First-time Sliding Failure in Weak Sandstones and Subsequent Evolution in a Fast-moving Flow

Following intense rainfalls, during spring 2013 a first-time sliding failure developed over a gentle cultivated slope in Northern Apennines. The slope is made of weakly cemented layered sandstones and subordinate siltstones and mudstones of Eocene age. The morphology is fairly regular with an average angle of 15° that increases up to 25° in the lower part of the slope where the pelitic fraction is no longer present in the bedrock succession. Bedding generally dips into the slope though the presence of faults locally complicate the setting. The sliding interested the upper, more gentle part of the slope with a maximum depth of 10 to 15 meters. Morphological evidences indicate a rotational component of movement and rocks outcropping along the scarp show a moderate degree of weathering. The main landslide body maintained its coherence and only a minor part of the toe mobilized as a fast-moving flow. No building nor infrastructure was affected by the movement but a railway viaduct is present in the lower part of the slope. Following the 2013 failure, movements and pore pressures were monitored in the landslide deposit to control the evolution of the phenomena. During the following winter, the abundant precipitation caused a reactivation of the movement that led to a much larger fast-moving flow. In this work, we document the sequence of events that has interested the slope during the years 2013-2014. Subsequent ground-based and aerial topographic surveys allow to estimate the volumes involved in the different phases of movement. Field evidences and investigations are used to interpret the geotechnical model of the slope and constrain the range of resistances mobilized at failure. The deformations preceding the 2014 reactivation are analyzed together with post-failure landslide deceleration. It is shown that displacements proceeded maintaining sustained moderate velocities for days before the catastrophic reactivation, making difficult to anticipate the acceleration. It took months, and the arrive of the dry season, to return displacement to slow to very slow rates.