Estimating the Scouring Depth of Bridge Piers by a Triaxial Accelerometer

Taiwan has suffered from extreme rainfall in recent years. The consequences of heavy rainfall have also aggravated the impact of floods, particularly the scours on bridge foundations. Therefore real-time monitoring of bridge piers is crucial for early warning for preventing disasters. However, the traditional underwater measurement instruments need to be installed in the soil and rock around the pier, which often costs massive financial and labor. That is also prone to damage by floods. Therefore, this study proposed a method to estimate the pier scour by a triaxial accelerometer instrument. First of all, we designed an instrument of triaxial accelerometer measuring device for continuously monitoring. Then, through signal conversion, the measured acceleration is converted into frequency. Then the formula derived from our study is imported to convert the real-time scours from the bridge pier. Finally, the measurement from flume experiments and the in-situ scouring monitoring of the Baibufan Bridge are examined for validation.
This study first conducted flume experiments to test our method. The results show that the horizontal vibration frequency is highly correlated with pier scour to the balanced scours, but there is no apparent correlation in the vertical direction. This study then established a dimensionless formula based on the vibration frequency, flow rate, pier diameter, median particle size of the bed, and scour depth. Finally, this study applied the in-situ scouring data from the float-out devices to re-establish the frequency-scour regression formula. The results revealed the relationship between vibration frequency and pier scour in the horizontal direction of the bridge pier over 0.86; the average error is 19.3%. The study also used LSPIV (Large Scale Particle Image Velocimetry) to calculate the flow velocity around the pier and shows that the revised dimensionless formula can be well used in field applications.