Landslides
Introduction | The Origins of Landslides
| Classification | Measuring
and Monitoring | Prevention And Mitigation
Of Landslides | Notable Slides |Links
- 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
- 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
- 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
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Development
of a rotational landslide in homogeneous material.
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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.
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Translational
landslides begin in bedrock, such as siltstone, which
is dipping in the same direction as the slope topography.
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| 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
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TYPE OF MATERIAL
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TYPE
OF MOVEMENT
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BEDROCK
|
ENGINEERING
SOILS
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Predominantly coarse
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Predominantly
fine
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Rock fall
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Debris fall
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Earth fall
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Falls
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Rock topple
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Debris topple
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Earth topple
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Topples
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Rock slump
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Debris slump
|
Earth slump
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Slides
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rotational
|
few
units
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Rock block slide
Rock slide
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Debris block slide
Debris slide
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Earth block slide
Earth slide
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translational
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many
units
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Rock spread
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Debris spread
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Earth spread
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Lateral
Spreads
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Rock flow
(deep creep)
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Debris flow
(soil creep)
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Earth flow
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Flows
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Combination of two or more principal types of movement
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Complex
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- 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
- 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.
- 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
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