GLG 110
Part 2
Chapter 7
Slope Processes, Landslides and Subsidence
7.1
Introduction to Landslides
Each year about 25 people die
Annual loss is about $1 billion dollars
Ground failure is a natural process
Humans increase frequency and number of ground failure
Mass wasting: any down slope movement of earth material due to gravity
All slopes are unstable and on the move! Some slower, some
faster.
7.2
Slope Processes and Types of Landslides
Slopes are the most common of land forms!
All slopes are dynamic and on the move!
Distinct elements of slopes (Fig. 7.2):
-
complex slope or crest
-
free face or cliff
-
debris slope at 30 to 35 degrees
-
lower concave slope or wash slope
Angle of repose: the steepest angle witch can be maintained
by a material without collapsing, is different for different materials
Different forms of movement (Fig. 7.3):
-
flowing
-
falling
-
sliding
-
subsidence
speed of movement: from very slow
(creep) to very fast (rock fall); transitional processes common
7.3
Slope Stability
Slope stability is dependent on:
-
type of material
-
slope angle (topography)
-
climate
-
vegetation
-
water content
-
time
Forces on slopes: driving forces vs. resisting forces
Most important driving force is weight of material
Most common resisting force is the shear strength of material
Slope stability is evaluated by computing a factor of safety (FS):
FS = resisting forces/driving forces
The Role of Earth Material Types:
Composition of material determines type and frequency of movement
Rotational slides or slumps
Translational slide
Soil slip
The Role of Slope and Topography
The steeper the slope the greater the frequency and speed of movement
The Role of Climate and Vegetation
Climate: amount of water in environment and the temperature
The warmer and wetter it is the greater the risk for slope failure
Vegetation: is a complex process, usually it stabilizes the slopes
The Role of Water
Water is the most important ingredient in landslides
Small amounts of water usually stabilizes a slope (cohesion)
Larger amounts of water destabilize the slope (lubrication)
Perched water tables cause land failure after strong rainfall
Shaking of certain materials leads to failure (e.g. quick clay, liquefaction)
The Role of Time
Seasonal changes of slope stability exist
Other changes will affect slope stability over time: e.g. weathering, water
content, road building, construction
Causes of Landslides
Real causes of landslides:
- increase in driving
forces or
- decrease in resisting forces
Immediate causes of landslides:
- earthquake shocks, vibrations, or sudden increase in
water entering a slope
External causes of landslides:
- increase in shear stress (e.g. loading a slope, steepening of slope, quake)
Internal causes of landslides:
- reduction of shear strength (e.g. increase in pore
water pressure, decrease in cohesion)
- intermediate causes: internal and external causes
mixed
7.4
Human Use and Landslides
Effect of humans on slope failure ranges from insignificant to major
The best approach is education: know as much as possible about environment and
human impact
Timber
Harvesting and Landslides
Controversial issue
Clear-cutting may result in increase of landslide frequency
Modern management practices intend to minimize effects
Urbanization and Landslides
Large cities with development on slopes are particularly affected (e.g.
7.5
Minimizing the Landslide Hazard
First step: identifying areas likely to fail
Then: design slopes or engineer structures to prevent slides, warn people, control slides that are on the move
Identification
of Potential Landslides
-
examination of geologic condition in the
field from aerial photographs
-
individual person: look for physical
evidence of movement
Prevention
of Landslides
-
natural landslides are hard to prevent
-
to minimize the hazard: use common sense
and good engineering
-
drainage control: surface and subsurface
draining of water leads to higher slope stability
-
grading: usually leads to higher slope stability
-
slope support: construction of retaining
walls, gabions (stone-filled cages), piles (similar to nails)
-
preventive methods are usually expensive
Landslide
Warning Systems
Do not prevent landslides but may allow people to evacuate on time
Surveillance is the simplest type
Electrical systems, tilt meters, geophones, shallow
wells, monitoring rainfall
Landslide
Correction
Best strategy is to attach the process that started the slide
In most cases correct water pressure by draining water
7.6
Snow Avalanche
Please read in textbook!
7.7
Subsidence
Subsidence: slow or rapid sinking or settling of earth materials
Most subsidence is a result of extraction of fluids (e.g. water) from the
ground
Also extraction of oil and gas, as well as injection of steam, cause subsidence
Compaction of grains is real mechanism of subsidence
Sinkholes
Caused by removal of subterraneous material (natural process)
Limestone and dolostone are the most severely affected
rocks leading to sinkholes
Collapsed ceiling of cave when water level is lowered (e.g. during a drought)
Salt
Deposits and Subsidence
Associated with mining of salt, coal, or other minerals
Also naturally possible (e.g. in layered salt deposits)
Coal
Mining and Subsidence
Especially in areas with long-time coal mining
Full recovery of coal leads to subsidence
Leaving some coal usually minimizes the potential damages
7.8
Perception of the Landslide Hazard
“It always happens to other people.”
Review
questions are on page 205.