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Nancy Van Wagoner, Acadia University 1 Mass Wasting Nancy A. Van Wagoner Acadia University.

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Presentation on theme: "Nancy Van Wagoner, Acadia University 1 Mass Wasting Nancy A. Van Wagoner Acadia University."— Presentation transcript:

1 Nancy Van Wagoner, Acadia University 1 Mass Wasting Nancy A. Van Wagoner Acadia University

2 Nancy Van Wagoner, Acadia University 2 Mass Wasting Defined u The down slope movement of material under the force of gravity

3 Nancy Van Wagoner, Acadia University 3 Agenda u Factors controlling mass wasting u Factors that can change slope stability u Types of mass wasting u Examples u Prediction and mitigation

4 Nancy Van Wagoner, Acadia University 4 Overview u mass wasting occurs throughout the world u The total global property damage from landslides in a single year equals that caused by earthquakes in 20 years.

5 Nancy Van Wagoner, Acadia University 5 Factors that control mass wasting u Steepness of the slope u Orientation of the rock layers u Strength and cohesion of materials u Pore water u Factors acting to change slope stability

6 Nancy Van Wagoner, Acadia University 6 Steepness of the slope u figure attached u factor of safety u example

7 f d n W Angle of the slope f = friction n = normal force (component of W perpendicular to slope) d = driving force (component of W parallel to slope) W = weight of the block = mass x gravity) Resisting Force = R = shear strength = internal resistance to movement = f x n Factor of Safety = R/d = F.S. Most building codes require F.S.>1.5 Problem: calculate d for 1000 kg block for slope angles of 60 degrees and 30 degrees Steepness of Slope

8 Orientation of rock layers (figure 10-20) u u dip slope vs rocks dip perpendicular to the slope DIP SLOPE = Rock layers are dipping or inclined in the same direction as the slope. VERY UNSTABLE Dip of rocks is perpendicular to the slope = better, more stable

9 Nancy Van Wagoner, Acadia University 9 Strength and cohesion of materials u strength = ability of material to resist deformation u cohesion = ability of particles to stick together u examples –unweathered granite vs poorly indurated sedimentary rock

10 Nancy Van Wagoner, Acadia University 10 Pore water u angle of repose = maximum slope or steepness at which loose material remains stable u The angle of repose for dry sand is about 35 degrees u Damp sand achieves slopes up to 90 degrees u Wet sand (saturated) has little strength, and almost no slope

11 Pore water (continued) u u why the difference in the slopes? – –water is a polar molecule, able to attract grains of sand by surface tension and hold them together if the pore water pressure is less than zero – –If the pore water pressure exceeds zero, the pressure exerted by the water, floats the particles away from each other

12 Nancy Van Wagoner, Acadia University 12 Factors acting to change slope stability (Triggering Events) u Change in the abundance of pore water u Earthquakes u Slope modification and undercutting u Volcanic eruptions

13 Nancy Van Wagoner, Acadia University 13 Change in abundance of pore water u increase pore water pressure –decrease cohesion of particles –increases the weight of the slope u ways of changing the amount of pore water –rain –housing development F water lawn F build swimming pool F septic field –change in groundwater level

14 Nancy Van Wagoner, Acadia University 14 Water (continued) u Liquefaction - the transformation of material to a liquid-like mass –results from a increase in water content F may be associated with ground shaking u Expansive clays (shrink-swell soils)

15 Nancy Van Wagoner, Acadia University 15 Earthquakes and other shocks u can lead to liquefaction u earthquakes frequently generate landslides –Grand Banks under water slide (turbidity current) –Peru 1970

16 Nancy Van Wagoner, Acadia University 16 Slope modification and undercutting u u road construction u u natural processes – –streams – –waves

17 Nancy Van Wagoner, Acadia University 17 Volcanic eruptions u u deposit unconsolidated debris rapidly u u may be associated with melting of glacier and/or rain

18 Nancy Van Wagoner, Acadia University 18 Types of Mass Wasting u Can occur slowly or rapidly –imperceptible (cm/yr) to rapid (400 km/hr) u 3 main types (figure 10-23) –Flow –Slide –Fall

19 Nancy Van Wagoner, Acadia University 19 Flows - 3 types u Unconsolidated material moves as a viscous fluid –creep (slow) –solifluction (slow) –debris flows, earthflows, mudflows (fast)

20 Nancy Van Wagoner, Acadia University 20 u Creep (figure to 10-27) –about 1 cm/yr –results from alternate expansion and contraction of surface materials due to F freezing and thawing F wetting and drying –may notice bent rock layers, tree trunks curved at the base, tilted fence posts or grave stones

21 Nancy Van Wagoner, Acadia University 21 u u Solifluction (fig ) – –important in frigid zones F F high latitude or high elevation when ground has a layer of permafrost F F summer or spring – –upper layers of permafrost may melt saturating the upper surface – –water can’t percolate downward because of ice below F F earth flows slowly downward on icy layers below, even on gentle slopes

22 Nancy Van Wagoner, Acadia University 22 u u Debris flow, Mudflow, Earth Flow, Lahar – –fluid motion of water saturated debris – –occur everywhere including F F semi-arid environments F F volcanoes – –viscous, able to float cars – –fast, up to 100 km/hr

23 Nancy Van Wagoner, Acadia University 23   Semi-arid flows (continued) – –infrequent/high precipitation rainstorms – –loose debris – –little vegetation – –easily saturated by rain – –acquires consistency of concrete, – –moves down slope

24 Nancy Van Wagoner, Acadia University 24 F F volcanic mudflows (Lahars) – –instant deposition of ash – –usually associated with rain and/or melting ice caps – –example: Nevado del Ruiz

25 Nancy Van Wagoner, Acadia University 25 Landslides - two types u slump u rock slide, or rock avalanche

26 u u Slump – –downward slipping of a mass of rock or unconsolidated material, moving as a unit, along a curved surface – –see figure 10-23, note F F slump block F F surface of fracture F F slump scarp

27 Nancy Van Wagoner, Acadia University 27 Slump example u Portuguese Bend-Abalone Cove Landslide u Southern California, coastal community u area = 10’s of square miles u very fancy homes, million $ range

28 Nancy Van Wagoner, Acadia University 28 Portuguese Bend geology u shale, dipping seaward overlain by poorly consolidated Portuguese Tuff u area of natural sliding, undercut and oversteepened by waves

29 Nancy Van Wagoner, Acadia University 29 Portuguese Bend urbanization u accelerated slumping u over steepened slopes-terraces, road cuts u added weight and water to slope –swimming pools –irrigation –sewage u Result = ground slumps along plane of weakness F 150 homes destroyed u Remediation = pumping excess groundwater

30 Nancy Van Wagoner, Acadia University 30 u Rock slide, rock avalanche- blocks of bedrock break loose and move down slope –fastest and most destructive type of mass movement u common conditions –steep slopes –dip slope u common triggers –earthquake –excessive rain and/or melting snow F lubricates and adds weight to the slope

31 Nancy Van Wagoner, Acadia University 31 Rock slide, avalanche examples u Gros Ventre landslide, Wyoming (fig ) u Frank Alberta (p. 268)

32 Nancy Van Wagoner, Acadia University 32 Gros Ventre u conditions –slope = degrees F river cut through sandstone, removing toe of the slope –Spring 1925 F heavy rains and melting snow –increase weight, increase pulling force –increase pore water pressure, decrease cohesion –lubricate sandstone/slay contact u result –38 million cubic metres of debris gave way –descended a vertical distance of 600 m –rose up 100 m on the opposite side to come to rest in the river bed, damming the river

33 Nancy Van Wagoner, Acadia University 33 Frank Alberta u show diagram


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