19 MountainRisks IntensiveCourse-Barcelona-08 Labiouse ... · EPFL -LMR Protective measures at the...

ÉCOLE POLY TECHNIQUEFÉDÉRALE DE LAUSANNE

L M RSeptember 3rd, 2008Mountain Risks Intensive Course

LABORATOIRE DEMÉCANIQUE DES ROCHES

V. Labiouse

Mitigation measuresProtective works

2

EPFL - LMR

Steps for Rock Fall Risk Management

Mitigation measures

Evaluation of rock fall propagation

Risk analysis and evaluation

Hazard mappingand land-use planning

Identification and characterisation of the departure zone

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Delineation of rock falls hazard zones according to an intensity-probability diagram

Return period (Probability)

Inte

nsity

[kJ]

30 years

100 years

300 years

High Medium Low

Hig

hM

ediu

mLo

w

High Danger

Medium D

anger

Low Dan

ger

⇒ 1. Constraints on land-use planning (built-up restrictions)2. Stabilisation and/or protection measures (if existing buildings)

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Constraints on land-use planning

g First priority is given to land-use planning, which includes the delineation of areas suitable for construction or zones where additional protection is required

No restrictionsLocal protection recommended.Protection required for sensitive

buildingsLow

Modifications only with increased safety measures (local

protection)

No new residential area permitted.In existing housings lots, construction

permitted with conditions (local protection)

Medium

Normal maintenance of building permitted.

Modifications only when Nb of persons is not increased

Construction of new buildings prohibitedHigh

Existing buildingsNew buildingsDegree of hazard

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Stabilisation and/or protection measures

g Only second priority is afforded to structural protection measures.g The protection requirements are defined on the basis of the following questions:

n What level of protection is required, for whom and why ?n What level of residual risk can be tolerated ?n What level of protection can we afford ?

Lateltin & al. 2005

Cost-benefit analysis

necessary

PLANAT, 2007

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Mitigation measures

Fearing the sky may fall on our

head…

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Protective measures at the source (cliff face)

g Removal of unstable rock elementsn clearing (manually or by explosives)n Slope reshaping

g Stabilization / reinforcement of unstable rock massesn Shallow and deep-site drainagesn Steel wire mesh or wire rope netsn Use of vegetationn Anchors or boltsn Shotcreten Retaining walls (buttress)

See the presentation« Mitigation measures- Protective works »by Eduardo Alonso

EPFL - LMR

Protective measures in the slope (propagation)

g Natural barriersn Protection forestsn Boulder-gathering ditches

g Protective engineering worksn Wire mesh or cable net drapesn Rigid barriers (wood, metal, concrete)n Low energy barrier fencesn High energies barrier fencesn Galleries n Embankments and reinforced dams

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Protection forests

g Danger significantly reduced (low and medium blocks), but not completely eliminated (large blocks)

g Natural protection to promote, before resorting to engineering works

g Preserving measures needed (forest maintenance)

Document WSL

Cemagreffilm

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Probable Residual Rockfall Hazard under a forested slopehttp://www.ecorisq.com/en/rockfornet.php

g Rock characteristicsn Dimensions, type, shape

g Slope characteristicsn Mean gradient of the slopen Height of the cliffn Length of the forestn Distance before the forest

g Forest characteristicsn Mean stand densityn Stand basal area

Rockfor.NET is a program developed by the Cemagref (Grenoble) to calculate the Probable Residual Rockfall Hazard (PRH)

under a forested slope, which is the percentage of rocks that surpasses the forested area of a slope.

PRH depends on:

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Boulder-gathering ditches

Vallorbe - Le PontRoad

ditch and dam(Visinand quarry)

2.5 m

Document Bonnard & Gardel

3 m

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Guidelines for bouders-gathering ditches

rollingbouncingfalling

Prevalent mode of rockfall motion (Ritchie, 1963)

Design chart for rock traps as suggested by Whiteside (1986) from work by Fookes and Sweeney (1976) based on Ritchie’s experiments (hundreds of full-scale rockfall tests)

Summary by Richards (1986) of main findings from Hong Kong rockfall tests by Mak and Blomfield(1986). Release of 1000 boulders (10-30 cm) on each of 13 different pre-split slopes.

1 and 1.5 m high barriers at 1.5 m from the slope toe will trap 95% and 100%

respectively of all boulders

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Protective measures in the slope (propagation)


g Protective engineering worksn Wire mesh or cable net drapesn Rigid barriers (wood, metal, concrete)n Low energy barrier fencesn High energies barrier fencesn Galleries n Embankments and reinforced dams

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Defensive works

Energy absorption

capacity[kJ]

RIGID BARRIERS

LOW ENERGY FENCES

FENCES+CABLE

GALLERIES

EMBANKMENTS

REINFORCED DAMS

Compiled by Prof. Descoeudres in 1997

CAN

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Cable net drapery

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Rigid barriers

Safe at a first glance...

very low energy absorption capacity!(from 30 to 50 kJ)

but...

Leissigen

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Mitigation measuresFlexible barriers

Photos BEG, Valais


g Protective engineering worksn Wire mesh or cable net drapesn Rigid barriersn Low energy barrier fencesn High energies barrier fencesn Galleries n Embankments and reinforced dams

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Barrier fences

g Low energy4 up to 200 kJ

g Medium energy4 between 200 et 1000 kJ

g High energy4 over 1000 kJ

Swiss guideline

Barrier classes depending on the energy absorption capacity

June 2001

European guideline 9 classes of falling rock protection kits depending on their

energy absorption capacityFebruary 2008

SEL [kJ]

MEL [kJ]

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Barrier fences – low energy

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Barrier fences – medium and high energies

1. By the net4 Elastic deformation4 No maintenance required

2. By the energy dissipators4 Concentrated plastic deformation4 Replacement of the dissipators

3. By other parts of the structure4 Damages to the structure (e.g. posts)

10 m

4 to 7 m

Energy dissipation

2

1

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Geobrugg nets

Before

After


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Isofer nets

Before

After


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European Technical Approval for barrier fencesETAG 027 on Falling Rock Protection Kits

g 2 testsite-geometries consideredn inclined or vertical

g Standardised test set up and executionn Three functional modulesn Block: form, weight, impact speedn Impact point defined for 1 + 2 SEL, MEL

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European Technical Approval for barrier fenceshttp://www.eota.eu/ETAGs/ETAG 027

g Service energy level (SEL)n objective : to check if the kit is able to accept successive impacts and

the reduction of the useful height is limited within an acceptable valuen 2 successive impacts in net fence without maintenance between testsn Residual height hR after 1 SEL-test: Threshold value of 70 % of

nominal height

2.

1.

block removed after the first launch

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European Technical Approval for barrier fenceshttp://www.eota.eu/ETAGs/ETAG 027

g Maximum energy level (MEL)n objective : to characterize the maximum capacity of the kitn 1 impact with three times the energy of the SEL-testn Residual height classes after MEL-test

4 A (≥50%), B (31-49 %), C (≤30 %) (% of nominal height)

SEL [kJ]

MEL [kJ]

carried out in the same kit used for SEL testingafter being repaired or in a new kit

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Mitigation measuresGalleries

Photo J. Jacquemoud

Photo Journal 24Heures



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Protection galleries

g Mainly reinforced concrete rigid structures (eventually prestressed)

g (usually) with a soil cushion above the slab to assure energy dissipation

Protection of linear infrastructures (roads, railways)

Structures used in the same way as a protection against snow

avalanches

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Traditional-design protection galleries

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Traditional design of the Swiss galleriesMost of the Swiss galleries are cast-in-place reinforced concrete structures

with a soil cushion above the slab to ensure energy dissipation

129

74

25

52

78

13

197

190

246

157

212

83

139

56

119

0% 20% 40% 60% 80% 100%

With cushionlayer

Precast

Prestressed

Arch

Adjacent totunnel

Yes No No data

Data base of Swiss galleries compiled by K. Schellenberg (ETHZ – IBK)

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Maximum impulsive force on the soil cushionPh.D. thesis of Sara Montani (LMR-EPFL, 1997)

g Similarities with Hertz’ law for elastic shocks:

g Influence of the friction angle not inferred from experiments, but deduced from FE computations

g Influence of the thickness of the soil cushion eg Penetration of the block d into the soil cushion:

Slab

e

mR

ME ϕ

H

M

Facc

( )F RR

eM Eacc E pot= ⋅ ⋅

⋅⎛⎝⎜

⎞⎠⎟ ⋅ ⋅ ⋅135

30 2 0 4 0 2 0 6. exp tan. . . .ϕ

53

53

525

1HWMR765.1P Emax ⋅⋅⋅⋅=

dF d Eacc pot⋅ = ⋅16.

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Maximum equivalent static force acting on the slabPh.D. thesis of Sara Montani (LMR-EPFL, 1997)

g Similarities with Hertz’ law for elastic shocks:E0.4 and (m g H)0.6

g Factor accounting for the slab stiffness k similar to a formula derived by Tonello (1988) based on momentum conservation considerations.

( ) 5.0HgmmM1

k2P ⋅⋅⋅+⋅

=

( )F R e Mk

m M gEE potint

. . . ..= ⋅ ⋅ ⋅ ⋅+ ⋅

⋅−0 13 0 8 0 1 0 4 0 6

Slab

e

mR

ME ϕ

H

FintM

Mk

m

Mk

m

P

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Design of protection galleries in Switzerland ??

Swiss FederalRoads AuthoritiesCFF

F Re

M EE potint. . .. tan= ⋅ ⋅ ⋅ ⋅ ⋅2 8

10 7 0 4 0 6ϕ

( )F R e Mk

m M gEE potint

. . . ..= ⋅ ⋅ ⋅ ⋅+ ⋅

⋅−0 13 0 8 0 1 0 4 0 6

g ASTRA and CFF guideline for the design of rock fall protection galleries (October 1997)

g Ph.D. thesis of Sara Montani (December 1998)

g Revised version of the ASTRA guideline (2008)Same formula to assess the equivalent static force acting on the slab

g Verification of existing rock fall protection galleries (ASTRA, 2004)Assessment of the robustness of galleries by means of 3 factors:

structure ductility, absorbing cushion, magnitude of the design action

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Innovative conceptPare-Pierre Structurellement Dissipant PSD

(Tonello Engineering)

Highly reinforced concreteslabs of high thickness (≈ 1 m)with stirrups against punching+ energy dissipators at each

column-slab contactNo soil cushion !

Recent-design protection galleries in FranceDesigned for an energy absorption capacity of 10´000 kJ at ULS

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Mitigation measuresEmbankments and reinforced dams

Photo Tissières



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Embankments and reinforced dams

JapaneseexperimentsProf. Yoshida

Impact of2700 kJ

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Embankments with pneusolelements

Dam reinforced by geotextiles and steel meshes (TENAX)

Capacity of 4500 kJ

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Dam in Saleudan (Valais)after rockfalls in July 1996

800 m3 of material retained by the dam

Block of 50 m3Document Tissières


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Simulations of rockfall impacts on embankments using a Discrete Element Model

F.-V. Donzé, Laboratoire 3S-R, Grenoble

g Calibration and validation on triaxial tests & impacts on granular cushionsg Parametrical study

n Influence of the translational kinetic energy4 the block gets over for an impact energy of 4000 kJ.

n Influence of the impact height4 a height of safety (about 1/4 of the dam height) is needed to contain the block.

n Influence of the rotational kinetic energy4 block’s rotation plays a major role. It is getting over for a ratio Erot/Etrans larger than 10%.

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The capacity of structural protective measures against rock falls is increasing…

Energy absorption

capacity[kJ]

RIGID BARRIERS

LOW ENERGY FENCES

FENCES+CABLE

GALLERIES

EMBANKMENTS

REINFORCED DAMS

Compiled by Prof. Descoeudres in 1997

CAN

EPFL - LMR

Bases for a correct risk management (FOWG):g An adequate assessment of hazards and risks is

essential. It requires knowledge about the nature of the governing processes and about the possibility of their control.

g Protection requirements depend on the potential consequences (damages). Protection of human life has the outmost priority.

g First priority applies to land use planning, which includes the delineation of areas suitable for construction or zones where additional protection is required.

g Only second priority is afforded to structural protection measures, in cases where they can be cost-effective.

g Maintenance of protection structures is imperative

… but sufficient emphasis should be put on preventive measures …

… and more than1000 trajectory runswere performed todesign adequatelythe rockfall barriers

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Residual risk reduction by detecting nets

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