Study On Nonlinearity Of Site Effects On Ground Motion Based On Vertical Borehole Array Data

The seismic site effect is still a hot topic in the field of earthquake engineering,and the soil nonlinear response under the strong motion is the most difficult.Using mathematical statistics and numerical simulation,scholars has proposed so many methods to evaluate the site effect,and brought them to practical application.Recently,great numbers of strong motion records are available with the fast development of strong motion observation technology and the layout of strong motion observation around the world,these records provide more and more abundant field data for the site effect study.The Ki K-net vertical arrays provide a rich data for study from both the surface and the downhole.Based on the strong motion records from the Ki K-net,the relation between the surface/downhole amplification factor and the ground motion intensity are statistically analyzed,and the nonlinearity between site condition and amplification effect of ground motion.is studied.Firstly,142 stations of which PGA>300gal are selected as the data base,these data are divided into 5 groups in each single station.Secondly,the surface/downhole amplification factor of all the stations are calculated,then the equivalent shear wave velocity are calculated to evaluate the relation with the amplification factor.Lastly,the reliability of acceleration response spectrum calculated by borehole strong motion record is verified by numerical simulation of a specific example.The main findings are as follows:?1?The result of the statistical analysis using the measured data of vertical borehole array indicates that: with the increase of ground motion intensity,the maximum value of surface/downhole ground motion acceleration response spectrum decreases gradually.The maximum value can be predicted by the following formula:C_sap=C₀*(1.00-0.176 log₁₀?ai/a0?±0.18) Where,C_sap is the maximum value of surface/downhole ground motion acceleration response spectrum when the peak acceleration is ai<300cm/s²?unit cm/s²?,C₀ is the average value of the maximum ground/downhole ground motion acceleration response spectrum when the peak acceleration is between0.3cm/s² and 3cm/s².and a₀=1cm/s²..?2?The result of the statistical analysis using the measured data of vertical borehole array indicates that: with the increase of ground motion intensity,the period value corresponding to the maximum surface/downhole ground motion acceleration response spectrum tends to increase gradually,but its dispersion is large.The following equation can be used to predict the period value corresponding to the maximum response spectrum of surface/downhole ground motion acceleration:T_sap=T₀+0.043 log₁₀?ai/a0?±0.03 Where,T_sap is the period value corresponding to the maximum response spectrum of surface/borehole ground motion acceleration when the peak acceleration is ai<300cm/s²?unit cm/s²?,T₀ is the corresponding period value of the maximum mean value of surface/borehole ground motion acceleration response spectrum when the peak acceleration is between0.3cm/s² and 3cm/s².and a₀=1cm/s².?3?Statistics show that the maximum value and corresponding period of the surface/borehole ground motion acceleration response spectrum are weakly correlated with the equivalent shear wave velocities Vs₂₀ and Vs₃₀,which is inconsistent with the previous cognition.This phenomenon needs to be confirmed by detailed statistical analysis.?4?A few examples of numerical simulation results show that the calculated base seismic response spectrum and imported motion response spectrum exist difference but the difference is not big,so within a certain precision,the surface/downhole seismic acceleration response spectrum is seen as the surface/incident?the 1/2 of surface ground motion?acceleration response spectrum.