Prediction of rail transit ground vibration with shear-wave velocity profiles

摘要

The Federal Transit Administration recommends testing procedures for evaluating environmentalimpacts of ground-borne noise and vibration from rail transit systems. These procedures rely onthe concept of a Line Source Response (LSR) for describing propagation of vibration throughsoils. The Line Source Responses are computed from measured transfer mobility functionsbetween force and velocity by conversion of transfer mobility responses to third octaveresponses, regression of the responses over distances, and numerical integration over a linesource representing the train. Numerically computed transfer functions, based on verticalprofiles of seismic wave velocities, have also been developed by the author for environmentalanalyses of vibration impacts at a number of rail transit projects. The methodology employs aseismic reflectivity model and measured velocity profiles, with a constant-Q assumptionregarding dissipation. The frequency range of ground vibration produced by rail transit systemsis roughly 8 to 80Hz, while seismic velocity profiles are obtained by measuring first arrivals ofshear and compression waves at higher frequencies. The shear and dilatational moduli are thuspotentially dependant on the dispersion relations used in the constant-Q model. Comparisons ofmodel predictions with measured ground vibration responses are presented, and the significanceof constant-Q dispersion is investigated for glacial tills with relatively high seismic velocity.Also discussed are the relative strengths of the near field, compression wave, and shear waveresponses.