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Evaluation of topographic site effect in slope stability under dynamic loading

Evaluation of topographic site effect in slope stability under dynamic loading. Hieu Toan NGUYEN Jean-Alain FLEURISSON Roger COJEAN. PLAN DE LA PRESENTATION. Introduction Parametric analyses Methodology outline Impact of the dimensionless frequency Impact of the slope angle

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Evaluation of topographic site effect in slope stability under dynamic loading

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  1. Evaluation of topographic site effect in slope stability under dynamic loading Hieu Toan NGUYEN Jean-Alain FLEURISSON Roger COJEAN

  2. PLAN DE LA PRESENTATION • Introduction • Parametric analyses • Methodology outline • Impact of the dimensionless frequency • Impact of the slope angle • Conclusions and perspectives

  3. 1. Introduction • Topography site effect • Cause: the irregularities of the morphology (slope, ridge, cliff...) → the interference of the incident and the reflected waves • Consequence: the spatial, spectral and temporal modifications of the seismic signal • Aggravation of the earthquake damages and the slope instabilities Las Colinas landslide triggered by the earthquake 13-01-2001 (Mw=7.6) Amplification De-amplification Amplification De-amplification Incident waves

  4. 1. Introduction • Eurocode 8 (2005) • Slope belong to the two-dimensional topographic irregularities: long ridges, cliff… • Slope height ≥ 30m • Slope angle ≥ 15° • French paraseismic code PS-92 • Remarks • Do not take into account the role of the geologic and seismic conditions of slope. • Do not take into account the characteristic of the seismic signal. • Study purpose • Identify the factors influencing the slope topographic site effect. • Determine the relationships between these factors and this phenomenon. • Provide a simple method to quantify this effect.

  5. 5H H/tan(a) 15H+H/tan(a) Free Field H Free Field a 20m Quiet boundary 2a. Methodology outline • Interpretation criteria • Amplifications factors • Proportion by area of the amplified zones • Dimension of the amplified zone at the slope crest: Hx, Dxc • Mesh size • N=30÷100  Numerical error < 3% • Boundary conditions • Free Field • Quiet boundary • Seismic excitation • SV wave, sinusoidal signal • PGA=0.4g, F=0.5÷10Hz S1 U S2 U S3 U S4 S1 U S2 U S3 2H² pSAS pSA 2H Ax Ay Dxc 3 1 Hx H 2 h 4 a

  6. 2b. Impact of the dimensionless frequency • Important parameters in a site effect study • Step-like slope with homogeneous, isotropic and elastic material • Morphologic parameters: H, a • Geologic parameters : E, n, r • Seismic characteristic parameter: x • Sinusoidal seismic signal • Amplitude : PGA • Frequency : F • Shaking duration : t • Dimensionless frequency (h) • Integrated parameters : H, E, F, n, r  (5/9) l H Vs (E, n, r), x a PGA

  7. 2b. Impact of the dimensionless frequency h constant • Parametric analyses • Change the value of 2 of the 5 parameters (H, E, F, n, r) each time  10 couples • Change 4 times for each couple  40 cases • Results • Evaluate the coefficient of variation (standard deviation/mean) • Conclusions • Ax=f(H) • Ay=f(H, n) • pS=f(H) • Hx=f(H) • Dxc=f(H, n) Dxc Ax Ay H,F H,E pSAS 3 1 Hx h pSA 2 a 4

  8. 2b. Impact of the dimensionless frequency H=25m H=50m H=100m H=200m Ax H=25m H=50m H=100m H=200m h=0.1 a=50° Ay n=0.2 n=0.25 n=0.3 n=0.35 H=40m H=80m

  9. 2b. Impact of the dimensionless frequency h variable • Parametric analyses • Same geomorphologic condition • h=0.05÷1.0 (Dh=0.05) • Conclusions • Horizontal amplification • h≤0.5 : amplification zones along the free surface • h≤0.15 : one amplified zone, at crest • h>0.15 : many amplified zones, at crest and behind the crest • h>0.5 : additional amplification zones inside the slope mass • Vertical amplification • h≤0.5 : highest amplified zone located along the slope • h>0.5 : highest amplified zone located at the crest 0.15 0.5 h=0.05 h=0.1 h=0.2 h=0.4 h=0.6 Ax H=40m a=50° Ay

  10. 2c. Impact of the slope angle Tendency of the given criterion when a increases Increase (I) Decrease (D) Non dependent (ND) • Parametric analyses • Slope angle a=30÷80 • Results • Conclusions • The amplification factors (Ax, Ay) increase with an increase of the slope angle • Dxc is not dependent on the slope angle • Other criteria (pSA, pSAS, Hx), the interaction between a and h should be considered Dxc Ax Ay pSAS 3 1 Hx h pSA 2 a 4

  11. 3. Conclusions and perspectives • Conclusions • Important role of the dimensionless frequency parameter • Summary table of the relationships between the affecting factors and the interpretation criteria • Graphs to estimate Ax, Ay, pSA, pSAS, Hx, Dxc as a function of h (h=0.05÷0.4) and a (a=30÷80°) • Perspectives • Improve the graphs by extending the value range of h and by integrating H and n • Develop a calculation method applicable to a real seismic signal with a large frequency band. • Extend the study to the cases of the slopes with the more complex and more realistic geomorphologic conditions

  12. Thank you for your attention

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