E3S Web of Conferences 415, 02006 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341502006

Laboratory experiments to analyse the influence of bridge
profiles on debris-flow impact forces
Caroline Friedl1*, Christian Scheidl1, Susanna Wernhart1 and Dirk Proske1,2
1University of Natural Resources and Life Sciences, Vienna, Department of Civil Engineering and Natural Hazards, Institute ofMountain Risk Engineering, Peter-Jordan-Straße 82, 1190 Vienna, Austria
2Bern University of Applied Sciences, School of Architecture, Wood and Civil Engineering, Pestalozzistraße 20, 3401 Burgdorf,SwitzerlandAbstract. Debris-flow impact plays a significant role in the failure of bridges in mountainous areas posing
a risk to human life and leading to high reconstruction costs. The aim of this study is to measure and quantify
the frontal impact forces of debris flows on bridge superstructures based on laboratory experiments with a
special regard to the comparison of two different bridge profiles and the presence / absence of a bridge pier.
To this end, we conducted 20 experiments, measuring the frontal impact forces on the bridge superstructurewith two 3-axis force sensors at the abutments of the miniature bridge. We found that the type of thesuperstructure does have an influence on the magnitude of the frontal impact forces.

1 Introduction
Mountainous areas exhibit a greater density of bridges
than flat regions due to their topography and mobility
requirements. At the same time, bridges in higher
altitudes are exposed to various dangers such as flash
floods, fluvial sediment transport processes, debris
floods and debris flows. A glance at the statistics ofbridge failure in the US shows that debris flows alonecause 3.33 % of bridge collapses. If debris-flow impacts
are added to general collisions, both sum up to over
15 % contribution to bridge failure [1].
Besides posing a threat to human life, frequent
destruction of bridges also entails the need for frequent
reconstruction, which consecutively leads to highfinancial expenditures. Since there is evidence that
endangered areas prone to debris-flow events are likely
to increase as a consequence of climate change as well
as spatial development [2], the topic of debris-flow
impact investigation is also relevant for the future safety
of infrastructure elements.
The influence of debris flows on bridge piers has
already been analysed in the past [3] – [6] whereas
mechanisms and consequences of debris-flow impact onbridge superstructures remain unclear. Debris-flow
impacts on masonry arch bridges have been studied
before, however, the investigations were based on a
separate examination of impact forces and the behaviourof the structure [7].
We hypothesize that frontal impact forces play aconsiderable role in bridge failure caused by debris-flow
impacts. We also conjecture that the type of the bridgesuperstructure, specifically the bridge profile has aninfluence on the occurring forces. We aim to measureand quantify the forces exerted on different bridge
profiles during debris-flow impact based on small scale
experiments. Additionally, we investigate the influenceof a bridge pier on the acting forces.

2 Methods
The experimental setup (1 in Figure 1) consists of asemi-circular channel with a length of 4 m and adiameter of 0.3 m. The channel is inclined at 20 ° andthe experiments feature a scale of 1:30. The debris-flow
material is released from a rectangular starting box in adam-break scenario. Travelling down the flume, thematerial forms a typical debris-flow habitus (granularhead, more liquid body and tail) before hitting theminiature bridge profile (2 in Figure 1) at the end of theflume.

Fig. 1. Experimental setup total (1) and detailed view of theminiature bridge mounted on a steel frame (2)* Corresponding author: caroline.friedl@boku.ac.at© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 02006 (2023)
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https://doi.org/10.1051/e3sconf/202341502006

This study features two miniature bridge profiles: thesolid slab profile (1 in Figure 2) and the trough profile(2 in Figure 2). Both profiles have a total length of0.33 m and a width of 0.4 m normal to the direction ofthe flow. The solid slab profile features cantilevers witha length of 0.05 m to both sides in and against the flowdirection. The total height of the solid slab profile is 0.02
m. The trough profile has no cantilevers and a totalheight of 0.06 m, which results in an impact area threetimes as large as that of the solid slab. The miniature
bridge profiles were fabricated of cement and aremounted on a steel frame that is decoupled from theflume to avoid the influence of vibrations of the channel.
The bridge pier is made of wood. It is not force-lockedwith the bridge profile (3 in Figure 2) and it can be easily
removed (4 in Figure 2).

maximum values 𝐹𝑚𝑎𝑥,𝑦𝑖 and 𝐹𝑚𝑎𝑥,𝑧𝑖 for each replicate
i. The resulting maximum frontal impact force 𝐹𝑖 wascalculated with the following equation (1).
𝐹𝑖 = √𝐹𝑚𝑎𝑥,𝑦𝑖 2 + 𝐹𝑚𝑎𝑥,𝑧𝑖 2(1)The setup with the solid slab profile is set to be thebase case. To enable an efficient comparison of thesetups, all values were related to the median of themaximum frontal impact force of the solid slab 𝐹̃
bc orbase case (A) (eq. 2), yielding the dimensionless related
frontal impact force Fri, which will be analysed in more
depth in next section.
𝐹𝑟𝑖 = 𝐹𝑖 /𝐹̃𝑏𝑐(2)3 Results and Discussion
Figure 3 shows the results within boxplots of the related
frontal impact forces Fri grouped by the investigatedfour setups: base case (A), base case + pier (B), trough(C) and trough + pier (D). Each group consists of five
replicates, yielding a total of n = 20.

Fig. 2. Solid slab profile (1), trough profile (2), setup with pier(3) and setup without pier (4)The debris-flow mixtures are based on previously
conducted experiments [8], [9]. The total mass is kept
constant at 50 kg and corresponds to a granular debrisflow. The solid-fluid-ratio in terms of volume is65:35 %.
A total of 20 experiments were carried out. There
were five replicates in each setup: solid slab, solid slab
with pier, trough, trough with pier.
Flow heights are measured at three locations withBaumer OADM 20I6480/S14F laser distance sensors.
The flow velocity is estimated based on the passage timeof the debris-flow front between the laser sensors 1 and2 [9]. Additionally, pore water pressures are gauged
with Keller 25Y piezoresistive pressure transmitters[10] below the laser sensors. The impact forces aremeasured with 3-axis force sensors (ME K3D120) at theleft and right abutment of the bridge.
The signals are amplified by means of a Quantum
MX1601B datalogger from HBM and postprocessedwith the corresponding software Catman V5.3.2.
2400 Hz was chosen as basic measurement frequency.
The logged signals were processed by applying a
Butterworth lowpass filter, the cut-off frequency todifferentiate the impact of single particles from the bulk
impact was set at 10 Hz based on the consideration offlow velocities and the maximum grain diameter.
To compare the frontal impact forces of the four
setups, we first acquired the maxima of the filteredforces in the horizontal (y) and vertical (z) direction (see2 in Figure 1) by summing the forces of the left and theright sensor in each timestep and then reading out theFig. 3. Boxplots of related frontal impact force Fri for thesetups base case (A), base case + pier (B), trough (C) andtrough + pier (D). ntotal = 20, nSetup = 5

We found that the median Fri in setup (B) is about
one quarter smaller compared to the base case (A). We
assume that the pier in setup (B) influences the debrisflow dynamics in a way that material is dammed which
reduces the flow velocity resulting in lower frontal
impact forces. The comparison between (A) and (C)shows that the related forces for (C) exceed the forces in(A) by more than a quarter. Thus, we conclude that theprofile of the superstructure has an influence on themagnitude of the frontal impact forces, which could beexplained by the extent of the impact area.
The presence of the pier results in lower forces forthe full slab bridge (B) and higher forces for the troughbridge profile (D). We suggest that this is caused by theabsence of a cantilever at the trough profile, provoking
a higher damming effect.
If we compare the pier-setups (B) and (D), we cansee that the forces in (D) are much greater than in (B).

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E3S Web of Conferences 415, 02006 (2023)
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https://doi.org/10.1051/e3sconf/202341502006

Again, this might be due to the impact area, which islarger for the trough profile.

4 Conclusion and Outlook
After conducting and evaluating 20 experiments withtwo different profiles and with and without a bridge pier,we can summarize that the forces are greater with thetrough bridge setups (C) and (D) than with the base casesetups (A) and (B). This may be attributable to theimpact area. Furthermore, the pier seems to have aninfluence on the frontal impact forces, but the directionof the influence may depend on the profile as well as theposition of the pier in relation to the bridge
superstructure.
We plan to extend the test series to a total of fiveprofiles in order to investigate the influence of theprofiles even more closely. In addition, further tests may
be carried out in the future to study not only the frontal
impact forces, but also other potential force components
such as uplift or frictional forces.
Acknowledgements:
This study was conducted as part of the project Nr. P 32511:“Debris-flow impact forces on bridge superstructures” fundedby the Austrian Science Fund (FWF). The experiments werecarried out at the Institute of Mountain Risk Engineering(IAN) at the University of Natural Resources and LifeSciences, Vienna.

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