engleski

Designing and stabilization of unstable parts at slope cut in Breccias

The major portion of Motorway A1 Zagreb-Split-Dubrovnik, Section IIIC-2 Žuta Lokva-Otočac was designed and constructed in Cretaceous limestones, while the analyzed area of the right cut incline from km 14+360 to km 14+820 was designed and subsequently stabilized in limestone breccia sediments. At a portion of approx. 500 m, the Motorway hacks into steep slope composed of surface sediments of high plasticity clay and of drowned breccia blocks, with the base formed of limestone breccia. The slope is at an incline of 40-50&ordm ; in the length over 300 m. In order to avoid high slope cuts, the subject side cut was originally designed and constructed in stages approx. 8 m high with benches approx. 4 m wide between the stages. An incline of stages varies from 1:1 to 2:1 at more shallow and worn portions of rock mass and 3:1 in deeper and less worn rock mass portions. Maximal height of cutting into slope has reached up to 55 m. This method of cut construction initially required protection of cut by anchors and shotcrete, with net against rockfall, by anchors and wire cables, particularly at lower cut stages. The highest portions of slope cut within relatively wide area had no slope protection. At the highest unprotected slope areas, after exploitation of motorway during approx. 7 years, regular landslide of bigger and smaller blocks was noted in clay material, furthermore, at portions formed of blocks, karst and clay the phenomenon of major surface erosion - relatively strong surface rinsing of clay material under numerous stone blocks at slopes - has been identified, what could potentially lead to separation of stone blocks and their collapse along the slope cut onto the motorway. Additional geotechnical prospecting and inspection of location at the potentially unstable portions of constructed and only partially protected slopes at the right side cuts, has identified, at the local level, the phenomenon of forming the potentially unstable blocks and rockfall of coarse and fine stones and blocks caused by permanent rinsing of clay covering layer. The basic problem was to prevent the unprotected slope portions from being additionally exposed to an intense erosion and rinsing of slope hinterland, and then, to safeguard the unprotected portions of slope by adequate protective methods and remove evidently and potentially unstable rock blocks located at and above slopes. Stabilization of unstable slope cut portions in breccias is a technical solution for the area of unstable rock mass surface which requires additional intervention - stabilization in order to grant road safety. Furthermore, protection of unprotected slope cut portions in breccias is necessary in order to prevent erosion of clay as well as formation of weak and unstable blocks

geotechnics; engineering geology

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