Chapter 6 Landslides & Related Phenomena

Learning Objectives Gain a basic understanding of slope stability and mechanisms of slope failure Understand the role of driving and resisting forces affecting slope stability Understand factors that affect slope processes: –Topography –Climate –Vegetation –Water –Time –(Gravity) –(rock type) Understand how human use of land affect landslides & slopes Familiarization with identification, prevention, warning, & correction of landslides Appreciation for processes related to land subsidence (Part B)

Terms & Definitions Landslide: –Rapid downslope movement of rock/soil as a coherent mass –Comprehensive term for any type of downslope movement of rock/soil Related terms: –Slope failure –Mass wasting

Slope Processes Slopes: –Common landform –Appearance as stable/static features –Typically dynamic, evolving systems Downslope movement of material Interaction with other systems –Rivers, stream valleys, floodplains –Soil formation –Eolian (wind) –Volcanoes

Slope Elements 1.Convex slope/crest Associated with creep 2.Free-face (steep/vertical) Associated with rockfalls 3.Debris slope Angle of repose  30 o -35 o 4.Concave slope or wash slope Produced by processes associated with running water

Types/Patterns of Downslope Movement

Patterns of Downslope Movement

Classification of Downslope Movements

Debris Flow

Slope Stability Relationship between driving & resisting forces –Driving forces (DF) Weight of rock, soil Weight of superimposed material –Vegetation –Fill –Buildings –Resisting forces (RF) Shear-strength of slope material acting along potential slip planes –Cohesion –Internal friction Ratio RF/DF = Factor of Safety (FS) –>1.0 = stable –<1.0 = unstable –Subject to changed conditions (see example; fig. 6.4)

Causes of Landslides Real Causes –Driving Forces > Resisting Forces Primary Forces –Gravity (downward force & downslope component) –Resistance = shear strength Factors and Causes: –Landslides occur when forces along potential rupture surfaces exceed the forces resisting movement (i.e., cohesion, frictional)

Functional Relationships The factors in S are: u = fluid pressure (pore water pressure) tan  = coefficient of internal friction  = angle of internal friction (frict. resist.)  n = normal stress (i.e., normal to surface or plane of discontinuity C = cohesion of material C and  depend on material type C, , and u vary with water content Relationships between driving force (weight) & resistance force (R) Resisting force (R) is proportional to shear strength of material (S) Where shear strength or resistance (S) S = C + (  n -u) tan  The main driving force is the downslope component of the weight (force) of the material above the potential slip plane = W sin  where W =Weight of material (above plane)  angle of the plane (from horizontal)

Factors/Controls Gravity/Amount of material (above a slip plane) –Weight (force); downslope component of the weight of the slope materials above the slip plane Downward Normal to surface or plane of discontinuity (  n ) Angle of repose (slope angle) Parallel to surface or plane of discontinuity Rock/Material Type Structure (layering, foliation, etc.) Water Others?

Some Implications, Relationships The steeper the angle of inclination from horizontal, the larger the shear stress Decreases in S  less strength, i.e., less force is required to cause a rupture C and   have different values for: –different types of soil or rock materials –dry materials vs. wet materials Angles of frictional resistence (  ) of dry rock materials vary from  >40 o for some igneous rocks, to  = 10 o -20 o for clay. For materials like soil and clay-rich rocks,  can be smaller by a factor of 2 for saturated conditions

Factors Resulting in Decreased Slope Stability Increased pore pressure (affects  n ); e.g., Storms, fluctuating groundwater Increased water content (reduces C,  ) Steepening of slopes (affects  n ) Loading of slopes (affects  n ) Earthquake shaking (reduces C,  ) Removal of material from the base of slopes (Directly reduces S) –Rivers, waves, man Changes in vegetation Change in chemical composition of pore water

Role of Slope & Topography Slope angle (slope); correlated with: –Driving force (e.g., >15% slope) –Correlation w/ geography Extent of landslide activity Steep slopes –Rockfalls, debris avalanches –Soil slips turn into mudflows (saturated slopes) Moderate slopes –Earthflows/mudflows, debris flow

Roles of Rock/Soil Type Patterns of movement –Rotational slides (slumps) occur along curved surfaces Produces topographic benches (see fig.) Commonly occur in weak rock types (e.g., shale) –Translational slides Planar Occur along inclined slip planes within a slope (6.2) –Fractures in all rock types –Bedding planes in rock slopes –Clay partings –Foliation planes (metamorphic rocks) –Soil Slips Type of translation slide Slip plane above bedrock, below soil Colluvium

Slope Material Strength Influences magnitude & frequency of slides –Creep, earthflows, slumps, soil slip Common on slope of: –Shale –Weak pyroclastic (volcanic) material Less common on slopes of more resistant rock –Well-cemented sandstone –Limestone –granite

Role of Water Almost always involved with slope failure –Soil water/moisture –Cohesion (unsaturated vs. saturated effects) –Chemical weathering effects on slope strength –Soil shear strength  1/(pore water pressure) pore water pressure  water saturation Rise in water table (incl. perched)  reduced shear strength Unsupported bank storage Liquifaction (of clay-rich sediments that loose shear strength)

Role of Climate & Vegetation Controls nature/extent of ppt., moisture content Vegetation effects (dependent on plant type) –Enhances infiltration/retards erosion –Enhanced cohesion –Adds weight to slope –Transpiration reduces soil moisture

Climatic Effects A.Semiarid regions or regions with resistant rocks –Steep slope profiles (free-faces) (6.1a) –Soil at crest & wash slope (not free face) B.Subhumid regions or areas of soft rocks –Gentler slope profiles (6.1b) –Soil thick at top & bottom parts of slope Thin in steeper middle portions

Role of Time Forces on slopes change w/ time –Moisture content –Water table –Weathering (reduced cohesion)

Causes & Prevention

Human Use and Landslides Timber harvesting Urbanization (Irrigation)

Minimizing Landslide Hazards Identification of potential landslides Prevention of Landslides –Drainage controls –Grading –Slope supports Warning systems Landslide correction

Mitigation/Prevention Slope Drainage Slope Reduction Engineering Methods/Structures –Grading of slopes –(see figures)

Features & Recognition of Landslides

Examples, Case Studies

Others (Subsidence-In Part B) Snow Avalanche Subsidence –Sinkholes –Salt deposits –Coal mining

Causes of Landslides Real Causes –Driving Forces > Resisting Forces Immediate causes (triggers) –Earthquake shocks –Vibrations –Sudden increase in water External Causes –Slope loading –Steepening –Earthquake shocks Internal Causes: Causes that reduce shear strength