Expressway Construction on Young Karst in Breccia (Vipava Valley, Slovenia)

During precise and longterm monitoring of expressway construction characteristics for Slovenia, relatively rare karst phenomena were discovered in breccia that lie on a sloping foundation of impermeable flysch. The authors distinguished characteristic types of caves and initial stages in the development of dolines. The largest and most frequent are caves that developed in breccia above the contact with flysch. Caves that occur in the middle of breccia are smaller and most often filled with fine–grained sediment. Of special origin are fissure caves across the slopes. Traces of continuous vertical percolation of water are less distinct. Caves also form in the flysch. The Vipava Valley lies between the high karst plateaus of Trnovski gozd and Mount Nanos to the north and the low plateau of the Classical Karst to the south. Mount Nanos is overthrust on flysch. Below its steep western edge on the sloping flysch, scree material accumulated and consolidated into breccia that developed into a special young karst. The surface of the slopes was formed by the mass movement and mechanical weathering of rock, which was accompanied on the flysch bedrock by landslides. Water that flowed above the flysch also dissected the slopes. The thickness of the layers of scree material or breccia varies from place to place. More or less vertical fissures developed in the breccia that indicate tension in the slopes. During expressway construction when the laying out cuts deeply into the slope, the contact between scree material and breccia and the flysch bedrock showed an extremely fragile balance.