Landslide remediation design on the DC32 state road
A landslide has been activated in Primorje-Gorski Kotar County, on the southwestern side of the DC32 state road, between the settlements of Prhci and Tršće, measuring approximately 25.0 meters in length and 35.0 meters in width. The main failure surface of the landslide is located along the edge of the newly asphalted section. The road section was constructed by partially filling the existing terrain and partially cutting into it. The existing embankments and cuts are unprotected, without any retaining structures.
No accumulation was identified through engineering and geological mapping, as it was likely washed away. The cause of the landslide is most likely a combination of inadequate drainage system implementation for both the slope and the roadway, traffic-induced vibrations, an unfavorable slope gradient, the load of the new asphalt and concrete layers, as well as heavy rainfall and recent snowmelt, which caused the movement of the unstable material.
Geotechnical investigation and testing
For the landslide remediation design, we conducted engineering and geological as well as geotechnical investigation and testing. The works included:
- geological and geotechnical prospecting of the wider area;
- engineering and geological mapping of the terrain;
- exploratory drilling;
- a standard penetration test (SPT);
- inclinometer measurements;
- geophysical testing;
- laboratory testing; and
- an analysis of previous research.
As part of the exploratory drilling, three boreholes (B-1, B-2 and B-3) were drilled on the site, each with a depth of 20.0 meters. Based on the analysis of all the data, a geotechnical report was produced, serving as a basis for the further development of the landslide remediation design. The geotechnical report provided insight into the shape and dimensions of the slip surface, as well as the materials present at the location and their characteristics.
A geotechnical review of the site and the drilled boreholes revealed that the geotechnical profile consists of four (4) geotechnical units: fill material (GW, GC — Geotechnical Unit 1); diluvial/eluvial deposits (CL, ML — Geotechnical Unit 2); highly to moderately weathered clayey siltstone — Geotechnical Unit 3; and moderately to slightly weathered sandy siltstone — Geotechnical Unit 4.
Landslide remediation measures
As part of the landslide remediation on the DC32 state road, the planned works include measures to increase the overall stability of the slope, the construction of a stormwater drainage system and the reconstruction of 40.0 meters of the road.
The landslide is to be remediated by constructing a bored, reinforced concrete pile wall with pile lengths of 5.0, 6.0, 7.0 and 8.0 meters and a nominal diameter of 620.0 / 540.0 mm. The piles are to be installed in two rows with an axial spacing of 1.50 m. The transverse spacing between the pile rows is to be 1.20 m. Above the piles, a reinforced concrete retaining wall of variable height between 2.55 m and 2.65 m is to be constructed, with its foundation serving to connect the piles (pile cap). Behind the wall, a backfill of stone material is planned. In addition to the measures for ensuring global slope stability, the project also includes road reconstruction and the implementation of a stormwater drainage system at the landslide location.
To assess the effectiveness of the designed interventions, a systematic observation plan is to be implemented during the execution of the works and subsequently during the exploitation phase. Additionally, testing of all the piles is planned to ensure their characteristics align with the design requirements.
Below is a BIM model of landslide remediation.
