Analysis of a complex slope failure in a granodiorite quarry bench

We describe a complex failure mechanism in a 30-m high bench in a granodiorite quarry. This mechanism included a circular failure in completely decomposed granodiorite at the bottom of an unstable area pushing on a block of granodiorite. This first block induced subsidence and failure of a wedge of highly decomposed granodiorite, which pushed a slender block below it, causing it to topple. This toppling block eventually pushed a lower block that slid out of the slope. In this rather complex mechanism, completely weathered rock interacted with rock blocks and ultimately produced a small landslide of around 1500 m3 of material. Our study highlights the need to consider the possibility of a complex failure mechanism when analysing rock slope instability. It also highlights the role of toppling in some part of a rock mass prone to instability. When a slender block located in the middle of an initially stable slope experiences a small amount of force, for instance, due to water pressure, that causes it to topple, all the weight is transmitted to the block immediately below the slender block, inducing a greater force that can render the entire area unstable.