LECTURE #12: Landslide Assessment, Mitigation and Case...
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Page 1/6 GEOL 0820 Ramsey Natural Disasters Spring, 2020 LECTURE #12: Landslide Assessment, Mitigation and Case Studies Date: 19 February 2020 V. General Types (continued from last class): 3. flow: materials breaks up and moves as a viscous fluid (differential shearing) o no distinct basal plane o greatest velocity at upper surface o can be the wettest, driest, fastest and slowest types of mass movements o falls & slides can undergo transitions to flows o flow types: creep very slow movement of soil down slope tell-tale indicators most important in terms of total volume initiated: - freeze-thaw cycles - wetting-drying cycles - biological displacement bent tree trunks: evidence of soil creep solifluction very slow movement of soil down slope water/ice saturated sediments most important in cold weather climates mud and debris flows rapid movement of soil and water water = ~ 30% total volume (mudflows) lahars, flash floods Lahar at Mt. St. Helens volcano
Transcript of LECTURE #12: Landslide Assessment, Mitigation and Case...
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GEOL 0820 Ramsey Natural Disasters Spring, 2020
LECTURE #12: Landslide Assessment, Mitigation and Case Studies
Date: 19 February 2020 V. General Types (continued from last class):
3. flow: materials breaks up and moves as a viscous fluid (differential shearing) o no distinct basal plane o greatest velocity at upper surface o can be the wettest, driest, fastest and slowest types of mass movements o falls & slides can undergo transitions to flows
o flow types:
creep very slow movement
of soil down slope tell-tale indicators most important in
terms of total volume initiated:
freeze-thaw cycles wetting-drying
cycles biological
displacement
bent tree trunks: evidence of soil creep
solifluction very slow movement of soil down slope water/ice saturated sediments most important in cold weather climates
mud and debris flows rapid movement of
soil and water water = ~ 30% total
volume (mudflows) lahars, flash floods
Lahar at Mt. St. Helens volcano
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debris avalanche unconsolidated debris moves rapidly down slope highest velocities semi-circular scape at the head, chute, and debris fan
example: Peru, 1970:
earthquake triggered avalanche deposit moved 7 miles in < 3 minutes 80m (~260 ft.) high wave of debris coming down the valley! 30,000 people killed
example #2: Socampa Volcano, Chile (7,000 years ago)
major collapse produced a massive debris avalanche 500 km2 coverage due to large uplift thrust fault 3 km blocks slid in-place smaller fragments flowed rather than sliding
Satellite image of Socampa Volcano, Chile showing the large debris avalanche. The flow stopped over 50km away (white
arrow).
~ 40km
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VI. Assessment • landslide potential maps - identify slide prone areas • distribution and characteristics of past landslides • identify potentially unstable slopes
o terminology susceptibility: the probable degree of response of rocks and soils to
natural or artificial cutting, loading of slopes, or anomalously high precipitation to slope failure high, moderate, and low susceptibility same categories used in classifying the incidence
incidence: the percentage of slope failure over a given area (e.g., county)
Case Study: La Conchita, CA (2005)
La Conchita, CA (1995)
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o example: La Conchita, CA (2005) prior slope failure in 1995 homes rebuilt excessive rain in January, 2005 1995 debris was re-mobilized destroyed 15 homes killed 12 people
La Conchita, CA (2005)
La Conchita, CA (2005)
o example #2: Oso, WA (2014) mud/debris landslide occurred engulfed a rural neighborhood covering 1 square mile 43 people were killed 49 homes and other structures destroyed
Oso slide, WA (2014)
maps of past slides in the Oso area
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VII. Prediction? • slopes
o over-steepened slopes can be a problem seismically active areas regions of large earth moving activity
o geology and structure weak base of slope orientation of bedding
planes, fractures evidence of prior falls or
slides
• surface water buildup
o saturation indicated by springs, wet ground, and pools of standing water
• vegetation o vegetation of different ages (why?) o fallen or bent vegetation
• accelerated creep
o occurs prior to failure
• visible damage to structures • morphologic features from old
slides o indicative of prior mass
movement o scarps & hummocky
topography VIII. Prevention & Mitigation
• slope drainage o interceptor drains
concrete lined drains that capture runoff and transport it away from slope
• perforated pipe o driven into slope to collect water
and drain it away from slope • wells driven into slopes
o pumped to remove water rapidly
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• slope reduction o reduce slope by grading o excavate terraces or benches into slope
• engineering structures
o seal crevices and apply shotcrete
o 8 - 10 cm thick layer
o prevents frost wedging
• retaining walls with drains
o stabilize base of slope
• rock bolts o steel rods in cement o stabilize inclined layers
• cable nets & wire fences
o for rock falls o intercept ditches or berms
• rock sheds and tunnels
o for landslides and avalanches
