Transcript Crosby/Krieger - Idaho State University

Mass Movement Types:
Falls and Topples
Rockfall near Wenchuan, China.
A massive earthquake hit the area
on May 12, 2008. This event closed
the entrance and exit to the region
on May 23, 2008.
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Photograph by Chua Chin Hon,
Straits Times, Singapore.)
Semester Schedule
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28-Oct
31-Oct
4-Nov
7-Nov
9-Nov
11-Nov
14-Nov
18-Nov
21-Nov
Thinning
25-Nov
28-Nov
2-Dec
5-Dec
Creep Processes
No Class
Style/Activity/Distribution
Type: Slides
FIELD TRIP
Type: Flow
Type: Topple and Rockfall
Causes: Earthquakes and Overloading
Causes: Water, Slope Angle and Glacier
Hazard Assessment
Landslide Risk
Landslide Risk
(Away?)
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For each category we will discuss
Definition
 Occurrence
 Relative Size/Range
 Velocity of Travel
 Triggering Mechanism
 Effects (direct/indirect)
 Mitigation Measures
 Predictability

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Much of this information comes from: Highland, 2008
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Guthrie, 2012
Classification Schemes
Definitions
Anti-dip Slope
Or
Anaclinal Slope
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Dip Slope
Or
Cataclinal Slope
Photo: Doug Sherman, Pbase, Banff National Park
Guthrie, 2012
Falls
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USGS, 2004
Yosemite Movie
 Induced rockfall
 Roadcut rockfall

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Highland, 2008
Modes of Transport
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Talus Cones formed by Rockfall
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Talus Cones formed by Rockfall
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Falls Defined:
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
Falls are abrupt, downward movements
of rock or earth, or both, that detach
from steep slopes or cliffs. Little or no
shear displacement (e.g. joints/fissures)

The falling material usually strikes the
lower slope at angles less than the angle
of fall, causing bouncing.

The falling mass may break on impact,
may begin rolling on steeper slopes, and
may continue until the terrain flattens.
Falls

Triggering mechanism
◦ Undercutting of slope by natural processes such
as streams and rivers or differential weathering
(such as the freeze/thaw cycle), human activities
such as excavation during road building and (or)
maintenance, and earthquake shaking or other
intense vibration.

Effects (direct/indirect)
◦ Falling material can be life-threatening. Falls can
damage property beneath the fall-line of large
rocks. Boulders can bounce or roll great distances
and damage structures or kill people. Damage to
roads and railroads is particularly high: Rockfalls
can cause deaths in vehicles hit by rocks and can
block highways and railroads.
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Far-traveled rock in Switzerland
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http://www.oobgolf.com/content/fore+play/
Falls

Occurrence and relative size/range
◦ Common worldwide on steep or vertical
slopes—also in coastal areas, and along rocky
banks of rivers and streams. The volume of
material in a fall can vary substantially, from
individual rocks or clumps of soil to massive
blocks thousands of cubic meters in size.

Velocity of travel
◦ Very rapid to extremely rapid, free-fall;
bouncing and rolling of detached soil, rock,
and boulders. The rolling velocity depends on
slope steepness.
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Falls

Video of net test
Corrective measures/mitigation
◦ Rock curtains or other slope covers,
protective covers over roadways, retaining
walls to prevent rolling or bouncing,
explosive blasting of hazardous target areas
to remove the source, removal of rocks or
other materials from highways and railroads
can be used. Rock bolts or other similar types
of anchoring used to stabilize cliffs, as well as
scaling, can lessen the hazard. Warning signs
are recommended in hazardous areas for
awareness. Stopping or parking under
hazardous cliffs should be warned against.
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Falls

Predictability
◦ Mapping of hazardous rockfall areas has been
completed in a few areas around the world.
Rock-bounce calculations and estimation
methods for delineating the perimeter of
rockall zones have also been determined and
the information widely published. Indicators
of imminent rockfall include terrain with
overhanging rock or fractured or jointed rock
along steep slopes, particularly in areas
subject to frequent freeze-thaw cycles. Also,
cut faces in gravel pits may be particularly
subject to falls. Figures 3 and 4 show a
schematic and an image of rockfall.
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Guthrie, 2012
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Guthrie, 2012
Topples
Topples
Topples: movements of rock, debris or
earth masses by forward rotation
about a pivot point.
 Movie1
 Movie1.1
 Movie2

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Topple
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Hungr, 2013
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Exponential acceleration of separation
Topple
Describable using rigid body
mechanics, but materials often break
during movement, violating modeling
assumptions
 Hard to know block geometries

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Physical Experimentation
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Adhikary et al, 1997
Topple

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A topple is recognized as the forward
rotation out of a slope of a mass of soil or
rock around a point or axis below the
center of gravity of the displaced mass.
Toppling is sometimes driven by gravity
exerted by the weight of material upslope
from the displaced mass. Sometimes
toppling is due to water or ice in cracks
in the mass. Topples can consist of rock,
debris (coarse material), or earth
materials (fine-grained material).
Topples can be complex and composite.
Two types…
Topple
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Rock block topple

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Forward rotation and overturning of
rock columns or plates (one or many),
separated by steeply dipping joints.
The rock is relatively massive and
rotation occurs on well-defined basal
discontinuities. Movement may begin
slowly, but the last stage of failure is
extremely rapid.
Rock flexural topple

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Bending and forward rotation of a rock
mass characterized by very closely
spaced, steeply dipping joints or
schistose partings. The rock is relatively
weak and fissile. There are no welldefined basal discontinuities that could
allow for rotation of blocks. The
movement is generally slow and tends to
self-stabilize. However, secondary
rotational sliding may develop in the
hinge zone of the toppling.
Amini, 2012
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Topple

Occurrence
◦ Known to occur globally, often prevalent
in columnar-jointed volcanic terrain, as
well as along stream and river courses
where the banks are steep.

Velocity of travel
◦ Extremely slow to extremely rapid,
sometimes accelerating throughout the
movement depending on distance of
travel.
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Topple

Triggering mechanism
◦ Sometimes driven by gravity exerted by
material located upslope from the
displaced mass and sometimes by water
or ice occurring in cracks within the mass;
also, vibration, undercutting, differential
weathering, excavation, or stream erosion.

Effects (direct/indirect)
◦ Can be extremely destructive, especially
when failure is sudden and (or) the
velocity is rapid.
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Topple

Corrective measures/mitigation
◦ In rock there are many options for the stabilization of
topple-prone areas. Some examples for reinforcement
of these slopes include rock bolts and mechanical and
other types of anchors. Seepage is also a contributing
factor to rock instability, and drainage should be
considered and addressed as a corrective means.

Predictability
◦ Not generally mapped for susceptibility; some
inventory of occurrence exists for certain areas.
Monitoring of topple-prone areas is useful; for
example, the use of tiltmeters. Tiltmeters are used to
record changes in slope inclination near cracks and
areas of greatest vertical movements. Warning
systems based on movement measured by tiltmeters
could be effective. Figures 5 and 6 show a schematic
and an image of topple.
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