Sacrificial Support External to Strata

Sacrificial Support External to Strata:-

This section relates largely to passive support placed in openings to prevent movement of the strata into the opening. It includes the oldest type of support, timber props and bars, and a whole variety of material placed in openings between roof and floor – timber chocks, roadside stone or pumped packs, cans, cementitious blocks or props, steel bars and legs, steel arches, etc. It could also include items such as steel straps or mesh, but these are generally used in conjunction with strata reinforcement and will be included in that section.

Some of these, notably timber props, provide minimal resistance to roof movement and really only serve to prevent failed material from falling. Others are able to provide a fair degree of resistance but require a considerable amount of roof/floor movement before high resistance is obtained, and are very dependent on the installation standards applied.

The use of this type of support is now limited to:

• Where a degree of failure has occurred or is occurring and rapid resistance is required to prevent further failure or to retain already failed material in place.
  
• Where a roadway is only required in the short term and space restrictions are not a problem e.g. a longwall tailgate immediately outbye the face.
  
• Where a roadway can be closed off, to prevent a possible failure extending into a roadway that is in use e.g. across a cut through which leads into a goaf area.
  

The advantage of these supports is that they are cheap and can be easily and quickly installed if they are suitable for the proposed application.

Once these supports have taken load they will be deformed and/or crushed and will not usually be suitable for re-use even if they can be safely recovered.

There is a type of support which fits in this section being "sacrificial" in the sense that it is never re-used, but which is often a long term or life of mine installation. This includes such support methods as concrete or steel lining in shafts, sometimes drifts and at times at particular locations which are life of mine work places. Sometimes such linings are designed to withstand high stresses and are true supports preventing major strata failures, but more frequently the linings serve the purpose of retaining strata in place so that minor failures do not develop into major ones. Preventing small pieces of rock material falling a long vertical distance is also an important safety function in shafts. The smooth finish of most types of lining has added benefits in reducing resistance to ventilation flow, improving the attachment of shaft or drift services (pipelines, cables, etc) and possibly facilitating cleaning, especially in drifts carrying a conveyor.