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Dealing with Unstable Ground: Mass Movements (Mass Movement Types (Flows,…
Dealing with Unstable Ground: Mass Movements
What are Mass Movements
down slope movement of soil or rock under the influence of gravity
referred to as a landslide
May be slow or fast, and are equally damaging
Understanding mass movements
Downslope driving force
A portion of the force of gravity is directed won the slop, shear stress
Frictional resisting force
A portion of the force of gravity presses the sediments against the bedrock and works with friction to prevent downhill sliding, shear strength
Depends on slope angle
Angle of repose
Steepest slope angle that loose materials can lie in without sliding down
Varies with size & shape of sediments, and the water content of the sediments
Water makes materials less cohesive
when a material is saturated , pore pressure is increased and reduces the effect of the frictional resisting force
Understanding slope stability through factor of safety
Ratio of frictional resisting force to the downslope driving force
FS = 1 when
Resisting force = driving force
The slope is at the angle of repose and mass movements can occur following a small trigger
Slope is unstable when = 1 or < 1
When FS > 1
Resisting force is greater than driving force
slope of the angle is less than the angle of repose
Slope is considered stable
Mass Movement Types
Flows
Shallow slope failure of loose material that acts like a fluid
Soil Creep: slew downward progression of rock and soil dow a low-grade slope; often the result of freezing and thawing
Earth-flows: associated with the flow of saturated fine-grained sediments(clay, silt, fine sand) The surface may only be partly disturbed & movement speeds are low
Mudflows: Characterized by a fluid motion of water, saturated, fine grained materials (sand, soil, volcanic ash) that has consistency of cement
Debris Flows: Fluid motion of water saturated, coarse-grained materials; half the material is bigger than sand size (granules, pebbles, cobbles, boulders)
Ex) La Conchita, California 1995
Slides
Movement of material as a solid mass that slips along one or more surfaces
Generall leave a scar at the top of the slope, called a scarp
Translational slides (rockslides) move by sliding directly downhill along a pre-existing, weak and flat slip plane below the grounds surface
Rotational slides (slumps) move along a curved, spoon-shaped slip plane created by pre-exisitng weak surfaces below ground
Material loosened by the slide may form a mudflow or a debris flow and the toe of the slide
Falls
describe when blocks fall down a cliff face, then typically break on contact and flow outwards
Primarily involve a vertical drop although there may be some rolling and bouncing once the rock hits the slope
Ex) Yosemite, California, Mount St. Helens
If rock falls from high enough it may turn into a debris avalanche
Subsidence
The sinking of a mass of Earth material below the level of surrounding material
On the contrary to flows, slides and falls, it is not a down slope movement but a vertical movement caused by loss of support
Rock Glacier: a lobe of ice-cemented rock debris that slowly moves downhill
Frost Heaving: Large contributor to creep in cold climates. When water saturated soils freeze, they expand pushing rocks and boulders upwards. When the soil thaws the boulders move down vertically, resulting in net down slope movement
Underwater: Also occur on steep slopes in the ocean. Slope failure can occur due to over-accumulation of sediment on a slope or as a result of a shock, like an earthquake
Decreasing the frictional resisting force and increasing the driving force
Decreasing frictional resisting force
Pore pressure
if hillside material becomes saturated with water, the fluid pore pressure is raised, reducing frictional forces
Too much water forces the grains apart and forces the material to collapse and flow like a fluid
Clay-rich soil layers are quasi-impermeable to water, so it collects above them and saturates the rocks increasing the pore pressure
Lack of Vegetation
Roots of trees and undergrowth are critical to bind slope material
helps preventing that hill slopes become saturated and unstable
vegetation lowers the water table by soaking up water
Negative: trees exposed by high wind can have a negative effect on the water table
Increasing the Driving force
Oversteepening
Increases the angle of the slope
Caused by river erosion, wave action, human excavations
Loading
increases the weight on the top of the hillside, destabilizing hillside by increasing the driving force, pushing material downhill
when buildings, material, or water are added to the top of a hill
Shocks and Vibrations
decrease cohesion of slope material by earthquakes and blasting
Lubrication
Water acts as a lubricant
Clay-rich layers are slippery when wet, making excellent slide planes
Subsurface collapse
Most are caused by human activities: pumping of water, crude oil, natural gas, collapse of underground mines or natural caves, groundwater loss, expandable soils, tectonic movement
Primary and Secondary Effects
Primary: physical damages, casualties
Secondary: Contamination of water, spread of water borne diseases, shortage of food
Prediction of Mass Movements
Hydrolgoic conditions: heavy rain
Earthquake triggered events
Volcanic eruptions, but this isn't helpful because volcanic eruptions can't be predicted
From undercutting and slope destabilization, but people living on the slope aren't trained to monitor it
Warning signs
springs, seeps, saturated water where it shouldn't be
new cracks/bulges in ground, floors, foundation
Soil moving away from a foundation, broken water lines
offset fence lines
Prevention and mitigation
Steep slopes can be covered or sprayed with concrete or wire mesh net
Retaining walls
Drainage pipes inserted into the slope to decrease pore pressure
Oversteepened slopes can be graded or terraced to reduce slope to natural angle of repose
Warning systems
Tilt meters to measure changes in slope angle
Geophones to measure vibrations
Groundwater monitoring wells to monitor saturation and pore pressure in soils
Building on deep foundations, support pillars