Triaxial creep testing on argillaceous weak rock for DSGSD movement prediction

Deep-Seated Gravitational Slope Deformations (DSGSDs), very slow-moving landslides, are widespread across Italy, from the Alps to the central Apennines. These slow movements, often associated with geological formations exhibiting viscous behaviour, can severely affect infrastructures. A significant example is the interaction between the Sparvo twin-tube motorway tunnel, located in central Italy, and a large DSGSD affecting the area. Such conditions require careful monitoring of the infrastructure and, when possible, the development of tools such as advanced numerical models to study and predict the time evolution of the interaction between the infrastructure and the complex landslide. This can be achieved using time-dependent constitutive models, whose most advanced formulations require the calibration of multiple parameters. The calibration process, in turn, relies on a detailed characterization of the time-dependent behaviour of the geological formations involved. This paper focuses on the characterization of the time-dependent behaviour of Palombini shale formation, an argillaceous weak rock responsible for the DSGSD movement interacting with the Sparvo tunnel. For this purpose, several triaxial multistage creep tests were carried out on Palombini shales samples. The tests, performed on specimens collected both within the landslide shear band and outside it, revealed a marked tendency of the material to develop time-dependent deformations and allowed the definition of strain–time curves under different stresses. Moreover, the results showed clear behavioural differences between the highly deformed in-situ material sampled from the shear band and the intact material collected outside the shear band, allowing a more comprehensive characterization of the time-dependent behaviour of the argillaceous weak rock.