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MASS WASTING

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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MASS WASTING

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  1. 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)

  2. 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

  3. 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

  4. 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.

  5. MASS WASTING

  6. 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

  7. 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

  8. 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

  9. 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?

  10. 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

  11. 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

  12. 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

  13. 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?

  14. 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

  15. MASS WASTING Pg. 33 4. UNDERCUTTING SLOPE Another link And another A) STREAM B) SHORELINE C) ROAD CUT

  16. 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

  17. 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

  18. 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

  19. 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?

  20. MASS WASTING Mass Wasting CASE HISTORIES Pg. 35 1) QUAKE LAKE-slide A) 1959 B) Earthquake induced C) Foliation/bedding parallels slope

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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

  27. 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

  28. MASS WASTING ADD Can we predict mass wasting? Like most natural disasters….

  29. 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?

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