15 - 1 Source: NRCan Fraser River canyon, BC MASS...

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ERTH2404/14 Mass movement 1 MASS M%&EMENT Instructor: Claire Samson, Ph.D., P.Eng. Source: NRCan Fraser River canyon, BC
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ERTH2404/14 Mass movement 2 Reading assignment • Please read Kehew’s book to complement the material presented in this lecture: Chap. 13 in 3 rd Edition Chap. 12 in 2 nd Edition
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ERTH2404/14 Mass movement 3 Lecture objectives • To understand the various causes of slope movement • To be able to describe the characteristics of the different types of mass movements • To reinforce the theoretical concepts presented by the study of case histories
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ERTH2404/14 Mass movement 4 Lecture objectives • To be aware of several mitigation strategies against mass movement
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ERTH2404/14 Mass movement 5 Contents • Causes of slope movement • Classification of mass movements – Falls and topples – Slides • Lateral spreads – Flows – Complex events
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ERTH2404/14 Mass movement 6 Contents • Case histories – Lateral spreads • Rissa, Norway (1978) • Lemieux, ON (1993) – Rock avalanche • Frank slide, AB (1903) • Mitigation
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ERTH2404/14 Mass movement 7 Causes of slope movement
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ERTH2404/14 Mass movement 8 Basic definition Mass movement : collective name for a variety of processes for the downslope movement of earth materials under the direct influence of gravity – Bedrock usually remain fixed in place but might move downslope if discontinuities occur – Soil particles are held poorly together and are more susceptible to downslope movement
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ERTH2404/14 Mass movement 9 Safety Factor (S.F.) • Quantitative measure of slope stability – Resisting forces tend to resist failure – Driving forces tend to cause movement ! Resisting forces ! Driving forces S.F. = When S.F. A 1 slope is stable
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ERTH2404/14 Mass movement 10 ! ! 6 sin ! 6 cos ! Driving force: downslope component (W sin " ) Normal force: force pressing surfaces together (W cos " ) Resisting force: b (W cos " ) F = µ ( 6 c o s ! ) b : static friction coefficient " : slope angle W: weight m: mass g: gravity acceleration 6 = m g
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ERTH2404/14 Mass movement 11 Causes of slope movement • Slope characteristics • Lithology with low static friction coefficient • Excess pore water
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ERTH2404/14 Mass movement 12 Slope characteristics Steepening of slope decreases stability Road cuts, erosion, etc. Less stable More stable &ery unstable
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ERTH2404/14 Mass movement 13 Slope characteristics • Discontinuities dipping in the slope direction decrease stability Less stable More stable
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ERTH2404/14 Mass movement 14 Slope characteristics • Removal of lateral support decreases stability Ref.: Kehew, A.E. 1995. Fig. 12-31. Shownwithpermission.
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ERTH2404/14 Mass movement 15 Friction • Static friction coefficient b [ ]: constant proportional to the force restricting the movement of an oBject that is stationary on a relatively smooth, hard surface b # 1 b = 0.33 ±rick on moist clay b = 0.5 ±rick on dry clay b = 0.6 Granite on granite b = 0.75 Limestone on limestone
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ERTH2404/14 Mass movement 16 Friction • Decreasing b – Decreases the resisting force – Decreases staBility 0.00 25.00 50.00 75.00 100.00 0 30 60 90 Slope angle [degree] Force [N] Driving force Resisting force (fric=0.75) Resisting force (fric=0.6) Resisting force (fric=0.5) Resisting force (fric=0.33)
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15 - 1 Source: NRCan Fraser River canyon, BC MASS...

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