Deciphering Heterogeneous Mechanical Stability in an Exhumed Fault Zone through a Structural-Geotechnical Approach: A Case Study from the Great Glen Fault, Scotland.

Inherent complex internal architecture within fault zones, governed by diverse geological factors, results in heterogeneous mechanical variability, causing uncertainties in subsurface ground conditions. Therefore, understanding the spatial variations in mechanical stability in a fault zone is crucial to appropriate engineering mitigation plans and cost reduction for surface or subsurface infrastructure projects. The Great Glen Fault (GGF) is one of the major NE–SW trending strike-slip faults in Scotland, exhibiting complex internal architecture resulted from the multiple reactivation events and exhumation. The Torcastle block, a fault-bounded sliver within the fault core of the GGF, contains heterogeneous micaceous shear zones, faults, and local dykes cutting foliated psammitic–pelitic gneiss and quartzite. At the Torcastle block, this study synthesizes the parallel structural geological and engineering approaches to decipher the relationship between structural features and mechanical stability by mapping structural domains, topological nodes, fracture densities, and engineering Q-values. Four fracture types are classified based on the spatial distribution pattern and geometrical relationships with local faults and foliations. The heterogeneous spatial patterns of faults, fractures, and foliations at the Torcastle block define several fault-bounded structural domains. The geometrical properties of fractures are highly variable, but they clearly relate to the dyke distribution and local foliation trend in each domain. Mechanically weak zones, represented by low Q-values, are highly heterogeneous but concordant with the areas of high fracture and topological X and Y node density. These mechanically unstable zones are typically related to the following structural features in a fault zone, including major shear or fault strands and embedded blocks, intruded igneous dykes, abutting areas of faults with different orientations, and highly rotated blocks showing re-oriented local foliations. Correlation analysis between Q-values and other parameters, including fracture density, RQD, Jn, Jr, and X and Y node density, reveals different contributing patterns of each parameter to mechanical stability in each structural domain. Especially, the zone of highly rotated local foliations exhibits lower mechanical stability, despite relatively low fracture densities compared to other mechanically weak zones, due to increased fracture orientation variability and connectivity. The results of this study highlight the heterogeneous internal architecture of fault zones and their relationships with mechanical stability distribution, which shed insight into forecasting mechanically weak zones in rock masses and reducing geotechnical risks for subsurface engineering projects.