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Workshop of the Committee on Geological and Workshop of the Committee on Geological and Geotechnical EngineeringGeotechnical Engineering
LIFECYCLE COSTS LIFECYCLE COSTS ROCK SLOPE STABILIZATION AND ROCK SLOPE STABILIZATION AND
PROTECTION MEASURESPROTECTION MEASURESNovember 28, 2007November 28, 2007
Duncan C. WyllieDuncan C. Wyllie
Wyllie & Norrish Rock EngineersWyllie & Norrish Rock EngineersVancouver, CanadaVancouver, Canada
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Rock FallsRock Falls
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3Rock fall bounced on asphalt and then into windshield
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4Rock fall in strong, blocky granite during heavy rainfall
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Trains can survive larger impacts than cars, but cannot swerve or brake to avoid hazard.
Rock fall modeling programs tend to calculate trajectories that are higher than reality.
This results in nets and fences being larger than required.
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Rock falls can be managed by one of Rock falls can be managed by one of three approaches:three approaches:
1.1. Design and construction of rock fall Design and construction of rock fall protection for new facilitiesprotection for new facilities
2.2. Slope maintenance during operationsSlope maintenance during operations3.3. Reactive Reactive –– clean slides after they happenclean slides after they happen
Decision analysis can be used to evaluate strategy to
minimize lifecycle costs
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Design and construction of rock fall Design and construction of rock fall protection for new facilities:protection for new facilities:
1.1. Uniform face with no benchesUniform face with no benches2.2. Rock fall containment ditchesRock fall containment ditches3.3. Controlled blasting on final facesControlled blasting on final faces
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Drill offset~2 to 3 ft.
Uniform face with no benches
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Serious accident due to rock bouncing off
remnant bench and landing on car
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Benched cuts in horizontally
bedded sedimentary
rock
Intermediate benches on weaker rock
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Design and construction of rock fall Design and construction of rock fall protection for new facilities:protection for new facilities:
1.1. Uniform face with no benchesUniform face with no benches
2.2. Rock fall containment ditchesRock fall containment ditches3.3. Controlled blasting on final facesControlled blasting on final faces
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Rock fall containment
system
Sloped ditch with loose gravel
cushioning layer
Posts to support Ringnet
fence
Planting between walls
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Ditch design chart –
depth/width related to slope
height and angle.
Also, Oregon ditch design
charts.
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Design and construction of rock fall Design and construction of rock fall protection for new facilities:protection for new facilities:
1.1. Uniform face with no benchesUniform face with no benches2.2. Rock fall containment ditchesRock fall containment ditches3.3. Controlled blasting on final facesControlled blasting on final faces
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Blasting in strong, blocky granite:• upper bench, extensive damage; •lower bench, controlled blasting
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Excavated rock faces with carefully aligned blast holes
andno benches
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Rock falls can be managed by one of Rock falls can be managed by one of three approaches:three approaches:
1.1. Design and construction of rock fall Design and construction of rock fall protection for new facilitiesprotection for new facilities
2.2. Slope maintenance during Slope maintenance during operationsoperations
3.3. Reactive Reactive –– clean slides after they happenclean slides after they happen
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Rock Cut Stabilization and
Protection
Stabilization Measures
Protection Measures
Reinforcement Rock Removal
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Hand scaling to remove loose rock.
Work does not need a crane for access which minimizes traffic delays,
and improves worker safety
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Bencher drill and spider cage –2-1/2 inch diameter hole,
40 ft. depth.Low cost, little
disruption to traffic
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Rock falls can be managed by one of Rock falls can be managed by one of three approaches:three approaches:
1.1. Design and construction of rock fall Design and construction of rock fall protection for new facilitiesprotection for new facilities
2.2. Slope maintenance during operationsSlope maintenance during operations
3.3. Reactive Reactive –– clean slides after they clean slides after they happenhappen
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Rock fall (just) contained in ditch
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Damn!
Slide exceeded capacity of ditch, but was detected by warning system
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High reliability, high cost stabilization and protection measures:
• Bolted and shotcreted faces
•Rock fall sheds
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Rock faces above powerhouse bolted,
shotcreted and covered with draped mesh.
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Rock sheds provide high level of protection against rock falls and
require little maintenance.
Super shed
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Decision analysis can be used to evaluate strategies to
minimize lifecycle costs
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Chance point
Decision Analysis to evaluate Decision Analysis to evaluate mitigation optionsmitigation options
Decision point
Strategy, S
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Thank youThank you
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Impact points on dockImpact points on dock
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Trajectory calculationsTrajectory calculationsRock fall trajectories at base of slope
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32Rock fall could be predicted from observations
of previous falls in housing development
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34“Super Rock Shed” – high ductility shed, capacity 800 tf m
Test loading:Mass = 44,000 lbHeight = 120 ft.
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35Pre-cast concrete shed
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Reinforced concrete buttress with
tensioned rock anchors to support
sliding blocks