Structural and Dynamic Evolution of Compound Rockslides – Insights from the Brienz Rockslide Collapse of June 2023

In May 2023 the village of Brienz/ Brinzauls, Switzerland was evacuated due to high landslide risk, drawing national and international attention.  On June 15, 2023, a significant collapse occurred at the site, with a volume of 2 Mm³.   This collapse followed a prolonged acceleration phase of a section of an old, partially active deep seated gravitational slope deformation (DSGSD).  The Insel compartment is composed of ductile clay-schist at its base, overlaid by porous rauhwacke and brittle dolomites. In the present work, we analyse the complete dynamic and structural evolution of the Insel compartment using data from various monitoring systems, including 3D displacement measurements from a robotic total station (RTS, operational since 2012), bi-annual LiDAR surveys, Doppler radar monitoring of rockfall activity, ground-based InSAR monitoring (since 2018), and automated digital image correlation of high-resolution time-lapse images. We additionally developed simple analytical dynamic models to investigate the behaviour of viscoplastic and frictional materials.

The current study identified three key phases of the Insel compartment's evolution: (i) the compartment formation phase, (ii) the Insel acceleration and (iii) the terminal phase. During the formation phase (2018-2022), the compartment extended laterally and in the down-slope direction, and a transition from toppling to sliding kinematics was observed.  The acceleration phase started in summer 2022 and was characterized by a prolonged exponential increase in displacement rates, occasionally interrupted with linear growth phases, persisting until early May 2023. In the terminal phase, four short-term surge episodes (lasting days to weeks) were noted, defined by rapid exponential velocity increases followed by stagnation. Surge episodes became more frequent towards the date of collapse and strongly influenced the short-term applicability of classical velocity prediction models such as the Voight’s model.

Based on the available data we developed a kinematic model of the Insel compartment, resulting in a two-wedge compound rockslide which moves on a bi-linear sliding plane. The upper, active wedge comprised brittle, heavily fractured dolomites, while the lower, passive wedge primarily consisted of ductile clay-schists. Intense subsidence at the top of the active wedge suggested the formation of a graben structure along pre-existing large-scale lineaments. The sliding planes dipped southward at 50° (active wedge) and 25° (passive wedge), with a sub-vertical internal shear/deformation zone (ISP) evolving at the kink point of the bi-linear sliding plane. The passive wedge exhibited decreasing displacement in downslope direction, indicating internal shearing and rupturing. At least one rupture plane formation was identified within the passive wedge, causing a rapid acceleration followed by velocity stabilization.

We could replicate the velocity characteristics of surge episodes by combining analytical dynamic models using viscoplastic and frictional materials. This led us to the conclusion that a complex interplay between rupturing within the passive wedge, displacements along the ISP and mass balance changes due to frontal collapses caused the complex dynamic evolution and hence the difficulties in the short-term applicability of Voight’s model. This comprehensive investigation offers new insights and valuable field observations of the complex interplay of structural, mechanical, and external factors driving the dynamics of compound rockslides.