Topographic p Effects

Transcription

1 Soil Amplification and Topographic p Effects Prof. Ellen M. Rathje, Ph.D., P.E. Department of Civil, Architectural, and Environmental Engineering University of Texas at Austin 18 November 2010

2 Seismic Design Framework Ground Motion Characterization Closest distance fault to site (Rcl) Local site conditions Source Characterization Locations of sources (faults) Magnitude (M w ) Recurrence R rup Soil conditions Topographic conditions Soil conditions and topographic conditions can affect ground shaking

3 Soil Amplification Local Effects Increase in ground motion intensity due to dynamic response of local soil layers Topographic p Amplification Increase in ground motion intensity due to focusing of waves within hillsides These effects typically applied to rock ground motions defined by seismic hazard assessment

4 1985 Michoacan Earthquake Example of soil amplification ( Site effects ) M w 8 along coastal subduction zone ~300 km west of Mexico City

5 Damage Patterns Some damage near coast Most damage in Mexico City Unusual to have severe damage 300 km from earthquake Ground shaking was significantly affected by soil conditions in Mexico City Mexico City built on ancient lake bed Very soft clays underlie much of the city

6 Mexico City N EQ waves

7 Mexico City

8 Mexico City

9 Ground Shaking

10 Ground Shaking 10x amp Amplification is different at each period 4x amp

11 Selective Building Damage Dynamic soil response in damaged areas Soil site period, T s ~ 2 s T s = 4 H / Vs. = 4(35 m)/70 m/s = 2 s Damaged Buildings Mostly taller buildings T bldg ~ 2 s Areas east with deeper soil, T s >> T bldg Soft Soil Vs~70m/s Hard Soil H~35 m

12 Soil Amplification Amplification Factor (AF) = Sa,soil / Sa,rock 3 Soil Profile Rock Factor (Soil/Ro ock) Amplification Site Period (T s ) Period (s) T =4H/Vs T s = 4 H / Vs H: thickness of soil Vs: avg Vs over H

13 Soil Amplification AF s are period dependent 3 Ampli ification Fa actor (Soil/ /Rock) Period (s)

14 Soil Amplification AF s are influenced by Vs profile Softer soil (smaller Vs) larger AF (generally) 5 5 tion Factor (Soil/ /Rock) Amplifica V s (m/s) tion Factor (Soil/ /Rock) Amplifica Period (s) 40 Period (s) Depth (m)

15 Soil Amplification AF s are influenced by level of rock motion Soil is nonlinear PGArock = 0.1 g PGArock = 0.4 g 3 3 l/rock) 2.5 l/rock) 2.5 Amplificatio on Factor (Soi Amplificatio on Factor (Soi Period (s) Period (s)

16 Accounting for Site Effects Simplified Methods Quantify site conditions based on simple parameters (e.g. Vs30) Develop estimates for amplification based on these parameters Wave Propagation Analysis (Site Response) Model full Vs profile of soil from bedrock (Vs~760 m/s) to the ground surface Apply motions at base of soil and compute expected amplification at ground surface Both methods assume a one-dimensional soil profile

17 Parameters Simplified Methods Vs30: average Vs over top 30 m Z1.0: depth to Vs=1.0 km/s 0 Vs (m/s) Vs (m/s) Vs30 = 345 m/s 5 5 Z1.0 =? (> 30 m) Depth (m) 15 (m) Depth Vs30 = 625 m/s Z1.0 = 16 m 25 30

18 Influence of Vs30: GMPEs Sp pectral Accelerati ion (g) 1.2 Vs30 = 760 m/s 1 Vs30 = 300 m/s Period (s) Moderate Rock PGA Amp lification Factor (Soil/Rock) Sp pectral Accelerati ion (g) 3 Rock PGA = 0.22 g 2.5 Rock PGA = g Period (s) Vs 30 = 760 m/s 0.2 Vs30 = 300 m/s Period (s) () High Rock PGA

19 Influence of Z1.0: GMPEs

20 Site Response Analysis Depth (m) Vs (m/s) Advantages: Model detailed velocity profile Model local soil types Increased Complexity: Measuring Vs down to bedrock Selecting input motions Defining i nonlinear soil properties Site response program Strata available for free at:

21 Integration with PSHA Define hazard in terms of an acceleration response spectrum on rock (Vs30 ~ 760 m/s) Apply soil amplification to rock response spectrum Building code procedure GMPE amplification p Site response analysis Increasing Complexity

22 Topographic Amplification Increase in ground motion intensity due to focusing of waves within hillsides Crest L=half-width Base H=height Amplification = Crest Motion/ Base Motion Shape Ratio = H / L

23 Topographic Amplification Amplification increases with increasing Shape Ratio Theoretical Values H/L Slope PGA Amp Geli et al. (1988)

24 Topographic Amplification Frequencies of maximum amplification: where wavelength equals mountain width 2L=width Wavelength of motion = Vs / f Mountain width = 2L Amplification frequency, f* ~Vs/2L Larger Vs or smaller L f* increases

25 Topographic Effects Field measurements of topographic effects generally larger than theoretical predictions PGA: Theoretical ~ 1.2 to 1.5; field ~ 1.5 to 3.5 At f*: Theoretical ~ 2.0 to 4.0; field ~ 4.0 to 10 Reasons for inconsistency Complexity of natural ridges vs. theoretical models Interaction of adjacent ridges Underlying velocity structure 3D geometry No standard procedure to predict topographic amplification

26 Summary Soil Amplification Amplification depends on soil properties and input intensity Amplification is period-dependentdependent Apply soil amplification factors to rock acceleration response spectrum Topographic Amplification Ridges can amplify motions Complex problem with no standard procedure for estimation