11. Emergency diagnosis method for scour damaged bridges

The stability of river bridge piers is significantly reduced due to a decrease in the bearing capacity of the foundation as a result of reduced penetration from scouring and the base becoming exposed. Impact and vibration tests conducted after water runoffs can be used to estimate the extent of penetration reduction based on changes in natural frequencies of the structure, but it is difficult to estimate the exposure rate of the base of the foundation. Additionally, there is no established method for estimating the bearing capacity after scouring, and the stability of the piers is currently evaluated directly by loading tests conducted in the field to resume train operations. As a result, it may take several months to resume operations, even if the deformations are minor (Figure 1).

Therefore, we proposed a method to estimate the base exposure rate by determining the rotation center of the foundation based on the amplitude ratio of vertical responses measured at the upstream and downstream sides of the pier crowns during the impact and vibration test. We verified the validity of this method through model experiments (Figure 2). We have also proposed a bearing capacity calculation method that takes into account the effects of the reduction of penetration depth, the reduction of base area from increased exposure rate and eccentricity, which are causes of bearing capacity reduction due to scour, and confirmed its validity by applying it to actual damaged piers (Figure 3).

By estimating the bearing capacity based on the estimated exposure rate, we proposed an emergency diagnosis method to evaluate the stability of piers subjected to scour and, if a certain level of stability is maintained, to shorten the time required to resume operation. The proposed method eliminates the need for on-site loading tests when the foundation base is exposed but still retains a certain level of bearing capacity. Additionally, if the bearing capacity is found to be insufficient, this method allows for a quick determination of an appropriate restoration approach. With this approach, the time required to resume operation is expected to be shortened by up to half.