3D Discrete Fracture Network Modelling from UAV Imagery Coupled with Tracer Tests to Assess Fracture Conductivity in an Unstable Rock Slope: Implications for Rockfall Phenomena

Abstract

The stability of a rock slope is strongly influenced by the pattern of groundwater flow through the fracture system, which may lead to an increase in the water pressure in partly open joints and the consequent decrease in the rock wall strength. The comprehension of the fracture pattern is a challenging but vital aspect in engineering geology since the fractures’ spatial distribution, connectivity, and aperture guide both the water movement and flow quantity within the rock volume. In the literature, the most accepted methods to hydraulically characterise fractured rocks in situ are the single borehole packer test, the high-resolution flow meters for fractures, and the artificial tracer tests performed in boreholes. However, due to the high cost a borehole requires and the general absence of wells along coastal cliffs, these methods may not be appropriate in rockfall-prone areas. In this study, an unsaturated rocky cliff, strongly affected by rockfalls, was investigated by combining kinematic analysis, Discrete Fracture Network (DFN) modelling, and artificial tracer tests. The DFN model and potential rock block failure mechanisms were derived from high-resolution 3D virtual outcrop models via the Structure from Motion (SfM) photogrammetry technique. An artificial tracer was injected using a double ring infiltrometer atop the recharge zone of the slope to determine the infiltration rate and validate the DFN results. The DFN and tracer test methods are frequently used at different spatial scales and for different disciplines. However, the integration of digital photogrammetry, DFN, and tracer tests may represent a new step in rockfall and landslide studies. This approach made possible the identification of groundwater flow patterns within the fracture system and revealed about a 10-day tracer transit time from the injection area and the monitored slope, with similar conductivity values gathered from both the DFN and tracer test. Planar and wedge failures with volumes ranging from 0.1 and 1 m3 are the most probable failure mechanisms in the areas. The results were consistent with the delay between the intense rainfall and the slope failures previously documented in the study area and with their mechanisms.

Combined Methodology for Rockfall Susceptibility Mapping Using UAV Imagery Data

“…Slope failure phenomena are characteristic of steep mountain slopes, often composed of strongly fractured sedimentary, igneous, and metamorphic rocks. The fracture network is a highly causative factor for rockfalls as it serves as a pathway for groundwater and determines erodibility and slope failure mechanisms [41,42]. The modelling of fracture system parameters like distribution, connectivity, length, and angle is crucial in slope instability studies [43].…”

“…The modelling of fracture system parameters like distribution, connectivity, length, and angle is crucial in slope instability studies [43]. The choice of method for determining cracks as well as tracing is based on a compromise between the required accuracy and detail and the absence of determination errors and processing costs [41,43]. Manual techniques are reasonable when the size of the study site is relatively small or high accuracy is required, while using them on large areas is time-consuming and impractical [44,45].…”

Biostratigraphic investigations assisted by virtual outcrop modeling: a case study from an Eocene shallow-water carbonate succession (Val Rosandra gorge, Trieste, NE Italy)

“…Among the techniques to produce virtual outcrop models, structure from motion-multiview stereo (SfM-MVS) digital photogrammetry (e.g., Arbués et al, 2012;James & Robson, 2012;Favalli et al, 2012;Bemis et al, 2014;Tavani et al, 2014), particularly thanks to the advances in drone imagery, has emerged as a flexible, precise and easy-touse methodology. This technique has progressively been applied in diverse disciplines including structural geology (Franceschi et al, 2015;Corradetti et al, 2017;Camanni et al, 2021), fracture stratigraphy (Manniello et al, 2023), geomechanics (Menegoni et al, 2019;Furlani et al, 2022;Schilirò et al, 2022;Mammoliti et al, 2023), geomorphology (Brodu & Lague, 2012;Carrivick & Smith, 2019;Devoto et al, 2020) stratigraphy (Nesbit et al, 2018) and sedimentology (Fabuel-Perez et al, 2010;Jablonska et al, 2021) among many others.…”

A Low-Cost and Fast Operational Procedure to Identify Potential Slope Instabilities in Cultural Heritage Sites

“…A-DInSAR, which can be ground-or satellite based, makes it possible to monitor ground movements with millimeters of accuracy [10]. Diversely, UAVs are often used to improve the interpretation of geological and geomorphological features, especially in inaccessible areas [11][12][13][14].…”