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MASS WASTING 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) MASS WASTING Pg.31 Mass wasting events can be geologic hazards *normal geologic processes that affect human

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slide1

MASS WASTING

  • pg. 31 1. Mass wastingplays an important roll
  • in eroding the Earth's surface

Scarp

Diagrammatic sketch of a landslide by David J. Varnes (1978)

slide2

MASS WASTING

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

slide3

MASS WASTING

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

slide4

MASS WASTING

Pg. 31 b. Slides- mass of material remains coherent (mostly) and moves along a well- defined (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.

slide6

MASS WASTING

  • 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
slide7

MASS WASTING

Pg. 32

Fr

Fr

Fd

Fn

Fn

Fd

W

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

slide8

MASS WASTING

  • 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
slide9

MASS WASTING

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?

slide10

MASS WASTING

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

slide11

MASS WASTING

(+)

(+)

(+)

H

H

H

H

H

H

O

O

O

(-)

(-)

(-)

Pg. 37d.) ADD The charge on water pushes negative clays apart

Flat platey Clay Particle

- to +Attract

-to - Repel

Flat platey Clay Particle

slide12

MASS WASTING

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

slide13

MASS WASTING

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?

slide14

MASS WASTING

  • Pg. 33 SLOPE STABILITY INDICATORS
  • 1. HISTORY OF LANDSLIDES
  • SOIL TYPE - SILT, CLAY, VOLCANIC ASH
    • This stuff is “slippery when wet”.
  • 3. ORIENTATION OF PLANES OF WEAKNESS- parallel to surface slope
  • A) BEDDING-rock layer alignment
  • B) FOLIATION-mineral alignment
slide15

MASS WASTING

Pg. 33 4. UNDERCUTTING SLOPE

Another link

And another

A) STREAM

B) SHORELINE

C) ROAD CUT

slide16

MASS WASTING

Pg. 337. 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

slide17

MASS WASTING

  • Pg. 34
  • 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
slide18

MASS WASTING

  • 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
slide19

MASS WASTING

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?

slide20

MASS WASTING

Mass Wasting CASE HISTORIES

Pg. 35

1) QUAKE LAKE-slide

A) 1959

B) Earthquake induced

C) Foliation/bedding parallels slope

slide21

MASS WASTING

Pg. 35

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

CASE HISTORIES

slide22

MASS WASTING

Pg. 35

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 evacuated-

some died when dam failed

ND pg. 251 Fig. 9.27

CASE HISTORIES

slide23

MASS WASTING

CASE HISTORIES

Pg. 35

4) Portuguese Bend, CA-earthflow

A) bentonite clay and water

B) rocks dip seaward

C) ocean undercuts toe

D) ancient slide surface - reactivated in 1950s

ND pg. 253 Fig. 9.30

slide24

MASS WASTING

CASE HISTORIES

Pg. 35

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

slide25

MASS WASTING

Pg. 35

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

CASE HISTORIES

slide26

MASS WASTING

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

slide27

MASS WASTING

Pg. 36

MITIGATION OF MASS WASTING

  • 4. DRAIN FLUIDS -not too much though
  • 5. RETAINING WALLS
  • 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
slide28

MASS WASTING

ADD Can we predict mass wasting?

Like most natural disasters….

slide29

MASS WASTING

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?