Transcript mass movement_final

MASS
MOVEMENTS
IMPORTANT CONCEPT:
ROLE OF GRAVITY
Gravity causes the downward and
outward movement of landslides
and the collapse of subsiding
ground. Eventually it will flatten
all slopes.
The force of gravity is the mass of a
body x the sine of the slope.
If the initial resistance to motion is
removed, the body will move.
Earthquake and heavy rain could
give initial energy.
IMPORTANT CONCEPT:
CREEP
Movement down slope of soil and uppermost bedrock
Most commonly seen by
its effects on
telegraph poles,
fences and trees.
The soil zone slips in ultra
slow movement as
particles shift in
response to gravity.
Block diagram showing the effects of creep.
IMPORTANT CONCEPT: CREEP
How creep works
Surface materials expand
perpendicular to slope (1 to 2) as
a result of freezing, wetting or
heat of sun.
Upon thawing,drying or cooling they
contract (2 to 3) under pull of
gravity vertically.
The surface materials do not go back
to original position. Thus have a
slow movement down slope i.e.
creep
IMPORTANT CONCEPT: LANDSLIDE
Fast moving mass-movement
Causes most fatalities.
Landslide is a mass whose
center has moved
downwards and
outwards.
Has a tear-away zone
upslope where material
has pulled away and a
pile-up zone where
material had
accumulated.
IMPORTANT CONCEPT: LANDSLIDE
Major topographic features
Features include crown, head scarp, basal surface of rupture,
transverse cracks, transverse ridges and radial cracks. All created in
the downward and outward movement
IMPORTANT CONCEPT: EXTERNAL
PROCESSES CAUSING FAILURE
On arcuate failure surfaces have balance between the driving mass and the
resisting mass. Changing either can create a landslide
Processes include: 1) steepen slope, 2) remove support from bottom of slope,
and 3) add mass high up on slope.
IMPORTANT CONCEPT:
INTERNAL CAUSES OF SLOPE FAILURE
Clay forms during chemical weathering due to acidic fluids such as
water, CO2 charged water and organic acids decomposing
minerals created at high pressures and temperatures.
Clay has totally different internal structure. Clay minerals are built
like books and have many unfilled atomic positions in the crystal
structure.
Typically, clay can have their strength dramatically reduced by
adding water which also causes expansion.
IMPORTANT CONCEPT: INTERNAL CAUSES
OF SLOPE FAILURE
Quick Clay:
Ontario, Canada
1993
Fine grained rock flour left behind during the retreat of the glaciers and
deposited in a nearby sea. The clay and silt particles are loosely
packed and held together as a rock by sea salts.
When the sea retreats, the sediments are uplifted and the glue
removed by fresh water. Anything can cause the house of cards to
collapse
IMPORTANT CONCEPT: INTERNAL CAUSES OF SLOPE
FAILURE (The five roles of water)
1)
2)
3)
4)
5)
Sediments have high porosities. When these void spaces are filled
with water the sediment is much heavier and the driving mass
increased.
Water is easily absorbed and attached externally to clay minerals
with a major decrease in strength.
Water flowing through rocks can dissolve the minerals that bind
the rocks together. The removal of the cement makes the rock
easier to move or a slope easier to collapse.
Water can physically erode loose material creating caverns.
Pressure builds up in water trapped in the pores of sediments
being buried deeper and deeper. Sediments can compress but
water does not compress. Get abnormally high pore-water
pressures which “jacks up” the sediment and makes it very easy
to move.
IMPORTANT CONCEPT: INTERNAL CAUSES
OF SLOPE FAILURE (The role of flowing water)
Schematic cross section of ground water flowing through
poorly consolidated rock. The water will carry sediments
to the stream creating a series of caverns that seriously
weaken a hill.
IMPORTANT CONCEPT: INTERNAL
CAUSES OF SLOPE FAILURE (Slope stability)
Addressed by use of Coulomb/Terazaghi equation where
s = c + (p - hw) tan ø
Where s = resistance due to shear, c = the cohesion of the sediment layer
p = load of sediment and water above a slide surface
hw = weight of water above the potential surface.
F = internal angle of friction.
Strength comes from cohesion + the weight of the sediment.
Weakness from the pore water pressure and the internaL angle of friction.
Clays have high cohesion but a very low failure angle.
Sands have poor cohesion.
Granites have very high failure angle.
IMPORTANT CONCEPT:INTERNAL CAUSES OF
SLOPE FAILURE (Quick sand)
Example of the Coulomb-Terazaghi equation.
The pore water pressure hw equals the weight of the sands p.
Leaves cohesionless sand with no shear stress.
With no shear stress you will sink into the sand when you walk on it.
IMPORTANT CONCEPT: ADVERSE
STRUCTURES
1) Ancient slip surfaces are weaknesses that tend to be reused over time.
These surfaces are especially slippery when wet.
2) The orientation of the sediment layers can create strong or weak conditions.
Sediment layers dipping into the hill are very stable, dipping in the same
direction but shallower than the slope have daylight bedding. Potentially
dangerous condition.
3) Rocks have inherent weakness that set-up slope failure. Lack of cement, clay
layers, soft rocks, splitting joints, faulting surfaces.
TRIGGERS
Basic causes bring slopes close to failure:
Rain, earthquakes or humans
MASS MOVEMENTS
CLASSIFICATION (Speed of movement and water flow)
On left have mass movement speed versus moisture content.
On right have rates of travel for mass movements
MASS MOVEMENTS
CLASSIFICATION (Falls, Flows, Slides and Subsides)
Falls and subsides involve
vertical drops. Slides
and flows involve
downward and
outward motion.
Sliding involves a
coherent mass.
Flowing involves the
moving mass
behaving like a
viscous fluid.
Fall
Creep
Avalanche
Slide
Flow
Slump
Most common
mass wasting
types
Avalanche
Creep
Flow
Slump
Rockslide
Slumps
Movement of coherent block of material along a curved surface.
More likely to occur when slope is undercut from below, material is
saturated.
Common on river banks where stream erosion oversteepens banks.
Debris flows, earth flows, mud flows
Flows are movements in which the material deforms chaotically as it
moves.
Debris flows -- contains large boulders, gravels
Earth flows -- sandy material
Mudflow -- mud with considerable water
Example of slump
Deposits of Mass Wasting
Mass wasting produces sediment deposits.
These deposits commonly contain a wide
range of sizes of particles (they are
"unsorted").
The deposits do not show layering (they
are "unstratified")
How do we prevent landslides and mass
wasting? This is NOT possible.
A better question is “how do we lessen the
effects” of mass wasting?
1. Remove weight from slope
2. Engineering controls
3. Vegetation and GeofabricTM
4. Cables and anchoring systems
5. Tunnels built over highways thus reducing
weight