Assessing Granular Debris-Flow Impact Forces on Bridge Superstructures

Abstract

In countries similar to Austria, bridges fail due to debris-flow impact on a regular basis. As there is a large uncertainty in terms of the acting forces during these events, this study examines debris-flow impact forces on bridge superstructures. We first set up an idealized load pattern for the load case of debris-flow impact on a bridge superstructure. Second, we conducted 60 experiments in a small-scale physical model (scale 1:30), where we measured debris-flow impact forces on six different bridge profiles in the presence and absence of a bridge pier with two 3-axis force sensors installed at the bridge abutments. Our findings indicate that there is an influence of the superstructure profiles on the magnitude of the effective frontal debris-flow impact force while the presence or absence of a bridge pier affects the direction of the effective frontal impact force, but not necessarily its magnitude. Our results also indicate that the effective frontal impact force acts mainly in the horizontal direction as the vertical proportion is mostly <10%. We found that the forces that act on bridge superstructures are on average between one- and three-fifths of the peak debris-flow force. Our findings can contribute to a better design of bridges against debris-flow impact in the future. A debris flow is a mixture of stones, plant parts, soil and other solids, and water that often causes great damage in mountainous regions, as it can transfer very large forces. When debris flows hit bridges, they are often destroyed. Because debris flows are expected to become more frequent due to climate change and therefore more damage to bridges can be expected in the future, it is necessary to gain a better understanding of the impact process of a debris flow on a bridge. Here, we looked at the forces acting on the deck of the bridge. To this end, we have carried out small-scale experiments and systematically investigated the magnitude of occurring forces and how they are influenced by different shapes of the bridge. We found that it makes a difference which bridge profile is impacted and that usually less than half of the total force of the debris flow acts on the deck of the bridge. Our discoveries can help to better construct bridges against debris-flow impact in the future.