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

This study aims to investigate the failure mechanisms and factors influencing cliff stability of soft rock cliffs, specially focusing on a clastic coastal area of the mid-western Adriatic Sea of the Abruzzo region, in Central Italy. The Punta Ferruccio cliff, characterized by conglomerate with notches, stands at a height of 25m, notch depth of 7m, and width of 4m, and serves as the focal point for the analysis. The research explores the impact of notches induced by basal erosion at the cliff and discontinuities, such as joints, on cliff stability. The numerical models of the cliffs are categorized into homogeneous material (without joints) and homogeneous material with joints. The Finite Element Method (FEM) is employed to analyze the progression of notch excavation and its influence on cliff stability using RS2 software. The Finite-Discrete Element Method is used to simulate two cliff types using Irazu 2D software, providing a more accurate depiction of the actual cliff collapse process and enabling comparison with RS2 software results. The Shear Strength Reduction (SSR) method is utilized in the FEM analyses of slope stability, presenting results in terms of the global critical Shear Reduction Factor (SRF) as an indicator of cliff stability. In the case of homogeneous vertical cliffs, varying heights ranging from 25 m to 2.5 m are analyzed, while maintaining constant notch depths of 5 m, 3 m and notch height of 4 m. Additionally, the stability of a homogeneous 25 m high cliff with a slope angle of 70˚ and varying notch depths (3 m, 5 m, 7 m, and 10 m) is compared to that of a vertical cliff. For homogeneous cliffs with joints, a parametric analysis is carried out and the discontinuities are introduced into the models by adjusting parameters such as joint length and horizontal locations of joint placement relative to the cliff face. This helps in understanding the influence of joints on failure mechanisms and cliff stability. Furthermore, a back analysis of the Punta Ferruccio cliff was conducted to assess the impact of varying joint persistence factors on cliff properties upon failure. The numerical results of the cliff models reveal that an increase in notch depth leads to cliff instability in homogeneous cliffs. Homogeneous cliffs with joint exhibit increased instability with the length and horizontal locations of joints, specially, the position of the joint is close to the notch end. These findings hold significance for further stability assessments and hazard analyses for soft rock coastal areas.