Landslide mitigation at Vela Voda

A landslide of approximately 110 m in width and 40 m in length occurred on a part of an unstable slope along the western edge of county road in Gorski Kotar, section Crni Lug – Mrzle Vodice – Gornje Jelenje. The village of Vela Voda is located only 0.5 km north of the landslide. Some instabilities such as embankment settlement and toe scars from old landslides on the slope beneath the embankment had been previously observed at the location. The road has been repaired multiple times with regards to pavement structure, culvert reconstruction, the replacing of existing road material with rock embankment material and drainage systems installation with for the purpose of lowering the groundwater level.

Geotechnical exploration and investigation works

We have carried out engineering geological and geotechnical exploration and investigation works in order to design the project documentation. Four (4) rotational boreholes of 10.0 m in length each and of 40.0 m total length were conducted. We have installed inclinometer tubes for stability control and observation of the exploration of the section in two of the boreholes.
The observation included 5 horizontal displacement measurements carried out at one-month intervals, with a total duration of 182 days. By analyzing the measured displacements, we were able to determine the location of the sliding surface at a depth of approximately 7 m.

prikaz horizontalnih pomaka u inklinometrima — *Figure 1. Image of horizontal displacements in inclinometers*

Based on the exploration and investigation works, we have concluded that the explored landslide was the result of many factors such as unfavorable slope of terrain, geotechnical profile of the location, groundwater seepage paths, road embankment load on the slope, and continuous unloading of the landslide toe due to the constant Vela Voda stream that erodes it.

sanacija klizišta - geotehnički presjek klizne plohe — *Figure 2. Sliding surface position (in red) on geotechnical sections*

In order to optimize the design-making process as much as possible, we have created a 3D geological model to provide better insight into the spatial slide distribution. This way it was possible to choose suitable landslide mitigation measures.

sanacija klizišta 3d geološki model — *Figure 3. The 3D geological model*

Landslide mitigation measures

Landslide mitigation included works carried out to increase the overall slope stability and the performance of the precipitation drainage system.

In order to perform landslide mitigation and achieve road stability, the construction of a pile wall of reinforced concrete piles D = 800.0 mm in diameter, L = 12.0 m in length and with a horizontal spacing of 1.60 m is planned. The piles are connected to each other by reinforced concrete capping beam. For additional stability of the pile wall, rock bolts of total length L = 15.0 m are to be installed.

In order to lower the groundwater level, it is necessary to reconstruct the precipitation drainage along with the planned intervention. As well as the cleaning of existing self-drilling drains above the road and the gutters at the location, plans include the reconstruction of the gap and the implementation of additional self-drilling drains and concrete canals below the existing road.

poprečni presjek klizišta — *Figure 4. The cross-section of landslide mitigation measures*

In order to check the success of the designed works, it is necessary to perform systematic observations during the execution and during the exploitation afterward. For this purpose, two vertical inclinometers and deformeters of 15.0 m in length are provided with the measuring of horizontal and vertical displacements. Also, it is planned to examine the integrity of all executed piles in order to compare their characteristics to design requirements.

The model with the abovementioned mitigation measures is shown below.

BIM