An underground seasonal heat storage facility is planned to be excavated at Nurmijärvi, Finland for the purposes of storing thermal energy in water for later use in winter. The facility is planned to be excavated under existing structures situated on the surface, which are commercial buildings with strict limits on uneven ground movements. Specific temperatures have to be maintained constantly within the buildings. The preliminary design phase of construction requires defining the potential mechanical and thermal effects induced by the facility, particularly related to any existing structures. This was investigated with rock mechanical simulations focused on identifying the critical effects of the project and studying how any identified rock mechanical risks could be reduced within the constraints of the project.

The simulation geometry included the new storage facility, new open-pit excavations and the building foundation excavations. The rock surface topography was interpreted from surveyed outcrops and from percussion drillings. Rock quality and rock joint orientations and properties were interpreted from geological surveys and core drillings. Intact rock and joint properties were based on samples tested in the laboratory and the in-situ stress field on stress measurements.

The main 3-dimensional thermomechanical continuum and discontinuum simulations were performed using FLAC3D, (Itasca 2023), where the discontinuum model incorporated a discrete fracture network using zone joints. Supplementary 2D simulations were also performed to enable faster comparison of different excavation options. Simulation results included the analysis of the evolution of the in situ stress state, its effects and displacements. Simulations also focused on the shearing / opening potential of joints and weakness zones as significant hydraulic connections between the storage caverns are not allowed. Based on the simulation results, the excavation and reinforcement designs were adjusted accordingly. Simulation results also provided the reference values for on-site displacement and deformation measurements during the excavation.