CHAPTER 4
MASS MOVEMENT
AND
AVALANCHE
Natural systems are noted for their attempt to seek steady state and
dynamic equilibrium
Gravity is an equalizing force that attempts to create this
equilibrium
--- massive amounts of material are moved down slope by the
force of gravity - a process we term mass movement
(or mass wasting – down slope movement of earth material
under the effects of gravity)
Aside
Mass movement / mass wasting ultimately serves a very important
function in that it transports material from the area of parent
material to locations where it is then subject to erosional forces
--- the vast percentage of this material is moved slowly, and for
the short-term – imperceptibility
… so to us, with very few exceptions, our hill slope
landscapes are static; stable
… what we “see” or “perceive” may lead to the conclusion
that Earth materials on slope surfaces remain static until
disturbed by natural events or man-induced change
--- this is not true, landscapes are constantly forced by both
internal and external that change them… esp. slope regions
--- Ex: When disasters occur in connection with landslides,
it is usually because of interference by humans with
such natural processes (Ebert. p 29 )
With limited exceptions, change across the span of human
perception and the human time scale is extremely slow
- The down slope movement of Earth material may take forms
ranging from slow internal creeping to catastrophic ground
failures [ex: gravity can move slope materials in excess of 100 mph]
--- many actions taken by Man either delay or accelerate
these processes
--- in addition to rendering Earth materials unstable in response
to natural forces, there are may situations where shear
stress [disproportionate force on one section of slope material absent in
other portions] in such materials is increased by human
activities
--- including: removal of vegetative cover, excavations,
dumping of mine materials, and by changing the hydrology
of an area
Many variable factors are involved in producing the physical state
at which a particular type of Earth material becomes instable
- The trigger mechanisms, the materials and specific sequences
vary according to case
- As in fault systems, mass movement is a factor of stress
--- in mass movement, stress is the relationship existing between
friction and gravitational pull
Slope because it is an element in both friction and gravity it must
then be a significant factor in rendering a mass unstable
--- its particular effect cannot be predicted as it is affected by
the physical properties of the mass
--- some form of failure will take place if the resisting forces
(friction and shear strength) within the material become
less than the driving force (gravity)
[shear strength defined through: (1) internal frictional forces; (2) cohesion]
[ratio between these opposing forces can be used to assess stability of materials]
Classification of Massing Wasting Processes
(1) Was movement fast or slow?
(2) What type of material was moved?
(3) What kind of motion took place?
[abstraction of Fig 9-15 and Table 9.2]
Slow: greater cumulative effect on hillslope
a. creep
b. solifluction
Fast: rapid; often spectacular; easily seen and understood
a. slide
b. fall
c. slump and earthflow
The Role of Gravity and Friction: Basic Mechanics of Creep
- Gravity induces materials to move via creep – the slowest, most
extensive form of slope failure and mass movement and can
involve whole mountainsides
--- creep is almost imperceptible; most commonly seen by effect
on fences, telephone poles, stone walls, deformed trees, etc
- What happens in creep? [Fig. 9.4 and 9.5, p. 236]
--- soil particles slip and slide past each other in response to:
(a) gravity and slope
(b) expansion and contraction of soil
Expansion and contraction of soil occurs in a couple of ways
(1) soil has porosity (void spaces between particles) which may
fill with water and subsequently expand (up to 9%)
[this can be spectacular when the water freezes to ice / frost]
(2) soils with significant clay content will expand when water
infiltrates and “swells” the clay
(3) hearing by the sun will increase soil volume
Conversely, soil will contract when it: dries, cools, thaws
Causes of Slope Failure
Most fatalities are caused by fast-moving mass movement
- commonly we give these fast mass movements the name
landslides
- typical landslide is a mass movement whose center has
moved downward and outward resulting in an upslope
tearaway zone [material has pulled away] and a downslope
pileup zone [gravity-moved material has accumulated]
- movement is downslope to an angle of repose where gravity
will move material no further
- slope stability dictates mass movement and thus the creation of
“tearaway” and “pileup” zone materials
Slope stability depends on both internal and external factors
External Causes
External processes that increase th odds of slope failure include:
(1) steepening the slope
(2) removing support
(3) additional materials high on the slope
Internal Causes
Long-term processes taking place below the surface of the Earth
… weakening processes
Internal causes of slope failure include:
(1) water
--- weight
--- interplay with clay materials
--- decreasing the cohesion of rocks
--- subsurface erosion
--- pressure in pores of rocks and sediments
(2) inherently weak materials
(3) deceasing cohesion
(4) adverse geologic structures
--- ancient slip surfaces
--- rock layering dipping less than slopes
--- structures within the rock and contact between soft and
hard rock
Classification of Mass Movements
Major types of Earth failures (Fig 9-16)
(1)
Falls (rockfalls) – occur when elevated rock mass separates along joints
and “come tumbling down”; where forces sufficiently weaken or reduce
underlying rock or where fissures and/or bedding planes result in
weakened zones
(2)
Slides (landslide, rock slide, debris slide) – a massive failure over a large
body of material - the most massive are in association with earthquakes;
tend to be extremely destructive and “media events”; a significant
modifier of the physical landscape
(3)
Flows (loess flows, debris flows, mudflows) – mass movements that behave
like fluids; range of particle size is enormous [from silt-to-boulders]
Snow Avalanches
A number of the principles discussed for earth materials also relate
to the development of avalanches
… primary difference is that snow may undergo rapid physical
change and can accumulate or diminish within a short
period of time
Four requisites for a snow avalanche
(1) accumulation of a critical mass of snow – what is a “critical
mass” is a function of:
characteristics of the snow
snow pack conditions
ground conditions
weather conditions
(2) structure change in the snow mass that reduces stability
(3) adequate slope to initiate gravity-induced flow (slope angle)
no consensus on how much is enough (15o to 50o)
a function of snow characteristics
(4) a triggering mechanism
a mechanical disturbance
frequently the fall of the leeward-facing snow cornice
Interesting
It is estimated that in the European Alps, 80% of all avalanche
victims are skiers
In Scandinavia studies indicate that permanent local populations
are at the most risk
The worst year for avalanche in Norway was 1679 when up to 500
people died… 2nd highest known was 1755 when 200 were
presumed to have died
(Gunnar Ramsli, Un. Of Oslo)
[reading his work makes a strong case that the mortality caused (though very
devastating short-term) is not the most significant aspect of avalanche]
Response to Threat
Response must be proactive
- practice avoidance
--- keep population out of susceptible areas
… possible? practical?
- maintain monitoring of snow depth and water volume
characteristics
… modify as necessary
- structurally practice snow control / threat diversion
(1) construction windward of the rupture zone
(2) construction in the rupture zone
(3) construction in the avalanche track
(4) construction in the deposition area
No.s 1 and 2 are intended to prevent the formation of avalanches
No.s 3 and 4 are intended to stop snow mass before it reaches the
threatened area, or to deflect them or carry them over the
exposed objects