Landslide risk increasing caused by highway construction

Hazard of landslides appearance is dependent on slope conditions described with geometry, soil composition, structure and soil parameters, so as hydrogeological, climate and vegetation conditions as a time variable. This paper presents experiences in the landslide hazard and risk increasing during the Adriatic Highway construction in the flysch slopes of Draga Valley near Rijeka, Croatia. The geological fabric of Draga Valley is very complex: steep slopes are consisting of limestone rocks and flysch deposits in the bottom, mainly made of siltstones and covered with clayey slope formations. The route of designed highway is lying on lower parts of the flysch slopes, where the alignment is formed partially by slope cutting and partially by embankments filling. The consequences are steeper slopes, deficiency of masses in lower parts of the slope, increasing of hydraulic gradients of ground water flows and decreasing of run-off surface waters coefficients with significant impact on pre-existing high landslide hazard in the area. Hazard increasing was resulting with numerous landslides during the highway construction. Similarities of landslides in dimension, shape, depth of slip surface, mechanism of sliding so as risk consequences were indicating on relatively explicit pattern. Based on these facts, it was possible to identify the landslide risk on similar flysch slopes.