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International Conference on Case Histories in (2004) - Fifth International Conference on Case Geotechnical Engineering Histories in Geotechnical Engineering

17 Apr 2004, 10:30am - 12:30pm

Shear Wave Velocity and Its Effect on Seismic Design Forces and Liquefaction Assessment

Thomas G. Thomann URS Corporation, New York, New York

Khaled Chowdhury URS Corporation, New York, New York

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Recommended Citation Thomann, Thomas G. and Chowdhury, Khaled, "Shear Wave Velocity and Its Effect on Seismic Design Forces and Liquefaction Assessment" (2004). International Conference on Case Histories in Geotechnical Engineering. 2. https://scholarsmine.mst.edu/icchge/5icchge/session03/2

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This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. SHEAR WAVE VELOCITY AND ITS EFFECT ON SEISMIC DESIGN FORCES AND LIQUEFACTION ASSESSMENT

Thomas G. Thomann Khaled Chowdhury URS Corporation URS Corporation New York, NY (USA) New York, NY (USA)

ABSTRACT

The shear wave velocity of soil and rock is one of the key components in establishing the design response spectra, and therefore the seismic design forces, for a building, bridge, or other structure. The shear wave velocity can be measured from in-situ field tests, such as cross-hole or downhole testing. The shear wave velocity can also be estimated based on empirical correlations with other field collected information. This paper presents case histories from 8 bridge projects performed in the northeastern United States where in-situ measurements of shear wave velocities were performed for site-specific ground motion studies. A comparison of these measurements with several empirical correlations indicates that the empirical correlations do not approximate the shear wave velocity very well. Therefore, the use of the empirically derived shear wave velocities may result in an inaccurate determination of the seismic forces imparted to the soils and the structure. Therefore, based on these results, it is concluded that the use of empirically derived shear wave velocities should be used as a preliminary assessment for development of response spectra and liquefaction susceptibility parameters.

INTRODUCTION PROJECT DESCRIPTIONS

It is well known that the soil conditions beneath a structure have A summary of the location and size of the bridges, the depth to an impact on the propagation of ground motions from the bedrock, and the method used to measure the in-situ shear wave bedrock to the ground surface. The soil conditions may amplify velocities is included in Table 1. Additional information certain spectral accelerations and attenuate spectral accelerations regarding the subsurface conditions at specific bridge locations at other periods. Utimately, the spectral accelerations are used are presented elsewhere in this paper. to estimate the earthquake induced forces that are imparted to a structure. In addition, the shear stresses imparted to the soil by the earthquake are affected by the soil properties and have a Table 1 – Project Descriptions direct effect on the liquefaction potential of the soils. The shear wave velocity (or shear modulus) of the soils is the soil property Site ID Location Bridge Depth Method of In- that has the greatest effect on the determination of an appropriate Number Size to situ Vs response spectra and estimation of shear stresses. Bedrock Measurements 1 Jersey City, 5000 ft 30 – Cross Hole This paper presents the results of in-situ shear wave velocity NJ long steel 125 ft measurements from eight bridge projects located throughout the girder northeastern United States. Comparisons between the measured 2 Ridgefield 500 ft 110 – Down Hole values and those derived based on empirical equations related to Park, NJ long steel 150 the N-values of test borings are presented. Site specific response girder analyses were performed using idealized shear wave velocities 3 Harrison, 7000 ft 80 Cross Hole based on the measured and empirical values. Comparisons NJ long steel between the response spectra and shear stresses from these site girder response analyses are made and conclusions are presented based 4 Brooklyn, 300 ft 500 Down Hole on the results of these analyses. NY long steel girder 5 Chelmsford, 50 ft long 50 Cross Hole MA steel girder

Paper No. 3.03 1 6 Bridesburg, 8500 ft 80 Cross Hole The above empirical relationships were developed for primarily PA long steel sandy soils. The results indicate that the use of the N-value in truss determining the shear wave velocity of sandy soils is not very 7 Perth 7000 ft 120 Cross Hole reliable. The results from specific projects indicates that the Amboy, NJ long steel average of the empirical equations sometimes predicts the shear girder wave velocity well but at other projects, the predicted shear wave 8 Brooklyn, 150 ft >300 Cross Hole velocities may be significantly different than the measured NY long values. bascule

SHEAR WAVE VELOCITY COMPARISONS SITE RESPONSE ANALYSIS RESULTS

A comparison between the measured shear wave velocities of the Site response analyses are performed for some of the project eight projects listed in Table 1 and those obtained from the use locations given in Table 1 for the purpose of evaluating the effect of an empirical equation using the N-value (N) from test borings of using empirically derived shear wave velocities and measured is shown in Figure 1. The N-values are from test borings shear wave velocities. Specifically, the response spectra at the performed at the location of the in-situ shear wave velocity foundation level is evaluated since this is used in determining the measurements. The shear wave velocity from the empirical seismic forces imparted to a structure. In addition, the shear equation (shown in Figure 1 as a line) is an average of the stress is evaluated since this is a critical component in evaluating following four empirical equations: liquefaction potential.

Vs = 280 * N0.348 (Ohta and Goto [1978]) (1) One dimensional site response analyses were performed using the ProShake computer program. A site response analysis requires Vs = 185 * N0.5 (Seed, et. al. [1983]) (2) an acceleration time history and a soil profile. The acceleration time history used for the analyses consists of an actual time Vs = 318 * N0.314 (Imai and Tonouchi [1982]) (3) history from the Magnitude 6.5 Saquenay earthquake in Quebec, Canada that occurred in October 1988. The time history has Vs = 350 * N0.27 (Sykora and Stokoe [1983]) (4) been matched to the New York City Department of Transportation (NYCDOT) recommended response spectrum for Soil Class B (Rock) with a 10% probability of exceedance in 50 2000 years (NYCDOT, [1998]). The soil profile includes the total unit weight and the shear wave velocity of the soils. In addition, 1800 relationships between the shear wave velocity and the shear 1600 strain are needed.

1400 Site response analyses were performed for Sites 3, 5, 7, and 8. These locations were selected so that a relatively wide range of 1200 subsurface conditions could be evaluated. In addition, with the 1000 exception of Site 8, in-situ shear wave velocity measurements were obtained for the entire soil column and the bedrock. At Site Vs (fps) 800 8, the influence of the bedrock depth on the site response analyses is relatively minor; therefore, a depth of 140 ft is used 600 as the top of the bedrock. For a given site, response analyses were performed using the 400 measured shear wave velocities and the empirically derived shear 200 wave velocities and the results are compared. All other parameters (e.g., bedrock motion, modulus degradation curves) 0 used in the site response analyses remain constant. The soil 0 20 40 60 80 100 profiles for the four sites are shown in Figures 2 through 5. N60 - Value (blf) Site 1 Site 2 The response spectra, located at a depth of 5 ft below the ground Site 3 Site 4 surface, for the various sites evaluated are included in Figures 6 Site 5 Site 6 through 9. A comparison of the response spectra for a given site Site 7 Site 8 Empirical Relationships indicate that in some cases, such as at Site 7, there is very little difference between the response spectra using measured and Figure 1 - Comparison of Measured and empirically derived shear wave velocities. As expected, the Empirically Derived Shear Wave Velocities measured and empirically derived shear wave velocities match relatively well at this location. However, at Site 8, the response

Paper No. 3.03 2 spectra using the measured shear wave velocities generally results in lower spectral accelerations. In general, the empirically derived shear wave velocities at this location are greater than the measured values. 0 0 Clayey Sand Sand g = 115 pcf 10 g = 120 pcf 20

20 Stiff Clay 40 g = 125 pcf 30 Varved clayey Silt 40 g = 118 pcf 60

50 Fine Sand Varved silty 80 g = 135 pcf 60 Clay g = 120 pcf 100 70 Decomp. Rock Depth below Ground Surface (ft) Depth below Ground Surface (ft) Sand, g = 125 pcf g = 125 pcf 80 120 Shale and Bedrock - 90 Sandstone Diabase g = 150 pcf 140 g = 150 pcf 100 0 1000 2000 3000 4000 0 1000 2000 3000 4000 5000 Shear Wave Velocity (fps) Shear Wave Velocity (fps) Measured Empirical Measured Empirical Idealized - Measured Idealized - Empirical Idealized - Measured Idealized - Empirical Figure 4: Soil Profile - Site 7 Figure 2 - Soil Profile - Site 3 0 Medium to fine 0 Sand(FILL) 20 g = 115 pcf

10 Clay, g = 115 pcf 40 Coarse to fine 20 Sand Sand, g = 120 pcf g = 120 pcf 60 30 Coarse to fine 80 Sand g = 125 pcf 40 Sand (Till) g = 125 pcf 100 50 Fine Sand 120 Depth below Ground Surface (ft)

Depth below Ground Surface (ft) Bedrock - g = 125 pcf Mica Schist 60 Clay g = 125 pcf g = 150 pcf 140 Assumed 70 BEDROCK 0 2000 4000 6000 160 g = 150 pcf Shear Wave Velocity (fps) 0 1000 2000 3000 4000 Shear Wave Veloclity (fps) Measured Empirical Measured Empirical Idealized - Measured Idealized - Empirical Idealized - Measured Idealized - Empirical Figure 5: Soil Profile - Site 8 Figure 3 - Soil Profile - Site 5

Paper No. 3.03 3 0.35 0.35

0.3 0.3

0.25 0.25

0.2 0.2

0.15 0.15 Spectral Acceleration (g) Spectral Acceleration (g) 0.1 0.1

0.05 0.05

0 0 0.01 0.1 1 10 0.01 0.1 1 10 Period (sec.) Period (sec.)

Measured Empirical Measured Empirical

Figure 6 - Response Spectra at 5 ft. BGS - Site 3 Figure 8 - Response Spectra at 5 ft. BGS - Site 7

0.35 0.25

0.3 0.2

0.25

0.15 0.2

0.15 0.1 Spectral Acceleration (g) Spectral Acceleration (g) 0.1

0.05 0.05

0 0 0.01 0.1 1 10 0.01 0.1 1 10 Period (sec.) Period (sec.)

Measured Empirical Measured Empirical

Figure 7 - Response Spectra at 5 ft. BGS - Site 5 Figure 9 - Response Spectra at 5 ft. BGS - Site 8

Paper No. 3.03 4 Similar trends are observed when comparing the peak shear stresses using the measured and empirically derived shear wave velocities, as shown in Figures 10 through 13. 0 0

10 20 20 40 30

40 60

50 80

60 100 Depth below Ground Surface (ft.)

70 Depth below Ground Surface (ft.)

120 80

90 140 0 100 200 300 400 0 100 200 300 400 500 Peak Shear Stress (psf) Peak Shear Stress (psf)

Measured Empirical Measured Empirical

Figure 10 - Shear Stress Curve - Site 3 Figure 12 - Shear Stress Curve - Site 7

0 0

20 10

40

20 60

30 80

100 40 120 Depth below Ground Surface (ft.) Depth below Ground Surface (ft.) 50 140

60 160 0 100 200 300 400 0 100 200 300 400 500 Peak Shear Stress (psf) Peak Shear Stress (psf)

Measured Empirical Measured Empirical

Figure 11 - Shear Stress Curve - Site 5 Figure 13 - Shear Stress Curve - Site 8

Paper No. 3.03 5 CONCLUSIONS

The shear wave velocity is an important parameter for the development of site specific response spectra and the evaluation of liquefaction potential at a site. The shear wave velocity can be estimated based on empirically derived relationships. However, these relationships can result in values that are significantly different than the measured values. As was shown herein, this may result in the improper estimation of the seismic response accelerations, and ultimately, the seismic forces imparted to the structure. In addition, the shear stresses from site response analyses based on empirically derived shear wave velocities may result in an incorrect assessment of liquefaction potential. Therefore, it is concluded that the use of empirically derived shear wave velocities should be used for a preliminary development of response spectra and liquefaction susceptibility parameters. Analyses should be performed where the shear wave velocity values are varied in order to evaluate the sensitivity of the results to changes in the shear wave velocity. If the results of these analyses indicate that the variation in the seismic forces or liquefaction potential are significant or are a concern, it may be prudent to perform more detailed analyses, including the measurement of in-situ shear wave velocities.

REFERENCES

Imai, T. and Tonouchi, K. [1982]. “Correlation of N-Value with S-wave Velocity and Shear Modulus”, Proceedings of the Second European Symposium on Penetration Testing, Amsterdam, The Nederlands, pp. 67-72.

NYCDOT [1998]. Seismic Design Criteria Guidelines, December 30, 1998.

Ohta, Y. and Goto, N. [1976]. “Estimation of S-Wave Velocity in Terms of Characteristic Indices of Soil”, Butsuri-Tanko (Geophysical Exploration) (in Japanese), Vol. 29, No. 4, pp. 34- 41.

Seed, H.B., Idriss, I.M., and Arango, I. [1983]. “Evaluation of Liquefaction Potential Using Field Performance Data”, Journal of the Geotechnical Engineering Division, ASCE, Vol. 109, No. 3, pp. 458-482.

Sykora, D.W., and Stokoe, K.H., II. [1983]. “Correlations of In Situ Measurements in Sands with Shear Wave Velocity”, Geotechnical Engineering Report GR83-33, The University of Texas at Austin, Austin, Tex.

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