Landslides

Learning Objectives

• Understand 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 Wasting

• Definition: 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

Slopes

• Weak and Soft Rock– Form slope with 3 segements

•Convex upper part•Straight central segment•Concave lower part

• Hard Rock– Form free face with talus slope at base

Important types of mass wasting

• Slide – downslope movement of coherent block of earth material

• Slump – is sliding along a curved slip plane producing slump blocks

• Fall – rocks fall from vertical face• Flow – Downslope movement of unconsolidated

material in which particles move about and mix within the mass

• Subsidence is the sinking of of a mass of earth material below the level of surrounding material

• Landslides are commonly complex combinations of slding and flowage– Upper slump block– Lower flow

Forces on Slopes

• The stability of a slope expresses the relationship between resisting forces and driving forces

• Driving forces – forces which move earth materials downslope– Downslope component of weight of

material including vegetation, fill material, or buildings

• Resisting forces – forces which oppose movement– Resisting forces include strength of

material

Potential Slip Planes

• Geologic surfaces of weakness in the slope material – bedding, foliation, fractures

Safety Factor

• The ratio of resisting forces to the driving forces– RF/DFSF > 1 Slope is stableSF < 1 Slope is unstable

Factors Affecting Slope Stability

• Type of earth material• Slope Angle and Topography• Climate• Vegetation• Water• Time

Rotational Slides

• Sliding occurs along a curved slip plane

Translational Slides

• Sliding occurs on a planar surface or on a slip plane

Slump (a type of slide)

• Indicators:– Scarp– “Hummocky”

terrain on and below (earthflow)

Slump

scarp

Debris Flow

• Debris flows are the downslope flow of relatively coarse material

• > 50% of particles in a debris flow are coarser than sand

• Movement may be very slow or very fast, depending on topographic conditions

• Mudflows, debris avalanches, and debris flows• Small to moderate magnitude events, occasional

large magnitude events

Debris Flow

Rockslide

• Rock moves because there’s nothing holding it back!• Generally requires a pre-existing low-friction surface...

Rockslide

• like a clay layer, once it’s wet...

“Earthquake Lake”, MT

• 28 deaths in 1959, triggered by earthquake

Role of Earth Materials

• Slopes formed by weak rocks such as shale or have thick soil deposits typically fail by rotational slides

• Slopes formed by hard rocks typically fail by translational slides

• Soil slips occur above bedrock and fail by translational slides

Role of Slope and Topography

• Hillslope angle is a measure of the steepness of a slope = slope gradient

• Steeper slope = increased driving forces• Steep slopes associated with rockfalls• Subarid to arid environments

Role of Vegetation

• In subhumid to humid environments, vegetation is thick and abundant

• Landslide 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 surface

– Vegetation has root systems that tend to provide an apparent cohesion which increases resistance to landsliding

– Vegetation adds weight to the slope increasing the driving forces

Role of Water

• Water can affect slope stability by:– Shallow soil slips can develop during

rainstorms when slopes become saturated

– Slumps or translational slides can develop months or years after slope is saturated

– Water can erode the base or toe of a slope decreasing slope stability

Role of Climate

• Climate influences the amount and timing of water in the form of water or snow

• Influences type and amount of vegetation

Role of Time

• Physical and chemical weathering can weaken slope materials decreasing resisting forces

Earthflow

• basically a very viscous (thick) debris flow• slow-moving

– faster in wetter weather

Earthflow

Creep

• very slow• result of freezing and thawing

Creep

shale

sandsto

ne

Creep

from D. Schwert, NDSU

Triggers for rapid Mass Wasting

• Rain• Oversteepening

– cutting at foot of slope– piling on head of slope

• Deforesting / Devegetating• Earthquakes