Engineering Geology and Geohazards Lecture 9: Landslides and Karsts Flashcards
How common are landslides
Landslides are the most widespread and common geological hazard on Earth.
– Earliest on record: Yi and Lo Rivers, Central China, 1767 BC
June and August prominent months for landslide deaths (due to rainfall).
Where do landslides occur?
Everywhere, especially:
– Young mountains
– Coastlines
– Volcanoes
– Remobilized ancient landslides
Describe the effects of landslides
- Loss of life
- Destruction of property
- Defence measures (e.g., barriers,
drainage) - Repairs
- Environmental effects:
– Fertile soils stripped away
– Ecosystem damage
Describe the secondary effects of landslides
● Tsunami/ waves
● Earth dams:
○ Collapse
○ Flooding
● Increased sedimentation into rivers
& deltas:
○ Fish stocks
○ Water quality
○ Flood risk
What are the landslide triggers
- Rainfall
- Earthquakes
- Volcanic deformation/eruption
- Erosion & Alteration
Describe an example of rainfall triggering a landslide
Hurricane Mitch, October 1998
* Hurricane stalls for 6 days over Central America as much as 2000 mm/m2 rainfall.
Floods;
~10 to the power of 6
Las Casitas volcano debris flow, 16 km long, destroyed at least four villages and killed 2,000+.
~10,000 deaths;
~1.5 million homeless
* Damage ~US$ 5 billion
* Most destructive Atlantic hurricane for 200 years
Describe an example of a landslide triggered by an earthquake
Huascarán, Peru, 1970
M 7.7, 150 km
15:23 local time, 31 May
50-100 million cu. m.
16 km at ~280 km/hr
Destroyed Yungay
and smaller communities.
Killed 18,000
Describe different landslide speeds
Sluggish m/hr - m/century
Intermediate m/hr to m/s
Catastrophic >m/s
Describe slides (in the context of landslides)
Slides
* Emplaced in minutes-hours
* Common: embankments, coasts
(Dorset)
* Volumes ~m3
to ~100,000’s m3
* Failure depths ~m to ~10’s m, hence
mostly soil and weathered rock
(regolith)
* Weak rock, easily mobilised by rain
Describe mudflows
Material flows downslope as mixture of rock fragments and wet mud/clay.
- Soils, clay-rich rocks, volcanoes.
- Volumes most ~10,000 cu.m or less; some 10 cu.km
- Speed: 0.1-10 km/hr
- Highly destructive
Describe the Storegga landslide
The Storegga Slide, occurred around ~8.1 ka(8.1 thousand years ago)
* Instantaneous detachment of 2400 to 3200 km3 of sediment, covering an area of 95000 km2.
- Increasingly detailed bathymetric mapping of the ocean floors has revealed many well-preserved landslide deposits of comparable volumes.
Typical hazards:
– destroy communications lines on sea bed
– trigger tsunami
– threaten shipping
– threaten sea-bed habitats
Describe Giant, catastrophic landsides / sturzstroms
Rare, unique type of landslide
consisting of soil and rock
- Great horizontal movement when compared to its initial vertical drop - as much as 20 or 30 times the vertical distance.
- Flow across land fairly easily, and their
mobility increases when volume
increases.
Define a Karst
Karst are landscapes created by the dissolution of soluble rocks
● Soluble rocks: rocks susceptible to dissolution
● Most frequent karst:
○ Carbonates: Limestones/Dolomite
○ Evaporites: Halite & Gypsum
○ Quartzite
Karts can cause collapse features: sinkholes (cenote, torca or dolines), uvalas and poljes
Define dissolution
The process in which solids (like minerals) are disassociated and the ionic components are dispersed in a liquid (usually water).
Dissolution speed depends on the rock type and environmental conditions
(acidity, temperature)
● Creates multitude of geomorphological features:
○ Underground - Typically cave systems
○ Overground- characteristic landscapes and depressions
● Soluble rocks pose a frequent and serious hazard to human live and activities
What variables affect the development and the rate of development of karsts
Speed of development and evolution depends on:
- Lithology (Gypsum, Halite, Limestones, Dolomites, Quartzite)
- Temperature and climate (Latitude)
- Ph
- Natural (organic matter/ vegetation)
-Anthropogenic (Pollution)
- Atmospheric CO2
What are the various types of karst
Caves
Tower karst
Cockpit karst
Describe the short and long term development and results in evaporites
Short term:
-Very high solubility makes it vulnerable to dissolution/collapse when it interacts with unsaturated water
-Collapse and flooding of salt mines
-Formation of ‘meres’ or ‘flashes’ when collapsed karsts flood
-Increased insurance for properties in karstic regions
Long term:
-Collapse of karsts (e.g. Dead Sea coastline) due to aggressive groundwater dissolution results in falling lake level and climatic desiccation
-Changes in groundwater chemistry as becomes
more saturated
-Damage to ecosystems due to increase in aridity
Describe Karsts when found in qaurtzite
Quartzite: metamorphic rock from quartz rich sandstone
● Quartzite Karst is typically found in mountainous regions where quartzite is exposed to weathering and erosion.
● There is much debate whether they are a karst or a pseudokarst
● Time for cave formation in quartzite seems to be only 10 times greater than in classical karst
Vertical Shaft Formation - enlarged by
lateral erosion-Further ceiling collapse
Sinkholes are rare
Describe karts impacts
● Loss of property
● Risk of harm to humans, potentially fatal
● Damage to infrastructure
● Changes to the landscape
● Changes to the hydrology of an area
What are the positive consequences of karsts
● Create aquifers where large volumes of water are stored underground
● Springs from karst aquifers provide fresh water
● Caves and karstic landscapes attract tourism to many countries
● Caves have been used as shelters throughout history
What are karsts mitigation methods
Limiting the water percolation may help with limiting the amount of dissolved
material
● Therefore we need
○ Land use planning (from hazard maps)
○ Engineering solutions
○ Emergency management
Engineering solutions to soluble
rocks:
● Relocating the structure on a safer site,
● Filling the voids and the fractures with concrete or chemical compound
● Improving the foundation ground with grouting and/or geogrids,
● Replacing foundation soil,
● Bridging the voids with rigid mats or beams
What is compaction grouting?
Compaction grouting is the injection of a very stiff, pumpable cementitious grout mixture
at very high pressure into specific areas in the soil. The expanding globular mass of
grout then compacts the soil around each point of discharge
Can be done with cement grout or chemical grout.
What is underpinning
Underpinning consists of releveling or stabilizing a structure through the use of steel piers.
● The piers are installed into deep, stable soils and used to hydraulically lift the concrete slab
or foundation.
● Underpinning is used when there is major structural damage to a building and is also
sometimes used in conjunction with a different type of sinkhole repair.
● This is not a common solution for sinkhole repair since it does not fix the underlying
problem.
What is permeation grouting
Used to fill voids, cracks, fractures, cavities, and to create barriers to the
movement of liquids through the ground.
Consist of the pressure injection of a fluid grout (chemical or cementitious) into
the ground to fill the spaces between particles.
Pressure grout methods are used for preventing water flow, stabilizing granular
material, encapsulating organic material, and improving the physical properties
of soil or rock.
The process is quite flexible and can be designed to cause minimal disruption
at the surface. Therefore, it is advantageous for use in urban areas or areas of
limited access.
What is an inverted filter
Used for sinkhole repair when the sinkhole is not located near structures that require advanced,
engineered support.
Steps:
● The sinkhole area is excavated to expose either bedrock or the throat of the sinkhole.
● A course of rock large enough to bridge the throat of the sinkhole is placed at the bottom of the excavation.
● Courses of progressively finer rock and gravel are compacted above the base course.
● A geotextile fabric may be placed above the finest gravel course to prevent excessive loss of the uppermost course, which may consist of sand and/or soil.
