横川駅付近における杭打衝撃による地盤ならびに建物の振動に関する一考察
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1. Vibration of Earth under piling Impacts and running Trains The stratum near Yokogawa Station where the writer and the others investigated the vibrations is chiefly composed of silty sand and there are hard layers of gravels in 6m and 18m depth. The stratum in this neighbourhood is almost similar. The driving concrete pile is 45cm in diameter and 18m in length. The weight of drop hammer is 2500kg and its drop height is about 2-3m. We measured the vibrations of earth by running trains to compare with those under piling impacts. According to the measuring results, the natural frequency of earth is 9-15cps, and its average value is about 13cps. The relation between the amplitude and the distance from the origin of vibration is shown in Fig. 1. table table The relation between the drop height and the amplitude at the point of 13.2m from the driven pile is shown in Fig. 2. Comparing both amplitudes of piling impacts and running trains, the magnitude of the former is about twice of the latter. 2. Vibrations of a Building (Yokogawa Apartment) We measured the vibrations of the four-storied reinforced-concrete building in the nearest distance of 12m from the driven pile. As the results the average value of measured amplitudes is 0.09mm at the bottom of the building and is 0.07mm at the top. According to the theoretical calculation the amplitude at the top of the building is 0.854 a, assuming the amplitude at the bottom as a. Therefore the result of the theoretical calculation is quite similar with the measured value. Calculating the stresses theoretically, σ_<max> is 0.43kg/cm^2 at the bottom and is 0.23kg/cm^2 at the top. So the values of stresses are very small. It is concluded that a concrete crack can't be produced by piling impacts if the concrete works are carefully done. Calculating the natural frequency of the column by longitudinal vibrations theoretically, its value is about 35.6cps. However the natural frequency of the concrete slab on the roof is about 14.3cps, and this valus is quite similar to the value of earth, So I feel the danger of resonance. I should like to strengthen the supports of slab. Calculating accelerations in order to clear the influence on human bodies, I found that the values are almost similar to those of running trains in velocity of about 60km/hr.
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