Unexpected and uncontrolled explosion gas venting unfortunately occurs in many blasting operations, especially where water-saturated ground is present. Common factors involved in such incidents include the confinement of the blast, such as in drop cuts, as well as inexperience of the personnel involved in the design and execution of confined wet blasts. It is usually found in these incidents that the personnel involved have not accounted for the role of the water and/or the extra confinement within the blast.
Water is a blasting parameter that is usually not explicitly considered in the blast design process except for the type of explosive used and potentially for using reduced timing. The amount of water and its ease of movement within the blast area can affect other blasting parameters. The reason for this is that water can vary the amount of energy required to adequately fragment and move the rock which makes maintaining control of movement and venting of the explosion gases very difficult.
The role of water should be considered in all blasting operations as it is unacceptable in any operation, never mind how remote and removed from personnel or infrastructure, to have uncontrolled venting of explosion gases due to the potential risks this produces such as flyrock as well as air overpressure exceedances.
ROLE OF DESIGN AND WATER
To achieve satisfactory excavation in drop cuts generally requires blast holes drilled on a closer than normal pattern (compared to free face blasts). Powder factors in this type of blast are usually between 0.55 kg/m3 and 1.2 kg/m3, depending on rock properties (softer sediments or weathered materials, etc can be less, harder more competent material can be higher) or approximately 0.1 kg/m3 to 0.2 kg/m3 higher than a production blast in the same area.
Blast designers normally only consider the presence of water when choosing the type of explosive to utilise or when considering the timing design, such as decreasing delay intervals to minimise the risk of cutoffs, dynamic desensitisation and dead pressing. It is well known that water can aid in the coupling of the explosive energy to the ground, thus potentially increasing fragmentation in certain scenarios. What is not thought about in practical everyday blast design is how saturated the ground is and how this fluid is going to influence the results when the explosive energy is applied to the rock mass. Water is usually considered as simply a hindrance to the drill and blast process and/or introduces different procedures and methods to the processes, all of which can add complexity but, more importantly, increased risk. It is common that in a confined blasting situation, such as drop cuts or toe blasts, there is a much higher chance of unsatisfactory results occurring. These unsatisfactory results are not necessarily diggability or fragmentation, but environmental exceedances and/or flyrock occurrences. The poor results are made worse when water-saturated ground is present.
ROLE OF CONFINEMENT
The role of confinement in association with water-saturated ground conditions results in a changed blasting environment requiring a modified set of blast parameters be applied. These changed blast parameters may not be necessary in all confined blasting geometries but are essential in heavily confined water-saturated scenarios.
In confined blasting scenarios, the gas energy liberated from detonation of the explosives has a reduced means of returning to a neutral state, ie the pathways for energy release are restricted. The explosion gases when released move towards the path of least resistance which in unconfined blasting is the nearest free face, usually the front face. In confined blasts, the path of less resistance for these explosion gases is predominantly upwards, as there is usually no front or side free faces. The control of venting of these gases is usually obtained by increased stemming lengths; however, most quarry personnel do not want oversize in the stemming region, therefore stemming lengths are not usually increased to fully contain explosion gas release. Water in these confined scenarios adds further to restriction of explosion gas pathways that can often lead to unexpected blowouts and flyrock generation.
Another contributor to confinement in drop cut blasts is the loss of blastholes, again especially in wet ground. Most drop cuts are in the bottom of pits to develop new levels, etc and this is usually the area of the pit with the most water. Water-saturated ground generally does not lead to good drilling conditions. One of the most common errors made in developing drop cuts is to try to conduct large shots with lots of blastholes. Often this leads to multiple blasthole collapse, especially if blastholes are left for any length of time before blasting. Any loss of any blasthole will increase confinement and reduce pathways of explosion gas release which will increase the risk of poor results (excavation and environmental).
No blast, confined or unconfined, will be fully successful and achieve all the required results (excavation, environmental and crushing) unless all blastholes are drilled as per the plan and can be loaded with explosive as per the design. In drop cuts, loss of blastholes cannot be tolerated. As mentioned, it has been common for designers and operators to try large drop cut shots. It is possible to do this in the right conditions (dry conditions, use of specialised drilling techniques for blasthole stability or large, >150mm, blastholes are used) however, in the bulk of confined blasting scenarios in quarries, it is not. If water is present in significant amounts, then starting off small to establish a small free face and sumps for water control is essential, then building up shot size. Therefore, generally the recommendation to achieve a successful drop cut is to:
1. Always provide a free face in water-saturated ground. A free face should be created in front of the blast by either blasting a sump to the full depth of the next bench in those applications that are not environmentally sensitive or to blast (or use a rock hammer) to create a small slot (as little as two metres wide) that acts as a free face. These sumps must be fully dewatered prior to blasting (and drilling) to provide relief otherwise it will not be effective.
2. Try to dewater the area as much as possible before and during blasting. This is not only to enable blastholes to stand but also to reduce the influence of the water on the explosion gas pathways.
3. Utilise larger than normal stemming as the hot explosion gases find this harder to fluidise. Use stemming twice the normal diameter or about 20 per cent of the hole diameter but only in water-saturated ground.
4. Use extra stemming if collar pipes are used as they will affect stemming confinement.
5. Detect the presence of broken ground as it will further influence confinement and stemming control.
6. Start with small blasts until dewatering sumps and effective free faces can be established, otherwise lost blastholes and loss of control of explosion gas venting is highly likely.
7. Increase stemming heights in super-saturated areas by 50 to 80 per cent above site-specific specification (however, this can influence fragmentation and diggability results), especially in environmentally sensitive areas or where infrastructure or personnel are in close proximity.
Incidents unfortunately occur in confined water-saturated drop cut blasting scenarios and unless blast designers recognise the causes and implement design changes to mitigate the risk, then adverse blasting outcomes will continue to occur. The readers are referred to more information on this subject, including some case studies, at www.maxam-corp.com.au
Shane Slaughter is the technical manager for Maxam Australia.