Study On Amplification Effect Of Ground Motion In Thick Loess Covered Site

Loess Plateau is the most developed region of Loess in China,which has complex topography,intense neotectonic movement,frequent strong earthquakes and wide coverage of typical thick loess,resulting in a large number of building damage and geological disasters in this region.In view of the severe situation of ground motion amplification effect and the aggravation of earthquake damage in thick loess covered site,it is of great scientific significance to carry out the soil layer seismic response analysis of thick loess covered site and the study on dynamic response of loess slope under earthquake action for important engineering construction and operation in the Loess Plateau.In this paper,the soil layer seismic response of thick loess covered site and the dynamic response characteristics of loess slope are analyzed by using the seismic damage data of thick loess covered site,strong motion data,field borehole test and indoor soil mechanics experiment,combined with a new method of soil layer seismic response analysis,numerical calculation and large-scale shaking table test.The main research work and conclusions are as follows:(1)The seismic damage of buildings and geological disasters in thick loess covered site under Minxian-Zhangxian earthquake,Wenchuan earthquake and Yongdeng earthquake are summarized,including the seismic damage characteristics and causes of rural buildings;the characteristics and causes of geological disasters of loess landslide,loess liquefaction and loess seismic subsidence.Based on shaking table slope model test with the geometric similarity ratio of 1:25,the acceleration response characteristics of loess slope are studied deeply under earthquake action.The experimental results show that the loess slope model has significant amplification effect on the free surface and vertical amplification effect under earthquake action,which provides physical model verification for the numerical calculation of loess slope in thick loess covered site under earthquake action.(2)The reliability of SOILQUAKE was verified by collecting the surface and underground measured records of two stations from the KiK-net strong earthquake network in Japan.After querying and screening a large number of borehole data,the boreholes with cover thickness of 25.2m,52.6m,89.6m and 235.1m were selected.Finally,SOILQUAKE was used for soil layer seismic response analysis in thick loess covered site and the calculation results of the peak ground acceleration and magnification were discussed.The results show that with the increase of cover thickness,the peak ground acceleration of borehole decreases under the same probability of exceedance,and the long period spectrum component is more obvious,and the response spectrum curve moves to the right.The ground motion amplification effect in thick loess covered site is mainly reflected in the increase of the long period of the site,which makes the predominant period of the site close to the natural vibration period of the building on the site,leading to the resonance between the building and the foundation.(3)The model of five loess slopes with different slopes were established by ABAQUS finite element software,and the dynamic response analysis of five loess slopes with different slopes was carried out by inputting horizontal seismic wave recorded by Minxian strong seismic station in Minxian-Zhangxian M_S6.6 earthquake at the bottom of the model with infinite element-finite element coupling boundary.The results show that under the action of the same horizontal seismic wave,the peak areas of the maximum principal stress S_maxof loess slopes with different slopes are generally consistent,and on the whole,maximum principal stress S_maxof slope model increases with the increase of slope.The dynamic response characteristics of acceleration are basically consistent with results obtained from shaking table test,and both of them show obviously amplification effect on the free surface and vertical amplification effect.The amplification effect of ground motion increases with increase of slope angle,and loess slopes with the slope of 60°-70°have the most significant amplification effect on ground motion.Under action of the same horizontal seismic wave,the free surface appeared in all slope models(except the 20°slope model),and with the increase of slope,the second free surface developed along foot of slope began to appear.