3/8/17. #20 - Landslides: Mitigation and Case Histories. Questions for Thought. Questions for Thought

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Clifford Anthony
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#20 - Landslides: Mitigation and Case Histories Web Exercise #3 (Volcanoes) Due

Exam 1 Scores Scores and exam key are posted Vaiont Dam Disaster (1963) Questions for

Questions for Thought Can landslide damage be reduced?

cost of landslide itself) Questions for Thought Can landslides be predicted as to where

Yes, pretty well; it is cheap to use preventative measures BEFORE building Landslide

1 #20 - Landslides: Mitigation and Case Histories Web Exercise #3 (Volcanoes) Due Wednesday There is a 2-point penalty for every day the assignment is late. Exam 1 Scores Scores and exam key are posted Vaiont Dam Disaster (1963) Questions for Thought Can landslides be completely prevented? No, you can't stop gravity. Questions for Thought Can landslide damage be reduced? Yes, landslide hazards are easy to detect and fairly easy to mitigate (much less than cost of landslide itself) Questions for Thought Can landslides be predicted as to where and approximately when they might occur? Yes, pretty well; it is cheap to use preventative measures BEFORE building Landslide Mitigation Before Building (Public Policy) 1) Mapping and Zoning (by engineering geologist) - Need detailed maps of known landslide areas; generate landslide hazard maps, which give potential for future landslides 1

Landslide Mitigation Before Building (Public Policy) Creation of Landslide Hazard Map

, 36-37:30) Landslide Mitigation Before Building (Public Policy) Case History: 2005 La

weakness Results can be extremely successful, e.g.

2 Landslide Mitigation Before Building (Public Policy) Creation of Landslide Hazard Map Optional Video - Engineering Geologist Personal video - Thunder On the Mountain (Nat. Geog., 36-37:30) Landslide Mitigation Before Building (Public Policy) Case History: 2005 La Conchita Mudslide La Conchita built in narrow strip between beach and steep cliffs along California coast. 2) Building Codes - Rules to ensure good construction practice, e.g., requirements for drainage, soil compaction, slope angle ( 27 ), dealing with planes of weakness Results can be extremely successful, e.g., Los Angeles Case History: 2005 La Conchita Mudslide During heavy rains in 2005, slope failed, heavy mud quickly flowed down, burying houses, 10 deaths. Slow-moving mudslide occurred in same area in 1995, only damage Optional Video: 2005 La Conchita Mudslide 2005 landslide at La Conchita, CA (YouTube, 0:18) 2

Landslide Mitigation Before or After Building:

allow water to build up within slope (< weight, >

drain water from within slope, surface drains, or

infiltration) 1) Improve Water drainage Clicker

from a slope decrease landslide risk? A.

It decreases the weight of the slope materials C.

Both of the above Control and Stabilization 2)

landslide (< driving force), place it at toe of

(small steps in slope) to prevent large landslides

3 Landslide Mitigation Before or After Building: Control and Stabilization 1) Water drainage - Don't allow water to build up within slope (< weight, > shear strength); install horizontal plastic pipes to drain water from within slope, surface drains, or impermeable seals on top of slope (prevent infiltration) 1) Improve Water drainage Clicker Question Review question: Why does draining water from a slope decrease landslide risk? A. It decreases pore pressure inside the slope. B. It decreases the weight of the slope materials C. Neither of the above D. Both of the above Landslide Mitigation Before or After Building: Control and Stabilization 2) Proper grading of slope - remove material from top of landslide (< driving force), place it at toe of landslide (> resisting force); also create benches (small steps in slope) to prevent large landslides Cross-section Landslide Mitigation: Proper Grading of Slopes - Benches 3) Retaining walls - steel-reinforced concrete wall at vertical face to strengthen slope (> resisting force); need holes in wall for drainage 3

3) Retaining walls - wire mesh fencing or sprayed

bedrock slopes and tunnels in bedrock, bolts can be

fencing Landslide Mitigation Before or After

vegetation - roots bind soil, canopy protects soil

water Unstable slope Slope Stabilization Step 1 -

- Regrow vegetation Clicker Question Which of the

Decrease infiltration of water into hillslope

4 3) Retaining walls - wire mesh fencing or sprayed concrete (shotcrete) 3) Retaining walls - For steep bedrock slopes and tunnels in bedrock, bolts can be installed to strengthen rock Shotcrete wire mesh fencing Landslide Mitigation Before or After Building: Control and Stabilization 4) Regrowing vegetation - roots bind soil, canopy protects soil from erosive impact by raindrops, plants take up water Unstable slope Slope Stabilization Step 1 - Multi-level retaining wall Slope Stabilization Step 2 - Regrow vegetation Clicker Question Which of the following is NOT a potential way to decrease the occurrence of landslides? A. Drain water from hillslope interiors. B. Decrease infiltration of water into hillslope interiors. C. Decrease loading on top of a slope D. Add a retaining wall to a slope E. None of the above, they ALL are potential factors. 4

Clicker Question Which A. It decreases pore pressure inside the slope. B.

Both of the above Hillside community (~200 houses) overlooking Pacific

From 1956-1986, area (+ most houses) moved steadily downslope (total of

) Many houses were abandoned, others still occupied, only with constant

Road dip due to landslide movement Cause?

5 Clicker Question Which A. It decreases pore pressure inside the slope. B. It decreases the weight of the slope materials C. Neither of the above D. Both of the above Hillside community (~200 houses) overlooking Pacific Ocean, ~ 40 km south of Los Angeles. From , area (+ most houses) moved steadily downslope (total of ~200 m, averaging cm per day!) Many houses were abandoned, others still occupied, only with constant maintenance (hydraulic jacks to keep houses, garages level). Road dip due to landslide movement Cause? Both hazardous local geology and negative impact of development. 1) Local bedrock is weak (shale, siltstone, bentonite/swelling clay) layering parallel to slope; waves undercut slope at sea cliff (< resisting forces). 5

Nearby 1929 slide: Sunken City Area was mapped as ancient landslide (before development).

strength); Los Angeles County put fill dirt at head of landslide (> driving force).

Fill Dirt Sedimentary layering ~parallel to slope Homeowners successfully sued LA County for adding fill dirt.

Improper site selection. World's worst dam disaster.

killed 2,500 people within 7 minutes Case History: Vaiont Dam, Italy Location = northeast Italy in Alps, Vaiont

6 Nearby 1929 slide: Sunken City Area was mapped as ancient landslide (before development). Housing development added risk: added weight of houses; Added water from septic tanks (> slope weight, << shear strength); Los Angeles County put fill dirt at head of landslide (> driving force). Result: Continuous, slow downslope movement of large block (slide). Fill Dirt Sedimentary layering ~parallel to slope Homeowners successfully sued LA County for adding fill dirt. Movement was greatly decreased in mid-1980s: 1) drainage wells installed, 2) slope regraded. Improper site selection. World's worst dam disaster. Enormous debris slide splashed much of water out of reservoir, created huge destructive wave (100 m high) that killed 2,500 people within 7 minutes Case History: Vaiont Dam, Italy Location = northeast Italy in Alps, Vaiont dam constructed in 1960 as world's highest thin arch dam (267 m high). Why? Hazardous local geology: Bedrock is fractured limestone (with caves) and mudstone folded with layering parallel to slope. Effects of reservoir filling; Dam built across mountain river valley with steep sides. BEFORE 6

7 Why? As reservoir filled, local water table rose, saturated rock pore spaces, < shear strength. Heavy rains in 1963 triggered massive landslide, filled half of reservoir with slide material. AFTER ~2,500 deaths in 7 minutes due to flooding. Longarone - After Longarone - Before Disastrous site selection. End of #20 7

GG101 Lecture 22: Mass Wasting. Soil, debris, sediment, and broken rock is called regolith.

GG101 Lecture 22: Mass Wasting Mass Wasting is the movement of rock and soil down a slope due to the force of gravity. Soil, debris, sediment, and broken rock is called regolith. Mass wasting creates broad