The $4.2-million project repaired and stabilized a roadway in Alton, Ill., situated on top of a 150-ft bluff with views of the Mississippi River. The road started showing signs of movement in July 2019, leading to its closure. The slope failed in January 2020, removing about 15 ft of the roadway and creating a deep scarp about 23 ft deep. Below the scarp, the slope of the bluff is about 55 ft long at a 53-degree angle below horizontal and it terminates at a 60-ft-tall vertical limestone face.

To repair the slope and restore the road, the team installed a soil nail wall; a geo-synthetically confined soil retaining wall; and a soil nail and mesh surface stabilization system. The soil nail wall was constructed near the center line of the road to provide lateral stability. A long reach excavator was used to install the wall, preventing the need for excessive excavation to achieve access to the front of the wall.

A 28-ft-tall geo-synthetically confined soil retaining wall was constructed to build back grade for the roadway that was lost in the landslide and excavated for the soil nail wall construction. To prevent the weight of the GCS wall from destabilizing the slope, the GCS wall is supported by micropiles and a structural micropile cap. The micropiles were also installed with the long reach excavator.

Photo courtesy GeoStabilization International

To protect the slope below the wall and prevent surface erosion from undermining the new walls a pin and mesh system was installed on the slope to the top of the rock. This system was drilled by workers on rappelling ropes with the drill suspended by cables anchored separately from the crew.

During excavation, three existing concrete deadmen were discovered. The width and depth of the deadmen were unknown and excavating them could have resulted in a large excavation of the roadway. Instead, the design of the soil nail wall was revised to accommodate the deadmen and incorporate them into the wall. Without direct line of sight to the drill from the excavator, the drill was set up with a remote control and operated on the platform at the drill location.

The project was completed on time and on budget with no OSHA recordable incidents and no lost-time accidents.