Soundness Evaluation Method of the Ground around the Pier Foundation against Scouring by Microtremor Measurements at Both Sides of the Bridge Piers

<b>Automatic Identification Method for Natural Frequency of Bridge Piers by Microtremor Measurement at Both Sides on Top of Pier</b>

“…In estimating the ground vibration, we obtain the position of the oscillation center provided that the bridge pier is oscillating around the rotational center (hereinafter referred to as primary vibration). Previous studies show that the position varies depending on the depth of penetration and does not exist necessarily at the bottom of the bridge pier [5]. The oscillation center position is calculated using the following method.…”

“…If the natural frequency at time t is f t , the velocities f bp x a (t), and f bp z a (t) where the primary vibration is dominant can be obtained by applying a bandpass filter to the velocity data x a (t) and z a (t) at time t if they include the frequency f t . The relationship between f bp x a (t) and f bp z a (t) can be approximately expressed as (5) from (4).…”

“…When the relationships between f bp x a (t) and f bp z a (t), and between f bp x b (t) and f bp z b (t) are plotted on a plane consisting of horizontal axis and vertical axis, (5) and 6draw a trajectory perpendicular to the center of vibration. In actual vibration, the trajectory varies, but since it can be approximated by a straight line passing through the origin, following (7) and (8) is obtained by the least-squares method.…”

“…The oscillation center position is usually located near the bottom surface and is slightly above when the penetration depth is larger [5], [7]. It is determined that the natural frequency of the bridge pier is not included in this frequency band when the oscillation center is significantly deeper than the bottom surface or near the top of the pier.…”

“…The authors propose a method to identify the natural frequency using a transfer function, which is obtained from the estimated ground vibration using microtremor measurement data measured at both ends of the pier top [3]. Furthermore, assuming the vibration of the bridge pier is a single-degree-of-freedom system, a new method for identifying the natural frequency was developed based on the premise that the transfer function is theoretically represented by a resonance curve [4], [5]. This paper describes the outline of the new identification method for analyzing disaster hazards, and its application to actual bridges to verify the validity of the method.…”