Landslides and other Mass Movements

LAST MODIFIED:

POSTED: 17 October 2018

• Mass Wasting

• a general term that describes the loss of "usable" land (land valuable to us in the widest sense)
• includes mass movements as defined in the classical sense (see below)
• includes erosion (of topsoil) and subsequent loss of navigable land (e.g. repeated rainfall over time causes deep gullies)
• also includes (man-made) deforestation and subsequent erosion (see below)



• Mass Movements

• a mass movement is essentially the downward movement of material pulled by gravity
• in the U.S., mass movements cause $1.5 billion in damage and 25 deaths annually
• most mass movements are associated with
• slope failure
• collapse of structures beneath



• The Urgency of Understanding Mass Movements

mass movements become an increasing threat because of
• population grows
• expansion of cities
• building on unstable ground

• Mass Movements in the U.S.

• areas of high precipitation
• areas of steep slopes
• areas of high run-off rates
• large meandering rivers (see also lectures on floods)

• Factors Controlling Landscape Development

• erosion: agents are water, ice and wind
• elevation: slope steepness determines the speed of erosion
• climate: determines the predominance of eroding agents
• life activity: conserving (tree roots) and weakening (bacteria) organisms
• plate tectonics: uplift, earthquakes
• substrate composition: controls response to erosion
• human activity: mining pits, housing and urban developments, dams, agriculture, deforestation and others

• The Role of Gravity

pull of gravity is constant and continuous by pulling down vertically on a tilted slope, pull of gravity is decomposed into components perpendicular (normal force) and parallel (downslope force) to the slope friction holds mass in place; friction causes a slope-parallel resistance force with opposite direction of the downslope force the balance between resistance force and downslope force holds a mass into place or lets it move downward if resistance force is smaller than downslope force, mass moves down the slope some events can lower friction and trigger mass movements: rainstorms, irrigation, earthquakes, volcanic eruptions




• The Angle of Repose

• maximum angle a pile of debris can retain without disintegrating
• up to 45°
• depends on properties of particles with the pile:
  • large, rough particles have large angles
  • small, smooth particles have small angles

NB: a substance that acts as cohesive increases the angle of repose.
Example: when building a sand castle, we use water to make the sand stick together. When the water dries, the castle starts to collapse because the walls are steeper than the angle of repose. So in this case, added water helps to stabilize a slope. Water can also be destabilizing, namely when we use too much of it and reduce the friction between particles or sliding blocks. This is discussed below.


• Surfaces of Weakness

The contact zone between the sliding mass and the ground below.
• faults
• joints (extensive cracks in layers of rocks)
• clay-rich layers
• soft rocks slipping on hard rock
• hard rock being broken apart by movement in underlying soft rock
NB: clay is a very fine sediment that results from the weathering of silica rich rocks. Clay particles have a book-like structure and it can absorb large amount of water. The clay expands and shrinks, depending on how much water it absorbs and releases, thereby weakening the ground.

• Basic Features of Slope Failure

scarp or tear-away zone upslope (head scarp) pile-up zone or zone of accumulation downslope (toe) glide horizon/surface of weakness transverse cracks where tear-away zone goes into zone of accumulation




• Internal Causes of Slope Failure

inherently weak materials (e.g. clay-rich layers) water in different forms (see below) decreasing cohesion (e.g. when buried layers get exposed) adverse geologic structures (e.g. preexisting ancient slide surface, weak zones that dip with the slope (daylight bedding), structures within the rock (rock not cemented together; clay lenses; joints)




• The Role of Water in Slumping

Water in general, especial when it penetrates the weak zones/glide horizon:
• interplay with clay minerals
• reducing of cohesion
• change in pore pressure
• reduction of friction and lubrication along glide horizon

	Rain and Irrigation increase in weight acceleration of surface and subsurface erosion
	Groundwater Table can rise after prolonged rain or by irrigation can also rise when pools, septic tanks or sewer lines leak increases slumping risk when water table rises into or above glide horizon

• External Causes of Slope Failure

• steepening of slope (e.g. by fault movement)
• removal from low end (toe) of a slope (e.g. river cutting into its banks)
• adding mass on the high end (head) of a slope (e.g. sediment accumulation)



• How Can Humans cause Slope Failure?

• mining, grading, streets, housing development
• steepening slopes beyond angle of repose
• adding mass to head (increasing potential energy)
• removing mass at toe (removing support)
• removing vegetation that used to anchor mass into ground
• dams
• change of pore pressure
• change of groundwater table
• increase in weight may open faults beneath or cause new faults
• inappropriate drainage; excess irrigation
• deforestation



• Deforestation

Deforestation is particularly devastating in the tropics and causes mass wasting. Contrary to what a lush tropical rain forest would suggest, the fertile humus layer is very thin so is easy to erode. In fact, the soil is good only for a few seasons so farmers have to move on to new sections in the forest. High precipitation rates accelerate erosion leaving behind huge gullies that cannot be used for farming etc (badlands). New tropical soil accumulates extremely slowly. A tropical rainforest takes thousands of years to grow back to maturity.



• Slow and Rapid Mass Movements

slow movement can cause extensive property damage but fast movements are the big killers



• Classification of Mass Movements

• typically by speed and water content (see appendix)
• Abbott's book classifies by how a mass moves
• • falls
  • slides
  • flows
  • subsidence or subsides



• Falls

• free fall of mass
• detachment of mass along near-vertical cliff
• dominantly vertical downward movement of mass
• mass moves in separate blocks
• possible triggers: rain, frost wedging, earthquakes



• Flows

• flow over landscape
• mass movements that behave like viscous fluids
• internal movement (turbulence) dominates
• can reach speeds > 320km/h(200mph) but some are slow (e.g. Portuguese Bend)
• no or only short-lived slip surfaces



• Slides

• movements above a failure surface (curved or planar)
• move as semisolid mass (no internal turbulence)
• some pre-slide coherence of mass is maintained during movements (i.e. can break apart somewhat)


• rotational slides/slumps:
• movement on curved failure surface
• short distance
• a resisting force at the toe works against the driving force (Swedish Circle)
• head scarp can cause more instability when water accumulates near top
• translational slides:
• slide on surface of weakness
• can move long distances
• type 1: remains coherent as one mass
• type 2: mass deforms and disintegrates into debris slide
• type 3: can spread laterally, when hard material at top breaks apart



• Subsidence

ground moves down
• slow:
• sags gently
• compaction of loose fluid saturated sediments
  (e.g. New Orleans 3m in last 50yrs)
• removal of ground water or oil
  (e.g. San Joaquin Valley 8.5m since 1925, Las Vegas 1-2m since 1935)
• fast:
• drops catastrophically
• compaction of loose water saturated sediments or rapid collapse of caves
• e.g. sink holes; cave collapses
  (e.g. Winter Park, FL; May 10, 1981) or
  (e.g. San Diego due to failing infrastructure)

1. gravity and buoyancy
2. kinetic and potential
3. downslope and resistance
4. acceleration and rotation

1. true
2. false

1. water table below glide horizon
2. water table above glide horizon

1. when water table rises above glide horizon
2. rain adds mass to the slumping block
3. rain lowers the friction on the glide horizon
4. all of the above
5. only b) and c)

1. adding mass to the head
2. removing mass at the foot
3. steepening the slope
4. adding mass to the foot

APPENDIX: OTHER CLASSIFICATION OF MASS MOVEMENTS

• Classification by Speed and Water Content

• solifluction
• creep
• slumps
• slides
• earth and mudflow
• debris and snow avalanches
• rock fall



• Solifluction

occurs in permafrost areas were only the top 1-3cm thaw in summers. Melt water cannot penetrate the ground building wet soggy unstable ground that moves down a hillside. rock glaciers: mixtures of rock fragments and ice.

• Creep

slowest but most widespread form of slope failure; thawing and freezing of water in soil changes its volume and makes mass move down slope; movement is so slow that it is imperceptible; can be observed through deformed structures such as fences, bent tree trunks, tilted telephone poles and grave stones; can be cause by swelling and shrinking of ground;
• swelling:
• freezing water in rock pores expands by 9%
• expandable material (e.g. clay) is soil
• heating by sun
• shrinking:
• soil thaws
• soil dries
• soil cools



• Debris Flows

debris flows can be extremely fast-moving. The physics of debris flow is currently not fully understood and there are several ideas why some flows can move as fast as they do.
• water may aid as lubricant; but some flows are dry!
• generation of steam as lubricant; considered unlikely because some flows contain ice
• frictional melting of material within moving mass; considered unlikely because some flows contain ice


• air cushions; popular but unlikely because similar sturzstroms are found on ocean floor (no air), Moon (no atmosphere) and Mars (thin atmosphere)
• trapped vibrational (acoustic) energy/acoustic fluidization; somewhat provocative; each particle remains in its relative position with respect to each other (as found in many flows) (i.e. laminar, not turbulent flow)



• Snow Avalanches

• similar to earthen mass movements though usually smaller
• heavy snow fall/end of season
• travel up to a few km at 370km/h (230mph)
• typically 0.6-0.9m (2-3ft) thick, 30-60m (100-300ft) wide; drop 90-150m (300-500ft)
• dry avalanches at 65-100km/h (40-60mph)
• wet avalanches at 30-65km/h (20-40mph)
• downhill skier can trigger snow avalanches
two basic types:
• type 1:
• loose powdery snow; 95% pore space
• start at one point and grow
• start at steepest points of slopes (30-45^o)
• then move down less steep slopes (>20^o)
• come to end in runout zone (<20^o)
• type 2:
• large avalanches from breaking free of slabs of cohesive snow
• analogous to translational slides
• ice slab contains many layers of packed snow
• melt surfaces are potenital failure surfaces
• mass breaks free and slides down as a whole on a glide horizon
• typically turn into flows during downhill movement