Effect of water and geological factors on the long-term stability of fracture zones in the Päijänne Tunnel, Finland: a case study

A high rate of groundwater inflow correlates to some degree with the level of original reinforcement. Even though groundwater inflow does not necessarily indicate support need, increased fracturing in bedrock generally provides more flow paths to water. Hydraulic properties of fracture zones are inferred to contribute to the deterioration in the tunnel but probably as a relatively minor factor compared with the rock support solutions and structural orientation sub-parallel to the tunnel. The orientation of fractures is inferred to play a role as variation is observed in inflow when linked with the strike of the fracture zone. The damage concentrates to zones of existing weakness and sections that have not been lined with shotcrete. Shotcrete prevents leaching of fracture fillings which can lead into loosening and falling of key-blocks. The application of steel-supported reinforcement in the fractured sections is a plausible explanation to why no consistent connection was observed between the occurrence of large block falls and fracture zone intersections. The impairment of the tunnel's condition, revealed during the repair inspection, is considered to have been influenced by the initially low level of reinforcement.