Mass Wasting: Geomorphic Impacts and Mechanics

1. Geomorphology Chapter 4: Mass Wasting

2. Chapter 4: Mass Wasting • Geomorphic context • Types • Mechanics • Causes • Effects on Landscape

3. Chapter 4: Mass Wasting • Geomorphic context • Most discussion of mass wasting in geology • pertains to landslides, mud flows etc. as “geohazards” or • engineering problems threatening humans or their structures. • We are more interested in the role mass • wasting has played in shaping the landscape over geologic • time scales.

4. La Conchita landslide, Anaheim, California,1995

5. Wasting a landscape

8. Chapter 4: Mass Wasting • Geomorphic context • Most discussion of mass wasting in geology • pertains to landslides, mud flows etc. as “geohazards” or • engineering problems threatening humans or their structures. • We are more interested in the role mass • wasting has played in shaping the landscape over geologic • time scales. Relates to the issue of scale-age

11. Chapter 4: Mass Wasting • Types

15. Chapter 4: Mass Wasting • Examples of different types

18. Lost River, WV

19. Chapter 4: Mass Wasting • Mechanics

20. Driving / Shearing Force W θ W cos θ W sin θ Driving Force (FD = FP) = Shear Force = W sin θ

21. W θ W cos θ W sin θ Force providing Shearing Resistance Force Normal to slope (FN) = Wcos θ Resisting Force (FR) = Shear Strength * Area = [ c + σN tan Φ ] * A = [ c + σN tan Φ ] * A = [ c + ( Wcos θ / A) tan Φ ] * A = c A + Wcos θ tan Φ

22. W θ W cos θ W sin θ Force providing Shearing Resistance Resisting Force (FR) = c A + Wcos θ tan Φ c= cohesion Φ = angle of internal friction

23. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ Block will slide when FR <=FD FR /FD = factor of safety = F F < 1 means failure likely F > 1 means slope is stable

24. Chapter 4: Mass Wasting • Causes • Given knowledge of mechanics, what are causes • or triggers of mass wasting? F = FR / FD F = [c A + Wcos θ tan Φ] / W sin θ

25. F = FR / FD F = [c A + Wcos θ tan Φ] / W sin θ • Effects of weight? • Effect of slope? • Effects of cohesion? • Effects of angle of internal friction? • How do different types of geologic materials affect the above? • How do environmental conditions affect the above?

26. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ Numerical Example 1: Rock slope of massive granite Area = 10,000 square feet Slope = 40 degrees Weight = 1,000 tons = 2,000,000 lbs Cohesion = 725,000 lbs/square foot Φ = 40 degrees

27. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ Numerical Example 1: Rock slope of massive granite Area = 10,000 square feet Height = 100 ft, Unit weight = 150 lbs/ft3 Slope (θ) = 40 degrees Weight = 75,000 tons = 150,000,000 lbs Cohesion = 8,500 lbs/square foot* Φ = 35 degrees* FD = (150 x 106 lbs) sin (40 deg) = 9.6 x 107 lbs FR = (8.5 x 103 psf)(1 x 104 ft2) + (150 x 106 lbs) cos (40 deg) tan (35 deg) = (8.5 x 107 + 8.1 x 107 ) lbs = 16.6 x 107 lbs *Hoek and Bray, 1981)

28. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ FR /FD = factor of safety = F FR /FD = (16.6 x 107 lbs) / (9.6 x 107 lbs) F = 1.7 F > 1 means slope is stable

29. Consider the effects of water • Not a lubrication effect • A pressure effect F = [c A + (Wcos θ – pA) tan Φ] / W sin θ

30. W θ W cos θ W sin θ Pore pressure, p, acts against normal component of weight, does not change downslope component FD = W sin θ FR = c A + Wcos θ tan Φ pA Block will slide when FR <=FD FR /FD = factor of safety = F F < 1 means failure likely F > 1 means slope is stable

31. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ Numerical Example 1 with water: Average height of water table above potential slide plane = 50 ft p = 50 ft x 64.3 lbs/ft3 = 3215 lbs/ft2 pA = 3215 lbs/ft2 x 10,000 ft2 = 3.2 x 107 lbs

32. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ pA Numerical Example 1: Rock slope of massive granite Area = 10,000 square feet Height = 100 ft, Unit weight = 150 lbs/ft3 Slope (θ) = 40 degrees Weight = 75,000 tons = 150,000,000 lbs Cohesion = 8,500 lbs/square foot* Φ = 35 degrees* FD = (150 x 106 lbs) sin (40 deg) = 9.6 x 107 lbs FR = (8.5 x 103 psf)(1 x 104 ft2) + [ (150 x 106 lbs) cos (40 deg) – - 3.2 x 107] tan (35 deg) FR = 14.4 x 107 *Hoek and Bray, 1981)

33. W θ W cos θ W sin θ FD = W sin θ FR = c A + Wcos θ tan Φ pA FR /FD = factor of safety = F FR /FD = (14.4 x 107 lbs) / (9.6 x 107 lbs) F = 1.5 (12% decrease compared to dry slope) F > 1 means slope is stable

37. Chapter 4: Mass Wasting • Effects on Landscape

40. Appalachian highlands are mantled by colluvial deposits that • preserve late Pleistocene and Holocene history (USGS, 2002). • Debris flows may be responsible for as much as 50% of total • physical denudation within some of the drainage basins in the • Appalachians (USGS, 2002). • For example, in the Blue Ridge study of these deposits is helping • understand some of the following: • soil development • extreme climate variation over last 35,000 years • periglacial processes • rates of landscape denudation • potential modern debris flows and their triggers

42. Style of mass wasting/sediment movement in Blue Ridge differs from that in Ridge & Valley. In Blue Ridge foothills, thick fans and flows of coarse alluvium/colluvium accumulate over weak and soluble materials of Valley and Ridge (Clark, 1989). Foothills composed of Quaternary fans, flows Blue Ridge Catoctin Fm. (basalts) Valley & Ridge Sediments / meta- sediments, esp. quartzites of Erwin Fm. weak / soluble rocks esp. carbonates , form sediment trap for colluvium

43. Style of mass wasting/sediment movement in Blue Ridge differs from that in Ridge & Valley.

44. In Valley and Ridge, accumulations of coarse material at base of ridges are thin or lacking because Massanuten Fm, a hard quartz arenite, is not a source for large clasts and limestones of valley don’t form sediment traps here (Clark, 1989).

46. Differences in geology between Ohio and Mississippi Valleys?