Earthquake Response Characteristics of Loess Sites in Wenchuan Earthquake
Large earthquake not only cause serious damage to structures in the near-fault sites,but also may cause damage to structures located far away from the epicenter due to the resonance effect.Especially large-scale structures such as high-rise buildings,nuclear power station,oceanic platform and large dam.Long period structures are sensitive to far-field earthquake ground motion,seismic design of long-period structures puts forward new requirements for design spectrum,and far-field ground motion has also become a must for seismic design.In the past,far-field ground motion is usually not considered or not to be considered with ordinary ground motion.The increasing number of structures with long natural periods makes it necessary to study characteristics of long-period earthquake ground motion,which can be reliably obtained from the records of digital accelerographs.However,because of lack of long-period seismic records all around the world,it is not well-developed on dynamic characteristics for long-period seismic motion.As a special seismic action or phenomenon,far-field ground motion shows different characteristics in every specific earthquake environment.Because of the filtering effect and amplification of thick soil layer,the part of long-period ground motion may be greatly enhanced.2008 Wenchuan earthquake caused serious damage to structures in loess region,the amplification effects of site conditions and topography on ground motion were significant in this area.After the Wenchuan earthquake,lots of digital strong motion records were obtained,especially far-field records.The research region are located on the Loess Plateau,with deep loess deposit.Loess is a kind of porous and weakly cemented soil,which shows a strong dynamic catastrophe characteristic of weak soil and high dynamic vulnerability.In loess area,the topography is complex,slopes are very developed,and the upper edge of high and steep slope has an obvious influence on the damage.In recent years,the study of topography,soil thickness and soil structure effects on seismic ground motion has been acquired substantial progress,while the influence of hydrogeological conditions on ground motion are still not clear.In this paper,a detailed analysis on earthquake response characteristics of loess sites in Wenchuan earthquake was carried out.The investigations including site inspection,site measurement,laboratory experiment,numerical simulation and shaking table test.The aim is to explore the earthquake damage characteristics and destruction reasons of high-rise buildings in far-field of large earthquake,to give some qualitative or quantitative analysis for the main factors(including earthquake ground motion characteristics,topographic feature,soil thickness and hydrologic conditions)affecting earthquake damages in the far-field.Combining with the actual earthquake damage characteristics,we also put forward some notable problems for seismic safety of buildings in loess area.The specific work is as follows:(1)In order to obtain earthquake damage characteristics and destruction reasons of high-rise buildings in Far-Field of large earthquake,several history earthquake disaster data of typical great earthquake are compared,key factors that control the destruction of high-rise buildings are found.The experience and lessons are summarized and the content of this paper are clarified.(2)Analyzing ground motion records of Wenchuan earthquake,change rule of peak ground acceleration,seismic response spectrum and duration time with propagation distance and site conditions are obtained,and characteristics of ground motion indices in Far-Field are clarified.And the deficiency of current design spectrum is pointed out,the modification of design spectrum considering far-field seismic action is discussed.(3)Choosing Wenxian slope as the typical loess site affected by the Wenchuan earthquakes for temporary strong motion array,12 aftershocks were recorded and analyzed using the traditional spectral method,and effects of altitude of loess slope on peak ground acceleration and response spectrum were analyzed.In addition,by using finite element analysis software,4 analysis models with different slope gradients are built and calculated,and dynamic response of slope with different slope gradients variating with altitude of loess slope are founded.(4)According to the typical loess-yuan site in Wenchuan earthquake,3 experiment models with different thickness of loess deposit are built and analyzed by shaking table test.In addition,based on datas of shaking table test and earthquake damage investigations,4 analysis models with different thickness of loess deposit are built and calculated by using the numerical analysis method,and characteristics of peak ground acceleration change with thickness of loess deposit and intensity of seismic ground motion are analyzed.(5)Constructing 10 groundwater level soil profiles and 11 surface water level soil profiles according to the typical loess site.Then using one-dimensional equivalent linearization wave motion method,seismic response of soil site by inputting 9 time histories of synthetic ground motions with 3 probability level were calculated,total about 189 cases.Effects of groundwater rise and surface water permeates on peak ground acceleration and acceleration response spectrum under different ground motion inputs were analyzed.The study of this paper have profound significance to determine the design ground motion parameters of construction site in loess region,and will improve the seismic fortification capacity of structures in loess region.