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

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

Lipponen, A. / Manninen, S. / Niini, H. / Rönkä, E.

Abstract

Potable water to the Helsinki metropolitan area in Finland is conveyed through Precambrian bedrock along the mostly unlined, rock-surfaced Päijänne Tunnel which has been in use since 1982. Two cave-ins in recent years and the damage observed during the inspection preceding the repair of the northern 64 km of the tunnel in 2001 demonstrates the active eroding processes. The extent of block falls inside the tunnel, interpreted from a recording made by a submersible robot, is compared with both measured and semiquantitatively documented groundwater inflow and characteristics of fracture zone, intersecting the tunnel that commonly appear as linear depression, in topography. Most of the damage observed in the tunnel is inferred to result from weathering, swelling clays, erosion and changes in groundwater or pressure conditions The main factor in triggering these physical and chemical changes is water that flows both along the tunnel and in the fractures of the surrounding bedrock. Foliation sub-parallel to the tunnel orientation seems to be a factor making tunnel sections more prone to the occurrence of block falls. 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 interred 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.