Comparative Study Of 2D And 2.5D Finite/infinite Element Approaches To Traininduced Soil Responses

The development of society and the desire of rapid transit have been the reasons for the construction and operation of high speed railways.In recent decades,the environmental vibration induced by moving trains has become the focus of engineers and researchers as it may affect the adjacent production lines and residents living quality.Among the numerous approaches previously used,Yang and Hung(2001)proposed the 2.5D finite/infinite element approach for studying the environmental vibration induced by moving loads,taking account of the fact that the 2D approach is incapable of simulating the load moving effect,while the 3D approach is computationally prohibitive.In this thesis,the basic concepts of both the 2D and 2.5D finite/infinite element approaches have been presented.Furthermore,relevant computation programs have been written and the results calculated have been compared with the previous ones.Specifically,attention has been paid to the computation of the Fourier transform for the 2.5D finite/infinite element approach.In view of practicality of the 2D approach in the state of initial design for design engineers and necessity to understand its characteristics,a comparative study has been performed between the 2D and 2.5D finite/infinite element approaches,in which parameters such as train speed,railway roughness and float slab track are included.To use the Fourier transform approach in the 2.5D approach correctly,the space range of the soil responses should be determined based first on the practical results.Then,one can calculate the time range of the soil responses with respect to train speed.Finally,he can determine a suitable time period and corresponding frequency resolution for the frequency response function.With these,he can obtain the real soil responses through the inverse Fourier transform approach.The major findings through comparisons are concluded as follows:(1)The results obtained from the 2D approach are larger than the 2.5D approach.(2)The soil responses derived from the 2D approach can be regarded as the special solutions of the 2.5D approach with infinite train speed.(3)The frequency response of the 2D approach doesn't contain the effect of railway roughness,i.e.the results of the 2D approach covers the whole range of frequency regardless of the railway roughness frequencies,while the results of the 2.5D approach are intensified mainly in the range of railway roughness frequencies.(4)After inserting the floating slab,the velocity and acceleration of the soil responses obtained by both of approaches are reduced significantly for the range of frequency higher than the natural frequency of the float slab track,although in the lower frequency range the 2D approach responses are enlarged more than the 2.5D approach.The contributions of this thesis can be summed as follows:(1)The Fourier transform approach applied in 2.5D finite/infinite element approach has been analyzed.It is important that suitable space and time ranges of the soil responses be determined for getting the real soil responses induced by trains,rather than the fictitious ones introduced by periodic extensions of responses.(2)A comparative study of the soiltunnel responses in time domain obtained through the 2D and 2.5D finite/infinite element approaches has been conducted.Moreover,the reason for the differences between the two results have been derived from the frequency domain responses.(3)Parameters like train speed,railway roughness and float slab track have been included in the process of comparisons.