(12) Environmental Geology (lecture 12): Slopes, Landslides and Subsidence I

Slopes

Surfaces in which one end is at a higher level than the other
Inclination from a vertical or horizontal line
Segments
- Free face phase
- Talus phase
- Convex phase
- Straight phase
- Concave phase

Free Face Phase

Straight vertical slope segment
- Usually hard rock section

Talus Slope Phase

Base of a slope
Accumulation of falling rock fragment

Convex Slope Phase

Curves outwards
Usually found in humid wet climate regions
Associated with softer rocks

Straight Slope Phase

Mid-section of slopes
Connects concave and convex portions

Concave Slope Phase

Curves inwards
Thick soil and vegetation cover
Usually found in humid wet climate regions
Associated with softer rocks

Slopes cont

Evolve through dynamic processes
- Time
- Climate
Involves material movement downslope
- Creep - slow movement
- Falls/Avalanche - Fast and rapid
Segments combination can be varied

Material Type

The physical characteristics of material involved in movement
Relative proportion of the different material involved in movement

Unconsolidated

loose materials consisting of solid and fluids
- Mud, Debris

Bedrock

solid materials
- Rocks

Motion Type - The manner of movement (Flow)

Viscous movement downslope
Water saturated

Motion Type - The manner of movement (Fall)

The free fall of detached individual pieces

Motion Type - The manner of movement (Slide)

Zone of weakness between stable and an unstable surface

Rotational

Concave-upwards rupture (spoon-shaped)

Translational

Movement along relatively flat surfaces, Fault,bedding plane

Slumps

This is the downward movement of soil and rock masses along
a concave slip surface
- Mass move downslope as intact unit
- Common in soils and rocks of low shear strength
- Can be triggered by slope undercutting
-- Wave action
-- Erosion
-- Human activity

Slide

This is the sliding of soil or rock mass downhill as an intact unit
- Often along inclined bedding planes or zones of weakness
- Movement usually along flat surfaces parallel to slope
- A rapid form of mass movement

Rockslide

Solid bed rock

Debris slide

unconsolidated materials

Flow

This is the rapid downhill movement of a mixture of rock, soil and water
- Can contain both fine and coarse materials
- Material are not intact
- Material behave like fluid

Mudflow

Contains materials finer than sand

Debris

Contains materials coarser than sand

Lahar flow

Resulting from volcanic eruption as lava movement

Avalanche

Involves the rapid downhill movement of snow

Major Types 1

Rock Slump
Soil Slump
Rock Slide
Soil Slide
Rock Fall
Soil Creep

Major Types 2

Earthflow
Debris flow
Debris avalanche
Lateral spread
Complex slide
Subsidence

Slope Stability

Resistance of inclined surfaces to movement
Potential of slopes to withstand and undergo movement
Based on forces acting on a slope

Driving Forces

moving materials downslope
- Weight

Resisting Forces

Opposing movement
- Friction

Slope Stability

Safety Factor
- Evaluation of Slope stability
- Based on forces acting on a slope
Safety Factor (SF)= Resisting Factor/Driving Factors
• SF > 1 = Stable Slope (Safe)
• SF < 1 = Unstable Slope (Unsafe)

Slope Stability Factors

Slope Steepness
Water
Vegetation
Climate
Time
Geology
- Structures
- Processes

Slope Steepness

Slope angle proportional to Mass
Movement susceptibility
- Varies with material types and
properties

Angle of repose

steepest angle at which loose materials remain stable

Water

Can increase weight leading to stress build up
Water adsorption can reduce friction
Can increase and reduce cohesion
Can dissolve minerals
- Reduce cohesion

Plant roots holds soil together (Vegetation)

Minimizes mass movement

Plant remove water from the soil (Vegetation)

Minimizes water adsorption

Trees canopies break rainfall impact (Vegetation)

Minimizes stress

Climate

Controls weathering
Temperature
- Cyclic heating
Precipitation
- Water supply
- Vegetation

Time

Exposure duration
- Degree of weathering
Seasons
- wet
- dry

Geology (Structures)

Dip of bedding planes
- Angular deviation from the
horizontal
Faults
Fractures

Geology (Processes)

Earthquakes
- Liquefaction
-- Fluid like behavior of water
saturated materials