Download landslides and related phenomena

El Salvador, Jan. 2001
How can Factor of Safety (FS) Change?
•Safety Factor(FS)
•FS =1.25; minimum for slope
•FS = 10; minimum for a structure
•Change of FS
•Time
•Erosion
•Water
FS = RF/DF
Finish Factor of Safety Example
from last lecture
FS = SLT/ W sine 
S = shear strength of clay
L = length of the slope
T = unit thickness
W sine  = driving force
Using numbers in text, the
SF is only 1.125. Needs to
be 1.25 or greater
Calculations show that
about 50 m3 must be
removed for FS = 1.25
Note Assumptions:
1.
Planar Slip surface
2.
Not considering effect of fluid pressure
– what is it?
Engineering Properties of Soils: Worked
Example of Soil Shear Strength
USGS Experimental
Debris-Flow Flume
Show flume video
Vaiont
Worked Example – Estimating debris
flow speed from the scar and deposit
Subsidence
• Causes
– Withdrawal of fluids (gas, oil, water)
– Collapse of surface soil and near-surface
rocks
• Karst topography (Kars Mts., Slovenia)
• Result from dissolution
– Limestone or dolomite
– Gypsum and salt
Carbonates and Landforms in Fla. Sinkholes
Sinkholes
• May be natural or man made
• Water table
– High
• Dissolution
• Vertical flow
• Pressure change
– Low
• Decreases support
SINKHOLE DAMAGE
Sinkholes
OrlandoWinter Park, FL
Sinkholes
West central Florida sinkholes
Sinkhole
Formation
Sinkholes
• Types of sinkholes
– Solution or subsidence sinkhole
•
•
•
•
Occur where limestone is exposed at or near the surface
Covered by soil and insolubles as the depression forms
Usually not large
Bowl shaped: steepness determined by rate of subsidence
Sinkholes
• Types of sinkholes
– Cover-subsidence sinkholes
•
•
•
•
Cover is mainly non-cohesive, permeable sand
Develop by gradual subsidence
Generally only a few feet in diameter and depth
Generally remain small, cavities are filled as fast as they develop
Sinkholes
• Types of sinkholes
– Cover collapse sinkholes
• Clayey, cohesive overburdens can bridge LS cavities
• Changes in water table level (pumping effect) weakens
the bridging overburden, resulting in collapse
• Lack of support on falling water table
• Upward pressure in the case of a rising water table
Sinkhole Map
Sinkholes
• Detection Methods
– Drilling
– Ground penetrating
radar
– Resistivity
Sinkholes
12/22/04: Deltona's Howell Blvd
The Informed Volusian
Sinkholes
Caves
• Common in karstic terranes
• Mostly form by the dissolution of
limestone
• Mechanisms
– Acidic meteoric and groundwater (most
caves)
• CaCO3 + H2CO3Ca+2 + 2 HCO3-1
– Biological Activity (more recent idea)
• H2S + 2O2 + Biological activity H2SO4
• H2SO4 + CaCO3 + 2H2O CaSO4•2H2O + HCO3-1
Crystals in Cave
Gypsum Crystals:
Selenite (variety)
Size: 20 X 50 feet
Other Subsidence
In Florida
• Oxidation of Histosols
Perception of
the Landslide Hazard
• Negligible
• Residents are not easily swayed by technical
information
Cause of Portuguese
Bend Landslide
Portuguese Bend
Oft reported mass movement
Mass movement resulting from
expansive soil.