CHAPTER 4 MASS MOVEMENT AND AVALANCHE

Natural systems are noted for their attempt to seek steady state and dynamic equilibrium Gravity is an equalizing force that attempts to create this equilibrium --- massive amounts of material are moved down slope by the force of gravity - a process we term mass movement (or mass wasting – down slope movement of earth material under the effects of gravity)

Aside Mass movement / mass wasting ultimately serves a very important function in that it transports material from the area of parent material to locations where it is then subject to erosional forces

--- the vast percentage of this material is moved slowly, and for the short-term – imperceptibility … so to us, with very few exceptions, our hill slope landscapes are static; stable … what we “see” or “perceive” may lead to the conclusion that Earth materials on slope surfaces remain static until disturbed by natural events or man-induced change --- this is not true, landscapes are constantly forced by both internal and external that change them… esp. slope regions

--- Ex: When disasters occur in connection with landslides, it is usually because of interference by humans with such natural processes (Ebert. p 29 ) With limited exceptions, change across the span of human perception and the human time scale is extremely slow - The down slope movement of Earth material may take forms ranging from slow internal creeping to catastrophic ground failures [ex: gravity can move slope materials in excess of 100 mph] --- many actions taken by Man either delay or accelerate these processes

--- in addition to rendering Earth materials unstable in response to natural forces, there are may situations where shear stress [disproportionate force on one section of slope material absent in other portions] in such materials is increased by human activities --- including: removal of vegetative cover, excavations, dumping of mine materials, and by changing the hydrology of an area

Many variable factors are involved in producing the physical state at which a particular type of Earth material becomes instable - The trigger mechanisms, the materials and specific sequences vary according to case - As in fault systems, mass movement is a factor of stress --- in mass movement, stress is the relationship existing between friction and gravitational pull

Slope because it is an element in both friction and gravity it must then be a significant factor in rendering a mass unstable --- its particular effect cannot be predicted as it is affected by the physical properties of the mass --- some form of failure will take place if the resisting forces (friction and shear strength) within the material become less than the driving force (gravity) [shear strength defined through: (1) internal frictional forces; (2) cohesion] [ratio between these opposing forces can be used to assess stability of materials]

Classification of Massing Wasting Processes (1) Was movement fast or slow? (2) What type of material was moved? (3) What kind of motion took place?

[abstraction of Fig 9-15 and Table 9.2] Slow: greater cumulative effect on hillslope a. creep b. solifluction Fast: rapid; often spectacular; easily seen and understood a. slide b. fall c. slump and earthflow

The Role of Gravity and Friction: Basic Mechanics of Creep - Gravity induces materials to move via creep – the slowest, most extensive form of slope failure and mass movement and can involve whole mountainsides --- creep is almost imperceptible; most commonly seen by effect on fences, telephone poles, stone walls, deformed trees, etc - What happens in creep? [Fig. 9.4 and 9.5, p. 236] --- soil particles slip and slide past each other in response to: (a) gravity and slope (b) expansion and contraction of soil

Expansion and contraction of soil occurs in a couple of ways (1) soil has porosity (void spaces between particles) which may fill with water and subsequently expand (up to 9%) [this can be spectacular when the water freezes to ice / frost] (2) soils with significant clay content will expand when water infiltrates and “swells” the clay (3) hearing by the sun will increase soil volume Conversely, soil will contract when it: dries, cools, thaws

Causes of Slope Failure Most fatalities are caused by fast-moving mass movement - commonly we give these fast mass movements the name landslides - typical landslide is a mass movement whose center has moved downward and outward resulting in an upslope tearaway zone [material has pulled away] and a downslope pileup zone [gravity-moved material has accumulated] - movement is downslope to an angle of repose where gravity will move material no further - slope stability dictates mass movement and thus the creation of “tearaway” and “pileup” zone materials

Slope stability depends on both internal and external factors External Causes External processes that increase th odds of slope failure include: (1) steepening the slope (2) removing support (3) additional materials high on the slope

Internal Causes Long-term processes taking place below the surface of the Earth … weakening processes Internal causes of slope failure include: (1) water --- weight --- interplay with clay materials --- decreasing the cohesion of rocks --- subsurface erosion --- pressure in pores of rocks and sediments

(2) inherently weak materials (3) deceasing cohesion (4) adverse geologic structures --- ancient slip surfaces --- rock layering dipping less than slopes --- structures within the rock and contact between soft and hard rock

Classification of Mass Movements Major types of Earth failures (Fig 9-16) (1) Falls (rockfalls) – occur when elevated rock mass separates along joints and “come tumbling down”; where forces sufficiently weaken or reduce underlying rock or where fissures and/or bedding planes result in weakened zones (2) Slides (landslide, rock slide, debris slide) – a massive failure over a large body of material - the most massive are in association with earthquakes; tend to be extremely destructive and “media events”; a significant modifier of the physical landscape (3) Flows (loess flows, debris flows, mudflows) – mass movements that behave like fluids; range of particle size is enormous [from silt-to-boulders]

Snow Avalanches A number of the principles discussed for earth materials also relate to the development of avalanches … primary difference is that snow may undergo rapid physical change and can accumulate or diminish within a short period of time

Four requisites for a snow avalanche (1) accumulation of a critical mass of snow – what is a “critical mass” is a function of: characteristics of the snow snow pack conditions ground conditions weather conditions (2) structure change in the snow mass that reduces stability

(3) adequate slope to initiate gravity-induced flow (slope angle) no consensus on how much is enough (15 o to 50 o ) a function of snow characteristics (4) a triggering mechanism a mechanical disturbance frequently the fall of the leeward-facing snow cornice

Interesting It is estimated that in the European Alps, 80% of all avalanche victims are skiers In Scandinavia studies indicate that permanent local populations are at the most risk The worst year for avalanche in Norway was 1679 when up to 500 people died… 2 nd highest known was 1755 when 200 were presumed to have died (Gunnar Ramsli, Un. Of Oslo) [reading his work makes a strong case that the mortality caused (though very devastating short-term) is not the most significant aspect of avalanche]

Response to Threat Response must be proactive - practice avoidance --- keep population out of susceptible areas … possible? practical? - maintain monitoring of snow depth and water volume characteristics … modify as necessary

- structurally practice snow control / threat diversion (1) construction windward of the rupture zone (2) construction in the rupture zone (3) construction in the avalanche track (4) construction in the deposition area No.s 1 and 2 are intended to prevent the formation of avalanches No.s 3 and 4 are intended to stop snow mass before it reaches the threatened area, or to deflect them or carry them over the exposed objects