Exam 4 Flashcards
(23 cards)
What is a landslide or mass wasting ?
Commonly used terms for down slope movement of material
What is the driving energy behind landslides ?
Gravity
How do gravity and the slope work together to promote landslides ?
As a slope becomes more steeper the tangential (slope parallel) component increases relative to the perpendicular component and the shear stress is larger
The steeper the slope the faster the material can move; landslides are more frequent on steeper slopes
What are some of the resistance of failure ?
Friction
Shear strength
What are the types of landslides ?
- What happens in each ?
Falls Topples Slides Lateral spreads Flows
Whatâs the difference between translational and rotational slides ?
Rotational- thick soils/ sediments
Rotates on a curved failure surface
Translational- soils/ sediments or bedrock
Slide surface is typically a plane of weakness
Which type has a concave up slip surface ?
Rotational
Whatâs the difference between a mudflow and a debris flow ?
Mudflow is a mass of water and sand-sized particles that can flow very rapidly
Debris flow are the moving material must be loose and capable of âflowâ, and more than half of the solids in the mass must be larger than sand grains
Explain creep
Creep is the slow downward progression of rock and soil down a low grade slope
What are internal causes of landslide failure ?
Water - role of water can be to increase or decrease likelihood of landsliding Loading - increase in likelihood Increase in water pressure - aka pore fluid pressure - increase in likelihood
How does water both increase and decrease the likelihood of landslides ?
Water is commonly present within rocks and materialâs near the earthâs surface
It can act to either increase or decrease the likelihood of mass wasting
What is material strength ?
- How does it contribute to or resist landslides ?
- Which types of materials are weak and strong ?
Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area)
What is material strength ?
- How does it contribute to or resist landslides ?
- Which types of materials are weak and strong ?
Strength measures the resistance of a material to failure, given by the applied stress (or load per unit area)
Clay materials- inherently weak
- they form during chemical weathering of rocks
Influenced by weathering
Explain:
- oxidation
- hydrolysis
- solution
Oxidation- process of rusting
- oxygen and water react with the minerals in a rock
Hydrolysis- H2O (water) is a unique molecule or compound. In the molecule the + charges (H) lie to one side and the â charges (O) lie to another side, a polar molecule, but the overall molecule is neutral. So, it attaches to things because of the charge and dissolves the mineral by carrying away tiny parts.
Solution- In this process CO2 combines with water to form carbonic acid. Over time this weak acid dissolves rocks, especially limestone.
â Limestone is a rock that forms mostly in the oceans, but also in lakes. It is made of calcium carbonate, and it can dissolve with vinegar, any acid.
What are the two types of weathering ?
- Describe them
- What are the two main types of physical/mechanical weathering
Physical/mechanical weathering- can be caused by the effects of changing temperature on rocks, causing the rock to break apart. The process is sometimes assisted by water.
- Freeze
- Exfoliation
Chemical weathering- any of various processes that cause exposed rock to undergo chemical decomposition, changing the chemical and mineralogical composition of the rock
What are the five common landslide triggers ?
Earthquakes Volcanic eruptions Slope modification Changes in hydrologic conditions Overloading
What are five critical aspects of prediction?
ĚHow does each work? Ě
E.g., Does a steeper or gentler slope promote landsliding? ĚWhat features of vegetation and topography can indicate that a past landslide happened in a location? Ě
How do changes in hydrologic characteristics relate to landslides?
Slopes - he steeper the slope, the greater its potential for downslope movement.
- Construction commonly oversteepens slopesârelates to selecting a home location
- If the artificial slopes are not protected, mass movement will occur.
Topographic features and vegetation- The material on and below a steep slope should be examined prior to construction to check for evidence of landslides -
- the scarp is a key
- hummocky topography is a key
- distinctly different ages of vegetation on either sides are a clue.
- also look for the turned / twisted up trees and vegetation tilting of recent power poles or fence posts
- road surfaces with cracks and fractures the continue off the road.
Geology and structure- Soluable rocks, like limestone, and weak rocks, like mudstone, have a greater potential for sliding. The potential is increased if layers (bedding) or joints or other fractures are inclined downslope - toward valleys
Water buildup- If surface materials on slope are wet, then failure could occur.
- How to determine without expensive drilling? Springs on the slope, areas of wet ground, pools of standing water = high water saturation
Natural triggers in area- earthquakes, volcano in the area, usually high rainfall or fast snowmelt, etc.
- Remember that in many areas landslide potential maps can be purchased or downloaded from a government agency like the U.S. Geological Survey
Why is not building in potential landslide areas a good mitigation technique ?
If a landslide happens again, the building can be destroyed
What are the components of a landslide early warning system and in what situations do they work ?
Slope movement sensor Rain gage Shallow poor pressure Battery Geophone Deep pore pressure When a potential landslide is about to occurs
How are potential landslides monitored ?
LiDAR is an âacronymâ for Light Detection and Ranging. Using a narrow laser beam to probe through dense ground cover, such as trees, LiDARcan produce accurate terrain maps even where forest cover gets in the way of traditional photography. The technique produces a very accurate Digital Elevation Model map (DEM)
How is drainage improved in potential landslide areas ?
- Why is that important
Drains are usually combined with other measures such as anchors and soil removal
What are other techniques used to prevent or minimize landslide damage ?
Slope reduction Protect surface from rain and snow Retaining walls Add vegetation Rock bolts Cable nets and wire fences Intercept ditches and berms Rock sheds and tunnels
What are key elements in landslide preparedness ?
- Before
- During
- After
Features that might be noticed before major landslides:
- Springs, seeps, or saturated ground in areas that have not typically been wet before
- New cracks or unusual bulges in the ground, street pavements or sidewalks
- Soil moving away from foundations
- Ancillary structures such as decks and patios tilting and/or moving relative to the main house
- Tilting or cracking of concrete floors and foundations
- Broken water lines and other underground utilities
- Leaning telephone poles, trees, retaining walls or fences - Offset fence lines
- Sunken or down-dropped road beds
- Sudden decrease in creek water levels though rain is still falling or just recently stopped.
- Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb
During Intense Storms: â˘
- Stay alert and stay awake! Many debris-flow fatalities occur when people are sleeping. Listen to a radio for warnings of intense rainfall. Be aware that intense short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
- If you are in an area susceptible to landslides, consider leaving if it is safe to do so. Remember that travel during an intense storm is hazardous.
- Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger flows.
- If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate debris-flow activity upstream, so be prepared to move quickly. Donât delay! Save yourself, not your belongings.
- Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.
What to do if you suspect imminent landslide danger:
- Evacuate, if possible.
- Contact your local fire, police, or public works department.
- Inform affected neighbors
After landslides occur:
- It is best to stay away from the slide area, as there may be danger of additional slides; however, this is not always possible or desirable. Check for injured and trapped persons near the slide area. Give first aid if trained, and call for help.
- Remember to help your neighbors who may require special assistanceâinfants, elderly people, and people with disabilities.
- Listen to a battery-operated radio or television for the latest emergency information.
- Remember that flooding may occur after a mudflow or a landslide.
- Check for damaged utility lines. Report any damage to the utility company.
- Check the building foundation, chimney, and surrounding land for damage.
- Replant damaged ground as soon as possible because erosion caused by loss of ground cover can lead to flash flooding.
- Seek the advice of geotechnical expert for evaluating landslide hazards or designing corrective techniques to reduce landslide risk
