Impact of water inflow and design solutions for construction of Karavanke tunnel

During the construction of West tube of 7,9 km Karavanke motorway tunnel in the 1980s, the builders experienced frequent water intrusions, which caused general tunnel instability. The most prominent cases occurred at chainages cca 700 m where tunnel nearly collapsed and at cca 3000m (Goliški fault zone) where collapse was inevitable due to enormous inflow of ground water that also acted at pressures as high as 60 bars. To counteract these conditions a careful approach was taken in design of the drainage measures during the construction of the East tube, which is currently ongoing. The water inflow at chainage 700m was managed mainly by the timely and correctly selected use of drainage boreholes at the head of the excavation. The article presents construction conditions on this section of the Karavanke tunnel describing effectiveness of drainage measures that enabled rapid progress of the tunnel excavation in demanding conditions. The experience gained will be readily applied for the drainage measures predicted for the high-water inflow expected at Goliški fault zone. As construction of the tunnel has not yet reached the critical position of the fault only design solutions, that include the use of pilot tunnel, are presented in the paper.