Particle Simulations as a Key Technology in Alpine Disaster Management

Heavy rainfall, rising temperatures, and extreme weather conditions are effects of climate change that promote rockfalls and debris flows in the Alpine region. This increases the risk to residential areas and infrastructure facilities in their run-off areas. Scientists at the Department of Geotechnical Engineering at the University of Innsbruck are conducting research into preventive protective measures.

In addition to geotechnical conditions, these scientists are primarily concerned with the energy released during rockfalls. The focus is on interactions, the effects on protective dams of various designs, and the spread and deposition patterns of debris after impact. The findings will help to ensure that dams are built to withstand the locally expected loads and provide maximum protection.

Experimentation and simulation instead of empiricism 

With this research work, the Innsbruck team has broken new ground. This is because there is hardly any known empirical preliminary work, nor are there any documented events on which to rely as a pillar. As a result, a better understanding of these powerful processes must be gained through experiments on models and simulations, as well as through the interaction of both methods.

The Innsbruck research team has examined various questions relating to flow and impact properties on the basis of around 200 model experiments and extracted formulas from the measurement results. Calibrated simulations using the discrete element method (DEM) are then used to check the plausibility of the data and correlations obtained. In addition, influencing parameters are analyzed, sensitivities are extracted, and the findings are scaled to other scenarios.

Model tests in the laboratory 

Using model structures, data and formulas were determined in the geotechnical engineering laboratory to describe gravitational mass movements and their impact on various protective dams. Since every rockfall is unique, the researchers defined representative variants with material types that differ in properties such as grain size, grain roughness, grain distribution, and friction. Other factors such as angle of inclination and subsoil were also varied in order to analyze the influence of velocities and flow depths.

To describe the mass movement from the moment of impact on the protective structure, four phases were identified:

• Phase 1, the impact of the material,
• followed by phase 2, the upward surge after impact (“run-up effect”),
• before it flows along the dam in phase 3 and a wedge of material (“dead zone”) is created,
• which redirects subsequent material to the side in phase 4 (“pile-up effect”).

What are particle simulations based on the discrete element method (DEM)?

Particle simulations are based on a numerical method called the discrete element method (DEM). They are widely used in various areas of industry and research to analyze and improve particle flows—from dust to bulk materials to debris. Ansys Rocky is a software program that implements DEM for practical use by engineers and process technicians. 

Ansys Rocky | DEM Processes for Particle Systems

Simulations with Ansys Rocky for modeling rockfalls and their interaction with their surroundings

Based on the findings from the experiments, numerical modeling was carried out using discrete elements. These were modeled using Ansys Rocky software, with the sum of all discrete elements resulting in the rockfall mass. The interaction of the particles with the environment was represented using a linear spring-damping model. Since DEM simulations are very computationally intensive, the simulation model was simplified and discrete particles with perfect spherical geometry and an additional rolling friction model were used.

Retrospective simulation of a real rockfall in Tyrol 

The quality of such a model can be determined by recalculating past events. This was done for the massive rockfall that occurred on Christmas Eve 2017 near the village of Vals in Tyrol, fortunately without causing any injuries. With a mass of several 10,000 tons, it buried the road to Vals, cutting off the 130 villagers from the outside world for two days.

Is it possible to use a numerical DEM model to simulate the rockfall process so that the real and simulated run-off areas match with sufficient accuracy? And can the findings from the simulation be used to determine the load on dam structures caused by rockfalls or the required height of the protective structure?

The results indicate a realistic simulation model. Image 2 shows, from left to right …

• … the terrain model with the debris (red) before and the run-off area (blue) after the rockslide. This geometric information forms the basis for calibrating realistic material parameters as boundary conditions for modeling the rock avalanche in Ansys Rocky.
• … the DEM simulation of the rock avalanche based on information from the terrain model and adapted values from the experiments.
• … the run-off area and deposition heights from the DEM simulation. These indicate a high degree of conformity with the documented debris distribution.

Together with other process parameters, in particular the phase model, the simulation model can serve as a basis for the planning of suitable protective structures.

The method developed by the Innsbruck scientists, based on experiments and DEM simulations, shows reliable results in the evaluated case and is a major step toward securing endangered locations in the future through the construction of suitable protective structures.

This article is based on 
Publications by the Geotechnical Engineering Department at the University of Innsbruck.