Mass Movements and Hillslopes

Transcription

1 Mass Movements and Hillslopes Erosion (or lack of) results from balance between internal resistance of materials & magnitude of external forces acting on them Evolution of landscapes depends largely on regional slope development Mechanics of slope erosion are related to processes of physical weathering the forces disintegrating rocks also lower the internal strength of the unconsolidated cover

2 Resisting forces (the properties of matter that resist the stresses generated by gravitational force) Shear strength 1) overall frictional characteristic, expressed as angle of internal friction, Φ a) plane friction: grains sliding past one another on planar surfaces; varies with moisture, smoothness of plane surface, mineralogy b) interlocking friction: particles move upward and over one another (greater resistance than plane friction); varies with moisture, mineralogy, density of packing

3 2) effective normal stress, δ, acts to hold material together and to increase internal resistance to shear total normal stress: δ = δ + μ effective normal stress pore pressure pore pressure can increase or decrease δ in unsaturated zone, water molecules attached to surface particles by tension increase weight of soil (eg. wet sand) in saturated zone, water exerts hydrostatic pressure upward & supports soil 3) cohesion, c, causes increase in shear strength when grains are packed or cemented together (eg. clay)

4 Properties of material change with increasing or decreasing moisture: water added to dry soil voids fill plastic behavior more water decreases cohesion all pores filled liquid behavior Plastic refers to the way the material responds to stress (force per unit area), in terms of strain (deformation) resulting from applied force stress y plastic failure B y: yield stress (permanent deformation begins) B: breaking strength (rupture occurs) strain

5 Atterberg Limits: indicate transition from solid to plastic state, & from plastic to liquid state liquid limit plastic limit expressed as moisture contents (wt. of contained water/wt. of dry soil) Range of water contents between two limits is plasticity index Atterberg limits function of types of clay minerals (eg. limits higher for montmorillonite than kaolinite) size of particles (limits increase with smaller particles) history of wetting and drying

6 debris flow along I-70 corridor near Georgetown, triggered by rainfall

7 landslide above Horsetooth Reservoir, 8/97 Soil slips along Rt. 287 triggered by rainfall, 8/97

8 Factors influencing shear stress & resistance in slope materials 1) Factors increasing shear stress (promote failure) removal of lateral support erosion (rivers, ice, waves) human activity (quarries, road cuts, etc) addition of mass natural (rain, talus, etc) human (fills, ore stockpiles, buildings, etc) earthquakes regional tilting removal of underlying support natural (undercutting, solution, weathering ) human activity (mining) lateral pressure natural (swelling, freezing expansion, water addition)

9 Huascaran, Peru (1973 Yungay slide) Seismically triggered slides Hebgen Lake landslide, Montana

10 2) Factors decreasing shear strength (promote failure) weathering disintegration (lowers cohesion) hydration base exchange solution drying pore water buoyancy capillary tension structural changes remolding fracturing G s = resisting/driving = shear strength/shear stress G s > 1 stable

11 Three basic types of mass movements: slides: cohesive blocks of material move on a well-defined surface of sliding, with no internal shearing within the sliding block flows: move entirely by differential shearing within the transported mass no clear plane at base of moving debris; velocity decreases from the surface down heaves: disrupting forces act perpendicular to the ground surface by expansion of material facilitates downslope movement & is the forerunner of more rapid mass movements leads to seasonal or soil creep very slow movement of material due to gravity when cohesion & frictional resistance are spasmodically lowered functions in upper few feet of soil evidence includes stone lines, structures, trees caused by swelling & contracting due to wetting/drying or freezing/thawing

12 Slides arcuate soil slips along ridge crest, northern California slumps on landslide toe, southern Poland failure, crest of sand dune

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14 Flows upper channel scoured to bedrock burned slope unburned swale forest fire & resulting debris flow, Huachuca Mountains, AZ

15 lower reaches of channels & alluvial fan, Huachucas

16 Buffalo Creek, Colorado fire, debris flows, floods 1996

17 debris flow fan, Langtang, Nepal debris flow, Idaho debris flow, Khumbu, Nepal Culebres cut, Panama Canal

18 Rio Quijos, Ecuador failures on dune face

19 Falls/flows debris cone, Banff National Park, Canada debris cone, Oi River, Japan

20 Heave tree response to soil creep, northern Montana

21 Classification of Mass Movement Processes flow wet river mudflow earthflow solifluction dry landslide slide rockslide fast talus creep soil creep slow heave

22 Vajont dam overtopping, Italy, m high; 260 million m 3 failure; 2,000 casualties

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25 Mitigation of mass movement hazards attempted landslide prevention, Seattle slope stabilization, Japan debris flow monitoring site, Japan

26 Yoho National Park, Canada slope stabilization, Japan

27 Individual Grain Movements Sediment is moved on the surface of slopes by raindrop impact (splash) and by overland flow (wash) flow is shallow & spread evenly across slope as uniform sheet Amount of soil moved by splash depends on i) kinetic energy of raindrops ii) type & amount of soil exposed iii) steepness of slope particles are dislodged, detached, dispersed Sheet wash doesn t last long because natural flow irregularities concentrate flow into deeper & shallower paths variable flow depths imply irregular eroding & transporting capabilities small rills begin to develop, but they periodically shift their position so that erosion is fairly even in the long run

28 Amount of soil eroded & transported is balance: driving resisting (gravity, force of vs (vegetation, soil flowing water) shear strength) Soil strength is referred to as erodibility an estimate of the ease with which soil can be eroded I e, index of erodibility I e = shear resistance x permeability Soil loss can be estimated using empirical equations, eg: Universal Soil Loss Equation erodibility slope length cropping A = K R L S C P soil loss rainfall steepness conservation

29 slope angle (in degrees) Slope angles are not uniformly distributed, but tend to cluster in groups probably represent stability regimes for slopes formed in particular climatic & lithologic settings relative distribution of slope angles 10 0

30 Major controls on slope form and evolution are time lithology climate process Two contrasting models of slope development focus on process and time process model: slope angle is time-independent depends more on properties of slope materials & mechanics of dominant slope processes; slope angle decreases with increasing erodibility of rock regolith evolutionary model: slope angle depends on time, & decreases with time

31 Influence of lithology on slopes coherent, resistant rocks = steeper slopes more massive bedding = steeper slopes alternating weak & strong strata = irregular profile Resistance of a particular rock type varies with climate (eg. limestone), and resistance depends on whether overlying slope is controlled by a) processes of weathering (resistance of rock = rapidity with which rock is weathered) b) processes of removal (resistance = rate at which regolith is eroded)

32 Influence of climate on slopes 1) Slopes in humid temperate regions tend to be convex straight concave upper convexity due to soil creep, lower concavity to soil wash applies after mass movements produce long-term angular stability, so that creep & wash become dominant slope processes 2) Slopes in semiarid/arid regions debris slope cliff plain less vegetation and precipitation mass movements occur at higher angles creep less important than wash

33 stepped slope profiles, Grand Canyon, Arizona

34 Rt. 125, Colorado spheroidal granite weathering & rounded slopes, Missouri Canyonlands, Utah

35 Slope Development with Time 1) slope decline: steep upper slope erodes more rapidly than basal zone, flattening the overall angle, with a convexity on the upper slope & a concavity on the lower slope 2) slope replacement: steepest angle is progressively replaced by upward expansion of gentler slope developed near base; enlarges overall concavity of profile, which can be segmented or smoothly curved 3) parallel retreat: maintain constant angles on steepest part of slope slope decline slope replacement parallel retreat