Analysis of sea cliff slope stability integrating traditional geomechanical surveys and remote sensing

Abstract

Abstract. An integrated approach to the geomechanical characterization of coastal sea cliffs was demonstrated at Mt. Pucci (Gargano promontory, Southern Italy) by performing direct traditional geomechanical and remote geostructural investigations via Terrestrial Laser Scanner (TLS). The consistency of the integrated techniques allowed us to achieve a comprehensive and affordable characterization of the main joint sets on the sea cliff slope. The observed joint sets were observed to evaluate the susceptibility of the slope to rock falls by attributing safety factors (SFs) to the topple- and wedge-prone rock blocks under three triggering conditions: (a) filling with static water, (b) seismic action, and (c) weathering of joint surfaces. The results of the susceptibility analysis for the topple-prone blocks show that the critical height of water filling of the joint is up to 50 cm and that the critical pseudo-static acceleration values vary in the range of 0.16–0.3 g depending on the block geometry and slope face orientation. For the wedge blocks, the critical height of water filling of the joint is generally up to several centimeters, and the critical pseudo-static acceleration values vary in the range of 0.05–0.8 g depending on the block geometry and slope face orientation. Moreover, the unstable conditions of the blocks due to weathering generally represent 60% of the joint degradation of the intact rock. The combined action of weathering and static water fill was also considered, resulting in a significant decrease of the SFs. Specifically, unstable conditions are associated with water levels lower than 47% of the water levels observed in intact joints, even if less than 60% of the weathering is attributed to the joints. Furthermore, remote survey analyses via Thermal InfraRed Camera and Terrestrial SAR Interferometry (TInSAR) were performed to evaluate the role of the surveyed joint sets in inducing instabilities in the Mt. Pucci sea cliff. The results of this study can be summarized as follows: (i) the thermal images allowed us to identify anomalies that correspond well to the main joints and to the slope material released due to recent collapses; and (ii) TInSAR monitoring revealed permanent displacements greater than 1 mm, and cyclic daily displacements of up to 1.2 mm were detected in certain sectors and were attributed to thermal variation of the rock mass.