Ideas on the design of earth mounds and dams to protect highways against snow avalanches

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PIARC International Winter Road

Ideas on the design of earth mounds and dams to protect highways against snow avalanches

Harald Norem, NTNU

Linda Brateng, PRA Norway

Arnold Hustad, PRA, Norway

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•Norwegian experiences with

retaining structures like mounds and dams

•Results of model experiments

•Transfering the results of the model experiments to the nature

Content of the presentation

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Protective measures on road 065

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Effect of protective earth works on county road 065, grouped for different types of avalanches,.

0 20 40 60 80 100 120 140

umber of documented avalanches (1986- 1993)

8 avalanches closed the road

101 avalanches were retained 6 avalanches closed the road

6 avalanches were retained

28 avalanches were retained 10 avalanches closed the road

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Present use of earth mounds

Earth mounds

• The height varies within 4 and 8 m

• The mounds are generally made of soil and have slope angles of 1:1,5 (34 degree)

• The mounds should be located in at least two rows in chess-board pattern

• The length/height ratio is recommended to 1

• Preferably, one to three mounds should be located higher up in the run-out zone

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Present use of collecting dams

• The height of the collecting dams should be close to the kinetic energy height of the avalanche, H=kv²/2g

• There should be sufficient storage area in front of the dam

• The dam should be located close to the road where the avalanche velocity is at minimum and the dam may offer protection for the powder part of the avalanche

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Experimental set-ups

1,35 m

1,65 m

0,3 m

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Test procedures

Recorded the retaining effect due to:

•Location in the run-out zone

•Height of the structures

•Steepness in front of the structures

•Combinations of dams and mounds

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Analyses of the experimental results

Graphical presentation of the Bernoulli equation

0 200 400 600 800 1000 1200 1400

0 200 400 600 800 1000 1200 1400 1600 1800

Height

0 5 10 15 20 25 30 35 40 45 50

0,40

Calculated velocity

Avalanche profile

Energy line

Road X=1560 Potential energy, z

Kinetic energy, v²/2g

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Comparing model and nature

450 500 550 600 650

Height Energy line, model

Energy line, nature

Profile, model

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Energy lines for different dam heights

0,00 0,10 0,20 0,30 0,40 0,50

1,5 1,7 1,9 2,1 2,3 2,5 2,7 2,9 Distance (m)

Height (m)

2V 1,5V 1V

Chute profile

Energy line for no structures

Energy lines for dams

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Singular energy loss due to mounds and dams

150 200 250 300 350

Height above sea level (m) e

Calculated energy line without protective measures

Avalanche path

Kinetic energy at X=1210 H=80 m

Road River

Estimated energy line after protection

Estimated run-out after protection

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Run-out distances for mounds located in a plow shaped pattern

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Energy dissipation of mounds and dams

0,00 0,20 0,40 0,60 0,80 1,00 1,20

1 1,5 2 2,5 3

Efficienfy relative to the kinetic energy at the structure

Dam, 2 cm and 45^o

Dam, 1 cm and 45^o

Mounds,1cm and 60^o

Mounds, 1 cm and 45^o Mounds, 2 cm and 30^o

y

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Energy dissipation of mounds

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Recommended protective structures

Mounds

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Estimated effect of constructing two rows of mounds and one dam close to the road

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Recommended protective structures

Mounds

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Cross section profiles

4,5 m+ 4,5 m high mounds

7 m high dam

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Conclusions

• The maximum energy dissipation by use of mounds and dams is 30-40 % of the kinetic energy at the site of the first row

• Earth mounds should preferably be used high up in the run-out zone, and should be combined with dams close to the road

• Structures should have steep slopes in the front, espacially in the upper part, where the kinetic energy is high

• The jumping distance may be calculated by simple theories for throwing. The second row should be located below the landing area of the masses

• Dams are recommended to be used close to the road where the kinetic energy is at the lowest and, to take benefit of the effect to reduce the impact of the powder part of the avalanche at the road level