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Earthquake Research in China Volume 28,Number 2,2014 Effect of Shear Wave Velocity on the Ground Motion Parameters of Site Surface1 Cao Junfeng,Feng Weidong,Meng Fanyue,and Dong Shuanglin Earthquake Administration of Anhui Province,Hefei 230031 ,China Based on the data from typical sites in the Jianghuai region,many kinds of soil layer seismic response are modeled by increasing and decreasing the measured values of shear velocity to a certain scale. The seismic response of soil layer sites are calculated using the one-dimensional equivalent linear method in the frequency domain by choosing the Taft, Kobe and El-centro records as the ground motion input. The results show that the impact of shear velocity variability on the surface ground motion is in relation to the soil layer structure and ground motion input parameters such as amplitude and spectral characteristic. With the increase of shear velocity,the PGA ( peak ground acceleration) on the surface of site will increase,however,the characteristic period of the acceleration response spectra is decreasing. Key words: Shear velocity; Peak ground acceleration; Characteristic period INTRODUCTION Because shear velocity can mirror the dynamic characteristics of soil, it has been an important index for evaluating the site conditions and an indispensable basic parameter of seismic response of soil layers in evaluation of seismic safety for engineering sites. At present,shear velocity of soil layers is obtained by an in-situ test,methods mainly including single hole layer detection method,cross hole method,transient surface wave method and the stable surface wave method. Due to the difference of drill site selection, construction methods, test method, instrument precision and artificial operation,the shear velocity test results have a large dispersion ( sometimes up to 30% ~ 40% or more) in the in-situ test,which can bring serious interference in determining the site ground motion parameters ( Gao Yufeng et al. ,2001; Liu Hongshuai et al. ,2005; Lan Jingyan et al. ,2006; Chen Guoxing et al. ,2007; Sun Rui et al. ,2009 ) . Therefore,it is of great significance to study the effect of shear velocity variation of soil layers on 1 Received on March 20,2013. This project was sponsored by the Anhui Natural Science Foundation Project (10040606Q24),and the Youth Seismic Research Program of Anhui Province,China (20120707) and (20140301). Earthquake Research in China 234 the ground motion parameters, as well as the corresponding influencing range and variation trends. Based on the data from typical sites in the Jianghuai region, and by increasing and decreasing the measured values of shear velocities according to certain proportions and the influence of shear velocity variations on the peak ground acceleration,the characteristic period and response spectrum is analyzed. This work can provide reference for rational determination of seismic fortification criteria for major construction projects in Anhui and other areas. 1 1. 1 ANALYSIS METHOD AND CALCULATION PARAMETERS Analysis M ethod At present,there are many methods for calculating seismic response of soil sites,such as the linear elastic wave ( and vibration ) analysis method, equivalent linearization wave motion method,and the direct time-domain nonlinear integral analysis method. The one-dimensional frequency-domain equivalent linearization wave method, which is also the national standard “Evaluation of Seismic Safety for Engineering Sites ( GB17741-2005 ) ” recommended approach ( Liao Zhenpeng et al. ,1989) ,is widely used in engineering. This method uses an equivalent shear modulus and damping ratio to replace all the shear modulus and damping ratio under different strain amplitudes,thus,converting the nonlinear problem into a linear problem. This method is used in this study. 1. 2 Basic Calculation Profile Based on the data from a typical site in the Jianghuai region,using the one-dimensional frequency-domain equivalent linearization wave method,we choose two typical drillings ZK4 and ZK24 as the basic calculation profiles. The overburden thickness of the first typical drilling ZK4 is 42. 3m,and ZK4 s covering soil layer is mainly composed of fill soil,clay,coarse sand and sandstone,which belongs to medium-hard soil. The overburden thickness of the second typical drilling ZK24 is 29. 4m,and ZK24 s covering soil layer is mainly composed of fill soil,silty clay and silt mass in silty clay,coarse sand,gravel and sandstone,which belongs to medium-soft soil. The measured shear velocity and soil layers information of the basic calculation sections are shown in Fig. 1. Nonlinear dynamic parameters of typical soil samples are obtained from the dynamic triaxial test,and some of the soil dynamic parameters are derived from classic recommended values ( Yuan Xiaoming et al. ,2000) . 1. 3 The Input Bedrock G round M otion According to the principle for selecting input ground motion,we choose the Taft,Kobe and El-centro strong motion records as the bedrock input ground motions ( Fig. 2) and the duration of those records is taken as 40s. The mean peak accelerations of strong motion records are adjusted to be 50gal,100gal,200gal and 400gal,and any time history with the amplitude damped to onehalf of its value is chosen as the bedrock input ground motions. 2 THE RESULTS OF SOIL LAYER SEISMIC RESPONSE ANALYSIS Based on the information of typical drilling ZK4,ZK24,nine soil layer response analysis models are established by increasing or decreasing the measured shear velocity by 5% ,10% , 15% ,20% ,respectively on the same profile. Combined with the three input ground motion records,seismic response of the soil layer sites is analyzed under 216 working conditions. The Volume 28,Number 2 235 Fig. 1 The drilling histogram of basic profiles PGA ( peak ground acceleration) and T g ( characteristic period of response spectrum) are shown in Table 1 ~ Table 2 ( where,A max and T g are the mean of the calculation results using the three input records with same peak strong motion) ,and T g ( s) is obtained from the abscissa periodic value at the right intersection point of the response spectrum platform value A max β m with the response spectrum curve. According to the teaching material of the “Seismic Ground Motion Parameters Zonation Map of China”( Hu Yuxian,et al. ,2001) ,the amplification factor of the response spectrum ( β m ) takes 2. 5. Fig. 3 and Fig. 4 show the ground acceleration response spectrum of the two drillings under different shear wave velocities of soil layers with E1centro ( NS direction) ground motion as the input. It can be seen from the figures that: (1) For first typical drilling ZK4,when the input bedrock PGA is less than 100 gal,ground acceleration response spectrum in 0 ~ 1. 0s showed slight variations under different shear wave velocities in soil layers. The shorter the cycle,the more significant changes of response spectrum. The effect of response spectrum larger than 1. 0s from the shear velocity of soil layers can be ignored. When the input bedrock PGA exceeds 200gal ( i. e. under violent earthquake ) ,the shape of the ground surface acceleration response spectrum within 0 ~ 6. 0s is affected to various extents by shear wave velocities of soil layers, and when shear wave velocity of soil layers decreases,the long period part of response spectrum moves to the right,the short period spectrum value decreases,and the long period spectrum increases in general. When shear wave velocity of soil layers increases,the short period spectrum value increases,the medium and short cycle part Earthquake Research in China 236 Fig. 2 The three input strong motion records of the response spectrum has a greater effect,and the response spectrum curve moves to the left. (2) The second typical drilling profile ZK24 contains silt soft soil layer,which shows strong nonlinearity,thus the input bedrock peak seismic motion has a very limited effect on the ground acceleration response spectrum. When the input bedrock peak is 50gal,the ground acceleration response spectrum differs greatly in the period range of 0 ~ 6. 0s. With the increase of the input bedrock peak,the shape of the response spectrum shows a larger difference,and its overall performance is consistent with the same tendency of ZK4 under a violent earthquake. Table 1 Basic profile ZK4 ZK24 The peak acceleration A max ( gal) under different shear wave velocities of soil layers Input PGA ( gal ) - 20% - 15% - 10% - 5% Measured value + 5% + 10% + 15% + 20% 50 75. 4 75. 4 78. 6 80. 2 81. 3 83. 5 81. 5 82. 2 83. 3 100 141. 2 144. 3 152. 2 152. 2 155. 5 159. 6 156. 5 157. 4 159. 8 200 283. 9 275. 5 270. 9 287. 4 296. 3 303. 6 295. 8 305. 4 306. 9 400 472. 1 534. 3 594. 5 603. 5 566. 6 547. 6 555. 7 591. 4 608. 2 50 89. 1 84. 9 81. 3 82. 9 87. 5 89. 7 92. 0 98. 7 102. 3 100 145. 7 164. 0 172. 1 171. 2 160. 5 157. 1 160. 6 160. 8 171. 5 200 165. 6 190. 3 214. 8 236. 6 253. 0 279. 0 304. 6 315. 5 307. 8 400 336. 7 355. 9 388. 6 423. 7 453. 0 475. 7 502. 7 524. 1 521. 0 Volume 28,Number 2 237 Table 2 The characteristic period of response spectrum T g ( s ) under different shear wave velocities of soil layers Basic profile ZK4 ZK24 Input PGA ( gal ) - 20% - 15% - 10% - 5% Measured value + 5% + 10% + 15% + 20% 50 0. 71 0. 69 0. 67 0. 65 0. 63 0. 62 0. 62 0. 60 0. 58 100 0. 72 0. 70 0. 67 0. 65 0. 64 0. 63 0. 62 0. 61 0. 59 200 0. 79 0. 77 0. 74 0. 70 0. 69 0. 67 0. 66 0. 62 0. 61 400 1. 02 0. 97 0. 87 0. 83 0. 80 0. 76 0. 74 0. 72 0. 67 50 0. 82 0. 80 0. 79 0. 75 0. 72 0. 69 0. 67 0. 61 0. 55 100 0. 98 0. 92 0. 86 0. 82 0. 79 0. 76 0. 74 0. 72 0. 68 200 1. 12 1. 05 1. 03 1. 01 0. 97 0. 93 0. 87 0. 83 0. 81 400 1. 17 1. 11 1. 06 1. 02 1. 01 1. 00 0. 99 0. 98 0. 97 Fig. 3 The response spectrum under different shear wave velocities of soil layers for basic profile ZK4 3 ANALYSIS ON THE EFFECT OF VARIABILITY OF SHEAR WAVE VELOCITY OF SOIL LAYERS In order to analyze the influence of shear wave velocity variation of soil layers on ground 238 Earthquake Research in China Fig. 4 The response spectrum under different shear wave velocities of soil layers for basic profile ZK24 motion parameters,the results of the adjusted shear wave velocity and measured shear wave velocity are compared and analyzed. In order to show the intuitive deviation,calculation results of measured shear wave velocity are taken as the reference,and the relative deviation of calculation results between adjusted and measured shear wave velocity are shown in Table 3,Table 4,Fig. 5 and Fig. 6. We can see from those results that: ( 1) In the same basic profile,if the measured shear wave velocity of soil layers increases or decreases by a range of 20% ,the results have a large difference between adjusted and measured shear wave velocity. The larger the difference in shear wave velocities,the greater the variations in peak ground acceleration and characteristic period of response spectrum. For the basic profile ZK4,the variation amplitude of peak ground acceleration is - 16. 7% ~ 7. 3% ,and the variation amplitude of response spectrum characteristic period is - 16. 7% ~ 27. 5% ; and for the basic profile ZK24,the variation amplitude of peak ground acceleration is - 34. 5% ~ 24. 7% ,and the variation amplitude of the response spectrum characteristic period is - 16. 8% ~ 23. 6% . (2) The peak ground acceleration is in positive correlation with the shear wave velocity of soil layers. When the shear wave velocity of soil layers decreases,the peak ground acceleration also decreases gradually; and when the shear wave velocity of soil layers increases,the peak ground acceleration also increases gradually. For example,for the basic profile ZK24 under the Volume 28,Number 2 239 condition of the bedrock input peak being 200 gal,when the soil layer shear wave velocity decreases by 5% to 20% in succession,the variance of peak ground acceleration also decreases from - 6. 5% to - 34. 5% ,compared with the results calculated from the measured shear wave velocity and vice versa. Table 3 Basic profile ZK4 ZK24 Table 4 Basic profile ZK4 ZK24 The comparison results of peak ground accelerations Variances in shear wave velocity ( relative to the measured values) Input PGA( gal ) - 20% - 15% - 10% + 5% + 10% + 15% + 20% 50 - 7. 2% - 7. 3% - 3. 3% - 1. 3% 2. 7% 0. 2% 1. 1% 2. 4% 100 - 9. 2% - 7. 2% - 2. 1% - 2. 1% 2. 7% 0. 6% 1. 2% 2. 7% - 8. 6% - 3. 0% 2. 5% - 0. 2% 3. 1% 3. 6% 6. 5% - 3. 4% - 1. 9% 4. 4% 7. 3% 5. 1% 12. 8% 16. 9% 200 - 4. 2% - 7. 0% 400 - 16. 7% - 5. 7% 50 1. 9% - 3. 0% - 9. 2% 2. 2% 100 4. 9% - 5% - 7. 1% - 5. 2% 7. 2% 2. 5% 6. 6% - 2. 1% 0. 0% 0. 2% 6. 9% 200 - 34. 5% - 24. 8% - 15. 1% - 6. 5% 10. 3% 20. 4% 24. 7% 21. 7% 400 - 25. 7% - 21. 4% - 14. 2% - 6. 5% 11. 0% 15. 7% 15. 0% 5. 0% The comparison results of characteristic period of response spectrum Variances in shear wave velocity ( relative to the measured values) Input PGA( gal ) - 20% - 15% - 10% - 5% + 5% + 10% + 15% + 20% 50 12. 2% 10. 1% 6. 3% 3. 7% - 2. 1% - 2. 1% - 4. 8% - 7. 4% 100 12. 0% 8. 9% 4. 2% 2. 1% - 1. 0% - 2. 6% - 5. 2% - 7. 3% 200 14. 5% 11. 1% 7. 2% 1. 0% - 3. 4% - 3. 9% - 10. 1% - 11. 6% 400 27. 5% 20. 8% 8. 3% 3. 3% - 4. 6% - 7. 5% - 10. 4% - 16. 7% 50 13. 9% 11. 1% 10. 2% 3. 7% - 3. 7% - 6. 5% - 15. 7% - 23. 1% 100 23. 6% 16. 0% 8. 4% 3. 4% - 3. 4% - 5. 9% - 8. 4% - 13. 5% 200 15. 1% 8. 2% 6. 5% 3. 8% - 3. 8% - 10. 7% - 14. 8% - 16. 8% 400 15. 5% 9. 6% 4. 6% 1. 0% - 0. 7% - 2. 0% - 3. 3% - 4. 3% (3) The characteristic period of response spectrum is in negative correlation with the shear wave velocity of soil layers. When the shear wave velocity of soil layers decreases, the characteristic period of response spectrum will increase; and when the shear wave velocity of soil layers increases, the characteristic period of response spectrum decreases accordingly. For example,for basic profile ZK4,under the condition of bedrock input peak of 100 gal,when the soil shear wave velocity decreases by 5% to 20% in succession,the variance of characteristic period of response spectrum also increases gradually from 2. 1% to 12% ,compared with the calculated results of measured shear wave velocity,and vice versa. (4) In contrast,the effect of decrease of shear wave velocity of soil layers is slightly larger than its increase on the peak ground acceleration and the characteristic period of response spectrum. ( 5) The impact of the soil layer shear wave velocity variation on the ground motion is related to the amplitude of bedrock input ground motion. Generally,with the increase of input bedrock peak ground motion, the variances in peak ground acceleration and characteristic period of response spectrum also increase gradually under different shear wave velocities of soil layers. ( 6) Under the same bedrock input ground motion,the variation of shear wave velocity of soil layer in the relatively soft soil profile ZK24 has a bigger effect on the peak ground acceleration and Earthquake Research in China 240 the characteristic period of response spectrum than that in profile ZK4. This shows that the impact of velocity variation on the ground motion is related to soil layer structure,and the shear wave velocity of soil layers can directly reflect the degree of “softness”and “hardness”of the soils. The softer the site,the greater the impact will be. Fig. 5 Effects of variability of the shear wave velocity of soil layers on peak ground acceleration Fig. 6 Effects of variability of the shear wave velocity of soil layers on characteristic period of response spectrum 4 CONCLUSION Based on the data from typical sites in the Jianghuai region,the influence of shear wave velocity variation on the peak ground acceleration and the characteristic period of response spectrum is analyzed with the one-dimensional equivalent linear method. Our results show that: (1) When the shear wave velocity of soil layers decreases,the peak ground acceleration decreases gradually,and the long period part of the response spectrum moves to the right,the long period spectrum value increases generally,and the characteristic period of response spectrum Volume 28,Number 2 241 increases correspondingly. (2) When the shear wave velocity of soil layers increases,the peak ground acceleration also increases gradually,the long period part of response spectrum moves to the left,and it has a greater effect on the medium and short period part of the response spectrum; the response spectrum curve moves to the left,and the characteristic period of response spectrum will be reduced accordingly. (3) In contrast,the effect of decrease of shear wave velocity of soil layers is slightly larger than its increase on the peak ground acceleration and the characteristic period of response spectrum. (4) The impact degree of velocity variation on the ground motion is related to the spectral characteristics and amplitude of bedrock input ground motion as well as the soil layer structure of the site,etc. Thus,the variation of shear wave velocity of the soil layer has significant influence on the ground motion. Obtaining objective shear wave velocity data is very important for seismic response analysis of the soil layer. This paper has been published in Chinese in the journal of Technology for Earthquake Disaster Prevention,Volume 8,Number 3,2013. REFERENCES Chen Guoxing,Liu Xuezhu,Wang Binghui. Effect of variability of soil dynamic parameters on ground motion parameters for deep soft sites [J]. Journal of Disaster Prevention and Mitigation Engineering,2007,27(1) : 1 ~ 10( in Chinese with English abstract) . Gao Yufeng,Liu Hanlong,Zhu Wei,Gu Changcun. Influences of shear wave velocity on the seismic response of sand ground [J]. Geotechnical Investigation & Surveying,2001,1: 39 ~ 42 ( in Chinese with English abstract) . General Administration of Quality Supervision,Inspection and Quarantine of the Peoples Republic of China. Evaluation of Seismic Safety for Engineering Sites ( GB17741-2005) [M]. Beijing: Chinese Standard Press, 2005 ( in Chinese) . Hu Yuxian,Gao Mengtan,Du Wei et al. The Teaching Material of Seismic Ground Motion Parameters Zonation Map of China [M]. Beijing: Chinese Standard Press,2001. 85 ~ 87( in Chinese) . Lan Jingyan,Bo Jingshan,Lv Yuejun. Study on the effect of shear wave velocity on the design spectrum [J]. Technology for Earthquake Disaster Prevention,2007,2(1) :19 ~ 24( in Chinese with English abstract) . Liao Zhenpeng,Li Xiaojun. A Equivalent Linearization Method for Calculating Earthquake Response of Ground Surface Soil Layers: Earthquake regionalization ( Theory and Practice) [M]. Beijing: Seismological Press, 1989. 141 ~ 153( in Chinese) . Liu Hongshuai,Bo Jingshan,Wu Zhaoying,Liu Dedong. Effects of soil parameters on ground surface peak acceleration and response spectra [J]. Journal of Seismological Research,2005,28 ( 2 ) : 167 ~ 171 ( in Chinese with English abstract) . Sun Rui,Yuan Xiaoming,Liu Xiaojian. Effects of dynamic shear modulus ratio and velocity on surface ground motion and their equivalent relations [J]. Chinese Journal of Geotechnical Engineering,2009,31(8) : 1267 ~ 1274( in Chinese with English abstract) . Yuan Xiaoming,Sun Rui,Sun Jing,Meng Shangjiu,Shi Zhaoji. Laboratory experimental study on dynamic shear modulus ratio and damping ratio of soils [J]. Journal of Earthquake Engineering and Engineering Vibration, 2000,20(4) :133 ~ 139( in Chinese with English abstract) . About the Author Cao Junfeng,born in 1983, is an engineer at the Earthquake Administration of Anhui Province. He is mainly engaged in earthquake engineering work. E-mail:caojunfeng2009 @ 126. com.