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12. Mass Wasting

12. Mass Wasting. Classifications (Definitions, processes and controlling factors) Examples (Appling knowledge of processes) Prevention of Mass Wasting (limiting and eliminating). Definitions Flow, Slide, Fall Controlling Factors Gravity Water Shear strength Examples

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12. Mass Wasting

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  1. 12. Mass Wasting Classifications (Definitions, processes and controlling factors) Examples (Appling knowledge of processes) Prevention of Mass Wasting (limiting and eliminating)

  2. Definitions Flow, Slide, Fall Controlling Factors Gravity Water Shear strength Examples Mt. St. Helens Yosemite Rock fall/slide Gros Ventre Slide St. Francis Dam, Calif. Be able to distinguish a flow from a slide. How do controlling factors increase or reduce tendency of mass wasting? How did the controlling factors lead to these events? How would you use your knowledge of the controlling factors to reduce risk? Terms and Concepts to Learn

  3. Classification of Mass Wasting

  4. Classification Material Velocity Creep Debris Imperceptibly Slow Earth Flow Debris Slope and Material Dependent <5 km/hr Mudflow Saturated Debris Avalanche Debris or Rock Very Fast 100 km/hr Rotational Slide Debris Slow-mod. (short) Rock Slide Bedrock Fast Debris Fall Debris Fast Rockfall Bedrock Fast Classification of Mass Wasting Flow Type of Movement Slide Fall

  5. Creep • Imperceptibly slow flow • Expansion - contraction • Heating – Cooling • Freeze – Thaw Fig. 9.6

  6. Debris (soil) both slides and flows Rotational Sliding Zone of Depletion Surface of Rupture Flow Zone of Accumulation Surface of Separation Earth Flow andRotational Slide

  7. Zones of Depletion Crown Main/Minor Scarps Head Transverse Cracks Longitudinal Fault Zone Zone of Accumulation Transverse Ridges and cracks Radial cracks Foot/Tip/Toe Crown Slump/Earth- Flow Left Flank Right Flank Head Foot Kehew, Fig. 12-22

  8. Bedrock may slide and/or fall Weathering reduces bedrock strength Chemical Mechanical Eventually gravity wins Earthquake triggered Rock Slide and Fall

  9. The result of Mechanical weathering Rock falls and slides Crushing and abrasion (more mechanical weathering) Rock Avalanches Slopes of rock fragments may let go and careen downhill as a very fast flow Talus Slopes

  10. E.g. Unavoidable Rock Fall • The Old Man of the Mountain, Cannon Mtn. NH

  11. Mt. St. Helens • Landslide triggers eruption • Reduced shear strength from earthquakes and bulging • Increased shear force as bulge grows and slopes steepen • Eruption causes Mudflows

  12. Gros Vantre Slide • Sandstone and debris on Impermeable shale • Saturation of sandstone and lubrication of shale • Both reduced shear strength (added to shear force) • Shear force overcomes shear strength • Sandstone and debris slide

  13. Shear Force vs. Shear Strength • Driving Forces i.e., Shear Force • Fs = W sin q Fs: Shear force W: Weight q : Dip of slope

  14. Shear Force vs. Shear Strength • Resisting Forces • Friction and Cohesion of Soil or Rock • Ff = m (W cos q) m: coefficient of static friction

  15. Shear Force vs. Shear Strength • Resisting Forces i.e., Shear Strength • Friction and Cohesion of Soil or Rock • S = C + se tan f S: Strength C: Cohesion se: Effective Stress f : Angle of internal friction

  16. Safety Factor S resisting forces S Driving Forces If S.F.>1 then failure occurs S.F.=

  17. Effective Stress and Strength sT sT Before RainfallAfter Rainfall • se = sT – P se = sT – (P+DP) sT: Total Stress P: Fluid pressure of ground water (or soil water) se: Effective stress (stress supported by the soil skeleton) Note: fluid pressure is negative (less than atmospheric) if unsaturated and becomes positive when saturated P P+DP

  18. Stability Analysis for Rotational Slides • S.F. = SLR / WX S: Shear Strength L: Langth of potential surface of rupture R: Moment Arm W: Weight of Block X: Length of gravity moment arm X R C.G. S L

  19. Use Knowledge of Mass Wasting to Avoid Risks • Be able to recognize geologically unstable situations

  20. Understanding Mass Wasting Development causes: • Increased shear force • Steepened slope • Added weight • Decreased shear strength • Devegetation • Reworking of fill • Saturation of soil

  21. Reduce Risks Some solutions include: • Increase shear strength • Re-compact soils • Re-vegetate soil slopes • Construct retaining wall with anchors • Prevent Saturation • Prohibit over-irrigation • Install surface drains • Install subsurface drains

  22. Reduce Risks • Increase shear strength with iron rods and anchors • Remove risk

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