Stepwise mitigation of the Macesnik landslide, N Slovenia

The paper gives an overview of the history of evolution and mitigation of the Macesnik landslide in N Slovenia. It was triggered in 1989 above the Sol ava village, but it enlarged with time. In 2005, the landslide has been threatening a few residential and farm houses, as well as the panoramic road, and it is only 1000 m away from the Savinja River and the village of Sol ava. It is 2500 m long and up to more than 100 m wide with an estimated volume in excess of 2 million m3. Its depth is not constant: on average it is 10 to 15 m deep, but in the area of the toe, which is retained by a rock outcrop, it reaches the depth of 30 m. The unstable mass consists of water-saturated highly-weathered carboniferous formations. The presently active landslide lies within the fossil landslide which is up to 350 m wide and 50 m deep with the total volume estimated at 8 to 10 million m3. Since 2000, the landslide has been investigated by 36 boreholes, and 28 of them were equipped with inclinometer casings, which also serve as piezometers. Surface movements have been monitored geodetically in 20 cross sections. This helped to understand the causes and mechanics of the landslide. Therefore, landslide mitigation works were planned rather to reduce the landslide movement so that the resulting damages could be minimized. The construction of mitigation works was made difficult in the 1990s due to intensive landslide movements that could reach up to 50 cm/day with an average of 25 cm/day. Since 2001, surface drainage works in the form of open surface drains have mainly been completed around the circumference of the landslide as the first phase of the mitigation works and they are regularly maintained. As a final mitigation solution, plans have been made to build a combination of subsurface drainage works in the form of deep drains with retaining works in the form of concrete vertical shafts functioning as deep water wells to drain the landslide, and as dowels to stop the landslide movement starting from the slide plane towards its surface. Due to the length of the landslide and its longitudinal geometry it will be divided into several sections, and the mitigation works will be executed consecutively in phases. Such an approach proved effective in the 800 m long uppermost section of the landslide, where 3 parallel deep drain trenches (250 m long, 8 to 12 m deep) were executed in the autumn of 2003. The reduction of the movements in 2004 enabled the construction of two 5 m wide and 22 m deep reinforced concrete shafts, finished in early 2005. In Slovenia, this sort of support construction, known from road construction, was used for the first time for landslide mitigation. The monitoring results show that the landslide displacements have been drastically reduced to less than 1 cm/day. As a part of the stepwise mitigation of the Macesnik landslide, further reinforced concrete shafts are to be constructed in the middle section of the landslide to support the road crossing th