The Study On Characteristic Of Ground Motion In Site Of Loess Valley

In the Midwest of China,the loess plateau region is earthquake pron area, where many cities,villages and towns as well as infrastructure projects are built in the loess valley area.According to seismic design code,the loess Liang, Mao slopes and valleys and gully edge belong to seismic risk area or unfavorable site,landslides collapse also tend to occur on this site caused by the eart-hquake. With the development and vigorously promotion of city and urbanization,the Midwest regional seismic safety problem is increasingly highlighted.This paper combining with the characteristics of regional seismic damage and the valley of the loess area as the research object,study the characteristics of ground motion under various types of seismic waves by the method of numerical simulation and field test,and try to reveal the earthquake the trigger mechanism of this kind of ground seismic geological disaster.The main research conclusions are as follows:(1)The typical valley field models are established, through the different amplitude, frequency spectrum characteristics of the input and the duration of the seismic wave, for there is rock and have the loess covered valley in the area for the seismic response analysis: I. under different earthquake intensity, bedrock and loess covered valley the ground motion increases with the input of the monitoring; Bedrock ground motion increases and relief were positively correlated; compared with the monitoring bedrock valley, loess covered valley in the spectrum amplitude of the ground motions are increased, and spectrum peak move to the high frequency. II. under different earthquake spectrum characteristics, the different locations of valley area have a clear different PGA magnification, and ground motion at each point increases with the energy of input fields vibrations. III. under different earthquake action duration, the same venue location of seismic spectrum shape with similar characteristics, along with the duration growth, the spectrum amplitude increases obviously, the spectrum energy to concentrate in near field characteristic frequency. All in all, under different earthquake ground motion action, the characteristics the of valley field amplitude spectrum at different positions of site controlled by the soil and terrain characteristic frequency.(2) The test loess valley site micro-tremor test and analysis results show that the valley spectral values variation with the elevation of the lower down from the shoulder to the bottom, each point basic frequency decrease with height to the high frequency spectrum is moving, the bottom ground pulsation spectrum frequency and shallow seismic exploration is to reveal the formation of wave impedance interface. Test site ground micro-tremor characteristics reveal the vibration characteristics of the type valley field(3)The loess valley type test site in the same position of the blasting vibration and the dynamic stress response of the observation results show that the dynamic stress response of the monitoring sites and their seismic amplitude have positive correlation; Upper part test slope under the action of blasting vibration from seismic wave energy gathering, the ground motion amplitude increases, the actual may first damage in the earthquake. According to the results of field investigation and shallow seismic exploration type loess valley proving ground finite element model is established, the calculation and analysis show that the input frequency and the basic frequency of venues to the distribution of the ground motions, the input frequency is together with the seismic source, propagation path and site condition, has a great deal of randomnes, the basic frequency and by site factors such as topography, soil structure, through the field test and numerical simulation of this paper found that in different ground motion input, site soil characteristics and terrain characteristics have different embodiment. Under the effect of ground motion, the top place tension failure happens first, as the earthquake duration increased, slope toe started to shear failure.