Wire-Mesh Lacing for Rockfall Protection
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, rockfall usually occur due to blasting induced vibration.
Rockfall protection systems refers to the structural elements used for the mitigation of the potential hazards resulting from un-controlled falls of ground.
Rockfall in general range from minor falls to larger falls that affect safety of people and may also cause damage to 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 mining industry rockfall can have a detrimental effect on the safety and economic position of the mining operations.
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.
Rockfall hazards mitigation systems aims at minimising the potential effects 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.
Rockfall protection systems includes:
- Open drapery wire mesh system
- Pinned drapery wire mesh system
- Attenuated wire mesh system
- Rockfall/debris flow protection barriers
There are many other types of rockfall mitigation systems; and these will be discussed in our next article. This article only focuses on the most commonly used rockfall protection systems. These are open drapery wire mesh or pinned drapery wire mesh systems.
In the open and the pinned drapery systems the wire mesh is installed by means of draping the individual rolls of wire mesh over the affected slope/excavation surface area.
The critical factors are the weight and dimensions of the wire mesh rolls. The weight of the individual wire mesh roll varies depending on the type of the product used as well as the dimensions of the individual roll. The dimensions of each roll of wire mesh is typically around 3,9 x 30m. The total weight is around 170 kg.
As indicated above, installation of the wire mesh involves draping each roll of mesh over a slope. This is followed by lacing using OEM connecting clips; typically on a spacing of less than 10cm. As you can imagine, this will become a very physically involving task for the technicians. At times, lacing covers a length of over 300m, and a total SQM area of 30 000. Keeping in mind that all the wire mesh lacing is done manually by hands. This is a toll order and a demanding task.
Typically, the wire mesh installation is carried-out by trained technicians (SPRAT or IRATA) using rope access. In other projects typical work at height technicians can prevail using hydraulic lifting systems such as cranes. However, note that in most cases there is no space to establish a stable base for the crane or lifting systems. Usually, the areas below the affected slopes is usually covered by public roads or haul roads in the case of a mine site. The implication is that the roads cannot be closed for extended periods of time. This constrain creates immerse pressure on the personnel who are employed to carry-out the installation.
Typically, the situation is compounded by the fact that the slopes may be undulating and creating unpredictable conditions which sometimes make the installation process to become difficult. This usually occurs when the individual rolls of mesh lands far apart from each other and usually requires physical aligning which is very time consuming. In some cases inexperienced contractors have abandoned projects just because it can become extremely difficult to complete the installation.
For all rockfall hazards mitigation projects wire mesh lacing presents significant financial risk and safety hazard and must be approached with careful considering and skill. A standard risk assessment might not be sufficient to prevent injuries and/or death. Proper job planning and technical impetus is required to carry-out this task safely and effectively.
Prepared by
Ernest Rankhododo
MSc. Civil Eng.
