Site Specific Response Analyses and Design Spectra for Soft Soil Sites

Steven F. Bartlett, Ph.D., P.E.

I-15 NATIONAL TEST BEDTECHNOLOGY TRANSFERSYMPOSIUM

Highway Bridge and Geotechnical Innovations

Salt Lake City, UtahSept 17-18, 2003

Amplification of pga on Soft Soil Sites

Figure 5. Relationsh ip for amax and amax rock for deep stiff so il sites using data from Loma Prietaand Northr idge earthquakes and calcu lations from ground response methods (after Chang et al.,1997).

Amplification of pga on Soft Soil Sites

Approach

1. Development of Subsurface Profile and Dynamic Soil Properites2. Development of a Target Rock Spectrum3. Selection of Time Histories4. Scaling or Spectral Matching of Time Histories to Target Spectrum5. 1-D Equivalent Linear Convolution Analysis6. Development of Final Enveloping Design Spectra

Step 1 - ExampleI-80 and 600 S. Soil Profile

SHALLOW PROFILE: 0 - 65m

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 100 200 300 400 500 600 700

Vs (m/s)

dep

th (

m)

SLC Airport East, Wong & Silva (1993)

Lacustrine-alluvial silt and clay (Northern CABay Mud), Wong et al. (2002, published)

Ashland & Rollins average

Ashland & Rollins + 1SD

Ashland & Rollins - 1SD

600 South, Gerber (1995)

I-80, Gerber (1995)

Deep ProfileTOP PORTION OF DEEP PROFILE: 0 - 1000m

0

100

200

300

400

500

600

700

800

900

1000

0 1000 2000 3000 4000 5000

Vs (m/s)

dep

th (

m)

SLC Airport East, Wong &Silva (1993)

Lacustrine-alluvial silt andclay (Northern CA Bay Mud),Wong et al. (2002,published)Interpreted cross section,distance = 15.5km, Hill et al.(1990)

Generic U.S. Rock, Boore &Joyner (1997)

Deep Profile I, this study

Deep Profile II, Wong et al.(2002, unpublished)

Semi-consolidated

Unconsolidated

End, Wong et al. 2002, published (152m)

DepthTo site class C(approx. 70 m)

The Vs model should be developedto a depth corresponding to site classC soils (1200 ft/s or 365 m/s)

Step 1Selection of Modulus and Damping Curves

• Vucetic and Dobry (1991) for shallow clayey and silty soils (requires estimate of PI)

• Electric Power Research Institute (1993) sands for deeper sediments (or other curves that account for effective stress dependency)

Step 2Development of Target Rock Spectrum

0.682

1.706

0.746

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Period (s)

Sp

ectr

al a

ccel

erat

ion

(g

)

2% Exceedance Probability in 50 yrs – Site Class B

Step 3Selection of Time Histories

Considerations a. fault mechanism and tectonic regimeb. earthquake magnitude (characteristic event)c. fault distance d. duration of strong ground motione. site conditions (select rock records)f. fault directivity pulse (sites < 10 km)g. number of independent time histories required (5 to 7)

Step 3Selection of Time Histories

Recommended Time Histories for Response Analysis of I-80 and 600 South Interchanges

E arthquake,Magnitude

Distance Site Channel Direction pga vmaxmax (cm/s)

SiteConditions

TectonicRegime

1992 Landers,Ca lifornia, M =7.3

1.1 kmclosest tofaultrupture

24Lucer ne

LCN 000

LCN 275

0 deg

275 deg

0.785 g

0.721 g

31.9

97.6

rock,U.S. G.S.site class A

str ike slip

1971 Sa nFer na ndo, M =6.6

2.8 km tofaultrupture

Station279PacoimaDa m

PCD 164

PCD 254

164 deg

254 deg.

1.226 g

1.16 g

112.5

54.3

Har d rock compressional

1980 Irpinia,Ita ly M = 6.5

11.2 km(r jb), 21 kmepicentral

Ca litr i C TR270

C TR000

EW

NS

0.176 g

0.132 g

18.7

16.7

rock extensional

1979 I mper ia lValley, Ms =6.9

14.2 km tofaultrupture,14.0 (r jb),

ParachuteTest site

H-PTS315H-PTS225

315 deg

225 deg

0.204 g

0.111 g

16.1

17.8

rock,U.S. G.Ssite class B

extensional

1992 Er zincan,Turkey, M =6.7

2.0 km tofaultrupture

Er inca n ERZ-NS

ERZ-EW

0 deg

90 deg.

0.515 g

0.496

83.9

64.3

st iff soil,U.S. G.S.site class C(V s = 180to 360 m/s)

extensional

Step 4Spectral Matching

• Spectral matching insures that the design time history has the proper amplitude and frequency content of the target spectrum.

• RSPMATCH (Abrahamson, 1992)

• RSPMATCH adjusts the initial accelerogram iteratively in the time domain to achieve compatibility with a specified target acceleration response spectrum.

• RSPMATCH adds wave packets having specified period ranges and limited durations to the input time history.

Step 4Spectral Matching

Comparison of Matched Spectrum to Target Spectrum

Step 5 – 1 D Convolution Analysis

Surface Soil Response

Bedrock

Site-Specific Profile

Spectrally matched time history assigned to bedrock layer as an outcropping rock motion.Assign 1 to 2 percent damping in the elastic bedrock layer.

Step 5Results of 200 m deep convolution Analysis

Step 6 – Development of Final Design Spectrum