Understanding spatial and temporal evolution of progressive excavation damage zone in granitic rocks – development of a novel mine-by and laboratory experimental framework at the BedrettoLab

Abstract. The brittle damage around tunnels in the form of spalling is common in massive crystalline rocks subjected to high in-situ stresses. Understanding the short-term development and long-term evolution of the Excavation Damage Zone (EDZ) is particularly important for deep geological repositories (DGRs) for heat-generating nuclear waste, as the EDZ may provide preferential pathways for radionuclide migration. This paper presents an overview of the PRECODE mine-by experiment at BedrettoLab, a newly developed underground research laboratory (URL) in southern Switzerland designed to investigate brittle fracturing and EDZ evolution in crystalline rocks under repository-relevant conditions. The PRECODE tunnel was excavated in Rotondo Granite using both controlled drill-and-blast (D&B) and line-drilling and rock-breaking (LDRB) methods to evaluate the influence of excavation technique on EDZ characteristics. The experiment combines multidisciplinary in-situ monitoring including acoustic emission, distributed acoustic and strain sensing, hydraulic monitoring, electrical resistivity tomography, and terrestrial laser scanning with laboratory-based fracture mechanics and time-dependent testing under varying environmental conditions. The main objectives are to characterize short-term EDZ formation, investigate long-term crack growth and stress corrosion processes, quantify permeability evolution, and assess the interaction between excavation-induced damage and nearby fault zones. Preliminary results indicate clear differences in AE activity and EDZ development between excavation methods, while observations from the mature EDZ of the more than 40-year-old Bedretto Tunnel suggest ongoing time-dependent brittle fracturing. The coupled in-situ and laboratory datasets provide a unique basis for improving predictive models and long-term safety assessments for nuclear waste repositories in crystalline rocks.