Presentation on theme: "Pg. 31 1. Mass wasting plays an important rollMass wasting in eroding the Earth's surface Diagrammatic sketch of a landslide by David J. Varnes (1978)"— Presentation transcript:
pg. 31 1. Mass wasting plays an important rollMass wasting in eroding the Earth's surface Diagrammatic sketch of a landslide by David J. Varnes (1978) Scarp
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 cliffFall b. Slides - mass of material remainsSlides coherent and moves along a well-defined surface
Pg. 31 b. Slides - mass of material remains coherent (mostly) and moves along a well- defined (pretty much) surfaceSlides 1. Surface parallel to slope = translation side 2. Surface curved = rotation = slumpslump c. Flow - mass of material moves as a viscous fluidFlow C.) Rate of motion fast or slow.
SLOWFAST MOVEMENTLess than 1 cm/yr 1 mm/day to 1 km/hr 1 to 5 km /hr Greater than 5 km/hr FLOWCreepEarthflowMudflow (water saturated Debris Avalanche Rock Avalanche (wet or dry) SLIDEDebris Slide Rockslide Debris Slide Rock Slide FALLDebris Fall RockFall 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 ofResisting forces material on slope, type of material C. Slope stability (SF) = resisting force driving force SF > 1 stable, SF< 1 unstable, SF = 1 balance
FnFn W FrFr FnFn FdFd FdFd Fr=Resisting Force (sticking, weight of material, etc.) W=Weight of material (constant) F d = Force due to weight in the direction of failure (driving force, increases with increasing slope) F n = force due to weight into the slope/land (decrease with increasing slope) *as slope increases F d gets larger and over comes F r W FrFr Pg. 32
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 reposeWater B little water OK C too much water reduces cohesion by removing cements-lubricant HOW DOES WATER INCREASE or DECREASE F r ?
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 F d ) b.) water can remove materials – piping-cave formation c.) increases pressure between pores H 2 O comes in and increases pressure Pressure pushes particles apart Weakens material Flat platey clay particle
Flat platey Clay Particle O (-) HH (+) O (-) HH (+) O (-) HH (+) - to +Attract Flat platey Clay Particle -to - Repel Pg. 37d.) ADD The charge on water pushes negative clays apart
Pg. 32 B. Type of material and features a. strength of material; 1.) mudstone vs. granite (particles) (interlocking crystals) 2.) poorly cemented etc.poorly cemented b. features of material; angle of bedding orangle of bedding fractures, ancient faults, ancient slide surfaces ALL can be surface of weakness especially if inclined parallel to the ground surface- daylighted beddingdaylighted 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?roots hold
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 WEAKNESS- parallel to surface slopePLANES OF WEAKNESS- A) BEDDING-rock layer alignment B) FOLIATION-mineral alignment
Pg. 33 4. UNDERCUTTING SLOPEUNDERCUTTING SLOPE Another link And another A) STREAM B) SHORELINE C) ROAD CUT
Pg. 33 7. Types of mass wasting A. Rockfall – Yosemite 1996Yosemite B. Slump - moves along curved planeSlump Ensenada 1976 B(oops). Rockslide – slow to rapid slide of bedrockRockslide Pt. Fermin 1929-block slide Gros Ventre 1925 ND pg. 251/fig. 9.27 C. Debris flow – may move down a channelDebris flow fluid like behaviour dry to sloppy wet up to 175 mph Turtle Mountains 1903
Pg. 34 D.Earth flowEarth flow low gradient hillsides moist-saturated slow motion – Portuguese Bend 1950sPortuguese Bend 1950s ND pg. 254 Fig. 9.31 E. Creep – very slowCreep How it works F. Fluidized Rock Flows Highly fluid and low viscosity Travel long ways sturzstroms-longrunout debris flows Nevados Huascaran 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 collapsesinkhole How it happens- MechanicsMechanics
Pg. 34 10. Triggers of mass wasting events A. Earthquake B. Remove support/modify slope a. Roadcut 1.) steepening slopesteepening slope 2.) undercuttingundercutting 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-slideQUAKE LAKE A) 1959 B) Earthquake induced C) Foliation/bedding parallels slope
Pg. 35 2) TURTLE MOUNTAIN/FRANK SLIDETURTLE 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
Pg. 35 3) GROS VENTURE-slideGROS VENTURE 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
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
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
CASE HISTORIES Pg. 35 6) Wasatch FrontWasatch Front A) 1970s B) fire-how decrease slope stability? C) built debris basin- can’t stop so hopefully can divert 7.) ADD Wastach FrontWastach Front a.) Cedar Hills near American Fork b.) Recativated slip surface c.) debris flow
Pg. 36 MITIGATION OF MASS WASTING 1. MAP AREAS -MAP AREAS SLOPE STABILITY LANDSLIDE/FALL POTENTIAL 2. ZONE AGAINST BUILDING 3. REDUCE SLOPE ANGLE OR AMOUNT OF MATERIAL ON SLOPE Remember F r, W, etc. all those variables? Unload the driving force Over/up load the resisting force Terrace the slope
4. DRAIN FLUIDS -not too much thoughDRAIN FLUIDS 5. RETAINING WALLSRETAINING WALLS 6.CATCH BASINS -try to route material into specified areas. 7.) Use bolts to “stitch” unstable ground together and to solid/stable layers deeperbolts stitch Images courtesy of California Coastal Commission 8.) Re-vegitation 9.) Use large nets catch-netscatch-nets-stop/redirect flows and falls stabilization nets Pg. 36 MITIGATION OF MASS WASTING
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?