Modeling Coupled Hydro-Mechanical Effects during PRECODE Tunnel Excavation in Rotondo Granite

The assessment of deep geological repositories in crystalline rocks requires a thorough understanding of coupled hydro-mechanical (HM) processes. This is especially true for the rock mass in close proximity to the tunnel, in the so-called Excavation Damage Zone (EDZ), where coupled processes are at the highest intensity. During the drill-and-blast excavation of the 10-meter PRECODE experimental tunnel, a new branch of the Underground Research Laboratory (URL) located in the Bedretto tunnel in Southern Switzerland, distinct changes in pore pressure were registered. These pore pressure increases and decreases are expected to be dominated by an undrained poro-elastic response of the rock mass around the tunnel opening during excavation as well as dynamic forces caused by drilling and blasting operations. Using the finite element modeling software MOOSE and input parameters from extensive field campaigns measuring rock mass permeability and in situ stress conditions, we develop a three-dimensional numerical model that captures the HM response of the porous rock medium to the tunnel excavation. We validate model results against pore pressure data registered at several intervals within a borehole drilled and instrumented a few meters from the PRECODE tunnel. Additionally, we use strain data from the Distributed Strain Sensing (DSS) cable for model validation. Using results from the HM model calibrated against a comprehensive set of field measurements recorded during tunnel excavation, we evaluate the shape and extent of the short-term EDZ development around the tunnel.