12. Mass Wasting Classifications (Definitions, processes and controlling factors) Examples (Appling knowledge of processes) Prevention of Mass Wasting (limiting and eliminating) I will give some more detail for the objective of the chapter I indicate the questions to focus on

Terms and Concepts to Learn 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?

Classification of Mass Wasting

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

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

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

Slump/Earth- Flow Zones of Depletion Zone of Accumulation Crown Left Flank 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 Right Flank Head Foot Kehew, Fig. 12-22

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

Talus Slopes The result of Rock Avalanches 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

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

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

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

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

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

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

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

Effective Stress and Strength P P+DP Before Rainfall After 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

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

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

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

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

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