Rock-shape and its role in rockfall dynamics

Rockfalls are an ongoing hazard to settlements and infrastructure across the Alps and worldwide. Rockfalls may become more prevalent due to climate change impacting mountain permafrost and slope stability in rockfall starting zones.

Managing rockfall hazards is a primary and urgent concern for government authorities. Rockfalls were responsible for five fatalities and numerous closures of roads and railways in Switzerland over the past year alone. To assess rockfall danger, geotechnical engineers apply trajectory models to determine runout paths, jump heights and velocities. Trajectory models help to investigate hazard scenarios, delineate rockfall intensity maps and to design rockfall barriers and dams that protect roadways and railway lines.

Figure 1: Julierpass (14.11.2012), 80t rockfall closes the cantonal higway (Photo:Maja Bless).

Video 1: Rockfalls in Grabengüfer Randa, canton Valais, 2011

One of the biggest challenges with rockfall modelling is that the terrain material properties and impact conditions of rockfalls are greatly uncertain and spatially variable. A key factor in this variability is the different rock shapes that are involved in rockfalls; which are strongly associated with the geological diversity of rockfall zones. To date the influence of rock-shape is poorly accounted for in rockfall trajectory models.

In this research we focus on how different rock shapes influence the dynamics and runout behaviour of rockfalls. This can be thought of as the difference in the way that a football and a rugby ball would roll over a playing field. We are interested to see if there are particular rock-shapes that tend travel faster or jump higher than one another; and to see which shapes travel the furthest and produce the most dispersion from their release point. This information is important for rockfall management where the positioning and design of rockfall protection barriers require information of rockfall impact loads and jump heights. 

We investigate the run-out behaviour of different rock shapes with detailed field studies of natural rockfalls (FIG 2, Video 2) and small scale laboratory experiments (FIG, rotation video). To observe the motion of the rockfalls we employ high speed video tracking techniques to resolve the velocities; and we use dynamic motion sensors which are imbedded in the centre of rock test bodies to capture the ground impact accelerations and angular velocity of the rocks. 

Video 2 and Figure 2: Rock rolling laboratory experiments on planar slope with different rock shapes and dynamic motion sensor.

With these measurements we are gaining better insights into the behaviour of different rock-shapes such as the relationships between the velocity of the rock-body and the resultant angular velocity after impact; in addition to identifying the preferred axes of rotation (FIG 3) for different rock-shapes. Through this we are noticing behaviour such as chart-wheeling of flatten plate like rocks (FIG, rotating rock) which leads to long and fast rockfall run-outs.

Figure 3: RAMMS::ROCKFALL simulation using different rock-shapes (below right) of a rockfall in Gurtnellen (UR) Switzerland displaying velocities (left); trajectory profile plot showing rock jump heights (upper right).

The results from this research underpin the calibration of a rockfall trajectory model part of a natural hazards simulations software package RAMMS (Rapid Mass MovementS) (FIG 3, Grutnellen) which the SLF are developing in collaboration with the Centre of Mechanics, Department of Mechanical & Process Engineering ETH Zürich. The model employs rigid-body mechanics which greatly differs to the current standard in rockfall modelling because it has the advantage of being able to explicitly account for rock-shape in its simulations. Laser scanned rocks from field studies are being converted into virtual rocks (Video 2, FIG 2) and used in rockfall simulations.

The rockfall model is soon ready for test release and will be trailed by rockfall engineers across Switzerland. The first RAMMS::rockfall workshop will be held at the WSL in Birmensdorf on the 04. Juli 2013.