by Dr. Martin Woodard, PhD PG PE, Director of Rockfall Services, GeoStabilization International
Rockfalls happen quickly and can be catastrophic. While sometimes they come with warning, that is not always the case – putting people and infrastructure in potential danger. Therefore, it’s critical to understand the integrity of a rock slope, the potential for a geohazard to occur, and techniques for remediation.
Modes of Failure
The risk of rockfalls typically occur because of discontinuities within the rock formation. These are breaks in the continuity of the rock mass, causing weakness to the slope. To identify discontinuities – such as bedding, joints, and faults – key information is needed, such as the dip and orientation of the rock slope, orientation of the discontinuities, infilling and roughness of the discontinuities, and water conditions. This information will help determine the potential and likelihood of a failure occurring.
Planar failures, for example, are most often controlled along a single plane. This type of failure has the potential to move down the slope, as if it is traveling down a slide. In layered rock slopes, we often see toppling failures – where the failure is initiating at the slope toe, commonly caused by undercutting of more erodible layers. The most prevalent failure we see, however, are wedge failures. This is where at least two discontinuities work in conjunction to create a failure. Understanding the mode of failure will provide insights into the best mitigation techniques. The location, scope of failure and geometry of the slope also help to determine how to approach remediation efforts.
The first step should always be scaling. Scaling uses rope-access systems to check discontinuities along the slope, removing any loose rocks. For some slopes, this may be the most effective method for remediation without the need for further mitigation. Additional mitigation measures for potential large block failures may require measures such as rock bolts, slope drains, or systems such as pinned mesh or shotcrete solutions may be necessary.
Draped mesh is a method of a steel wire mesh sheet loosely draped over a rock slope. This system safely guides falling rocks down the slope into a catchment area, where they can later be removed. Pinned mesh, on the other hand, is used when a catchment area isn’t available or possible to create. This system uses the same steel wire mesh, but instead of draping it over the slope, anchors cinch the mesh in place and hold loose rock to the slope. To further stabilize an area and help stop erosion from happening, shotcrete can be added to pinned mesh.
With failures of smaller scope, creating a containment area is often the most cost-effective mitigation method while still reducing risk of harm from rockfalls. For example, if the area below a vertical slope is 20-feet or greater between the slope base and roadway, that space can act as an effective containment
area without further remediation. If that space isn’t large enough, berms made from rock and soil can also be effective, if they are built at an appropriate height and distance from the base. These are most often seen during mining operations.
When a stronger, more permanent wall is necessary, rigid barriers such as GSC walls can be an effective solution. Depending on the energy of the falling debris, however, rigid barriers can break under increased pressure. In these instances, flexible barriers can help contain rockfalls while bending with the weight of the debris versus breaking. With each of these methods, prior to creating the containment area, scaling to remove potential loose rock is recommended.
Whether due to inconsistencies within the rock slope or geologic processes that set the stage, rockfalls will happen. But they don’t always have to be disastrous. Through understanding the integrity and geometry of the slope, and the likelihood a rockfall will occur, remediation best practices can be put in place to mitigate the risk of the geohazard and protect people and infrastructure from potential danger.