Application of passive multichannel analysis of surface waves method at sites close to underground railways - Problems and a case study

摘要

The Passive Multichannel Analysis of Surface Waves (MASW) method is a non-intrusive and cost effective technology adopted in civil and earthquake engineering for site geological and geotechnical survey. However, the application of passive MASW method under urban environments meets some challenges. One of interesting problems is associated with underground railways, which can be treated theoretically as buried sources. Since Rayleigh wave energy is dominant among that of surface waves, it is critical to design a proper geophone configuration during field tests at a site close to the metro line to ensure significant Rayleigh wave energy and therefore reliable dispersion images. This paper studied the propagation of Rayleigh waves generated from a buried source and proposed an empirical formula to estimate the nearest offset, which was then verified by numerical simulations. A case study was implemented in Shanghai, China at a site close to a subway line. With a linear spread geophone configuration, the influence of different parameters, including nearest offset, spread length and geophone interval, was investigated regarding their influences on dispersion images. Field tests were conducted by following the geophone configuration designed based on the proposed estimation method. The one dimensional (1D) shear velocity structure, being inverted from field vibration measurements, matches relatively well with the geologic feature obtained from borehole testing. Study results are expected to help facilitate the application of the passive MASW method for cases in which Rayleigh waves are generated by the buried source like underground traffics in cities. (C) 2019 Elsevier B.V. All rights reserved.