Study Of In Situ Soil Dynamic Parameters And Site Response In Geotechnical Array

Site response and in situ dynamic soil parameters are two important issues in earthquake engineering and geotechnical earthquake engineering,which are quite critical in seismic design of engineering structures.During the last few decades,the study of site response mainly relies on numerical simulation,seismic hazard investigation and ground motion data recorded by surficial seismic stations,whereas the research of dynamic soil parameters highly depends on laboratory experiments and in situ test methods.In recent years,with the increasing number of geotechnical arrays,numerous ground motion data has been produced,which lays a solid foundation for the study of site response and in situ dynamic soil parameters.Records from geotechnical arrays could be used to assess site effects and reveal the mechanism of site response.Besides,geotechnical arrays provide data necessary for a better understanding of in situ dynamic soil behavior under real earthquake motion,covering loading paths and excitation levels that are not covered by laboratory experiments and in situ test methods.Besides,geotechnical arrays also provide a check on the accuracy and reliability of site response analysis methods currently used,and shed light on the calibration and improvements of laboratory experiments and in situ test methods.In this paper,280 groups of ground motion data from Garner Valley Downhole Array(GVDA)and 14 groups of ground motion data from Treasure Island Geotechnical Array were used.First,shear wave velocity profiles were extracted using seismic interferometry by deconvolution,and compared with that from in situ test.Second,the relationships of shear stress,shear strain and shear modulus ratio were obtained utilizing strong motion records to observe the site's nonlinear behavior under different levels of excitation and its variation with depth,the results fitted well with the results from laboratory experiments.Last,the variation of site amplification with depth was studied using Surface to Borehole Spectral Ratio(SBSR)method.The results are as follows:(1)At both arrays,the extracted shear wave velocity profile varies at certain range,which is in consistent with in situ tests.At GVDA,anisotropy of shear wave velocity is observed between east-west direction and north-south direction.(2)The relationship between shear modulus ratio and shear strain in shallow layers indicates that,the pseudo shear modulus starts to degrade when pseudo strain reaches about 10-6,becomes obvious when pseudo strain comes up to approximately,10-5 and strong nonlinearity can be detected when pseudo strain ascends to around10-4.While at deeper layers,the pseudo shear modulus does not show clear evidence of degradation.(3)As the seismic wave travels from bedrock to free surface,different vibration modes of soil layers are excited.The variation of site amplification with depth is in accordance with characteristics of different vibration modes.At GVDA,the shift of fundamental frequency to lower range is observed in two earthquakes that have peak ground acceleration exceeding 40 gal.