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/DF SF > 1 Slope is stable SF < 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

sandstone shale

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