - has state and local regulations regarding landslides and public use.
    
    • 92-97% reduction in landslide losses for new construction
      
• Preventive Medicine
  
  • Prediction is the first necessity
    
    • This is done with subsurface data by geologists
      
  • Emphasis has changed from correction to prevention
    
    • Required by law
      
    • Cheaper for the developer
      
    • Population explosion in hillside areas
      
• Property Purchase Stage
  
  • Developer should secure a geologic feasibility report before purchasing hillside property
    
    • This is generally done with successful development
      
    • Report may be requested by home buyer
      
  • Private individuals may hire geologists for property inspection
    
• Planning and Design Stages
  
  • Geologic factors should be considered in zoning, road access, storm drainage, sewage disposal, etc.
    
  • It is best to modify the natural landscape as little as possible
    
• Treatment of Existing Natural Landslide Areas
  
  • Massive grading is the most economic correction method
    
    • Driving force must be reduced
      
    • Resisting force must be increased
      
  • Buttresses or buttress fills may be required to restrain unstable areas
    
  • Proper drainage is required
    
    • Maintains the natural flow
      
    • Prevents saturation of the fill
      
• Treatment of Cut Slope Stability Problems
  
  • Dip slope problem is one of the most common
    
    • Temporary solutions include guniting the slope, deep rooted vegetation, sandbagging toe, and plastic sheeting
      
  • Remedies for cut slope instability
    
    • Flatten the slope to the dip angle
      
    • Construct a buttress with a compacted backfill
      
    • Construct a buttress fill with subdrainage
      
• Treatment of Erosion Problems
  
  • Slopes must be properly prepared and drained
    
  • Terraces and interceptor drains must protect raw cut and fill slopes
    
  • Established root systems and continuous vegetation will prevent slope erosion
    
    • Leaving vegetation in its natural state adjacent to home will reduce erosion
      
  • Basic responsibility is the homeowners
    
    • Poor drainage may produce rills and gullies as well as landslides
      
• Steps the Homeowner may take
  
  • Check that thorough geologic soil studies were made prior to development
    
  • Check that all recommendations were heeded
    
  • Check that the community has strong building and grading codes
    
  • Check that someone (developer or community) maintains drainage control devices
    
  • Don't make major land modifications without professional advice
    

Introduction | The Origins of Landslides | Classification | Measuring and Monitoring | Prevention And Mitigation Of Landslides | Notable Slides |Links | top | Geol 357 Lecture Home