Methodology

Purposes of the study

Reactivated in 2004, the landslide of Pont Bourquin is studied by the university of Lausanne since 2006. It’s situated in the canton of Vaud in Switzerland (Figure 1). It’s an active landslide 300m long and 40m wide.

Figure 1. Situation of the sliding of Pont Bourquin (topographic bottom: Géodonnées © Swisstopo). It’s on the road to the ski station “Glacier 3000” .

Study Area

M. Jaboyedoff (1), MH. Derron (1), C. Michoud (1)P. Nicolet (1), T. Lacour (1,2)(1) Institute of Earth Sciences (ISTE), University of Lausanne, Switzerland.EGU General Assembly 2013

EGU2013-1518, 2013

Geologic settingIts substratum consists of ultra-Helvetic nappes, composed by cornieule and altered gypsum (located at the toe of the landslide) subjected to intense dissolution inferring a loss of stability of the overlying land. Furthermore, flysch and shales, at the top of the slide, are subject to the sagging due to their orientation and their mechanical properties.

Eight representative samples were studied by laboratory tests (water content, liquid limit, plasticity limit, angle of friction ф and the cohesion). And we performed site-specific rheological parameters (yield stress τ, viscosity n, and shear stress y) using the rheometer Haake Rheostress 1. These last parameter allow to study the fundamental properties of flow under an applied strength, and to know threshold parameters (from which there is flow).

These rheological parameters are input in software of propagation: Bing and Dan3D. These simulation tools provide the run-out and the critical height of possible zone of departure. Three zones of release have been studied (NS, NLS and NRG) and one is the event of 2007 (NS) which mobilised 11 000 m3 of mud and allows us to calibrate models.

Knowing the hazard, a study of risk was established on this sliding according to these scenarios.

Risk along a road from rheological data : application on Pont Bourquin landslide

The landslide of Pont Bourquin is investigated, both in the field and in the lab, to assess its potential impact on population, infrastructures or environment.

In particular, rheological properties are characterized because important earth flows have already occurred on this site and may occur again in a near future. Rheology is used to better constrain the flow dynamics and propagation, and to access to the risk.

Dan3D

Contact : philippe.limousin@mines-ales.fr

NS

NL S

NR G

τ0 (Pa) 28 9 25 1 62 6

k (Pa. s ) 11 10 13 9 1 28 0

n 0 . 1 5 0 . 1 5 0 . 2 8

yr (s -1) 1 , 3*1 0^ - 4 1 , 1*1 0^ - 5 17 Appar en t vis cos ity

(Pa. s ) 25 30 16

Financial Humans

Moto BusCar

1 to 51 to 2 10 to 60

Nber of occupant of vehicules

Wounded DeathsWounded DeathsWounded Death

1/102/51; 2 if 5 occupants

11 if 2 occupants

Clearing ofthe road

Compensations

Ski station Trucks

Rents trucks + drivers

Fuel

WinterOther periods

17’000 CHF35

’000

CH

F

48’

000

CH

F 3’500 CHF

Nber of vehicules

Car Autobus

4 per day

2700 per day

18 per day

Time of residence

Speed of the véhicule

Length of the vehicule

Car AutobusMoto

30Km/h40Km/h

20Km/h 4m2m 13,5m

Car AutobusMoto

Moto

Day Night

Winter

Summer

Car Autobus

220 per day

575 per day

28 per day

Moto

Car Autobus

1 per day

300 per day

2 per day

Moto

Car Autobus

10 per day

200 per day

2 perday

Moto

Winter

Summer

Vulnerability

MotoBus

Car

90% 90% 60%

NLS

NS NRG

60% 60% 30%

NLS

NSNRG

30% 30% 10%

NLS

NSNRG

Probability that the road

is reached

Rate of covering of the road

Frequency of occuring

100% 64% 82%

NLS

NSNRG

80% 90% 40%

NLS

NS NRG

1/4 in winter

3/4 in summer

7% 19% 11%

NLS

NS NRG7% 19% 11%

NLS

NSNRG

Risk

Hazard

Exposure Vulnerability

Potential losses

Figure 2. Frictionnal result for NS (ф=28.2°)

Figure 3. Bingham result for

NS (τ0=2000 Pa & n=5.09 Pa.s)

Figure 4. Frictionnal result for NLS (ф=26°)

Figure 6. Frictionnal result

for NRG (ф=18.7°)

Figure 5. Bingham result for NLS (τ0=2451 Pa &

n=6.17 Pa.s)

Figure 7. Bingham result for NRG (τ0=1273 Pa &

n=3.96 Pa.s)

Dan3D is a model used to simulate the release of a slide and to visualize its distribution in the three directions of the space. Two types of simulation were made : Frictionnal (ф) and Bingham (τ0 & n (τ=τ0 +ny)) rheologies. Frictionnal rheology work correctly on this software with few parameters and the comparison between the two models allow to confirm results obtain with rheometer.

NLS

NSNRG

Topography

All earth flows reach the road and “NS” fit the event that occured on July 2007 (2m on the road).

Potential losses

y = 1,2239x-2,906b

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Criti

cal H

eigh

t (HC

)

Liquidity index

Critical height versus liquidity index

Hc

Power (Trend curve)

y = 188.34x-2.533

R² = 0.9525

0

5

10

15

20

25

30

0.0 5.0 10.0 15.0

τ (P

a)0

Liquidity index

Yield stress versus liquidity index

τ0

Figure on the left shows rheometer results (rheogram) for various water content (Il) for a sample obtain with the rheometer with the trend curve of Herschel Bulkley (continuum curve):

Globally the regression law fit well to our tests (R2 from 0,881 to 0,957) .

Tests in laboratory were made and thanks to them, we were able to use the results for the propagation of possible zones of departure.The correlation between the tests in laboratory and the sliding of 2007 on both softwares (Bing and DAN3D) shows the importance which takes the rheology of materials to study their flow. Our risk analysis takes into account several possibilities for the final calculation. So even if the results are rather approximate and numerous, it gets closer to the reality.

Institute of Earth Sciences

Risk Results1320

1340

1360

1380

1400

1420

1440

1460

1480

0

0.5

1

1.5

2

2.5

3

3.5

4

0 50 100 150 200 250 300 350

Alt

itu

de

(m)

Hei

gh

t (m

)

Distance (m)

Source NRG

Source NLS Source NS

Bulge

road

Hazard

Exposure

Frequency that in case of event, the flow reaches the road determined thanks to the simulation on Bing.

Rate of covering of the road thanks to simulations led on Dan3D.

Temporal occurence (in function of the season) by analysing the report of the number

of events over the period of activity and the index of the previous precipitation (IPA).

o

o

o

Hazard is the :

Two types : Financials and humans.

The first for the road and the ski station

and compensation.

The second depend on the type of

vehicle.

The exposition depend on the type of vehicle

particularly its time of residence on the risk zone and

its number per day.

The vulnerability is in function of the

height on the road of the landslide and

depend on the type of vehicle

A 2m Hc, corresponding to the critical height of the event of 2007 is used. y is the unit weight.

To find the liquidity index which we extrapolate our rheological results with the relation by Hampton (1972) :

The three release zones are called: secondary scarp (NS), secondary lateral scarp (NLS) and terminal scarp (NRG).

Parameters used for Bing are those calculated with rheometer (y0, τ0 and n). Others are : mud density = 1834 kg/m3; ambient fluid density = 1.23 kg/m3; number of Nodes : 30.

NS NL S

NR G

τ0 (Pa) 20 0 0 24 5 1 12 7 3

n (Pa. s ) 5 . 0 9 6 . 1 7 3 . 9 6 I n ter n al fr iction

an gle (deg)

28 . 2 26 18. 7

NS NLS NRG

They show similar results and confirm our values obtain with rheometer. Moreover the bingham result of the NS correspond to the event of 2007 and fit with the “reality”.

NS

NLS

NRG

Parameters used in simulations :

Results :

(2) Institute of risks sciences, École des Mines d’Alès, France

For compensations of deaths and

wounded people, the price is fixed

to 5’000’000 CHF by death and

5’000 CHF for a wounded person

By mapping the detailed of lithology, past events and recent observations of ground, we decided to keep three release zones to apply the propagation models.

Rheometer applies a shear stress to the sample, generally of low characteristic size and allows to study the fundamental properties of flow in answer to an applied strength. Samples were analyzed by means of the plane-plane geometry. Once tests made, the rheometer allows to know threshold parameters: shear rate y0(s-1), yield strength τ0 (Pa) and viscosity n (Pa.s).

Author : philippe Limousin (1,2) Co-author :

About the financial risk, clearing the road, compensating the death and the wounded people, and balancing the loss of the ski station cost up to 540 000 CHF a year.Regarding the human risk, a fatality due to this landslide happens appoximately every 50 year.

Simulations on Bing

Risk induced Rheological parameters Zone of departure

Conclusions

Risk = Hazard X Vulnerability X Exposure X Potential losses

McDougall, S. and Hungr, O. 2004. A model for the analysis of rapid landslide motion across three-dimensional terrain. Canadian Geotechnical Journal, 41: 1084-1097 p.Hampton M.A., 1972. The role of subaqueous debris flow in generating turbidity currents. Jour. Sed. Petrology, 42 : 775-793 p. Jaboyedoff M., Loye A., Oppikofer T., Pedrazzini A., Güell i Pons M. and Locat J. 2009: Earth-flow in a complex geological environment: the example of Pont Bourquin, Les Diablerets (Western Switzerland). In: Malet J.-P., Remaître A. and Bogaard T. (Eds.): Landslide processes 131-137

Imran J., Parker G., Locat J. et Lee H., 2001. 1D numerical model of muddy subaqueous and subaerial debris flows. Journal of hydraulic engineering, 127(11):959-968 p. Herschel W.H., Bulkley R., 1926. Konsistenzmessungen von Gummi-Benzol-Loesungen, Kolloid Z.,39, 291-300 p.Mainsant G., Larose E., Brönnimann C., Jongmans D., Michoud C., Jaboyedoff M. 2012:Ambientseismic noise monitoring of a clay landslide: toward failure prediction. JGR-ES, 117, F01030, 12 PP. doi:10.1029/2011JF002159 (pour la a fluidisation observée --> intro)