What is a mine portal: A “mine portal” represents the immediate area surrounding the entrance of an underground mine excavation from “surface” leading up to a tunnel or adit. However, tunnels and adits may also be excavated from the bottom of a vertical mine shaft.
Mine portals are usually “driven-in” a few benches from surface or within an old or active open pit mine operation. Whether a mine portal was “driven-in” from surface or from an old underground working; the general method of excavation entails some form of drill and blast method. Drill and blast methods are associated with some form of rockfall inducted blast damage; and sometimes blast freezes; also known as blast overhangs. These usually leads to a high risk of slope failure or rockfall hazards. Because of these, mine portals should be automatically classified as high-risk areas and proper slope stability monitoring and rockfall hazards mitigation must be implemented from the on-set.
Define a rockfall: 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 netting systems for mine portals.
Rockfall netting systems refers to the structural elements used for the mitigation of the potential hazards resulting from un-controlled falls of ground.
In general, rockfalls 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 mine portal, a simple rockfall event 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 for a mine portals aims at minimising the potential occurrence and effects of rockfall events through the design and installation of rockfall protection systems. Rockfall netting systems are engineered to be durable, effective, and to save lives.
The design for mine portal protection usually starts-off with data collection from the site mapping to determine rock characteristics, slope morphology, rockfall frequency and trajectory, which is then used in 2D or 3D modelling to determine the correct structure or combination of structures and the location thereof. These can be active or passive hazards mitigation structures. These structures should then be designed; and could consist of rockfall barriers, rockfall embankments, drapery systems, shotcrete (teckcrete, mine-crete etc).
If a rockfall protection or debris flow barrier is chosen; the barrier design must determine the position, maximum energy levels versus the service energy levels, downslope deflection, elongation, anchors, post foundations; and type of corrosion protection coating for the steel components. An embankment design must determine the type, position, size, and the global and internal stability of the embankment.
In cases where a rockfall meshing systems is used; the design should determine if the systems should be an open drapery, simple drapery; or secured/pinned drapery system. The design must also determine the type of drapery mesh, its strength and durability, configuration, and type of anchors and cables required. The anchor design should include more specifics such as the size, depth; and spacing required.
