Characteristic Of Ground Motion At Sites Influenced By Soil Dilatancy

In the field of civil engineering,site seismic effect and ground motion characteristics have always been an important research topic.As reliable field data,strong motion records and station site information provide a basis for the study of site seismic propagation and ground motion characteristics.During the several great earthquakes occurred in recent years,such as the Wenchuan earthquake in 2008 and the Tohoku earthquake in 2011,there appeared a considerable number of strong motion records with spike waveform.Such strong motion records have never so frequently appeared in previous earthquakes,and their peak ground acceleration(PGA)is usually caused by the spike contents.The formation mechanism and engineering characteristics of such strong motions and their effects on the structure dynamic response have become a topic worthy to study.Therefore,in this thesis the formation mechanism,engineering characteristics of spike-like ground motions and their influence on the structure response is studied.The main work and conclusions are summarized as follows:(1)Based on the strong motion records from the Tohoku earthquake,the characteristics of amplitude,PGA amplification coefficient and response spectrum of strong motions with spike waveform are studied.It is found that the PGA of such records obviously exceeds the value predicted according to the existing attenuation equations,and the amplitude of the“spike”part shows a sudden increase,and the amplification effect of the“spike”in high or medium to high frequency range is significant.Combining the existing formation mechanism of spike waveform and the characteristics of spike-like strong motions,other causes of spike waveform are excluded.It is found that the nonlinear,dilatancy and pore water pressure of sand layer are the major cause of spike-like strong motions frequently appeared in the Tohoku earthquake.(2)Using a high-level nonlinear constitutive model in which both the dilatancy and pore water pressure of the sand layer can be considered,the refined site modeling of two stations(MYGH04 and Onahama)with typical spike strong motion records from the Tohoku earthquake is carried out.The propagation of seismic wave in the soil layer is simulated,and the spike acceleration,response spectrum and amplification of high-frequency components recorded on the surface are accurately reproduced,it is revealed that the mechanism of acceleration spike is caused by the cyclic mobility effect due to dilatancy of the sand layer.(3)Based on the findings from above simulation and the correlation between the spike acceleration recorded on the ground surface and the cyclic mobility effect due to dilatancy of sand layer,an index and method based on support vector machine(SVM)is put forward in this thesis to quickly identify the ground motions which are significantly influenced by the cyclic mobility due to sand dilatancy from a large number of ground acceleration records.Then,the strong motion records from the K-NET and KIK-net seismic networks are successfully divided into two categories:the records with significant cyclic mobility due to dilatancy(DCM records)and the records without or with not iobvious cyclic mobility due to dilatancy(NDCM records).(4)The characteristics of amplitude,frequency contents and nonstationarity of two type of ground motions(DCM and NDCM ground motions)are studied,and the influence of cyclic mobility effect due to dilatancy to them are explored.The results show that in terms of amplitude attenuation,all notions of amplitude of DCM records are larger than those of NDCM records,and the attenuation with fault distance is also slower.In the range of fault distance 50?200km,the PGA ratio of DCM records to NDCM records is 2.5?5.5,the ratio of PGV is 2?3.5,the ratio of EPA is 2.5?5.8,the ratio of EPV is 1.5?2.7,and the ratio of Arias intensity is 5?17.In terms of frequency contents,the comparison of H/V spectrum ratio between two types of records shows that DCM records have a larger amplification than NDCM records in the whole frequency domain generaly,the most significant difference for class II sites locates in the range of 0?5Hz,the largest ratio for DCM records is 1.7 times of that for NDCM records at about 3Hz,the most significant difference for class III sites is in the range of0?3Hz,and the largest ratio for DCM records is 1.8 times of that for NDCM records at about 1Hz.Compared with NDCM records,DCM records have smaller T_g and larger?_max.The comparison of the response spectra of the two types of records shows that DCM records have more significant influence on structures with a natural period less than 1s,but have no significant influence on the structure with a natural period more than 2s.In terms of nonstationarity in frequency contents,DCM records have more high frequency components and smaller decrease in instantaneous frequency than NDCM records.Comparison of the instantaneous spectral ratio of the two types of records and the records from liquefaction sites,it is shown that there are significant difference in the instantaneous frequency of DCM records,NDCM records and the liquefaction site records,and the different changing rules of the instantaneous eigen frequency of the site can also be used as an index to discriminate these three types of records.(5)The typical DCM and NDCM strong motion records are selected as the input ground motions,and three kinds of structures with different periods are designed to investigate the influence of two types of ground motions on the structural response.The elastoplastic time history analysis of structures shows that:the spike acceleration has an obvious influence on the response of short-and medium-period structures.The maximum story shear force and story drift response of structures under DCM input is about 2 times of that under NDCM input.The spike acceleration has no significant effect on the response of long-period structure.The maximum story shear and story drift responses under DCM input are basically the same as those under NDCM input except the slight amplification of response at the upper part of the structure.(6)The spike records before and after spike elimination are used as input ground motions to explore the influence of spike waveform on the response of short-and long-period structures.The results show that the spike has a great influence on the response of short-period structures,the shear force,displacement response,damage degree and energy of each story under spike inputs are greater than those under inputs without spike waveform,the maximum response is consistent with the occurrence time of the spike waveform indicating that the maximum response of the structure is mainly caused by the spike.The spike waveform has no obvious effect on the response and damage of long-period structure.The innovations of this thesis are summarized as follows:(1)It is found that the non-linearity,dilatancy and excess pore water pressure of the sand soil layer are the main reasons for the strong motion records with spike waveform commonly existing in the records from the Tohoku earthquake.In this thesis,the fine modeling of two typical stations with spike-like strong motions is carried out,the propagation of seismic wave in soil layer is simulated,the spike acceleration,response spectrum and significant amplification of in high frequency contents are accurately reproduced,and the formation mechanism of spike waveform inside the soil layer is revealed,it is caused by cyclic mobility effect due to dilatancy of sand layer.(2)In this thesis,the index and method of quickly identifying the records which are significantly influenced by the cyclic mobility effect due to dilatancy from a large number of surface acceleration records are proposed,and records from the K-NET and KIK-net network are successfully divided into two types,i.e.,DCM and NDCM records.(3)The differences of characteristics in amplitude,frequency contents and nonstationarity between DCM and NDCM records are compared comprehensively,and the influence of cyclic mobility effect due to dilatancy on the characteristics of ground motions and dynamic response of structures with different natural periods is explored.(4)Based on the research findings of nonstationarity in frequency contents,a method is proposed to accurately identify the dilatancy effect of the site by using the instantaneous spectrum ratio,this method can also identify the situation of site liquefaction and without dilatancy at the same time.