In nature, faults rarely occur as discrete slip surfaces but instead as zones of fractured and displaced rock centered around an approximate central fault surface. These fault zones introduce altered geotechnical strength and hydraulic permeability to the surrounding rockmass, making their characterization important to geotechnical engineering projects such as hard rock tunneling. Inherent heterogeneity of the fault zone composition is common, with synthetic (parallel to fault zone) and antithetic (perpendicular to fault zone) faults producing a complex internal fracture network. This heterogeneity results in subjectivity when defining the boundaries of fault zone components such as the damage zone and fault core. This study demonstrates a semi-automatic, quantitative workflow for characterizing the fracture network comprising a fault zone focusing on assessing the network geometry using principal fracture sets and the rockmass structure using areal fracture density. In an outcrop study of fault zones above the Eisenhower-Johnson Memorial Tunnels (EJMT) in Colorado, USA, point clouds derived from unmanned aerial vehicle (UAV) photogrammetry were collected over historically mapped fault zones. These data are used in accordance with prior knowledge on the nature of fault zones in the project locality to quantitatively characterize the fault zone internal structure and geometry.
Widespread access to high-resolution, photogrammetric point cloud data and the ongoing development of open source point cloud processing software programs have created a unique opportunity for enhancing site characterization projects in geological engineering (Buyer et al., 2020; Fekete & Diederichs, 2013; Ferrero et al., 2009), structural geology (Thiele et al., 2019) and reservoir modelling (Bisdom et al., 2017).
A variety of tools have been developed in specific fields (e.g., block analysis, fracture network connectivity) providing a diverse toolset that have been synthesized into a workflow for analyzing the structure and geometry of the fracture network comprising a fault zone (i.e., the zone of fractured and/or displaced rock surrounding a fault surface). In the context of this work, the term ‘geometry’ refers to the orientation and nature of the principal fracture surfaces, while the term ‘structure’ refers to numerical characteristics of the rockmass such as fracture intensity.