Sublevel Stoping Method

Sublevel Stoping Method

(orBlastholeStoping)

Introduction

Figure shows the general approach. Drilling and blasting is the main means of breaking the ore. The method extracts ore and leaves the stope empty.

The stope is often very large with the largest dimension in the vertical direction. The method only applies to vertical or steeply inclined ore bodies.

To prevent the collapse of the hanging wall, large ore bodies are divided into 2 or more stopes. Pillars are left between the stopes to ensure the stability of the hanging wall. These pillars could be horizontal or vertical.

The thickness of these pillars depends on many factors such as:

-Competence of the hanging wall

-Size of the stopes

-Method of drilling and blasting

-Importance of subsidence

-Value of the ore, and many other minor parameters.

In some cases, pillars can be designed to be extracted later, partially or fully. In such cases, this would be done during the final stages of mining activities, when the collapse of the hanging wall does not affect mining operations.

Main levels are driven at pre-determined places and between these, sublevels are driven. The ore is attacked from both main levels and sublevels.

The ore is drilled and blasted from the sublevel and main level drifts. Large pieces of ore fall into the empty stope and transported from the bottom of the stope to the loading stations.

Applications

The most important conditions of the ore body in order to be able to use sublevel stoping method are:

1)The ore body must have a steep dip i.e the inclination of the footwall must exceed the angle of repose.

2)The hanging wall and the footwall must be strong. Weak hanging wall means collapse and dilution. Weak foot wall means distortion of operations and plans.

Developments

The method needs a relatively comprehensive set of developments:

-A transport drift below the stope

-Raises between the sublevels for access to sublevels

-Production drifts or sublevels

-An undercut below the stope

-A loading system

Production

Drilling and blasting is the means of production. Generally there are two methods of production:

1)The traditional and conventional method called Ring Drilling. The method uses long hole extension rods to achieve the required length.

These rods are normally 1.2 – 1.8 m (4 – 6 ft) long. The bit

diameter is approximately 5 cm. The hole length varies but it cannot usually exceed 24m. Longer holes are possible but due to the increased risk of deflection, they are not usually applied.

2)Recently the application of down-the-hole hammers has made the Parallel Drilling more attractive.

In this system the hole diameter is 10-18 cm and the usual maximum applied is 20 cm.

Larger holes cover more volume of ore. Therefore fewernumber of holes are required because spacing between the hole is more. This means that fewer drilling machines are needed. The mine applying this system can therefore afford to buy more advanced machines.

Also, larger holes allow deeper holes to be drilled since the technique minimizes deflection. Holes of 45-60 m long are easy and the maximum applied is 90-100 m.

Longer holes are advantageous because the vertical spacing is increased between the sublevels. Therefore fewer developments are needed.

To date, no disadvantage has been noted by he use of these machines.

In sublevel stoping, drilling can be done independently of the other operations. Large sections of the ore can be drilled and

left in place for blasting later. Machine utilization factor is therefore high and hence the method can afford to use highly specialized andexpensive machines. These machines allow controlled and accurate hole alignment.

Mineral Transport

One of these systems can be used and at the bottom of the stopeAppropriate development layout should be prepared:

i) The ore may be loaded through chutes and directly into the

cars.

The fact that sublevel stoping may produce boulders to great

extent, makes this system of transport inconvenient. If

blockage happens in the chutes, blasting has to be done there.

This can be complicated and troublesome. Generally, in such

cases, the overall production can be affected seriously.

ii) Slushers may be used to draw the ore from the draw points

and load it into mine cars. This system solves the problem of

boulders.

iii) Overhead loaders can be used to load the ore from the draw

point into mine cars. LHD loaders are used in practice in

many large mines that have a trackless transport design.

Conclusions

With normal short hole drilling, the method is not particularly attractive. However, the recent introduction of the long and large holes has made sublevel stopingsomewhat popular.

The complicated and comprehensive development is a disadvantage.

Production operation is efficient since drilling, blasting and loading can each be done independently of the others. This gives the method potentials for using high degree of mechanization.

Mechanizability is high. Concentration is also high because there are few faces with a high output from each one.

The method in itself is not continuous in production but with a few stopes (2 or 3) the mine output can become continuous.

Generally, continuous production means highest production.

Sublevel stoping requires a regular layout of the stopes and ore boundaries. Stopes have to be in exact cubical or other similar shapes. Inside the shape, everything is mined and outside the stope nothing can be mined. Therefore no selective miningis possible at all. After blasting separation is also impossible. Sublevel stoping therefore needs regularity of the ore body more than most other methods.

Key factors in successful operation of sublevel stoping are:

-A thorough knowledge of the geology of the area. This is important when designing the stope. Also important when mining. It is enhanced by concentration.

-Good knowledge of ore boundaries

-Careful hole alignment control specially in parallel system

Another characteristic in production is that output is low in the beginning, increase towards the middle of the stope and then decreases again towards the end of production in each stope. Careful production planning is therefore necessary.

Recovery expected from a non-extracting pillar sublevel stoping is 35-50%. Pillars usually represent the majority of the ore lost.

If pillars are to be mined, thensuch decision must be made in the early stage of planning so that the same developments can be used for this operation.

If pillars are to be extracted, it should be done as soon as possible after the stope is mined out.

Advanced sublevel Stoping

The following are said with mechanized modern systems in mind.

Stope design

1)Length and width

Indeed the most important decision in the design process is the dimensions of the stope.

Factors affecting the length and width of the stope are:

-Ore body geometry

-Principal stress direction

-Competence of the hanging wall

-Optimum drill pattern

-Production drift layout

In sublevel stoping, planning is done before entry to the mine i.e. the stope dimensions are decided in advance. Therefore utmost care must be taken in design. What is required is good knowledge of the area and preferably some local experience.

2)Stope height

Factors affecting the decision are

-Competence of stope pillars and walls

-Slenderness ratio (pillars/empty stopes)

-Ore body dip

-Ore body thickness

-Hole depth capacity of drilling machines

-Fragmentation characteristics of the ore

-Level intervals in existing mines

In practice, where the ground is known to be competent, i.e. recent experience available etc, then the drill-hole length and its accuracy is the limiting factor for the stope height.

With the machinery available now, often the sublevels are eliminated. This represents a considerable saving in development cost.

3) Drawpoint location and design

There are important matters to note:

- Optimum spacing of drawpoints, within the constraints of

thestope dimensions. This is for uniform movement of

broken ore and hence maximum recovery.

-Drawpoints should be designed with ground stability in

mind. They should be made to last for the life of mine and when recovering pillars. They must be solid and durable.

-Floor surface, grade (for water) etc must be considered.

-Orientation with the main transport level for optimum

loader maneuverability and ground stability at intersections.

-Length, to allow articulated loaders to load when standing

straight.

Generally, in sublevel stoping, careful design and construction of drawpoints are essential for successful operation. Therefore:

-Extra care must be taken when driving the developments i.e. in blasting etc since blasting produces cracks in the strata.

-Rockbolts, grouts, wire mesh and shotcrete are all useful to increase the life of drawpoints.

Also from safety point of view, it is essential that these are kept stable and safe since the presence of workers is more in here than anywhere else.

More particularly, when pillars are to be mined, all efforts to construct the draw points well, are justified.

It must be noted that secondary blasting of boulders in chutes weakens the structure of the drawpoints greatly.

The best design for floor is a smooth surface, reinforced with concrete and poured with grout with an inclination of 3-4% towards the broken ore for:

-Water to run away from the drawpoint and roadways.

-Better penetration of the bucket loaders into the ore.

Undercutting

Proper and efficient undercutting is essential to the success of sublevel stoping. It affects blasting (better and higher output) and helps loading.

Undercut should not be more than a few (2-4) steps in front of the blasting on the sublevels (for back stability).

Undercut could be eliminated, but its presence is always preferred. In narrow stopes it is sometimes not made, mainly due to difficulty in access and operational activities caused by limitation of space.

Production drilling

This is the most important operation in the production process. The factors affecting the drilling operation and design are:

a)Drillability of the ore:

- Hardness

-Abrasiveness

-Fractures

-Specific gravity

b)Fragmentation characteristics of the ore

c)Type of hole and accuracy expected from drilling machine

d)Orientation and spacing of holes

e)Hole diameter and length

As mentioned before, sublevel stoping has recently become attractive, mainly because of the new drilling machines. Since the 1970’s the ring system has started to be replaced by parallel and hence using one of these drilling machines:

-High pressure pneumatic down-the-hole drills

-Electro-hydraulic rotary drills

-Electro-hydraulic rotary-percussive drills

These machines are capable of drilling 20 cm diameter holes to the depth of 100 m with less than 2% deviation.

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