pg. 31 1. Mass wasting plays an important roll
in eroding the Earth's surface
Scarp
Diagrammatic sketch of a landslide by David J. Varnes (1978)
Pg.31
Mass wasting events can be geologic hazards
*normal geologic processes that affect human
life or property
A. $1.5 billion damage per year ~ 25 fatalities
B. Predictability is poor to good
C. Mitigation - numerous methods depending on
the hazard
Mass wasting - downslope movement of rock and
unconsolidated material in response to gravity
4. Types of mass wasting are classified by 3 criteria
ND pg. 245 fig. 9.16
A. type of material - solid rock or unconsolidated
material (soil-dirt)
B. type of movement
a. Fall - material free falls down a cliff
b. Slides - mass of material remains
coherent and moves along a
well-defined surface
Pg. 31 b. Slides - mass of material
remains coherent (mostly) and moves along a welldefined (pretty much) surface
1. Surface parallel to slope =
translation side
2. Surface curved = rotation = slump
c. Flow - mass of material moves as a viscous
fluid
C.) Rate of motion fast or slow.
SLOW
FAST
MOVEMENT
Less than 1 mm/day to
1 cm/yr
1 km/hr
1 to
5 km /hr
Greater than 5
km/hr
FLOW
Creep
Earthflow
Mudflow
(water
saturated
Debris
Avalanche
Rock
Avalanche
(wet or dry)
Debris Slide
Rockslide
Debris Slide
Rock Slide
Debris Slide
Rock Slide
SLIDE
FALL
Debris Fall
RockFall
LANDSLIDE
LANDSLIDE
LANDSLIDE
5.Mass Wasting is the relationship between resisting
forces and the driving force.
Pg. 32A. Driving force is gravity - the downslope
component of weight of slope material
including anything on it.
B. Resisting forces - strength and cohesion of
material on slope, type of material
C. Slope stability (SF) = resisting force
driving force
SF > 1 stable, SF< 1 unstable, SF = 1 balance
Pg. 32
Fr
Fr
Fd
Fn
Fn
W
Fd
W
Fr=Resisting Force (sticking, weight of material, etc.)
W=Weight of material (constant)
Fd= Force due to weight in the direction of failure
(driving force, increases with increasing slope)
Fn= force due to weight into the slope/land
(decrease with increasing slope)
*as slope increases Fd gets larger and over comes Fr
pg. 32 D.) A slope will become unstable if reduce
resisting force and/or increase the
driving force
HOW DO WE DO THAT???
GENERAL IDEA
ND pg. 238 Fig. 9.7
6. Factors that affect resisting force and driving force
A Water –angle of repose
B little water OK
C too much water reduces cohesion
by removing cements-lubricant
HOW DOES WATER INCREASE or DECREASE Fr?
pg.32D.) clay minerals are weakened by water:
may absorb water and spread grains,
or absorb water and expand
a.) water also adds weight – increase driving
force downslope (bigger Fd)
b.) water can remove materials – piping-cave formation
c.) increases pressure between pores
H2O comes in and
increases pressure
Flat platey clay particle
Pressure pushes particles apart
Weakens material
Pg. 37d.) ADD The charge on water pushes
negative clays apart
Flat platey Clay Particle
H
(+)
O
H
H
(-)
(+)
O
(-)
-to - Repel
H
H
(+)
O
H
- to +Attract
(-)
Flat platey Clay Particle
Pg. 32
B. Type of material and features
a. strength of material;
1.) mudstone
vs. granite
(particles)
(interlocking crystals)
2.) poorly cemented etc.
b. features of material; angle of bedding or
fractures, ancient faults, ancient slide surfaces
ALL can be surface of weakness especially if inclined
parallel to the ground surface- daylighted bedding
ND pg. 242 Fig. 9.13
Pg. 33 C. Angle of slope/topography
a. steep or vertical vs. flat
b. Over steepened slope
D. Climate – precipitation – sudden rains
E. Vegetation - roots hold material how
change Fr?
Pg. 33 SLOPE STABILITY INDICATORS
1. HISTORY OF LANDSLIDES
2.SOIL TYPE - SILT, CLAY, VOLCANIC ASH
This stuff is “slippery when wet”.
3. ORIENTATION OF PLANES OF WEAKNESSparallel to surface slope
A) BEDDING-rock layer alignment
B) FOLIATION-mineral alignment
Pg. 33 4. UNDERCUTTING SLOPE
Another link
And another
A) STREAM
B) SHORELINE
C) ROAD CUT
Pg. 33 7. Types of mass wasting
A. Rockfall – Yosemite 1996
B. Slump - moves along curved plane
Ensenada 1976
B(oops). Rockslide – slow to rapid slide of bedrock
Pt. Fermin 1929-block slide
Gros Ventre 1925 ND pg. 251/fig. 9.27
C. Debris flow – may move down a channel
fluid like behaviour
dry to sloppy wet
up to 175 mph
Turtle Mountains 1903
Pg. 34
D.Earth flow
low gradient hillsides
moist-saturated
slow motion – Portuguese Bend 1950s
ND pg. 254 Fig. 9.31
E. Creep – very slow
How it works
F. Fluidized Rock Flows
Highly fluid and low viscosity
Travel long ways
sturzstroms-longrunout debris flows
Nevados Huascaran 1962, no perceptible trigger
ND pg. 259 fig. 9.37
Pg. 34
8.) How can humans make a hillside more
vulnerable to mass wasting?
How can these hazards be mitigated?
9. Subsidence- settling of land changing slope.
A.) Slow – oil or groundwater withdrawl
B.) Catastrophic – sinkhole collapse
How it happens- Mechanics
Pg. 34
10. Triggers of mass wasting events
A. Earthquake
B. Remove support/modify slope
a. Roadcut
1.) steepening slope
2.) undercutting
b. stream undercut slope
c. ocean undercut cliff
ND pg. 238 Fig. 9.7 again
d. devegetate by fire
C. Sudden and heavy rain-recall what type
of material this especially influences?
Mass Wasting CASE HISTORIES
Pg. 35
1) QUAKE LAKE-slide
A) 1959
B) Earthquake induced
C) Foliation/bedding parallels slope
Pg. 35
CASE HISTORIES
2) TURTLE MOUNTAIN/FRANK SLIDE
A) 1903
B) joints/cracks parallel to slope
C) mining weaken toe?
D) 90 million tons
E) raced 2 mi. across valley
F) 400ft up other side
G) buried south end of town
H) killed 70
I) Sturzstrom -long runout debris flow
sturz=“fall” strom= “stream or storm”
viscosity?
ND pg. 256 fig. 9.33
Pg. 35
CASE HISTORIES
3) GROS VENTURE-slide
A) 1925
B) Sed rocks dip parallel to slope
daylight bedding
C) water and clay
D) Blocked Gros Ventre River
E) 3 weeks it was 60meters deep
F) May 1927 lots of rain lake rose
G) Town of Kelly evacuatedsome died when dam failed
ND pg. 251 Fig. 9.27
CASE HISTORIES
Pg. 35
4) Portuguese Bend, CA-earthflow
A)
B)
C)
D)
bentonite clay and water
rocks dip seaward
ocean undercuts toe
ancient slide surface - reactivated in 1950s
ND pg. 253 Fig. 9.30
Pg. 35
CASE HISTORIES
5) Pt. Fermin, CA-blockslide
A) 1929
B) bedding dip seaward-daylighted bedding
C) toe undercut by ocean waves
D) clay layer
E) ½ mile long block of land
ND pg. 248 Fig. 9.22
And
ND pg. 249 Fig 9.23
Pg. 35
CASE HISTORIES
6) Wasatch Front
A) 1970s
B) fire-how decrease slope stability?
C) built debris basin- can’t stop so
hopefully can divert
7.) ADD Wastach Front
a.) Cedar Hills near American Fork
b.) Recativated slip surface
c.) debris flow
Pg. 36
MITIGATION OF MASS WASTING
1. MAP AREAS SLOPE STABILITY
LANDSLIDE/FALL POTENTIAL
2. ZONE AGAINST BUILDING
3. REDUCE SLOPE ANGLE
OR AMOUNT OF MATERIAL ON SLOPE
Remember Fr, W, etc. all those variables?
Unload the driving force
Over/up load the resisting force
Terrace the slope
Pg. 36
MITIGATION OF MASS WASTING
4. DRAIN FLUIDS -not too much though
5. RETAINING WALLS
6. CATCH BASINS -try to route material into
specified areas.
7.) Use bolts to “stitch” unstable ground together
and to solid/stable layers deeper
Images courtesy of California Coastal Commission
8.) Re-vegitation
9.) Use large nets
catch-nets-stop/redirect flows and falls
stabilization nets
ADD Can we predict mass wasting?
Like most natural disasters….
KINDA
1.) Measure pore pressure- as pore pressure increases???
2.) Measure movement of land-extensiometers
3.) Precipitation- how does this effect pore pressure??
4.) Geophone-measure “rumbling” of movement similar
to small earthquake -enough time?