centre for engineering geology and hydrogeology school of earth and environment faculty of...

CENTRE FOR ENGINEERING GEOLOGY AND HYDROGEOLOGYSchool of Earth and EnvironmentFACULTY OF ENVIRONMENT

Can topographic amplification be predicted from Rock Mass

Classification?

Dr Bill Murphy,The School of Earth & Environment

The University of Leeds.Leeds. United Kingdom

…and the implications for earthquake induced landslides.


Outline• Brief acknowledgements.• Topographic amplification – what is it?• Rock mass classification and theoretical

background• Do calculated wave velocities appear consistent

with other models and field observations?• What are the implications for landslides caused by

earthquakes.• Quo Vadis?


Topographic amplification

• A seismic response of slopes giving accelerations 2-5 times above background motions (Faccioli; 2002);

• A function of wavelength, orientation and slope geometry (Ashford et al 1997).

• Case examples seen during Northridge (Tarzana); and Chi Chi earthquakes.


From: Sepulveda, S. A., Murphy, W., Jibson, R. W., & Petley, D.N. 2005. Seismically induced rock slope failures resulting from topographic amplification of strong ground motions: The case of Pacoima Canyon, California Engineering Geology, 80, 336– 348


From: Sepúlveda, S.A., Murphy, W. & Petley, D.N. 2005.Topographic controls on coseismic rock slides during the 1999 Chi-Chi earthquake, Taiwan. QJEGH, 38, 180-196


From: Ashford, S. A., Sitar, N., Lysmer, J. and Deng, N. 1997 . Topographic Effects on the Seismic Response of Steep Slopes. Bulletin of the Seismological Society of America, 87, (3), 701-709.


Rock Mass classification

or

can ‘static’ empirical models ever link to ‘dynamic’

processes and responses?


What is rock mass classification?•Observational methods of classifying rock masses

based on material properties, fracture state and water content.

•Traditionally based on deep tunnels and mines and has been used to predict stand-up times.

•Empirical correlations exist between rock mass classification schemes and Mohr-Coulomb and Hoek-Brown Strength parameters and elastic properties.

•Correlations are normally derived for ‘static’ rather than ‘dynamic’ mass properties


GSI classification (Hoek, E. & Brown, E. T. (1997). Estimation of rock mass strength. Intl Jl Rock Mech, Min Sci Geomech Abs, 34, 1165-1186) and (right) the GSI system modified by Sonmez and Ulusay (1999).


Hoek. E & Diederichs, M. S. 2006. Empirical estimation of rock mass modulus. International Journal of Rock Mechanics & Mining Sciences, 43, 203–215.


MPafor

DGPaEm ci

GSI 10010)2

1()( 40/)10((

MPafor

DGPaEm ci

GSIci 10010100

)2

1()( 40/)10((

)1(

2 RMRM

RMSRM

EV


(top left) Impact of disturbance factor on Rock Mass Modulus of Elasticity and 4b Comparison between rock mass deformation modulus (each point is the average of

multiple tests at the same site) from China and Taiwan with values calculated values with average D=0.5. (Hoek and Deiderich, 2006)


)21(3

Ek

)21(2

E

)(i

rmirm E

E

Where is Bulk modulus; E is Young’s Moduls; is Poisson’s Ratio, is the shear modulus; RM is Poisson’s Ratio for the intact material; ERM is Young’s Modulus for the rock mass.


Vs /)3

4( kVp

Where Vp is the of P wave velocity; Vs is the shear wave velocity; is the bulk modulus; is the shear modulus and is the density of the material.


Simple hypothesis…. Given these relationships can rock mass data predict shear wave velocities and then topographic amplification?


Main areas of uncertainty• Rock mass

density

• Rock mass Poisson’s Ratio

• Variations in D both spatially and stratigraphically


•Steeply dipping sedimentary sequences

• strong (sandstones and siltstones) and weak (cleaved mudstones / slates) rocks juxtaposed

• medium to thickly bedded

• close to large spaced discontinuities

•Slopes are large enough to be considered as homogenous and isotropic


• Slopes were broken down on the basis of their geomorphology into slope facets.

• Vs and Vp were calculated from rock mass data incorporating geotechnical uncertainty.

• Seismic wave frequency from nearby records of ground motion were used to calculate seismic wavelength

• A simple slope ratio was calculated.


0

5

10

15

20

25

30

35

40

45

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9SRs

fre

qu

en

cy

unstable slopes

Stable slopes

0

5

10

15

20

25

30

35

40

45

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9SRp

fre

qu

en

cy

unstable slopes

Stable slopes

NB. The frequency of calculated wave velocities incorporates geotechnical uncertainty NOT just discrete instability records.


NB. The frequency of calculated wave velocities incorporates geotechnical uncertainty NOT just discrete instability records.

0

5

10

15

20

25

30

35

40

45

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9SRs

fre

qu

en

cy

unstable slopes

Stable slopes

0

5

10

15

20

25

30

35

40

45

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9SRp

fre

qu

en

cy

unstable slopes

Stable slopes


Summary so far….


• Excellent rock mass and lab data existed for the sites at Techi and Puli. Assessments of rock mass elasticity were constrained with lab measurements.

• In spite of this a large range of uncertainty was encountered due to imprecision in determining disturbance factor.

• While there appeared to be reasonable correlation between rock mass determined Vs and amplification as indicated by Ashford and Sitar, 1997, for the Techi site, for the Puli stream catchment site these relationships were poorer


• The close proximity of a strong motion instrument at Techi provided a good estimate of frequency of vibration.

• The lack of a good strong motion makes calculation of velocity at the Puli site prone to large errors.

• There were also large variations in Disturbance Factor due to weathering of the rock mass at the Puli site – another source of uncertainty.


Büch, F. 2008.Seismic Response Of Little Red Hill – Towards An Understanding Of Topographic Effects On Ground Motion And Rock Slope Failure. Unpublished PhD Thesis, Department of Geological Sciences University of Canterbury, Christchurch, New Zealand.


• Seismological data were exceptionally high quality

• Rock mass data were poor.• Results for this analysis were poor

overall because of poor rock mass data.• At current state of knowledge poor rock

mass constraints are more important than seismological constraints


• Slopes at Techi that failed during the Chi Chi earthquake showed evidence of disturbance. It is unclear whether such disturbance extended as deep as the landslides.

• The failure of the slope at Las Colinas occurred in highly heterogenous materials with large velocity contrasts between lavas of andesitic composition (Vs c. 1500-1800ms-1), pyroclastics (Vs c. 700-800 ms-1) and palaeosols.

• Reports from Large Open Pits that have been subjected to strong earthquakes suggest that “loose, weathered materials” both inside LOPs and in natural slopes in the vicinity, are prone to failure during earthquakes but engineered ‘undisturbed’ slopes are not (pers. Comm. Dr John Read, CSIRO, 2008).

Anecdotal Evidence


0

2

4

6

8

10

12

0 1000 2000

Vs m/s

De

pth

PailengFm East

PailengFm West

• Calculated shear wave velocities with depth based on disturbance factor (D).

• Disturbance Factors can be used as an analogue for weathered rock masses in naturally weathered slopes.

• Amplitude of particle exciting increases as towards the free surface as energy cannot be transmitted out of low velocity zones as quickly as it arrives.

•QUESTION – Does increasing wave amplitude cause anelastic deformation of the rock mass?


.

.

..

.

...

.

.

..

-25

Vs / UCS / SPT N

dep

th

-25

weathering/ D/ stress release

dep

th

-25

dep

th

Vs

‘strength’


Where now????

What are we all doing for the next 10 years???


Rock engineering issues• There needs to be a better way of

estimating shear wave velocity in the absence of Vs30 measurements.

• We need more data on E and for rock masses, especially for dynamic conditions.

• Disturbance Factor needs to be constrained more tightly, there is too much subjectivity for such and important parameter.


science issues• We need to know how earthquake induced

landslides ACTUALLY happen. Approximations are fine for Practice but these should be based on a good science case – not what we have done for 40 years!

• How do weathered and disturbed rock masses – material, fractures AND water interact in the field?

• Do slopes ‘yield’ without failing and if so what is the seismic response of these slopes? Field evidence suggests this happens.


Procedural issues• Inventories – do we need more

information? Geotechnical data? Depths of sliding?

• A common reporting mechanism – worldwide.

• A central searchable database. A seismically induced landslide equivalent of NEIC earthquake database.

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