Better prepared for the tsunami TU Braunschweig investigates research gaps in construction guidelines

Family homes destroyed beyond repair, ships wedged between buildings, meter-high debris at the gates of a warehouse, washed-out walls – the earthquake-triggered tsunami of September 28, 2018, hit the Indonesian city of Palu with full force. Scientists from the Technische Universität Braunschweig, Canada, the USA and Japan have documented the damage on site and studied the effects on infrastructure and buildings. The aim of their studies is, among other things, to improve the design approaches for tsunami-proof cities in building guidelines.

The earthquake tsunami with a wave between approximately two and seven meters high devastated large parts of the city of Palu and other places along the coast. On site, the scientists saw how the goods from several warehouses were scattered over a large area by the tidal wave and piled up on load-bearing building columns. As a result, the columns were severely strained and often collapsed.

This accumulation of debris, also known as a “debris dam,” was also observed for ships by the research team in Palu: For example, the tsunami pushed two interlocking boats between two buildings. This hydrodynamic load was transmitted through the boats to the buildings, and at the same time the water level in front of the boats was raised. “Both effects lead to an increase in building loads, which to date has not been taken into consideration in the construction guidelines and therefore needs to be further investigated,” said Clemens Krautwald, a research associate at the Leichtweiß Institute of Hydraulic Engineering and Water Resources (LWI) at TU Braunschweig, who traveled to Asia with his Canadian colleague Jacob Stolle for the field investigations a month after the tsunami disaster.

The American Society of Civil Engineering’s (ASCE) “Minimum Design Loads and Associated Criteria for Buildings and Structures” (ASCE 7-16, 2017) are the base. They are considered the global benchmark for the design of tsunami-safe structures. Research gaps and unresolved issues related to structural loads from tsunami are revealed by the scientists in two recent papers published in peer-reviewed journals.

Scouring of reinforced concrete structures

Not only floating debris and possible hydrodynamic loads affect the stability of structures after such a catastrophe. Clemens Krautwald cites scouring, i.e. the formation of a water-filled depression, on structures as a further problem. This applies to reinforced concrete structures, for example, which remained mostly intact in Palu but were less stable after the tsunami because load-bearing soil was lost.

The scientists in Indonesia measured scouring at the corners of buildings, around columns and behind overflowing walls. The researchers observed that building structures were anchored differently in the ground, depending on the soil composition. “During a tsunami inundation, high pressure is built up, which is transferred to water in the pores in the soil. When the pressure drops abruptly due to the passing tsunami, the soil must relieve this excess pressure. Adverse soil compositions can prevent pressure reduction, resulting in increased erosion or scour,” says Clemens Krautwald.

Experiments in the Large Wave Flume

Although these processes can be described and their damage assessed in field studies, they cannot always be substantiated with reliable data. However, they are necessary to know how deep the piers of a bridge, for example, need to be built into the ground.

Scientific investigations into such questions require the processes to be reproducible in large-scale experimental facilities, for example in the Large Wave Flume (GWK) of the Coastal Research Center (FZK) of the Technische Universität Braunschweig and the Leibniz Universität Hannover.

Therefore, an international research team consisting of scientists from the Leichtweiß Institute of Hydraulic Engineering and Water Resources at TU Braunschweig, the Ludwig-Franzius Institute at Leibniz Universität Hannover, and the University of Ottawa in Canada conducted experiments on scour formation around pier-like structures during tsunamis. The temporal evolution of pore water pressures and scour depths during a tsunami-like inundation represents a research gap that will be further investigated with this research project.

A magazine article provides insights into the experiments with tsunami-like waves, including videos:
https://magazin.tu-braunschweig.de/m-post/tsunami-im-wellenkanal/