15. Bridge pier stability monitoring method using microtremor data

The stability of bridge piers can be assessed by using the natural frequency of the pier obtained through impact and vibration tests, however, these tests involve creating impacts using weights, which from a practical point of view is not viable for periodic monitoring. As a solution to this problem, a monitoring method was developed using pier microtremor measurements.
However, in the case of large ground vibrations, pier vibrations were drowned out by the ground vibrations, so the method could not be applied.
Consequently, in contrast with the single measurement location used in this method, another microtremor monitoring method was developed based on two (Fig. 1) microtremor measurement locations on the top on the pier, and then subtracting the estimated ground vibration, to make it possible to monitor the natural frequency of the pier even in conditions where influence of ground vibrations is significant.

The new method (Fig. 2) works as follows:
① two microtremor gauges are placed upstream and downstream on the top of the pier, and two microtremors are measured;
② Ground vibration is estimated using measured values;
③ The estimated ground vibration is then subtracted from the measured values;
④ The values are then converted to a curvilinear regression using a theoretical equation to identify the natural frequency of the bridge pier.

The new method was used to identify the natural frequency of 12 bridge piers, which were compared with the natural frequency obtained through separately conducted impact and vibration tests.
The results of this comparison demonstrated that apart from one pier that was in exceptionally good condition, it was possible to identify the natural frequency of each of the 11 other piers even in conditions heavily influence by ground vibrations to within 1 Hz.

Given that this new method does not require prior measurement of the natural frequency through impact and vibration tests, it is much more practical to implement.
In addition, since microtremors values are used, it is possible to monitor to the piers even with rising water levels.