Rockfall remediation, Hrvatska Kostajnica

Hrvatska Kostajnica is located on the very south-eastern edge of central Croatia, on the border with Bosnia and Herzegovina. In the early hours of March 11th, 2018, a rockfall occurred from the hill Djed above the family houses in Davor Trstenjak Street, along with the north-eastern edge of the promenade Dr. Franjo Tuđman and the river Una. The volume of rockfalls behind the houses is about 500 m3, and behind them are other significant overhangs of rock mass which may cause the potential rockfall of another approx. 500 m3 of material in the overhangs.

Due to the risk of new potential rockfalls, we conducted geotechnical exploration and investigation works and made main and detailed rockfall remediation design. Remediation of unstable overhang directly adjacent to existing objects will be carried out by installing the three-shotcrete layer 5.0 cm thick and the self-drilling anchors with a load capacity of 36.0 t. The reinforced concrete pillows made of rein-forced mesh Q-188 and shotcrete will be used for stabilization of rock mass overhangs above the existing buildings. Manual removal of unstable blocks of up to 50 kg in weight precedes the execution of reinforced concrete pillows.

Due to its size, the overhead rock mass over the initiated rockfall will not be secured. Instead, it will be removed and the rock slope will be profiled with a 3V: 1H inclination. Unstable blocks will be carefully removed. The rock overheads will be removed in phases, with the maximum permissible volume of the rock block at each stage of 5.0 m3. The technical-technological study of overhang removal will be executed before removing the overhang. Safety against possible rockfall during removal of the overhang and the other possible rockfalls includes the performance of a rockfall barrier with possible energy absorption (energy capacity) of 3000.0 kJ, height H = 5.0 m and length L = 50.0 m at the foot of the slope.

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