| Site seismic effect is one of the research hotspots in the field of seismology and seismic engineering.Its research has important theoretical significance and engineering value for the numerical simulation and spatial distribution characteristics of ground motion,the location of buildings（structures）and seismic fortification.However,since soil is a heterogeneous multiphase medium,it will show complex dynamic behavior under the condition of large dynamic strain.Therefore,how to determine the dynamic characteristics of in-situ soil is one of the main challenges in the study of site seismic effects.In order to further evaluate the in-situ soil dynamic properties of soils at different sites,especially the nonlinear dynamic behavior under strong earthquakes,three aspects of in-situ soil dynamic parameter estimation under small strain,site nonlinear soil dynamic characteristics and its coseismic and post earthquake change process are studied by using the ground motion observation data of three geotechnical arrays in the United States and KIK net network in Japan.The main work and achievements of the paper are as follows:（a）Assessment of in-situ soil dynamic parameters under small strain1.There are obvious deficiencies in the seismic interferometry by deconvolution used by predecessors,such as:using band-pass filter to remove the high-frequency components,unable to identify the shear wave velocity and compression wave velocity in the thin soil layer,the deconvolution function under weak motion is unstable,the identification accuracy is limited and the error is large.In this paper,the method is improved for these problems,and now it can stably identify the shear wave velocity and compression wave velocity of thin soil layers under weak motion,and improve the travel time identification accuracy to ten thousandths of a second.Utilizing the weak motion data of three geotechnical arrays located at the Crockett-Caquinez Bridge,Delaney Park,and Treasure Island in the United States,and using the improved seismic interferometry by deconvolution in this paper,the in-situ compression wave velocity and shear wave velocity,and in situ soil dynamic parameters such as Poisson’s ratio,elastic modulus,bulk modulus and shear modulus are evaluated.The calculated shear wave velocity and compression wave velocity are similar to the measured wave velocity profile.The calculated in-situ soil dynamic parameters reflect the soil dynamic characteristics under the actual seismic load,such as in-situ stress,confining pressure and pore water pressure.2.Although it is a general consensus among geotechnical engineers and geologists that geotechnical materials have anisotropy,site anisotropy is rarely considered in practical engineering applications.Using the improved seismic interferometry by deconvolution method and the borehole to ground spectral ratio method,the relationship between the anisotropic characteristics of shear wave velocity of three geotechnical arrays in the United States and the geological landform and sedimentary environment of the site,as well as the anisotropy of site effect are studied.The causes of shear wave velocity anisotropy of three American geotechnical arrays can be summarized as follows:the three American geotechnical arrays are mainly affected by river hydrodynamics,sedimentation,glaciation or faults,and human factors.Among them,the hydrodynamic effect of the river caused the shear wave velocity anisotropy of the three arrays to reach 5.78-13.05%,which was related to the flow direction of the river;the shear wave velocity anisotropy of the marine and lacustrine sediments was relatively small;the shear wave velocity anisotropy caused by glaciation and faults is 50.85%and 32.86%,and the shear wave velocity anisotropy caused by human factors can reach 4.10%and 3.34%.The ratio between the maximum value and the minimum value of the spectral ratio peak amplification factor of the upper and lowest layers of the three geotechnical arrays is between 1.17 and 1.59.The seismic effect of the site also has obvious anisotropy.（b）Research on Nonlinear soil dynamics characteristics of Sites1.Due to the enhancement of global seismic activity and the construction of strong earthquake observation network in recent years,a large number of high-intensity strong ground motion observation data have been accumulated,which lays a foundation for the systematic study of nonlinear site response based on a large number of strong earthquake observation data.Using 45186 sets of acceleration records of 28 Ki K-net arrays from February 1,2008 to November 30,2021,the relationships between kappa parameters,damping ratio,shear modulus ratio,compression modulus ratio,fundamental frequency decline and strain is studied.And the empirical relationships between them are established.2.There are obvious differences in the threshold research results of different scholars on the nonlinear effect of soil,even for the same site.So far,the volume threshold PGA（PGAtv）of soil has not been studied by using strong motion data.Combining experimental data with strong vibration data to estimate linear threshold PGAtl and volumetric threshold PGAtv is an urgent problem to be solved.Based on 6shear modulus degradation curves tested by Vucetic and Dobry（1991）in the laboratory and the strong motion data of 28 vertical arrays of Ki K-net,the empirical relationships between PGA and in-situ shear modulus ratio and strain of the soil above the arrays are established respectively.The relationships between in-situ linear,significant nonlinear and volumetric threshold shear strains(i.e.,γtl,γtsnlandγtv)and plasticity（PI）,as well as the empirical relationships between the corresponding threshold PGA(i.e.,PGAtl,PGAtsnland PGAtv)and plasticity index（PI）and the measured average shear wave velocity(Vs,soil)of the soil above the array.Using the measured average shear wave velocity(Vs,soil)and plasticity index（PI）,the extremely weak ground motion,weak ground motion and medium to strong ground motion are classify to estimate the soil behavior and possible geotechnical disasters during the earthquake.（c）Research on co-seismic and post-seismic change process of site nonlinear soil dynamics characteristics of siteDifferent scholars have disputes on the issue of site recovery time,ranging from tens of seconds,minutes to tens of minutes,one day,several months to several years.At present,many scientists generally believe that the site recovery process is divided into two stages（short-term rapid recovery stage and long-term slow recovery stage）,but how long it can recover,to what extent,and whether it can recover to the pre-earthquake level are still issues that need to be resolved.1.Using the ground motion data recorded by 142 arrays of the Ki K-net network of the 2011 earthquake of the Pacific coast of Tōhoku on March 11,2011,the coseismic variation processes of shear wave velocities（Vs）,fundamental frequencies and amplifications of 5 groups of sites with different shear wave velocities and different intensities are studied.The results show that:with the increase of PGA and shear wave velocity（Vs）of sites,the maximum decreasing amplitude of shear wave velocity increases,the recovery degree decreases,and the recovery amplitude increases.2.Using the data of four earthquakes with magnitudes above 6.9 in the east coast of Honshu Island,Japan,the relationship between the maximum decreasing amplitude,the recovery degree and the amplitude of the shear wave velocity and the ground motion intensity is studied.The results show that the shear wave velocity of the site basically recovers to more than 90%of the pre-earthquake level at the end of the earthquake load.The maximum decrease and recovery amplitudes of shear wave velocity both increase with the increase of intensity.Under the same PGA,the shear wave velocity recovery amplitudes of different earthquakes and different groups of pre-earthquake shear wave velocity are similar.When the PGA is less than 20cm/s2,the average recovery amplitude of the shear wave velocity is about 1%,when the PGA reaches 200cm/s2,the average recovery is about 6%,and when the PGA reached 500cm/s2,the average recovery is about 12%.And takeγtl=1×10-5andγtv=1×10-4as the boundary to divide the soil behavior（linear elastic,nonlinear elastic and elastic-plastic stages）during the the seismic load,the shear wave velocity recovery process of the arrays with average shear wave velocity less than 760m/s is divided into instantaneous,rapid and long-term recovery stages.As a strong nonlinear material,bedrock has a lower degree of recovery than soil at the end of the coseismic change process of shear wave velocity of a single earthquake,but the strain in bedrock is small,which is not enough to cause damage to bedrock.This phenomenon is a slow dynamic characteristic.3.Using the same data of 28 Ki K-net arrays used in Chapter 3,the long-term recovery process of shear wave velocity,compression wave velocity,fundamental frequency and amplification factor after earthquake is studied.When the shear strain is greater than 1×10-4（volume threshold strain）and there is no seasonal variation and equipment adjustment,the shear wave velocity and fundamental frequency of most arrays have not fully recovered to the pre-earthquake level 11 years after the earthquake,the permanent damage to the soil cannot be fully recovered.The amplification factor only decreased when the site is subjected to strong vibrations,but returned to the pre-earthquake level at a very fast speed,which also reflected that the wave impedance ratio of the bedrock and overlying soil is less affected by the earthquake.4.Combining the results of the coseismic short-term fast recovery process and the post-earthquake long-term slow recovery process,the nonlinear recovery of the four parameters（i.e.,shear wave velocity,compression wave velocity,fundamental frequency and amplification factor）is divided into two distinct stages.During the coseismic short-term rapid recovery of shear wave velocity,most arrays（except those with shear wave velocity greater than 1500 m/s before the earthquake）can recover to more than 90%of the pre-earthquake level at the end of the seismic load;In the study of the long-term recovery process after the earthquake,the shear wave velocity and fundamental frequency of most arrays can recover to more than 95%of the pre-earthquake level one month after the strong earthquake,and then recover slowly.The shear wave velocity and fundamental frequency recover to more than 99%of their pre-earthquake levels in about 10 years after the strong earthquake. |
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