engleski

Instability phenomena in the Čabranka Valley, Croatia

The Čabranka river is a bordering river between the Republic of Croatia and Republic of Slovenia. River valley is cutted in broken anticline consisted of Permian clastic sedimentary rocks. Rock mass has lithological heterogeneity. It mostly consists of micaceous shale’ s, sandstones and quartz conglomerates. This sedimentary rock mass is folded, faulted and extensively fractured which enabled separation and disintegration of rocky blocks, and accumulation of falling material on the scarp foot and in Čabranka riverbed respectively. Bedrock is covered by debris formations which are consist from sandy and silty clay mixed with coarse fragments and boulders. The natural slopes near road are very steep and debris formations are potentially unstable. Geohazad of mass movement in the Čabranka valley is very high. The regional road between Čabar and Zamost is mostly located along and near the southern coast of Čabranka channel. This road is up to 5 m wide and is inadequate to modern traffic, and road fill is permanently undercutted by river flow. For the purpose of its reconstruction, undercutting into the very steep slopes was designed and constructed. The previous geotechnical investigations were not performed before design of road reconstruction. Due to a heterogeneous geological composition and a complex tectonic structure, the undercutting of the slope resulted in occurrence of several different types of instabilities in rock mass and in slope deposits. For this reason there was a need for urgent remedial measures. Individual planar slides in rock mass and smaller rock falls were improved quickly and relatively simply by adequate support systems. More complex geotechnical problem has been reactivated transgressive landslide. The large part of landslide body is saturated by water witch penetrates through slope formation and flows on contact with bedrock. Groundwater flow has direction toward untaken spring, which is situated on the toe of unstable slope. Groundwater has had great impact on strength properties of material, and effective stress quantity on failure surface. There were also activated some debris falls formed as a result of removing a material from a toe of the slopes. The landslide and debris falls were improved with relatively massive retaining structure combinated with reinforcing systems of rock mass in bedrock and complexly drainage systems. All of accepted remedial works were executed using an active design approach. Monitoring of the established network of piezometers, inclinometers and landmarks is been carried out during the remedial works. In this paper we will present descriptions of some phenomena of instability in the Čabranka valley in Croatia occurred during the reconstruction of regional road and designed and performed remedial works. We will also present the concept of accepted active design approach during execute of remedial works on the reactivate landslide.

slope stability; lanslide; Čabranka valley

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