1. US 550, Bondad Hill
2/14/2016

2. Consider how steep a slope can be and remain stable
Sand dune
Angle of repose for dry sand
15.07.b
Some parts of slope steeper than angle of repose
EXERCISE
Can bring in sand to pour in class to demonstrate angle of repose, and then undercut slope
NOTES
Sand: Morocco
Talus: Tibet
Scoria: Northern Arizona
Scoria cone
Talus slope
Coarse material can have a steeper angle of repose

3. Factors that control slope stability
Amount of water
Angle of repose for material
EXPLANATION
Loose materials, like soil may have typical angle of repose; solid rock can hold steeper slopes
Adding too much water to loose materials can weaken them
Rock is weaker if discontinuities are oriented so rocks can slide downhill
EXERCISE
Draw sketches of hills with different orientations of layers and have students decide which side is least or most safe
Discontinuities: fractures, cleavage and bedding
15.07.c

4. Some ways that slopes fail
Earthquake activity
Undercut cliff
NOTES
Undercut cliff: Yampa River, Utah
Earthquake: Alaska
Steep slope: Tibet
Landslide: El Salvador
Undercut slope: Pacific Palisades, California
Too steep a slope
Undercut slope

5. Classification of Slope Failures
Mechanism of Movement
Fall
Flow
Type of Material
Solid rock
Unconsolidated
NOTES
Fall: Colorado National Monument, Colorado
Flow: La Conchita, California
Solid rock: Monument Valley, Arizona
Unconsolidated: Grand Canyon
Fast: Venezuela debris flows
Slow: Slumgullion, Colorado
Rate of Movement
15.08.b1
Fast
Slow
15.08.b

6. Observe what happens when rocks fall or slide
Rock fall and debris fall
Rock slide
EXPLANATION
Fall: must be a cliff; large blocks below cliff may indicate the potential for fall
Rock slide: somewhat intact block at start of movement; typically detaches along bedding plane, fault, or other discontinuity
Rotational slide: curved surfaces in either rock or loose materials; also called a slump
Rotation tilts layers if present
NOTES
Rock fall: Eiger, Swiss Alps (hidden behind other photos but shows up first)
Rock slide: Naxos, Greece (undercut by highway)
Rotational slide: Toreva, Arizona
Rotational slide
15.09.a1

7. Observe the types of slope failure that involve flow: Part 1
Creep
Debris slide
EXPLANATION
Creep: slow, continuous movement in weak materials; occurs on most slopes but at different rates
Debris slide: Unconsolidated but detaches along some interface
Earth flow: weak, with matrix of fine-grained materials; moves like wet concrete; also called mudflow
NOTES
Debris slide: British Columbia
Earth flow: Thunder Mountain, Idaho
Earth flow
15.10.a

8. Observe the types of slope failure that involve flow: Part 2
Debris flow
Rock or debris avalanche
EXPLANATION
Debris flow: loose slurries with various sizes of clasts; can move rapidly
Rock/debris avalanche: collapse of steep slope followed by high velocity flow; shattered pieces of rock
EXERCISE
Read aloud great description of debris flow filling house in Control of Nature by John McPhee
NOTES
Debris flow: Venezuela
Rock avalanche: Peru (1970 earthquake shook loose side of mountain; killed 18,000 in one town)
15.10.a

9. Slide Lake, Utah: What kind of movement is this?
Moved 14 ft between 2004 and 2009

10. Egypt
New Zealand
Japan

11. Landslides and La Conchita
15.10.m
1995
2005
EXPLANATION
Coastal California town hit by landslides 1995 and 2005 (and one earlier in 1980’s)
Same area or adjacent area with identical setting hit each time
Geologists recommend abandoning town

12. Slope Failures in the U.S.
Earth flow - New York
Debris avalanche - Alaska
Rock slide - Wyoming
EXPLANATION
Alaska: magnitude 7.9 magnitude earthquake in 2002 unleashed rock and debris avalanches
Tully Valley, New York: landslide (earth flow) in 1993 mobilized clays deposited in glacial lake
Gros Ventre, Wyoming: tilted beds undercut by river; mountain made noises well before rockslide in 1925
Thistle landslide, Utah: costliest landslide in U.S. history; buried railroad, 2 highways, and town in 1983
15.11.a
Landslide - Utah