12 Mass Wasting A Classifications Definitions processes and

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

Terms and Concepts to Learn n Definitions n n n Flow, Slide, Fall Controlling Factors n n Gravity Water Shear strength n n Examples n n Mt. St. Helens Yosemite Rock fall/slide Gros Ventre Slide St. Francis Dam, Calif. n 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?

Classification of Mass Wasting

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

Creep n n Imperceptibly slow flow Expansion - contraction n n Heating – Cooling Freeze – Thaw Fig. 9. 6

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

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

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

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

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

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

Gros Vantre Slide n n n 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

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

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

Shear Force vs. Shear Strength n Resisting Forces n n 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

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

Effective Stress and Strength s. T P P+DP After Rainfall se = s. T – (P+DP) Before Rainfall n se = s. T – P s. T: 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

Stability Analysis for Rotational Slides n 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

Use Knowledge of Mass Wasting to Avoid Risks n Be able to recognize geologically unstable situations

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

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

Reduce Risks n n Increase shear strength with iron rods and anchors Remove risk