Study On Low Frequency Characteristics Of Environmental Vibrations Due To Metro Trains

In recent years, environmental vibrations induced by metro trains have been paid more and more attention; especially the low frequency (<20 Hz) vibrations might affect the operation of sensitive equipment and high-tech machinery. It is necessary to further study the characteristics of the propagation of potential environmental low frequency vibrations due to metro trains. How to adopt reasonable vibration mitigation measures to minimize the impact of low frequency vibrations is an urgent problem to be solved.Based on the steel spring floating slab track (FST) and the ordinary track (direct fixation fasteners) in the tunnel, the characteristics of the propagation of low frequency vibrations have been studied, as well as the influence of key parameters on the low frequency characteristics of environmental vibrations. Using the combination of laboratory test, theoretical analysis and numerical method, the coupled track-tunnel-soil system is modelled to analyze the low frequency vibrations. The analysis formula of low frequency ground vibrations is derived. The critical parameters and their influence on low frequency ground vibrations are evaluated.Low frequency vibration tests on the steel spring floating slab track (FST) and the ordinary track in the Laboratory of Track Vibration Abatement and Control on the campus of Beijing Jiaotong University are carried out to study: (a) the attenuation of low frequency vibrations in the tunnel; (b) the variation of low frequency vibrations in the free field; (c) the response of low frequency vibrations in an adjacent building; (d) the influence of varied key parameters including the stiffness and spacing of steel springs; (e) the comparison of vibration isolation efficiency between the FST and the ordinary track.Dynamic characteristics of the FST and the ordinary track are studied in detail, using the modal analysis method. Based on the theory of orthogonal experimental design, a detailed parametric study is performed to analyze the influence of various parameters on the dynamic characteristics of the FST. The parameters are related to the length and thickness of the slab, the stiffness and spacing of the slab bearings and the stiffness of the rail pads. The transmission ratios of different tracks are obtained and compared.Based on measurement data, the coupled FST-tunnel-soil system and the coupled ordinary-track-tunnel-soil system are respectively modelled by using the finite element method to further analyze the vibration level, insertion loss and transfer function of low frequency vibrations.On a basis of test data and numerical results, the influence of the excitation frequency (1-20 Hz), the distance (0-100 m) from the track and the buried depth (10-30 m) of the tunnel on the low frequency ground vibrations are analyzed, using the theory of regression analysis. Hence, the analysis formula of low frequency ground vibrations is derived.Maximum values of vibration acceleration level calculated from the analysis formula are divided into four-level response zones consisting of I, II, III and IV. The relationship between the distribution of response zones and critical parameters including the buried depth of the tunnel, the distance from the track and the basic frequency of the track is elaborated.