Mine Design Relating to Dimensions and Shape

Mine Design Relating to Dimensions and Shape:-

The aim of this aspect of strata control is to make the strata self supporting as far as possible, or if not, to minimize the extra support work required.

With regard to opening size, this involves designing minimum practical widths for whatever operations are carried out and could involve modifying the design of equipment to fit into smaller openings. It is perhaps more common to install extra support to stabilize an opening that is suited to available equipment rather than design and manufacture equipment to suit the opening. It is likely that the economics of the latter alternative are seldom examined closely.

Mining sequences can be designed to allow intersections to be mined across existing roadways and minimse breakaways which are always bigger excavations. Equipment still needs to be able to turn the corners however.

Opening size is always going to be a compromise between a desire to minimise excavation and maximize stability versus minimizing ventilation resistance and maximizing the available work space.

The height of excavations also needs to be considered – is it better initially to mine less than the final working height in a thick seam for the benefit of more stable ribs? In the event of rib failure openings effectively become wider to the depth that the failure extends into the rib.

With regard to pillar design, the aim is usually to design pillars large enough to remain stable under increased vertical load caused by redistribution of the load previously carried by the extracted coal. Note that there may be several stages to this load redistribution as first and second workings are undertaken.

There are some cases where pillars are actually designed to yield (i.e. at least partially fail) in order to relieve stress on adjacent roadways.

During second workings with continuous miners, remnant pillars or stooks may be designed to remain stable for only a very short time and then be allowed to fail in the longer term (in fact this may be desirable to improve caving).

With regard to pillar stability, it is not only the plan area which is of importance but also the height to width ratio – a tall, thin pillar is more likely to fail than a short, fat one. The length and/or width required for a stable pillar is therefore going to increase as the working height increases.

The shape of an opening also affects its stability. A circular opening is the most naturally stable shape and has been used at mines, notably for shafts and drifts. While a circular profile may be more stable a flat floor is required for most purposes – generally there is little point in removing strata in the lower portion only to re-fill it again afterwards. An arch shape provides the benefit of a circular profile in the upper section while retaining a flat floor. The drawbacks of an arch section are:

• Because the width of an arch narrows towards the top, an arched roadway may need to be mined wider and/or higher than a rectangular roadway to obtain the dimensions required for given equipment to pass.
• Typical continuous miners have wide cutting heads to maximize production (coal being soft enough to not require excessive cutting power). Mining an arch section, particularly in stone requires a narrower head machine to excavate the profile and meet potential power limitations.
• In laminated or banded strata, the curved portions of roof often fall away in part so the shape tends to a rectangle of its own accord.
• An arched profile would be incompatible with the current design of longwall gate-end supports.

For these reasons, nearly all mines cut rectangular profile openings, apart from in shafts and drifts