What are Rockfall Hazards?
Rockfalls are a major hazard in rock cuts for highways and railways in mountainous terrain. While rockfalls do not pose the same level of economic risk as large-scale failures which can and do close major transportation routes for days at a time, the number of people killed by rockfalls tends to be of the same order as people killed by all other forms of rock slope instability. It is important to note that rockfall is a natural catastrophic erosional process that has been occurring in steep terrain for as long as the Earth has existed. Young mountain ranges such as those in Colorado do not weather away grain by grain, rather, they tumble down in a punctuated but perpetual sequence of rockfalls, rockslides, landslides, and debris-laden floods over millions of years.
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Rockfalls are generally initiated by some climatic or biological event that causes a change in the forces acting on a rock. These events may include pore pressure increases due to rainfall infiltration, erosion of surrounding material during heavy rain storms, freeze-thaw processes in cold climates, chemical degradation or weathering of the rock, root growth or leverage by roots moving in high winds.
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Recognizing a Rockfall:
​Many areas where rockfall may occur are relatively easy to recognize. Other areas where rockfall is a potential hazard are difficult to identify and evaluation of the degree of hazard present may be virtually impossible. Potential rockfall areas are those where relatively steep or barren cliffs rise above less steep talus or colluvial slopes. The talus slope and areas adjacent to it, occupied by larger angular randomly oriented rocks, constitute the long-term potential rockfall danger zone even though the talus may be partially overgrown with vegetation. Active rockfall areas are those showing evidence of recent falling and rock movement. Rock displaced or damaged vegetation, fresh “tracks” of rocks rolling down-slope, fresh scars on cliffs, anomalous or disoriented lichen growth on rock blocks, eyewitness accounts, and damage to fences or man-made works are some common criteria for identifying active rockfall areas.
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Rockfall Protection:
Active mitigation is necessary when there's a real danger of falling rocks. The designed rockfall protection systems must be capable of absorbing the impact of the falling rock and must be able to contain it in such a way as to reduce danger and present safeguards for people and property.
Passive mitigation is necessary when the scenario is intended as a preventive way of controlling rockfalls. This method is usually used ​in areas that are not fractured or where slopes have already been worked with slope stabilization measures.
Hexagonal mesh rockfall protection is different from gabion mesh. The hexagonal mesh is a double-twisted safety net that provides greater restraint from stretching. It allows the rock to move freely inside the mesh when it's properly installed.
The double twist on the mesh also ensures that the net won't fail if one wire is cut. Anchoring the mesh to the surface can be accomplished by different procedures and depends on the soil characteristics. This method is recommended for rocks varying in size from 18 inches to 2 feet in diameter. Soil embankments are used as rockfall protection systems when it's impossible to install drapery or mesh systems. In construction areas that are inaccessible or where part of the slope can't be reached, soil embankments might provide the safety protection that you're looking for.
Rockfall soil embankments built using local fill material to create a "green" embankment will reduce the environmental impact of the system. This type of protection can withstand high energy impacts. The advantages of these systems are their low installation cost, the minimum maintenance that's required, and the integration of local resources with the environment.
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