Jim Dowling - Mining Engineer

 

 

UNDERGROUND MINE PLANNING: An introduction to practical mine planning for small and medium sized mines

 

"Underground Mine Planning" by Jim Dowling

 

 

Extract from the Preface:-

Underground mining takes place in a uniquely challenging environment. Rock is excavated in confined spaces by drilling and blasting, or mechanical cutting, using increasingly powerful and sophisticated machinery. There is the constant risk of inflammable and poisonous gases, ionising radiation, high humidity, increasing heat and dust. These facts alone call for careful preparation, planning and control of operations. There are, however, further constraints which make meticulous planning vital, relating to geology, the environment, external financial influences and corporate pressure.

The target, the ore reserve, is, at best, only partially understood. It cannot be fully defined until mining is well underway. There are points of knowledge, mainly from exploratory drilling intersections, and large gaps with no data in between. There is a balance struck between more drilling, increasing exploration cost and reducing mining uncertainty. Some geological settings, for example seams of coal, are fairly uniform and predictable – but others such as metallic ore veins are not. These can vary dramatically over short distances, for instance in width, gradient or value, all of significant consequence in mine planning.

Safety is an important issue and is given the highest regard in mine planning decision making. Minimising accidents and health problems is a legal and civilized necessity. It is also cost-effective through lowering insurance premiums and avoiding stoppages to investigate dangerous occurrences.

Pressure on mines to protect the environment is increasing. Restrictions may be imposed on hours of work, location of surface facilities, size of blasts, employment of local people, for example. This all requires careful and continuous planning.

Then there is economics. Interest rates affect cost of capital and thereby justification for capital investments. Mineral prices vary and directly influence revenue from sales. Both of these are outside the mining company’s control.

There is, finally, over-riding pressure to maximise financial performance in order to give the best possible return to investors.

So given that underground mining is technically challenging, potentially hazardous, geologically uncertain, exposed to external factors and financially demanding, it comes as no surprise that thorough and wide-ranging forward planning is essential to achieve the most successful outcome.

Ore reserves are finite and new deposits are unlikely to be as valuable as those already discovered. As the largest and richest reserves are being mined out, there are fewer new ones left to find. Many smaller ore reserves have been identified but they are too small to support large scale mining. Yet as demand for most minerals is increasing and because small ore reserves can only be worked by small mines, it seems inevitable that there will be a resurgence in small and medium scale underground mining, to bring on stream these smaller deposits. This book is an attempt to address these trends.

Planning determines the optimum sequence of ore extraction, what to mine from where and at what point in time? At large mines some planning engineers will be devoted solely to this, working with specialist mine planning consultants and sophisticated software. But there are other aspects requiring planning, for instance:

Production - rate (tonnes/annum), cut-off grade.

Shafts and/or Drifts
- location, size, shape, length, equipment.

Developments
- number, direction, size, shape, spacing, gradient.

Transport
- type, unit sizes, number of units, tracked or trackless?

Hoisting
- type, skip or car and cage, dealing with spillage.

Drainage
- reducing inflow, location of pump stations, avoiding inrushes.

Stoping
- choice of method, stope planning, decline planning.

These questions will normally fall within the remit of the planning department. However, at smaller mines, line management, engineering and the geology and survey departments will deal with some of these. And there are other reasons why planning is different at smaller mines:

Smaller mines are generally more self-sufficient, relying less on external contractors.

They tend to use techniques specific to their smaller size – narrow vein stoping, small section developments, slushers, chutes, smaller gauge rail track, steeper drifts, rope haulages, short hole blasting, hand held drills, car and cage hoisting and so on.

At the same time, lower grade ores and decreasing economies of scale imply that higher standards of mine planning are required for these smaller mines.

As the subject matter is fairly broad, it has occasionally been difficult to avoid digressing into descriptions of mining methods. In some sections it has been impossible and a little background explanation has been inevitable. This is particularly so regarding Development, Rock Handling and Transport, where it is felt that mine planning procedures, to decide the best way forward, needed a brief summary of available options. As far as possible though, only planning matters have been included. For instance, planning aspects of shafts are covered in detail, techniques of shaft sinking are not. It is assumed the reader already has a basic knowledge of underground mining.

Some topics such as Ventilation, Support and Management do not have their own specific chapters. While these are important, in smaller mines they by and large do not determine the overall mining plan, but rather are influenced by it. They are, nevertheless, referred to throughout in their relevant planning contexts.

An important feature is the provision of intentionally simple line drawn sketches. Most of the topics are very graphical and for engineers to adequately demonstrate their knowledge they need to be able to produce simple drawings such as these. Subjects illustrated are mostly three dimensional and, for the avoidance of doubt, these orthographic projection terms are used:

Plan View is as seen looking vertically downwards.

Section
is as seen looking horizontally.

Cross Section
is as seen looking horizontally in the direction of the object’s longest horizontal dimension, if it has one, for instance the strike direction of a dipping vein.

Long, or Longitudinal, Section
, is as seen looking horizontally in a direction which is perpendicular to the direction of the object’s longest horizontal dimension.

Isometric View
is looking at an object along a sight line typically less than 45°  to the length/width/height axes, to indicate its three dimensional shape.

Perspective View
is an isometric view including size varying with distance.

Plane of Lode (POL)
is a view where the sight line is tilted to show the subject as if it were horizontal and a true plan view, so the sight line is perpendicular to the plane of the object, usually a mineral vein. In this view of workings within a dipping vein (lode), dimensions up-dip and along strike are accurate and can be scaled off, as if it were a plan view of a horizontal subject.

The role of mine planning is to maximise returns on capital. There are several criteria which are used to measure success. Two of the most widely accepted, and the ones used here, are Net Present Value (NPV) and Internal Rate of Return (IRR), as calculated by Discounted Cash Flow (DCF) analysis.

Planning is also necessary to prepare for actions required in the event of an emergency. Although emergency planning is not directly concerned with financial performance, it is the responsibility of mine planners, so a brief introduction to Emergency Planning is included as an Appendix.

Computer software is widely available for surface mine planning and, increasingly, for underground. Production and development schedules, extraction sequencing, optimisation and cash flow analyses can all be carried out rapidly and accurately with computers. They do not replace mine planners, but they undoubtedly improve the speed and accuracy of planning processes.

 

 

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