Formation phases and structural failure of a landslide compartment at Brienz/Brinzauls, Switzerland

This study analyses the formation of a surficial compartment of the Brienz/Brinzauls deep-seated landslide in eastern Switzerland and reveals the process transition leading to its failure. Based on point-cloud-derived, high-resolution 3D displacements, we recorded local changes in displacement direction and velocity and documented the development and temporal activity of shear and rupture surfaces. This gave us unique insight into the processes occurring during different phases of the compartment's evolution. We could observe the slow formation of a bedding-parallel rear rupture surface and the resulting onset of a compressional regime in the toppling slope foot. A large scale compression ratio of around 8 % in the direction of movement caused a progressive disintegration of the slope foot and a slow propagation of a basal rupture surface over several months. This led to a slow transition from toppling to sliding of the slope foot and ultimately to basal failure. We provide direct and detailed field observations of the physical processes governing the transition from uniform displacements to accelerated rock slope instability and provide practical insights to improve landslide hazard assessment in complex scenarios. • Three phases of rock creep are documented by high resolution 3D displacement data. • Fundamental changes in the displacement process caused transition to tertiary creep. • The start of exponential acceleration can be anticipated in advance. • Rock fracturing under compressional regime appears in point cloud data. • Massive slope compression and rock fracturing prevailed over years.