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 30o-35o

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 discontinuityC = cohesion of material-----------------------------------------C and depend on material typeC, , 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 >40o for some igneous rocks, to = 10o-20o 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