Is the visual inspection enough? In some rock masses with no known history of fretting or collapse, then potentially yes. However, this is not usually the case. How do we judge what is safe and what is not? A clean smooth face with no overhangs may look safe but is it? Unfortunately, the answer can often be ?No?!
This is not the answer most people want to hear, as a visual inspection is often the only tool they have in making this judgement.Maxam routinely provides advice or designs for the safest and most stable final walls, but because this can be a last minute thought by the customer, it is always given with the explanation that damage may have been done to the wall due to past practices. It must always be remembered the best blast design and execution in the world will not overcome poor planning or poor practices.
We all come into contact with or work under or near high walls in most quarriesevery work day. It must be recognised thata high wall is not only the 10m to 15m bench or the final pit wall but can be the two metre face at the top of the ramp. Any wall or incline with the potential to unravel needs to be considered when the question is asked: ?How safe is this wall??
A common problem in quarries is the lack of planning put into the design and blasting of high walls (or pits) up until the final pit wall is about to be exposed. Often in these cases, the blast designer is requested to have a smooth stable final wall. Maxam always recommends to the designers how to produce a design that will achieve the most effective and safest wall possible with the tools and the rock mass at hand. However, a smooth wall may not be a safe wall. We cannot see into the rock mass and gauge what is behind the visible face.
We only need to look at some mining pit walls for examples where considerable timeand money had been spent blasting for smooth walls only for them to fret or, worse, fail and collapse. Unfortunately, in some rock masses, it is often too late to conduct effective blasting when there are only a few metres of burden left to the final wall.
If wall design is considered from the start of the process, there is a higher likelihood of achieving safe stable walls. Jointing in the rock mass fundamentally affects wall stability. All care must be taken well in advance of any final wall to reduce blasting effects on these joints, otherwise considerable time, money and effort could be wasted.
Consider the rock mass shown in Figure 1 (on page 14). It is all basalt yielding high-class aggregates. However, it is in two differing forms. The top rock mass is a highly fractured balled basalt with little or no preferred jointing planes. The lower rock mass is a columnar basalt. Each rock mass will require different designs (high wall angles) and blasting techniques to yield safe walls.
It is easy to achieve a smooth wall in the top rock mass but is this safe if it is a final wall intended to stand for the next 50 years? Unfortunately, this material weathers and frets, especially under the influence of water. If the wall was to stand for a few months or a year then normal blast design would likely produce a stable safe wall.
However, if the wall is to stand for a period in excess of this, the overall wall angle should be decreased and/or trim blasting considered to reduce damage to the wall, and water management at the top of the wall needs to be in place as well as extra care in the excavation process to dislodge loose material. The lower rock mass will stand at a higher overall design angle. However, it is prone to toppling failure. For this reason, limiting damage to this rock mass in not only the immediate blast in front, but for all blasting within at least 50 metres of the wall, must be considered.
When safe stable walls are thought about, then presplitting is often suggested. Presplitting is used in areas where visual ascetics are required in addition to safety. Presplits are a tried and true method of producing stable walls. However, they are not the only means to achieving stable safe walls.
In some rock types, presplitting is very difficult to effectively implement or is, in other rock types, an excess and waste of time, money and effort for no increase in safety. Maxam alerts its customers of the high potential to impact upon environmental limits from the use of presplitting.
Effective presplitting usually requires that the presplit blast holes are left un-stemmed. This is just not possible for most Australian quarries. Apart from the high air overpressure levels that would be recorded from un-stemmed blast holes, they also produce considerable amounts of dust. As well as this, the best presplits are produced when the entire presplit line is initiated at once. Again, this is not possible at most Australian quarries as the MIC will produce vibration concerns.
Therefore, to achieve acceptable results from presplitting and meet environmental and community expectation does require compromise of designs. As such, these compromises need to be incorporated into planning earlier rather than later.
We all need to achieve safe stable high walls in our operations. Always remember that just because a final wall ?looks pretty? and is smooth does not mean it is safe and stable. Never think about final high walls when you are about to blast them. This will lead to less than optimal results as the planning and execution of final limits designs should be integral to all pits planning and is the result of a recorded and managed process, not just one blast.