A Department of Transportation (DOT) in the Mid-Atlantic region of the United States contacted GeoStabilization International to stabilize a series of shallow landslides above a river. At first glance, the site appeared to be a typical cut/cast failure with a slide plane on the bedrock/soil interface. However, further exploration revealed that the roadway had been originally constructed when a karstic section of bedrock was blasted, the remnants pushed over the side of an undercut river cliff, and then the talus covered with soil. The soil, talus, and the bedrock all contained voids and channels that connected hydraulically to the river below. GeoStabilization’s geotechnical engineers determined that in order to stop the movement and settlement of the roadway, the talus would need to be consolidated and densified, and additionally lateral tensile elements would also be required. First, GeoStabilization’s crews used a specialty-drill rig to install steel casing vertically into the unstable sections of roadway. Next, they injected low mobility grout to create columns and densify the surrounding talus. Low-mobility grout was chosen over high-mobility grout or flowfill for two reasons: 1) filling of the voids would not have been economical; and 2) high-mobility grout would likely travel through the talus into the scenic river below.
After grouting, a GeoStabilization SuperNailer™ installed a series of SuperNails® laterally into the slope to prevent further slope movement. Due to the remaining voids, difficult drilling conditions, and certainty of collapsing drill holes, the SuperNails® turned out to be the optimal technology choice for providing the required lateral tensile capacity.
As mining advances and undergoes operational changes, it is crucial to re-evaluate modes of slope failure potential. In addition to continually ensuring the integrity of the final highwall, this re-evaluation will also reduce the likelihood of instability and rockfall hazards.