Rockfall Hazard Mitigation

In general, protection measures can be distinguished into two main categories: engineered and non-engineered measures (Turner and Schuster, 2012). Engineered protection measures are interventions that diminish the occurrence or the effects of rockfalls. Non-engineered measures are interventions which do not directly affect the rockfall process; these include maintenance, warning signs and monitoring programs.

Engineered measures are further distinguished into three categories regarding their philosophy: Stabilization, protection, and avoidance. Stabilization measures aim to reduce the potential for rockfalls to initiate by securing the rock blocks in place or by removing them in a controlled manner. Protection measures aim to intercept falling blocks and restrict them from entering the protected area. Avoidance measures diminish the hazard for the user or the facility by relocating them to areas with less risk. 

Stabilization measures

Stabilization measures include interventions on the slope that reduce the likelihood of a rockfall occurrence. This can be accomplished by removing loose rocks in a controlled manner (removal), by changing the configuration of the slope (re-sloping), by securing the rock blocks in place (slope reinforcement) or by slope drainage.

Removal refers to partial or complete removal of loose or unstable rocks to reduce the occurrence of rockfall. It may include the modification of the slope profile; horizontal benches are very effective as they reduce the tensional forces of the slope, decelerate erosion rate, and stop falling blocks depending on the width of the bench. Partial removal (scaling) is performed by hand tools, air bags, light blasting, water blasting or excavators. Large-scale removal and slope re-shaping are usually performed with blasting. Scaling is particularly effective for short-term protection but becomes less effective with time. Therefore, it may require periodic repetitions and/or additional protection measures to maintain the desired level of protection. Blasting and re-sloping generally result in a more stable rock face with reduced maintenance costs.

Rigid barriers are structures that either contain or deflect the falling blocks. The structure is sufficiently stiff to withstand the impact imposed by the falling rock. Rigid structures undergo relatively limited deformation due to impact and therefore, they can be placed close to the assets they are protecting. There is a variety of barrier types in use today with a wide range of energy capacities depending on their materials and geometry. The basic types are:

• Earthen embankments (berms) can be constructed in a range of shapes and with various reinforcing elements and facing materials (soil, rip-rap). They act through a combination of deformation and internal compaction to absorb the energy imposed by falling rock blocks. They are capable of withstanding multiple impacts with very high energy.
• Structural walls are steep-faced rigid structures that may be constructed of concrete, timber, steel, or gabion baskets. They generally have a smaller footprint and cross-sectional area than earthen embankments. Barriers constructed with stiffer materials (concrete, timber, steel) are generally suitable for lower energy impacts since they present limited ability to deform.

Flexible barriers are lightweight structures that aim to contain the falling rock by significantly deforming to dissipate the energy of the rock block. A fence consists of a net panel that is suspended from a series of posts and cables that are anchored into the ground; several energy-absorbing components, such as breaking elements, are incorporated into the system to allow energy dissipation before transferring it to the ground. It is common practice today to use fence systems that have undergone standardized physical testing to verify their capacity. Testing allows the manufacturer to specify the energy level the fence is capable of withstanding, as well as its residual height. Because of the considerable expertise, time and cost involved in developing flexible barrier systems, they are proprietary systems consisting of several components, which are placed on the market together with one common quality certificate, by a small group of manufacturers.

An attenuator is a flexible fence system type, intended to slow falling rocks rather than to capture them. Attenuators reduce the energy of falling rocks by controlling their trajectory over a part of their path. The reduction in energy allows the falling rock to be more easily captured by other measures that are situated downwards. Attenuators are usually constructed using flexible barrier systems that are modified to incorporate a draped tail net. The blocks travel beneath the draped tail, forcing them to impact the ground, losing energy with each impact.

Drapes are flexible wire and/or cable meshes that are suspended over a rock face. They are used to intercept rocks, attenuate their energy, and guide them into a catchment area. They are useful for high frequency rockfalls and debris. Their maintenance requirement is analogous to the event’s frequency. They can be installed in difficult access locations more easily compared to other measures. However, they are limited to rock blocks less than 1.2m in size, and they are affected by corrosive environments.

Rock sheds or rockfall protection galleries are built over the asset at risk to cover it. They are mostly made from reinforced concrete, or steel. The roof slab is either covered with an energy-absorbing material layer or is significantly inclined to deflect the falling rocks over the asset at risk. Rock sheds are one of the costliest types of protection measures and are commonly constructed to protect roads and railway lines situated below steep-sided valley walls with frequent rockfalls. 

References

Slope geometry design as a means for controlling rockfalls in quarries, 2007, Alejano, Pons, Bastante, Alonso, and Stockhausen. International Journal of Rock Mechanics and Mining Sciences 44 (6), 903-921

Pierson, Lawrence, Davis, and Vickle, 1990, Rockfall Hazard rating system: implementation manual. No. FHWA-OR-EG-90-01.

Turner and Schuster, 2012, Rockfall Characterization and control, Transportation Research Board