Landslides

Learning ObjectivesUnderstand basic slope processes and the causes of slope failure

Understand the role of driving and resisting forces on slopes and how these are related toslope stability

Understand how slope angle and topography, vegetation, water, and time affect both slope processes and the incidence of landslides

Understand how human use of the land has resulted in landslides

Know methods of identification, prevention, warning, and correction of landslides

Understand processes related to land subsidence

Mass WastingDefinition: mass wasting refers to a downslope movement of rock or soil as a more or less coherent mass. Comprehensive term all inclusive term for any downslope movement of earth materials

SlopesWeak and Soft RockForm slope with 3 segementsConvex upper partStraight central segmentConcave lower part

Hard RockForm free face with talus slope at base

Important types of mass wastingSlide downslope movement of coherent block of earth materialSlump is sliding along a curved slip plane producing slump blocksFall rocks fall from vertical faceFlow Downslope movement of unconsolidated material in which particles move about and mix within the massSubsidence is the sinking of of a mass of earth material below the level of surrounding material

Landslides are commonly complex combinations of slding and flowageUpper slump blockLower flow

Forces on SlopesThe stability of a slope expresses the relationship between resisting forces and driving forces

Driving forces forces which move earth materials downslopeDownslope component of weight of material including vegetation, fill material, or buildings

Resisting forces forces which oppose movementResisting forces include strength of material

Potential Slip PlanesGeologic surfaces of weakness in the slope material bedding, foliation, fractures

Safety FactorThe ratio of resisting forces to the driving forces RF/DFSF > 1 Slope is stableSF < 1 Slope is unstable

Factors Affecting Slope StabilityType of earth materialSlope Angle and TopographyClimateVegetationWaterTime

Rotational SlidesSliding occurs along a curved slip plane

Translational SlidesSliding occurs on a planar surface or on a slip plane

Slump (a type of slide)Indicators:ScarpHummocky terrain on and below (earthflow)

Slump

Debris FlowDebris flows are the downslope flow of relatively coarse material> 50% of particles in a debris flow are coarser than sandMovement may be very slow or very fast, depending on topographic conditions

Mudflows, debris avalanches, and debris flowsSmall to moderate magnitude events, occasional large magnitude events

Debris Flow

RockslideRock moves because theres nothing holding it back!Generally requires a pre-existing low-friction surface...

Rockslidelike a clay layer, once its wet...

Earthquake Lake, MT28 deaths in 1959, triggered by earthquake

Role of Earth MaterialsSlopes formed by weak rocks such as shale or have thick soil deposits typically fail by rotational slidesSlopes formed by hard rocks typically fail by translational slidesSoil slips occur above bedrock and fail by translational slides

Role of Slope and TopographyHillslope angle is a measure of the steepness of a slope = slope gradientSteeper slope = increased driving forcesSteep slopes associated with rockfallsSubarid to arid environments

Role of VegetationIn subhumid to humid environments, vegetation is thick and abundantLandslide activity includes deep complex landslides, earthflows, and soil creep.Vegetation influences slope stability by:Providing a cover that cushions the impact of rain falling on slopes and retards erosion on surfaceVegetation has root systems that tend to provide an apparent cohesion which increases resistance to landslidingVegetation adds weight to the slope increasing the driving forces

Role of WaterWater can affect slope stability by:Shallow soil slips can develop during rainstorms when slopes become saturatedSlumps or translational slides can develop months or years after slope is saturatedWater can erode the base or toe of a slope decreasing slope stability

Role of ClimateClimate influences the amount and timing of water in the form of water or snowInfluences type and amount of vegetation

Role of TimePhysical and chemical weathering can weaken slope materials decreasing resisting forces

Earthflowbasically a very viscous (thick) debris flowslow-movingfaster in wetter weather

Earthflow

Creepvery slowresult of freezing and thawing

Creep

shalesandstone

Creep

Triggers for rapid Mass WastingRainOversteepeningcutting at foot of slopepiling on head of slopeDeforesting / DevegetatingEarthquakes