Analysis Of Site Seismic Characteristics And Roadbed Dynamic Response In The Qinghai-Tibet Plateau Engineering Corridor In Permafrost Region

National significant basic projects,containing already existed and proposed structures,are all densely located in permafrost engineering corridor of Qinghai-Tibet Plateau.Due to important geographic position in this region,specific engineering mechanics properties of permafrost,frequent earthquake activity,rapid global warming and continuous artificial disturbance,permafrost degradation becomes faster,which has made stability problems of structures and ecological environment problems becoming increasingly prominent in permafrost engineering corridor,where the width is less than 10 km.By the coupling influence of multiple factors and interaction of various structures themselves,lifeline engineering along engineering corridor would face huge risks and challenges under the superposed function of earthquake dynamic loading.Therefore,carrying out site seismic characteristic and dynamic response of proposed expressway roadbed along engineering corridor has urgent practical and scientific significance for seismic safety evaluation of major construction engineering in this region.In this paper,in order to solve the problems related to site characteristic and dynamic response mechanism of roadbed along permafrost engineering corridor of Qinghai-Tibet Plateau under seismic loading,wave velocity characteristic of soil in permafrost site along the engineering corridor,dynamic characteristic of frozen Qinghai-Tibet silty clay,seismic characteristic parameters of site and dynamic response of roadbed were analyzed by using following methods including summarizing field test data,dynamic triaxial laboratory test and numerical simulation.The main work and conclusions of research are summarized as follows.(1)According to the borehole data of wave velocity along Qinghai-Tibet engineering corridor,wave velocity characteristics of frozen soil have been analyzed in permafrost site.Inductive data can be concluded that wave velocity value of frozen soil is generally greater than that of ordinary soil,the ratios of vertical and horizontal wave velocity is approximately 1.5.Wave velocity of frozen soil increases with decreasing ground temperature and increasing ice content of frozen soil.(2)By dynamic triaxial test under cycle loading for remolded frozen Qinghai-Tibet silty clay,dynamic characteristics of remolded frozen soil influenced by confining pressure,negative temperature,moisture content and frequency were studied systematically.The experimental results are summarized as follows.The damping ratio of frozen soil decreases with decreasing confining pressure,negative temperature and moisture content,increasing frequency.And the ratio of dynamic shear modulus decreases with increasing confining pressure,negative temperature and frequency,decreasing moisture content.(3)Based on one-dimensional soil layer seismic response theory,according to seismic geological conditions,geophysical characteristics and seismicity in Qinghai-Tibet plateau,soil layer seismic response study are calculated in permafrost area.And the seismic time history of bedrock,seismic parameters of design site and earthquake response spectrum are obtained under the level of seismic waves with exceeding probability for 63%,10% and 2% in 50 years.(4)By using two-dimensional nonlinear dynamic finite element analysis method and modeling cross sections of integral and separated roadbed respectively,acceleration amplification effect,stress response characteristics and displacement response characteristics of two typical models are analyzed respectively under the function of the Qinghai-Tibet seismic waves with the exceeding probability for 63%,10% and 2% in 50 years.Meanwhile,by changing dividing distance of separated roadbed,dynamic response characteristic effects are obtained under different dividing distance conditions.