FDEM numerical calibration of mechanical properties of tuffaceous rocks for slope stability analysis

The characterisation of tuffaceous soft rocks represents a substantial challenge for slope stability evaluation. The geomechanical behaviour of these materials depends heavily on the microheterogeneity of pyroclastic rocks and varies considerably based on the degree of alteration and water-rock interaction processes.

Tuffaceous lithologies outcrop in a wide variety of areas worldwide, often forming landslide-prone vertical cliffs, both in coastal and continental settings. These instability processes are controlled by progressive rock failure (PRF) mechanisms that govern the enucleation and propagation of fractures, the evolution of which can lead the slope to a state of instability. PRF process can be numerically analysed by adopting hybrid stress-strain numerical solutions able to capture the transition from continuum to discontinuum behaviour. However, these tools require micromechanical input parameters governing the fracture mechanics (e.g., fracture toughness/energy), which are often difficult to calibrate numerically.

This study focuses on the tuffaceous cliffs of Ventotene Island (Italy), highly susceptible to rock-falls and topples and exposed to sea-wave actions, combining laboratory testing and numerical modelling. In the site the mechanisms of progressive fracturing in this type of soft rock and its relationship with environmental forcings are deepened through the design and implementation of the “Ventotene Field Laboratory”. This natural field laboratory, part of EPOS Field-Scale Laboratories, allows the integration of field observation, in situ monitoring data and numerical investigations.

To characterise the mechanical behaviour of the Ventotene tuffs, uniaxial compressive strength (UCS), indirect tensile (Brazilian) and fracture toughness (FT) tests were performed at Aachen Rock Mechanics Laboratory on representative rock samples under both dry and saturated conditions. The laboratory results highlight a strong influence of water content on the mechanical properties of the tuff, with a marked reduction in strength and stiffness under wet conditions. In addition, the thermal properties of the material were also investigated to support thermo-mechanical analyses.

The laboratory test results were used to provide (micro)mechanical input parameters to a FDEM slope numerical model using the Irazu software (Geomechanica Inc.). Overall, the results show that numerical calibration is essential to obtain a tuned parametrisation of tuffaceous soft rocks and to bridge the gap between laboratory-scale measurements and field-scale responses.

The laboratory tests were numerically simulated, and the calibrated parameters have been transferred to a numerical domain representative of the Ventotene sea cliffs. This latter model served to perform accurate slope stability analysis of coastal cliffs, by combining the action of different (marine and environmental) controlling factors.

The calibrated micromechanical parameters also provide a robust basis for future modelling FDEM studies since calibrations of this nature have rarely been conducted on tuffaceous lithologies.