Rockfall refers to quantities of rock falling freely (uncontrolled) from a natural slope or man-made excavation. The term is also a synonym for collapse of rock from an underground mine roof or open pit mine high-wall, also known as fall of ground. Rockfalls are also major hazard in excavations for highways and railways in mountainous terrain. A rockfall event is generally initiated by climatic and biological event that causes a change in the forces acting on a rock. In a mining environment, rock fall usually occur due to blasting induced vibration.
A rockfall event may be the result of factors both internal and external to the rock slope combined with the action of gravity. The external factors are chemical changes in the rock surface, temperature variations, surface scour by water and wind action, forces exerted by plant roots, and blasting/excavation methods. The internal factors are connected with residual geological stresses in the rock and with fractures or discontinuities. Of all the foregoing factors the one which has the greatest effect is undoubtedly the action of water and the freeze/thaw cycles.
In general, rockfalls range from minor falls to larger falls that affect safety of people and may also cause damage to public infrastructure. Rockfall may also include substantial slope failures that cause serious damage to critical public and private infrastructure.
The effect of rockfall events range from a simple damage to private property, injury or loss of live. It may also lead to disruption of critical economic systems such as closure of a mine or critical road access. In a mine, a simple rockfall event can have a detrimental effect on the safety and economic position of the mining operations.
Rockfall protection systems are aimed at reducing the potential occurrence of rockfall events through the design and installation of rockfall protection systems. Rockfall protection systems are engineered to be durable, effective and to save lives.
The design of a rockfall protection system usually starts-off with site investigation. Site investigation will usually involve geological mapping and topographical surveys to obtain accurate slope morphology. Site investigation data is required in order to determine rock mass characteristics, slope morphology and topographical profiles. Geological and topographic data is then imported in a 2D or 3D modelling software (e.g. rocscience’s rocfall) to determine rockfall trajectories and energies. The 2D modelling process is used to assist with determining the most suitable rock fall protection solution. But it should be noted that the selection of the most appropriate rockfall protection system is always based on technical experience rather than software and the software aids in the process of determining the most appropriate rockfall protection system.
After site investigation, rockfall protection structures are selected through rational design. The design of the rockfall protection structures usually consist of individual elements, and/or a combination of rockfall barriers (catch fences), rockfall embankments, open or simple wire-mesh drapery systems, pinned wire-mesh drapery systems; and attenuated wire-mesh systems. In some instances the rockfall protection systems may also consist of shotcrete.
If the rational design process opt for rockfall netting preferred option, the designer should determine if the rockfall netting system should be an open drapery or pinned drapery rock fall netting system.
The open (simple) drapery rockfall netting is anchored at the top of the slope using a continuous wire-rope cable anchored on a series of 9ft grouted gewi bars or rope anchors. The spacing and thickness of the gewi bars or rope anchors depends on the depth of the slope and the weight of the mesh to be installed. The simple drapery rockfall netting system is a passive system capable of containing the rockfall at the bottom of the slope. The bottom of the open drapery rockfall netting system can either be normal steep bar weights or it will be pinned with grouted gewi bars or cable anchors.
The designer must also determine the type, strength and durability, configuration and type of top-anchor cables required. The top-anchor design should include more specifics such as the size, depth; and spacing required. The design of the crest anchors may be divided into two different steps. The first allows designing the anchor diameter and it takes into consideration the sheared load transmitted from the system, composed of the mesh and cable. The second is the definition of the minimum anchor length, which depends on geotechnical parameters and the hole diameter.
The design and installation of rockfall netting system is usually carried-out by a specialist geo-hazards mitigation contractor; and require special equipment and skilled workers. In the hands of unskilled workers, the rockfall netting project could result in fatal injuries and damage of expensive rockfall netting materials. If it is on a public road, the liability can be very serious for anyone involved. So this type of work is reversed for the experienced contractors.
Prepared by
Ernest Rankhododo (MSc. Eng).
