Rockfall and slope remediation on county road ŽC5031, section Čabar-Mandli (6 locations)
Rockfall and slope remediation on county road ŽC5031, section Čabar-Mandli (6 locations) is currently underway. We have carried out engineering geological and geotechnical prospecting and made the main and the detailed rock slope protection design.
The ŽC5031 section in question extends from the town of Čabar in the north to the village of Mandli in the south, along the right side of the Čabranka river. The existing road is mostly made by cutting into the terrain, which opened up numerous slopes up to 30.0 m in height, and partly by filling the embankment with local plant cover. The slopes are predominantly on the right side of the road, they are subvertical up to a steep slope of about 50-70˚, and are mostly overgrown with local vegetation cover.
Remediation is currently underway at 6 locations on the road. The project documentation covers a section of 7.2 km in total.
Geotechnical exploration and investigation works
In order to prepare the construction design (geotechnical part) of the rock slope protection, we have carried out geotechnical and engineering geological mapping and slope categorization.
Due to the section length, it was decided that the section would be divided into several parts (phases), depending on the slope remediation method.
A part of the ŽC5031 section is located on conditionally stable slopes that are mostly overgrown with vegetation and/or where there are certain protection measures already carried out (conditionally stable). The rest of the section is located on unstable slopes which endangers traffic safety.
Rockfall and slope remediation – remediation measures
Rockfall and slope remediation measures included works carried out to increase slope stability and rockfall protection. The rockfall and slope protection included the application of the following remediation measures:
- reinforced concrete filling
- riprap execution
- rock bolts
- self-drilling rock bolts
- protective mesh
- protective mesh reinforced with steel ropes
- reinforced retaining structures with self-drilling rock bolts
Before implementing the rockfall and slope protection, the slope was cleared of vegetation, clay, stones and loose blocks up to 50.0 kg (manually or by blowing under pressure).
Instabilities were observed at the end of the slope, and a scar of fallen fragments of 3.0 m in height and 2.0 m in length was formed at the location where a rock wall stood. The remediation was conducted by riprap and water drainage. The dimensions of rock blocks ranged from 20x20x20 cm to 50x50x50 cm. The water drainages are made using hard PVC perforated tubes or similar material, with a minimum nominal diameter of DN 90. The water drainages are inclined at a 5° angle from the horizontal downwards.
There is a gap between the existing walls at the location. Instabilities were observed in the form of collapsing rock blocks (approx. 1 m3) down the slope. A reinforced retaining structure for the wall with a height H=2.0-5.0 m is planned. The reinforced retaining wall is secured with self-drilling rock bolts of 32 mm diameter, with a length of L=6.0 and 9.0 m.
Instabilities were observed in the form of collapsing rock blocks (approximately 1-2 m3) down the slope at the location of the existing wall (3.0 m in height, the slope behind the wall up to 24.0 m in height). A wedge was created in the slope that increased material erosion. Remediation was performed by installing rock bolts (diameter 32 mm, length L=6.0 m), a protective mesh and steel ropes. Galvanized double twisted hexagonal wire mesh Type 8×10 was reinforced with Ø 12.0 mm steel wire rope. The steel rope was mounted diagonally and vertically in both directions, and it was attached to specially designed bolts plates.
Instabilities were observed in the form of collapsing loosened rock blocks the size of 0.5 to 1.0 m3 at the location of the existing wall and the slope behind the wall up to 10.0 m high. Remediation was performed by installing rock bolts with a diameter of 32 mm and a length L=6.0 m, protective mesh and steel ropes. Self-drilling rock bolts with a diameter of 40 mm and a length L=9.0 m were installed on the unstable block. A reinforced concrete filling was installed below the unstable block. Reinforced concrete fillings were built by twisting the Q-188 reinforcement mesh and placing it in the scars of fallen fragments on the slope. The reinforcement was attached to the slope with rock bolts, and shotcrete was applied.
Instabilities were observed in the form of collapsing loosened rock blocks of size 0.5 to 1.0 m3 at the location of the existing slope with a height of 5.0 to 7.0 m. Remediation was conducted by installing a protective mesh. A galvanized double twisted hexagonal wire mesh Type 8×10 was installed. The mesh was further reinforced with concrete weights at the bottom. The weights are made of concrete with the compressive strength class C 25/30, measuring 30x30x10 cm.
Conclusion
At the moment the rockfall and slope remediation is 65% complete. Works on the reinforced retaining structure, rock bolts, protective mesh and steel rope installation are underway.
Read more: Engineering geological mapping of the slope
