Numerical study on the failure mechanisms of rock cliffs subjected to basal erosion

This study aims to investigate the failure mechanism and factors influencing the cliff stability in clastic soft rock coastal areas of the mid-western Adriatic Sea (Abruzzo, Central Italy). The study selects the Punta Ferruccio cliff, one of the cliffs on conglomerate with notches in the area, characterized by a height of 25 m, notch depth of 4 m, and width of 4 m, for analysis. The research focuses on the influence of notches induce by basal erosion at the cliff base and discontinuities, such as joints, on cliff stability. The numerical models of the cliffs are classified into two types: homogeneous material and heterogeneous material with the presence of joints. The numerical analyses are carried out using RS2 software from Rocscience, facilitating the investigation of the progression of notch excavation and its impact on cliff stability. In addition, Shear Strength Reduction (SSR) method is employed for slope stability analysis, presenting the results in terms of critical Shear Reduction Factor (SRF) as an indicator of the stability of the cliff. In the case of homogeneous cliffs, variations in cliff heights are made, ranging from 25 m to 2.5 m, while maintaining three constant notch depths of 10 m, 7 m, and 3 m. For heterogeneous cliffs, joints are introduced into the models by adjusting parameters such as joint length, tensile strength of joints and horizontal locations of joint placement with respect to the cliff face. This provides an understanding of how joints influence the failure mechanism and cliff stability. Furthermore, a back analysis of the Punta Ferruccio cliff is conducted by altering the joint persistence factor, resulting in defining the properties of joints at the time of cliff failure. The numerical results of the simplified model of the cliffs revealed that, in homogeneous cliffs, an increase of notch depth induces cliff instability, as expected. The failure mechanisms of homogeneous cliffs can be divided into low cliffs and high cliffs, based on the difference in the development of failure. Importantly, cliffs became more susceptible to cliff instability when the joints were placed in the heterogeneous models. The length and horizontal locations of joints play key roles in influencing the stability of the cliff. Lastly, the result of back analysis was consistent with the numerical analysis conducted by Calista et al. (2019), allowing for the determination of joint properties at the failure, including cohesion, friction angle, and tensile strength. These results can be useful for further stability assessment in the soft rock coastal areas.