lifecycle costs rock slope stabilization …wnrockeng.com/files/lifecycle costs-rock stabilization...

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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

3Rock fall bounced on asphalt and then into windshield

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

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.

35Pre-cast concrete shed

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Reinforced concrete buttress with

tensioned rock anchors to support

sliding blocks