Understanding the Regional Stress in Active Tectonic Regime Using 3D Numerical Modeling, Case Study of BedrettoLab, Switzerland

Understanding in situ stress magnitudes and orientations is critical for the design of underground engineering projects, including tunnels, geological nuclear waste repositories, and enhanced geothermal systems (EGS). This study presents the results of a regional 3D numerical stress model developed using FLAC3D, created to analyze stress variations along the Bedretto Tunnel in the southern Switzerland. The Bedretto Tunnel is a new underground research facility designed to host in situ experiments related to deep geothermal energy utilization, earthquake physics, and tunneling, with each field relying on robust estimates of stress magnitudes and orientations. The model considers the effects of topography, active tectonic stresses, and rock properties to assess local and regional stress patterns. The model is first calibrated against local in situ stress measurements obtained via the mini-frac method conducted near the BedrettoLab. By evaluating the characteristics of the excavation damage zone (EDZ) around the Bedretto Tunnel, we were able to further validate the simulated stress states along the tunnel. After calibration, we analyze the in situ stress variations in the surrounding area to evaluate the impact of topography and active stress on the local stress distribution. Modeling results reveal that stress magnitudes and orientations along the tunnel are strongly influenced by topography and tectonic forces, transitioning from strike-slip faulting near the portal (TM 0-1900) to normal faulting (TM 1900-5200). The results also show that topography influences stress orientation: at shallow to intermediate depths (up to 1.5 km), principal stresses are non-vertical, transitioning to more vertical and horizontal orientations at greater depths. The results indicate that the K-ratio (horizontal to vertical stress) depends on topography, exhibiting a wide range at shallow depths and tending toward unity at greater depths. Although the modeling results capture the overall variations of the stress state in the Bedretto region and can serve as a potential base model in this region, the local stress heterogeneity observed from in situ stress measurements could not be modeled, highlighting the necessity of incorporating geological structures, including faults and fractures, as well as tunnel-induced perturbations.