Landslide Remediation in Skrad on the DC3 State Road

In Primorje-Gorski Kotar County, on the DC3 state road, in section 0014, and at chainage from 22 + 500 km to 22 + 600 km, a landslide has activated, affecting both the traffic lanes of the roadway. The road section was constructed partly by filling over the existing terrain and partly by cutting into the existing ground to enable road construction.

The main slip surface affects the roadway across a width of approximately 25 m and extends over both traffic lanes, while the total width of the landslide is about 100 m. The length of the main landslide portion is approximately 45 m, with a depth ranging from 4 m on the western side to 8 m towards the east (at the boundary between highly and moderately weathered marls), where it terminates in a settlement zone near the Skrad rest area.

Cracks are visible on the roadway, and they have widened from 5 cm to 20 cm over a period of four months. To the left of the scarp, there is a culvert that does not adequately drain stormwater, while to the right lies an intermittent watercourse. Active seepage has been recorded from the colluvium of the landslide.

The cause of the instability is inadequate drainage of the slope and roadway over an impermeable substrate, combined with traffic-induced vibrations and an unfavorable slope gradient. Additional destabilization has occurred because of a new asphalt layer and intense precipitation.

Geotechnical investigation and testing

For the purpose of developing the landslide remediation design, engineering geological as well as geotechnical investigation and testing were carried out, consisting of:

geological and geotechnical prospecting of the wider area;
engineering and geological mapping of the terrain;
exploratory drilling;
a standard penetration test (SPT);
geophysical testing;
laboratory testing; and
an analysis of previous research.

exploratory drilling — *Photo 2 Exploratory drilling*

In order to prepare the geotechnical report, four (4) exploratory rotary boreholes were drilled, with individual depths ranging from 11.0 m to 20.0 m, and with a total drilled length of 60.0 m. Within the investigation area, seismic refraction testing was performed along one (1) seismic profile with a total length of 55.0 m.

Based on the analysis of all the collected data, a geotechnical report was prepared, serving as the basis for the landslide remediation design. The report provided insight into the configuration and dimensions of the slip surface as well as the materials present on site and their geotechnical properties.

*Photo 3 Dolomites and clayey soils on the cut*

Based on the geotechnical site inspection and the conducted investigation and testing, it was determined that the geotechnical profile at the location consists of five (5) geotechnical units: colluvium (geotechnical unit 1), eluvium (geotechnical unit 2), highly weathered marl (geotechnical unit 3), moderately weathered marl (geotechnical unit 4), and slightly weathered marl (geotechnical unit 5).

geotechnical cross-section — *Photo 4 Geotechnical cross-section*

Landslide remediation measures

As part of the landslide remediation on the DC3 state road, works are planned to improve the global stability of the slope along a length of 40.0 m (between local chainages 0+010.00 and 0+060.00).

The remediation plan includes the construction of a pile wall consisting of reinforced concrete bored piles with lengths ranging from 8.0 to 15.0 m, nominal diameters of 620.0 mm / 540.0 mm, and a center-to-center spacing of 1.0 m. The piles are to be constructed using concrete of compressive strength class C 30/37. Above the piles, a reinforced concrete capping beam with dimensions 62.0 cm × 70.0 cm (B × H), also using concrete of class C 30/37, is planned.

In addition to the works aimed at ensuring global slope stability, the reconstruction of the stormwater drainage system and the roadway at the landslide location is also planned.

In order to verify the effectiveness of the designed interventions, systematic monitoring is planned during the execution of works and subsequently throughout the period of use. Furthermore, the testing of all the constructed piles is planned to confirm that their characteristics comply with the design requirements.

Landslides