# Mass Wasting Nancy A. Van Wagoner Acadia University.

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Mass Wasting Nancy A. Van Wagoner Acadia University

Mass Wasting Defined The down slope movement of material under the force of gravity

Agenda Factors controlling mass wasting
Factors that can change slope stability Types of mass wasting Examples Prediction and mitigation

Overview mass wasting occurs throughout the world
The total global property damage from landslides in a single year equals that caused by earthquakes in 20 years.

Factors that control mass wasting
Steepness of the slope Orientation of the rock layers Strength and cohesion of materials Pore water Factors acting to change slope stability

Steepness of the slope figure attached factor of safety example

Steepness of Slope f d n W
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 n 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) W

Orientation of rock layers (figure 10-20)
dip slope vs rocks dip perpendicular to the slope Show overhead 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

Strength and cohesion of materials
strength = ability of material to resist deformation cohesion = ability of particles to stick together examples unweathered granite vs poorly indurated sedimentary rock

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

Pore water (continued)
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

Factors acting to change slope stability (Triggering Events)
Change in the abundance of pore water Earthquakes Slope modification and undercutting Volcanic eruptions

Change in abundance of pore water
increase pore water pressure decrease cohesion of particles increases the weight of the slope ways of changing the amount of pore water rain housing development water lawn build swimming pool septic field change in groundwater level

Water (continued) Liquefaction - the transformation of material to a liquid-like mass results from a increase in water content may be associated with ground shaking Expansive clays (shrink-swell soils)

Earthquakes and other shocks
can lead to liquefaction earthquakes frequently generate landslides Grand Banks under water slide (turbidity current) Peru 1970 Grand Banks, Newfoundland: 1929 Earthquake generated a turbidity current (submarine landslide) The turbidity current broke underwater telephone cables Based on the succession of breaks, determined turbidity current was: at least 150 km wide traveled at least 470 km from source max vel = 93 km/hr Peru, 1970 Earthquake triggered debris avalanche that roared 3.5 km down the side of Mt. Huascaran. speed = up to 400 km/hr destroyed towns of Yungay and Ranrahica Killed 20,000 people Eyewitness account from a geophysicist: from Murck, et al., 1996 (attached)

Slope modification and undercutting
road construction natural processes streams waves

Volcanic eruptions deposit unconsolidated debris rapidly
may be associated with melting of glacier and/or rain

Types of Mass Wasting Can occur slowly or rapidly
imperceptible (cm/yr) to rapid (400 km/hr) 3 main types (figure 10-23) Flow Slide Fall

Flows - 3 types Unconsolidated material moves as a viscous fluid
creep (slow) solifluction (slow) debris flows, earthflows, mudflows (fast)

Creep (figure 10-24 to 10-27) about 1 cm/yr
results from alternate expansion and contraction of surface materials due to freezing and thawing wetting and drying may notice bent rock layers, tree trunks curved at the base, tilted fence posts or grave stones

Solifluction (fig. 10-29) important in frigid zones
high latitude or high elevation when ground has a layer of permafrost summer or spring upper layers of permafrost may melt saturating the upper surface water can’t percolate downward because of ice below earth flows slowly downward on icy layers below, even on gentle slopes

Debris flow, Mudflow, Earth Flow, Lahar
fluid motion of water saturated debris occur everywhere including semi-arid environments volcanoes viscous, able to float cars fast, up to 100 km/hr

Semi-arid flows (continued)
infrequent/high precipitation rainstorms loose debris little vegetation easily saturated by rain acquires consistency of concrete, moves down slope

volcanic mudflows (Lahars)
instant deposition of ash usually associated with rain and/or melting ice caps example: Nevado del Ruiz

Landslides - two types slump rock slide, or rock avalanche

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

Slump example Portuguese Bend-Abalone Cove Landslide
Southern California, coastal community area = 10’s of square miles very fancy homes, million \$ range

Portuguese Bend geology
shale, dipping seaward overlain by poorly consolidated Portuguese Tuff area of natural sliding, undercut and oversteepened by waves

Portuguese Bend urbanization
accelerated slumping over steepened slopes-terraces, road cuts added weight and water to slope swimming pools irrigation sewage Result = ground slumps along plane of weakness 150 homes destroyed Remediation = pumping excess groundwater Figure B2.1, p.165 add also overhead drawing

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

Rock slide, avalanche examples
Gros Ventre landslide, Wyoming (fig ) Frank Alberta (p. 268)

Gros Ventre conditions result slope = 15-20 degrees Spring 1925
river cut through sandstone, removing toe of the slope Spring 1925 heavy rains and melting snow increase weight, increase pulling force increase pore water pressure, decrease cohesion lubricate sandstone/slay contact 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 show figure 10-31

Frank Alberta show diagram