Factors influencing suffusion-induced delayed displacement of river bridge piers

Abstract

In recent years, local scour has occurred on the pier foundations of river bridges during heavy rain and river flooding, often resulting in bridge collapse and outflow. This study focused on the characteristic displacement, called “delayed displacement”, of the river bridge pier. The critical displacement of the piers was first observed several days after the flood when the train passed and not immediately after the flood. The authors hypothesized that one of the possible reasons for the delayed displacement is the suffusion of the supporting ground beneath the pier foundation during the flood, followed by a compressive behavior due to the collapse of the soil skeleton under repeated traffic loads. Accordingly, this study performed erosion tests simulating flood and cyclic loading tests simulating train passage using a triaxial test apparatus to check the validity of this hypothesis. In some test cases, suffusion without any deformation occurred in the erosion test but deformed in the cyclic loading test just after the erosion test. This behavior matches the behavior of delayed displacement. It was also suggested that the risk of the delayed displacement becomes high when the soil skeleton was assumed to primarily comprise fine particles, and the void ratio and hydraulic gradient were high. By contrast, when the soil skeleton was assumed to primarily comprise coarse particles, suffusion occurred in the erosion test, but did not deform in the subsequent cyclic loading test. Thus, the risk of delayed displacement is considered to be low when coarse particles are dominant. Furthermore, clear relationship between suffusion and the consequent reduction in soil stiffness cannot be observed. This result indicates that no significant change in the stiffness occur in the supporting ground of the pier foundation at the stage immediately before the delayed displacement. Thus, identifying the deterioration in the stability of the piers through impact loading test, which is based on the concept that local scour reduces the natural frequency of the bridge pier, is difficult.