工事振動への経験的予測手法の適用性に関する検討

 There are many reports applied the theory of wave propagation in an elastic half-space medium to predict environmental vibrations such as, traffic vibrations, railway-induced ground vibrations, factory vibrations and construction vibrations. In general, the empirical methods simplified the equation by Bornitz were practically used in the engineering fields; however, the applicability of those empirical methods were not clear enough, due to less usage of velocity of subsurface layer which is the most important key-parameter to predict environmental vibrations in a site. To simply and accurately evaluate construction vibrations in one of the environmental vibrations, therefore, we investigated on the applicability of the empirical prediction method based on the theory of wave propagation to construction vibrations, using velocity of subsurface layer.<br> At first, we performed linear array measurements for 4 types of construction vibrations at a site. We deployed 8 to 10 stations, which are composed of vibration level meters and data loggers, with an interval distance of 5m in a linear array. We recorded the vibrations induced by large-sized construction equipment such as a power shovel, a bulldozer and silent piler. The accelerations for 4 types of construction vibrations had the same tendency that the amplitudes were dominated in a frequency of 4 to 40Hz at a distance of 2m or 5m from construction sources, especially at a frequency of 10Hz in a linear array. Also, we performed a surface wave exploration and array microtremor observations to estimate phase velocity of Rayleigh waves and S-wave velocity structure at the site. As to the surface wave exploration, we deployed 24 stations with an interval distance of 2m in a linear array. As to array microtremor observations, we deployed 7 stations in a double equilateral triangle and its center with a side length of 2 to 60m. We applied the F-K method to data of surface wave exploration and the SPAC method to vertical data of array microtremor observations respectively, to obtain the phase velocity of Rayleigh waves in a broadband. We inverted S-wave velocity structures from the phase velocity in the inversion technique of the Genetic algorithm.<br> We applied the wave propagation equation to construction vibrations, to identify wave types of construction vibrations and investigate the validity of velocity of subsurface layer. In case of the identification of wave types, we found that construction vibrations were dominated by surface waves, especially Rayleigh waves in vertical components. In case of the validity of velocity of subsurface layer, we found that the phase velocity of surface waves in a subsurface layer was the most important key-parameter to accurately predict construction vibrations. We calculated the distributions of amplitudes of the fundamental mode Rayleigh waves against depth for frequencies of 5 and 10 Hz at the site and therefore; we found that the construction vibrations dominated at a frequency of 10Hz were significantly affected by the subsurface layer to a depth of 3 m. Finally, we evaluated the peak ground acceleration (PGA) of construction vibrations again distances from construction sources, by the empirical prediction method based on the theory of wave propagation. By properly substituting the frequency dominated in construction vibrations and the phase velocity of surface waves in a subsurface layer at the site to the equation of wave propagation, we could accurately reproduce construction vibrations against distances from construction sources.