The Study On The Seismic Response Of The Gravel Formation On Chengdu Plain

An earthquake of Ms 8.0 hit Wenchuan, on May 12, 2008 ,which caused heavy casualties and property losses, and induced a large number of geohazards. At the southeast of Longmen Mountain, the Chengdu Plain, known as rich plains and large population, is classified as Quaternary gravel formation. Through field investigation, it was found that the damage in the Chengdu Plain was less than the area of Longmenshan. For these reasons, the study on the seismic response of the gravel layer is exceedingly important and extremely meaningful.The thesis uses the seismic design code of EduShake to analyze seismic response of gravel, and explores the effects on the peak acceleration, predominant period and weak intercalation. The results show:(1) In Chengdu Plain, the gravel formation is a formation of hard and short seismic period, so the amplification to short-period seismic wave is stronger than long-period wave. The model makes short part of response spectrum near ground surface significantly enlarge, and makes predominant period move towards short-period. At the same time, the model weakens the details of Seismic waves, but it concentrates the distribution of energy.(2) Although there is non-relationship between amplification and the thickness of the gravel layer, but the amplification of the top layer of non-gravel will enlarge with its own thickness. With the depth increase of the non-gravel, the predominant period spectrum will move towards long-period, which part is excellent amplified. While response spectrum display the decay phenomenon, and several significant periods distribute balanced.(3) The peak ground acceleration of response spectrum grows with that of input spectrum, but it displays "negative index" changes with the depth increase of the gravel. If the thickness of the lower gravel formation is more than 58m, the 5m thick non-gravel layer contributes scarcely to the peak ground acceleration.(4) From the comparative analysis between Soft soil and hard soil, it can be learnt that the weak intercalation, of which the thickness is thin, impacts little on the ground response spectrum, so this part could be regarded as hard formation. With the increase in the thickness of the weak intercalation, the model displays the decay phenomenon, then the curve of the ratio of peak ground acceleration goes up, especially to middle-period and long-period Seismic waves.