The Study On Seismic Stability Of Fangjiawan Loess Tunnel Structure Under Viscoelastic Boundary Condition

Tianping railway project is one of the key construction projects of the Eleventhfive-year plan of Gansu province. The construction has an important significance onthe implementation of Western regions development. It also promotes thetransformation of Gansu province from the resources advantage to the economicadvantage, and pulls the regional economic development. The tunnels of the project areall located in the loess area, so whether the safety degree could meet the normal usestate is an important issue. For FangJiaWan tunnel is a double large-span tunnel, so itsstability under earthquakes is analyzed in this paper.For the foundation absorption of scattering wave energy can be simulated byviscoelastic artificial boundary. And the elasticity restore performance of the mediumcan be simulated by it too. Also it has a very good calculation stability. According tothe engineering practice experience, the tunnel structure stability analysis should focuson surrounding rock and the first lining structure, but for loess tunnel structures, thestability of surrounding rock mass structure is related directly to the whole structure.And this paper puts the emphasis on the following aspects:(1) According to the actual situation of the FangJiaWan loess tunnel, through thebasic soil physical indexes and the existing literatures, the surrounding rock materialparameters and shear strength parameters are determined. Using the method ofdynamic fem static strength reduction, the seismic stability analysis of FangJiaWanloess tunnel structure is studied. Also the seismic stability under different earthquakeintensities are studied.(2) Using the software of ANSYS, the impetus and static calculation analysismodel of viscoelastic artificial boundary are both established, and they are realized byadding COMBINE14unit and MASS21unit on the cut artificial boundary, Using themethod of dynamic fem static strength reduction, the seismic stability analysis ofFangJiaWan loess tunnel structure is studied. Also the seismic stability under differentearthquake intensities are studied.(3) The safety factors of the boundaries are curved and analyzed, also theengineering measures are put forward. The seismic safety situation of the tunnels isestimated quantitatively, thus a reliable guarantee is provided for the normal operation.