Identification of line-source transfer mobility and force density by ground acceleration measurements

Abstract

Abstract Ground-borne vibration caused by railway traffic has been a research concern due to its possible side effects on nearby residences. The force density and line-source mobility can effectively characterize the generation and transmission of train-induced vibrations, respectively. This research proposed a frequency-domain method for identifying the line-source mobility and force density using measured vibrations at the ground surface. It was achieved by minimizing the difference of force densities formulated using measurements at two different locations, in which the genetic algorithm was adopted for solving the optimization problem. The proposed method was later applied to and validated by a case study at Shenzhen Metro in China, where a total of seven fixed-point hammer impacts with 3.3 m equal intervals were used to represent the vibration excitation from one vehicle. The fixed-point loads' assumption was validated by comparing the identifications with the predictions based on the train-track coupled dynamic model and theoretical derivation. Causes for different dominant frequencies can be traced by separating the dynamic characteristics of vibration excitation and transmission. It was found in the case study that at a location 3 m away from the track, the peak at 50 Hz was caused by excitations, while that at 63 Hz was attributed to transmission efficiency related to the soil properties. Finally, the identified line-source transfer mobility and force density levels were applied to the forward problem of making predictions. The predicted ground vibrations at different locations were compared to field measurements, with good agreement, which experimentally validated the identification method. The identification results of the case study can be employed by similar railway systems as a good reference.